Training/ exercise

Animals take in energy in the food they eat. This energy has to be transferred into a useful form to power movement, growth, or to keep the body warm. Energy is released from food by a process called respiration.

Respiration
Respiration is a carefully coordinated series of chemical reactions. It happens in all plant and animal cells. Most living things need oxygen for respiration. When you exercise, respiration happens faster to release more energy. You must get more oxygen into your body, so you breathe faster.
Your body is a demanding animal. Billions of cells each carry out thousands of chemical reactions every second to keep you alive. These cells need a constant supply of energy to drive these reactions. The food you eat provides you with molecules to make new cells. But it is also a store of chemical energy. This is converted by respiration into energy your cells can use.
Most of your energy comes from aerobic respiration. During aerobic respiration glucose from your food reacts with oxygen. The reactions release energy from the glucose. Respiration can be summarized by this equation:
glucose + oxygen => carbon dioxide + water (energy released)
C6H12O6 + 602 => 6CO2 + 6H2O

Respiration without oxygen
Many organisms can release energy from food without oxygen. This is called anaerobic respiration. Many animals use anaerobic respiration for short bursts of energy. When you exercise hard (e.g. sprinting) your muscle cells need large amounts of energy. The oxygen demand for aerobic respiration may be greater than the body can provide. If this happens the muscle cells can use anaerobic respiration. This releases energy from glucose without oxygen. This can be an advantage if your muscles need extra energy very quickly. Anaerobic respiration can be summarized by this equation:

glucose => lactic acid (energy released)

Anaerobic respiration can only be used by muscles for a short period of time. It releases much less energy from each gram of glucose than aerobic respiration does. Also, the waste product lactic acid is toxic if produced in large amounts. It builds up in muscles, making them feel sore and tired.

The 100m sprint takes about 10 – 11 seconds for top athletes. The heart and lungs cannot increase the oxygen supply to the muscles fast enough, so most of the energy required during the short race comes from anaerobic respiration.

Getting rid of lactic acid
Toxic lactic acid cannot be left in muscle cells. After anaerobic respiration it is broken down into carbon dioxide and water. This reaction needs oxygen. So in human beings anaerobic respiration only provides energy without oxygen for a short period of time. The oxygen must be ‘paid back’ eventually. The amount of oxygen needed to get rid of the lactic acid is called the oxygen debt. Once you have finished exercise you carry on breathing deeply. Your breathing rate returns to normal when all the lactic acid has been broken down.

The effect of exercise
Working muscles need more energy that relaxed muscles. When you exercise, aerobic respiration in muscle cells must happen faster to provide this extra energy. To do this the muscle cells need more oxygen and glucose. The cells also need to get rid of more waste carbon dioxide made during respiration.
Like other chemicals, oxygen, glucose, and carbon dioxide are transported around your body in your blood. So two things happen when you exercise:

Your breathing increases – more oxygen is brought into your body, and more carbon dioxide can be got rid of.
Your heart rate increases – oxygen and glucose are transported to your muscle cells faster, and carbon dioxide is removed faster.

What happens to the energy from respiration?
All respiration releases energy from glucose. This energy will be needed by many processes in the cell. There must be a way of sorting the energy temporarily, so that it can be moved to the part of the cell that needs it. This is achieved by a very important chemical – ATP.
ATP is made by the cell using energy released during respiration. When energy is needed for reactions in the cell, some ATP is broken down, releasing energy. So ATP can be described as the ‘energy currency’ of the cell.