Mass Transport in Plants

Question 1

What substances does Cohesion-Tension Theory (transpiration) transport

Answer 1

Water and inorganic ions

Question 2

What substances does Mass-Flow Hypothesis (translocation) transport?

Answer 2

Organic solutes

Question 3

Where is the root hair cell located?

Answer 3

Epidermis

Question 4

Advantage of root hair cell structure

Answer 4

Thinner cellulose walls so shorter diffusion distance

Question 5

How does water travel from the root hair to the xylem?

Answer 5

Travels across the cortex cells via the apoplast pathway

Question 6

What is the casparian strip?

Answer 6

A thick waxy waterproof band on the endodermis which blocks the apoplast pathway

Question 7

What are xylem pits?

Answer 7

Small holes in the xylem which allow water to move laterally between vessels

Question 8

Explain how water enters xylem from endodermis in the root and is then transported to the leaves (6)

Answer 8

• Casparian strip blocks apoplast pathway and only allows symplast pathway
• Ions and salts actively transported across cell membrane into xylem
• Down water potential gradient
• Transpiration from leaves creates cohesion between water molecules
• Which adhere to the xylem vessel walls
• Creating a continuous flow of water

Question 9

Root pressure moves water through the xylem. Describe what causes root pressure (4)

Answer 9

• Active transport by endodermis (symplast pathway)
• Of ions/SALTS into xylem
• Lowers water potential
• Water enters by osmosis

Question 10

Describe how a high pressure is produced in the leaves (3)

Answer 10

• Water potential becomes lower as sugar enters phloem.
• Water enters phloem by osmosis
• Increased volume of water causes increased pressure

Question 11

What to use to measure rate of transpiration?

Answer 11

Potometer

Question 12

Give 2 precautions the student should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot

Answer 12

• Cut shoot at a slant
• Cut shoot under water

Question 13

Xerophyte adaptations

Answer 13

• Reduced number of stomata
• Stomata in pits
• Leaves reduced to spines
• Rolled leaves
• Thin hairs to trap water
• Thick waxy cuticle

Question 14

What does phloem transport?

Answer 14

Sucrose and organic substances (eg: amino acids)

Question 15

Sucrose structure

Answer 15

• Disaccharide of glucose and fructose
• Glycosidic bonds (condensation reaction, loss of water)
• Carbon, Hydrogen, Oxygen

Question 16

2 cell types in phloem tissue

Answer 16

• Companion cells
• Sieve tube elements

Question 17

What are sieve tube elements?

Answer 17

LIVING cells that form the tube for transporting solutes. Have no nucleus and few organelles.

Question 18

What are companion cells?

Answer 18

Each siebe tube element has its own companion cell which carries out living functions for them.

Question 19

Companion cell features:

Answer 19

Contain many mitocondria to synthesise ATP through AEROBIC RESPIRATION for the active transport of solutes.

Question 20

How are sieve tubes connected?

Answer 20

Sieve plates

Question 21

What is translocation?

Answer 21

Movement of solutes to where they are needed in a plant (e.g: shoots, roots)

Question 22

What is an assimilate?

Answer 22

A solute that gets incorporated into the plant tissue

Question 23

What is source to sink?

Answer 23

Translocation moves solutes from source (where assimilates are produced in higher concentrations) to sink (where assimilates are used, so lower concentration)

Question 24

How is a concentration gradient maintained from source to sink?

Answer 24

Enzymes convert solutes at the sink to other storage substances (e.g: starch) so there is always a lower concentration at the sink than the source.

Question 25

What is Mass Flow Hypothesis?

Answer 25

Best supported theory of how solutes are transported from source to sink by translocation.

Question 26

Describe the mass flow hypothesis for the mechanism of translocation in plants (5)

Answer 26

• Companion cells
• Actively transport sucrose into phloem sieve tubes
• Lowering water potential in sieve cells + water enters via osmosis
• Increase in hydrostatic pressure causes mass movement towards sink end
• Sucrose used in respiration or stored as starch.