Life processes and making food

Is it alive?

A dog that runs is clearly alive. But what about a dog that sleeps, or a tree that does not seem to move at all? Inside them, molecules never stop moving. Cells are repaired, and new materials are made. All this keeps going only because living things use energy.

The processes that keep a living thing alive are called life processes. They are

• nutrition: getting food, for energy and for the materials of the body;
• respiration: breaking down food in the cells to release energy;
• transport: carrying food, oxygen and wastes from one part of the body to another;
• excretion: removing the harmful wastes that the cells make.

A single cell, such as Amoeba, takes in what it needs through its surface. A big body has many cells far from the outside, so it needs special organs for each job.

Two ways of getting food

Food is the source of energy and of the materials for growth. Living things get it in one of two ways.

• Autotrophs make their own food from simple raw materials: carbon dioxide and water. Green plants and some bacteria are autotrophs.
• Heterotrophs take food made by other living things. Animals and fungi are heterotrophs.

So all heterotrophs depend, in the end, on autotrophs.

Photosynthesis

Green plants make food by photosynthesis. They take carbon dioxide from the air and water from the soil, and use the energy of sunlight. Chlorophyll, the green pigment, traps the light. In words,

and as a balanced equation,

The plant soon turns the glucose into starch, a store of food. That is why we test a leaf for starch to see if it has made food.

Photosynthesis happens in three events. They need not come one after the other at once.

1. Absorbing light energy by chlorophyll.
2. Changing light energy into chemical energy, and splitting water into hydrogen and oxygen.
3. Reducing carbon dioxide to carbohydrates, with the hydrogen.

Desert plants open their stomata only at night, when it is cool. They take in carbon dioxide in the dark and store it in another form. In the day, they use it with the energy of light.

Where it happens

Chlorophyll is in small green bodies called chloroplasts. Most of them are in the cells near the upper surface of a leaf, which get the most light.

Carbon dioxide enters through tiny pores called stomata (one pore is a stoma). Most stomata are on the lower surface of the leaf. Each stoma lies between two guard cells, which open and close it.

• When water flows into the guard cells, they swell and curve, and the pore opens.
• When the guard cells lose water, they shrink, and the pore closes.

A plant loses a lot of water through open stomata. So the pores close when the plant does not need carbon dioxide, and when water is short.

The other raw materials

• Water comes from the soil, taken up by the roots.
• Carbon dioxide comes from the air, through the stomata. A water plant takes it from the water.
• Nitrogen, phosphorus, iron and magnesium come from the soil. Nitrogen is taken as nitrates or nitrites, or from bacteria that turn the nitrogen of the air into these.

Testing a leaf for starch

To see where a leaf makes food, we test it for starch.

1. Keep a potted plant in the dark for three days. It uses up the starch that it had stored.
2. Put the plant in sunlight for about six hours. A leaf that is part green and part white is best.
3. Boil the leaf in alcohol in a water bath, to remove the chlorophyll. Alcohol catches fire easily, so never heat it straight over a flame.
4. Dip the leaf in dilute iodine solution. Starch turns blue-black.

Only the parts that were green turn blue-black. So chlorophyll is needed to make food. If a plant is kept in a bell jar with potassium hydroxide, which absorbs carbon dioxide, its leaves make no starch. So carbon dioxide is needed too.

You should find that the rate never passes the shorter bar. With light at $80\%$ and carbon dioxide at $30\%$, more light does not help. Only more carbon dioxide does.

Try it in Python

A chemical equation is balanced when each element has the same number of atoms on both sides. This program counts them for photosynthesis.

Practise with new numbers

Common mistakes

• Thinking a plant photosynthesises at night. It needs light. At night it only respires.
• Thinking the oxygen comes from carbon dioxide. It comes from splitting water.
• Saying the leaf is tested for glucose. The glucose is quickly stored as starch, so we test for starch.
• Mixing up the guard cells. They open the pore when they take in water, not when they lose it.

Remember these

• Life processes: nutrition, respiration, transport and excretion. They need energy all the time.
• Autotrophs make their own food; heterotrophs take food from others.
• Photosynthesis: carbon dioxide and water, with light and chlorophyll, give glucose and oxygen.
• The three events: absorbing light, changing it to chemical energy and splitting water, and reducing carbon dioxide.
• Stomata let gases in and out. Guard cells open them when they fill with water.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four life processes.

2. Name the two raw materials of photosynthesis, and say where each comes from.

3. Why is a plant kept in the dark before its leaf is tested for starch?

4. A guard cell loses water. What happens to the stoma?

5. Where does the oxygen given out in photosynthesis come from?