MODULE 3: EXCHANGE AND TRANSPORT - TRANSPORT IN PLANTS

Question 1

Why do multicellular plants need transport systems?

Answer 1

To get substances like water, minerals + sugars (to live)
Small SA:Volume ratio

Question 2

What is a vascular bundle?

Answer 2

The transport systems in plants, made up of the xylem and phloem

Question 3

What is the function of the xylem?

Answer 3

• Transports water and mineral ions up that plant
• Supports the plant

Question 4

How are xylem vessels adapted for transporting water and minerals?

Answer 4

• Xylem vessels are long, tube-like structures formed from cells (vessel elements) joined end-to-end
• No end walls on these cells, making an uninterrupted tube that allows water up the middle easily
• The cells are dead - no cytoplasm or organelles - hollow tube
• Walls thickened with woody substance (LIGNIN) which supports xylem vessels and stops inward collapsing (LIGNIN MAY BE IN A SPIRAL OR RINGS)
• Amount of lignin increases as cell gets older
• Water ions move into and out of vessels through small pits where there’s no lignin
• Xylem may have xylem parenchyma next to tube which stores food and contains tannin (bitter chemical, defence against herbivores)

Question 5

What is the function of the phloem?

Answer 5

• Transports assimilates (sugars) up + down (source (leaves/seeds) to sink (anywhere else)) the plant (sucrose)
  ^— sucrose- less reactive + not used up un respiration
• Alive (to do active transport for translocation)

Question 6

Describe the structure of the phloem.

Answer 6

• Sieve tube elements - living cells that form the tube for transporting solutes through the plant
  ^— joined end-to-end to form sieve tubes, no nucleus, very thin layer of cytoplasm
• Companion cells - keeps phloem alive since sieve tube elements don’t have a nucleus
  ^— companion cells carry out living functions for themselves and sieve cells (e.g. they provide energy for active transport of solutes)

Question 7

What is the name of the substance that thickens the walls of xylem vessels?

Answer 7

Lignin

Question 8

What is the function of companion cells?

Answer 8

• They keep the phloem alive since sieve tube elements don’t contain a nucleus
• They also provide energy for active transport

Question 9

How does water move from the roots up the plant?

Answer 9

• Water enters the root through the root hair cells via osmosis (travels down a water potential gradient) - soil around plant generally has high water potential and leaves have a lower water potential since water constantly evaporates
• Water moves across (root) cells in 2 pathways - Symplast pathway: goes through living parts of cell (cytoplasm) and through plasmodesmata (small channels in cell walls linking cells). Apoplast pathway: goes through non-living part of cells (cell walls). Water diffuses through walls and carries solutes + move from areas of high hydrostatic pressure to areas of low hydrostatic pressure
• When water in the apoplast pathway gets to endodermis cells in root, its path is blocked by waxy strip (Casparian strip), so has to take symplast pathway. Plasmodesmata filters out toxins
• Ions are moved into the xylem first by active transport (therefore endodermis no has higher water potential so moves down water potential gradient into xylem via osmosis). Root pressure generated —> pushes water up xylem

Question 10

In terms of water potential, why does water move into the roots from the soil?

Answer 10

Soil has a high water potential in comparison to plants, which have a lower water potential due to water in the leaves constantly evaporating. Therefore, water moves down the water potential gradient from the soil into the roots

Question 11

What is the Casparian strip?

Answer 11

A waxy strip embedded in the cell wall that means water travelling through the apoplast pathway must move into the cytoplasm and travel through the symplast pathway

Question 12

What is transpiration?

Answer 12

• Loss of water vapour by evaporation through stomata
• Cools the plant down
• Causes transpiration stream/pool (the movement of water and minerals up the xylem from the roots)

Question 13

What is the transpiration stream/pool?

Answer 13

the movement of water and minerals up the xylem from the roots

• When water from a cells is evaporated, other cells with higher water potentials will transport water via osmosis to the cell with a lower water potential. This will continue

Question 14

What is a disadvantage of transpiration?

Answer 14

• IT LOSES WATER
  ^— looses turgor pressure and eventually plant will wilt if it doesn’t get more water

Question 15

Describe the cycle of the stomata

Answer 15

• Stomata open, allowing carbon dioxide to enter
• Oxygen is lost
• Water is lost (diffusion)

Transpiration is a side effect of photosynthesis

Question 16

Describe the transpiration stream within the xylem.

Answer 16

Cohesion - Hydrogen bonds between the water molecules
- Adhesion - hydrogen bonds between water the the xylem wall

Between the two, they help lift the water up the xylem

Question 17

What are the factors affecting transpiration rate?

Answer 17

Temperature - Increase = faster transpiration rate
Humidity - Lower humidity = faster transpiration rate (water potential gradient increases because air is drier than plant)
Air movement - Windier = increase transpiration (blows away water molecules from around the stomata)
Light - Increase light = faster transpiration rate (stomata open when it gets light)

Question 18

What is a potometer?

Answer 18

Measures water uptake by plant

Question 19

What is translocation?

Answer 19

The transport of assimilates around the plant (form source to sink)

Question 20

What is the process of translocation? (1)

Answer 20

• Protons are pumped out of companion cell through proton pumps by active transport (increases proton conc. outside companion cell)
• Protons want to diffusion back into companion cell - gets co-transported with sucrose into companion cell through co-transporters
• Higher sucrose concentration in companion cell than sieve tube so sucrose diffuses across plasmodesmata into sieve tube down sucrose concentration gradient
• because substances are entering sieve tube, water potential in sieve tube is decreasing. So water in companion cell moves into sieve tube by osmosis down water potential gradient
• generates turgor pressure for mass flow

Question 21

What is the process of translocation? (2)

Answer 21

• The source has a high pressure and the sink has a low pressure. Assimilates flow from source to sink
• Phloem loading is a passive process
• Due to sugar concentration gradient, sucrose moves from sieve tube to cell (sink, e.g. root)
• Due to loss of assimilates, water potential increases and will move from an area of high water potential to an area of low water potential (either sink or adjacent xylem for transpiration stream)

Question 22

What is turgor pressure?

Answer 22

The force within the cell that pushes the plasma membrane against the cell wall

A.k.a hydrostatic pressure