Textbook

Question 1

Why is water a more limiting resource for plants than animals?

Answer 1

Plants use far more water due to transpiration.

97% of water absorbed is lost through transpiration, 2% is kept for growth, and 1% is consumed in biochemical reactions.

Question 2

What inevitable consequence of terrestrial life has affected the evolution of plants?

Answer 2

Water loss to the atmosphere in a ratio of 400 water molecules lost for every carbon dioxide molecule gained.

Question 3

How does hydrogen bonding affect the properties of water?

Answer 3

1. VERY STRONG IMFs: can form up to 4 hydrogen bonds with adjacent water molecules or other electronegative atoms
2. UNIVERSAL SOLVENT: decreases interactions between solutes to increase solubility of ions and polar molecules
3. HIGH THERMAL PROPERTIES: high heat capacity and latent heat of vaporization ensure temperature fluctuations are buffered
4. CAPILLARITY: cohesion, adhesion, and surface tension generate physical forces that pull water through the vascular system
5. HIGH TENSILE STRENGTH: ability to withstand pressure allows water to move efficiently through the vascular system

Question 4

What is turgor pressure?

Question 5

What is transpiration?

Question 6

What is surface tension?

Question 7

What is the difference between cohesion and adhesion?

Answer 7

Though both are the result of hydrogen bonding, cohesion is the mutual affection between like molecules, while adhesion is the attraction of water to a solid surface (ex. cell wall).

Question 8

What is capillarity?

Question 9

What is cavitation?

Answer 9

The expansion of gas bubbles due to tension.

Question 10

What is diffusion?

Answer 10

Diffusion is the spontaneous movement of substances from higher concentrations to lower ones and is most efficient over short distances (ex. cell).

Question 11

What is osmosis?

Answer 11

Osmosis is the net movement of water across a selectively permeable membrane due to differences in solute concentration.

Question 12

What is free energy and how does it relate to water potential?

Answer 12

Free energy represents the potential for performing work. When input into a plant, free energy drives biochemical reactions, solute accumulation, and long-distance transport.

Water potential is a measure of the free energy of water per unit volume (equivalent to pressure, Pa).

Question 13

What are the three major factors that contribute to water potential in cells?

Answer 13

1. Solutes
2. Pressure
3. Gravity

Question 14

What is the effect of solute potential on water potential?

Answer 14

Increasing solute potential results in decreasing water potential, since the presence of solutes increases the entropy of the system. This effect is independent of the nature of the solute.

Question 15

What is the effect of hydrostatic pressure on water potential?

Answer 15

Positive pressure (turgor) increases water potential, while negative pressure decreases water potential (tension).

Question 16

How is the gravitational component of water potential calculated and when is it relevant?

Answer 16

For small plants and at the cellular level, gravity is negligible. When dealing with larger plants where gravity has an effect, each meter increases potential by 0.01 MPa.

Question 17

What happens when a cell with higher solute concentration is put in a solution of lower solute concentration?

Answer 17

The water potential of the solution is higher than that of the cell, so water will move into the cell. As the cell enlarges, the cell wall resists deformation and thus increases turgor pressure within the cell. The water potential of the cell will eventually reach equilibrium with the water potential of the solution.

Question 18

What is the typical water potential of plant cells?

Answer 18

Plant cells generally have water potentials of less than 0 MPa, which means that the free energy within the cell is less than it would be in standard conditions outside the cell. As the components of water potential change around the cell, water will enter or leave via osmosis.

Question 19

What happens when a cell with lower solute concentration is put in a solution of higher solute concentration?

Answer 19

The water potential of the cell is higher than that of the solution, so water will move out of the cell. As the cell shrinks, the pressure of the cell decreases until the water potential reaches equilibrium with the water potential of the solution.

Question 20

What happens when a cell is squeezed within a solution of equal water potential?

Answer 20

Squeezing the cell will increase pressure potential, which causes water to move out of the cell in order to balance the water potential with that of the solution. If half of the water is squeezed out of the cell, a new equilibrium will be reached, but with different components. The solute potential is even more negative than before, since water was removed while the solutes stayed within the cell.

Question 21

What determines the direction and rate of water flow?

Answer 21

The direction of water flow is determined by the direction of a water potential gradient, with the rate being proportional to its magnitude (therefore, the rate will slow down as the water potential gradient is lessened through proceeding diffusion). The rate is also dependent on hydraulic conductivity, which refers to how readily water can move across a membrane (increased conductivity is caused by large SA:V ratios and stiff cell walls).

Question 22

What is the role of aquaporins in water movement?

Answer 22

Aquaporins (integral membrane proteins that form water-selective channels through the membrane) facilitate faster water movement across membranes. They increase the rate of movement but have no impact on direction or cause of movement.

Question 23

What are the three components of water potential in soil?

Answer 23

1. Osmotic potential (typically negligible)
2. Hydrostatic pressure (increases with moisture)
3. Gravitational potential (proportional to elevation)

Question 24

How does capillarity relate to soil and water uptake?

Answer 24

Water tends to cling to the surfaces of soil particles due to adhesive forces that increase as moisture in the soil decreases. Water recedes into the interstices between soil particles, where surface tension creates concave menisci with negative pressure potentials. The curvature of each air-water interface deepens as more water leaves the soil, causing pressure potentials to become even more negative.

Question 25

What is bulk flow and why do plants use it?

Answer 25

Bulk flow is the concerted movement of molecules from regions of higher pressure to lower pressure.

Question 26

How does water move in soil?

Answer 26

Water moves predominantly by bulk flow from regions of higher water content to lower ones (more air-water interfaces). Some water also moves due to the diffusion of water vapour (particularly in dry soils).

Question 27

What two factors determine the rate of water flow in soil?

Answer 27

1. Size of the pressure gradient throughout the soil
2. Soil hydraulic conductivity (ease of movement due to soil composition)

Question 28

How does air impact soil hydraulic conductivity?

Answer 28

As water leaves soil, it is replaced by air. The presence of air in soil channels forces water to move only along the periphery of the channel, restricting water flow throughout the soil (therefore decreasing soil hydraulic conductivity).

Question 29

What is the purpose of root hairs?

Answer 29

Root hairs are filamentous outgrowths that provide greater surface area for the absorption of ions and water from soil. Water enters most readily near the root tip, since mature regions are less permeable to water due to the development of protective tissue.

Question 30

What are the three pathways of water movement through roots?

Answer 30

1. Apoplast
2. Symplast
3. Transmembrane