Respiration

Respiration is not breathing

This is the confusion that costs more marks than any other idea in the topic, so get it straight now. Breathing is the mechanical movement of air into and out of the lungs, produced by the ribs, the diaphragm and the intercostal muscles. Respiration is the chemical reactions inside cells that break down nutrient molecules and release the energy stored in them.

Breathing is a physical process that happens in the lungs. Respiration is a chemical process that happens in cells. A yeast cell respires and has no lungs at all; a plant respires in the dark and never breathes. Breathing exists only to serve respiration: it takes in the oxygen that aerobic respiration needs and removes the carbon dioxide that respiration produces.

Respiration is defined as the chemical reactions in cells that break down nutrient molecules, such as glucose, and release energy for metabolism. The energy is not made from nothing - it was locked in the bonds of the nutrient molecule and is now released.

What cells use the released energy for

The energy released by respiration drives every process in the body that needs energy. Cambridge lists seven uses and you should be able to state any of them.

The energy is released in a series of small steps rather than all at once, so the cell can capture it and use it. Some of the energy is always released as heat, and in mammals this heat is used to maintain a constant body temperature.

• Muscle contraction - to move the body and to keep the heart beating.
• Protein synthesis - to build enzymes, hormones and new cell structures.
• Cell division - for growth and for replacing worn-out cells.
• Active transport - to move substances across membranes against a concentration gradient, which always needs energy.
• Growth - making new cells and new material.
• The passage of nerve impulses - to carry electrical messages along neurones.
• Maintaining a constant body temperature - using the heat released by respiration.

Aerobic respiration and its equations

Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Glucose is broken down completely, so it releases a relatively large amount of energy per glucose molecule.

The word equation is: glucose + oxygen -> carbon dioxide + water. This is what the Core paper asks for, and it must be learned exactly: the reactants are glucose and oxygen, and the products are carbon dioxide and water. Energy is released, so it is not written as a product.

The Extended paper also wants the balanced chemical equation: C6H12O6 + 6O2 -> 6CO2 + 6H2O. Check that it balances - 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side.

Anaerobic respiration in muscles: lactic acid and oxygen debt

Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. Because the glucose is only partly broken down, it releases much less energy per glucose molecule than aerobic respiration.

In muscles during vigorous exercise the word equation is: glucose -> lactic acid. Exercise makes the muscles respire faster than the blood can deliver oxygen, so they switch to anaerobic respiration for the extra energy.

The lactic acid produced builds up in the muscles and in the blood. This build-up is what causes an oxygen debt: the extra oxygen that the body must take in afterwards to break the lactic acid down.

Anaerobic respiration in yeast: fermentation

Yeast also respires anaerobically, but the product is different. The word equation is: glucose -> alcohol + carbon dioxide.

The Extended paper wants the balanced chemical equation: C6H12O6 -> 2C2H5OH + 2CO2. Here C2H5OH is ethanol (alcohol) and CO2 is carbon dioxide.

This process is called fermentation, and humans use it in the food industry. In bread making the carbon dioxide produced makes the dough rise; in brewing the alcohol produced is the alcohol in beer and wine.

Exercise: why breathing rate and heart rate rise

During exercise the muscles contract more often and respire faster. They need more glucose and more oxygen, and they produce more carbon dioxide, so the body has to move gases faster.

Breathing rate and depth increase so that more oxygen enters the lungs and more carbon dioxide is removed. Heart rate increases so that oxygen and glucose are carried to the muscles faster and carbon dioxide is carried away faster.

If the muscles respire so hard that the blood cannot deliver oxygen quickly enough, they begin to respire anaerobically as well. That is why you are out of breath at the end of hard exercise and why you keep breathing hard afterwards: you are repaying the oxygen debt.

Investigating respiration in yeast

Yeast is used in the practical because it respires quickly and the carbon dioxide it produces is easy to see and measure. A suspension of yeast in glucose solution is placed in a test tube or flask, and the gas produced is collected.

The rate of respiration is measured by the carbon dioxide produced - for example by counting the bubbles released per minute, or by measuring the volume of gas collected in a set time. More carbon dioxide per minute means a faster rate of respiration.

The effect of temperature: as temperature rises the rate of respiration increases, because the enzymes controlling respiration have more kinetic energy and collide more often. The rate reaches an optimum and then falls sharply, because at higher temperatures the enzymes are denatured and can no longer catalyse the reactions. Because the temperature is the variable being changed, it is the independent variable; the volume of carbon dioxide is the dependent variable; and the volume and concentration of the yeast and glucose solutions, and the time measured, must all be kept the same as control variables.