Exploring Biology, Chemistry & Physics Grade 9

Derek McMonagle

Series editor: Marlene Grey-Tomlinson

24 Substances transported in plants

Plants obtain the substances they need to flourish in a completely different way to animals.

We are learning how to:

• identify substances transported in plants

• Animals eat complex chemicals that are broken down to provide those substances

• Plants absorb and use simple chemicals to create those substances themselves

Consequently substances are transported in different ways.

Transport of gases

You learned in the previous unit that:

• all organisms carry out respiration to obtain energy; this process requires oxygen and produces carbon dioxide

• green plants carry out photosynthesis to make food; this process requires carbon dioxide and produces oxygen

ProcessGas neededGas produced respirationoxygencarbon dioxide photosynthesiscarbon dioxideoxygen

TABLE B 3.24.1

Plants have no organ corresponding to the lungs or gills of an animal. They rely on diffusion for transport of gases. Oxygen and carbon dioxide pass into and out of the plant through tiny pores. Many of these pores, or stomata, are located on the underside of leaves.

This is not an active method of transport so gases take much longer to move through a plant than through an animal.

FIG B 3.24.1 Section through a leaf

Transport of substances in solution

Plants cannot ingest solids in the same way that animals can, so all of the chemicals they need must be absorbed in solution from the soil.

Soil water containing dissolved minerals is absorbed into root hair cells by osmosis. The solution passes from cell to cell in the root by the same process until it reaches the stem.

Inside a plant stem there are two types of vessels called the xylem and the phloem. You will learn more about these vessels later in this unit.

• The solution of minerals passes up from the roots to the rest of the plant through the xylem.

• Solutions of substance pass from one part of a plant to another through the phloem.

Plants have no organ corresponding to the heart of an animal. Solutions of substances are not pumped around a plant in the same way as blood circulates in an animal and consequently transport takes much longer.

Check your understanding

1. a) How are gases transported in and out of plants?

b) Why does the movement of gases in plants take much longer than it does in animals?

2. a) How are nutrients transported from the soil through the roots of a plant?

b) Through which vessels are substances transported in a plant?

Key terms

diffusion net movement of particles spreading from an area where they are in high concentration to one where they are in low concentration

osmosis net movement of water from an area of high concentration to one of low concentration through a partially permeable membrane

root

FIG B 3.24.2 Movement of water from soil into plant root and stem

25 Plant roots

We are learning how to:

• identify different types of plant roots

• describe the functions of a plant root

The roots are the part of the plant that is in the soil. They absorb water containing dissolved minerals from the soil.

water film covering soil particles and roots, also between particles

A plant, such as a pepper plant, might have a fibrous root that divides into thinner and thinner roots, or it may be more like a carrot and have a tap root, which is a large swollen root with smaller roots growing from it.

Plant roots are ideally suited to absorbing water from the soil. They divide many times so that there is a large surface area available for absorption.

Plant roots consist of plant root cells. Each cell has a root hair that grows out into the gaps between soil particles, increasing the surface area of the roots even more. Water containing minerals is absorbed through the root hairs.

water

root hair

root hair cell

soil particle

FIG B 3.25.1 Fibrous roots (left) and tap roots (right)

FIG B 3.25.3 Plant root cells

FIG B 3.25.2 Plant roots divide many times

Activity B 3.25.1

Examining roots

Here is what you need:

• recently germinated pea or bean plant

• hand lens

• legume plant

Here is what you should do:

1. Carefully remove a plant from a pot so that soil remains around the roots.

2. Look carefully at the roots. Notice how they are spread out to provide the plant with a firm anchor, and also to absorb water and nutrients from different parts of the soil. Notice how the roots divide a number of times to finally produce very thin root hairs.

3. Knock away some of the soil and examine the area immediately around the roots. Can you see any root hairs?

4. Make some drawings of the roots showing their different features.

5. [STEAM] The roots of a group of plants called legumes are associated with fixing or converting atmospheric nitrogen into nitrogenous compounds in the soil. Obtain a legume plant and draw its roots showing how they differ from other plant roots.

Check your understanding

1. Fig B 3.25.4 shows two different types of root system.

Key terms

fibrous root root that divides many times, producing a network of tiny roots

tap root systemfibrous root system

a) Which root stores more nutrients?

b) In what ways are the root systems designed to gather water and nutrients efficiently?

tap root large thick root swollen with food reserves root hair finger-like projection on a root cell, which increases its surface area for absorption

FIG B 3.25.4 Different types of root system

26 Xylem and phloem

We are learning how to:

• identify the xylem and the phloem in a plant stem

You have already seen how water containing dissolved minerals is absorbed by the roots and passes to other parts of the plant. However, there is also a flow of other substances in the plant.

The leaves use energy from sunlight to make food in the form of glucose. The glucose is then distributed to the other parts of the plant either to provide the plant’s cells with energy, or to be stored as starch.

The plant therefore needs two vessels to transport dissolved substances:

• The xylem transports water and water-soluble minerals from the soil to different parts of the plant

• The phloem transports glucose, proteins and other organic chemicals within the plant

In the root of a plant the xylem and phloem together form a vascular cylinder.

Inside the vascular cylinder the xylem forms the centre, which is surrounded by the phloem.

A cross section of a plant stem shows that the vascular cylinder forms a series of vascular bundles. These are arranged symmetrically around the stem but the pattern is different in different plants. Each vascular bundle consists of xylem cells towards the centre of the stem and phloem cells towards the outside. These cells are separated by vascular cambium cells.

The flow through the xylem vessels is upwards from the roots towards other parts of the plant. The flow

bundle

FIG B 3.26.3 Vascular bundle in a plant stem

FIG B 3.26.2 Section through vascular cylinder in the plant root

root hair phloem xylem

FIG B 3.26.1 Vascular cylinder in the plant root

vascular cylinder phloem xylem xylemphloem

FIG B 3.26.4 Direction of flow in a vascular bundle

through the phloem may be in either direction as substances are carried between the different parts of the plant.

Activity B 3.26.1

Arrangements of vascular bundles

Here is what you need:

• poster-sized sheet of paper

• different coloured card × 2

• scissors

• glue

Here is what you should do:

1. Cut 20 discs of two different colours and sizes from coloured card. 10 will represent the xylem and 10 the phloem.

2. Arrange the discs to show how the xylem and phloem are arranged in the root of a plant and in the stem of a plant.

3. When you are satisfied with your arrangement, glue your discs in place so your poster can form part of a classroom display.

Check your understanding

1. Fig B 3.26.5 shows a section across a plant stem.

Key terms

xylem vessels carrying solution of minerals from the soil to different parts of the plant

phloem vessels carrying solution of nutrients and other chemicals around the plant

a) Name the parts labelled A, B and C.

b) What is carried in: i) A ii) B?

c) In which direction is the flow through: i) A ii) B?

vascular bundle structure containing xylem and phloem vessels

vascular cambium cells between the xylem and phloem in a vascular bundle

FIG B 3.26.5 A section across a plant stem

27 Movement of substances through the xylem and phloem

We are learning how to:

• describe the movement of substances along the phloem and the xylem of a plant

Both the xylem and the phloem appear to be long thin tubes, like narrow drinking straws, but their structures are very different.

Structure of the phloem

Phloem tissue consists of two main types of cells; sieve elements and companion cells, together with other types of cells that make up the structure.

Sieve elements are long narrow cells that join together to form a sieve tube. At the ends of each sieve element there is a sieve plate. Sieve plates are porous and therefore allow substances to flow between cells along the sieve tube.

Sieve elements have no nuclei but they do have thick rigid cell walls made of cellulose. These are needed to withstand the hydrostatic pressures that bring about the flow through the phloem.

Sieve elements would not be able to function without companion cells. The companion cells make the movement of materials through the sieve plates possible.

Substances can move in both directions through the sieve tubes. Movement is the result of hydrostatic pressure from the xylem.

Structure of the xylem

The xylem consists of stacks of dead cells, which are joined together to form tubules.

water and minerals

no end walls between cells thick walls stiffened with lignin

sieve element companion cell

FIG B 3.27.1 Sieve elements and companion cells in the phloem

FIG B 3.27.2 A stack of dead cells makes up a xylem vessel

one-way only

xylem vessel

Since the xylem doesn’t contain living cells it is not possible for the solution of mineral salts to pass up the plant by osmosis. Instead, the solution of minerals rises up through the xylem by capillarity. This occurs when liquids are in tubes that have very small diameters.

Activity B 3.27.1

Examining the tubes that carry water up a stem

Here is what you need:

• solution of red ink

• stick of celery

• sharp knife

Here is what you should do:

1. Cut off the bottom part of a fresh celery stalk and stand it in the red ink solution at the start of a lesson.

2. Near the end of the lesson take the stalk out of the ink solution and cut across the middle with a sharp knife.

3. Examine the cut part of the celery. Look for the cut xylem tubes which will look like red dots where the ink has been drawn up through the stem.

Check your understanding

1. Decide whether each of the following statements describes the xylem, the phloem, or both the xylem and the phloem.

a) Carries substances dissolved in solution.

b) Substances travel in two directions.

c) Substances can only travel by capillarity.

d) Contains sieve elements.

e) Consists of stacks of dead cells.

Interesting fact

Cut flower stems can absorb solutions that will alter the natural colour of the flowers.

Florists use this trick to make unusual floral displays.

Key terms

sieve tube a tube composed of sieve elements in the phloem capillarity movement of a liquid up a narrow tube

FIG B 3.27.3 The narrower the tube the greater the effect of capillarity

FIG B 3.27.4

28 Transpiration

The absorption of water and minerals from the soil is only one part of a bigger process called transpiration.

Transpiration is the movement of water through a plant. Water is absorbed by the roots and passes through the plant. It is eventually lost by evaporation, mainly through the leaves. This is sometimes called a transpiration stream.

We are learning how to:

• explain the importance of transpiration to the movement of dissolved minerals through a plant

If the stem and leaves of a plant are sealed in a polythene bag and left for 24 hours, condensation forms inside the bag. The water is not coming from the soil, but from the leaves of the plant.

As a result of transpiration, water is continually being lost from the surface of the leaves and more water is drawn up through the xylem to replace it. In turn, soil water containing minerals is absorbed by the plant roots to replenish the water in the xylem. This is called transpiration pull.

Interesting fact

Although carnivorous plants, like the Venus flytrap, absorb water through their roots just like other plants, they do not rely on soil water for minerals.

Carnivorous plants obtain nutrients by trapping and digesting insects.

FIG B 3.28.2 Transpiration causes condensation inside a polythene bag

water evaporates from leaf surface

water travels through plant

water absorbed by roots

FIG B 3.28.1 Transpiration through a plant

FIG B 3.28.3 A Venus flytrap

Activity B 3.28.1

Investigating water loss from both surfaces of a leaf

Here is what you need:

• plant with green leaves

• blue cobalt chloride paper

• plastic paper clip

• scissors

Here is what you should do:

1. Cut out two pieces of blue cobalt chloride paper roughly the same size as a leaf. (They do not have to be exactly the same size as the leaf.)

2. Using a paper clip, attach the cobalt chloride paper to each side of a leaf that is attached to a plant, as shown in Fig B 3.28.4.

3. Make sure the plant is well watered and leave it for several hours.

4. Observe the colour of the cobalt chloride paper covering the upper surface and the lower surface of the leaf. Blue cobalt chloride paper turns pink in the presence of water.

5. [STEAM] Devise an experiment to measure the pressure created by transpiration as water passes up a leafy shoot.

In addition to allowing the uptake of minerals, transpiration has some more important benefits for a plant.

• Transpiration allows the plant to get rid of excess water.

• Transpiration creates hydrostatic pressure, which allows substances in water to be transported around the plant.

• When water evaporates from the surface of a leaf it removes heat energy. A plant can increase the rate at which water is lost from the leaves in order to cool its leaves and prevent them being damaged by heat from the Sun.

Check your understanding

1. In an experiment carried out to demonstrate transpiration, a polythene bag was placed over the leaves and stem of a plant. The open end of the bag was sealed around the stem, as shown in Fig B 3.28.5.

a) Predict how the plant and bag will appear after 24 hours.

b) Explain your prediction.

Key terms

transpiration movement of water through a plant evaporate when a liquid changes to a vapour below its boiling point

polythene bag

paper clip blue cobalt chloride paper

leaf attached to plant

FIG B 3.28.4 Investigating water loss experiment

FIG B 3.28.5 Transpiration experiment