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TRANSPORT AND SUPPORT IN PLANTS

Plants have a transport system, in some ways similar to an animal's blood circulatory system. However, it is rather different in several important ways. For example, there is no pump like the heart, no circulating cells and liquids do not continuously move round and round.

The substances which are transported - mineral salts (ions) from the soil, and the products of photosynthesis from the leaves - are dissolved in water (as an "aqueous solution"). The transport system basically consists of 2 types of conducting tissue, each of which is made from cells which have been modified for their special purpose. Some cells die as a result of this modification, and they may also lose some of their internal components.

When you look at sections of plant material, you will see this tissue as a mass of cells, rather than easily defined tubes like blood vessels in animals!

Water and mineral salts enter a plant through special cells called root hair cells. The water is taken up by a special form of diffusion called osmosis , but the mineral salts (ions) may also be taken up by active transport which uses some of the plant's energy to concentrate them.

Xylem vessels carry water and minerals on into the plant, but only in an upward direction. These vessels form a continuous "pipe" from the root to the stem and leaves.
In the laboratory, this may be shown by the uptake of dyes by plant material such as celery "stalks".

Where do the water and minerals come from? (at least 2 possible answers)

> roots > soil

Xylem consists of fairly wide tubes with thick walls. These xylem vessel elements were originally cells which died due to the buildup of a tough substance called lignin in their cell walls, and then lost their end walls and cytoplasm.

Phloem tubes carry sugars (sucrose is more likely than glucose) and other "foods" such as amino acids produced as a result of photosynthesis.
The direction of this movement can be either upwards or downwards from the leaves.

Name 2 destinations for this "food".

> buds/leaves > roots/storage organs/flowers/fruits

Why cannot each of these make their own foods?

> (buds)-leaves furled >(roots)-no light under ground

no light for photosynthesisnot green - no chloroplasts

For what purposes (life processes) will plants use these substances?

> growth > reproduction

Phloem consists of cells called sieve tubes because their end walls have several small holes with strands of cytoplasm running vertically through them. Sieve tube cells have no nuclei, but they are not dead, and their activity is controlled by companion cells alongside.

THE DISTRIBUTION OF XYLEM AND PHLOEM IN PLANTS

These 2 conducting tissues are to be found in roots, stems and also in leaves. However, the pattern of their arrangement differs in each.

Root structure

When you were studying cell division, you were shown a vertical section of a root.

You should be able to label the various tissues in the transverse section of a root below. Note that the parts coloured yellow, and light brown, are also composed of cells, of different types.

Stem structure

The diagram of a stem similarly shows a transverse section of a stem. Here you see that the conducting tissue is grouped differently, into vascular bundles.

You should be able to label the various tissues in the transverse section of a stem below. Note that the part coloured pale yellow is also composed of "packing" cells.

TS Stem - no labels

Leaf structure

The diagram above shows the distribution of conducting tissues (again in vascular bundles), and green tissue in the centre of the leaf, which is in some ways like the packing tissue in the other parts of the plant.

Describe the wall of a xylem vessel.

> thick & strong

Describe the wall of a phloem tube.

> thin

What sort of tissue is in the centre of a root?
i.e. name it > xylem

What sort of tissue is in the centre of a stem?;

> packing tissue

What sort of tissue is in the centre of a leaf? (tricky!)

> mesophyll (palisade + spongy)

What is the function of cambium cells?

>grow into more xylem and phloem

What is the function of root hair cells?

> to take in water and mineral salts

What is the function of root cap cells?

> to protect growing tip

What is the name given to the process of development of specialised cells like these?

> differentiation

Support systems

Plants do not have skeletons like many animals. Instead, they use the turgidity of cells within packing tissue, and the strongly constructed conducting tissue, for support.

Small plants (herbs) rely on cells in packing tissues pressing against one another, to make most of the plant firm (turgid). This depends on a good supply of water which enters all the cells, and passes from one cell to the next.

What process is involved in water moving into the cells (and from one to the next)?

> osmosis

What happens if a plant does not get enough water? (before it dies!)

> it wilts / cells become flaccid

Osmotic pressure establishes a tension between cells which enables fairly flimsy structures such as the hollow flower stalks (scapes) of dandelions to stand upright. The contribution made by osmosis can be seen when sections are cut lengthways into 4 ("quadrant") sections and placed into either water or into sugar solutions - the resulting curvature being due to differential expansion or contraction of the inner and outer layers of the cells making up the normally tubular structure.

Larger plants, like trees and shrubs, use the strength of xylem to hold themselves up. However, as the walls become thicker, the cells die, and rings of new xylem vessels have to be produced each year as a tree grows.

Which cells divide to make more xylem?

> cambium

What is wood mainly composed of? (many possible answers!)

> cells / cell walls / cellulose + lignin / xylem

TRANSPIRATION

This is the process by which plants lose water. Mostly, it escapes from the spaces inside leaves in the form of water vapour, and passes out through the pores called stomata , on the underside of the leaves. Transpiration is therefore caused by ordinary evaporation , but with two important refinements: it is a potentially continuous process bringing up large amounts of water, and it can be turned off by closing the stomata. The flow of water up the plant is called the transpiration stream . Some of the water is needed as a raw material for the process of photosynthesis. It carries, in solution, mineral salts which the plant needs. Water also supports small plants (herbs) by keeping their cells turgid ( firm ). It may also be said that evaporation of water keeps leaves cool in hot weather.

Under what (weather/atmospheric) conditions does the transpiration stream flow fastest? > warm > dry > sunny > windy

Transpiration can be measured on the basis of the weight of water lost from a plant, or even from leaves removed from a plant.
Simple comparisons of the rate of transpiration of a shoot may be made using apparatus called a potometer.

How stomata work

A stoma is a single hole or pore, through which gases can enter or leave a plant leaf. [Stomata is the plural]. There are usually more stomata on the lower side of a leaf (and fewer on the top side).

It is possible to see (impressions of) stomata when nail varnish is applied to leaves, then peeled off when dry and examined under a microscope. Numbers of stomata on upper and lower surfaces can thus be compared.

Similarly, anhydrous cobalt chloride paper (which turns from blue to pink when exposed to moisture) can be used to compare water loss from upper and lower surfaces of leaves.

What surrounds each stoma? > 2 guard cells

What causes stomata to become open? > extra water attracted by osmosis into guard cells due to products of photosynthesis/K+ causing guard cells become curved, leaving gap between them

What causes stomata to close? > loss of water

List as many structures (cells, tissues) as you can that water passes through on its way up the plant. > root hair > root > xylem in root > xylem in stem > xylem in leaf > mesophyll

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