AP Biology

Multicelled, eukaryotic, photosynthetic autotrophs

Life cycle characterized by alternation of generations

Ancestral plants lived in aquatic areas but are now mostly on land

Non-vascular; lack xylem and phloem

Absorb water through diffusion

Contain flagellated sperm that swim through water to fertilize an egg

Lack lignin, tissue that supports tall plants restricted to moist habitats and are tiny

Grow on rocks, soil and trees

Can be used as fuel (sphagnum, peat moss)

Examples: mosses, liverworts, hornworts

Plants with vascular tissue Xylem and phloem for transport Lignified transport vessels to support

plant Roots to absorb, anchor and support Leaves, increasing surface for

photosynthesis Dominant sporophyte generation

during developmen

Seed plants More advanced and far more numerous Gymnosperms (bearing cones) Angiosperms (bearing flowers and

fruits)

Seedless plants Ferns

Seedless tracheophytes Primitive plants reproducing with

spores Homosporous, producing only one type

of spore which develops into a bisexual gametophyte

Restricted to moist habitats Sperm are flagellated and must swim to

fertilize an egg

Heterosporous, producing megaspores and microspores

Megaspores = female gametophytes

Microspores = male gametophytes Sperm are not flagellated, and

therefore are not restricted to moist environments

First seed plants on earth Seeds are “naked”, as they are not

enclosed inside a fruit, like angiosperms Seeds are exposed on modified

leaves that form cones (dry environments)

Modifications include needle-shaped eaves

Depend on wind for pollination

Pines, firs, redwoods, junipers and sequoia

Seed plants whose reproductive structures are flowers and fruits

90% of all plants Color and scent of flowers attracts

animals that will carry pollen from one plant to another

After pollination and fertilization, ovary becomes fruit and ovule becomes seed

Fruit protects seeds and aids in their dispersal

Examples Maple trees have seeds with wings helping

them travel greater distances Some plants have burrs on their fruits that

cling to fur or clothing Animals eat and digest fruit while tough

seeds pass through digestive tract, eventually being deposited with feces as fertilizer

Characteristic

Monocot Dicot

Cotyledons (seed leaves)

One Two

Vascular bundles in stem

Scattered In a ring

Leaf venation Parallel Netlike

Floral parts Usually in 3’s Usually in 4’s or 5’s

Roots Fibrous roots Taproots

Plants moved to land as competition for resources increased

Problems: supporting plant body while absorbing and conserving water

Cell wall made of cellulose to give support and maintain shape

Roots and root hairs absorb water and nutrients from soil

Stomates open to exchange gas and close to prevent water loss

Cutin, waxy coating on leaf, prevents excess water loss from leaves

Some plants have protective jacket of cells called gametangia, protecting gametes and zygotes as well as preventing drying out

Sporopollenin, a tough polymer, is resistant to environmental damage and protects plants Found in walls of spores and pollen

Seeds and pollen have a protective coat preventing desiccation

Xylem and phloem vessels enable plants to grow tall

Lignin embedded in xylem and other plant cells provide support

Meristem Enables plants to grow as long as they live Continually divides and generates new

cells Apical meristem

In tips of roots and buds of shoots is the source of primary growth, the elongation of the plant down into the soil and up into the air

Lateral meristem Provides secondary growth, or increase in

girth

Herbaceous (nonwoody) plants Only primary growth

Woody plants Secondary growth responsible for thickening

of roots and shoots

Dermal tissue Covers and protects plant Includes epidermis, guard cells, root

hairs, cuticle cells Vascular tissue

Xylem and phloem, transporting water and nutrients

Ground tissue Support, storage, photosynthesis

Water and mineral-conducting tissue Consists of two types of elongated cells:

Tracheids and vessel elements Both dead at functional maturity

Tracheids Long, thin cells that overlap and taper at

the ends Water passes from one cell to another

through pits, with no secondary cell wall Areas with secondary cell wall are

hardened with lignin providing support and transport

Vessel elements Wider, shorter, thinner walled, less tapered Align end to end Ends are perforated to allow free flow

through vessel tubes Seedless vascular plants and

gymnosperms have only tracheids; most angiosperms have both tracheids and vessel elements

Xylem makes up wood of plants

Carries sugars from photosynthetic leaves to rest of plant by active transport

Consist of chains of sieve tube members or elements whose end walls contain sieve plates Facilitate flow of fluid from one cell to

the next Alive at maturity Lack nuclei, ribosomes and vacuoles

Connected to each sieve tube member is at least one companion cell Contains a full set of organelles Nurtures sieve tube elements

Support, storage, photosynthesis Three cell types

Parenchyma, scelernchma, collenchyma

KEEP IN MIND THAT FORM RELATES TO FUNCTION

Parenchymal cells Primary cell walls that are thin and flexible Lack secondary cell walls Contain one large vacuole Some contain chloroplasts and carry

out photosynthesis Parenchymal cells in roots contain

plastids and store starch When turgid, give support and shape

Parenchymal cells After a plant is injured, parenchymal

cells retain ability to divide and differentiate into other cell types

An entire plant can be regenerated or cloned from one parenchymal cell (in a lab)

Collenchymal cells Unevenly thickened primary cell walls

but lack secondary cell walls Mature cells are alive Support growing stem “Strings” of a stalk of celery are

collenchymal cells

Sclerenchymal cells Thick primary and secondary cell walls

fortified with lignin Support plant Two forms are fiber and sclereids

Fibers are long thin and occur in bundles; make rope and linen

Sclereids are short and irregularly shaped; make up tough seed coats and pits

Absorb nutrients from soil, anchor plant and store food

Made of specialized tissues and structures

Epidermis covers surface of roots; modified for absorption

Root hairs extend out from each cell and increase surface area

Cortex consists of parenchymal cells that contain plastids to store starch and organic substances

Vascular cylinder or stele of root consists of xylem and phloem surrounded by tissue called pericycle, from which lateral roots arise

Endodermis surrounds vascular cylinder; each endoderm cell is wrapped with the Casparian strip, a continuous band of suberin, waxy material impervious to water

Endoderm selects which minerals can enter

Apical meristem which provides primary growth (up and down)

Three zones of cells during primary growth: Zone of cell division (bottom) Zone of elongation (middle) Zone of differentiation (top)

Root tip is protected by a root cap, which secretes a substance that digests the earth

Zone of cell division Meristem cells actively divide

Zone of elongation Cells elongate and push root cap deeper

into soil Zone of differentiation

Protoderm becomes epidermis, ground meristem becomes cortex, procambium becomes xylem and phloem

Types of Roots Taproot

Single large root that gives rise to branch roots

Common in dicots

Fibrous root system Holds plant firmly in place Common in monocots

Types of Roots Adventitious roots

Roots that arise above ground

Aerial roots Found in marshes Stick out of water and give air to root cells

Prop roots Grow above ground out from base of stem Support plant

Stems Primary stem tissue

Vascular tissue exists as vascular bundles Xylem inside, phloem outside, meristem tissue in

between

Monocots Vascular bundles scattered throughout stem

Dicots Vascular bundles arranged in a ring

Stems Ground tissue

Consists of cortex and pith Used for storage

Secondary Growth in Stems Produced by lateral meristem

Replaces dermal tissue with bark

Second lateral meristem adds vascular tissue Wood is secondary xylem accumulation

The Leaf Epidermis: upper and lower protection for leaf

Cuticle: waxy coating minimizing water loss

Guard cells: contain chloroplasts; control opening of stomates

Stomates: tiny openings allowing for gas exchange

The Leaf Mesophyll

Palisade layer: cells packed tightly; contain many chloroplasts

Spongy layer: cells packed loosely, allowing for diffusion of gases; less chloroplasts

Vascular bundles (veins): found in mesophyll; carry water and nutrients Specialized bundle sheath cells surround

veins

Stomates

Accounts for majority of water loss

Opened and closed by guard cells If guard cells absorb water by osmosis and

become turgid, they curve and open stomates

If guard cells lose water and become flaccid, stomate closes

Stomates

Why Stomates Open Loss of CO2 within air spaces of leaf

Occurs when photosynthesis begins Increase in potassium ions

Lowers water potential, causing water to diffuse into them

Stimulation of blue light in a guard cell Stimulates proton pumps which promote

uptake of potassium ions Active transport of H+ out of guard cells

Why Stomates Close Lack of water

Guard cells become flaccid and close High temperatures

Increases cellular respiration which increases concentration of carbon dioxide

Abscisic acid Produced in response to dehydration

Transport in Plants Xylem rises against gravity without

using energy Transpirational pull pulls water up Root pressure pushes xylem upward a few

yards Water droplets on leaves in the morning result

from root pressure, a process called guttation

Root Pressure

Transpirational Pull Transpiration is the evaporation of

water from leaves This causes negative pressure to develop in

xylem Cohesion of water helps pull a column of

water upward Absorption of sunlight causes water to

evaporate

Transpirational Pull

Transpirational Pull-Cohesion Tension Theory For each molecule of water that

evaporates, another molecule of water is drawn from the root to replace it

Factors affecting transpiration: Humidity: increased slows, decreased speeds Wind: reduces humidity, speeding up

transpiration Increased light: increased photosynthesis,

increased water vapor, increased transpiration Stomates: if closed, stops transpiration

Absorption of Nutrients and Water Apoplast and symplast

Performs lateral movement of water and solutes Symplast

continuous cytoplasmic system interconnected by plasmodesmata

Long-distance transport Apoplast

Network of cell walls and intercellular spaces Short-distance, extracellular transport

Absorption of Water and Nutrients Mycorrhizae

Symbiotic structures consisting of the plant’s roots intermingled with hyphae (filaments) of fungus

Increase amount of nutrients to be absorbed

Absorption of Water and Nutrients Rhizobium

Symbiotic bacterium living in nodules on roots of specific legumes

Fix nitrogen gas from air into a form that plants can use

Nitrogen-fixing bacteria

Absorption of Nutrients and Water Bulk flow

Movement of fluids across great distances within a plant

Translocation of Phloem Sap Phloem sap travels through plant from

sugar source to sugar sink This is called translocation

Source is where sugar is being produced (leaves)

Sink is where sugar is stored or consumed (roots and fruit)

Plant Reproduction Asexual

Accomplished through vegetative propagation

A piece of the vegetative part of the plant (root, stem or leaf) can produce a new plant genetically identical to the parent

Grafting (plant parts are fused together) Cuttings (taking plant stem cells and placing them

in soil) Bulbs (underground buds) Runners (stem grown underground)

Sexual Reproduction in Flowering Plants Flower is the sexual organ Process begins with pollination

One pollen grain contains three haploid nuclei (one tube nucleus, two sperm nuclei)

Grain lands on the sticky stigma of the flower Pollen grain absorbs moisture and sprouts,

forming a pollen tube that burrows down the style into the ovary

Sexual Reproduction in Flowering Plants

Two sperm nuclei travel down the pollen tube into the ovary

Once inside, two remaining sperm nuclei enter the ovule through the micropyle

One sperm nucleus (haploid) fertilizes the egg (haploid) and becomes the embryo (2n)

The other sperm nucleus fertilizes the two polar bodies and becomes the triploid (3n) endosperm, food for the embryo

Sexual Reproduction in Flowering Plants

This process is double fertilization After fertilization, ovule becomes the

seed and ripened ovary becomes the fruit

Monocots: food reserves remain in endosperm; in coconuts, endosperm is the liquid

Dicots: mature seed lacks endosperm since they are transported

The Seed Protective coat, embryo and cotyledon

or endosperm

Embryo Hypotcotyl: becomes lower part of stem Epicotyl: becomes upper part of stem Radicle (embryonic root): first organ to

emerge from the germinating seed

Alternation of Generations Haploid (n) and diploid (2n) generations

alternate

Gametophyte (n) produces gametes by mitosis

Fuse during fertilization to form 2n zygotes Each zygote develops into a sporophyte (2n),

which produces haploid spores (n) by meiosis Each haploid spore forms a new gametophyte

Mosses and Bryophytes Alternation of generations Haploid generation is dominant; diploid

generation lives a short time and depends on the gametophyte for food

Mosses and Bryophytes Overview

Gametophyte dominates Archegonia and antheridia develop on tips of

gametophyte Sporophyte grows out of the top of the

gametophyte and obtains nutrients from it Haploid spores are formed in mature sporangia

Ferns Sporophyte generation is larger than and

independent from gametophyte Archegonia and antheridia both develop

underneath heart-shaped haploid gametophyte Sperm swim from antheridia to archegonia (of

different plants) to form a diploid zygote Zygote grows into a large diploid sporophyte

plant Haploid spores emerge and land on the ground

to sprout into gametophytes

Seed Plants Flowering plants

Gametophyte generation exists inside the sporophyte generation and depends totally on it

Meiosis occurs inside anthers and pistils Anthers produce microspores, forming male

gametophytes Ovules produce megaspores, forming female

gametophytes

Fertilization of Seed Plants Occurs in ovary, forming zygotes that develop

into sporophyte embryos in the ovule

Ovule becomes the seed, carrying embryo and food for it

Seed Plants Gymnosperms (conifers)

Sporophytes whose sporangia are packed inside cones

Gametophyte generation develops from haploid spores

Small pollen cones produce microspores, which become male gametophytes

Larger ovulate cones produce megaspores, which become female gametophytes

Plant Responses to Stimuli Hormones

Coordinate growth, development and environmental responses

Signal transduction pathways amplify hormone signals and connect them to specific cell responses

Auxin Responsible for phototropisms Enhances apical dominance (upward growth) Stimulates stem elongation and growth Indoleacetic acid Spraying synthetic auxin on tomato plants

induces fruit production without pollination (seedless tomatoes)

Cytokinins Stimulate cytokinesis and cell

division Delay senescence (aging) by

inhibiting protein breakdown Produced in roots and travel

upward

Gibberellins Promote stem and leaf elongation Work with auxins to promote cell

growth Induce bolting, the rapid growth

of a floral stalk

Abscisic Acid Inhibits growth Enables plants to withstand

drought Closes stomates during stress Promotes seed dormancy, keeping

seeds from sprouting until spring

Ethylene Gas hormone Promotes fruit ripening (positive feedback) Produced during stressful times Facilitates apoptosis, or cell death Promotes leaf abcission

If a leaf falls from the plant, a scar forms to prevent pathogens from entering

Tropisms Growth of a plant toward or away from

a stimulus Thigmotropism (touch) Geotropism (gravity) Phototropism (light)

Growth toward a stimulus is positive tropism while away from it is negative tropism

Signal Transduction Pathway Reception, transduction and response

Receptor is stimulated and activates a second messenger

Secondary messengers are cyclic nucleotides (AMP and GMP) that transfer and amplify signals

This messenger leads to a response by altering factors and targeting specific cells

Photoperiodism Physiological response to the photoperiod Relative lengths of day and night Biological clock set to 24 hour day

(circadian rhythm) Long-day plants (short night

plants)flower when light period is longer than a certain number of hours

Short-day plants and day-neutral plants flower regardless of length of day

Photoperiodism Phytochrome is the pigment

responsible for keeping track of the length of days and nights Pr (red light absorbing)

Phytochrome is synthesized in this manner When the plant is exposed to light, it convers to…

Pfr (infrared light absorbing) In the dark, Pfr reverts back to Pr Enables plant to keep track of time