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
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