Chapter 7 Transport in Plants

Question 1

Structure and function of xylem vessel elements

Answer 1

• Hollow/ no cell contents and no end walls: continuous water flow
• Wide lumen: large amounts of water can be transported
• Lignified cell walls: prevents collapse of vessels, impermeable to water
• Cellulose cell walls: allows adhesion of water molecules to xylem walls
• Pits: allows lateral movement of water and connect to all parts of plant
• Narrow diameter: for adhesion and prevents air locks

Question 2

Structure of phloem sieve tube elements

Answer 2

• Sieve plates
• Sieve pores
• Mitochondrion
• Cellulose cell wall
• Cell surface membrane
• Cytoplasm
• Endoplasmic reticulum

Question 3

Structure of companion cells

Answer 3

• Cellulose cell wall
• Cell surface membrane
• Cytoplasm
• Mitochondrion
• Rough endoplasmic reticulum
• Plasmodesmata
• Vacuole
• Tonoplast
• Nucleus

Question 4

Transport of water from the soil to the xylem through apoplast pathway

Answer 4

• Water moves through the cell walls and intercellular spaces, without crossing cell membranes
• The cell walls are made of cellulose, which is hydrophilic and allows water to pass freely
• This pathway offers least resistance and is the fastest route for water movement
• As water travels through the apoplast, it is eventually blocked at the endodermis by the Casparian strip

Question 5

Transport of water from the soil to the xylem through symplast pathway

Answer 5

At the endodermis, water in the apoplast is forced into the symplast because of the Casparian strip

• Water enters the cytoplasm of cells through partially permeable membranes by osmosis
• It then moves from cell to cell via plasmodesmata
• This movement is controlled and allows selective uptake of minerals

Question 6

What is transpiration?

Answer 6

• Loss of water vapour from a plant to its environment; mostly takes place through the stomata
• Passive process

Question 6

Process of transpiration

Answer 6

• Water vapour in air spaces diffuses out of the leaf through the stomata down the water potential gradient
• Water evaporates from cell walls of spongy mesophyll cells into the intercellular air spaces. This causes water to move from the cell’s cytoplasm into the cell wall
• Water from neighbouring xylem vessels, through pits, move into mesophyll cells by osmosis, down a water potential gradient
• A cohesion tension and transpiration pull is created at the top of the plant due to the evaporation of water, as the hydrostatic pressure at the top of the xylem is reduced. This causes water to move up the xylem from roots to soil

Question 7

Role of hydrogen bonding in xylem water movement

Answer 7

• COHESION: transpiration pull
• ADHESION: Water molecules also form hydrogen bonds with cellulose molecules in the cell walls of xylem vessels. This adhesion helps to resist the downward pull of gravity and supports vertical movement of water, especially in narrow vessels

Question 8

Explain how xerophytic plants are adapted to reduce water loss by transpiration

Answer 8

• Thick cuticle: increases distance of diffusion, acting as a barrier for transpiration. reflects heat, lowering temperature
• Rolled leaf: increases humidity around the stomata
• Sunken stomata: traps moist air in pits
• Hairs/trichomes on surface: trap moisture to reduce water potential gradient
• Small leaves, reduced to spikes: lessens surface area for transpiration

Question 9

What is assimilation?

Answer 9

Assimilation in plants is the range of processes by which the plant converts its inorganic nutrients into organic compounds

Assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes

Question 10

Explain how companion cells transfer assimilates to phloem sieve tubes

Answer 10

• Using active transport
• PROTON PUMPS: located in the plasma membrane of companion cells. Use ATP to actively pump H⁺ ions out of the companion cell into the cell wall (apoplast), creating a high concentration of H⁺ ions outside the cell. This establishes a proton gradient (electrochemical gradient)
• COTRANSPORTER PROTEINS: Located in the companion cell membrane and the plasmodesmata connections to sieve tube elements. Allow H⁺ ions to flow back into the companion cell down their concentration gradient. As H⁺ ions re-enter, they carry sucrose molecules with them using cotransport (symport). Sucrose then moves through plasmodesmata into the sieve tube element

Question 11

Explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

Answer 11

AT THE SOURCE:
- Sucrose is actively loaded into the phloem sieve tubes by companion cells using ATP and cotransport
- This increases the solute concentration in the sieve tubes
- Water enters the sieve tubes by osmosis from the nearby xylem, creating a region of high hydrostatic pressure

IN THE PHLOEM:
- The phloem sap (sucrose + amino acids dissolved in water) moves from high pressure at the source to low pressure at the sink
- This movement is called mass flow, as the entire fluid moves together along a pressure gradient

IN THE SINK:
- Sucrose is actively or passively unloaded from the phloem into sink cells
- This lowers the solute concentration in the sieve tube at the sink
- Water leaves the phloem by osmosis (often re-entering the xylem), reducing hydrostatic pressure at the sink end.