Topic 9: Plant Science

9.1: Plant Structure and Growth

• 9.1.1:Draw and label plan diagrams to show the distribution of tissues in the stem and leaf of a dicotyledonous plant.

9.1.1

IB Question: The main parts of growing plants are roots, stems and leaves. Draw a plan diagram to show the arrangement of tissues in the stem of a dicotyledonous plant. [5] M10/4/BIOLO/HP2/ENG/TZ1/XX

9.1.2:Outline three differences betweenthe structures of dicotyledonous and

monocotyledonous plants.

Describe the differences in the structures of dicotyledonous plants andmonocotyledonous plants. [5] M11/4/BIOLO/HP2/ENG/TZ2/XX

9.1.3: Explain the relationship between thedistribution of tissues in the leaf and

the functions of these tissues.

Draw a labeled diagram showing the tissues present in a dicotyledonous leaf. [4] M09/4/BIOLO/HP2/ENG/TZ2/XX

Leaf tissues Function

Epidermis

The epidermis is an outer support tissue that holds the leaf together. The upper epidermis is exposed to direct sunlight so it has a waxy coating (called the waxy cuticle) to prevent water loss. The lower epidermis has specialized guard cells. Guard cells form adjustable pores that control the rate of transpiration (water loss) as well as gas exchange.

Pallisade mesophyll

The center of a leaf is composed of mesophyll, which has 2 layers: palisade mesophyll and spongy mesophyll. Palisade mesophyll is an upper layer of elongated cells that contain many chloroplasts to absorb light and carry out photosynthesis.

Spongy mesophyll

The spongy mesophyll is a lower layer containing loosely packed cells. The loose arrangement of cells allows water, O2 and CO2 to diffuse easily. Rapid diffusion is necessary for transporting CO2 for the light independent reactions of photosynthesis.

Vascular tissue

Leaves contain vascular bundles in the spongy mesophyll layer. Vascular bundles contain xylem tissue and phloem tissue. Water moves through xylem tubes and dissolved glucose moves through phloem tubes. The phloem transports the products of photosynthesis to other parts of the plant

9.1.3: Explain the relationship between the distribution & function of leaf tissues

9.1.4:Identify modifications of roots, stemsand leaves for different functions:

bulbs, stem tubers, storage roots andtendrils.

9.1.5: STATE: Dicotyledonous plants have apical and lateral meristems

9.1.6: Compare growth due to apical andlateral meristems in dicotyledonous

plants.

Apical meristemsLateral meristems

Occur in the tips of stems and roots

Occur between xylem and phloem in stems

Produces soft tissues Produces hard xylem tissue: wood

Lengthens roots and stemsWidens stems to support the weight of tall plants

Allows plants to develop special structures like leaves and flowers

Allows trees to grow tall, helping them to compete effectively for light.

Found in all phyla of plants Absent in mosses and horsetails.

9.1.7: Explain the role of auxin inphototropism as an example of the

control of plant growth.

Explain the role of auxin in phototropism. [8] M10/4/BIOLO/HP2/ENG/TZ2/XX+

9.1.7: Explain the role of auxin in phototropism as an example of plant growth

Plants use hormones to control the growth of roots and stems. When a plant releases a growth hormone in response to an external stimulus we call the resulting directional growth a tropism.One type of tropism is phototropism: growth in response to light. Phototropism may be either positive (towards the light) or negative (away from the light). Phototropism requires the absorption of light by proteins known as phototropins. Phototropins change to a new conformation (a new shape) when they absorb certain wavelengths of light. The new shape causes phototropins to act as ‘on switches’ for a gene that regulates the activity of auxins.Auxins cause cells to become longer. Therefore, by releasing auxins on one side of a stem but not on the other side, a stem will bend because one side becomes longer than the other. When a stem detects directional light it moves auxins from its sunny side to its shady side, which causes the shady side to bend toward the light. Bending toward light allows plants to absorb more sunlight and be able to photosynthesize at a faster rate.Auxins cause cells to become larger in the following way: 1) they cause cells to actively transport hydrogen ions out of the cell, making the outside acidic; 2) the acid outside the cell makes the cell wall softer; 3) softer cell walls make the cells more stretchable; and 4) stretchy cells are bigger because the internal pressure inside the cell causes the cell wall to bulge out.

auxin is a plant hormone; produced by the tip of the stem/shoot tip; causes transport of hydrogen ions from cytoplasm to cell wall; decrease in pH / H+ pumping breaks bonds between cell wall fibres; makes cell walls flexible/extensible/plastic/softens cell walls; auxin makes cells enlarge/grow; gene expression also altered by auxin to promote cell growth; (positive) phototropism is growth towards light; shoot tip senses direction of (brightest) light; auxin moved to side of stem with least light/darker side causes cells on dark side to elongate/cells on dark side grow faster; [8 max] Accept clearly annotated diagrams for phototropism marking points.

9.2: Transport in angiospermophytes

• 9.2.1: Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs.

9.2.1

Outline the adaptations of plant roots for absorption of mineral ions from the soil. [5] M10/4/BIOLO/HP2/ENG/TZ1/XX

9.2.2: List ways in which mineral ions in thesoil move to the root.

• Mineral ions move through soil to the roots of plants by one of three means:

1) Diffusion

2) Mass flow of water in the soil carrying ions

3) Mutualistic association with hyphae of fungi

9.2.3: Explain mineral ion absorption from the soil into roots by active transport

Plants take up mineral ions by active transport. In active transport, mineral ions are moved against a concentration gradient, which requires: 1) numerous mitochondria in root hair cells for ATP production; 2) protein channels in the cell membrane for active transport; and 3) oxygen in the soil that is absorbed by root hairs for cell respiration.In order for mineral ions to be pumped into the roots the mineral ions must make physical contact with protein pumps on cell membranes of root hair cells.The mineral ions move into contact with root hair proteins in one of two ways: 1) diffusion and 2) mass flow. Mass flow is when draining water carries minerals.Diffusion and mass flow are slow processes because mineral ions bind to the surface of soil particles. Therefore many plants evolved mutualistic relationships with fungi to improve the rate of mineral absorption. The long thread-like hyphae of the fungus intertwine with the root hairs of the plant, and extend into the soil. Hyphae are highly efficient at absorbing mineral ions from the soil, which they share with the plant roots in exchange for sugars. The photo below shows the thread-like hyphae of a fungus growing amongst a plant's root hairs

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

9.2.5 : Transpiration is the loss of water vapour from he leaves and stems of plants

Root pressure

adhesion

transpiration pull

9.2.6: Explain how water is carried bythe transpiration stream, including

the structure of xylem vessels,transpiration pull, cohesion, adhesion

and evaporation.

Describe how water is carried by the transpiration stream. [7] M09/4/BIOLO/HP2/ENG/TZ1/XX

Explain the effect of light intensity and temperature on the rate of photosynthesis. [8] M09/4/BIOLO/HP2/ENG/TZ2/XX

Photosynthesis and transpiration occur in leaves. Explain how temperature affects theseprocesses. [8] M10/4/BIOLO/HP2/ENG/TZ1/XX

Define the term transpiration and explain the factors that can affect transpiration in atypical terrestrial plant. [9] M11/4/BIOLO/HP2/ENG/TZ2/XX

9.2.6: Explain how water is carried by the transpiration stream

The xylem is a system of long hollow tubes responsible for replacing water lost during transpiration and photosynthesis. The xylem is made of two kinds of cells: tracheids and vessels. Xylem cells die before they are functional: after they die they become long, narrow tubes with pores at each end that allow water to pass through them. The xylem sap moves from roots, through the stem, to the leaves without any energy being spent by the plant.

Three processes cause water to rise up the xylem tube:Root pressure: Water moves into roots by osmosis because the roots have high concentrations of solute. This causes a positive pressure that forces sap up the xylem towards the leaves. Root pressure is highest in the morning before the stomata open and allow transpiration to begin. Capillary action: The xylem is a long tube that is microscopically thin. When water molecules contact the surface of the xylem there is adhesion. Adhesion tends to pull water molecules upward by a process called capillary action.Transpiration pull: When water molecules evaporate from leaves the water potential drops at the stomata. The low pressure then pulls new water molecules towards the stomata from the xylem vessels. As these water molecules move they pull on water molecules behind them due to cohesion (caused by hydrogen bonding). The pull is transmitted from one water molecule to the next, all the way to the roots.

9.2.7: STATE: Guard cells can regulate transpiration by opening and closing stomata.

9.2.8: STATE: The plant hormone abscisic acid causes closing of the stomata.

9.2.9

9.2.9: Explain how abiotic factors affect transpiration rate in a terrestrial plant

Transpiration is the loss of water (by evaporation) from the leaves and stems of plants. In a typical terrestrial mesophytic plant, the rate of transpiration:

Decreases: when humidity increases because at high humidity, the air is saturated with water so evaporation stops.

Increases: when light intensity increases because more stomata open in strong light to maximize the rate of photosynthesis.

Increases: when temperature increases because water molecules evaporate faster creating negative pressure at the stomata.

Increases: as wind speed increases because air movement carries water vapor away from stomata creating negative pressure at the stomata.

9.2.10: Outline four adaptations ofxerophytes that help to reduce

transpiration.

Outline adaptations of xerophytes. [4]N09/4/BIOLO/HP2/ENG/TZ0/XX

9.2.11: Outline the role of phloem in activetranslocation of sugars (sucrose)

and amino acids from source(photosynthetic tissue and storage

organs) to sink (fruits, seeds, roots).

9.3: Reproduction in angiospermophytes

9.3.1: Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower.

9.3.2Distinguish between pollination,fertilization and seed dispersal.

Pollination is the attachment of a pollen grain on the stigma of a flower, by wind or by an animal. After pollination, a pollen grain grows a long pollen tube that stretches down the style to the ovary. The pollen tube enters the ovary through a small opening, the micropyle, and releases sperm to fertilize the eggs.

Fertilization is the fusion of an egg and sperm to form a zygote. A zygote develops into an embryo, and in flowering plants the embryo is packaged in a seed coat with food reserves.

Mature seeds typically spread out from their parent plant, a process called seed dispersal. Plants have evolved diverse methods of seed dispersal: some seed pods are explosive, some seeds are attached to sails that blow in the wind, some seeds spin like helicopter blades, and others rely on birds to eat them and spread them in their droppings.

9.3.3: Draw and label a diagram showingthe external and internal structure ofa named dicotyledonous seed.

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

Mature seeds are dormant with very few metabolic processes going on. The resumption of growth in a seed is called germination.

Seeds need oxygen, water and warmth to germinate.

Without water, enzymes are not activated.

Without oxygen, cellular respiration isn't possible.

Warmth is also important for germination because the enzymes involved in growth are more active at warmer temperatures.

Germination is usually triggered by a change in the environment; e.g., warmer temperature, wetter soil, erosion of the seed coat by fire, etc.

Explain the conditions that are needed to allow a seed to germinate. [5] M11/4/BIOLO/HP2/ENG/TZ1/XX

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

1. Absorption of Water

2. Plant hormone called gibberellin is produced in the cotyledons of the seeds.

3. Gibberellin stimulates the production of amylase.

4. Amylase catalyzes the digestion of starch into maltose. Maltose is transported to grow plant in different regions of seedling including embryo root and the embryo shoot.

5. Maltose is hydrolysed into glucoseused in aerobic cell respiration as a source of energy used to synthesize cellulose (which is necessary to produce the cell wall of new cells) or substances needed for growth.

When the leaves of seedling reach the light, it can start processing photosynthesis which can then supply the seedling with foods. 

Draw the external and internal structure of a named dicotyledonous seed. [4] M08/4/BIOLO/HP2/ENG/TZ2/XX N07/4/BIOLO/HP2/ENG/TZ0/XX+

9.3.5: Explain how flowering is controlledin long-day and short-day plants,including the role of phytochrome.

9.3.6: Explain how flowering is controlled in long-day and short-day plantsPhytochrome is a protein pigment found in most plants. It acts as a photoreceptor, which means it detects light.When PR absorbs red light (660 nm) it gets converted into PFR; and when PFR absorbs far-red light (730 nm) it switches back to PR.Switching to PFR from PR is a fast process; but changing from PFR to PR is a slower process. Thus, in sunlight, PR is quickly converted into PFR; but at night PFR is slowly converted into PR. This difference in conversion rates means that PFR levels are highest in plants at the peak of summer, when day-length is greatest. Thus plants can use PFR levels like a calendar, to determine the date of mid-summer.

Long-day plantsPlants that start to flower in mid-summer are called long-day plants. Long-day plants use PFR to trigger the flowering process. In mid-summer, nights are too short to convert all of the PFR into PR. This results in many PFR proteins becoming bound to receptor proteins, which in turn ‘turn on’ genes that produce flowers.

Short-day plants Plants that start to flower in autumn are called short-day plants. Short-day plants use PFR to inhibit the flowering process. Thus, near mid-summer, the protein receptors of short-day plants act to ‘turn off’ the flower-producing genes. By autumn, day-length is short so PFR levels drop too low to inhibit flowering, thus flowering begins.

9.3.6

SLOW (in darkness)

FAST (in light)

660 nm 730 nm

High Pfr concentration is an inhibitor in short day plants but will allow flowering when levels Pfr drop to a “critical” level

High Pfr concentration is the trigger for flowering in long day plants by turning on genes for flowering

In Long Day Plants Pfr accumulates and acts as a transcription factor, turning on the genes for flowering. E.g. Clover

When Pfr levels fall low enough (depending on the species) short day plants will flower. E.g. Strawberry

Explain how flowering is controlled in long-day and short-day plants. [7] M09/4/BIOLO/HP2/ENG/TZ1/XX