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In this section you will learn :

Food as a fuel

All living things need to have a source of chemical energy in order to live. A fuel is a substance that can be used as a source of chemical energy. Animals eat food as their fuel . Plants make food to act as their fuel, using the energy of sunlight and a process called photosynthesis. Both animals and plants then release the energy from food by using the process of respiration.

Aerobic respiration

Respiration means the release of energy from food. This is a process carried out by all living things. The chemical process involved works most efficiently if oxygen is used. Just like a fire needs an oxygen supply to burn the fuel, so aerobic respiration needs oxygen. Without oxygen the respiration does not release all the energy and is called anaerobic respiration. If there is a plentiful supply of oxygen then aerobic respiration takes place.

The aerobic respiration equation

When food is burned to release energy using oxygen their are two waste products produced. These are carbon dioxide and water. These are the same waste products produced when a fuel, such as coal or wood is burned. The process of aerobic respiration can be represented by the word equation :

The food usually used as an energy supply is glucose, a type of sugar produced when most carbohydrates are digested.

Location of aerobic respiration

Aerobic respiration takes place inside the cell. All cells need an energy supply to carry out their functions. The food and oxygen are transported to the cells in humans by the blood in the circulatory system. The oxygen comes from the lungs of the respiratory system and the food comes from the small intestine of the digestive system. Parts of the cell called the mitochondria is the actual location for aerobic respiration.

Questions (p2)

  1. What do we use as a fuel for our body ?

  2. Which system of our body absorbs this fuel ?

  3. What process breaks this fuel down to produce energy ?

  4. Where does this breakdown of the fuel take place in the body ?

  5. What other substance is needed to help break down the fuel efficiently ?

  6. What are the two waste products when the fuel is broken down efficiently ?

  7. Write a word equation for this process.


In order to get the oxygen to the cells humans need to breathe air into the lungs. The lungs are where gas exchange takes place between the air and the blood.

Therefore breathing is only a physical action at the start of the whole respiration process for humans. Breathing is often called external respiration. The chemical process of aerobic respiration in the cell is often called internal respiration. This can be summarised by the flow diagram :

The lungs and gas exchange

The lungs are the organs where gas exchange takes place. The breathing movement occurs when the diaphragm and intercostal muscles move causing the chest cavity to change in size. An average individual takes about 15,000 breaths per day. The lungs themselves do not move but are inflated and deflated by changes in pressure.

A diagram of the human lungs

Inside the lungs the bronchi tubes split into smaller and smaller tubes called bronchioles. These finish in a dead end formed from a cluster of small microscopic bubble-like sacs called alveoli. It is in an alveolus where gas exchange takes place.

A diagram of an alveolus

In the alveolus oxygen diffuses from the air into the blood and carbon dioxide diffuses in the opposite direction. This structure of about 300 million alveoli greatly increases the surface area of the lungs so speeding up the rate of gas exchange. The typical human lungs have a surface area of about 70m², about the size of a tennis court. The alveoli are kept moist by a liquid called mucus. This allows the oxygen from the air to dissolve and so also speeds up the gas exchange. The walls of the alveoli are very thin so gases do not have far to move to get through to the blood. Finally the alveoli have a very rich deoxygenated blood supply embedded within their walls as a network of blood capillaries. This means the blood and the air are separated by a membrane only one micron (0.001mm) thick.

Gas exchange occurs by diffusion which is a slow process. Diffusion occurs because of the movement of particles. Gases diffuse from the air into the blood in solution. The lung structure greatly enhances the rate of diffusion in the following four ways :

As well as oxygen diffusing into the blood from the air, carbon dioxide also diffuses out of the blood into the air. This is one of the processes of excretion.

Questions (p4-5)

  1. Which system of our body functions to absorb oxygen ?

  2. Label the diagram below to show what happens in the lungs during gas exchange.

  3. Describe four ways in which lung structure makes gas exchange more efficient?

  4. By which physical process does gas exchange take place ?

Answers (p2)

  1. Food is used as the fuel for our body. Carbohydrates such as the sugar glucose is the most common fuel.
  2. The digestive system absorbs the fuel. Food is absorbed by the small intestine of the digestive system. Carbohydrates such as starch need to be digested to form glucose before they can be absorbed into the blood.
  3. The process of respiration breaks down the food to produce energy. There are two types of respiration, anaerobic and aerobic.
  4. The process of respiration takes place in the cells. Many small mitochondria in the cell is the actual location for respiration.
  5. Oxygen is needed for respiration to take place efficiently. When oxygen is used the process is called aerobic respiration.
  6. The two waste products of aerobic respiration are carbon dioxide and water.
  7. The word equation for aerobic respiration is :

Answers (p4)

  1. The respiratory system functions to absorb oxygen using the lungs.
  2. The four ways in which the lungs are structured to aid gas exchange are :
    1. millions of alveoli to increase surface area for diffusion
    2. moist alveoli surface to dissolve oxygen and aid diffusion
    3. thin walls of alveoli reducing diffusion distance
    4. a rich deoxygenated blood supply to absorb oxygen
  3. Below is a labelled diagram to show how gas exchange takes place.

  4. The process by which gas exchange takes place is diffusion. This physical process occurs because of the motion of particles of gas in solution.



aerobic respiration

The process whereby energy is released from food by the use of oxygen. In this process all the available energy is released from the food.


Microscopic sac-like structures in the lung tissue where gas exchange between the air and the blood takes place.

anaerobic respiration

The process whereby energy is released from food without the use of oxygen. In this process the food is only partially utilised, some available energy remaining in the waste products.



The air carrying tubes that enter the lungs. The singular form of the word is bronchus. The windpipe in the human splits into two bronchi one entering each lung.


The smaller air tubes branching off from the bronchi. They eventually lead to the alveoli.



The smallest blood vessels in the body. They are only about 10 microns across (0.01mm) just big enough for red blood cells to pass through. Capillary networks link up the arteries from the heart to the veins returning back to the heart.


A group of food molecules that contain only the elements carbon, hydrogen and oxygen. The hydrogen and oxygen being in the same proportion as in water (2 to 1). They are grouped into sugars and polysaccharides (starch and cellulose).

carbon dioxide

A gas made of carbon and oxygen. It is the main waste product of aerobic respiration and burning of fuels.


The smallest living unit. Humans are multicellular living things made up of billions of cells of different types.

chemical energy

A form of energy that is stored in compounds such as sugar.

chest cavity

The space surrounded by the rib cage. It contains the lungs and heart. The bottom is separated from the abdomen (belly) by the muscular diaphragm.

circulatory system

The system in the body concerned with transport of food and oxygen. It consists of tubes called blood vessels connected to the heart which acts as a pump. Blood is forced from the heart to the body tissues through arteries and returns back to the heart through veins.



This means without oxygen. Blood without oxygen travels to the lungs where it loses carbon dioxide and gains oxygen.


The arched muscular sheet that separates the chest cavity from the abdomen (belly). It is used during breathing where it contracts and lowers so increasing the space in the chest cavity when inhaling. It relaxes to cause exhalation.


The movement of a substance from where it is in high concentration to where it is in low concentration. The movement happens because the particles have kinetic energy.

digestive system

The system of the body concerned with the breaking down (digesting) and absorption of food. It consists of a tube called the alimentary canal that connects the mouth to the anus and glands that secrete digesting enzymes onto the food as it passes through.



The removal of waste products from the body.

external respiration

The action of breathing to take air into the body and gas exchange to absorb oxygen into the blood and excrete carbon dioxide out of the blood.



A substance that contains a lot of energy. Fuels can be burned to release the energy as heat and light. Food is a fuel used by living things to supply useful energy for movement and keeping warm.


gas exchange

The exchanging of gases that takes place between the air and the blood in the alveoli of the lungs. It occurs through diffusion. Oxygen moves from the air into the blood whilst carbon dioxide moves out of the blood into the air.


A type of sugar most commonly used as food for respiration. It can be found naturally and is the product of digestion of starch.


intercostal muscles

The muscles between the ribs. They are used to expand and contract the size of the chest cavity for deep breathing during exercise.

internal respiration

The release of energy from food that takes place inside mitochondria of cells.



The organs where gas exchange takes place.



A thin layer of tissue only a few cells thick.


The parts of a cell where internal respiration takes place. Each cell has many mitochondria. The singular form of the word is mitochondrion.


A sticky liquid secreted by the cells lining the respiratory and digestive system tubes. In the respiratory system it ensures that oxygen dissolves in the lungs before diffusing into the blood.



The gas needed by all living things that carry out aerobic respiration.



The process by which green plants build up complex food materials from the simple materials carbon dioxide and water using the energy of sunlight absorbed by the green substance chlorophyll.


Force applied to a surface. In the chest cavity pressure is reduced to cause us to breathe in and is increased to cause us to breathe out.


respiratory system

The system of the body concerned with external respiration. ie. breathing and gas exchange. It consists of the air passages, lungs, rib cage and associated muscles.


small intestine

The tube in the digestive system where food is absorbed into the blood.


One of the groups of carbohydrate. It is used as a food high in energy. Glucose is the commonly used form in the body.

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