Plant Transport

Question 1

Draw an label the cross section of a stem, root and leaf

Question 2

What is the function of the xylem?

Answer 2

• Transports water and mineral ions up the plant (transpiration pull)
• Supports the plant, lignin is deposited onto cell walls, keeping the plant rigid
• Dead
• No organelles so a hollow tube

Question 3

Describe how lignin supports the xylem

Answer 3

• Strengthens the cell wall
• Waterproof
• Transpiration pull causes high pressure, so cells may not be able to withstand the pressure of water uptake

Question 4

Describe the holes in the xylem wall

Answer 4

• Non lignified pits
• Allow water and mineral ions to leave and move to surrounding cells if necessary

Question 5

What are the functions of the xylem parenchyma cells?

Answer 5

• Stores food
• Contains tannin - bitter chemical - defence against herbivores as it stops them from eating the plant due to the bitter taste

Question 6

What are some properties of the phloem?

Answer 6

• Transports assimilates (sugars up and down the plant eg sucrose)
• From the source (leaves/seeds/fruit) to the sink
• Sucrose instead of glucose as it is less reactive and not used up in respiration
• Alive so it can do active transport for translocation

Question 7

Describe the structure of the phloem

Answer 7

• Stack of cells where the cell walls are partially broken down
• Individual elements called the sieve tube element, cell wall called the sieve plates
• Companion cell keeps the phloem alive, with gaps called plasmodesmata. Companion cell also involved in translocation

Question 8

Describe the 4 stages of how water moves across a root

Answer 8

1+2 - Water moves across root cells in 2 pathway. Main pathway - apoplast pathway through the cell wall by tension. Also the symplast pathway through the cytoplasm by diffusion and plasmodesmata by diffusion
3 - Water moves across endodermis. Moves through apoplast pathway. Casparian strip embedded within cell wall, and it is made of waxy (so waterproof) suberin. Water therefore has to move through symplast pathway and is forced across the plasma membrane, which filters out toxins
4 - Water moves into the xylem. Mineral ions move in first by active transport to reduce the water potential of the xylem. Water can therefore naturally move into the xylem by osmosis. This generates root pressure

Question 9

What is the effect of root pressure?

Answer 9

• Pushes water up the xylem

Question 10

What is transpiration?

Answer 10

• A loss of water vapour by evaporation through stomata

Question 11

What are the good things and downsides to transpiration?

Answer 11

+ Cools plants down as water vapour absorbs heat and diffuses out of stomata
+ Causes transpiration stream which delivers water and mineral ions
- Means the plant is losing water, so losing turgor pressure and may wilt and die

Question 12

What is the transpiration stream?

Answer 12

• Pull of water and ions up the stem due to transpiration
• Occurs due to a lower water potential in the leaves as water leaves through transpiration

Question 13

Why does a plant wilt?

Answer 13

To reduce the surface area of leaves and stem exposed to the surrounding environment, hence reduces transpiration

Question 14

What 4 factors increase transpiration rate?

Answer 14

• Increased temperature due to increased kinetic energy
• Lower humidity due to a steeper water vapour concentration gradient
• Increased air movement as it reduces humidity
• Increased light intensity due to more stomata open

Question 15

Describe a potometer

Answer 15

• Measures water uptake by a plant
• Estimates transpiration rate
• Air bubble into capillary tube, which is attached to a reservoir which resets the air bubble position, and a fresh shoot
• Measure how far up the ruler the air bubble travels in different conditions

Question 16

Why is the potometer reading an estimate, and not 100% accurate?

Answer 16

Water can be used for photosynthesis or maintaining turgor pressure, not just transpiration

Question 17

How do you calculate volume of water uptake using a potometer?

Answer 17

πr^2 x d
where d is the ruler length measured

Question 18

What is translocation?

Answer 18

Transport of assimilates from source to sink in plants

Question 19

Describe phloem loading in translocation

Answer 19

• Phloem loading through apoplast pathway
• Hydrogen ions are actively pumped out of the companion cell using ATP for active transport
• This increases hydrogen ion concentration outside the companion cell so they can move back in down the conc gradient through co transporters
• Co transporters have to contain sucrose for this to occur, therefore sucrose concentration in the companion cell increases, so the sucrose diffuses through the plasmodesmata into the sieve tube
• This decreases the water potential, so water moves into sieve tube element by osmosis
• This generates a turgor pressure, so water can move up and mass flow takes place

Question 20

What is mass flow

Answer 20

Assimilates flow from source to sinks down pressure gradient

Question 21

Describe phloem unloading

Answer 21

• Passive process
• Sucrose diffuses from phloem into sinks
• Sucrose is then moves into other cells or converted into other forms to maintain concentration gradient
• Loss of assimilates increases water potential of sieve tube elements, so water moves out of sieve tube into cells by osmosis, or enters xylem’s transpiration stream