Oxygen and carbon dioxide

When people are asked to name a gas in the air, they usually reply "Oxygen!" This is the first gas that most people learn about. They learn about it because it is the most vital gas. Without oxygen most living things would die. People usually learn about carbon dioxide a little later. They learn about it being exhaled in the breath. This suggests that carbon dioxide is just a waste that animals and humans produce. While this may be true, carbon dioxide is vital to the survival of green plants. When you look more closely at how oxygen and carbon dioxide behave, you find that they are linked to one another in a way that lets animals and green plants survive together on the Earth.

Imagine a particle of oxygen gas. It is being bounced around with other oxygen particles, nitrogen particles and the particles of other air gases. There is a sudden current of air and it is rushed along into a person's body. The oxygen particle passes down the air passages and enters the lungs. It moves through the lungs and into the blood. Oxygen travels round the body in the blood. When it reaches a part of the body that needs it, the oxygen leaves the blood. It enters the part of the body and is ready for action.

Oxygen is an active chemical. It is as if it likes to take part in changes. It takes part in a special change inside the body of all living things. This process is called respiration. In respiration, the oxygen takes part in a change with a food called sugar. During the change, energy is set free. The body uses this energy to stay alive. When oxygen takes part in respiration, it changes. It joins with a substance called carbon from the sugar. The carbon and oxygen form a substance called carbon dioxide.
When carbon dioxide is produced in the body it dissolves in the body fluids. It travels round the body in the blood.

When the carbon dioxide reaches the lungs it leaves the blood. The carbon dioxide enters the air in the lungs and becomes a gas. When a person breathes out, the chest squeezes on the lungs and pushes on the air. This flows out of the lungs taking the carbon dioxide with it. Once it is in the air, the particle of carbon dioxide is jostled with all the other air gases.

Imagine that the particle of carbon dioxide reaches the leaf of a plant. There are holes on the underside of the leaf and the carbon dioxide enters one of them. When it gets inside the leaf, the carbon dioxide takes part in a change with water. The carbon dioxide and water are changed to sugar and oxygen. Energy is needed for this process and the plant collects it from sunlight. As light is used to make or synthesise food the process is known as photosynthesis.

Although the process of photosynthesis takes place inside a plant, it is useful to animals in two ways. It provides them with food and it provides them with oxygen. In return the animals provide the plants with carbon dioxide.

Does oxygen travel in the blood as bubbles of gas?
No. The blood contains a substance called haemoglobin. It collects the oxygen and forms a substance called oxyhaemoglobin. This is bright red. The oxyhaemoglobin takes the oxygen round the blood. When the blood reaches a part that needs oxygen, the oxyhaemoglobin lets go of the oxygen and returns to the lungs for some more. Haemoglobin is dark red and when it flows through blood vessels called veins it makes them look purple. When the oxygen leaves the blood, it dissolves in the liquids in the body. It does not form bubbles at any time.

Is carbon dioxide carried in the haemoglobin?
No. When it forms it dissolves in liquids in the body. The blood has a watery part called the plasma. The carbon dioxide dissolves in this to travel to the lungs.

When you breathe in do you just take in oxygen?
No. You take in the whole mixture of air gases. They can all dissolve in your blood but the haemoglobin lets you take in extra amounts of oxygen.

When you breathe out do you just breathe out carbon dioxide?
No. When the air is in the lungs it does not remove all the oxygen, and the other gases in the air remain in the lungs too. When you breathe out you breathe out less oxygen, but the same amount of the other air gases along with the carbon dioxide.

Plants are living things. Don't they need oxygen too?
Yes, they do. They use oxygen in their body for respiration just as animals do. In daylight when photosynthesis is occurring, the plants make more oxygen than they need so they let it escape into the air. At night, when photosynthesis is not occurring, plants take in oxygen so they can keep respiring.

Are there enough plants on land to make all the oxygen that animals need?
Most of the oxygen in the atmosphere is made by tiny plant-like living things called algae. Huge numbers of them live in the upper sunlit waters of all the seas and oceans. Even though you may
not live by the coast, some of the oxygen you are breathing right now was made by algae in the sea.

How does oxygen make a fire burn?
The substance that burns in a fire is called the fuel. Fuels contain carbon. When the fuel is warm enough oxygen changes the carbon to carbon dioxide. During this change large amounts of energy are released. Most of it is in the form of heat and light. If other substances are hot enough they will burn in air. For example, sulphur is a yellow solid found on the vents of volcanoes. If it is heated with oxygen it burns to form sulphur dioxide.

Why do some fire extinguishers say CO2 on the side?
CO2 stands for carbon dioxide. Things will not burn in carbon dioxide. Also carbon dioxide is heavier than air. When a fire extinguisher containing carbon dioxide is used on a fire, the carbon dioxide flows over the fire and pushes the oxygen out of the way. As the fuel and oxygen are separated the fire goes out.

Does oxygen make iron rust?
Yes, it does, but the metal must also be damp. Water and oxygen are needed for the rusting of iron. When iron rusts it changes to a brown substance called iron oxide. It seems as if oxygen likes to join with iron whenever it can. Iron in the minerals in rocks is attacked by oxygen and changed to iron oxide. The change makes the rocks brown.