Life Processes

In Grade 6 you learned that respiration is one of the seven life processes — it releases energy from food, and it is not the same as breathing. Now we look at exactly how it works.

The most common type is aerobic respiration. The word aerobic means "with oxygen". It happens inside cells, in the mitochondria, all day and all night.

Cells use a sugar called glucose as their fuel. Glucose reacts with oxygen to release the energy stored inside it. That energy powers everything a living thing does — moving, growing, keeping warm, even thinking.

You can write the process as a word equation:

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

The arrow means "turns into". The two raw materials are on the left; the two waste products are on the right; energy is released as the reaction happens.

Both animals and plants respire — every living cell needs energy, so respiration never stops, even in a plant at night.

Animals must eat to get food, but green plants make their own. This food-making process is called photosynthesis — photo means light, synthesis means making.

Photosynthesis happens in the chloroplasts of plant cells. Chloroplasts contain a green substance called chlorophyll, which traps light energy from the Sun.

The plant uses that trapped energy to join two raw materials together: carbon dioxide (taken from the air) and water (taken up by the roots). The result is glucose — the same sugar that respiration uses as fuel — plus oxygen, which is released into the air.

The word equation is:

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

Light is not a substance, so it is written above or beside the arrow rather than in the equation itself. Without light, photosynthesis stops — which is why it only happens in the daytime.

Photosynthesis matters far beyond the plant. It is the source of nearly all the food on Earth, and it supplies the oxygen that living things breathe.

Both respiration and photosynthesis use and produce gases, so living things must constantly swap gases with their surroundings. This swapping is called gas exchange.

In animals, gas exchange happens in the lungs. Oxygen passes from the air into the blood for respiration; carbon dioxide passes out of the blood to be breathed away.

In plants, gas exchange happens through tiny holes in the leaves called stomata. During the day, photosynthesis is faster than respiration, so a leaf takes in carbon dioxide and gives out oxygen overall. At night, only respiration happens, so the leaf takes in oxygen and gives out carbon dioxide.

Wherever it happens, gas exchange works best across a large, thin surface — that is why lungs and leaves are both built that way.

Animals get their glucose and other materials by eating. To stay healthy, you need a balanced diet — the right amounts of every food group, not too much and not too little.

The seven food groups and what each one does:

• Carbohydrates — your main source of energy (bread, rice, pasta).
• Proteins — used for growth and repairing the body (meat, fish, beans).
• Fats — a store of energy and used to keep the body warm.
• Vitamins — small amounts keep the body working properly.
• Minerals — needed for healthy bones, teeth and blood.
• Fibre — keeps food moving through the digestive system.
• Water — needed for almost every process in the body.

Eating too much or too little of any group can lead to ill health, so balance is the goal.

Look carefully at the two word equations and something remarkable appears.

The two processes are almost mirror images:

• Photosynthesis uses carbon dioxide and water to make glucose and oxygen.
• Respiration uses glucose and oxygen to make carbon dioxide and water.

The products of photosynthesis are the raw materials of respiration, and the other way round. This forms a natural cycle. Plants make glucose and oxygen; every living thing uses them in respiration; the carbon dioxide and water released are then used again in photosynthesis.

There is one key difference, though. Photosynthesis stores energy from sunlight inside glucose; respiration releases that energy for cells to use.

Understanding these life processes now opens the door to later topics:

• Organ systems — the breathing, circulatory and digestive systems all exist to support respiration and nutrition.
• Ecosystems — energy flow through food chains starts with photosynthesis in producers.
• Plant growth — you will study how light, water and carbon dioxide affect photosynthesis.
• Health — understanding a balanced diet helps you understand diet-related illness.

Whenever a later topic feels tricky, return to the two word equations and ask where the energy and the gases are going. Take your time — really understanding these processes is worth far more than rushing through them.