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Plants: Form and Function

Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The Internet. Provided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. By David Knuffke. Modified by Eric Friberg
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Eric Friberg

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Transcript of Plants: Form and Function

Plants
Bryophytes: Mosses, liverworts, etc.
Seedless Vascular Plants: Ferns, horsetails, etc.
Gymnosperms: Evergreens, Ginkos, etc.
Angiosperms: Flowering plants
General Characteristics:
eukaryotic, multicellular, photoautotrophic
cell walls made of cellulose
Characteristics: No vascular tissue or seeds
the gametophyte generation is dominant, with a dependent sporophyte
Characteristics: Vascular tissue, but no seeds.
Sporophyte is dominant, dependent gametophyte.
Motile, free-swimming gametes.
Characteristics: Vascular tissue, seeds develop exposed or in cones
Sporophyte is dominant, microscopic gametophyte
Includes all "evergreen trees"
Characteristics: Have flowers. Vascular tissue, seeds develop in floral ovary (fruit)
Sporophyte is dominant, microscopic gametophyte
Classified into 2 major groups: monocots and dicots
Most diverse group of plants (why so successful?)
Vascular Tissue!
Pollen & Seeds!
Flowers & Fruit!
What makes a plant?
What it is:
What they are:
What they are:
molecular analysis suggests that plants evolved from green algae
Plants grow continuously during their life cycle.

"meristem": permanently undifferentiated tissue. Site of plant growth
"Alternation of Generations" Life cycle:
2 multicellular forms (haploid & diploid)
Evolutionary trends in plant forms:
evolutionary time
Tissue that transports water (xylem) and sugar (phloem) throughout the plant.
Why does it matter?
"Vascular Bundles" of xylem and phloem in a celery stalk
Why do they matter?
Pollen: a water-free mode of dispersing sperm.

Seeds: a protective, nourishing, dispersive place for early embryo development
typical pollen structure:
Dandelion seeds are adapted for wind dispersal:
"naked seed"
gymnosperms include the largest plants on Earth
Why do they matter?
Flowers: a structure that contains both reproductive organs of the plant and is adapted for pollination.

Fruit: a structure that houses fertilized seeds and aids in their dispersal.
"Mmm... Nom Nom Nom!"
fruit consumption by animals = seed deposition in fertilizer
"Male part"
"Female part"
Learn your floral anatomy!
It makes me want to Sing and Dance!
Transport
Overview
Root Processes
Transpiration
Phloem Processes
Regardless of size, plants have similar transport requirements

Oxygen and Carbon Dioxide are subject to diffusion. This works in opposition at the leaf and at the root.

Soil nutrients must enter the plant in the roots.

To effectively photosynthesize, water must move in to the roots of the plant, through the stem and up to the leaves.
Water Potential Revisited
Water potential is a function of the
pressure potential
and the
solute potential
.
Water will move from areas of less solute (
higher solute potential
) to areas of more solute (
lower solute potential
)
Positive pressure (
higher pressure potential
) will push water into an area of negative pressure (
lower pressure potential
).
More solute (lower solute potential) has an opposing effect on water movement to positive pressure (higher pressure potential).
Plant Physiology is fundamentally dependent upon the movement of water into the plant at the roots and out of the plant at the leaves.
By controlling internal solute concentration and pressure, plants control the direction of water and solute movement
Root Tonicity:
Roots are
hypertonic
to the soil.

Cells of the
root cortex
actively transport soil nutrients in to the root.

This maintains tonicity and keeps water (and dissolved nutrients) moving in to the root.
Symplast vs. Apoplast
Two routes of transport through plant cells:
Symplastic
: transport through cytoplasm and plasmodesmata.
Apoplastic
: transport through cell wall channels.

Material can also travel through both means (the "
transmembrane route
")
Prior to transport into the vascular tissue of the root (and therefore into the rest of the plant), all material must move into the symplast of the
endodermis
that surrounds the vascular cylinder.

This is accomplished by the means of a waxy strip of material (the "
Casparian strip
") that lines the cell wall of the endodermis. Apoplastic material can not traverse the Casparian strip and must move into the symplast.
This forcing of the symplastic route prior to transport to vessels serves a means of controling the material that moves in to the vessels of the plant
The continual movement of water into the roots of a plant does not usually provide enough pressure to move water through the entire plant.

At some point (somewhere ~ 4 feet off the ground), gravity begins to counterbalance the positive root pressure.

And yet, there are plants taller than 4 feet....how is this possible?

Transpiration
: The movement of water from roots to leaves and in to the atmosphere.

A major part of the water cycle.
How transpiration happens...
Root:
The hypertonicity of the roots (low solute potential) keeps water moving in to the plant.

Water is sent to the xylem.

This process creates a positive root pressure, which keeps water moving up the xylem of the plant (to a point).
Stem:
Water continues to move through the stem in response to the the positive root pressure and
cohesional attraction
to water molecules that are moving up the xylem.

Water molecules also have
adhesional attraction
to the walls of the xylem, which keep them moving up (or at least not moving down)
Leaf
Water moves from the xylem in to the
mesophyll
of the leaf, where it can be used for photosynthesis.

Excess water moves in to the atmosphere via
stomates
in the leaf.

Cohesional attraction between water molecules keeps water moving through the stomates ("
Transpirational Pull
")
Transpiration is made possible by the fact that as water moves through the plant, it is always moving to an area of lower water potential.

The area of highest water potential in the plant is the roots. Plants spend a lot of energy maintaining the osmotic potential of the roots by actively transporting solutes in to the roots (which keeps the roots hypertonic to the soil and serves as a source of nutrients).

As water moves up the plant, it is moving into increasingly lower water potential, due mainly to a decreasing pressure potential.

The atmosphere has the lowest pressure potential.
At the leaf, water will saturate the cell wall fibers (
micofibrils
). As water evaporates into the air spaces of the leaf, adhesive attractions among water molecules and cohesive attractions to the cell wall fibers will bring more water in to the boundary between cells and the atmosphere.
At night, transpiration does not typically occur.

As water continues to enter the roots of the leaf, the postive root pressure can cause water to be pushed out of the plant, forming droplets on the surface of the leaf ("
guttation
").
Stomatal Control
Opening and closing stomates is the major way that plants are able to control transpiration and gas exchange.
Stomates are controlled by
guard cells
, which line the opening of the stomate.

Turgid guard cells: Open stomate

Flaccid guard cell: Closed stomate.
Typical stomatal density is anywhere from 100-1000 stomates per square millimeter of leaf surface (!)
The active transport of potassium ions into guard cells leads to turgidity.

Flaccid guard cells stop actively transporting potassium ions, which causes water loss.
Water loss prevention adaptations
Xerophytes
: Plants that have adapted to live in arid climates.
The white bristles of the Old Man Cactus reflect sunlight.
Ocotillo only leafs after heavy rainfall. The leaves are small and fall off quickly.
Oleandar stomates are found in recessed "crypts", which increases humidity at the stomate and delays water loss.
Phloem tissue transports sugars ("
sap
") made at the leaf through the rest of the plant.
As sugar is produced by mesophyll, it travels through symplastic and apoplastic channels to the phloem.

Sugar is actively loaded into the phloem through proton/sucrose cotransporter proteins.
Once loaded into the phloem, the high sugar concentration at the leaf (
source
), causes water to diffuse into the phloem.

This leads to a positive pressure which moves phloem sap through the plant until it reaches an area of low sugar concentration (
sink
), at which point the sap diffuses out of the phloem.
Experiments that utilized aphid mouthparts (
stylets
) demonstrated evidence to support the "
pressure flow
" hypothesis of phloem transport.

Stylets closer to leaves had faster rates of sap extrustion.
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Theory
How Cells Communicate
The 3 Phases of Signal Reception
Local Signals
Distance Signals
Molecules are Required!
Two Kinds of Hormone Molecules
Advanced
Considerations
The ligand isn't important.
The Response is!
Cells
Amplify
A Message
The Role of the "
Second Messenger
"
Things Get Complicated Quickly
Released by cells, recieved by neighbors
Different Chemistry
Different Reception
Different Responses
An
Epinephrine
("
Adrenaline
") Receptor
What kind of hormone is epinephrine?
Epinephrine signal transduction is mediated by
G-Protein linked receptors.

G-Proteins are very common signal relay proteins for membrane receptor ligands.

"G-Protein" means it is associated with
GTP

AMP is high when ATP is low
Lipid Hormones Tend To Have Longer Term Effects than Peptides
Estrogen
is responsible for long term changes in sexual cycles (menstruation, puberty).

Estrogen activates long term responses by changing protein expression in cells.

{
Vitellogenin
is a precursor to nutritive egg yolk proteins present in all egg producing female animals}
The Systems that send distance signals are the nervous and endocrine systems.

These are the major regulatory systems in animals.

There is a good deal of interplay between all signalling systems
Generally speaking, nervous and endocrine systems have complementary purposes.

Nervous system
: quick, fast, regulation. More on that later.

Endocrine system
: long term, regular regulation.

Both systems interact
Internal signalling molecules released due to external ("first") signals
Epinephrine is a polar amine ligand
Note:
This is a review of "Cellular Communication"
"
ligands
"
Have Hormones, Too!
Plants
Auxin & Apical Dominance
Gibberellin and lots of things
Absisic Acid Prevents Growth
Ethylene is Awesome!
Leaf Absicion is Complex!
Some Major Plant Responses
Auxin
Production at the
Apical Bud
determines the direction of plant growth
Gibberellin
treatment leads to "
bolting
" in growing plants & accelerated fruit ripening
Intake of water in germinating seeds leads to gibberelin production and the breakdown of stored starch to power
cotyledon
growth
Decreasing [
ABA
] leads to
germination
precocious!
Ethylene
is a gas!
It triggers growth
& Fruit Ripening
Increased ethylene is associated with the "
triple response
" in young plants, a reaction to encountering an overhead blockage during growth
Leads To
Ethylene response mutants
Abscision
(loss of leaves),
involves interactions between many hormones, including ethylene, gibberelin, and absisic acid
Other Organisms
Fungi & Plants
Prokaryotes
It is thought that restriction enzymes protect prokaryotes from bacteriophage infections.
Fungi and Plants rely on the production of a wide variety of chemicals that can cause unpleasant effects in would-be pathogens and predators.
The "Death Cap" mushroom produces
alkaloid
chemicals that cause irreversible liver failure in humans.
The Ppenicillium genus of fungi produce
antibiotics
(like penicillin) to protect against bacterial infections.
Plants have systemic mechanisms to prevent the spread of viral infections
The diversity of chemicals that plants can produce in response to pathogens is remarkable!
a. By
methylating
their own restriction sites, prokaryotes can protect agains phage infection.







b. Of course, phages can evolve mechanisms to evade prokaryotic defenses.
Nutrition
Overview
Roots
Adaptations
Plant Nutrients
Soil
Structure
Symbiosis
Nutritional
Symbiotic
With the exception of Carbon Dioxide and Oxygen, plants recieve most nutrients from the
soil
Hydroponic
techniques are utilized to determine the effects of specific nutrient deficiencies on plant growth.
While all organisms require SPONCH to live, plants are particularly sensitive to Potassium deficiencies as well,
Soil is a complex mixture of organic and inorganic compounds of both biological and physical origin.

Soil is produced and replenished through natural processes including deposition, weathering and decomposition.

There is a directionality to typical soil production, with newer, nutrient-rich soil being deposited in the top
horizon
.
The "Dustbowl" in the American Midwest: An example of accelerated
soil depletion
due to human ignorance combined with natural drought cycles.
Contour farming
: one example of a practice that can slow erosion due to farming.
Plant roots are the structures responsible for soil nutrient absorption.

Root Hairs
: Major absorptive surface of roots.
Cation Exchange
Protons are pumped out of plant roots into the soil.

The protons displace nutritive cations (K+, Mg++, etc.) in the soil, which are then absorbed into the roots.

Nutrients move in to roots via diffusion, mostly.
Plants rely upon bacteria and fungi to provide them with some nutrients in the soil.
Bacteria
Fungi
Bacteria play a major role in the
Nitrogen Cycle
The action of
nitrogen-fixing
bacteria in the soil converts atmospheric nitrogen into biologically useful forms, which can be incorporated from soil into amino acids and nucleotides by plants.

This is how all nitrogen enters the food chain.

Legumes
: plants that have specialized
root-nodule
adaptations that contain populations of nitrogen-fixing bacteria.
Mycorrhizae
: Symbiotic fungal-root associations.

Increase nutrient absorption by roots.
Development & Structure of Root Nodules
"Carnivorous" Plants
Live in nutrient-poor soils.

Adapted to catch and digest small animals as a supplementary source of soil nutrients (particularly nitrogen).
Epiphytes
Plants that live on other plants.

Typical in rainforests and other soil-poor areas.

Can be mutualistic, commensal, or parasitic.
Parasites
There are many different plant parasites of other plants, with a wide diversity of parasitic adaptations.
Data from an experiment (right) looking at the effects of invasive garlic mustard (left) on the ability of native plants to form mycorrhizal associations
"all that remains is a husk"
Note:
Need to click the link, since embedding is disabled.
Sound effects are a bit over the top
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http://www.youtube.com/watch?v=O7eQKSf0LmY
Plant Structure
Major functions are anchorage, and nutrient/water absorption.
Can be modified for other purposes
Plants demonstrate differentiation of cells.
Plant physiology can be understood in terms of
tissue-level
,
organ-level
, and even relatively simple
systems-level
organization.
2 Systems
3 Organs
Cells & Tissues
3 Types of Plant Cells
Most of a plant's cells are
parenchymal
cells.

Parenchymal
cells are responsible for photosynthesis, while
collenchyma
and
sclerenchyma
provide structure and support.

Schlerenchymal
cell walls are filled with
lignin
, a structural polymer. The lignification process leads to the death of the sclerenchymal cells at their functional maturity.
3 Types of Plant Tissues
Dermal
Ground
Vascular
Specialized parenchyma. Includes
stomates
& a waxy layer ("
cuticle
") to prevent dessication.
Includes all three cell types. Involved in photosynthesis (at leaves), storage of food, support of plant,
Not found in bryophytes. Contains specialized cells that comprise the
Xylem
and
Phloem
of the plant.
Xylem
Phloem
transport water.
Made of
tracheids
and
vessel elements
.
dead at functional maturity.
transport carbohydrates.
Made of
sieve-tube elements
and
companion cells
.
sieve-tube elements are dead at functional maturity.
companion cells regulate sieve-tube element function.
Leaf
Major function is photosynthesis.
Can be modified for other functions.
Stem
Major functions are support & transport.
Can be modified for other functions.
Root
Shoot
Root
All above-ground parts of the plant.

Photosynthesis, floral reproduction, etc.
All below-ground parts of the plant.

nutrient/water absorption, etc.
Organismal Biology

The physiological processes at work in any organism are constrained by the environment and adapted by evolution.
Example 1:
Materials Exchange
Unicellular and microscopic organisms are able to exchange materials directly with their environment

Plants and animals have evolved adaptations to accomplish these exchanges internally (usually through maximizing surface area)
Example 2:
Energy Considerations
The energetic considerations of an organism's enviornment have consequences for physiology and behavior.
Example 3:
Convergence
Similar environmental constraints often result in similar adaptive solutions.
Full transcript