transport in plants

Why do plants need a transport system?

• Metabolic demands : many parts of the plant do not photosynthesise so they need oxygen and glucose to be transported to them.

• Size : some plants can be very large so they need an effective transport system to move substances up and down.

• Surface area to volume ratio : the SA:V is too small for diffusion to be an effective form of delivery to cells

What are dicotyledonous plants (dicots)?

Plants that make seeds that contain to cotyledons.

How do dicots transport substances?

They have a vascular bundle which contain the xylem and phloem.

How can you observe xylem vessels in living plant stems?

1. Make a clean transverse cut across the stem with a sharp blade on a white tile.

2. Stain the plant.

3. Observe and draw the position of the xylem vessels which would show up as coloured spots.

What is the xylem?

The non-living vascular tissue which transports water and mineral ions up the plant from the roots to the shoots and leaves.

What is tannin?

a bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores.

What does lignin do?

Lignin provides structural support and mechanical strength.

What is the phloem?

The living vascular tissue that transports food in the form of organic solutes around the plants from the leaves where they are made.

The phloem supplies cell with the sugars and amino acids needed for cellular respiration and for the synthesis of other molecules.

What are sieve tube elements?

Living cells that form a tube to transport solutes.

Between the cells the walls become perforated to form sieve plates.

What are companion cells?

Cells that are linked to the sieve tube elements by many plasmodesmata which links the cytoplasm of adjacent cells.

They act almost as a life support system for the sieve tube cells.

What are plasmodesmata?

Microscopic channels through the cellulose cell walls.

Why is water key in plants?

• Turgor pressure provides a hydrostatic skeleton to support the stems and leaves.

• Turgor drives cell expansion.

• The loss of water via evaporation helps cool the plant down.

• Mineral ions and the products of photosynthesis are transported in aqueous solutions.

• Water is a raw material for photosynthesis.

What are root hair cells?

The exchange surface in plants where water is taken in from the soil.

They have root hairs which is a long thin extension from a root hair cells.

How are root hairs adapted?

• Microscopic so they can fit between soil particles.

• Large surface area to volume ratio.

• Thin surface layer so diffusion can take place quickly.

• Concentration of solutes in the cytoplasm of root hairs cells maintains a water potential gradient between the soil water and the cell.

Why does water move into a cell via osmosis?

Soil water has a very high water potential and the root hairs cells contain many different solvents including sugars, so the water potential is lower. This means there is a water potential gradient and water can move into the root hairs cells contain cells by osmosis.

What is the symplast pathway?

Water moves through the continuous cytoplasm of the plant cells connected by plasmodesmata by osmosis.

The root hair cell has a higher water potential than the next cell along, so the water moves into the next cell.

This process continues until the xylem is reached.

What is the apoplast pathway?

Water moves through the cell walls and intercellular spaces. Water fills the space between the loose and open network of cellulose.

As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to cohesion.

This creates a tension that ensures there is a continuous flow of water.

How does water move into the xylem?

Water moves through either the apoplast or symplast pathways until it reaches the endodermis.

The water potential of the xylem is much lower than the water potential of the endodermal cells connected so the rate of water moving into the xylem by osmosis increases.

What is the Casparian strip?

A band of waxy material called suberin that runs around each of the endodermal cells, forming a waterproof layer.

At this point water in the apoplast pathway is forced into the cytoplasm.

Why is the Casparian strip useful?

It forces water to go through a selectively permeable cell surface membrane which filters any potentially toxic solutes.

What does water do after it enters the vascular bundle?

Water returns to the apoplast pathway to enter the xylem and move up the plant.

What is root pressure?

The active pumping of minerals into the xylem to produce movement of water by osmosis.

It gives water a push up the xylem.

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

• Some poisons affect the mitochondria and prevent the production of ATP. If poison is applied to root cells there is no energy supply and the root pressure disappears.

• Root pressure increases with a rise in temperature, suggesting chemical reactions are involved.

• If levels of oxygen fall, root pressure falls.

What is transpiration?

The loss of water vapour from leaves and stems.

What is the transpiration stream?

The movement of water from the roots through the xylem and out of the leaves.

How does the transpiration stream work?

• Water molecules evaporate into the air spaces in the leaf and move out the stomata into the surroundings by diffusion.

• The loss of water lowers the water potential so water moves into the cell from an adjacent cell.

• This is repeated across the leaf to the xylem. Water moves out of the xylem by osmosis into the cells of the leaf.

Water molecules form hydrogen bonds which the carbohydrates in the walls of the xylem vessels (adhesion) and with each other (cohesion)

What is capillary action?

The process by which water can rise up a narrow tube against the force of gravity.

What is the transpiration pull?

Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation. This results in tension which helps to move water from the soil across the roots.

What is evidence for the cohesion-tension theory?

• When transpiration is at its height during the day, the tension in the xylem vessels is high too which means that the tree shrinks in diameter.

• When a xylem vessel is broken, air is drawn into the xylem rather than water leaking out.

• If air does move into the xylem then water can no longer move up the stem as a continuous stream because the cohesive forces have been broken.

How do you measure transpiration?

By using a potometer which measures the uptake of water of a plant.

How do you calculate the rate of water uptake?

distance moved by air bubble / time taken for air bubble to move that distance.

How do stomata open and close?

It is a turgor driven process. When turgor is low the asymmetric configuration of the guard cell walls closes the pore.

How does light intensity affect the rate of transpiration?

Increasing the light intensity will increase the rate of transpiration since more stomata will be open for gas exchange and photosynthesis to take place.

How does temperature affect the rate of transpiration?

Increasing the temperature increases the rate of transpiration because water molecules have more kinetic energy so evaporate quicker.

How does humidity affect the rate of transpiration?

Increasing the humidity lowers the rate of transpiration because there will be a lower water potential gradient.

How does air movement affect the rate of transpiration?

Increasing air movement increases the rate of transpiration because it maintains a steep water potential gradient.

How do plants transport organic compounds?

In the phloem from sources to sinks in a process called translocation.

What is translocation?

An active process that requires energy to take place and transports substances up or down the plant.

What are assimilates?

The products of photosynthesis that are transported.

What are the main sources of assimilates in plants?

• Green leaves and green stems

• Storage organs such as tubers and tap roots

• Food stores in seeds when they germinate

What are the main sinks in a plant?

• Roots that are growing/actively absorbing mineral ions

• Meristems that are actively dividing

• Any parts of the plant that are laying down food stores such as developing seeds

What are the two ways in which plants load assimilates into the phloem?

The symplast route and the apoplast route.

How does sucrose move through the symplast route?

In some species of plants the sucrose from the source moves through the cytoplasm of mesophyll cells and into the sieve tubes by diffusion through plasmodesmata.

— This process is passive.

How does sucrose move through the apoplast route?

In many plant species sucrose travels through the cell walls and intercellular spaces to the companion cells and sieve elements by diffusion.

The concentration gradient is maintained by the removal of sucrose into the phloem vessels.

In the companion cells sucrose is moved into the cytoplasm across the cell membrane in an active process.

Hydrogen ions are actively pumped out of the cell into surrounding tissue using ATP.

The H+ ions return to the companion cell down a concentration gradient via a co-transport protein which also co-transports sucrose.

How are companion cells adapted for active loading?

Many infoldings in their cell membranes to give an increased surface area, Many mitochondria to supply the ATP needed for the transport pumps.

Why does water move into the phloem?

As a result of the build up of sucrose in the companion cell and sieve tube element water moves in by osmosis.

This leads to a build up of turgor pressure and the water carrying the assimilates moves into the tubes of the sieve elements, reducing the pressure in the companion cells. The water moves up or down the plant by mass flow to areas of lower pressure. (sinks)

Why is pressure higher at the source?

Solute accumulation in the source leads to an increase in turgor pressure that forces sap to regions of lower pressure in the sinks.

The pressure differences in plants allow for solutes and water to be rapidly transported.

How is sucrose unloaded?

The main mechanism of phloem unloading is diffusion of the sucrose into the surrounding cells. Sucrose is rapidly converted into another substance in order to maintain the concentration gradient.

Why does water leave the phloem?

When sucrose is unloaded at the sinks there is a sudden rise in the water potential of the phloem which creates a water potential gradient and so water moves out into surrounding cells via osmosis.

— Some of the water is drawn into the transpiration stream in the xylem.

What is the evidence for translocation?

— Advances in microscopy allow us to see adaptations of companion cells.

— If the mitochondria of the companion cells are poisoned, translocation stops.

— The flow of sugars is 10 000 times faster than it would be by diffusion alone.

— Aphids can be used to show that there is a positive pressure in the phloem.

What are some problems with the translocation theory?

Not all solutes move at the same rate, solutes can move in both directions.

What are xerophytes?

Plants that are adapted to living in areas where water is in short supply.

What adaptations to terrestrial plants have?

Waxy cuticle, stomata on the underside of the leaf, long roots.

What are the adaptations of xerophytes?

— A thick waxy cuticle

— Sunken stomata

— Reduced number of stomata