3.3.4.2 Mass transport in plants

Question 1

Explain the relation between stomatal opening and photosynthesis

Answer 1

• stomata allow uptake of carbon dioxide
• carbon dioxide required for photosynthesis

Question 2

How is xylem tissue adapted for its function?

Answer 2

• long cells with no end wall
• continuous water columns
• no cytoplasm and no organelles
• to obstruct flow for easier flow
• walls thickened with lignin
• to support xylem to withstanding tension and waterproof so keeps water inside cells
• pits and wells
• to allow lateral (sideways) movement of water around blocked vessels

Question 3

Why does xylem tissue contain hollow tubes?

Answer 3

Unrestricted water flow in a continuous column so it maintains the transpiration stream

Question 4

Why is lignin present in xylem cell walls?

Answer 4

• Provides support and strength
• prevents water loss
• maintains column of water due to adhesion

Question 5

Explain the cohesion tension theory

Answer 5

• Water evaporates into air spaces and diffuses out of stomata of a leaf
• it transpires from leaves
• this reduces water potential in leaf cells/mesophyll
• So water drawn out of xylem by osmosis
• this creates tension (negative pressure) on water in xylem
• So water pulled up as a continuous column
• because of cohesive forces, due to hydrogen bonding, between molecules
• adhesion of water molecules to walls of xylem

Question 6

Describe the ringing experiments for phloem

Answer 6

• ring of protective layer and phloem removed from stem (just xylem left)
• sucrose in phloem build up above the ring - swelling
• dead tissue below region (interrupted sucrose flow)
• if xylem was used in transpiration this wouldn’t have happened

Question 7

Describe the tracer experiment for phloem

Answer 7

• Radioactive isotope 14C to radioactively label 14CO2
• isotope 14C incorporated into sucrose
• So the sucrose can be identified and traced as they move through plant using autoradiography
• stem cross section placed on x ray film
• turns black if exposed to 14C
• only phloem turned black

Question 8

How does water travel across the leaf?

Answer 8

• Water moves through cells through air spaces and out through stomata
• by diffusion
• down a water potential gradient

Question 9

Describe and explain how water in the mesophyll cells passes out of the leaf

Answer 9

• Water leaving the mesophyll cells
• reduces the water potential in and across cells
• So water is drawn out of xylem by osmosis

Question 10

Describe the mass flow hypothesis

Answer 10

• souce cells (leaf) produce sugars/sucrose from photosynthesis
• in source sucrose is actively transported into the sieve tube element of phloem by companion cells
• this lowers water potential of sieve tube element so water enters by osmosis from xylem
• this increases hydrostatic pressure and causes a mass flow movement in the phloem (from source to sink)
• sucrose is actively transported out of phloem by companion cells into sink cells
• sucrose used for respiration or converted into starch for storage

Question 11

What evidence supports mass flow?

Answer 11

• when cut, sap is released meaning it is under pressure
• concentration of sucrose is higher in leaves than roots
• downwards flow only takes place in the light
• if sucrose concentration increases in leaves, a similar increase is involved in phloem shortly after
• a lack of oxygen or inhibiting metabolism stops translocation of sucrose
• companion cells have a high number of mitochondria

Question 12

What evidence doesn’t support mass flow?

Answer 12

• sieve plates seem to hinder mass flow
• solutes move at different speeds, this should be the same if they are the same flow
• sucrose delivered at the same speed everywhere - mass flow would suggest that areas needing more sucrose have a lower hydrostatic pressure and the flow there would be greater