10. Exchange in Plants

Question 1

What is parenchyma?

Answer 1

Packing tissue that fills spaces between other tissues.

Question 2

What is sclerenchyma?

Answer 2

Sclerenchyma cells have lignified walls and are used to strengthen stems and leaf midribs.

Question 3

What is collenchyma?

Answer 3

Collenchyma cells have thick cellulose walls and are used to strengthen the vascular bundles and outer parts of stems whilst allowing some flexibility

Question 4

What does the transport system in plants consist of?

Answer 4

Specialised vascular tissue
Water and soluble mineral ions travel upwards in xylem tissue
Assimilates, such as sugars travel up or down in phloem tissue

Question 5

How is the vascular bundle arranged in the root?

Answer 5

The vascular bundle is found in the centre of the root
There is a central core of xylem in an X shape.
The phloem is between the arms of the X-shape which gives the root tensile strength.
There is a sheet of cells surrounding the bundle called the endodermis.

Question 6

How is the vascular bundle arranged in the stem?

Answer 6

The vascular bundle is found on the outer edge which provides strength and flexibility to withstand the bending forces that the stems are exposed to.
The xylem is found on the inside of each bundle then a layer of cambium then phloem.

Question 7

What is cambium?

Answer 7

A layer of meristem cells that divide to produce xylem and phloem

Question 8

What is the structure of xylem?

Answer 8

Lignin impregnates cell walls killing them.
Lignin makes the cells waterproof and ensures they won’t collapse.
The end of walls decay leaving a long column of cells.
When lignification is not complete it creates boarded pits aligned into adjacent vessels to allow water to move in and out of the vessel.

Question 9

How is xylem adapted to its function?

Answer 9

The tubes are narrow so that the water column does not break easily
Boarded pits in the walls allow water to move side waves from one vessel to another
The flow of water is not impeded because there are no cross walls, there is no cell contents, and the lignin thickening prevents the walls from collapsing.

Question 10

What is the function of xylem?

Answer 10

Transports water and mineral ions from the roots to the rest of the plant.

Question 11

What is the function of the phloem?

Answer 11

Transports assimilates around (up and down) the plant

Question 12

What is the structure of phloem?

Answer 12

Elongated sieve tube elements are lined up to form sieve tubes
They contain a nucleus and very little cytoplasm, leaving space for mass flow.
The end of the sieve tube elements have perforated cross-walls called sieve plates.
Between the sieve tubes are small companion cells, with a large nucleus and dense cytoplasm.
Their many mitochondria produce the ATP needed to actively load assimilates into the sieve tubes.

Question 13

What are the 3 pathways water can take across plant cells?

Answer 13

The apoplast pathway
The symplast pathway
The vacuolar pathway

Question 14

What is the apoplast pathway?

Answer 14

Water passes through the spaces in the cell walls and between the cells.
It does not pass through any plasma membrane into the cells.
This means that the water moves by mass flow rather than by osmosis.

Question 15

What is the symplast pathway?

Answer 15

Water enters the cell cytoplasm through the plasma membrane.

It can pass through plasmodesmata from one cell to the next

Question 16

What is the vacuolar pathway?

Answer 16

Similar to the symplast pathway, but the water is not confined to the cytoplasm of the cells.
It also passes through the vacuoles

Question 17

How does water move from cell to cell in plants?

Answer 17

Water moves by osmosis from a region of higher water potential to a region of lower water potential. In a plant cell, the cytoplasm contains mineral ions and sugars that will reduce the water potential so water moves by osmosis into the cell. As water diffuses by osmosis into the cell it increases this water potential so it will then diffuse by osmosis into the next cell which has a lower water potential.

Question 18

How is water taken up into the root of the plant?

Answer 18

Because plant cells have low water potential, water molecules will move down the water potential gradient into the cell.
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.
This reduces the influx of water.

Question 19

How is water lost from the plant?

Answer 19

If plant cells have a higher water potential than the solution, water will diffuse by osmosis out of the plant.
This causes the cytoplasm and vacuole of the plant to shrink.
The plasma membrane will lose contact with the wall in a process called plasmolysis.
The tissue is now flaccid.

Question 20

What is transpiration?

Answer 20

The loss of water from the upper parts of the plant, particularly leaves

Question 21

How does water usually leave the leaves of plants?

Answer 21

Water enters the leaf from the xylem and moves by osmosis into the cells of the spongy mesophyll of along the cell walls via the apoplast pathway.
Water evaporates from the cell walls of the spongy mesophyll.
This lowers their water potential, causing water to enter them by osmosis from neighboring cells.
Water diffuses out of the leaf through the open stomata down the water vapour potential gradient.

Question 22

What is the importance of transpiration?

Answer 22

Transports useful mineral ions up the plant
Maintains cell turgidity
Supplies water for growth, cell elongation, and photosynthesis.
Water can keep the plant cool on a hot day

Question 23

What factors effect the rate of transpiration?

Answer 23

Light intensity, temperature, relative humidity, air movement, water availability in the soil

Question 24

How does light intensity affect the rate of transpiration?

Answer 24

In light, more energy is supplied to carry out photosynthesis, the stomata open to allow gaseous exchange for photosynthesis.
Higher light intensity increases the rate of transpiration.

Question 25

How does temperature effect the rate of transpiration?

Answer 25

A higher temperature will increase the rate of transpiration in 3 different ways…

1. Increase the rate of evaporation from the cell surfaces so the water vapour potential in the leaf rises.
2. Increases the rate of diffusion through the stomata because the water molecules have more kinetic energy.
3. Decreases the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf.

Question 26

How does relative humidity effect the rate of transpiration?

Answer 26

Higher relative humidity in the air will decrease the rate of water loss. This is because there is a smaller water vapour potential gradient between the air spaces in the leaf and the air outside.

Question 27

How does air movement effect the rate of transpiration?

Answer 27

Air moving along the outside of the leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a high water vapour potential gradient.

Question 28

How does water availability in the soil effect the rate of transpiration?

Answer 28

If there is little water in the soil, then the plant can not replace the water that is lost. If there is insufficient water in the soil, then the stomata close and the leaves wilt.

Question 29

What is the transpiration stream?

Answer 29

The movement of water from the soil, through the plant, to the air

Question 30

What does the cross section of the root in the plant consist of?

Answer 30

Outside- Root hair cells which take up water and minerals from the soil.
Cortex- A layer of cells which the water and minerals travel along.
Endodermis- Layer of cells which each have a casparian strip.
Inner core- Consists of vascular tissue (medulla, xylem and phloem)

Question 31

How are the cells in the roots adapted to their function?

Answer 31

Endodermis contains starch granules for energy.
Casparian strip blocks the apoplast pathway.
Ensures that water and mineral ions have to pass into the cytoplasm then into the medulla and xylem.
Transporter proteins in the plasma membrane actively pump mineral ions into the medulla and xylem.
Water follows by osmosis.
Once substances enter the medulla, they cannot go back.

Question 32

How does water move up the stem?

Answer 32

Water and mineral ions move up the xylem by mass flow.

Question 33

What 3 processes help water move up the stem?

Answer 33

Root pressure, transpiration pull, capillary action

Question 34

How does root pressure help water move up the stem?

Answer 34

Pressure in the root medulla builds up and forces water into the xylem and up the xylem

Question 35

How does transpiration pull help water movement up the stem?

Answer 35

Water molecules are attracted to each other by cohesion that holds the water in a long chain.
The tension created by water loss from the leaves pull water up the xylem.
This is called the cohesion-tension theory.

Question 36

How does capillary action help water movement up the stem?

Answer 36

The same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel.
This is called adhesion.
Because the xylem vessels are very narrow, these forces of attraction can pull the water up the sides of the vessel.

Question 37

What is translocation?

Answer 37

Occurs in the phloem, and is the movement of assimilates throughout the plant (both up and down).

Question 38

What is the source?

Answer 38

The part of the plant that loads materials into the transport system.
The leaves photosynthesise and the sugars made are moved to other parts of the plant

Question 39

What is the sink?

Answer 39

The part of the plant where materials are removed from the transport system.
e.g. when sucrose is stored as starch in the roots

Question 40

What is active loading?

Answer 40

The active process that loads sucrose into the sieve tube

Question 41

How does active loading take place?

Answer 41

Hydrogen ions are actively transported out of the companion cells to create a concentration gradient.
In a process known as cotransport, the hydrogen ions now accompanied by sucrose molecules re-enter the companion cells by cotransporter proteins.
As the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube.