3.1.3 transport in plants

Question 1

Why is there a need for transport systems in multicellular plants?

Answer 1

Metabolic demands= many parts of the plant do not photosynthesis.
Size= some plants are very big and tall.
Surface area: volume ratio= relatively small SA:V ratio.

Question 2

How are vascular bundles arranged in the stem, root, and dicot leaf?

Answer 2

Stem= vascular bundles are around the edge to give strength and support.
Root= vascular bundles are in the middle of the plant to help withstand the tugging strains resulting from wind.
Dicot leaf= the midrib of a dicot leaf is the main vein carrying vascular tissue through the organ. Helps support the structure of the leaf.

Question 3

What is the structure and function of the xylem?

Answer 3

Xylem vessels are long hollow structures made of several columns of cells fused together end to end.
Made up of dead cells.
Transportation of water and mineral ions (roots to leaves).
Lignin spirals running around the lumen of the xylem (helps reinforce the xylem vessels so they do not collapse under the transpiration pull.

Question 4

What is the structure and function of the phloem?

Answer 4

Living tissue which transports food in the form of organic solutes (sugars and amino acids).
Up and down the plant.
Phloem sieve tube elements joined together, walls become perforated to form sieve plates.
Companion cells hold the organelle components that the phloem does not hold.

Question 5

Why is water key for the structure and metabolism of a plant?

Answer 5

Turgor pressure/hydrostatic pressure= provides a hydrostatic skeleton to support stem and leaves.
Turgor drives cell expansion= enables plant roots to force their way through concrete and tarmac.
The loss of water by evaporation helps keep plants cool.
Mineral ions and the products of photosynthesis are transported in aqueous solutions.
Water is a raw material for photosynthesis.

Question 6

How are root hair cells adapted for exchange surfaces?

Answer 6

Microscopic size= penetrate between soil particles.
Large SA:V ratio= thousands of hairs on each growing tip.
Thin surface layer= diffusion and osmosis can take place quickly.
Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between soil water and the cell.

Question 7

What is the symplast pathway?

Answer 7

Water moves through the symplast- continuous cytoplasm that is connected through the plasmodesmata by osmosis.
Root hair cell has a higher water potential than the next cell along.
As water leaves the root hair cell by osmosis, the water potential of the cytoplasm falls again, maintaining a steep water potential gradient.

Question 8

What is the apoplast pathway?

Answer 8

Water moves through the apoplast- the cell walls and the intercellular spaces.
As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to cohesion forces between water molecules.

Question 9

What is the casparian strip?

Answer 9

The casparian strip is band of waxy material (suberin) that runs around each of the endodermal cells forming a waterproof layer.

Question 10

How does water get transported when faced with the casparian strip?

Answer 10

Water is forced into the cytoplasm of the cell, joining the water in the symplast pathway.

Question 11

What is the evidence for the role of active transport in root pressure?

Answer 11

Some poisons affect the mitochondria and prevent the production of ATP.
Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions involved.
If levels of oxygen or respiratory substrates fall, root pressure falls.
Xylem sap may exude from the cut end of stems at certain times.

Question 12

What is the process of transpiration?

Answer 12

Transpiration is loss of water vapour from the leaves and stem of plants.
When the stomata are open to allow an exchange of carbon dioxide and oxygen between the air inside the leaf and external air, water vapour also moves out by diffusion and is lost.

Question 13

What is the transpiration stream?

Answer 13

The transpiration stream is the water vapour which moves into the external air through the stomata along a diffusion gradient.
It moves water up from the roots of a plant to the highest leaves.

Question 14

How does the transpiration stream work?

Answer 14

1. Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and out the stomata into surrounding air.
2. The loss of water by evaporation from the mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cell via osmosis.
3. This is repeated across the leaf to the xylem. Water moves out the xylem by osmosis into the cells of the leaf.
4. Water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels (adhesion). Water molecules form hydrogen bonds with each other and stick together (cohesion)-results in capillary action.
5. Water can be drawn up against gravity. Water is drawn up the xylem in a continuous stream to replace water lost (transpiration pull).
6. The transpiration pull results in a tension in the xylem, which helps move water across the roots from the soil.

Question 15

What is the evidence for the cohesion-tension theory?

Answer 15

Changes in the diameter of trees= when tension is lowest (night) the diameter of the tree increases.
When a xylem vessel is broken= air is drawn in to the xylem rather than water leaking out, the plant can not longer move water up the stem as the continuous stream of water molecules has been broken.

Question 16

How does the stomata control the rate of transpiration?

Answer 16

Turgor-driven process.
When turgor is low, the asymmetric configuration of the guard cell walls closes the pore.
When conditions are favourable, guard cells pump in solutes by active transport increasing their turgor.
Cellulose hoops prevent the cells from swelling in width, so they extend lengthways.
When water becomes scarce, hormonal signals from the roots can trigger turgor loss from guard cells, which closes stomata to conserve water.

Question 17

What are the factors affecting transpiration?

Answer 17

Light= more stomata open when there is a higher light intensity (photosynthesis).
Humidity= a relatively high humidity will lower the rate of transpiration.
Temperature= increase in temperature increases the kinetic energy of water molecules which increases rate of evaporation, and increases the concentration of water vapour that the external air can hold before it becomes saturated.
Air movement.
Soil-water availability.

Question 18

What practical measures transpiration?

Answer 18

Potometer.
All joints should be sealed with petroleum jelly to make sure any water loss is a result of transpiration only.
Fresh shoot cut under water and transferred to apparatus to avoid introducing air bubbles to the stem.

Question 19

What are the main sources of assimilates in a plant?

Answer 19

Green leaves and green stems.
Storage organs- tubers and tap roots.
Food stores in seeds when they germinate.

Question 20

What are the main sinks in a plant?

Answer 20

Roots that are growing and/or actively absorbing mineral ions.
Meristems that are actively dividing.
Any parts of the plant that are laying down food stores e.g. developing seeds, fruits, or storage organs.

Question 21

What is phloem loading?

Answer 21

Loading of soluble products of photosynthesis into the phloem from the sources by an active process.

Question 22

What is the apoplast route (translocation)?

Answer 22

Sucrose from the source travels through the cell walls and inter-cell spaces to the companion cells and sieve elements by diffusion down a concentration gradient.
In companion cells sucrose is moved into cytoplasm across the cell membrane in an active process. Hydrogen ions are actively pumped out of the companion cell into surrounding tissue using ATP-the hydrogen ions return to the companion cell down a concentration gradient via a co-transport protein.
As a result of the build up of sucrose in the companion cell and sieve tube element, water also moves in by osmosis.
The water carrying the assimilates moves into the tubes of sieve elements reducing the pressure in companion cells.

Question 23

What is phloem unloading?

Answer 23

Sucrose is unloaded from the phloem at any point into the cells that need it.
Main mechanism is diffusion of the sucrose from the phloem into surrounding cells.
Loss of solutes from the phloem leads to a rise in water potential of the phloem.

Question 24

What are xerophytes?

Answer 24

Plants in dry habitats that have evolved adaptations to live in low water availability conditions.

Question 25

What are hydrophytes?

Answer 25

Plants that live in water or on the surface of water.

Question 26

What ways do xerophytes conserve water?

Answer 26

A thick waxy cuticle= minimise water loss.
Sunken stomata= reduce air movement (produces a microclimate of still humid air).
Reduced numbers of stomata= reduce water loss by transpiration.
Reduced leaves= reduced SA: ratio e.g. thin needles for conifer leaves.
Hairy leaves= create a microclimate of still humid air.
Curled leaves= confines all stomata within a microenvironment of still humid air.
Succulents= specialised parenchyma tissue.
Leaf loss= loss leaves when water is not available.
Root adaptations= long tap roots which can penetrate several metres, or a mass of shallow roots with a large surface area able to absorb any water before a rain shower evaporates.

Question 27

What adaptations do hydrophytes have?

Answer 27

Very thin or no waxy cuticle= do not need to conserve water.
Many always-open stomata on upper surfaces= maximises gas exchange.
Reduced structure to the plant= water supports the leaves and flowers.
Wide, flat leaves= capture as much light as possible.
Small roots= water can diffuse directly into the stem and leaf tissue.
Large surface areas of stems and roots underwater= maximises area for photosynthesis and for oxygen to diffuse into submerged plants.
Air sacs= enable leaves/flowers to float to the surface of the water.
Aerenchyma= specialised parenchyma tissue forms in the leaves. It makes the leaves and stems more buoyant, and forms a low-resistance internal pathway for the movement of substances below the water.