3.2.2: Transport in plants

Question 1

Why do plants require a transport system?

Answer 1

1) Metabolic demands
2) Size: diffusion alone would take too long/distance substances would need to diffuse is too great
3) Low SA:V ratio: cannot rely on diffusion alone

Question 2

What does the phloem transport?

Answer 2

Sucrose dissolved in water (forms sap)

Question 3

What does the xylem transport?

Answer 3

Water and dissolved mineral ions

Question 4

Describe the structure of the xylem vessel

Answer 4

• Long, hollow columns of dead cells fused together end to end
• There are no cell contents to obstruct flow
• Walls of the vessel are thick and lignified
• Lignin and fibres prevent collapse under transpiration pull
• Bordered pits allow movement between vessels

Question 5

Why do the xylem vessels have bordered pits?

Answer 5

To allow the sideways movement of water between xylem vessels or into other plant cells.

Question 6

What is translocation?

Answer 6

The movement of assimilates in the phloem.

Question 7

Assimilates

Answer 7

Products of photosynthesis

Question 8

Source

Answer 8

Part of the plant that releases sucrose into the phloem.

Question 9

How does sucrose move into the phloem?

Answer 9

1) Companion cells use ATP to transport H+ ions into surrounding tissues (LOADING)
2) H+ ions diffuse back into companion cell down conc. gradient through co-transporter protein
3) Sucrose travels with H+ in co-transporter proteins into companion cells
4) Conc of sucrose in companion cells increases –> sucrose diffuses into phloem through plasmodesmata

Question 10

Transpiration

Answer 10

The loss of water vapour from a plant’s leaves and stem

Question 11

Transpiration stream

Answer 11

Movement of water through a plant until it is lost by evaporation from the leaves

Question 12

Describe the structure of the phloem

Answer 12

• Sieve tube elements –> lined up end to end, walls perforated by many pores so as not to obstruct sucrose mass flow
• Companion cells perform metabolic processes to keep sieve tube elements alive

Question 13

Why do plants need water?

Answer 13

• Photosynthesis
• Maintain cell turgor
• Medium to transport dissolved minerals
• Solvent for reactions
• Hydrolysis rections
• Coolant (evaporation)

Question 14

Root pressure theory

Answer 14

Active pumping of minerals into the xylem causes water to move in. This results in root pressure which “pushes” water part way up the xylem.

Question 15

Evidence for root pressure theory

Answer 15

• Cyanide –> no ATP –> no root pressure
• Increased temp. –> increased root pressure –> increased kinetic energy, ROR, ATP production
• When stem cut (and cohesion stopped) sap can exude from cut end

Question 16

Sucrose moves by

Answer 16

mass flow

Question 17

Sympoplast

Answer 17

Water moves through cytoplasm, passes into next cell via plasmodesmata. Changing water potential of cells; highest water potential = outside root cells

Question 18

Apoplast

Answer 18

Passing through cellulose cell wall

Reaches endodermis –> must enter cytoplasm to avoid being stopped by Casparian strip

Question 19

Factors affecting transpiration

Answer 19

• Light –> photosynthesis, stomata open
• Relative humidity –> relate to water vapour potential gradient
• Temperature –> molecules have more kinetic energy = higher rate evaporation//air can hold more water before saturation
• Air movement (water vapour potential gradient)
• Soil water availability: more water = higher rate of transpiration

Question 20

How are stomata opened

Answer 20

1) K+ ions pumped into guard cell (active transport)
2) Water moves in via osmosis
3) Turgor increases
4) Inner wall less flexible than outer –> becomes bean shaped, stomata open

Question 21

Adaptations of xerophytes

Answer 21

• Sunken stomata
• Needles/spines
• Hair on leaves

Question 22

Hydrophytes adaptations

Answer 22

• Stomata on upper surface of leave
• Air spaces in stem (low resistance internal pathway for moving substances)
• Small roots (decrease SA, reduce water uptake)
• Wide leaves w/ air spaces (capture light, easier for gas exchange)