B7

Question 1

What is water?

Answer 1

A polar molecule, unequal distribution of charge leads to strong intermolecular, cohesive forces between water molecules.

Question 2

What is chemical potential?

Answer 2

All chemicals have the capacity to do work as they have free energy. Chemicals will diffuse from areas of high chemical potential to areas of low chemical potential

Question 3

What is water potential?

Answer 3

The chemical potential of water, diffusion from regions of high water potential to low water potential. Pure water at atmospheric pressure has a water potential of zero. Water potential in a system can be increased and decreased in a number of ways. Anything that increases the potential of water to diffuse in a system, increases water potential. Water potential can be expressed in pressure units (Pa, mbar).

Question 4

What affects water potential?

Answer 4

Water potential is increased by heat, positive pressure, LSA, high conc gradient, smaller particles
Is decreased by dissolved solutes, adhesion

Question 5

What are the important components of water potential?

Answer 5

Osmotic potential - from solutes in vacuole
Pressure potential - from cell wall
Matric potential - from soil

Question 6

What are some important properties of plant cells?

Answer 6

They are bound by a selectively permeable cell membrane and surrounded by a cell wall. Contain dissolved solutes, cell membrane is permeable to water but largely impermeable to solutes. The cell wall is permeable to water and also largely permeable to solutes

Question 7

What is a plant cell vacuole bound by?

Answer 7

Selectively permeable tonoplast. Water potential of plant cells varies from 0 to <0

Question 8

What is osmosis?

Answer 8

A special case of diffusion. Water moves across a semi-permeable membrane from a region of high osmotic potential (low dissolved solute concentration) to a region of low osmotic potential (high dissolved solute concentration).

Question 9

Osmotic potential of plant cells

Answer 9

If there is no pressure potential or matric potential involved, the osmotic potential is the water potential of the cell. If this continues the cell will take up a large amount of water and burst.

Question 10

Turgor pressure in plant cells

Answer 10

Cells in water swell up and become turgid, cells in concentrated salt solution shrink and shrivel.

Question 11

Osmotic potential at 25 degrees c for a non-ionic solution?

Answer 11

0.01 mol L-1 water = -0.0248MPa
0.10 mol L-1 water = -0.248MPa
1.00 mol L-1 water = -2.48MPa
Seawater = -2.8MPa
For an ionic substance e.g. NaCl -> Na+ + Cl-
Effectively twice as many dissolved particles

Question 12

Define hypotonic?

Answer 12

A solution which contains a lower concentration of dissolved solutes I.e. higher water potential

Question 13

Define hypertonic?

Answer 13

A solution which contains a higher concentration of dissolved solutes I.e. lower water potential.

Question 14

Define isotonic?

Answer 14

The same concentration of dissolved solites

Question 15

What is pressure potential?

Answer 15

When plant cells are placed in pure water, they take up water until they become fully turgid. Plant cell walls have a high elastic modulus and they can develop high turgor pressures. When plant cells are fully turgid, the cell wall exerts a back pressure which cancels the osmotic potential. Thus osmotic + pressure potential = 0, thus in a fully turgid plant cell water potential = 0MPa

Question 16

What happens when a plant cell is in hypotonic solution?

Answer 16

Cell wall exerts a back pressure which cancels the osmotic potential

Question 17

What is plasmolysis?

Answer 17

If a turgid plant cell is placed in a hypertonic solution with low osmotic potential, water will move from the cell to the solution. When this happens, pressure potential decreases and the cell membrane eventually plasmolyses. At this point when pressure p becomes 0, water potential = 0.

Question 18

What happens to a plant cell in a hypertonic solution?

Answer 18

Hypotonic solution e.g. salt water with low osmotic potential, pressure p decreases to 0MPa and the cell plasmolyses. Water potential = negative MPa

Question 19

In plasmolysis what happens when the cells are surrounded by a hypertonic solution?

Answer 19

Water diffuses out

Question 20

How do you estimate osmotic potential of plant cells?

Answer 20

Plant tissue is placed in solutions of varying osmotic potential, degree of plasmolysis is recorded, solution in which plasmolysis is 50% is estimated from graph, this gives approximate osmotic potential of cells in tissues.

Question 21

How does water move in the whole plant?

Answer 21

Absorbed by the roots, moves across root to the xylem of the vascular cylinder. Ascend plant via xylem to the shoot and eventually, to the leaves. Lost from leaves to atmosphere via stomata, by process of transpiration. Water transport through the plant is one way. Transpiration stream also transports minerals in xylem.

Question 22

Water in the soil?

Answer 22

Water content and water movement in soil depends on the type of soil. Soil is a mixture of minerals and organic particles, water, solutes and air. In clay soils air is held tightly low matric potential by particles. When saturated with water, a soil is at field capacity after water is allowed to drain.

Question 23

What is the availability of water in soil spaces?

Answer 23

More readily available, at field capacity a clay soil will have matric potential between -0.01MPa and -0.1MPa. As water is depleted, water potential decreases.

Question 24

What do root hairs do and how?

Answer 24

Absorb water and nutrients from the soil, water absorbed through younger parts of root through the epidermis. Root hairs are extensions of epidermis, providing enormous surface area for absorption.

Question 25

What are the three passages of water across a root?

Answer 25

Apoplastic - via cell wall and intercellular spaces Symplastic - from protoplast to protoplast via plasmodesmata Transcellular - from cell to cell

Question 26

What is the endodermis and what does it consist of?

Answer 26

Layer of cells that separates the cortex of the root from the central stele. Endodermis cells have a band of Suberin called the casparian strip, on their radial walls. Water cannot follow an apoplastic route into the stele across the endodermis. So must go via a symplastic or transcellular route.

Question 27

What is the xylem and what does it consist of?

Answer 27

All vascular plants have tracheids, flowering plants have tracheids and vessel elements. Water moves from tracheid to tracheid via pits. Vessels have perforation plates at their ends, water can move from vessels to tracheids and vice versa.

Question 28

What three methods help water move upwards from the root to the top of the shoot?

Answer 28

Root pressure, capillarity, transpiration cohesion theory.

Question 29

What is root pressure?

Answer 29

Root pressure probably results from a hydrostatic pressure which builds up in the xylem of the root. Not all plants exhibit root pressure and not a strong enough force anyway - a pressure of 0.2MPa will raise water by 10m. Water will move up xylem to top of a shoot if root is cut off.

Question 30

What is capillarity?

Answer 30

Water will rise in a tube of narrow diameter as a result of adhesion. Capillarity continues until it is balanced by the force of gravity acting downwards. Capillary rise is inversely proportional to the radius of the tube. Tracheids and vessels are not narrow enough to raise water in big trees.

Question 31

What is transpiration?

Answer 31

The evaporation of water from moist cell walls in the leaf into the substomatal space. Diffusion of water vapour from the substomatal space, via stomatal pore, into the atmosphere. Transpiration through cuticle is usually very low - depends on thickness.

Question 32

Transpiration - what is cohesion theory?

Answer 32

As water evaporates from the wet cell walls in the leaf, a negative pressure develops which pulls water from adjacent cells. The tension is transmitted back to the vascular xylem and pulls water back up. This creates a negative pressure which is transmitted all the way to the root.

Question 33

What are embolisms and cavitation?

Answer 33

Embolisms are gas bubbles which can form when dissolved gases come out of solution in a water column under tension which results in cavitation which is the break in water column. Xylem vessels have a torus which seals off bordered pits. Embolism is contained. Surface tension helps to prevent gas bubble from passing through perforations in side or end walls of vessels. Water follows detoured roots, through pits, avoiding embolism. Gas bubbles cannot move through small pores of pit membranes, since vessels are interconnected, water flow is not stopped but moves through neighbouring vessels.

Question 34

What are stomata?

Answer 34

May be on both or one side of leaf, stomatal pore is regulated by guard cells on either side. Two types of stomata, elliptic and dumbbell. The potassium levels of open guard cells are very high. But very low in closed guard cells. Active import/export of K+ regulates water potential of guard cells. When K+ is high water diffuses in along water potential gradient. When K+ is low, guard cells lose turgidity and stomata close