1. Transport in Plants

2. Warm up questions-Xylem or Phloem Which is nearest the centre of a root? Which type of vascular tissue has walls reinforced with lignin? Which transports sucrose from source to sink? Which transports sucrose from roots to leaves? Which is involved in the transpiration stream?

3. A whistle-stop tour of transport in plants The leaf Photosynthesis takes place in the chloroplasts which are mainly in the palisade mesophyll cells Glucose is made by photosynthesis and can be converted into sucrose and starch During the hours of daylight leaves are a source of carbohydrate

4. Transport of sucrose (translocation) Sucrose enters the companion cells of the phloem by active transport Companion cells use ATP to transport H+ out into surrounding cells This sets up a concentration gradient of H+ high concentration outside the companion cell, low concentration inside

5. Transport of sucrose (translocation) The H+ diffuse back into the companion cells through co-transport protein channels in the plasma membrane. Co transport means that the H+ transport sucrose molecules through as they enter the companion cells. This leads to a high concentration of sucrose in the companion cells

6. Transport of sucrose (translocation) The sucrose diffuses through plasmodesmata (small gaps in the wall between the companion cells and the phloem sieve tube) This lowers the water potential in the sieve tube so that water enters the sieve tube by osmosis. The hydrostatic pressure in the sieve tube rises so pushing the sucrose solution down (or up) to an area where sucrose is being used (the sink). The movement of the sucrose is called mass flow

7. Try these questions Which two molecules are needed by the leaf for photosynthesis? How does carbon dioxide get into the leaf? Does sucrose move up or down a concentration gradient when moving from the companion cells to the sieve tube vessel? Which molecule provides an immediate source of energy for active transport? Which organelles would you expect to be most abundant in the companion cells?

8. Transport of sucrose Evidence that sucrose is transported in the phloem Supply radioactive CO₂ to the tree, radioactive sucrose can be detected in the phloem soon afterwards Cut away a ring of bark, sucrose will accumulate above the ring causing a swelling around the trunk. Evidence that transport in phloem is an active process When plants are given a metabolic poison that inhibits respiration, transport of sucrose stops. Companion cells have many mitochondria

9. Back to the leaf Leaf cells need water Water enters plants through roots Water is lost from plants through leaves by transpiration. Transpiration involves evaporation followed by diffusion of water vapour down a water vapour potential gradient

10. Transpiration Most water vapour diffuses through stomata which are open during daylight hours to allow carbon dioxide to enter the leaf. Water moves from the xylem across the mesophyll cells by the symplast route (by osmosis from cell to cell) and by the apoplast route(along the bundles of cellulose fibres forming the cell wall).

11. Tension The diffusion of water out of the stomata allows more water to evaporate from the surface of the mesophyll cells. This draws water along the symplast and apoplast routes creating suction (tension) in the xylem

12. Cohesion -Tension The tension set up in the leaf draws water up the xylem vessels in the stem of the plant. Water molecules form hydrogen bonds causing cohesion. This produces an unbroken column up the xylem. A stronger rate of evaporation leads to more tension in the water column and a faster rate of water movement up the trunk

13. Measuring the rate of water uptake by plants The potometer measures the rate of water uptake by a cut shoot. This can be used as an estimate of the rate of transpiration. Why is the uptake of water only an estimate of transpiration rate?

14. Structural factors affecting the rate of transpiration Number and size of plant leaves-more and bigger leaves……………….. Thickness of waxy cuticle- thicker cuticle covering leaf…………………. Number, size and position of stomata - fewer, smaller stomata on the lower surface of the leaf………………………………….

15. Environmental factors affecting the rate of transpiration Temperature- a higher temperature increases the rate of transpiration because…………… Relative humidity- high humidity reduces the rate of transpiration because……………….. Wind- more air movement increases the rate of transpiration because……………………… The rate of transpiration is more rapid in bright light because……………………………

16. Xerophytic adaptations Plants need water to remain turgid- when there is a shortage of water in the soil plants will wilt and eventually die Xerophytes are adapted to survive in dry conditions Like this cactus they have leaves reduced to needles to reduce the surface area for transpiration

17. Xerophytic adaptations Marram grass has rolled leaves to trap water vapour in the air inside the roll. Hairs on the leaves also help prevent the water vapour from being blown away from the leaf surface. The water vapour potential gradient becomes less steep. The stomata are sunken into pits also helping to trap humid air next to the stomata. Thick waxy cuticle.

18. And finally- the uptake of water from soil by plant roots Root hairs increase the surface area for the absorption of water and dissolved mineral ions. Mineral ions move into root cells by active transport. This lowers the water potential so…. Water moves from the soil into the root cells by osmosis.

19. Movement of water across the plant root

20. The end