Plants

Plants

Definition of a Plant: A multicellular, eukaryotic,

photosynthetic autotroph. The cell walls contain cellulose and they store excess glucose as starch. They also exhibit alternation of generations – one will be dominant over the other. The sporophyte generation is 2n (diploid) and the gametophyte generation is n (haploid).

Evolution of Plants Plants evolved from aquatic green algae

about 500 mya. Paleozoic era Adaptations to dry land Most plants are land plants today

Two Groups of Plants Non-vascular – Bryophytes – no xylem

or phloem Vascular – Tracheophytes – contain

xylem and phloem – conducting tissue Xylem conducts – water Phloem conducts – dissolved sugars

Primary Functions of Plants

Absorb carbon dioxide Release Oxygen Hold soil in place Provide for the transfer of energy from

the sun to other organisms – AKA – Food Provides habitats for animals

Division - Bryophyta Non-vascular Mosses, horn-worts, liverworts They lack any lignin-fortified tissue so

they can’t grow very tall. The gametophyte generation is

dominant in bryophytes (it is the green part that you see and the sporophyte is the stalk coming out of the gametophyte.)

Bryophyte continued Female gametophyte has an Archegonium that

produces the egg (n) Male gametophyte has Antheridia that

produces sperm (n)- These are motile sperm and must have water in order to fertilize the egg.

Fertilization occurs within the archegonium and produces the zygote. The Sporophyte then grows from the zygote out of the gametophyte and produces sporangia that produce spores.

What do Bryophytes do? Stabilize and form soil from rocks – are

pioneers in primary succession and are necessary to form the soil.

Used as fuel (peat moss) Retains moisture when mixed with soil

Tracheophytes are: Vascular plants Xylem and Phloem used for transport of water

and sugars Xylem and Phloem contain lignin that gives the

plant support so that it can stand up against gravity

Roots absorb water and prevent desiccation (drying out), and anchor in the soil giving extra support

Leaves increase the photosynthetic area Sporophyte is the dominant generation

Two Groups of Tracheophytes Seedless Plants – Ferns – Pteridophytes

Homosporous – They produce only one type of spore

Stay fairly small because they must have water to reproduce due to the fact that sperm are flagellated and swim to get to the archegonium to fertilize the egg.

Ancient Ferns were tree size They reabsorbed so much carbon dioxide during the

Carboniferous period that the Earth experienced Global Cooling

Most turned into Coal that is now one of the causes of Global Warming

Life Cycle of a Fern

Seed Plants Heterosporous – Produce megaspores –

female gametophyte and microspores – male gametophyte

Male gametophyte does not have flagella so they don’t need water

Gymnosperms – Cone bearing – naked seed (conifers – pine, firs, redwoods, junipers, and sequoia) – long-lived organisms

Gymnosperms First seed plants to appear Seeds are not enclosed in a fruit Replaced the ferns because they were

better adapted for land Have needle shaped leaves with a waxy

covering that helps to prevent drying Use the wind for pollination

Angiosperms Flowering plants – seeds are enclosed in a

fruit Most diverse and abundant of all plant

species Color and scent of flowers attracts animals

for pollination and dispersal of seeds Ovary becomes the fruit (fruit is ripened

ovary) Ovule becomes the seed

Seed/Fruit adaptations Maples have wings so that the wind can

carry them Fruit has burrs that stick to animal fur Brightly colored and sweet so that animals

eat them and then poop the seeds with fertilizer so it grows rapidly

Some float on water (coconut) Fruit prevents drying out of seeds and

protects them until they are ready to be dispersed.

Two types of Angiosperms Monocots – have one cotyledon (seed

leaf) Scattered vascular bundles Parallel veins in the leaves Flower parts in 3s Fibrous roots

Dicots – have two cotyledons (seed leaves) Vascular bundle is arranged in a ring Veins are netlike or branched Flower parts are in 4s or 5s Have taproots

Plants: Part II –How plants Grow

Plants can only grow from meristematic tissue (meristem) – it is embryonic tissue (stem cell)

Located at the tip of the roots and in the buds of shoots is apical meristem.

The plant’s roots grow down into the soil from this tissue and grow taller from the top of the plant.

Primary Growth Zone of cell division – apical meristem: actively

dividing cells Zone of elongation – Cells are not dividing but are

getting longer and push the root cap down deeper into the soil

Zone of differentiation: Cells become specialized into three tissue systems A. The protderm becomes the epidermis B. The ground meristem becomes the cortex for

storage C. The procambium becomes the primary xylem and

phloem.

Vascular Cambium is what makes woody plants thicker. Responsible for secondary growth.

Types of Plant Tissue: 1. Dermal Tissue 2. Vascular Tissue 3. Ground Tissue

Dermal Tissue 1. Covers and protects the plant.

Endodermis Epidermis Modified cells like guard cells, root hairs,

and cells that make waxy cuticle

Vascular Tissue Xylem – conducts water and dissolved

minerals (cells are dead at maturity) Consists of tracheids and vessel elements Both types of cells are dead at maturity Tracheids – long thin cells that overlap

and are tapered at the ends. Water passes from one cell to another through pits.

Cells walls are hardened with lignin. Xylem is what makes up wood.

Tracheid

Vessel Elements – wider than tracheids, but also shorter than tracheids.

Aligned end to end and the ends are perforated to allow free flow through the vessel tubes.

Seedless vascular plants and gymnosperms have only tracheids.

Seed plants have both tracheids and vessel elements.

Comparison of Tracheid and Vessel Element

Phloem Carries sugars from the leaves to the rest of

the plant by active transport. Made of sieve tube members (elements)

with sieve plates at the end to connect them.

These cells are alive at maturity but lack a nucleus, ribosomes and vacuoles.

Companion cells are connected to each sieve tube member and provides the sieve tube cells with what they need.

Ground Tissue Most common type of tissue in a plant Functions in 1)support 2) storage 3)

photosynthesis Three types of ground tissue

Parenchyma Sclerenchyma Collenchyma

Parenchymal Cells They are like a regular plant cell but

they lack secondary cell walls Are totipotent Many contain chloroplast and carry out

photosynthesis – mesophyll cells in the leaf

Collenchymal Cells Unevenly thickened primary cell walls

and lack secondary cell walls and lignin to harden them

They are alive at maturity and function in support of the growing stem.

They are the strings in celery

Sclerenchymal Cells Have thick primary and secondary cell

walls Function in support Two types: 1) fibers 2) sclerids Fibers – in bundles and are used in

making rope Sclerids – short and irregular and make

up the rough seed coat and pits. They give fruit the gritty texture like in pears.

Roots and their Function1) absorb water and nutrients

2) anchor the plant in the soil

3) store food

Dicot Root

Parts of the Root and their Fuctions

Epidermis- covers and protects, absorbs Root hairs – increase surface area

Cortex – Storage of starch and sugar Stele – consists of vascular tissue and is

surrounded by the pericycle (contains meristematic tissue) Lateral roots arise from the stele

Endoderm – Surrounds the vascular cylinder

Casparian strip – a band of cell wall containing suberin and lignin, found in the endodermis. It restricts the movement of water across the endodermis.

Suberin – a waxy like substance that surrounds the casparian strip and acts as a barrier to water and solute movement across the casparian strip.

Types of Roots 1) Drop – Type of aerial 2) taproot 3) fibrous 4) Adventitious roots 5) Prop 6) Aerial – pneumatophores - snorkel

The Leaf

Parts of the leaf Cuticle – covered with cutin (a wax) to prevent

water loss Guard Cells – modified epidermal cells that

surround the stomata and help to control the opening and closing of the stomates. They do contain chloroplast.

Stomata – Openings on the underside of the leaf where gases, CO2 is taken in, O2 is released and water vapor is lost by transpiration. 90% of water escapes through the stomates

Pallisade and spongy mesophyll – pallisade is underneath the epidermis and the spongy is in the middle of the leaf. The primary function is photosynthesis.

Vascular bundles – veins – located in the mesophyll and carry water and nutrients from the soil to the leaves and carry sugars from the leaves to the rest of the plant.

Control of Stomata by Guard Cells

In the light, guard cells actively pump protons out, and this increases the uptake of potassium and chloride ions. (No light, potassium and chloride diffuse out of guard cells)

Higher concentrations of potassium and chloride gives guard cells a negative water potential.

This causes water to flow into the cells and increases the turgor pressure.

Increased turgor pressure stretches the cells and opens the stoma. (Decreased turgor pressure (flacid), the stomata closes.

Basically, the cellulose fibers are arranged radially so that when the cells absorb water they curve outward and open the stomata. They curve inward when flacid and close the stomata.

What will make the guard cells open?

Decreased carbon dioxide stimulates stomata to open. Happens when photosynthesis begins.

Increase in potassium ions in the guard cells which lowers the water potential

Stimulation of the blue light receptor Active transport of H+ out of the guard

cells

What will make stomates close?

1. Lack of water 2. High temperatures – stimulates

cellular respiration and increases carbon dioxide

3. Abscisic acid – produced by mesophyll cells in response to dehydration and guard cells close the stomata

Transport of Water in Plants through the Xylem

Water rises in the xylem against gravity.2 forces accomplish this:

1) Root Pressure2) Transpirational Pull

Transpirational Pull Transpiration is the evaporation of water

through the stomates in the leaves. Causes negative pressure as the water

leaves. This negative pressure is also called tension.

Water molecules cling to each other due to hydrogen bonding. This is called cohesion and to the walls of the xylem which is called adhesion.

Transpirational pull-cohesion theory

The negative pressure created when water evaporates pulls other water molecules up the xylem. This is because the water molecules are clinging to each other.

1)Water diffuses out of the stomata by evaporation. This is called transpiration.

2)Water evaporates from mesophyll cell walls 3)Tension pulls water from the veins into the apoplast

surrounding the mesophyll cells 4) This pulls water in the veins of the leaves upward and

outward 5)This pulls the water in the xylem of the shoot and root

upward 6)Cohesion between water molecules forms a continuous

column of water from the roots to the leaves 7)Water enters the root and moves into the xylem by osmosis

Apoplast and Symplast: The movement of water across a plant is called lateral movement and it occurs along symplast and apoplast.

The symplast is a continuous system of interconnected cells via plasmodesmata.

The apoplast is the network of cell walls and intercellular spaces within a plant body that allows extracellular movement of water within a plant.

When it gets to the endodermis it can continue to the xylem through the symplast, but water in the apoplast must pass across the endodermis by diffusion.

Translocation The movement of carbohydrates and

other solutes through the plant in the phloem.

Moves from sources to sinks. A source is an organ that produces

by photosynthesis sugars A sink is an organ that uses sugars

such as a flower, fruit, etc.

Active transport of sugars

Sources and Sinks can change roles.If photosynthesis is not occurring then sugars can be released from the roots or stems to the leaves for use in cellular respiration.

Plant Reproduction Asexual – vegetative propagation

Stem Leaf Root Produces an entirely new plant that is a

clone Used in grafting, cuttings, runners

Sexual Reproduction 1. Pollination

Pollen grain contains 3 monoploid nuclei, one tube nucleus and 2 sperm nuclei

Lands on stigma Pollen absorbs moisture and sprouts a

pollen tube that goes down the style 2 sperm nuclei travel down the tube to the

ovary One sperm fertilizes the egg and becomes

the embryo

Fertilization continued The other sperm fertilizes the two polar

bodies and becomes the triploid (3n) endosperm.

This is called double fertilization The endosperm becomes the food for the

developing embryo The ovule becomes the seed In dicots the food in the endosperm is

transported to the cotyledons In monocots the endosperm is liquid

The seed

Seed coat – protective outer layer

Embryo – hypocotyl, epicoltyl, and radicle

Radicle – becomes the root

Hypocotyl becomes the lower stem and the epicotyl becomes the upper stem

Seed Germination

Steps in Germination 1) Seed coat imbibes water, swells and

ruptures 2) Radicle emerges 3) epicotyl elongates 4) radicle grows down 5) epicotyl pulls plumule out from between

the cotyledons 6)pulls plumule (first bud) backwards

through the soil so leaves aren’t damaged 7) epicotyl straightens and leaves open out

Plant Hormones Coordinate growth, development, and response

to environmental stimuli

Auxins 1) Auxin – IAA – Indoleacetic acid 2) responsible for phototropism 3)Enhances apical dominance – the preferrential

growth upward rather than outward (terminal bud suppresses the lateral buds)

4) stimulates stem elongation and growth by softening the cell wall

First hormone discovered Synthetic Auxin is roundup a weed killer Can also be used as a rooting powder Sprayed on tomatoes causes fruit

production without pollination – produces seedless tomatoes

Cytokinins Stimulates cytokinesis and cell division Works with auxins to promote growth Works against auxins in relation to

apical dominance Delays senescence (aging) by inhibiting

protein breakdown Produced in roots and travel upward in

the plant

Gibberellins Promotes stem and leaf elongation Works with auxins to promote growth Induces bolting or rapid growth of stalk

Abscisic Acid

1) inhibits growth 2) Enables plants to withstand drought 3)closes stomata during water stress 4) works in opposition to growth

promoting substances 5)Promotes seed dormancy

Ethylene Gas 1)Promotes fruit ripening

Positive feedback 2) Is produced in times of stress in large

amounts 3) promotes apoptastic programmed

cell death – cell reuses chemicals from the breakdown of cell parts to survive during stress ( drought, flood, injury, infection)

Promotes leaf abscission – leaf falling off –

A scar forms where the leaf falls off and prevents and prevents pathogens from entering. This is due to an increase in ethylene and a decrease in auxin.

Ethylene works in opposition to auxin.

Tropisms The growth of a plant towards or away

from a stimulus Phototropism – towards light Thigmotropism – towards touch Geo/gravitropism – towards or away from

gravity Positive tropism – towards the stimulus Negative tropism – away from the

stimulus

Phototropism – due to unequal distriubution of auxins which accumulate on the side of the plant away from the light. This side grows longer while the top side doesn’t grow, causing it to bend toward the light.

Geotropisms occur due to interaction of auxins with statoliths which are specialized plastids with dense starch granules.

Signal Transduction Pathway 1) Reception 2) Transduction 3) Response

A receptor is stimulated which undergoes a conformation change and is known as a ligand

This activates a second messenger (cyclic AMP, or cyclic GMP) and this messenger goes to the nucleus where transcription occurs.

The gene is then translated into a response for the plant.

Adaptations to Land 1) Cell walls – support 2) Roots and root hairs – absorption of water and

nutrients 3) Stomates – gas exchange 4) Waxy Cuticle 5) Gametangia – prevents drying of zygote 6) Sporopollenin – a tough polymer that is

resistant to environmental damage and protects the plants in a harsh terrestrial environment and is found in the walls of spores and pollen

7) Seeds and Pollen – protective coat that prevents drying and are how offspring are dispersed.

8) Reduced gametophyte generation 9) xylem and phloem 10) Lignin for support

Photoperiodism