Transport In Plants 8

Question 1

Transpiration

Answer 1

Evaporation of water from a plants surface through the stomata when they open to allow the entry of carbon dioxide for photosynthesis

Question 2

Environmental factors affecting transpiration

Answer 2

• light (more light more transpiration)
• temperature (high temp more kinetic energy more transpiration)
• humidity (increase in humidity decrease in transpiration, water potential gradient decreases)
• air movement (more air movement more transpiration)

Question 3

What adaptations do xerophytes possess that limit water loss?

Answer 3

Thick cuticle - long diffusion pathway
Hairs - trap air, becomes saturated with water vapour reducing WP
Rolling of leaves - traps air reducing WP
Reduced SA to volume (PINE NEEDLES) - reduces SA for water loss
Epidermal pits/grooves - reduce exposure to air currents, air trapped becomes saturated reducing water potential

Question 4

Rate of transpiration

Answer 4

Pi r2 d
D = distance bubble moves
R = lumen of capillary tubing

Question 5

Cohesion tension theory

Answer 5

1. Solar heat energy causes transpiration of water from leaves & water evaporates from mesophyll cells and diffuses out thorough stomata
2. Water potential is more negative compared to inner mesophyll cells so water moves from inner to outer mesophyll by osmosis along WP gradient
3. Water potential gradient extends across mesophyll cells to xylem vessels. Water is drawn from xylem creating a negative pressure, pulling up the water
4. Water column maintained by cohesive and adhesive forces (cohesive is the attraction of water molecules to each other due to hydrogen bonding) (adhesive is the attraction of water molecules to xylem walls)
5. Upward movement of water from xylem maintains the water potential gradient providing the mechanism for water uptake via Osmosis

Question 6

How does the structure of the xylem aid the movement of water?

Answer 6

Xylem tissue is dead, no cell contents leaving hollow tubes so there is minimal resistance
Cell wall strengthened by lignin making cell walls more rigid and provides support

Question 7

Evidence of movement of ions in xylem

Answer 7

Radioactive isotopes are used as tracers to show that the transport of mineral ions occurs mainly in the xylem
Xylem and phloem are separated using a wax cylinder to prevent lateral transport
Roots are supplied with radioactive potassium ions
Amount of radioactivity is greater In xylem indicating that transport of potassium occurs in this tissue

Question 8

Transport in phloem

Answer 8

Transport of photosynthetic products is known as translocation and occurs in phloem
Carbohydrates transported as sucrose, proteins as amino acids and lipids as fatty acid and glycerol
Molecules produced in leaf (SOURCE) then transported To the SINKS eg respiring cells, growing areas and storage areas

Question 9

What does the phloem consist of?

Answer 9

Sieve elements are joined end to end to form sieve tubes
End walls of sieve elements are sieve plates and possess pores
Next to each sieve element is a companion cell with dense cytoplasm and many mitochondria

Question 10

Mass flow hypothesis

Answer 10

1. Product is actively transported to sieve tubes in phloem from leaf cells by transfer cells
2. WP of phloem becomes more negative so water moves into phloem by osmosis from surrounding cells, creating high hydrostatic pressure
3. In SINK ions are being used up by respiration, making WP less negative, meaning water exits sink creating a low hydrostatic pressure
4. Product can be transported from the source to the sink along this pressure gradient

Question 11

Ringing experiments

Answer 11

Provided evidence that translocation occurs in phloem
Involves removing a complete ring of phloem around part of plant preventing transport through phloem
A slight swelling develops above ring
Swelling is linked to build up of photosynthetic products from leaves which are prevented from being transported

Question 12

Using radioactive isotopes as tracers to develop ringing

Answer 12

Detected using autoradiography

Place each plant between X-ray film. Film is exposed by the radioactively labelled compounds showing their location

Question 13

Evidence supporting mass flow hypothesis

Answer 13

Cutting stem results in sap being released indicating hydrostatic pressure in sieve tubes
Lowering temp or use of respiratory inhibitors reduces rate of translocation indicating active transport is involved

Question 14

Evidence against mass flow hypothesis

Answer 14

Structure of sieve tubes and sieve plates is not required

Movement of substances in opposing directions in same sieve tube has been observed