Download Plant Form and Function

AP Biology
Ms. Girvan
Date: _________________
Plant Form and Function
The information in this note was taken from:
Barron’s: How to Prepare for the AP Biology Exam (2004)
A. Introduction to Plants:
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multicelled, eukaryotic, photosynthetic, autotrophic
cell walls made of cellulose and surplus carbohydrate is stored as starch
life cycle characterized by alternation of generations
gametophyte generation: all cells are haploid (n)
sporophyte generation: all cells are diploid (2n)
believed modern plants evolved from green algae (Chlorophyta)
B. Strategies That Enabled Plants to Move to Land
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Plants moved to land as competition for resources increased
Two big problems: supporting body of plant and absorbing/conserving water
o Cell walls made of cellulose lend support to plant
o Root and root hairs absorb water and nutrients from the soil
o Stomates open to exchange photosynthetic gases and close to minimize excessive
water loss
o The waxy coating on the leaves, cutin, helps prevent excess water loss from leaves
o In some plants, gametes and zygotes form within a protective enclosure called
gametangia that prevents drying out
o Sporopollenin (tough polymer) is resistant to almost all kinds of environmental
damage (found in walls of spores and pollen)
o C-4 and CAM plants can survive in dry environment
o Seeds and pollen have a protective coat that prevents desiccation (drying out) – also a
means of dispersing offspring
o The gametophyte generation has been reduced
C. Plant Tissue:
A plant consists of three types of tissue, each with different functions: dermal tissue, vascular
tissue, and ground tissue
1. Dermal Tissue:
- covers and protects the plant
- it includes endodermis, epidermis and modified cells like guard cells, root hairs and cells that
produce waxy cuticle
2. Vascular Tissue:
- consists of xylem and phloem
- transports water and nutrients around plant
o Xylem
 Water and mineral conducting tissue
 Consists of two types of cells: tracheid and vessel elements (both are dead at
functional maturity)
 Tracheids are long, thin cells that overlap and are tapered at the ends
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Water passes from one cell to the next through pits (areas with no secondary
cell wall)
Tracheid cell walls are hardened with lignin which gives plant support
Vessel elements are generally wide, shorter, thinner walled and less tapered
than tracheids
Seedless vascular plants and most gymnosperms have only tracheids; most
angiosperms have both
Xylem is what makes up wood
Phloem
 Carries sugar from the photosynthetic leaves to the rest of the plant by active
transport
 Phloem vessels consist of chains of sieve tube members or elements whose
walls contain sieve plates that facilitate the flow of fluid from one cell to the next
 Cells are alive at maturity
 Companion cells (attached to each sieve tube member) have a full
complement of cell organelles for sieve tube elements
3. Ground Tissue
- most common type in plant
- functions mainly as support
o Parenchymal Cells
 Look like traditional plant cells
 Have primary cell walls that are thin and flexible
 Lack secondary walls
 Contain one large vacuole
 Some contain chloroplasts (mesophyll in leafs)
 Others in roots contain plastids and store starch
 When turgid with water give support and shape
 Retain ability to divide and differentiate other cell types after plant has been
injured
o Collenchymal Cells
 Have unevenly thickened primary cell walls and lack secondary cell walls
 Have lignin to harden them
 Mature cells are alive and function to support growing stem
o Sclerenchymal Cells
 Have very thick primary and secondary cell walls fortified with lignin
 Function is to support plant
D. Stomates
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~90% of the water a plant loses escapes through the stomates (which are only ~1% of the
surface of leaf)
Guard cells (modified epithelium containing chloroplasts) control the opening and closing of
stomates by changing their shaper
Cells wall of guard cells are not uniformly thick
Cellulose microfibrils are orientated so that when guard cells absorb water by osmosis and
become turgid, they curve, causing the stomata to open
When guard cells lost water and become flaccid, the stomata closes
 Several factors cause stomates to open:
o Depletion of CO2 within the air spaces of the leaf, which occurs when
photosynthesis begins, triggers the stomates to open.
o An increase in potassium ions in the guard cells, which lowers their water potential,
causes water to diffuse into them. As they become turgid, the guard cells swell and the
stomata opens.
o The stimulation of the blue light receptor (sensor) in a guard cell stimulates the
activity of ATP-powered proton pumps in the plasma membrane of the guard cells,
which in turn, promote the uptake of K+ ions. This causes the stomates to open.
o Stomate opening correlates with active transport of H+ out of guard cells and into
the surrounding epithethial cells
 Other factors cause stomates to close:
o Lack of water causes the guard cells to lose their turgor, become flaccid, and close the
stomata.
o High temperature also close the stomata presumably by stimulating cellular respiration
and increasing CO2 concentration within the air spaces of the leaf.
o Abscisic acid, which is produce in the mesophyll cells in response to dehydration,
signals guard cells to close the stomates
E. Transport in Plants
1. Transport in Xylem
- xylem fluid rises in a plant against gravity and requires no energy
- the fluid can be pushed up by root pressure or pulled up by transpirational pull
- Root pressure results from water flowing into the stele (vascular tissue of root) from the soil
as a result of the high mineral content in the root cells (known as guttation)
- Transpirational pull can pull fluid up the tallest trees
- Transpiration, the evaporation of water from leaves, causes negative pressure (tension) to
develop in the xylem tissue from the roots to the leaves
- The cohesion of water due to strong attraction of water molecules makes it possible to pull a
column of water from above within the xylem
- The absorption of sunlight drives transpiration by causing water to evaporate from the leaf
- Transpirational pull-cohesion tension theory states: for each molecules of water that
evaporates from a leaf by transpiration, another molecule of water is drawn in at the root to
replace it
- Several factors that affect the rate of transpiration:
o High humidity slows down transpiration, while low humidity speeds it up.
o Wind can reduce humidity near the stomates and thereby increase transpiration.
o Increased light intensity will increase photosynthesis and thereby increase the amount
of water vapor to be transpired and increase the rate of transpiration.
o Closing stomates stops transpiration.
2. Absorption of Nutrients and Water
- Apoplast and symplast: the movement of water and solutes across a plant (lateral
movement)
- Mycorrhizae: in mature plants that lack root hairs
o Supply the plant with water and minerals
o Symbiotic structures consisting of the plant’s roots intermingled with the hypae
(filaments) of a fungus that greatly increase the quantity of nutrients that a plant can
absorb
- Rhizobium: symbiotic bacterium that lives in the nodules on roots of specific legumes that
fixes nitrogen gas from the air into a form of nitrogen the plant requires (NO 3)
3. Translocation of Phloem Sap
- Travels around the plant from sugar source to sugar sink
- Transport is called translocation
- Leaves are the primary source of sugar
- Growing roots and fruit are sugar sinks
- Storage organs like tubers and roots are sugar sinks in growing season, but become the
source during the spring
F. Plant Response to Stimuli
Plant hormones help coordinate growth, development, and responses to environmental stimuli. They
are produces in small quantities, but can have a profound effect on the plant because the hormone
signal is amplified. Signal transduction pathways amplify hormonal signal and connect it to specific
cell responses.
1. Auxin
- responsible for phototropisms due to unequal distribution of auxin
- enhances apical dominance, growth of plant upward rather than laterally
- stimulates stem elongation and growth by softening cell wall
- was the first plant hormone discovered
2. Cytokinins
- stimulate cytokinesis and cell division
- work in concert with auxins to promote growth and cell division
- works against auxins in relation to apical dominance
- delay senescence (aging) by inhibiting protein breakdown (florists spray cut flowers with cytokinins
to keep them fresh)
3. Gibberellins
- promote stem and leaf elongation
- 100 different gibberellins have been identified
- work in concert with auxins to promote cell growth
- induce bolting (rapid growth of floral stalk) – ex. broccoli (ensures pollination and seed dispersal)
4. Abscisic Acid (ABA)
- inhibits growth
- enables plants to withstand drought
- closes stomates during times of water stress
- works in opposition to the growth-promoting plant hormones
- promotes seed dormancy (prevents seeds from sprouting until spring)
5. Ethylene
- this plant hormone is a gas
- promotes fruit ripening (example of positive feedback – ethylene gas promotes ripening, which
triggers increased production of ethylene gas: “one bad apple spoils the barrel”)
- produced in large quantities in times of stress (drought, flooding, injury, infection)
- facilitates apoptosis (programmed cell death) – helps salvage chemical components to reuse
- promotes leaf abscission (leaf dies and falls from plant – prevents pathogens from entering plant)
- works in opposition to auxin
G. Tropisms
A tropism is the growth of a plant toward or away from a stimulus.
Examples:
1. Thigmotropism (touch)
- movement in which an organism moves or grows in response to touch or contact stimuli.
- Touched cells produce auxin and transport it to untouched cells. Some untouched cells will
then elongate faster so growth bends around the object.
2. Geotropism or gravitropism (gravity)
- result from an interaction of auxins and statoliths (specialized plastids containing dense starch
grains)
3.
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Phototropisms (light)
result from an unequal distribution of auxins
accumulation of auxin on the side of the plant away from the light
auxins cause cell growth, so cells on shady side enlarge and stem bends towards light
Positive Tropism – growth of plant towards stimulus
Negative Tropism – growth of plant away from stimulus
H. Polar Transport
- responsible for transporting auxins from the shoot tip to the base of the stem
- requires energy and specific carrier proteins built into plant cell membranes
- ATP provides the energy for a proton pump that pumps protons out of the cytoplasm, creating
a pH gradient necessary for the transport of auxins throughout the plant
- Another example of work being done in a cell by coupling one process (pumping protons
across a membrane) with another process (transport of auxins in a plant)
I. Photoperiodism
The environmental stimulus a plant used to detect the time of year in the photoperiod, the relative
length of day and night.
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plants have biological clock (24 hours) known as circadian rhythm
physiological response to the photoperiod, such as flowering, is known as photoperiodism
some plants only flower when light period is longer than a certain number of hours – called
long-day plants
some plants are short-day plants and some are day-neutral and will flower regardless of the
length of day
in 1940s, realized that plants actually respond to length of darkness
phytochrome (pigment) is a photoreceptor responsible for keeping track of day and night
length
two forms of phytochrome: Pr (red-light absorbing) and Pfr (infrared light absorbing)
when plant is exposed to light, Pr converts to Pfr
in dark, Pfr converts back to Pr
conversion allows the plant to keep track of time
plant senses concentration of the two phytochromes and responds accordingly
J. Classifying Plants
 Non-vascular Plants/Bryophytes – have no vessels, no roots, no stems or leaves (ex. mosses and
liverworks)
 Vascular Plants – have vessels to transport food and water, have roots, stems and leaves (ex.
grasses, corn, trees, flowers, bushes)
Monocots
Have 1 seed leaf (cotyledon)
Parallel veins on leaves
Flower parts in multiples of 3s
Fibrous roots
Vascular bundles scattered
Dicots
Have 2 seed leaves (cotyledons)
Net-like veins on leaves
Flower parts in multiples of 4-5
Taproots
Vascular bundles organized in ring
K. Plant Reproduction
Flower:
- reproductive organ of the plant
- usually both male and female
- the male part is the STAMEN
- the female part is the PISTIL
 Sexual Reproduction:
- pollen is produced in the stamen
- pollen moves away from the plant via the wind or other pollinators (birds & bees)
- pollen lands on the pistil of another plant and fertilizes the eggs within the ovary
- flower petals fall off, the ovary develops into a FRUIT that encloses the seeds
- fruits are dispersed in a variety of ways (wind, animals)
- fruits are not always edible (anything with a seed is considered a fruit – ex. helicopter, acorns,
dandelions)
 Asexual Reproduction:
- many plants clone themselves, a process called VEGETATIVE PROPAGATION
- strawberry plants and other vine like plants send out RUNNERS, which grow into new plants
- some plant clippings will grow new plants
L. Alternation of Generations
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alternating haploid (gametophyte) and diploid (sporophyte) organisms in a life cycle of a plant
depending on the type of plant, one of these generations is the dominant form of the organism,
and the other generation is nutritionally dependent upon it or just grows as a smaller plant.
bryophytes/nonvascular plants ONLY group that has a dominant gametophyte generation
see attached diagrams of various life cycles
M. Plant Growth
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Germination occurs when a seed sprouts (usually caused by change in temperature and
moisture)
Plants grow only at their tips in regions called MERISTEMS
APICAL meristems: primary growth at root and stem; makes plant taller
LATERAL meristems: secondary growth (adds woody tissue); makes plant wider
M. Anatomy of a Leaf