Water Transport in Plants

Root pressure can best be described as (A) A high pressure potential driven by accumulation of ions and water by osmosis (B) The loss of water molecules from menisci, creating a large negative pressure (C) The adhesion of water molecules to the sides of xylem, creating a pull upward, and by cohesion with water molecules below (D) The movement of water from a region of low water potential to high water potential

A

Water flow in xylem is always unidirectional. (A) True (B) False (C) True, but water flow can easily reverse to supply the roots

A

Root pressure can best be described as (A) A high pressure potential driven by accumulation of ions and water by osmosis (B) The loss of water molecules from menisci, creating a large negative pressure (C) The adhesion of water molecules to the sides of xylem, creating a pull upward, and by cohesion with water molecules below (D) The movement of water from a region of low water potential to high water potential

A plants typically have a higher solute concentration (lower water potential) than the soil, causing water to move into the plant from the soil. during the day, stomata are open and plants are transpiring as they synthesize sugars from carbon dioxide. at night, in the absence of sunlight to drive photosynthesis, stomata close and transpiration stops. water continues moving into the roots at night because of the lower solute potential in the roots compared to the soil. as water continues to move in but is not transpired out, pressure builds up in the xylem eventually forcing water up the xylem. in short plants, this pressure is strong enough to push water out of the leaves in a process called guttation. root pressure is sufficient to move water up xylem at night and in short plants; however, it is not strong enough to move water up tall trees. movement of water up tall trees occurs as a result of the cohesion-tension model. cohesion is attraction among like molecules; in the case of water, cohesion is mediated by hydrogen bonding. surface tension is a force at the air-water interface that causes stronger hydrogen bonding among the water at the interface with the air, causing a concave surface boundary called a meniscus. this surface tension pulls against gravity due to the force of the interactions among the water molecules. when water evaporates out of the stomata in the leaves, it deepens the meniscus causing greater pull on the water below the meniscus (tension) and thus an upward force on all the water in the entire water column, which all interact with each other via hydrogen bonds (cohesion). thus transpiration is driven by evaporation in combination with cohesion and tension in the plant water column from leaf cell to root hair. adhesion (attraction of water to the cell walls of the xylem) combined with cohesion and surface tension together cause capillary action, or the movement of water up a narrow tube. capillary action alone is also not sufficient to move water the length of a tall tree.

Under which of the following conditions would the rate of transpiration INCREASE? (A)When the weather changes and air becomes drier (B) When the temperature of a leaf decreases (C) When stomata close at night (D) During a storm, when atmospheric pressure is low

A and D anything that increases evaporation of water from leaves increases transpiration. drier air will increase evaporation, and thus increase transpiration. in addition, low atmospheric pressure increases transpiration in much the same way that water moves from areas of high pressure to areas of low pressure. temperatures decrease in leaves as a RESULT of transpiration, not a cause of it (much like evaporation causes cooling for animals) transpiration occurs through stomata, and thus stomata must be open for transpiration to occur. closing stomata at night, or during a drought, decreases transpiration rates.

What drives the flow of water through the xylem? (A) active transport by tracheid and vessel elements (B) the evaporation of water from the leaves (C) passive transport by the endodermis (D) active transport by sieve-tube elements

B

Which of the following is NOT a key component of the cohesion-tension process in xylem? (A) Lignin in xylem cell walls (B) Open stomata (C) Use of ATP (D) Water between mesophyll cells

C

[Blank] bonds are responsible for the cohesion of water molecules (A) Polar covalent (B) Nonpolar covalent (C) Hydrogen (D) Ionic

C

Which of the following is NOT a key component of the cohesion-tension process in xylem? (A) Lignin in xylem cell walls (B) Open stomata (C) Use of ATP (D) Water between mesophyll cells

C cohesion is attraction among like molecules; in the case of water, cohesion is mediated by hydrogen bonding. surface tension is a force at the air-water interface that causes stronger hydrogen bonding among the water at the interface with the air, causing a concave surface boundary called a meniscus. this surface tension pulls against gravity due to the force of the interactions among the water molecules. adhesion is attraction among unlike molecules; in this case the attraction of water to the cell walls of the xylem. when water evaporates out of the stomata in the leaves, it deepens the meniscus causing greater pull on the water below the meniscus (tension) and thus an upward force on all the water in the entire water column, which all interact with each other via hydrogen bonds (cohesion). thus transpiration is driven by evaporation in combination with cohesion and tension in the plant water column from leaf cell to root hair. because the water column in a plant is continuous from root to leaf, the menesci (plural of meniscus) are present between the mesophyll cells, not actually within the xylem tubes. thus it is evaporation at these menisci inbetween the leafe mesophyll cells that drive movement via the cohesion-tension process. the cohesion-tension process puts strong negative pressure on the xylem, like sucking on a straw. lignin reinforces the walls of the xylem, which would otherwise collapse as a result of this negative pressure this cohesion-tension model does not require any input of energy on the part of the plant (energy that causes water to evaporate comes from the sun); there is no active transport to move water through xylem. this is because, in most cases, water potential in plants follows a gradient where it is highest in the soil and lowest in the leaves. specifically, it is lower in the mesophyll cells of the leaf than in xylem, and because of this gradient it is pulled from the soil into the roots.

Which gravity-defiant transport system is responsible for the height of trees?

Cohesion-tension

How does cohesion-tension work?

Combination of capillary action with transpiration. Transpiration is water leaving the leaves through the stomata. Since water is leaving, the pressure of the cell goes down (negative pressure). This creates tension that pulls the water upwards. Cohesion of water molecules helps fill in gaps.

What forces are responsible for capillarity? (A) high pressure potentials created by the entry of ions and water during the night, when transpiration rates are low, followed by an influx of water (B) cohesion of water molecules in a continuous flow from leaf to root (C) gravity and wall pressure (from the sides of xylem cells) (D) adhesion of water molecules to the sides of xylem cells, cohesion of water molecules to each other, and surface tension

D

Which of the following would tend to increase transpiration? (A) a thicker cuticle (B) sunken stomata (C) spiny leaves (D) higher stomatal density

D

Compared to plants from other environments, the cells of many desert plants contain high concentrations of solutes. This helps them survive in their arid surroundings because the high solute concentrations create relatively [blank], which help reduce water loss. (A) low pressure potentials (B) high pressure potentials (C) high solute potentials (D) low solute potentials

D (but I think B would also be correct?) Ψ (water potential) is the sum of ΨP (pressure potential) and ΨS (solute potential). water moves from areas of high water potential to areas of low water potential. water potential is related to two phenomena: pressure and solute concentration. A higher pressure means higher water potential; water moves from areas of high pressure (high water potential) to low pressure (low water potential). A higher concentration of solutes means lower water potential; water moves from areas of low solute concentration (high water potential) to high solute concentration (low water potential). desert soil is very dry and thus has very low water potential; this makes it difficulty for plants to absorb water from the soil because the soil may have lower water potential than the plant; this would cause water to move out of the plant and into the soil. as a result of selection in a dry environment, one adaptation in desert plants is very high solute concentrations in their tissues. the higher the solute concentration, the lower the solute potential, and thus the more likely water is to move into the plant from the soil: this low solute potential in the plant can drive the water potential of the plant lower than the water potential of the dry soil.

Under normal conditions, the _______ region of plants will always have the lowest water potential, whereas ____________ region will always have the highest. (A) root hair cells, mesophyll of leaves (B) root xylem, cork phloem (C) cork phloem, root xylem (D) mesophyll of leaves, root hair cells (E) pure water, air (F) air, pure water

D; water potential moves from high to low

How does root pressure work?

Root pressure system relies on positive pressure at the roots. The soil surrounding the roots has less solutes than the roots (which have a lower solute potential). The water from the soil will move into the roots which pressures higher pressure (positive). This high pressure at the bottom of the plant, pushes water up the xylem. Only works for a few meters

What are the three types of systems that move water against gravity?

Root pressure, capillary action, cohesion-tension

What happens to water potential when you add solutes?

The water potential decreases. Solute potential is negative, so the more solutes you have, the lower your water potential gets.

What happens to water potential when you increase pressure?

The water potential increases

What is the relationship between solute and pressure potential?

They work opposite each other. As one goes up, the other goes down. For example, if you add more solute to water, the solute potential "decreases" (becomes more negative), water is going to flow into the cell (water is going from high to low concentration) and so the overall pressure increases (pressure potential).

How does capillary action work?

This is the tendency of a liquid to move up against gravity when confined within a narrow tube. This is due to surface tension, adhesion(attraction between water molecules and xylem cells), and cohesion (attraction between the water molecules)

What is symplast?

shared cytoplasm, waters and minerals that can move through cells via plasmodesmada

What is an apoplast pathway?

water and minerals move through porous cell walls in plant cells

What is transmembrane pathway?

water moves through water channels present in the plant cell plasma membrane, from one cell to the next

What is the equation for water potential?

Ψ = Ψs + Ψp