Plants Form and FunctionChapter 28

Plants grow only at meristemsLeaf 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 SubstancesOver Short or Long Distances

Transport begins with the movement of water and solutes across a cell membrane. Solutes diffuse down their electrochemical gradients

Electrochemical gradients are the combined effects of the concentration gradient of the solute and the voltage or charge differential across the membrane.

Passive Transport requires NO energy Example- Diffusion

Active Transport requires energy Example- Proton pump

Different Mechanisms Transport SubstancesOver 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 SubstancesOver 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– 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.

1. 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 Regulatedby 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.– Reduces loss of water– Shuts down photosynthesis

2. Open when carbon dioxide concentrations are low inside the leaf.

– Allows photosynthesis to occur

3. Close at night and open during the day4. 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 hypothesis1. Sugars enter sieve-tube members2. Water enters sieve-tube members3. Pressure in sieve-tube members at the source move

water and sugars to sieve-tube members at the sink through sieve-tubes.

4. Pressure is reduced in sieve-tube members at the sink as sugars are removed for utilization by nearly cells.

Soil and Plant NutritionChapter 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 Relationshipswith 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 SignalsChapter 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 light-

absorbing 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

The following observations have been made for many plants

1. Pfr appears to reset the circadian-rhythm clock

2. Pr is the form of phytochrome synthesized in plants cells.

3. Pfr and Pr are in equilibrium during daylight.

4. Pr accumulates at night5. At daybreak, light rapidly converts the

accumulated Pr to Pfr

Photoperiodism

6. Night length is responsible for resetting the circadian-rhythm

- 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. These plants flower when daylight exceeds a critical

length. Short-day plants flower in late summer and early

fall when daylight is decreasing 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.