Respiration as the release of energy from glucose, the role of ATP, an outline of aerobic respiration in the cytoplasm and mitochondria, anaerobic respiration and lactate production in humans, and the measurement of respiration and energy needs.

What this dot point is asking

CCEA wants you to define respiration as the release of energy from glucose, explain the role of ATP as the energy currency, give an outline of aerobic respiration in the cytoplasm and mitochondria, describe anaerobic respiration and lactate production in humans, and understand how energy needs and respiration are measured. It links the respiratory and cardiovascular systems (which deliver oxygen and glucose) to the use of that energy in muscles and other tissues.

Respiration and ATP

ATP is made by adding an inorganic phosphate to ADP, which requires energy from respiration. When the cell needs energy, ATP is hydrolysed back to ADP and inorganic phosphate, releasing a small, usable amount of energy on demand. This makes ATP ideal as a currency: it can be made and broken quickly, it releases energy in convenient amounts, and it is used everywhere energy is needed, such as muscle contraction, active transport, nerve transmission and synthesising large molecules. Because ATP cannot be stored in large amounts, it must be made continuously, so cells with high energy demands contain many mitochondria.

Aerobic respiration

The word equation summarises it: glucose plus oxygen gives carbon dioxide plus water, releasing energy used to make ATP. The mitochondrion is adapted for this with a folded inner membrane (cristae) that increases the surface area for the reactions of oxidative phosphorylation, and a fluid matrix containing the enzymes of the Krebs cycle. The large ATP yield is why aerobic respiration is the main energy supply when oxygen is plentiful, for example in resting muscle and during sustained moderate exercise.

Anaerobic respiration and oxygen debt

When oxygen cannot be supplied fast enough, for example in muscles during vigorous exercise, cells respire anaerobically in the cytoplasm. Glycolysis still splits glucose into pyruvate, but with no oxygen the pyruvate is converted to lactate (lactic acid), regenerating the molecules needed to keep glycolysis going. This yields only a small amount of ATP (a net 2 per glucose) and produces no carbon dioxide. Lactate build-up lowers muscle pH and causes fatigue. After exercise, extra oxygen is taken in (the oxygen debt or excess post-exercise oxygen consumption) to break down the lactate, which is why breathing stays fast and deep for a time after stopping.

Examples in context

Example 1. Sprinting versus a marathon. A sprinter works so hard that oxygen cannot be delivered fast enough, so muscles respire anaerobically, producing lactate and incurring an oxygen debt repaid by hard breathing after the race. A marathon runner works at a pace where oxygen supply keeps up, so muscles respire aerobically, releasing far more ATP per glucose and avoiding rapid lactate build-up.

Example 2. Mitochondria in different cells. Cells with high, sustained energy demands, such as cardiac muscle and the muscle that moves the wings of a flying bird, are packed with mitochondria. The number of mitochondria reflects how much ATP a cell needs, directly linking structure to the rate of aerobic respiration.

Try this

Q1. State where glycolysis occurs and name its product. [2 marks]

• Cue. In the cytoplasm; the product is pyruvate (with a small ATP yield).

Q2. Explain why anaerobic respiration produces much less ATP than aerobic respiration. [2 marks]

• Cue. It stops at glycolysis; the energy-rich mitochondrial stages (Krebs cycle and oxidative phosphorylation) do not occur without oxygen.

Q3. State what is meant by the oxygen debt. [1 mark]

• Cue. The extra oxygen taken in after exercise to break down the lactate produced during anaerobic respiration.