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Living things need energy for everything they do such as:

• growth and repair
• movement
• control of body temperature in mammals

Respiration is a chemical reaction that happens in all living cells, including plant cells and animal cells.

It is the way that energy is released from glucose so that all the other chemical processes needed for life can happen. Do not confuse respiration with breathing (which is properly called ventilation).

Glucose and oxygen react together in cells to produce carbon dioxide and water and release energy.

Here is the word equation for aerobic respiration:

glucose + oxygen → carbon dioxide + water + energy

This is called AEROBIC Respiration – because it needs oxygen from air for it to work

Respiration can also take place without oxygen and is called ANAEROBIC respiration.Anaerobic respiration can take place in

• Muscle cells
• Yeast cells

Unlike aerobic respiration, anaerobic respiration does not need oxygen.

Anaerobic respiration is the release of a relatively small amount of energy in cells by the breakdown of food substances in the absence of oxygen.

Anaerobic respiration happens in muscles during hard exercise:

The word equation for anaerobic respiration. In muscles is:

glucose → lactic acid + energy

Glucose is not completely broken down, so much less energy is released than during aerobic respiration.

There is a build-up of lactic acid in the muscles during vigorous exercise, causing pain and tiredness. This can lead to cramp. The lactic acid is converted to carbon dioxide and water later.

This requires oxygen – (known as excess post-exercise oxygen consumption (EPOC)) - that needs to be ‘repaid’ after the exercise stops. This is why we keep on breathing deeply for a few minutes after we have finished exercising. This is oxygen debt.

Anaerobic respiration also happens in plant cells. Anaerobic respiration in yeast is used during brewing and bread-making:

glucose → ethanol + carbon dioxide + energy

Ethanol is the alcohol found in alcoholic drinks like beer and wine. In bread-making, bubbles of carbon dioxide gas expand the dough and help the bread rise.

In muscle:

• Lactic acid produced causing pain and tiredness
• Only occurs during strenuous exercise
• Much less energy released

In yeast:

• Alcohol and carbon dioxide produced
• Much less energy released
• Used in the production of bread and wine/beer

The table summarises some differences between the two types of respiration.

Aerobic respiration releases 19 times more energy than anaerobic respiration from the same amount of glucose.

Breathing and respiration can sometimes be confused – but they are two very different processes, so be careful.

Remember:

• Respiration is a chemical process that takes place in cells to release energy.

• Breathing is the physical process where you inhale and exhale air in and out of your lungs.**

Aerobic respiration needs oxygen, and it produces carbon dioxide as a waste product.

The human respiratory system contains the organs that allow us to get the oxygen we need from air and to remove the waste carbon dioxide we do not need. It contains these parts:

• two lungs
• tubes leading from the mouth and nose to the lungs ( trachea, bronchus and bronchiole)
• various structures in the chest that allow air to move in and out of the lungs (diaphragm, intercostal muscles and ribs)

Ventilation is the process of breathing. Air gets drawn into our lungs (inhalation) by the intercostal muscles contracting causing the ribs to move up and out. At the same time, the diaphragm contracts and pulls down. This causes the volume in the lungs to increase, and the pressure to decrease. This leads to air being drawn in from the atmosphere down into the alveoli. When we exhale (breathe out), the intercostal muscles relax causing the ribs to move back down and in, and the diaphragm returns to its original domed shape. The volume has decreased, causing the pressure to increase and the air gets forced back out of the lungs.

Ventilation or breathing involves movements of the ribs, intercostal muscles and diaphragm to move air into and out of the lungs:

• when we breathe in, we inhale
• when we breathe out, we exhale

The table describes the changes involved:

Air passes from the mouth into the trachea, often called the windpipe. The trachea divides into two bronchi, with one bronchus for each lung.

Each bronchus divides further in the lungs into smaller tubes called bronchioles.

At the end of each bronchiole, there are tiny balloon-shaped air sacs called alveoli. They greatly increase the surface area of the lungs. You have about 480 million alveoli, located in clusters at the end of the bronchioles. They cover a total surface area that can measure up to 100 m² – about half the size of a singles tennis court.

Features of the alveoli

The alveoli are adapted to make gas exchange in lungs happen easily and efficiently. Here are some features of the alveoli that allow this:

Remember, diffusion is the movement of a substance from an area of High concentration to an area of Low concentration.

We need to get oxygen from the air into the blood, and we need to remove waste carbon dioxide from the blood into the air. Moving gases like this is called gas exchange and occurs by diffusion.

Some water vapour is also lost from the surface of the alveoli into the lungs - we can see this condensing when we breathe out on cold days.

Breathing ensures that the oxygen concentration in the alveoli is higher than in the blood so oxygen moves from the alveoli to the blood. Carbon dioxide diffuses in the opposite direction.

A lung model can be used to demonstrate the process of breathing.

• the balloons represent the lungs,
• the glass jar represents the chest
• the glass tubes represent the trachea and bronchi
• a rubber sheet represents the diaphragm

The lung model shows inhalation:

• When the diaphragm (rubber sheet) moves down, the volume inside the glass jar (chest) increases
• This increase in volume causes a decrease in pressure
• The lungs (balloons) inflate as air enters until the pressures inside and outside are equal

The lung model shows exhalation:

• When the diaphragm (rubber sheet) moves up, the volume inside the glass jar (chest) decreases
• This decrease in volume causes an increase in pressure.
• The lungs (balloons) deflate as air exits until the pressures inside and outside are equal.

Our rate of breathing is affected by:

• Exercise
• Excitement
• Mood
• Stress
• Fever

Although the normal rate of breathing can vary slightly from one person to the next, there is a range that doctors and nurses consider usual.

The normal rate of breathing for healthy individuals between 12 and 18 years old is between 12 and 16 breaths per minute at rest.

The rate for healthy adults is between 12 and 20 breaths per minute.

At this breathing rate, the carbon dioxide exits the lungs at the same rate that the body produces it.

When we exercise we need:

• more oxygen to carry out more respiration and release more energy
• to get rid of more carbon dioxide

This is achieved by:

• breathing in and out faster
• breathing more deeply

Measuring breathing rate

Pick a friend or classmate to observe. You will need a stopwatch too.

Make sure they are at rest – even a short walk will increase the rate of breathing.

To take an accurate measurement, watch the person’s chest rise and fall.

One complete breath is one inhalation, when the chest rises, followed by one exhalation, when the chest falls.

To measure the rate of breathing, you will count the number of breaths for an entire minute.

• start the stopwatch
• carefully count the number of breaths in one minute
• stop the stopwatch.
• record this value in a suitable table

Repeat the measurement three times and calculate the average.