Transport In Plants 3.3

Question 1

What are plasmodesmata?

Answer 1

-gaps in the cell wall containing cytoplasm that connects two cells

Question 2

What is the apoplast pathway?

Answer 2

-water passes through the spaces in the cell walls and between the cells
-water moves by mass flow rather than by osmosis, also dissolved mineral ions and salts can be carried with the water
-moves due to the cohesive force of water , water molecules stick together forming a continuous stream of water
-this pathway transports water faster than symplast pathway

Question 3

What is symplast pathway?

Answer 3

-water enters the cell cytoplasm through the plasma membrane, it can then pass through the plasmodesmata from one cell to the next
-water moves by osmosis towards the xylem as each successive cell’s cytoplasm has a lower water potential

Question 4

Is the water potential in a plant cell negative or positive?

Answer 4

-water potential of plant cell is always negative as the cytoplasm will contain mineral ions and sugars (solutes)

Question 5

What happens once the cell has taken up sufficient water?

Answer 5

-once the cell is full of water it is described as turgid
-the water inside the cell starts to exert pressure on the cell wall called the pressure potential
-as the pressure potential builds up it reduces the influx of water

Question 6

What is a potometer?

Answer 6

-a device that can measure the rate of water uptake as a leafy stem transpires

Question 7

What is transpiration?

Answer 7

-the loss of water vapour from the aerial parts of a plant, mostly through the stomata in the leaves

Question 8

Describe the process of transpiration

Answer 8

1-water enters the leaf through the xylem and moves by osmosis into the cells of the spongy mesophyll. It may also pass along the cell wall via the apoplast pathway
2-water evaporates from the cell walls of the spongy mesophyll
3-water vapour moves by diffusion out of the leaf through the open stomata. This relies on a difference in the concentration of water vapour molecules in the leaf compared with outside the leaf. This is known as the water vapour potential gradient. There must be a less negative (higher) water vapour potential inside the leaf than outside

Question 9

What is transpiration a consequence of ?

Answer 9

-transpiration is a consequence of gaseous exchange

Question 10

Why is transpiration important?

Answer 10

As water vapour is lost from the plant it draws water up the stem as a transpiration stream. This movement:
-transports mineral ions up the plant
-maintains cell turgidity
-supplies water for growth, cell elongation and photosynthesis
-supplies water that, as it evaporates , can keep the plant cool on a hot day

Question 11

What are the environmental factors that affect transpiration rate?

Answer 11

-light intensity - in light the stomata open to allow gaseous exchange for photosynthesis. Higher light intensity increases the transpiration rate
-temperature - a higher temperature will increase the rate of transpiration e.g.it increases the rate of evaporation from the cell surfaces so that the water-vapour potential in the leaf rises
-relative humidity - higher relative humidity in the air will decrease the rate of water loss, because there will be a smaller water vapour potential gradient between air spaces in the leaf and the air outside
-air movement (wind)- air moving outside the leaf will carry away water vapour that has just diffused outside the leaf, this will maintain a high water vapour potential gradient
-water availability -if there is little water in the soil then plant cannot replace the water that is lost. If there is insufficient water then the stomata will close and the leaves wilt

Question 12

What is adhesion?

Answer 12

-the attraction between water molecules and the walls of the xylem vessel

Question 13

What is cohesion?

Answer 13

-the attraction between water molecules caused by hydrogen bonds

Question 14

What processes help water move up a plant?

Answer 14

-root pressure-the action of the endodermis moving minerals into the medulla and xylem by active transport draws water into the medulla by osmosis. Pressure in the root medulla builds up and forces water into the xylem ,pushing water up the xylem, root pressure can push water a few metres up the stem
-transpiration pull- loss of water by evaporation from the leaves must be replaced from water coming up the xylem. Water molecules are attracted to each other by forces of cohesion. These cohesion forces are strong enough to hold water molecules together in a long chain or column,the whole column is pulled up as one chain. The pull from above creates tension in the column of water (why xylem is strengthened by lignin to prevent vessel collapsing under tension) This is called cohesion-tension theory
Capillary action -water molecules are also attracted to the lignin in the xylem walls. This is called adhesion. As the xylem walls are very narrow these forces of attraction can pull the water up the sides of the vessel.

Question 15

What does cohesion-tension theory rely on?

Answer 15

-the plant maintaining an unbroken column of water all the way up the xylem
-if water column is broken in one xylem vessel, then the water column can still be maintained through another vessel via the bordered pits

Question 16

What are assimilates?

Answer 16

-substances made by the plant, using substances absorbed by the environment

Question 17

What is translocation?

Answer 17

-an energy- requiring process transporting assimilates ,especially sucrose, in the phloem between sources (e.g. leaves) and sinks (e.g roots, meristem)

Question 18

How does active loading occur during translocation?

Answer 18

Sucrose enters the sieve tube in the phloem in a process known as active loading where companion cells use ATP to transport hydrogen ions into the surrounding tissue, thus creating a diffusion gradient, which causes the H+ ions to diffuse back into the companion cells. It is a form of facilitated diffusion involving cotransporter proteins which allows the returning H+ ions to bring sucrose molecules into the companion cells, thus causing the concentration of sucrose in the companion cells to increase. As a result of that, the sucrose diffuses out of the companion cells down the concentration gradient into the sieve tube elements through plasmodesmata.

Question 19

How are assimilates removed at the sink?

Answer 19

As sucrose enters the sieve tube elements, the water potential inside the tube is reduced, therefore causing water to enter via osmosis, as a result increasing the hydrostatic pressure of the sieve tube. Therefore, water moves down the sieve tube from an area of higher pressure to an area of lower pressure. Eventually, sucrose is removed from the sieve tube elements by diffusion or active transport into the surrounding cells, thus increasing the water potential in the sieve tube. This in turn means that water leaves the sieve tube by osmosis, as a result reducing the pressure in the phloem at the sink.
-therefore the mass flow of water from the source to the sink down the hydrostatic pressure gradient is a means of supplying assimilates such as sucrose to where they are needed