G3 Transport in multicellular plants

AS Level and A Level Biology

Chapter 10

Pages 124-141

Learning outcomesCandidates should be able to:

• (a) explain the need for transport systems in multicellular plants and animals in terms of size and surface area to volume ratios;

• (b) define the term transpiration (see page 42) and explain that it is an inevitable consequence of gas exchange in plants;

• (c) [PA] describe how to investigate experimentally the factors that affect transpiration rate;

• (d) [PA] describe the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants;

• (e) [PA] describe the structure of xylem vessel elements, sieve tube elements and companion cells and be able to recognise these using the light microscope;

• (f) relate the structure of xylem vessel elements, sieve tube elements and companion cells to their functions;

• (g) explain the movement of water between plant cells, and between them and their environment, in terms of water potential ( no calculations involving water potential will be set);

• (h) describe the pathways and explain the mechanisms by which water is transported from soil to xylem and from roots to leaves;

• (i) outline the roles of nitrate ions and of magnesium ions in plants;

• (j) [PA] describe how the leaves of xerophytic plants are adapted to reduce water loss by transpiration;

• (k) explain translocation as an energy-requiring process transporting assimilates, especially sucrose, between the leaves (sources) and other parts of the plant (sinks);

• (l) explain the translocation of sucrose using the mass flow hypothesis;

Introduction• Requirements of plant cells:

– carbon dioxide for photosynthesis,– oxygen for respiration,– organic nutrients by those cells that do not photosynthesise,– inorganic ions and water.

• Because of the lower energy requirements of plants, compared to mammals, plants can manage with a slower transport system.

• Plants have two transport systems:– One for carrying water and inorganic ions from roots to parts above

ground,– One for carrying products of photosynthesis from leaves to other areas.

1. The transport of water

From soil to root hair

• Water moves into root hairs down a water potential gradient.

• The large number of very fine root hairs provides a large surface area in contact with soil water, increasing the rate at which water can be absorbed.

• Many plants, especially trees, have fungi located in or on their roots forming associations called mycorrhizas, which serve a similar function to root hairs. In return, the fungi receive nutrients from the plant.

From root hair to xylem

• Water can take two routes through the cortex:– apoplastic pathway,– aymplastic pathway.

• Once water reaches the endodermis, water can only move using the symplastic pathway because of suberin deposits in cell walls.

• The older the endodermal cells the more extensive the suberin deposits, except in certain cells called passage cells.

Xylem tissue

• In angiosperms (all flowering plants except conifers), xylem tissue contains:– vessel elements and tracheids which are

involved in water transport,– fibres which are dead, elongated cells with

lignified walls that help support the plant,– parenchyma cells which are standard plant

cells with unthickened cell walls and no chloroplasts.

Xylem vessels

Tracheids

• Tracheids, like vessel elements are dead cells with lignified walls but the do not have open ends.

• They have pits in their walls so that water can pass from one tracheid to the next..

• Tracheids are the main conducting tissue in relatively primitive plants, i.e. ferns and conifers. Angiosperms mainly rely on vessels for their water transport.

• Whereas in the root the xylem vessels are in the centre, in the stem they are nearer the outside.

From leaf to atmosphere - transpiration

• The loss of water from the leaves of a plant is called transpiration.

• It is affected by :– humidity levels,– wind speed,– temperature,– how many stomata are open/closed.

From xylem to leaf

• The removal of water from the top of xylem vessels reduces the hydrostatic pressure.

• This pressure is lower at the top of the xylem vessel than at the bottom, therefore the pressure difference causes water to move up the xylem vessel.

• The movement of water up through the xylem vessels is by mass flow, i.e. all water molecules move together.

• Mass flow occurs because of cohesion of water molecules with each other and adhesion to lignin in xylem cell walls.

• Pits in xylem vessel walls allow water to pass through to other vessels and also into surrounding living tissue.

• Root pressure can be increased by actively secreting solutes into the water in the xylem vessels in the root using active transport.

• However, root pressure is not essential for movement up the vessels. Water transport is largely a passive process fuelled by transpiration from the leaves.

Comparing rates of respiration

Xerophytes

2. Translocation• Translocation is the transport of soluble organic substances within

a plant.

• These substances are made by the plants themselves and are sometimes called assimilates.

• Assimilates are transported in sieve elements of phloem tissue.

• Phloem tissue is composed of:– sieve elements,– companion cells,– parenchyma,– fibres.

Sieve elements and companion cells

The contents of phloem sieve tubes

• The liquid inside phloem sieve tubes is called phloem sap or just sap.

• When phloem tissue is cut the sieve elements rapidly block the sieve pores in a process sometimes called ‘clotting’.

• The pores are first blocked by phloem protein and within hours by the carbohydrate callose.

How translocation occurs

• Phloem sap moves by mass flow just like water in xylem.

• However, phloem transport requires active transport to create the pressure differences needed for the mass flow.

• In leaves, sucrose is loaded into sieve elements and water moves in by osmosis.

• In roots, sucrose is removed from sieve elements and water also moves out by osmosis.

• This creates a pressure difference with high hydrostatic pressure in the sieve tube in the leaf and low pressure in the sieve tube in the root. Therefore water flows from the area of high pressure to an area of low pressure taking with it the solutes.

• Any area of a plant in which sucrose is loaded into the phloem is called a source.

• Any area where sucrose is taken out of the phloem is called a sink.

Loading of sucrose into phloem

Unloading of sucrose from phloem

• Little is known but it is possible that this happens by diffusion.

• Once in the tissue, the sucrose may be converted to another molecule to maintain the concentration gradient.

Evidence for the mechanism of phloem transport

• The evidence that phloem transport occurs by mass flow is considerable – the rate of transport is about 10 000 times faster than it would be if substances moved by diffusion.

• Circumstancial evidence exists for active loading of sucrose into sieve tubes:– phloem sap always has a relatively high pH of around

8,– there is a difference in electrical potential across the

plasma membrane of around -150mV inside,– ATP is present in large amounts in sieve elements.

3. Differences between sieve elements and xylem tissue