Transport in Plants

Question 1

Why do plants need a transport system?

Answer 1

To meet their metabolic demands, to efficiently move substances up and down and compensate for their relatively small Surface area : volume ration. Diffusion alone is not enough to supply all cells with nutrients

Question 2

Functions of xylem

Answer 2

1) Transports dissolved minerals and water around the plant.
2) Structural support
3) Food storage

Question 3

Function of xylems lignified cell walls

Answer 3

Adds strength to withstand hydrostatic pressure so vessels do not collapse, impermeable to water

Question 4

Function of xylem having no end plates

Answer 4

Allows the mass flow of water and dissolved solutes as cohesive and adhesive forces are not impended

Question 5

Function of xylem having no protoplasm

Answer 5

Does not impede the mass flow of water and dissolved solutes

Question 6

Function of pits in the wall of xylem

Answer 6

Allows continual flow in case of air bubbles forming in vessels

Question 7

Function of the small diameter of vessels

Answer 7

Helps prevent the water column from breaking and assists with capillary action

Question 8

Function of phloem

Answer 8

Transports organic compounds, from the source to the sink

Question 9

What is phloem made out of?

Answer 9

Sieve tube elements and companion cells

Question 10

Function of sieve plate with sieve pores

Answer 10

Allow the continuous movement of organic compounds

Question 11

Function of cellulose cell wall in sieve tube elements

Answer 11

Strengthens the wall to withstand the hydrostatic pressures that move the assimilates

Question 12

Function of sieve tube elements having no nucleus, vacuole or ribosomes in mature cells

Answer 12

Maximises the space for translocation of the assimilates

Question 13

Function of sieve tube elements having a thin cytoplasm

Answer 13

Reduces friction to facilitate the movement of assimilates

Question 14

Function of nucleus and other organelles in companion cells

Answer 14

Provides metabolic support to sieve tube elements and helps with loading and unloading of assimilates

Question 15

Function of transport proteins in plasma membrane of companion cells

Answer 15

Moves the assimilates into and out of sieve tube elements

Question 16

Function of large numbers of mitochondria in companion cells

Answer 16

To provide ATP for the active transport of assimilates into or out of cells

Question 17

Function of plasmodesmata in companion cells

Answer 17

The link to sieve tube elements which allows organic compounds to move into sieve tube elements

Question 18

Describe the apoplast pathway

Answer 18

It is the series of spaces running through the cellulose cell walls. The water moves by diffusion. It moves from cell wall to cell wall directly or intercellular spaces. When it reaches the endodermis, a waxy band of suberin blocks the apoplast pathway (Casparian strip) forms a barrier. The water must then take the symplastic pathway.

Question 19

Describe the symplast pathway

Answer 19

Water moves through the cytoplasm and plasmodesmata or vacuole of the cells. The water moves by osmosis into the cell

Question 20

Describe transpiration

Answer 20

Transpiration refers to the loss of water vapour from a plant to its environment by diffusion. Water evaporates from mesophyll cells to spaces in the leaves

Question 21

Describe transpiration pull

Answer 21

The water vapour lost by transpiration lowers the water potential in the air spaces surrounding mesophyll cells. The water within the mesophyll cells evaporates into the air spaces resulting in a transpiration pull. The pull from the water moving through mesophyll cells results in water leaving the xylem vessel through pits, which then cause the water to move up xylem vessels

Question 22

Cohesion and adhesion

Answer 22

The polar nature of water and the H-bonds that form between the water molecules result in cohesion between molecules and adhesion between the cellulose in the cell walls and water molecules. The cohesive forces results in a continuous column of water with high tensile strength and adhesive forces stop the water column from pulling away from the walls of the xylem

Question 23

Adaptations of xerophytic leaves

Answer 23

1) Reduced number of stomata: less water loss due to fewer pores.
2) Thick waxy cuticle on leaves: water loss reduced via cuticle
3) Leaves reduced to scales, spines or needles: Reduced transpiration due to reduced surface area

Question 24

What does phloem sap consist of?

Answer 24

Sugars, water, amino acids and hormones

Question 25

Why are carbohydrates mainly transported in the from of sucrose?

Answer 25

Less reactive than glucose no intermediate reactions occur and allows for increased energy storage

Question 26

Describe the process of loading assimilates

Answer 26

If sucrose molecules are taking the apoplastic pathway then modified companion cells pump hydrogen ions out of the cytoplasm via a proton pump into their cell walls. The large concentration of H+ in the cell wall results in the H+ moving down the concentration gradient back into the cytoplasm, thy move through a cotransporter protein. While transporting the H+ this protein also carries sucrose molecules into the cell. The sucrose then moves into the sieve tube elements via the plasmodesmata

Question 27

Describe mass flow on phloem

Answer 27

Tissue energy is required to create pressure differences for the mass flow of organic solvents. The pressure difference is generated by actively loading sucrose into the sieve elements at the source which lowers the water potential of sap. Water moves into sieve tube elements. The presence of water increases the hydrostatic pressure at the source and as solutes are unloaded from the sieve elements water follow and and a hydrostatic gradient occurs. The pressure difference between the source and sink results in the mass flow