Mass transport in plants

Question 1

What does xylem transport and what are some of its features?

Answer 1

Water and mineral ions

• Dead
• Lignified (strong, water-proof)
• No end walls
• Pits (water movement)

Question 2

How does water move from soil to xylem in 4 steps?

Answer 2

1. Water moves up xylem
2. Reduction of water potential
3. Water moves through cortex cells down water potential gradient
4. Water drawn into root hair cells down water potential gradient

Question 3

What is the apoplast pathway of water moving through roots?

Answer 3

1. Water molecules move through cell walls and can’t move through walls at the casparian strip
2. Water into cells across membrane –> transport control

Question 4

What is the symplast pathway of water moving through roots?

Answer 4

Water moves from cytoplasm to cytoplasm and moves between cells via plasmodesmata

Question 5

What is transpiration?

Answer 5

Water evaporation and diffusion from leaves and cells down a water potential gradient. Water replaced through roots

Question 6

What 4 factors affect transpiration and how?

Answer 6

1. Temperature - particles have more K.E which increases rate of evaporation
2. Humidity - higher humidity = higher water potential so rate of evaporation decreases because H2O will stay in cells
3. Wind - increases transpiration rate because H2O outside stomata blown away so water potential outside cells decreases
4. Light intensity - more photosynthesis so stomata will be open which increases SA so faster transpiration

Question 7

What are the 7 steps of cohesion tension theory?

Answer 7

1. Evaporation of water from cells in leaves
2. Reduced water potential in leaves which creates water potential gradient between roots and leaves
3. Cohesion between H2O molecules caused by hydrogen bonds
4. Adhesion between H2O molecules and sides of xylem vessels
5. Tension created within xylem
6. Column of water pulled up the xylem
7. Water replaced through roots

Question 8

What is assumed when measuring rate of transpiration?

Answer 8

That all water taken up is transpired when some of it could be used for photosynthesis

Question 9

What must be avoided when measuring rate of transpiration?

Answer 9

Air bubbles - plant must be cut underwater

Question 10

What is the structure of phloem?

Answer 10

Branched plasmodesmata provide nutrients for companion cells, companion cells provide nutrients for sieve tube elements

Question 11

Source definition

Answer 11

Where sugars are made

Question 12

Sink definition

Answer 12

Where sugars are used (converted for starch)

Question 13

What are the 7 steps for sucrose being transported in the phloem by mass transport?

Answer 13

1. Active transport of sucrose into phloem
2. Reduces water potential
3. Water moves in down water potential gradient by osmosis
4. Creates high hydrostatic pressure
5. Pressure gradient from source to sink (mass flow)
6. Sucrose removed at sink (storage or respiration)
7. Water moves out down water potential gradient by osmosis

Question 14

What are the arguments for translocation theory?

Answer 14

1. Ringing experiments
2. Aphids (feed directly from phloem)
3. Companion cells have many mitochondria (evidence for active transport)

Question 15

What are the arguments against translocation theory?

Answer 15

1. Sucrose travels to many different sinks
2. Not all solutes in phloem move at same rate
3. Role of sieve plates is unclear (provide a barrier to mass flow)