1.7 - Aerobic & Anaerobic Respiration

What respiration is and what it is for

• Respiration is the process that releases energy from glucose. It goes on in every living cell - in animals, plants, and microorganisms - all the time.
• It's not the same as breathing. Respiration is a series of chemical reactions inside cells, each controlled by enzymes, that break glucose down and transfer its energy into a form the cell can use.
• Because it happens continuously in every cell, living things have a constant supply of energy for everything they do.

ATP - The cell's energy currency

• The energy released is carried within the cell by a molecule called ATP.
• The reactions of respiration release energy from glucose and use it to make ATP. The cell then uses ATP to power its activities.

What cells use the energy for

• Contracting muscles - Lets animals move
• Building larger molecules from smaller ones - Such as making proteins from amino acids
• Cell division - Making new cells, and synthesising new substances for growth and repair
• Transmitting nerve impulses
• Active transport - Moving substances across cell membranes
• Keeping body temperature steady - In mammals and birds, when the surroundings are cold

Respiration is exothermic

• Respiration is an exothermic reaction, which means it transfers energy to the surroundings.
• Some of this energy is released as heat.

Aerobic respiration

• Aerobic respiration is respiration that uses oxygen.
• It is the most efficient way to release energy from glucose, transferring far more energy per glucose molecule than respiration without oxygen.
• It is the form of respiration cells use whenever enough oxygen is available, and happens all the time in the cells of plants and animals.
• Glucose reacts with oxygen and is fully broken down, releasing energy and producing carbon dioxide and water as waste products.

Word equation

glucose + oxygen → carbon dioxide + water (+ energy released)

The two stages of respiration

1. In the cytoplasm - Each glucose molecule is first broken down into two molecules of pyruvate, releasing enough energy to make two molecules of ATP.
2. In the mitochondria - When oxygen is available, the pyruvate passes into the mitochondria, where it is broken down completely to carbon dioxide and water. This releases a large amount of energy and makes many more molecules of ATP.

This second stage is why aerobic respiration transfers so much more energy per glucose molecule than respiration without oxygen.

Where it happens

• Most of the reactions take place inside the mitochondria - the structures where aerobic respiration is carried out.
• Cells that need a lot of energy tend to contain large numbers of mitochondria.

Anaerobic respiration

• Anaerobic respiration is the release of energy from glucose without using oxygen ("anaerobic" means "without oxygen").
• Because no oxygen is involved, the glucose is only partially broken down rather than fully oxidised.
• So it releases much less energy per glucose molecule than aerobic respiration - it is far less efficient.
• Cells use it when they can't get enough oxygen - for example a muscle working very hard during vigorous exercise.

Where it happens and the ATP yield

1. Like all respiration, the pathway begins in the cytoplasm, where each glucose molecule is broken down to two molecules of pyruvate, releasing enough energy to make two molecules of ATP.
2. Without oxygen, the pyruvate is not passed into the mitochondria.
3. Instead the whole pathway, called fermentation, is completed in the cytoplasm, and no further ATP is made.
4. So anaerobic respiration yields only those first two molecules of ATP per glucose - far fewer than aerobic respiration.

Anaerobic respiration in animal cells

• Glucose is broken down to lactate (also called lactic acid).
• The lactate builds up in the muscles during hard exercise.

The word equation is:

glucose → lactate (+ energy released)

Anaerobic respiration in yeast and plant cells

Instead of lactate, it produces ethanol (alcohol) and carbon dioxide.

The word equation is:

glucose → ethanol + carbon dioxide (+ energy released)

• In yeast, as in all cells, anaerobic respiration is called fermentation. Yeast are single-celled organisms.
• Fermentation by yeast is widely used in food and drink production:
  • The carbon dioxide it releases makes bread rise.
  • The ethanol it produces is the alcohol in beer and wine.