Mass Transport In Plants

Question 1

Explain the role of xylem in plants

Answer 1

It is the tissue that transports water in the stem and leaves

Question 2

Explain how xylem tissue is adapted for its function

Answer 2

Function: facilitates transpiration stream as: has dead, hollow vessels with no end walls that allow a continuous column of water, has lignin which provides strength/ support and withstands tension of water/high pressure. No cytoplasm or organelles to obstruct flow

Question 3

Describe how water is moved through a plant according to the cohesion tension theory

Answer 3

1. Water evaporates/ transpires from leaves 2. This reduces water potential in cell so water is drawn out from xylem, this creates tension 3. Water particles are polar and cohesive so water is pulled as a continuous column 4. Water also adhesive and attracted to xylem walls which results in tension as water is pulled up stem, pulling in walls

Question 4

What is the tissue that transports organic substances in plants

Answer 4

Phloem

Question 5

What is some evidence that supports cohesion tension theory

Answer 5

1. During day, transpiration is at its highest, as there is increased light intensity. Adhesion of water to walls results in tension which pulls them inwards and causes tree trunks to shrink in diameter but at night there is less tension so diameter increases 2. Of xylem broken and air enters, a tree can’t draw up water as column is broken 3. When xylem broken water doesn’t leak out but air is drawn in

Question 6

Describe the mass flow hypothesis

Answer 6

1. sucrose) produced by source are actively transported into sieve cells in phloem phloem by companion cells 2. This lowers water potential of sieve cell so water enters from xylem via osmosis which creates a high hydrostatic pressure . 3. Increase in pressure causes mass movement towards sink/root. 4. Sucrose used in root for respiration/storage (e.g sucrose/solutes removed from phloem) this raises W.p in phloem cells so water diffuses out by osmosis which decreases pressure

Question 7

Outline the use of tracers and ringing experiments to investigate transport in plants as evidence for mass flow hypothesis

Answer 7

Ringing experiments: cut phloem in photosynthesising plants, as result, there is a build up of sucrose above the ring cut and there is stem swelling but there’s an absence of sucrose below the ring
Plants exposed to radioactively labelled co2 show presence of radioactive organic compounds in phloem as shown by photographic film over transverse section of stem

Question 8

Explain why the transpiration rate is higher when it’s windy and when it’s warmer

Answer 8

When windy: it removes water vapour/ moisture saturated air which increases the water potential gradient so more evaporation occurs. When warm: increases the KE so water molecules move faster which increased evaporation