HL Topic 9 Plant Biology

9.1 Transport in the xylem of plants:

Understandings:

1. Transpiration is the inevitable consequence of gas exchange in the leaf.

2. Plants transport water from the roots to the leaves to replace losses from transpiration.

3. The cohesive property of water and the structure of the xylem vessels allow transport under tension.

4. The adhesive property of water and evaporation generate tension forces in leaf cell walls.

5. Active uptake of mineral ions in the roots causes absorption of water by osmosis.

Applications and skills

A1. Adaptations of plants in deserts and in saline soils for water conservation

A2. Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.

S1. Drawing the structure of primary xylem vessels in sections of stems based on microscope images.

S2. Measurement of transpiration rates using potometers.

S3. Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.

Retro-Biology questions

• Explain the light dependent chemical reactions of photosynthesis.

• Explain the light independent chemical reactions of photosynthesis.

• Annotate a diagram to indicate the adaptations of a chloroplast to its function.

• Added: Calvin’s experiment to elucidate the carboxylation of RuBP

9.2 Transport in the phloem of plants

Understandings

1. Plants transport organic compounds from sources to sinks.

2. Incompressibility of water allows transport along hydrostatic pressure gradients.

3. Active transport is used to load organic compounds into phloem sieve tubes at the source.

4. High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.

5. Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.

Applications and skills

A1. Structure-function relationships of phloem sieve tubes.

S1. Identification of xylem and phloem in microscope images of stem and root.

S2. Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.

9.3 Growth in plants

Understandings 1. Undifferentiated cells in the meristems of plants allow

indeterminate growth.

2. Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.

3. Plant hormones (auxin is the only one expected) control growth in the shoot apex.

4. Auxin efflux pumps can set up concentration gradients of auxin in plant tissue.

5. Auxin influences cell growth rates by changing the pattern of gene expression.

Applications and skills:

A1. Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones.

A2. Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species.

9.4 Reproduction in plants

Understandings:

1. Flowering involves a change in gene expression in the shoot apex.

2. The switch to flowering is a response to the length of light and dark periods in many plants. E.g. Chrysanthemums are short-day plants, stimulated by long nights rather than short days.

3. Success in plant reproduction depends on pollination, fertilization and seed dispersal.

4. Most flowering plants use mutualistic relationships with pollinators in sexual reproduction. 87 out of 115 leading global crops depend to some degree upon animal pollination, including bees. This accounts for one-third of crop production globally.

Applications and skills

A1. Methods used to induce short-day plants to flower out of season.

S1. Drawing internal structure of seeds

S2. Drawing of half-views of animal-pollinated flowers.

S3. Design of experiments to test hypotheses about factors affecting germination.

9.0.1 Draw and label plan diagrams to show the distribution of tissues in the, root, stem and leaf of a dicotyledonous plant

Sunflower Stem

Plan diagrams of dicot stems and roots

Plan Diagram of Sunflower

Xylem

Cambium

Phloem

Plan diagram of leaf: NB use Privet Leaf and draw V shaped leaf with central vascular bundle

9.0.2 Outline 3 differences between the structures of dicots and monocot plants

Adventitious

Lateral branches

9.0.3 Explain the relationship between the distribution of tissues in the leaf and functions of

these tissues.

• The function of leaves is to produce food for the plant by photosynthesis.

• The leaf is adapted by its structure to carry out photosynthesis efficiently.

• The main part of the leaf is the leaf blade or lamina. It has a large surface area to absorb sunlight but is very thin (0.3mm). It is composed of 4 thin tissue layers with veins at intervals.

Leaf Anatomy

9.0.4 State that dicot plants have

• apical and lateral meristems

• Apical: primary meristems

• Lateral meristems: cambium

• Meristems generate new cells for growth of the plant.

9.0.5 Compare growth due to apical and lateral

meristems in dicot plants.

Meristems: allow growth throughout the plant’s life. This property is termed indeterminate growth. Two main types:

1) apical meristems (all flowering plants): located at the tips of roots and in the buds of shoots. Enable the plant to grow in length.

Called primary growth: allows roots to extend throughout soil and shoots to increase exposure to light and carbon dioxide; can produce new leaves and flowers.

Lateral meristem

• Present only in dicots. Growth in thickness = secondary growth.

• Caused by activity of lateral meristems called the vascular cambium and cork cambium.

• Extend along the length of roots and stems.

• Vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem.

• Cork cambium replaces the epidermis with periderm, which is thicker and tougher.

9.0.6 Describe mitosis and cell division in the shoot apex.

• Mitosis and cell division in the shoot apex (apical meristem) provide cells needed for extension of the stem and development of leaves.

• Some cells always remain in the meristem, continue through cell cycle. With each division, one cell remains in the meristem while the other increases in size and differentiates as it is pushed away from the meristem region.

• These cells are displaced to the edge of meristem.

Apical meristem can give rise to additional meristems including:

• Protoderm (epidermis); Procambium (vascular tissue); ground meristem (pith).

• Cells at edge stop dividing and undergo growth and differentiation to become either stem or leaf tissue.

• Leaves are initiated as small bumps at the side of the apical dome. Leaf primordia.

9.0.7 Draw the primary xylem vessels in sections of stems based on microscope images: Labels include xylem vessels, pits, spiral lignin, cell wall,

hollow lumen, remains of end walls. Tracheids include tapered end.

9.1.1 Explain the process of mineral ion absorption from the soil into roots by active transport: Co-

transport Please don’t label the cotransporter as Sucrose!

Why does transport have to be active?

• What kind of mineral ions are brought in by active co-transport?

• What does each mineral ion require?

• What follows? Process?

• How does plant increase its surface area for absorption?

9.1.2 State that terrestrial plants support themselves:

• Thickened cellulose

• Cell turgor

• Lignified xylem

9.1.3 Define transpiration

• The loss of water vapor from the leaves and stems of plants.

9.1.4 Explain how water is carried by the transpiration stream

• Define transpiration stream.

• Include the structure of xylem vessels

• Transpiration pull

• Cohesion

• Adhesion

• Evaporation

Xylem

9.1.5 State that guard cells:

• Can regulate transpiration by opening and closing stomata.

9.1.5 State that this plant hormone causes stomata to close

• The plant hormone abscisic acid causes the closing of stomata

9.1.6 Explain how abiotic factors affect the rate of transpiration in a typical terrestrial plant

• Light:

• Increased = increase transpiration; linked to photosynthesis and gas exchange.

• Temperature:

• Increased = increase transpiration; linked to increased evaporation; linked to evaporative cooling necessary to prevent protein denaturation.

• Wind:

• Increased = increase transpiration; linked to blowing away saturated air around stomata and steepening water vapor concentration gradient from spongy mesophyll to outside.

• Humidity:

• Increased = decreased transpiration; linked to making concentration gradient less steep.

9.1.7 Outline 4 adaptations of xerophytes that help to reduce transpiration

• Reduced leaves

• Rolled leaves

• Spines

• Deep roots

• Thickened waxy cuticle

• Reduced number of stomata

• Water storage tissue

• Low growth form

• CAM physiology C4 physiology

9.1.7 Outline 4 adaptations of halophytes that help reduce transpiration

• 1. Many become succulent by storing water, thus diluting the salt concentrations.

• 2. Several species (mangrove) secrete salt through salt glands.

• 3. Some species are able to compartmentalize Na+ and Cl- in the vacuoles of their cells, thereby preventing NaCl toxicity.

• 4. Sunken stomata on thickened leaves reduce water loss by creating a higher humidity near the stomata. Often include a more developed cuticle to minimize water loss.

• 5. The surface area of the leaves is reduced.

• 6. Like xerophytes, may actually close stomata using the action of the guard cells.

• 7. Some concentrate salts in leaves that later die and drop off.

• 8. Some short-lived plant species complete their reproductive life cycle during periods (such as rainy season) when the salt concentration is low (salt avoidance rather than tolerance).

9.1.8 Describe models of water transport in xylem using simple apparatus including blotting or filter

paper, porous pots and capillary tubing.

9.1.9 Be able to describe the construction of potometers in order to measure transpiration rate

• Include: (also define transpiration)

1. Fresh cut shoot held under water and transferred to apparatus under water to avoid air bubbles.

2. Apparatus includes rubber tubing which is sealed with small clamp after shoot is attached. Also includes glass pipette with 0.1 ml markings.

3. Apparatus is filled with water prior to shoot attachment.

4. As water is lost through shoot, meniscus in pipette moves lower. Change is recorded in 30 minutes.

5. Surface area of leaves is measured using tracing on cm paper or using mass of leaf, mass of known surface area ( 5 x 5 cm) to determine surface area of leaves using proportions.

9.1.10 Be able to design an experiment to test hypotheses about the effects of temperature or

humidity on transpiration rates. 1. The rate of transpiration is difficult to measure directly. Instead the rate of

water uptake is usually measured, using a potometer.

2. You should illustrate your potometer. Rubber tubing attached to glass pipette marked in 0.1 ml increments. Entire assembly filled with water to which freshly cut plant that was held under water and has been attached (again under water to avoid air bubbles) at the unused end of rubber tubing.

3. Initial level of water marked on pipette. 30 minutes allowed to stand. Final level marked.

4. What factor will you investigate?

5. How will you vary the level of this factor?

6. How many results do you need, at each level of the factor that you are varying?

7. How will you keep other factors constant, so that they do not affect the rate of transpiration?

8. How will you analyze your results?

• What is your initial hypothesis?

• Briefly explain the reasoning behind your hypothesis.

9.2.1 Outline the role of phloem in transport (active translocation)

• What is being translocated?

• Sugars (sucrose) and amino acids, hormones, small rna molecules; produced by plant or not

• Where is it moving?

• From Sugar source to sugar sink

• (source: photosynthetic tissue and storage organs) depends on season

• (sink: fruits, seeds, roots) net storer or user of energy.

Phloem- Sieve-tube member and companion cells. Explain structure and function

Phloem anatomy: describe

• Translocation is the transport of any biochemical in phloem whether produced by the plant or not.

• Phloem cells are sieve tube members which are arranged end to end to form long sieve tubes. Between the cells are sieve-plates, porous cross-walls that allow the flow of sap along the sieve tube.

• Alongside each sieve-tube member is a nucleated companion cell. This cell is a non-conducting cell which is connected to the sieve tube member by numerous channels, the plasmodesmata.

Movement of Sucrose

• Phloem loading of Sucrose: movement through cytoplasm and cell walls followed by active translocation using proton pumps and sucrose co-transporters.

Pressure flow model of bulk transport • Things to note about

this model:

• Pressure is highest at the source end and lowest at the sink end.

• Concentration of sucrose is also highest at the source end and lowest at the sink end.

• Water moves in through osmosis and out/recycled

9.2.3 Analyze data from experiments measuring phloem transport rates using aphid stylets and

radioactively-labelled carbon dioxide.

• Method of obtaining samples of phloem sap from single sieve tubes.

• High pressure inside sieve tubes pushes phloem sap out through the stylet and into gut of aphid.

• In 1940s began using 14CO2 in leaf for photosynthesis. Labels the sucrose produced.

I think there are errors in this table. Either it is 32.5 cm/hour or 325 mm/hour as a rate in exp. 1, etc.

9.3.1 Explain the Role of Auxin in phototropism as an example of the control of plant growth.

Begin by defining terms!

9.3.1 Explain the role of auxin in phototropism as an example of the control of plant growth

• Auxin – any chemical substance that promotes elongation; purified by Kenneth Thimann at Cal Tech and determined to be indoleacetic acid (IAA).

• IAA is transported down the stem from shoot apex at 10mm/hr. Seems to be transported directly through parenchyma tissue unidirectionally.

• Caused by polar distribution of auxin transport proteins (auxin efflux carriers) in the cell. Concentrated at the basal end of cell.

• Synthesized by apical meristem of a shoot. • Proton pumps play a major role in growth response of cells to auxin.

Auxin stimulates plasma membrane’s proton pumps. • Pumping of H+ increases voltage across membrane and lowers the

pH in cell wall which activates enzymes called expansins that break the cross-links (hydrogen bonds) between cellulose microfibrils. Increased ion uptake, uptake of water, increased turgor, increased cell wall plasticity.

• Auxin also alters gene expression and causes new protein production.

9.3.2 Describe micropropagation and its uses.

• Micropropagation: in vivo procedure that produces large numbers of identical plants. Much faster/takes less space than traditional methods of propagation

• 1: Cells from the shoot apex are cultured on nutrient agar. (derived from plant with some desirable feature). Depends on totipotency of plant tissues.) Usually meristems. Requires sterile plant parts.

• 2: Growth hormones are used to achieve maximum growth and quantity of particular types of plants. (auxin and cytokinin)

• 3. Rare and endangered species can be maintained.

• 4. Used in the case of orchids, which have very small seeds, to grow plants more reliably in sterile cultures.

• 5. Must maintain pathogen-free environment when culturing meristematic tissue.

• 6. Has been used to develop virus-free strains of existing plants. Allows international exchange of plant materials.

• 7. Can be used in conjunction with gene transfer creating GMO plants.

• 8. Micropropagation is very expensive.

9.4.1 Draw and label a diagram showing the structure of a dicot animal-pollinated flower

sepal, petal, anther, filament, stigma, style, ovary AND name of flower. I also want: Carpal, ovules

Stamen 11 marks

9.4.2 Draw and label a diagram showing the external and internal structure of a named dicot seed

• Should be non-endospermic. testa, micropyle, embryo root, embryo shoot, and cotyledons. I want the scar or hilum and the plumule at the end of embryo shoot.

• We used Phaeseolus multiflorus. 2 drawings

Bean Seed

9.4.1 Distinguish between pollination, fertilization, and seed dispersal

• For the egg within the embryo sac to be fertilized (definition: the formation of a zygote by the union of a male gamete with a female gamete inside the ovule) the male and female gametophytes must meet and their gametes must fuse.

• Occurs when pollen released from anthers and carried by wind or animals lands on a stigma (not necessarily on the same flower or plant).

• Each pollen grain produces a structure called a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac resulting in fertilization of the egg. Goes through micropyle.

• Definition of pollination: the transfer of pollen from an anther to a stigma.

Co-evolution of plants and animal pollinators

• What type of species-species interaction is this?

• Mutualism

• Example?

• Vanilla orchid and Melipona bee

• Advantages if only one animal pollinator?

• Plant: Assures pollen distributed to same species.

• Animal: nectar = sugar; pollen=protein

• Disadvantage: if either species goes extinct or becomes endangered.

Seed dispersal: associated with fruit; dispersed via wind or animals • Advantage of animal involvement:

• Seeds distributed away from source

• Less competition of seeds for nutrient resources.

9.4.4 Explain the conditions needed for the germination of a typical seed

• Water

• Oxygen

• Temperature

9.4.5 Outline the metabolic processes during germination of a starchy seed

• Inbibition of water (absorption): rehydrates the tissues

• Synthesis of gibberellin in the embryo’s cotyledon.

• Which stimulates amylase production

• Which catalyses the breakdown of starch to maltose.

iv) Maltose (like any good disaccharide used for transport) is transported from the food stores to the growth regions of the seedling, including the embryo root and the embryo shoot.

v) Maltose conversion: maltose is converted to glucose, which is either used in aerobic cell respiration as a source of energy, or is used to synthesize cellulose or other substances needed for growth.

vi) Photosynthesis kicks in as soon as the leaves of the seedling have reached light and have opened. Photosynthesis supplies the seedling with foods and the food stores of the seed are no longer needed.

9.4.6 Design experiments to test hypotheses about factors affecting

germination • Be aware of safety

• Some of the factors you should consider: temperature, plant hormones, salt concentrations, light, wavelengths of light, water levels, possible toxins (heavy metals, pesticides).

• Independent, dependent (length of shoot or root at X days) controlled variables (temp., oxygen, water, # seeds/dish/container, amount of time, method of measurement)

• Hypothesis

• Materials and procedures

• Data collected; method of analysis (t-test; regression)

9.4.7 Explain how flowering is controlled in long-day and short-day plants including the role of :

Phytochromes

Consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains.

1. Photoreceptor: covalently bonded to a nonprotein pigment or chromophore.

2. Kinase: photoreceptor domains interact with kinase domains to link light reception to cellular responses triggered by the kinase.

9.4.7 Explain how flowering is controlled in long-day and short day plants including the role of phytochrome.

• Conversion of Pr (Phytochrome red absorbing) to Pfr (far-red absorbing) in red or white light

• The gradual reversion of Pfr to Pr in darkness, and

• The action of Pfr as a promoter of flowering in long-day plants

• An inhibitor of flowering in short-day plants

• Long-day plants: flower in summer when nights have become short enough.

• Short-day plants: flower in autumn, when nights have become long enough.

• Pfr is active form and binds to receptor proteins in cytoplasm. Pr does not bind.

• In long-day plants, large enough amounts of Pfr remain at end of short nights to bind to the receptor, which then promotes transcription of genes needed for flowering. Pfr is a promoter in these plants.

• In short-day plants, the receptor inhibits the transcription of genes needed for flowering when Pfr binds to it. Pfr is an inhibitor in these plants.

• However, at the end of long nights, very little Pfr remains, so the inhibition fails and the plant flowers.

9.4.8 Outline methods used to induce short-day plants to flower out of season

• EG Pointsettia, Chrysanthemum

• Can maintain plants in pots in greenhouses with blinds.

• When the nights are not long enough to induce flowering, the blinds are closed to extend the nights artificially.

• Some growers will cover individual plants with black cloth for 12-15 hours a day until the flower buds begin to show color.

9.4.9 Describe 3 methods of plant hormone detection

• Hormones in plants, like hormones in animals, are regulators of gene transcription (transcription factors).

• Concentrations at which plant hormones are active can be as low as picograms/gram of plant tissue (1 million millionth of a gram).

• Five groups of hormones which are very diverse and require different extraction methods.

• ELISA (Enzyme linked immunosorbent assays): an antibody is synthesized to the hormone (which acts as the antigen). A primary antibody will bind to a sample of phloem or tissue that has been absorbed onto a plastic well. A secondary antibody that has an enzyme linked to it will bind to the primary antibody/hormone complex. The substrate of the enzyme is added so that there is a color change. The color is read and interpreted.

Flow scheme of gas/liquid chromatography

• Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition.

Microarray analysis detects changes in the pattern of gene expression. Begins by isolating RNA samples from cells and synthesizing DNA copies. Hybridizing

cDNAs linked to fluorescent dyes to small surface with millions of DNA probes. Laser light makes the fluorescent dyes give off light.

9s Plant Structure and Growth

1 Draw and label plan diagrams to show the distribution of tissues in root, stem and leaf of a dicotyledonous plant.

2. Outline five differences between the structures of dicotyledonous and monocotyledonous plants. Be able to distinguish cross sections of roots and stems in both monocots and dicots.

3. Explain the relationship between the distribution of tissues in the leaf and the functions of these tissues.

4. State that dicotyledonous plants have apical and lateral meristems.

5. Compare growth due to apical and lateral meristems in dicotyledonous plants.

6. Describe mitosis and cell division in the shoot apex. 7. Draw the structure of primary xylem vessels in sections of stems based on microscope images.

Review of Photosynthesis

Retro 3: Annotate a diagram to indicate adaptations of a chloroplast to its function

Organization of the thylakoid membrane

Retro 1: Explain the light dependent chemical reactions of photosynthesis

Retro 2: Explain the light independent chemical reactions of photosynthesis (Stroma)

Glycerate 3 phosphate

Triose Phosphate

Retro 4: (Topic 8 in review guide or textbook) Melvin Calvin’s experiments to

elucidate the carboxylation of RuBP • See website

• Lollipop apparatus

• Radioactive Carbon labelled carbon dioxide

• Sampling at various times after injection of label

• Chromatography of organic substances in solution.

• Exposure of chromatograph to photographic paper to find where the radioactivity was and determine what molecule the spot represented.

• First: Glycerate-3-Phosphate, then Hexose phosphate, then sucrose

Go to slide 42

2 Groups of Angiosperms (Flowering Plants)

• Commonly recognized and are named for the number of cotyledons, or seed leaves, present on the embryo of the plant.

• One group are the monocots which include orchids, bamboos, palms, lilies, and yuccas as well as the grasses such as wheat, corn, and rice.

• The other group is the dicots which include roses, beans, sunflowers, and oaks.

Monocots 1. One cotyledon

2. Veins, which carry vascular tissue, usually parallel IIIII

3. Vascular bundles usually complexly arranged/ spread throughout/random

4. Fibrous adventitious root system. (Unbranched roots grow from stems)

5. Floral organ parts usually in multiples of 3 (stamens, petals)

6. Pollen grain with one opening.

Dicots

1. Two cotyledons.

2. Veins usually netlike.

3. Vascular bundles usually arranged in ring.

4. Taproot with lateral branches usually present.

5. Floral parts usually in multiples of four or five.

6. Pollen grains with 3 openings.

Flowering Plant

The Plant Organs: Ch. 35

• Plants have three basic organs: root, stems and leaves.

• These organs are composed of different tissues, and these tissues are teams of different types of cells.

Tissue Systems

1. Tissue systems

a) Dermal tissue or epidermis: single layer of tightly packed cells that covers and protects all young parts of the plant.

The epidermis of leaves and most stems secrete a waxy coating called the cuticle that helps the aerial parts of the plant retain water.

Root hairs are epidermal extensions found near the tips of roots and are important in the absorption of water and minerals. Increase surface area.

Leaf Anatomy

Tissues continued

b) Vascular tissue: continuous throughout the plant, is involved in the transport of materials between roots and shoots.

Two types: i) xylem which conveys water and dissolved minerals upward from roots into the shoots

ii) phloem, which transports food made in mature leaves to the roots and to nonphotosynthetic parts of the shoot system, such as developing leaves and fruit. (More generally, transport of sugars from sugar source to sugar sink).

Xylem elements

• Tracheids and vessel elements.

• These are elongated cells that are dead at functional maturity.

Primary xylem: Helical or ring-shaped thickenings

• Thickenings made of the cellulose cell wall are impregnated with lignin. This makes them hard to resist inward pressures.

• Pores in the outer cellulose cell wall conduct water out of the xylem vessel and into cell walls of adjacent cells.

Tracheids and Vessel Elements

Phloem

• In phloem, sucrose, other organic compounds and some mineral ions are transported through sieve tubes formed by chains of cells called sieve-tube members which remain alive at functional maturity.

Phloem Sieve Tube Members

• Connected to nucleated companion cells which are connected to phloem via plasmodesmata.

• All that remains of sieve tubes are plasma membranes.

• Also have porous end walls.

Tissues continued

c) Ground tissue: tissue that is neither dermal tissue nor vascular tissue.

In dicot stems, ground tissue is divided into pith, internal to the vascular tissue, and cortex, external to the vascular tissue.

Among the functions of pith are photosynthesis, storage and support.

Perpetually embryonic tissues

d) Meristems: allow growth throughout the plant’s life. This property is termed indeterminate growth. Two main types:

1) apical meristems (all flowering plants): located at the tips of roots and in the buds of shoots. Enable the plant to grow in length.

Called primary growth: allows roots to extend throughout soil and shoots to increase exposure to light and carbon dioxide; can produce new leaves and flowers.

Apical meristems: can continue to increase in length and produce any number of extra branches throughout its life.

• Some cells always remain in the meristem, continue through cell cycle.

• Other cells are displaced to the edge of meristem.

• Cells at edge stop dividing and undergo growth and differentiation to become either stem or leaf tissue.

• Leaves are initiated as small bumps at the side of the apical dome. Leaf primordia.

2.) Lateral meristems

• Present only in dicots. Growth in thickness = secondary growth.

• Caused by activity of lateral meristems called the vascular cambium and cork cambium.

• Extend along the length of roots and stems.

• Vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem.

• Cork cambium replaces the epidermis with periderm, which is thicker and tougher.

2. Cell types

a) Parenchyma cells: have primary walls that are relatively thin and flexible.

Have a large central vacuole, are the least specialized cells; perform most of the metabolic functions of the plant.

For example, photosynthesis occurs within chloroplasts of parenchyma cells in the leaf.

Leaf cross section

More cell types

b) Collenchyma cells: have thicker primary walls than parenchyma cells. Grouped in strands or cylinders, collenchyma cells help support young parts of the plant shoot

e.g. “strings” of a celery stalk

Functioning collenchyma cells are living and flexible and elongate with the stems and leaves they support.

c) Sclerenchyma cells: have thick secondary walls usually strengthened by lignin; much more rigid then collenchyma cells.

Mature sclerenchyma cells cannot elongate and they occur in regions of the plant that have stopped growing in length.

Many are dead at functional maturity. The water-conducting vessel elements and tracheids in the xylem are sclerenchyma cells called fibers and sclereids that specialize entirely in support.

E.g. fibers: hemp fibers used for making rope and flax fibers for weaving into linen.

E.g. sclereids (shorter): impart hardness to nutshells and seed coats and the gritty texture to pear fruits.

3. Organs: cross sections of roots, stems and leaves

• A. Structure and function of roots: Roots absorb mineral ions and water from soil.

They anchor the plant in the soil and are sometimes used for food storage.

The structure of root systems gives them a large surface area for absorption by branching and by the growth of root hairs.

Primary Root Growth

Dicot vs. Monocot Root

Buttercup root Maize root

B. Structure and Function of Stems

• Stems connect the leaves, roots and flowers of plants and transport materials between them using xylem and phloem tissue.

• Stems support the aerial parts of terrestrial plants.

• Xylem tissue provides support especially in woody stems. Cell turgor also provides support, with both pith and cortex containing many cells that are usually turgid.

Dicot vs. Monocot Stem

Sunflower stem Maize stem

Plan diagrams show the distribution of tissues in the stem, root and leaf in a generalized dicotyledonous plant

From microscope image to plan diagram

C. Structure and function of leaves

• The function of leaves is to produce food for the plant by photosynthesis.

• The leaf is adapted by its structure to carry out photosynthesis efficiently.

• The main part of the leaf is the leaf blade or lamina. It has a large surface area to absorb sunlight but is very thin (0.3mm). It is composed of 4 thin tissue layers with veins at intervals.

Leaf Layers: Upper and lower epidermis

1. Upper epidermis: continuous layer of cells covered by a thick cuticle. It prevents water loss from the upper surface even when heated by sunlight.

Lower epidermis: is in a cooler position and has a

thinner waxy cuticle.

Both upper and lower epidermis: First line of

defense against physical damage and pathogenic

organisms.

Barrier is interrupted only by stomata and guard

cells.

2. Palisade mesophyll (Parenchyma)

• Densely packed cylindrical cells with many chloroplasts.

• Main photosynthetic tissue and is positioned near the upper surface where the light intensity is highest.

3. Spongy Mesophyll • Loosely packed rounded cells with few chloroplasts.

• Labyrinth of air spaces through which gases circulate up to the palisade region. Air spaces are large near the stomata.

• Provides the main gas exchange surface so must be near the stomata in the lower epidermis. Photosynthesis depends on gas exchange over a moist surface.

• Spongy mesophyll cell walls provide this surface. Water often evaporates from the surface and is lost in a process called transpiration.

• Transpiration is the loss of water vapor from the leaves and stems of plants. There are adaptations that minimize the amount of transpiration.

Leaf continued:

4. Stoma: A pore that allows carbon dioxide for

photosynthesis to diffuse in and oxygen to diffuse out.

5. Guard cells: this pair of cells can open or close the stoma and so control the amount of transpiration.

6. Xylem: brings water to replace losses due to transpiration.

7. Phloem: transports products of photosynthesis out of the leaf.

8. Both xylem and phloem are centrally located to be close to all cells. The vascular infrastructure also functions as a skeleton that reinforces the shape of the leaf.

Leaf Anatomy

Plan diagram of leaf

Leaf Transpiration Pull

9.1 Transport in xylem of plants 1. Explain the process of mineral ion absorption from the soil into roots by active

transport. 2. State that terrestrial plants support themselves by means of thickened cellulose,

cell turgor and lignified xylem. 3. Define transpiration (the loss of water vapour from the leaves and stems of

plants). 4. Explain how water is carried by the transpiration stream, including the structure

of xylem vessels, transpiration pull, cohesion, adhesion and evaporation. 5. State that guard cells can regulate transpiration by opening and closing stomata.

State that the plant hormone abscisic acid causes the closing of stomata. 6. Explain how the abiotic factors light, temperature, wind and humidity, affect the

rate of transpiration in a typical terrestrial plant. 7. Outline 4 adaptations of xerophytes and halophytes that help reduce

transpiration. 8. Describe models of water transport in xylem using simple apparatus including

blotting or filter paper, porous pots and capillary tubing. 9. Be able to describe the construction of potometers in order to measure

transpiration rates. 10. Be able to design an experiment to test hypotheses about the effect of

temperature or humidity on transpiration rates.

Transport in Angiospermophytes

Transport in plants occurs on three levels: 1. Uptake and loss of water and solutes by individual

cells such as the absorption of water and minerals from the soil by cells of a root.

2. Short-distance transport of substances from cell to cell at the level of tissues and organs, such as loading of sugar from photosynthetic cells of a mature leaf into the sieve tubes of phloem.

3. Long-distance transport of sap within xylem and phloem at the level of the whole plant.

Plant transport

Root absorption of mineral ions from the soil: Active Transport

• The pumping of solutes across membranes against their electrochemical gradients.

• The cell must expend metabolic energy (ATP) to transport a solute uphill.

• Active transport in root cells is involved in the absorption of potassium, phosphate, nitrate and other mineral ions from the soil. The concentration of these ions in the soil is usually much lower than inside root cells.

Electrogenic Pump

Proton Pump

Major pump in plant cells. Used to generate a hydrogen ion gradient and membrane

potential (voltage). Inside of the cell is negative while the outside of the cell is

positive. This is a form of stored energy that can be harnessed to perform cellular work.

Plants use this energy storage in the proton gradient and membrane potential to drive the transport of many different solutes.

This mechanism is called co-transport because the transport protein couples the downhill passage of one solute (H+ ions) to the uphill passage of another (e.g. nitrite).

Co-Transport

Substitute K+

or Na+ or Cl- or any

Ion or mineral

“Coattail”

• This “coattail” effect is also responsible for the uptake of the sugar, sucrose, by plant cells (other e.g. cells accumulate anions (nitrate) by coupling their transport to the inward diffusion of hydrogen ions through a cotransporter).

Differences in water potential drive water transport in plant cells:

• The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane.

• There are 2 factors that influence the direction of water movement in plant cells.

1) Water will move from a hypotonic (lower solute concentration) to a hypertonic (higher solute concentration) area.

2) The cell wall adds a second factor affecting osmosis: physical pressure. Physical pressure causes water to move. If a solution is separated from pure water by a selectively permeable membrane, external pressure on the solution can counter its tendency to take up water due to the presence of solutes.

Movement of Water in Animal and Plant Cells

Turgor

• When plant tissue is placed in pure water, the cell begins to swell and push against the cell wall, producing a turgor pressure.

• The partially elastic wall pushes back against the pressurized cell. When this wall pressure is great enough to offset the tendency for water to enter because of the solutes in the cell a dynamic equilibrium will be reached and the cell will be turgid.

• Healthy plant cells are turgid most of the time. Their turgor contributes to support in non-woody parts of the plant.

Wilted and Turgid

Terrestrial Plants and Support

• Terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem.

Selective Channels for Water

• As of 1990, scientists have suggested that even water transport is mediated by selective channels. They have since found these channels in both plant and animal cells.

• The specific channels for passive traffic of water are transport proteins called aquaporins.

• They do not affect the gradient or the direction of water flow, but rather the rate at which water diffuses down its gradient.

So: to summarize

• Nutrients (mineral ions) are actively co-transported from soil to root.

• Water flows into roots through osmosis and is inhibited by cell wall pressure that results in turgidity.

Absorption of Water and Mineral by Roots

• Route: enter through epidermis of roots, cross the root cortex, pass into the stele, flow up xylem vessels to the shoot system.

• Root tips: epidermis is permeable to water. Much of the absorption of water and minerals occurs near root tips.

• Root hairs are extensions of epidermal cells, and account for much of the surface area of roots.

Lateral Transport

• Water and solutes move from one location to another within plant tissues and organs.

• This occurs when water and minerals are absorbed by a root from the outer cells and moved to the inner cells of the root.

Root Lateral Transport

Role of the Endodermis

• In order for water and minerals to pass from the soil and the root cortex to the rest of the plant, they must enter the xylem of the stele .

• The endodermis surrounds the stele and functions as a last checkpoint for the selective passage of minerals from the cortex into the vascular tissue.

• Minerals in the cytoplasm continue through the endodermal cells via plasmodesmata and into the stele.

• Minerals traveling via the cortex encounter a dead end that blocks their passage into the stele.

Casperian Strip

• In the wall of each endodermal cell is the Casperian strip, a belt of suberin, a waxy material that is impervious to water and dissolved minerals.

• Material must cross the plasma membrane of the endodermal cell and enter the stele via the cytoplasm.

• This assures that all solutes must pass through at least one cell membrane before entering the xylem.

• Water then passes into the tracheids and vessel elements of the xylem using both diffusion and active transport.

Long-Distance Transport • Bulk flow functions in long-distance transport: • Diffusion is efficient for transport over distances defined by

cellular dimensions but is too slow for long-distance transport e.g. from roots to leaves.

• Water and solutes move through xylem vessels and sieve tubes by bulk flow, the movement of a fluid driven by pressure.

• In xylem, it is actually tension (negative pressure) that drives long-distance transport. Transpiration and evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots.

• In phloem, for example, hydrostatic pressure is generated at one end of a sieve tube, forcing sap to the opposite end of the tube.

Transport of Xylem Sap:

• Anatomy of xylem: tracheids are spindle-shaped, elongated cells with pits through which water flows from cell to cell.

• They are dead at functional maturity.

• When the living interior of a tracheid or vessel element disintegrates, the cell’s thickened cell walls remain behind forming a nonliving conduit through which water can flow.

• Water moves from cell to cell without having to cross thick secondary walls.

• The secondary walls of xylem are strengthened with lignin so tracheids function in support as well as water transport.

• Vessel elements are generally wider, shorter, thinner walled, and less tapered than tracheids. They are aligned end to end forming long micropipes, the xylem vessels.

• The end walls of vessel elements are perforated, enabling water to flow freely through xylem vessels. Water streams from cell to cell through perforated end walls and can also migrate laterally between neighboring vessels through pits.

Cohesion Adhesion Xylem

Other Features of Xylem

• There are no plasma membranes in xylem vessels, so water can move in and out freely.

• The lumen of the xylem vessel is filled with sap because the cytoplasm and nuclei have disintegrated.

• There are pores in the outer cellulose cell wall to conduct water out of the xylem vessel and into cell walls of adjacent leaf cells.

• Cellulose rings with lignin make xylem hard so that they can resist inward pressures.

Ascent of Xylem Sap

• Depends mainly on transpiration and the physical properties of water. Must rise against gravity.

1) At night, when transpiration is very low or zero (stomata are closed, temperatures are lower), root is still using energy to pump mineral ions into xylem. Water will flow in from root cortex, generating a positive pressure that forces fluid up the xylem. This upward push of xylem sap is called root pressure.

Transpirational Pull

2) In most plants, root pressure is not the major mechanism driving the ascent of xylem sap. At most, root pressure can force water upward only a few meters.

For the most part, xylem sap is not pushed from below by root pressure, but pulled upward by the leaves themselves.

• a) transpirational pull: water vapor diffuses from the moist air spaces of the leaf to the drier air outside via stomata.

• Evaporation from the water film coating the mesophyll cells maintains the high humidity of the air spaces.

• This loss of water causes the water film to form menisci (singular meniscus) that become more and more concave as the rate of transpiration increases..

• The tension of water lining the air spaces of the leaf is the physical basis of transpirational pull, which draws water out of the xylem.

b) Cohesion and Adhesion

• The cohesion of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water separating.

• Also helping to fight gravity is the strong adhesion of water molecules (H bonds) to the hydrophilic walls of the xylem cells.

C) Summary

• Long distance transport of water from roots to leaves occurs by bulk flow, the movement of fluid driven by a pressure difference at opposite ends of a conduit (xylem vessels or chains of tracheids).

• The pressure difference is generated at the leaf end by transpirational pull, which lowers pressure (increases tension) at the “upstream” end of the xylem

D) Contrast with short distance transport

• Differences in both solute concentration and pressure contribute to osmotic movement of water from cell to cell.

• Bulk flow depends only on pressure. Osmosis moves only water. Bulk flow moves the whole solution.

• Bulk flow is powered by the absorption of sunlight that drives transpiration by causing water to evaporate from the mesophyll cells. Ascent of xylem sap is ultimately solar powered…flow is passive.

Models can be used to investigate the transport of water in xylem

1. Models allow one factor or aspect to be studied independently of other factors.

2. Water has adhesive properties. Glass capillary tubes can be used to model adhesion between water and xylem vessel walls. Water adheres to glass so rises up the capillary tube. Mercury does not adhere so does not rise.

Evaporation of water can cause tension

3. Porous pot can be used to model flow in a xylem vessel due to transpiration from the leaf.

Porous pot is similar to leaf cell walls as water adheres to it and there are many narrow pores. Water evaporates from the surface of the pot so more water is drawn into the pot to replace losses. Water rises up in the tube.

4. Water is drawn through capillaries in cell walls.

• Blotting paper or a porous pot can be used to model capillary attraction or adhesion. Strip of blotting, filter or chromatography paper is suspended by a rubber stopper from the top of a test tube into a small amount of water at the bottom of the test tube.

• Paper is made of cellulose (like cell walls) so water rises up through it against gravity in pores in the paper.

Regulation of transpiration

90% of plant water loss through stomata Stomata are flanked by a pair of guard cells. Guard cells control the diameter of the stoma by changing

shape. By taking in water, guard cells become more turgid and bowed

which increases the size of the pore between the guard cells. Changes in turgor pressure that open and close stomata result

from the reversible uptake and loss of potassium ions by guard cells.

Potassium ions stored in vacuole, so tonoplast also plays a role in regulation.

Stomata are stimulated to open by (a) light, (b) depletion of CO2 and (c) circadian rhythms and biological clocks.

Stomata are stimulated to close by the plant hormone, abscisic acid which inhibits growth, promotes seed dormancy, and closes stomata during water stress.

Factors that Affect the Rate of Transpiration

1.) Light-leads to photosynthesis, the need for gas exchange, and increases in transpiration. Light stimulates stomatal opening. Guard cells close the stomata in darkness.

2.) Temperature-transpiration assists the plant in evaporative cooling and prevents the leaf from reaching temperatures that could denature various enzymes involved in photosynthesis and other metabolic processes.

So increases in temperature lead to increases in transpiration. Another way to look at it: heat is needed for evaporation of water from the surface of spongy mesophyll cells, so as temperature increases, evaporation increases, and transpiration (water loss) increases.

3.) Wind – leads to increase in evaporation. Wind blows the saturated air away and leads to increases in the rate of transpiration – which leads to an increased movement of water through the xylem and an increase in transpiration. High wind velocities can cause stomata to close. Still air, reduces the rate of transpiration.

4.) Humidity – water diffuses out of the leaf when there is a concentration gradient between the air spaces inside the leaf and the air outside. The air spaces inside are always nearly saturated. The lower the humidity outside the leaf, the steeper the gradient and therefore the faster the rate of transpiration. The higher the humidity outside, the less steep the gradient and transpiration rate is decreased.

5.) Number, size, and distribution of stomata 6.) Surface area of leaf. 7.) Carbon dioxide levels in air.

Potometers

Design an experiment to test hypotheses about the effect of temperature or humidity on transpiration

rates.

• Keep everything else constant except the independent variable.

• If examining temperature you can use 2 plants with 2 lights, one putting out light and heat and the other one putting out just light at the same wattage (compact fluorescent bulbs do not put out heat).

• If examining humidity you can also use 2 plants with and without use of a mister and a plastic bag. Can monitor humidity using kestrel. Control leaf exposed to air of known humidity. Experimental leaf bagged with plastic bag, misted with 5 ml of water and sealed.

• In both cases describe the potometer set up and/or the gas pressure probe set up.

Adaptations of xerophytes

• E.g. Cereus giganteus, the Saguaro or giant cactus that grows in deserts in Mexico and Arizona.

• Xerophytes are plants adapted to arid climates. They have various leaf modifications that reduce the rate of transpiration.

Adaptations of Xerophytes

1. They have small, thick leaves, an adaptation that limits water loss by reducing surface area relative to leaf volume.

2. A thick cuticle gives some of these leaves a leathery consistency and reduces water loss.

3. Stomata are concentrated on the lower (shady) leaf surface, and they are often located in depressions that shelter the pores from the dry wind. These pits are surrounded by “hairs”.

Structural Adaptation: Oleander

Xerophyte Leaf

Adaptations of Xerophytes

4. During the driest months, some desert plants shed their leaves.

5. Other plants such as cacti, subsist on water the plant stores in its fleshy stem during the rainy seasons (these modified stems are the photosynthetic organs of cacti; the spines are modified leaves).

CAM plants

6. Succulents: CAM stands for crassulacean acid metabolism. These plants have a metabolic adaptation that allow them to incorporate carbon dioxide into organic acids during the night.

During the daytime, the organic acids are broken down to release carbon dioxide in the same cells, and sugars are synthesized by the conventional C3 pathway.

Because the leaf takes in its carbon dioxide at night, the stomata can close during the day, when transpiration is most severe.

7. Root adaptations

• Some plants have very wide-spreading network of shallow roots to absorb water after rains.

Halophyte adaptations: Mangroves • Halophytes are adapted to

grow in water with high salinity. They are a promising biofuel because they do not compete with food crops for resources.

• Only 2% of all plant species.

• Come in contact with saline water through its roots or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs and seashores.

Glycophytes: not salt-tolerant and damaged fairly easily by high salinity. Halophytes: exhibit salt tolerance or

salt avoidance.

Adaptations of halophytes • 1. Many become succulent by storing water, thus diluting the salt

concentrations.

• 2. Several species (mangrove) secrete salt through salt glands.

• 3. Some species are able to compartmentalize Na+ and Cl- in the vacuoles of their cells, thereby preventing NaCl toxicity.

• 4. Sunken stomata on thickened leaves reduce water loss by creating a higher humidity near the stomata. Often include a more developed cuticle to minimize water loss.

• 5. The surface area of the leaves is reduced.

• 6. Like xerophytes, may actually close stomata using the action of the guard cells.

• 7. Some concentrate salts in leaves that later die and drop off.

• 8. Some short-lived plant species complete their reproductive life cycle during periods (such as rainy season) when the salt concentration is low (salt avoidance rather than tolerance).

9.2 Transport in the phloem of plants

• 1. Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).

• 2. Explain the structure and function relationships of phloem sieve tubes.

• 3. Analyze data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.

Role of phloem in active translocation of sugars (sucrose) and amino acids, hormones and small

RNA molecules • Translocation is the transport of any biochemical in

phloem whether produced by the plant or not. This includes plant hormones and small RNA molecules (a recent finding).

• Phloem cells are sieve tube members which are arranged end to end to form long sieve tubes. Between the cells are sieve-plates, porous cross-walls that allow the flow of sap along the sieve tube.

• Alongside each sieve-tube member is a nucleated companion cell. This cell is a non-conducting cell which is connected to the sieve tube member by numerous channels, the plasmodesmata.

Phloem

Companion Cells

• The nucleus and ribosomes of the companion cell may serve not only the cell but also the adjacent sieve-tube member, which has no nucleus or ribosomes of its own.

• In some plants, companion cells in leaves also help load sugar produced in the leaf into the sieve-tube members. The phloem then transports the sugar to other parts of the plant.

Tapping Phloem

• Phloem sap is an aqueous solution in which the prevalent solute is sugar, primarily sucrose. It may be as high as 30% by weight. Phloem sap may also contain minerals, amino acids, and hormones in transit from one part of the plant to another.

Phloem transport using aphids • Method of obtaining

samples of phloem sap from single sieve tubes.

• High pressure inside sieve tubes pushes phloem sap out through the stylet and into gut of aphid.

• In 1940s began using 14CO2 in leaf for photosynthesis. Labels the sucrose produced.

Can measure transport rates by timing when radioactive sucrose emerges from severed aphid stylets at different distances from

leaf.

Direction of phloem sap:

• Variable, but always from a sugar source to a sugar sink.

• Sugar source: sugar is produced by photosynthesis or through the breakdown of starch. Mature leaves are major sugar source.

• Sugar sink: an organ that is a net consumer or storer of sugar. Growing roots, shoot tips, stems, and fruit are sugar sinks supplied by phloem.

Modified Shoots

• A storage organ, such as a tuber (potato) or bulb (e.g. onion) may be either a source or a sink, depending on the season. Other solutes, such as minerals, may be transported to sinks along with sugar and later transported to developing fruit.

Phloem Loading and Unloading • (sieve tubes have lost their nuclei and ribosomes but

not their plasma membrane and transport proteins). • Sugar must move into sieve tube members before it

can be exported to sugar sinks. • Loading: 1) Sugar can move through the cytoplasm

(plasmodesmata) through diffusion. 2) Sugar can move through a combination of the cytoplasm and cell wall pathways.

• Can accumulate sucrose to concentrations 2 to 3 times higher than concentrations in mesophyll. Thus phloem loading requires active transport. Proton pumps do the work that enables the cells to accumulate sucrose. (N.B. Co-transport).

Phloem Sucrose Load

Sieve Tube Pressure

Phloem loading

Unloading

Concentration of free sugar in the sink is lower than that in the sieve tube. As the result of this gradient, sugar molecules diffuse from the phloem into the sink tissues, and water follows by osmosis.

Mechanism of translocation: bulk flow drives pressure (synonym is pressure flow).

Pressure Flow

Step 1: phloem loading results in a high solute concentration at the source end of a sieve tube, which lowers the water potential and causes water to flow into the tube.

Step 2: hydrostatic pressure develops within the sieve tube, and the pressure is greatest at the source end of the tube.

Step 3: at the sink end, pressure is relieved by the loss of water which follows the exodus of sucrose.

Step 4: water is recycled back from sink to source by xylem vessels.

Summary of sugar movement in plants

a) Cellular level: active transport across plasma membranes in phloem cells.

b) Short distance level of lateral transport within organs: sucrose migration from mesophyll to phloem via the cytoplasm through plasmodesmata or through cell walls.

c) Long distance level of transport between organs: bulk flow within sieve tubes.

9.3 Growth in Plants

• 1. Explain the role of auxin in phototropism as an example of the control of plant growth.

• 2. . Describe micropropagation and its uses.

Growth and Plant Hormones

• Plant development involves extensive coordination amongst individual cells within a plant.

• Requires communication within the plant.

• Many factors affect plant development and growth.

a) environmental factors such as day length and water availability.

b) receptors, which allow the plant to detect certain environmental factors.

c) genetic makeup of plant

d) hormones which are chemical messengers

More on plant hormones

• Have varying effects depending on the receptor’s location in the plant.

• Often a great deal of interaction between the different hormones to bring about the most appropriate response.

• In plants, a hormone may be produced throughout the plant, not just in an individual organ.

Control of Plant Growth Example: Auxin and phototropism

• Growth response that results in curvatures of whole plant organs toward or away from stimuli is called a tropism.

• Growth of a shoot toward light is called positive phototropism. Learned from studies of grass seedlings (oats).

• Shoot of a sprouting grass seedling is enclosed in a sheath called the coleoptile.

• Coleoptile grows straight up if kept in dark or lit uniformly from all sides.

• If illuminated from one side, it grows toward light as a result of a differential growth of cells on opposite sides of the coleoptile; cells on darker side elongate faster than cells on the brighter side.

Fritz Went’s 1926 experiments

Went named chemical

messenger, Auxin

Auxin (Greek – to increase) • Auxin – any chemical substance that promotes

elongation; purified by Kenneth Thimann at Cal Tech and determined to be indoleacetic acid (IAA).

• Auxins are found in the embryo of seeds, the meristems of apical buds and young leaves.

• IAA is produced by all cells in the stem on the side towards the light. It is transported into the nuclei of cells on the side of the stem opposite the light.

• Caused by distribution of auxin transport proteins (auxin efflux carriers) in the cell. Synthesized by apical meristem of a shoot.

• The auxin and a receptor in the nuclei form a complex that activates a proton (hydrogen ion) pump.

• Proton pumps play a major role in growth response of cells to auxin. Auxin stimulates plasma membrane’s proton pumps.

• Proton pump moves hydrogen ions into the spaces of the cell wall.

• Pumping of H+ increases voltage across membrane and lowers the pH in cell wall which activates enzymes called expansins that break the cross-links (hydrogen bonds) between cellulose microfibrils. Increased ion uptake, uptake of water, increased turgor, increased cell wall plasticity.

• This results in the elongation of the cells on the side away from the light.

• Auxin also alters gene expression and causes new protein production.

• Auxins have many functions within a plant. They are involved in:

• A) Stimulation of cell division in most meristematic tissues.

• B) Differentiation of xylem and phloem

• C) Development of lateral roots in tissue culture.

• D) Suppression of lateral bud growth when present in the apical bud.

• E) Stimulation of growth of flower parts.

• F) Induction of fruit production without pollination.

Micropropagation: Application of our knowledge of plants, plant meristems, and hormones to produce large numbers of progeny.

Micropropagation: in vivo procedure that produces large numbers of identical plants. Much faster/takes less space than

traditional methods of propagation • 1: Cells from the shoot apex are cultured on nutrient agar. (derived from plant

with some desirable feature). Depends on totipotency of plant tissues.)

• 2: Growth hormones are used to achieve maximum growth and quantity of particular types of plants. (auxin and cytokinin)

• 3. Rare and endangered species can be maintained.

• 4. Used in the case of orchids, which have very small seeds, to grow plants more reliably in sterile cultures.

• 5. Must maintain pathogen-free environment when culturing meristematic tissue.

• 6. Has been used to develop virus-free strains of existing plants. Allows international exchange of plant materials.

• 7. Can be used in conjunction with gene transfer creating GMO plants.

• 8. Micropropagation is very expensive.

Micropropagation stages

9.4 Plant Reproduction in Flowering Plants

Flowers, fruit, seeds and germination

9.4 Reproduction in plants • 1. Draw and label half-views of animal-pollinated flowers.

• 2. Draw the internal structure of seeds

• 3. Distinguish between pollination, fertilization and seed dispersal. Why is each vital?

• 4. Explain the conditions needed for the germination of a typical seed.

• 5. Outline the metabolic processes during germination of a starchy seed.

• 6. Design experiments to test hypotheses about factors affecting germination.

• 7. Explain how flowering is controlled in long-day and short-day plants, including the role of phytochromes.

• 8. Outline methods used to induce short-day plants to flower out of season.

• 9. Describe 3 methods of plant hormone detection. (ELISA, gas/liquid chromatography, microarray analysis).

Flower Structure: Review guide Lamium album; Plum flower ; Azalea

Reproduction in Flowering Plants

• A. Flowers: specialized shoots bearing the reproductive organs of the angiosperm sporophyte (2N stage ).

• Female gametes are contained in ovules in the ovaries of the flower while pollen grains, produced by stamens contain the male gametes.

• The 4 kinds of flower organs, in sequence from the outside to the inside of the flower are the sepals, petals, stamens and carpels.

i. Stamens:

• Male reproductive organ consisting of a stalk called a filament and a terminal structure called the anther.

• The male gametophytes (1N stage) are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers.

ii) Carpels:

• Female reproductive organ has ovaries at its base and a slender neck called the style.

• At the top of the style is a sticky structure called the stigma that serves as a landing platform for pollen.

• Within the ovary are one or more ovules. The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries.

iii. Sepals and iv. Petals

• Non-reproductive structures: sepals enclose and protect the floral bud before it opens, are usually green and more leaflike in appearance than the other floral organs.

• Petals are more brightly colored than sepals and advertise the flower to insects and other animal pollinators.

Fertilization

• For the egg within the embryo sac to be fertilized (definition: the formation of a zygote by the union of a male gamete with a female gamete inside the ovule) the male and female gametophytes must meet and their gametes must fuse.

Pollination • Occurs when pollen, released from anthers and carried

by wind or animals, lands on a stigma (not necessarily on the same flower or plant).

• Each pollen grain produces a structure called a pollen tube, which grows down into the ovary via the carpel and discharges sperm nucleus into the embryo sac resulting in fertilization of the egg.

• The pollen tube completes its growth by entering an opening (micropyle) at the bottom of the ovary.

• The sperm moves from the tube to combine with the egg of the ovule to form a zygote.

• Definition of pollination: the transfer of pollen from an anther to a stigma.

Pollen grain

Pollen grain

Pollen Tube

Mutualism in Pollination

• 85% of the world’s 250,000 species of flowering plants depend on insects or other animal pollinators for reproduction.

• Appears that the first angiosperms were pollinated by insects. Convincing fossil evidence that angiosperms and insects coevolved.

• Most flowering plants use mutualistic relationships with pollinators in sexual reproduction.

Mutualism • Plant benefits: flowers are pollinated so that they form

seeds and pass on their genes.

• Pollinators benefit: obtains nectar which is a source of energy and pollen, which is a source of protein.

• Co-evolution often entails developing a mutualistic relationship for pollination with one specific species of insect.

• E.G. Vanilla orchid pollinated by a species of Melipona bee. Advantage: insect will transfer pollen from flower to flower of the species and not to other species.

Bee orchid mimics bee and has scent to attract males

Flowers can employ various means to attract their vector:

• Red flowers are conspicuous for birds

• Yellow and orange flowers are noticed by bees

• Heavily scented flowers can be located at night

• If wind pollinated: often have inconspicuous, odorless flower parts.

Fertilization in flowering plants

• Actually a double fertilization:

• One of the two sperms produced by the pollen grain combines with the egg

• The other combines with two polar nuclei within the ovary to produce a triploid (3N) endosperm. The endosperm has the function of storing nutrients for the young plant.

Reproductive Adaptations: Fertilized ovules develop into seeds.

• Include reduced gametophyte, advent of the seed, and the evolution of pollen.

A seed consists of a sporophyte embryo packaged along with a food supply within a protective tough coat.

Ovaries containing fertilized ovules develop into fruits.

Ovule to Seed

E.G. Bean Seed

• Parts of the seed include the seed coat, called the testa, the embryo root (radicle), the embryo shoot (epicotyl) which has at its tip, plumule which consists of the shoot tip with a pair of miniature leaves, and the cotyledons.

• The cotyledons of the bean plant are fleshy before the seed germinates because they absorbed food from the endosperm when the seed developed.

• The endosperm is a food-storing tissue of the seed.

• Remember that you have to be able to draw the external and internal structure.

Unopened seed

Bean Seed

Embryonic Development

• The zygote gives rise to an embryo, and as the embryo grows, the ovule that contains it develops into a seed.

• The entire ovary, meanwhile, develops into a fruit containing one or more seeds.

• Fruits, carried by wind or by animals, help disperse seeds some distance from their source plants. So, the function of fruit is to disperse seeds.

Germination:

• As a seed matures, it dehydrates and enters a phase referred to as dormancy, a condition of extremely low metabolic rate and a suspension of growth and development.

• Conditions required to break dormancy vary between plant species. Some seeds germinate as soon as they are in a suitable environment. Others remain dormant and will not germinate, even if sown in a favorable place, until favorable environmental cue causes them to break dormancy.

Plants and breaking dormancy

• Desert plants germinate only after a substantial rain.

• Where natural fires are common, many seeds require intense heat to break dormancy.

• Where winters are harsh, seeds may require extended exposure to cold.

• Some require light for germination.

• Some seeds have coats that must be weakened by chemical attack as they pass through an animal’s digestive tract.

General Factors Needed for Seed Germination

1. Water to rehydrate the dry tissues of the seed.

2. Oxygen for aerobic cell respiration. Some seeds respire anaerobically if oxygen is not available but ethanol produced in anaerobic respiration usually reaches toxic levels.

3. Temperature suitable for germination. Because germination involves enzymes, temperature cannot be too low (low collision of enzyme’s active site with substrate) or too high (denaturation of enzyme). Seeds stay dormant when temps are too high or too low.

Be able to design experiments that evaluate the effect of some factor affecting seed germination

• Be aware of safety

• Some of the factors you should consider: temperature, plant hormones, salt concentrations, light, wavelengths of light, water levels, possible toxins (heavy metals, pesticides).

• Independent, dependent (length of shoot or root at X days) controlled variables (temp., oxygen, water, # seeds/dish/container, amount of time, method of measurement)

• Hypothesis

• Materials and procedures

• Data collected; method of analysis (t-test; regression)

Metabolic Events During Germination

i) Imbibition: the uptake of water causes the seed to expand and rupture its coat and also triggers metabolic changes in the embryo that enable it to resume growth.

ii) Gibberellin production: the plant hormone, gibberellin, is synthesized in the cotyledons of the seed.

iii) Gibberellin stimulates the production of the digestive enzyme, amylase, which catalyzes the digestion of starch into maltose in the food stores of the seed.

More Metabolic Events During Germination

iv) Maltose (like any good disaccharide used for transport) is transported from the food stores to the growth regions of the seedling, including the embryo root and the embryo shoot.

v) Maltose conversion: maltose is converted to glucose, which is either used in aerobic cell respiration as a source of energy, or is used to synthesize cellulose or other substances needed for growth.

vi) Photosynthesis kicks in as soon as the leaves of the seedling have reached light and have opened. Photosynthesis supplies the seedling with foods and the food stores of the seed are no longer needed.

Control of Flowering

• Plants can detect not only the presence of light but also its direction, intensity, and wavelength (color).

• Two major classes of light receptors: blue-light photoreceptors and phytochromes, photoreceptors that absorb mostly red light.

Phytochromes Consists of two identical proteins joined to form

one functional molecule. Each of these proteins has two domains.

1. Photoreceptor: covalently bonded to a nonprotein pigment or chromophore.

2. Kinase: photoreceptor domains interact with kinase domains to link light reception to cellular responses triggered by the kinase.

Phytochrome

• Chromophore of a phytochrome is photoreversible: can be converted back and forth between 2 isomers.

• Pr inactive absorbs red light of wavelength 660 nm or white light (400-700 nm)

• Pfr absorbs far-red light 730 nm.

• When Pr absorbs red it is converted to Pfr

• When Pfr absorbs far-red light, it is converted back to Pr

• In normal sunlight: Pr rapidly converted to Pfr. Pr more stable than Pfr, so in darkness Pfr very gradually changes into Pr.

Phytochrome Swing

Control of flowering

• When a plant has roots, stems and leaves only = vegetative phase.

• When a plant produces flowers = reproductive phase.

• This change occurs when meristems in the shoot start to produce parts of flowers instead of leaves.

• This involves a change to the pattern of gene expression in cells produced by the meristem.

• In some plants, the trigger is a particular day length.

• Experiments done in growth chambers where the length of light and dark periods are controlled showed that it is the length of darkness that matters, not the length of daylight.

Long-day (short night) vs. short-day (long night) plants

• Long-day plants: flower in summer when nights have become short enough.

• Short-day plants: flower in autumn, when nights have become long enough.

• Pfr is active form and binds to receptor proteins in cytoplasm. Pr does not bind.

• In long-day plants, large enough amounts of Pfr remain at end of short nights to bind to the receptor, which then promotes transcription of genes needed for flowering. Pfr is a promoter in these plants.

• In short-day plants, the receptor inhibits the transcription of genes needed for flowering when Pfr binds to it. Pfr is an inhibitor in these plants.

• However, at the end of long nights, very little Pfr remains, so the inhibition fails and the plant flowers.

Methods used to induce short-day plants to flower out of season:

• EG Pointsettia, Chrysanthemum

• Can maintain plants in pots in greenhouses with blinds.

• When the nights are not long enough to induce flowering, the blinds are closed to extend the nights artificially.

• Some growers will cover individual plants with black cloth for 12-15 hours a day until the flower buds begin to show color.

Flower Photocontrol

Photoperiod Response

Detecting Traces of Plant Hormones

• Hormones in plants, like hormones in animals, are regulators of gene transcription (transcription factors).

• Concentrations at which plant hormones are active can be as low as picograms/gram of plant tissue (1 million millionth of a gram).

• Five groups of hormones which are very diverse and require different extraction methods.

Analytical techniques include

• ELISA (Enzyme linked immunosorbent assays): an antibody is synthesized to the hormone (which acts as the antigen). A primary antibody will bind to a sample of phloem or tissue that has been absorbed onto a plastic well. A secondary antibody that has an enzyme linked to it will bind to the primary antibody/hormone complex. The substrate of the enzyme is added so that there is a color change. The color is read and interpreted.

Elisa microplate.

• If you inject an antigen (hormone) into an animal (goat, rabbit, mouse) the animal will make antibodies from B lymphocytes which can be cultured and antibodies extracted. These antibodies are used to detect the “antigen”

Flow scheme of gas/liquid chromatography

• Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition.

Microarray analysis detects changes in the pattern of gene expression. Begins by isolating RNA samples from cells and synthesizing DNA copies. Hybridizing

cDNAs linked to fluorescent dyes to small surface with millions of DNA probes. Laser light makes the fluorescent dyes give off light.

Phloem

Xylem: tracheids and vessels

9.3 Reproduction in angiospermophytes 9.3.1 Draw and label a diagram showing the structure of a

dicotyledonous animal-pollinated flower.

9.3.2 Distinguish between pollination, fertilization and seed dispersal.

9.3.3 Draw and label a diagram showing the external and internal structure of a named dicotyledonous seed.

9.3.4 Explain the conditions needed for the germination of a typical seed.

9.3.5 Outline the metabolic processes during germination of a starchy seed.

9.3.6 Explain how flowering is controlled in long-day and short-day plants, including the role of phytochrome.

Phototropism

Lettuce Seed Germination

Sleep Movement

Experiment Flowering Hormone

Biological Clock Collage

Modified shoots

Modified leaves

• 1. 9.1: Transport in the xylem of plants http://131.229.88.77/microscopy/Portfolios/Arlene/Portf olio4_files/Portfolio2.html Orange xylem (magnified 2500x) Essential Idea: Structure and function are correlated in the xylem of plants

• 2. Understandings Statement Guidance 9.2 U.1 Transpiration is the inevitable consequence of gas exchange in the leaf 9.2 U.2 Plants transport water from the roots to the leaves to replace losses from transpiration 9.2 U.3 The cohesive property of water and the structure of the xylem vessels allow transport under tension 9.2 U.4 The adhesive property of water and evaporation generate tension forces in leaf cell walls 9.2 U.5 Active uptake of mineral ions in the roots causes absorption of water by osmosis.

• 3. Applications and Skills Statement Guidance 9.1 A.1 Adaptations of plants in deserts and in saline soils for water conservation. 9.1 A.2 Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing. 9.1 S.1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images 9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7) 9.1 S.3 Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.

• 4. http://www.greenpeace.org/international/Global/international/photos/forests/2010/7/01amazon-clouds.jpg 9.1 U.1 Transpiration is the inevitable consequence of gas exchange in the leaf

• 5. 9.1 U.1 Transpiration is the inevitable consequence of gas exchange in the leaf • Exchange of these H2O and CO2 gases must take place in order to sustain photosynthesis • Stomata – pores • Transpiration – loss of water vapor from the leaves and stems of plants • Guard cells – found in pairs (1 in either side stoma), control stoma and can adjust from wide open to fully closed • Liverworts – exception of stomata Click4biology.com

• 6. 9.1 U.1 Transpiration is the inevitable consequence of gas exchange in the leaf • Stomata ( singular Stoma ) are pores in the lower epidermis formed by two specialized Guard Cells. • The epidermis and its waxy cuticle is impermeable to carbon dioxide and water. • If the water loss is too severe the stoma will close. This triggers mesophyll cells to release abscisic acid (hormone). Which stimulates the stoma to close. Click4biology.com

• 7. 9.1 U.2 Plants transport water from the roots to the leaves to replace losses from transpiration • Water evaporates into the air • The water loss from the leaf draws new water vapor from the spongy mesophyll (symplastic & apoplastic movement) into air space. • In turn the water molecules of the Mesophyll space draw water molecules from the end of the xylem. • Water molecules are weakly attracted to each other by hydrogen bonds (Cohesion). Therefore this action extends down the xylem creating a 'suction' effect.

• 8. 9.1 U.3 The cohesive property of water and the structure of the xylem vessels allow transport under tension Plants are however more 'architectural' in their Structure with adaptations which provide support for a static structure, much in the same way as seen in buildings. Thickening of the cellulose cell wall and lignin rings

• 9. 9.1 U.3 The cohesive property of water and the structure of the xylem vessels allow transport under tension • In the diagram to the above left the xylem shows a cylinder of cellulose cell wall with annular lignification in rings. • The photograph to the left show the thickening of the cellulose walls of the xylem. Click4biology.com Click4biology.com

• 10. 9.1 U.4 The adhesive property of water and evaporation generate tension forces in leaf cell walls

• 11. 9.1 U.5 Active uptake of mineral ions in the roots causes absorption of water by osmosis. Pathways for water movement: (a) Water enters epidermal cell cytoplasm by osmosis. The solute concentration is lower than that of soil water due to the active transport of minerals from the soil water to the cytoplasm. Symplastic Pathway (b) to (c): water moves along a solute concentration gradient. There are small cytoplasmic connections between plant cells called plasmodesmata. In effect making one large continuous cytoplasm. Apoplastic Pathway (d) to (e):water moves by capillarity through the cellulose cell walls. Hydrogen bonding maintains a cohesion between water molecules which also adhere to the cellulose fibers. Click4biology.com

• 12. 9.1 U.5 Active uptake of mineral ions in the roots causes absorption of water by osmosis. Loading water into the xylem: • Concentration of mineral ions in the root 100 times greater than water soil • Active transport, protein pumps • Minerals are actively loaded into the xylem in the roots which in turn causes water to enter the xylem vessel. • Chloride for example is actively pumped creating a water potential gradient that moving water passively into the xylem. • Pressure within the xylem increases forcing water upward (Root Pressure).Click4biology.com

• 13. 9.1 U.5 Active uptake of mineral ions in the roots causes absorption of water by osmosis. • Fungus grows on the surface of the roots (sometimes in the cells of the root) to overcome the problem of ions slow movement as they bind to the surface of soil particles. http://www.sanniesshop.com/images/symbiosis-3.jpg http://planthealthproducts.com/wp-content/uploads/2013/04/mycroot.gif

• 14. 9.1 A.1 Adaptations of plants in deserts and in saline soils for water conservation. • Plants adapted to reduce water loss in dry environments. Examples of such water stress habitats include: • Desert (high temp, low precipitation) • High Altitude & High Latitude (low precipitation • Tundra where water is locked up as snow or ice. • Areas with sandy soil which causes water to rapidly drain. • Shorelines that contain areas of high salt levels

• 15. 9.1 A.1 Adaptations of plants in deserts and in saline soils for water conservation. Waxy Leaves: •The leaves of these plant have waxy cuticle on both the upper and lower epidermis •The waxy repels water loss through the upper and lower epidermal cells. If an epidermal cell has no cuticle water will rapidly be lost as the cellulose cell wall is not a barrier to water loss.

• 16. 9.1 A.1 Adaptations of plants in deserts and in saline soils for water conservation. Firs and Pines: •Confers have their distribution extended beyond the northern forests. Plants in effect experience water availability more typical of desert environments. •Needles as leaves to reduce surface area. •Thick waxy cuticle •Sunken stomata to limit exposer. •No lower epidermis. http://fc08.deviantart.net/fs71/i/2014/127/1/c/pine_needles_by_neelfyn-d7hj7z4.jpg

• 17. 9.1 A.1 Adaptations of plants in deserts and in saline soils for water conservation. Succulent •The leaves have been reduced to needles to reduce transpiration. •The stem is fleshy in which the water is stored. •The stem becomes the main photosynthetic tissue. http://upload.wikimedia.org/wikipedia/commons/c/c9/Echinocactus_grusonii_kew.jpg

• 18. 9.1 A.1 Adaptations of plants in deserts and in saline soils for water conservation • Species of grass occupying sand dunes habitat. • Thick waxy upper epidermis extends • Leaf rolls up placing, containing hairs. The stomata in an enclosed space not exposed to the wind. • The groove formed by the rolled leaf also acts as a channel for rain water to drain directly to the specific root of the grass stem. http://upload.wikimedia.org/wikipedia/commons/a/a2/ AmericanMarramGrassKohlerAndraeStateParkLake Michigan.jpg click4biology

• 19. 9.1 A.2 Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.

• 20. 9.1 S.1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images https://pw-biology-2012.wikispaces.com/file/view/Xylem.png/354386126/800x479/Xylem.png https://classconnection.s3.amazonaws.com/263/flashcards/1226263/jpg/6959590336_b28341ed7a_z1354494841351.jpg

• 21. Transpiration •Transpiration is the loss of water from a plant by evaporation •Water can only evaporate from the plant if the water potential is lower in the air surrounding the plant •Most transpiration occurs via the leaves •Most of this transpiration is via the stomata. 9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7) In the plant: factors affecting the rate of transpiration 1.Leaf surface area 2.Thickness of epidermis and cuticle 3.Stomatal frequency 4.Stomatal size 5.Stomatal position

• 22. A SimpleA Simple PotometerPotometer 1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’ Air tight seals Plastic tubing Graduated scale Capillary tube Leafy shoot cut under water Water evaporatesWater evaporates from the plantfrom the plant Movement of meniscus is measured over time 9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7)

• 23. 1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’ The rate of water loss from the shoot can be measured under different environmental conditions volume of water taken upvolume of water taken up in given timein given time Limitations •measures water uptake •cutting plant shoot may damage plant •plant has no roots so no resistance to water being pulled up Water is pulled upWater is pulled up through the plantthrough the plant 9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7)

• 24. 6 Environmental Factors Affecting Transpiration6 Environmental Factors Affecting Transpiration 1. Relative humidity:- air inside leaf is saturated (RH=100%). The lower the relative humidity outside the leaf the faster the rate of transpiration as the Ψ gradient is steeper 2. Air Movement:- increase air movement increases the rate of transpiration as it moves the saturated air from around the leaf so the Ψ gradient is steeper. 3. Temperature:- increase in temperature increases the rate of transpiration as higher temperature • Provides the latent heat of vaporisation • Increases the kinetic energy so faster diffusion • Warms the air so lowers the Ψ of the air, so Ψ gradient is steeper 9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7) 4. Atmospheric pressure:- decrease in atmospheric pressure increases the rate of transpiration. 5. Water supply:- transpiration rate is lower if there is little water available as transpiration depends on the mesophyll cell walls being wet (dry cell walls have a lower Ψ). When cells are flaccid the stomata close. 6. Light intensity :- greater light intensity increases the rate of transpiration because it causes the stomata to open, so increasing evaporation through the stomata.

• 25. 9.1 S.3 Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.

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