Chapter 36

Question 1

How does water move up to the top of a 10-story high tree?

Answer 1

Water first enters the roots.​

Then moves to the xylem.​

Innermost vascular tissue.​

Water rises through the xylem because of a combination of factors.​

Most of that water exits through the stomata in the leaves.

Question 2

Long-Distance Movement

Answer 2

Local changes result in long-distance movement of materials​

Water and dissolved minerals travel great distances in xylem​

Most of the force is “pulling” caused by transpiration.​

Evaporation from thin films of water in the stomata.​

Occurs due to cohesion (water molecules stick to each other) and adhesion (stick to walls of tracheids or vessels).

Question 3

Movement of Water at Cellular Level

Answer 3

Water can diffuse down its concentration across a plasma membrane by osmosis​

If the cell is placed into a hypotonic solution (concentration of solutes inside cell greater than that of the external solution)​

The rate of water movement into or out of cells is enhanced by membrane water channels called aquaporins​

Aquaporins speed up water movement across a membrane, but do not change its direction

Question 4

Aquaporins

Answer 4

Water-selective pores in plasma membrane increase the rate of osmosis by facilitating the diffusion of water

Question 5

Water Potential

Answer 5

Measured in units of pressure called megapascals (MPa)

is used to predict which way water will move​

Water moves from higher to lower​

Potentials are a way to represent free energy

Question 6

Osmosis

Answer 6

If a single plant cell is placed into water.​

Water moves into cell by osmosis.​

Cell expands and becomes turgid.​

If cell placed in high concentration of sucrose.​

Water leaves cell.​

Cell shrinks – plasmolysis.

Question 7

Water potential has two components

Answer 7

Physical forces, such as gravity or pressure on a plant cell wall. The contribution of gravity is small, but the turgor pressure is significant. This is the pressure potential​

The concentration of solute in each solution determines the solute potential​

Total water potential is the sum of its pressure potential and solute potential​

Water will always move, via osmosis, in the direction of lower water potential

Question 8

Pressure Potential

Answer 8

As turgor pressure increases, ​Wp increases

Solutions that are not contained within a membrane cannot have turgor pressure and always have a​ Wp of 0 MPa

Turgor pressure generated from fluid within a cell pushing against the cell wall gives a turgid cell a​ Wp > 0 MPa

Question 9

Solute Potential

Answer 9

Pure water has a​ Ws of 0 MPa

As a solution increases in solute concentration, it decreases in​ Ws making it < 0 MPa

When solutes are added, water molecules interact with the solute molecules.​

Fewer free water molecules are available to move, which decreases the water potential.​

Question 10

Determining Water Potential

Answer 10

Ww = Wp + Ws

The total water potential of a plant cell is the sum of its pressure potential​ and solute potential

Represents the total potential energy of the water in the cell.​

When Ww inside the cell equals that of the solution, there is no net movement of water​

Question 11

Water Absorption

Answer 11

Most of the water absorbed by the plant comes in through the region of the root with root hairs​

Surface area further increased by mycorrhizal fungi.​

Once absorbed through root hairs, water and minerals must move across cell layers until they reach the vascular tissues​

Water and dissolved ions then enter the xylem and move throughout the plant

Question 12

Three transport routes exist through cells

Answer 12

Apoplast route – movement through the cell walls and the space between cells​

Avoids membrane transport.​

Symplast route – cytoplasm continuum between cells connected by plasmodesmata​

Transmembrane route – membrane transport between cells and across the membranes of vacuoles within cells​

Permits the greatest control.

Question 13

Inward Movement of Water

Answer 13

Eventually on their journey inward, molecules reach the endodermis​

Any further passage through the cell walls is blocked by the Casparian strips​

Apoplast route is blocked by waterproof material called suberin.​

Molecules must pass through the cell membranes and protoplasts of the endodermal cells to reach the xylem

Question 14

Movement of Ions

Answer 14

Mineral ion concentration in the soil water is usually much lower than it is in the plant​

Active transport across endodermis is required for increased solute concentration in the stele.​

Plasma membranes of endodermal cells contain a variety of protein transport channels​

Proton pumps transport specific ions against even larger concentration gradients.

Question 15

Xylem Transport

Answer 15

The aqueous solution that passes through the endodermal cells moves into the tracheids and vessel elements of the xylem​

As ions are actively pumped into root or move via facilitated diffusion, their presence decreases the water potential​

Water then moves into the plant via osmosis, causing an increase in turgor pressure

Question 16

Root Pressure

Answer 16

Caused by the continuous accumulation of ions in the roots at times when transpiration from leaves is low or absent​

Often at night.​

Causes water to move into plant and up the xylem despite the absence of transpiration​

Guttation is the loss of water from leaves when root pressure is high​

Root pressure alone, however, is insufficient to explain xylem transport​

Transpiration provides the main force.

Question 17

Regulation of Water Movement

Answer 17

Water potential regulates the movement of water through a whole plant​

Water moves from the soil into the plant only if water potential of the soil is greater than in the root​

Water in a plant moves along a Ψw gradient from the soil to successively more negative water potentials in the roots, stems, leaves, and atmosphere

Question 18

Cohesive Water Forces

Answer 18

Water has an inherent tensile strength that arises from the cohesion of its molecules​

The tensile strength of a water column varies inversely with its diameter​

Because tracheids and vessels are tiny in diameter, they have strong cohesive water forces​

The long column of water is further stabilized by adhesive forces

Question 19

Effect of Cavitation

Answer 19

Tensile strength depends on the continuity of the water column​

A gas-filled bubble can expand and block the tracheid or vessel (process called cavitation)​

breaks the tensile strength of a water column.​

Damage can be minimized by anatomical adaptations​

Presence of alternative pathways.​

Pores smaller than air bubbles.

Question 20

Tracheids and vessels are essential for the

Answer 20

bulk transport of minerals​

Ultimately the minerals are relocated through the xylem from the roots to other metabolically active parts of the plant.​

Phosphorus, potassium, nitrogen, and sometimes iron may be abundant in xylem.​

Calcium, an essential nutrient, cannot be transported elsewhere once it has been deposited in a particular plant part.

Question 21

Rate of Transpiration

Answer 21

Over 90% of the water taken in by the plant’s roots is ultimately lost to the atmosphere​

At the same time, photosynthesis requires a CO2 supply from the atmosphere​

Closing the stomata can control water loss on a short-term basis​

However, the stomata must be open at least part of the time to allow CO2 entry

Question 22

Guard Cells

Answer 22

Only epidermal cells containing chloroplasts​

Have thicker cell walls on the inside and thinner cell walls elsewhere​

Bulge and bow outward when they become turgid.​

This causes the stoma between two guard cells to open.​

Turgor in guard cells results from the active uptake of potassium​ chloride and malate

Addition of solute causes water potential to drop.​

Water enters osmotically and cells become turgid.

Question 23

Changing Transpiration Rates

Answer 23

Transpiration rates increase with temperature and wind velocity because water molecules evaporate more quickly​

Several pathways regulate stomatal opening and closing​

Abscisic acid (A B A) initiates a signaling pathway to close stomata in drought.​

Opens K+. Cl-, and Malate channels
Water loss makes guard cells flaccid

Question 24

Also Affecting Stomatal Opening

Answer 24

Close when CO2 concentrations are high​

Open when blue wavelengths of light promote uptake of​ K+ by the guard cells

Close when temperature exceeds 34°C and water relations unfavorable​

Crassulacean acid metabolism (C A M) plants conserve water in dry environments by opening stomata and taking in CO2 at night​

Question 25

Water Stress Responses

Answer 25

Many morphological adaptations allow plants to limit water loss in drought conditions​

Dormancy.​

Loss of leaves – deciduous plants.​

Covering leaves with cuticle and wooly trichomes.​

Reducing the number of stomata.​

Having stomata in pits on the leaf surface.

Question 26

Plants have adapted to flooding conditions which deplete available oxygen

Answer 26

Flooding may lead to abnormal growth.​

Oxygen deprivation most significant problem.​

Plants have also adapted to life in fresh water.​

Form aerenchyma, which is loose parenchymal tissue with large air spaces.​

Collect oxygen and transport it to submerged parts of the plant.

Question 27

Aerenchyma

Answer 27

This tissue facilitates gas exchange in aquatic plants. Water lilies float on the surface of ponds, collecting oxygen and then transporting it to submerged portions of the plant. Large air spaces in the leaves of the water lily add buoyancy. The specialized parenchyma tissue that forms these open spaces is called aerenchyma. Gas exchange occurs through stomata found only on the upper surface of the leaf.

Question 28

Growth in Saltwater

Answer 28

Plants such as mangroves grow in areas flooded with salt water​

Must supply oxygen to submerged roots and control salt balance.​

Pneumatophores – long, spongy, air-filled roots that emerge above the mud.​

Provide oxygen to submerged roots.​

Succulent leaves contain large amount of water to dilute salt.​

May secrete salt or block salt uptake.

Question 29

Mangroves

Answer 29

The black mangrove, Avicennia germinans, grows in areas that are commonly flooded, and much of each plant is usually submerged. However, modified roots called pneumatophores supply the submerged portions of the plant with oxygen because these roots emerge above the water and have large lenticels. Oxygen diffuses into the root through the lenticels, passes into the abundant aerenchyma, and moves to the rest of the plant.

Question 30

Growth in Saline Soil

Answer 30

Halophytes are plants that can tolerate soils with high salt concentrations​

Some produce high concentrations of organic molecules in their roots​

This alters the water potential enhancing water uptake from the soil.

Question 31

Phloem Transport

Answer 31

Most carbohydrates produced in leaves are distributed through phloem to rest of plant​

This process, called translocation, provides building blocks for actively growing regions of the plant

Question 32

Demonstrating Phloem Transport

Answer 32

Aphids feed on the nutrient-rich content of the phloem, which they extract through their piercing mouthparts called stylets. When an aphid is separated from its stylet and the cut stylet is left in the plant, the phloem fluid oozes out of it and can then be collected and analyzed.

Question 33

Phloem transports

Answer 33

hormones.​

mR N A.​

a variety of sugars.​

amino acids.​

organic acids.​

proteins.​

ions.

Question 34

Most widely accepted model describing the movement of carbohydrates in phloem

Answer 34

Pressure-Flow Hypothesis

Dissolved carbohydrates flow from a source to a sink.​

Sources include photosynthetic tissues.​

Sinks include growing root and stem tips as well as developing fruits.​

Food-storage tissue can be sources or sinks.

Question 35

Phloem-Loading

Answer 35

Phloem-loading occurs at the source​

Carbohydrates enter the sieve tubes in the smallest veins at the source.​

Sieve cells must be alive to use active transport to load sucrose.​

Water flows into sieve tubes by osmosis.​

Turgor pressure drives fluid throughout plant.​

At sink, sucrose actively removed and water follows by osmosis.​

Water may be recirculated in xylem or lost.