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KS3 Bitesize

Science

Food chains

In this Revision Bite we'll look at the feeding relationships between living things. These can be shown in food chains and food webs.

All food chains start with green plants. Green plants use sunlight to produce the source of their energy. This energy is then passed along the food chain as one organism eats another.

Introduction

This Revision Bite covers:

Plants and photosynthesis

Before we look at food chains we will go over the way green plants capture energy from the Sun to make food. This is the start of all the food chains we will look at.

Plants and photosynthesis

Animals eat food to get their energy. But green plants don't. Instead they make their own food, glucose, in a process called photosynthesis. We say that plants can photosynthesise.

These are the things that plants need for photosynthesis:

  • carbon dioxide

  • water

  • light

These are the things that plants make by photosynthesis:

  • glucose

  • oxygen

We can show photosynthesis in a word equation, where light energy is shown in brackets because it is not a substance:

carbon dioxide + water (+ light energy)glucose + oxygen

Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots. Light energy comes from the sun.

The oxygen produced is released into the air from the leaves. The glucose produced can be turned into other substances, such as starch, which is used as a store of energy. This energy can be released by respiration.

Where does photosynthesis take place?

Photosynthesis takes place inside plant cells in small things called chloroplasts. Chloroplasts contain a green substance called chlorophyll. This absorbs the light energy needed to make photosynthesis happen. Plants can only photosynthesise in the light.

The plant cell is surrounded by a cell wall for structure. Inside this is the cell membrane and this contains the cytoplasm. There are chloroplasts and one nucleus in the cytoplasm, and in the centre of the cell is a large vacuole.

Getting carbon dioxide, light and water

Remember that the equation for photosynthesis is:

carbon dioxide + water (+ light energy)glucose + oxygen

Let's see how plants get the carbon dioxide and water they need for this process.

Carbon dioxide

Plants get carbon dioxide from the air through their leaves. The carbon dioxide diffuses through small holes in the underside of the leaf called stomata. (One of these holes is called a stoma. The plural is stomata.)

Shows how the leaf is structured: Upper layer, palisade layer, spongy layer, lower layer. Sunlight hits the upper layer. Gases are exchanged through the stoma in the lower layer

The lower part of the leaf has loose-fitting cells, to allow carbon dioxide to reach the other cells in the leaf. This also allow the oxygen produced in photosynthesis to leave the leaf easily.

Light

A leaf usually has a large surface area, so that it can absorb a lot of light. Its top surface is protected from water loss, disease and weather damage by a waxy layer.

The upper part of the leaf is where the light falls, and it contains a type of cell called a palisade cell. This is adapted to absorb a lot of light. It has lots of chloroplasts and is shaped like a tall box.

Water

two plants - one straight and one wilting

Turgid plant that's watered regularly and a flacid plant without enough water.

Plants get the water they need for photosynthesis through their roots.

The roots have a type of cell called a root hair cell - these project out from the root into the soil. Roots have a big surface area and thin walls, which allow water to pass into them easily.

Note that root cells do not contain chloroplasts, as they are normally in the dark and cannot photosynthesise.

Diagram shows a root hair cell. The water can pass through the thin cell wall of the root hair.

The water absorbed by the root hair cells passes through the plant in xylem tubes, and eventually reaches the leaves. If a plant does not absorb enough water, it will wilt or go floppy. Without water it may also not photosynthesise quickly enough, and it may die.

Respiration in plants

All living things get the energy they need to live from a chemical reaction called respiration. This process needs glucose as a starting point. First we'll see how respiration works, and then look at how plants use respiration in combination with photosynthesis.

Respiration

Living cells respire. Aerobic respiration is the chemical reaction used to release energy from glucose. It is called aerobic because oxygen from the air is also needed.

Here is the word equation for aerobic respiration. Energy is put in brackets because it not a substance:

glucose + oxygencarbon dioxide + water (+ energy)

Notice that the word equation for respiration is the reverse of the word equation for photosynthesis. Check back if you are not sure of this.

Plants

Plant cells respire, just as animal cells do. If they stop respiring, they will die. Remember that respiration is not the same as breathing, so take care - plants do not breathe.

As we can see from the word equations respiration and photosynthesis are opposites. Respiration uses oxygen and produces carbon dioxide. Photosynthesis uses carbon dioxide and produces oxygen.

So what happens to a plant overall? This depends on whether it is in the dark or the light, and how bright the light is.

Plants respire all the time, whether it is dark or light. They photosynthesise only when they are in the light.

Conditions Photosynthesis v respiration Overall result
Dark

Respiration
No photosynthesis

Oxygen taken in
Carbon dioxide given out

Dim light Photosynthesis rate equals respiration rate Neither gas is taken in or given out, as each cancels the other out
Bright light Photosynthesis rate greater than respiration rate

Carbon dioxide taken in
Oxygen given out

Photosynthesis usually results in a net food gain (net glucose gain), once respiration has been accounted for. This means that there is an increase in the biomass of the plant.

Plants that lose their leaves in winter store food produced during the summer by photosynthesis. They store enough food to last them over winter, and to provide energy reserves for new growth in the spring.

Food chains

A food chain shows the different organisms that live in a habitat, and what eats what.

Producers and consumers

A food chain always starts with a producer, which is an organism that makes food. This is usually a green plant, because plants can make their own food by photosynthesis.

A food chain ends with a consumer, which is an animal that eats a plant or another animal. Here is an example of a simple food chain:

grasscowhuman

Take care - the arrow points to the organism that is doing the eating. If you get the arrows the wrong way round, instead of showing that humans eat cows, you are showing that cows eat humans, and that grass eats cows.

Other words in a food chain

There are several words used to describe the organisms in a food chain. Study this food chain:

Grass Grasshopper Frog Hawk
grass   A grasshopper   a frog   a hawk
Producer   Consumer   Consumer   Consumer
    Primary consumer   Secondary consumer   Tertiary consumer
    Herbivore   Carnivore   Carnivore

The plant is the producer and the animals are consumers.

Notice that the first consumer in the chain is also called the primary consumer, the next one is the secondary consumer and the one after that is the tertiary consumer.

A consumer that eats plants is called a herbivore, and a consumer that eats other animals is called a carnivore. An omnivore is an animal that eats both plants and animals.

Predators and prey

A predator is an animal that eats other animals, and the prey is the animal that gets eaten by the predator. In the food chain above:

  • the frog is a predator and the grasshopper is its prey.

  • the hawk is a predator and the frog is its prey.

Pyramids of numbers

The population of each organism in a food chain can be shown in a sort of bar chart called a pyramid of numbers. The more organisms there are, the wider the bar. The producer in the food chain always goes at the bottom of the pyramid of numbers.

Think about this food chain:

cloversnailthrushhawk

Clover is a plant and it is the producer in this food chain. Its bar goes at the bottom of the pyramid:

Shows a large number of clover, a slightly smaller number of snails, a smaller number of thrushes and an even smaller number of sparrowhawks

Energy is lost to the surroundings as we go from one level to the next, so there are fewer organisms at each level in this food chain. A lot of clover is needed to support the snail population. A thrush eats lots of snails, and a hawk eats lots of thrushes, so the population of hawks is very small.

Other pyramid shapes

Sometimes the pyramid of numbers doesn't look like a pyramid at all. This could happen if the producer is a large plant such as a tree, or if one of the animals is very small. Remember, though, that whatever the situation, the producer still goes at the bottom of the pyramid.

Here are two examples like this:

Shows a single oak trea, a greater number of insects, and a slightly smaller number of woodpeckers

An oak tree is very large so many insects can feed on it

Shows a large number of grass, a much smaller number of rabbits, and a larger number (but not as large as the amount of crass) of fleas.

Fleas are very small and lots of them can feed on a rabbit

Food webs

When all the food chains in a habitat are joined up together they form a food web. Here is an example of a food web:

Food web - The producer is grass, which is eaten by rabbits, insects and slugs. The rabbit is eaten by the fox. The insects are eaten by frogs, voles and thrushes. Thrushes also eat the slugs. Frogs, voles and thrushes are eaten by a hawk. Frogs and voles are also eaten by the fox.

Although it looks complex, it is just several food chains joined together. Here are some of the food chains in this food web:

grass → insect → vole → hawk

grass → insect → frog → fox

grass → insect → vole → fox

Notice that the frogs, voles and insects have more than one predator, but the rabbits and slugs have just one predator.

This leads to some interesting effects if the population of a particular organism in the food web decreases. Some animals can just eat more of another organism if food is in short supply, while others may starve and die. This in turn can affect the populations of other organisms in the food web.

Changes to food webs

Let's look at the food web again and ask some questions about its predator-prey relationships, with some possible answers.

Food web - The producer is grass, which is eaten by rabbits, insects and slugs. The rabbit is eaten by the fox. The insects are eaten by frogs, voles and thrushes. Thrushes also eat the slugs. Frogs, voles and thrushes are eaten by a hawk. Frogs and voles are also eaten by the fox.

What would happen if the grass died?

The grass is the producer, so if it died the consumers that feed on it - rabbits, insects and slugs - would have no food. They would starve and die unless they could move to another habitat. All the other animals in the food web would die too, because their food supplies would have died out. The populations of the consumers would fall as the population of the producer fell.

What would happen if the population of slugs decreased?

Slugs, rabbits and insects all eat grass. If there were fewer slugs there would be more grass for the rabbits and insects. With more food the populations of rabbits and insects would increase. However, the thrushes would have to eat more insects to maintain their population, so it is also possible that the population of insects could decrease. This is turn may reduce the populations of voles and frogs.

What would happen if the population of insects decreased?

There would be more food for the rabbits and slugs, so their populations would increase. However, there would be less food for the frogs and voles, so their populations would decrease. This means less food for the foxes and hawks. However, there are likely to be more rabbits and thrushes for them to eat, so their populations are likely to stay the same.

Toxic materials in the food chain

Toxic materials are poisonous. Some quickly break down into harmless substances in the environment. Others are persistent and do not break down. Instead, they accumulate in the food chain and damage the organisms in it, especially the top predators. Mercury and DDT are two persistent toxic materials.

Mercury

Shows how a trace of mercury in plant plankton moves through the food chain: Plant plankton - animal plankton - small fish - larger fish - tune

Mercury compounds were used until quite recently to make insecticides (chemicals that kill the insects that damage crops) and special paints that stop barnacles growing on the hulls of ships.

Unfortunately, when it gets into the food chain mercury damages the nervous systems and reproductive systems of mammals, including humans. The diagram shows how mercury can accumulate in the food chain.

Tiny plankton in the sea absorb the mercury compounds. When the plankton are eaten by small fish, the mercury they contain stays in the fish. As the fish need to eat a lot of plankton, the concentration of mercury in them becomes higher than its concentration in the plankton.

The small fish are eaten by larger fish, and these are eaten by still larger ones, such as tuna fish. This creates a high concentration of mercury in the tuna. People eating contaminated tuna may get mercury poisoning. Mercury is now banned from many chemical products and mercury use in industry is carefully regulated.

DDT

DDT is an insecticide that can pass up the food chain from insects to small birds, and then from the small birds to birds of prey, like hawks. It can accumulate in the birds of prey, giving them a large amount of DDT. Concentrations of DDT in birds causes weakness in their eggs, and reduces their population. DDT is now banned because of this.

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