Plants Form and Function
Chapter 28
 Plants grow only at meristems
 Leaf anatomy relates to photosynthesis.
 The role of root hairs and mycorrhizae in resource
acquisition.
 Roots, stems, and leaves
Plants grow only at meristems
 Meristems are perpetually embryonic tissues
 Growth occurs only as a result of cell division in the meristems.
 Apical meristems
 Tips of roots and in buds of shoots

Allowing the plant’s stems and roots to extend.
 Primary growth
 Lateral meristem
 Results in growth that thickens the shoots and roots

Secondary growth
Leaf anatomy relates to photosynthesis.
Leaf anatomy relates to photosynthesis.
 How the structures enhance functions, such as, gas
exchange, photosynthesis, and reduction of water
loss.
 Stomata



Formed by guard cells which open and close the stomata.
Allows for gas exchange- CO2 and O2
Loss of water- Transpiration
The role of root hairs and mycorrhizae in
resource acquisition.
 Root system beneath the ground that is a
multicellular organ that anchors the plant, absorbs
water and minerals, and often stores sugars and
starches.
 Root hairs are located at the tips of roots, which
are extension of root epidermal cells.

Increase the surface area, making efficient absorption of water
and minerals possible.
 Symbiotic relationship with fungi at the tips of the
roots called mycorrhizae.

Assist in the absorption process
Resource Acquisition and Transport in Vascular
Plants- Chapter 29
 How passive transport, active transport, and




cotransport function to move materials across plant
cell membranes.
The role of water potential in predicting movement
of water in plants.
How the transpiration cohesion-tension mechanism
explains water movements in plants.
How bulk flow affects movement of solutes in plants.
Mechanisms by which plant cells communicate with
other distant cells.
Overview of Resource Acquisition and Transport
Different Mechanisms Transport Substances
Over Short or Long Distances
 Transport begins with the movement of water and
solutes across a cell membrane.

Solutes diffuse down their electrochemical gradients


Passive Transport requires NO energy


Electrochemical gradients are the combined effects of the
concentration gradient of the solute and the voltage or charge
differential across the membrane.
Example- Diffusion
Active Transport requires energy

Example- Proton pump
Different Mechanisms Transport Substances
Over Short or Long Distances
 The uptake of water across cell membranes occurs
through osmosis (passive transport)

Water moves from areas of high water potential to low water
potential.

Water potential includes the combine effects of solute
concentration and physical pressure.
Different Mechanisms Transport Substances
Over Short or Long Distances
 Ψs = solute potential
 Ψs of pure water is O.
 Adding solutes to pure water always lowers water
potential, therefore, solute potential of a solution is
always negative.
 Ψp = pressure potential


Pressure potential is the physical pressure on a solution.
An example of positive Ψp occurs when the cell contents press
the plasma membrane against the cell wall, a force termed
turgor pressure.

If the cell loses water, the pressure potential becomes more
negative, resulting in wilting.
Transport of Water
Three mechanisms are involved in the movement
of water.

1.
Osmosis
–
1.
Concentration gradient (soil to root)
 Continuous movement of water out of the root by xylem
 Higher mineral concentration inside the stele maintained
by the selective passage of ions through the endodermis.
Root pressure (osmotic force)
Transport of Water
 Three mechanisms are involved in the movement of
water.
2. Capillary action
Rise of liquids in narrow tubes
 Contributes to movement of water up xylem
 Adhesion forces
 Meniscus formation

Transport of Water
 Three mechanisms are involved in the movement
of water.
3. Cohesion-tension theory

Transpiration




Evaporation of water from plants
Removes water from leaves
Causing negative pressure or tension to develop within the leaves
and xylem tissue
Cohesion

Produces a single, polymerlike column of water from roots to leaves
Transport of Water
 Three mechanisms are involved in the movement of water.
3. Cohesion-tension theory

Bulk flow
Occurs as water molecules evaporate from the leaf surface.
 When a water molecule is lost from a leaf by transpiration, it
pulls up behind it an entire column of water molecules.
 In this way, water moves by bulk flow through the xylem by a
pulling action generated by transpiration

• Transpiration is caused by the heating action of the sun, therefore,
the sun is the driving force for the ascent of water and minerals
through the plants.
The Rate of Transpiration is Regulated
by Stomata
 Stomata opening and closing
 Influences gas exchange, transpiration, the ascent of
water and minerals (sap), and photosynthesis.
 Closed stomata
 Water and carbon dioxide are not available, and
photosynthesis cannot occur.
 Open stomata
 Carbon dioxide can enter the leaf.
 Water is delivered by the pulling action of transpiration
 Problem: the plant risks desiccation from excessive
transpiration.
Control of Stomata
Mechanisms that control the opening and
closing of stomata.

1.
Close when temperatures are high.
–
–
2.
Open when carbon dioxide concentrations are low
inside the leaf.
–
3.
4.
Reduces loss of water
Shuts down photosynthesis
Allows photosynthesis to occur
Close at night and open during the day
Stomata opening occurs by a diffusion of potasium
ions into guard cells, creating a gradient for the
movement of water into guard cells.
Transport of Sugars

Translocation


Movement of carbohydrates through the phloem from
a source (leaves) to a sink (a site of carbohydrate
utilization or storage).
Mechanism = Pressure-Flow hypothesis
1.
2.
3.
4.
Sugars enter sieve-tube members
Water enters sieve-tube members
Pressure in sieve-tube members at the source move water and
sugars to sieve-tube members at the sink through sieve-tubes.
Pressure is reduced in sieve-tube members at the sink as
sugars are removed for utilization by nearly cells.
Soil and Plant Nutrition
Chapter 29
 Mutualistic relationships between plant roots and
the bacteria and fungi that grow in the rhizosphere
help plants acquire important nutrients.
 Plants also form symbiotic relationships that are not
mutualistic.
 Interactions between population (such as
competition, predation, mutualism, and
commensalism) can influence patterns of species
distribution and abundance.
Plant Nutrition Often Involves Relationships
with Other Organisms
 Mutualistic relationship between nitrogen-fixing
bacteria and plants.

Rhizobium bacteria fix atmospheric nitrogen into a form used
by the plant.

Plants provide food into the root nodule where bacteria live.
 Mycorrhizae another example of mutualistic
relationships with roots.

Roots and fungi in the soil

Fungus receives sugar from the plant and the fungus increases the
surface area for water uptake, selectively absorbs minerals that are
taken up by the plant, and secretes substances that stimulate root
growth and antibiotics that protect the plant from invading
bacteria.
Nitrogen Cycle
Plants also form symbiotic relationships that are
not mutualistic.
 Parasitic plants
 Example- Dodder
They are not photosynthetic
 Rely on other plants for their nutrients.

 Epiphytes
 These plants are not parasitic but grow on the surfaces of other
plants instead of the soil.

Example-Orchids
 Carnivorous plants
 These plants are photosynthetic, but they get some nitrogen
and other minerals by digesting small animals.

Commonly found in nitrogen-poor soil, like bogs.
Plant Responses to Internal and External Signals
Chapter 31
 The three components of a signal transduction
pathway and how changes could alter cellular
responses.
 The role of auxins in plants.
 How phototropism and photoperiodism use changes
in the environment to modify plant growth and
behavior.
 How plants respond to attacks by herbivores and
pathagens.
Signal Transduction Pathway
Involves Three Steps:
 Reception

Cell signals are detected by receptors that undergo changes in shape
in response to a specific stimulus.

Two common plasma membrane receptors are G proteins coupled
receptors and receptor tyrosine kinase.
 Transduction


Multistep pathway that amplifies the signal.
This allows a small number of signal molecules to produce a large
cellular response.
 Response

Cellular response is primarily accomplished by two mechanisms:
Turning genes on or off and thereby increasing or decreasing mRNA
production.
 Activating existing enzyme molecules

Signal Transduction Pathway
Signal Transduction Pathway
Signal Transduction Pathway
Plant hormones help coordinate growth,
development, and responses to stimuli
 Hormones are defined as signaling molecules
produced in small amounts in one part of an
organism and transported to other parts.

Act as chemical messengers that coordinate the different parts
of a multicellular organism.
 Tropism: plant growth response that results in the
plant growing either toward or away from a stimulus.

Results from hormone production
Phototropism (positive/negative)
 The growth of a shoot in a certain direction in response to light
 Hormone = Auxin
 Gravitropism

Plant hormones help coordinate growth,
development, and responses to stimuli
 Auxins


Stimulate elongation of cells within young developing shoots
Produced in the apical meristems, activates proton pumps in the
plasma membrane, which results in a lower pH (acidification of the
cell wall)

This weakens the cell wall, allowing turgor pressure to expand the cell
wall, resulting in cell elongation.
 In phototropism, the plant moves towards the sunlight
to have its leaves at 90° angles to the light rays.


This response results from cells on the dark side elongating faster
than the cells on the light side.
Causes the shoot to grow faster on the shady side, bending it toward
the light.
Responses to light are critical for plant success
 Plants can detect not only the presence of light, but
also its direction, intensity, and wavelength.

Action spetra reveal that red and blue light are the most
important colors in plant responses to light.
 Blue-light photorecepetors initiate a number of
plant responses to light including phototropisms and
light-induced opening of the stomata.
 Light receptors termed phytochromes absorb
mostly red light.
Photoperiodism
 Response of plants to changes in the
photoperiod (the relative length of daylight and
night)
 Plants maintain a circadian rhythm (a clock
that measures the length of daylight and
night), in order to respond to changes in the
photoperiod.
 The mechanism is endogenous (it is an internal
clock that continues to keep time even if external
cues are absent.

External cues, such as dawn and dusk, reset the
clock to maintain accuracy.
Photoperiodism
 Mechanism for maintaining the circadian rhythm
 Phytochrome = a protein modified with a lightabsorbing chromophore

Two forms: Pr (P660) and Pfr (P730)


Absorb wavelengths of light: red @ 660 nm and far-red @ 730
nm
Photoreversible
 When
Pr is exposed to red light, it is converted to Pfr
 When Pfr is exposed to far-red light it is converted back to
Pr
Photoperiodism

1.
2.
3.
4.
5.
The following observations have been made for
many plants
Pfr appears to reset the circadian-rhythm clock
Pr is the form of phytochrome synthesized in
plants cells.
Pfr and Pr are in equilibrium during daylight.
Pr accumulates at night
At daybreak, light rapidly converts the
accumulated Pr to Pfr
Photoperiodism
6. Night length is responsible for resetting the circadianrhythm
If daylight is interrupted with a brief dark period, there is
no effect on the circadium-rhythm.
In contrast, flashes of red or far-red light during the
night period can reset the clock.
If a plant is exposed to a flash of re light during the night,
Pr is converted back to Pfr, a shorter night period is
measured, and the circadium rhythm is reset.
If a flash of far-red light follows the red light, then the
effect of the red light is reversed, and the night length is
restored to the night length in effect before the far-red
flash.
Photoperiodism
- In a series of alternating flashes of red and far-red
light, only the last flash affects the perception of
night length.
- Thus, red light shortens the night length and far-red
restores the night length.
Photoperiodism
 Many flowering plants initiate flowering in
response to changes in the photoperiod.
 Three groups:

Long-day plants flower in the spring and early summer
when daylight is increasing.


Short-day plants flower in late summer and early fall
when daylight is decreasing


These plants flower when daylight exceeds a critical length.
These plants flower when daylight is less than a critical length
Day-neutral plants do not flower in response to daylight
changes.

Some other cues, such as temperature or water, triggers flowering
Photoperiodism
Plants respond to a wide variety
of stimuli other than light
 Gravitropism
 Plant’s response to gravity
Roots show positive gravitropism and grow toward the source
of gravity
 Shoots show negative gravitropism and grow away from
gravity


Auxin plays a key role in gravitropism in both the roots and
stems.
 Thigmotropism
 Directional growth in response to touch

Vines display thigmotropism when their tendrils coil around
supports.
Plants respond to attack
by herbivores and pathogens
 Physical defenses against predators (herbivores)



Thorns
Chemicals such as bitter or poisonous compounds
Airborne attractants to recruit other animals to kill the herbivores
 Epidermal layer acts as a barrier (like humans) against
viruses.
 Plants immune responses involve both localized, specific
responses as well as plant-wide responses.



The lesions or dead spots you may have seen on leaves can be the
result of the plant responding to a pathogen by sealing off the
pathogen then killing the cells in the area.
This kills the pathogen and prevents the spread of the disease to the
rest of the plant.
Both plants and animals immune systems depend heavily on signal
transduction pathways.