Overview of respiration and glycolysisEdexcel International A Level Biology: Revision notes

Respiration releases the energy stored in organic molecules and uses it to make ATP. The most common respiratory substrate is glucose.

In aerobic respiration the substrate is split and oxidised:

• carbon dioxide is released as a waste product (by decarboxylation)
• hydrogen removed from the substrate is combined with atmospheric oxygen to form water
• a large amount of energy is released, which is used to make ATP

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ + energy

The hydrogen is carried from the substrate to oxygen by the coenzyme NAD (and FAD), which becomes reduced NAD.

Glucose is not simply burned. Respiration is a series of small steps, each controlled and catalysed by a specific intracellular enzyme (the names of the enzymes are not required).

This matters because:

• energy is released in small amounts that can be captured in ATP, whereas burning would lose most as heat
• a large release of heat would damage the cell and denature enzymes
• each step can be controlled, so the rate matches the cell's demand for ATP

Glycolysis is the first stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm and does not need oxygen. One glucose (a hexose, 6C) is converted to two pyruvate (3C).

1. Phosphorylation: two ATP are used to add phosphate to the hexose. This makes it more reactive and starts its breakdown.
2. The phosphorylated hexose is split and oxidised. Hydrogen is removed and accepted by NAD, forming 2 reduced NAD.
3. 4 ATP are made by substrate-level phosphorylation, where a phosphate group is passed from a substrate directly to ADP.

Net yield: 4 − 2 = 2 ATP, 2 reduced NAD and 2 pyruvate per glucose.

With oxygen (aerobic): pyruvate enters the mitochondrion for the link reaction and Krebs cycle, and reduced NAD passes its hydrogen to the electron transport chain, where much more ATP is made.

Without oxygen (anaerobic): oxygen cannot accept hydrogen at the end of the electron transport chain, so reduced NAD cannot be reoxidised there. Instead pyruvate accepts the hydrogen from reduced NAD and is converted to lactate in animals (ethanol and carbon dioxide in yeast).

This regenerates NAD, so glycolysis can keep making a small amount of ATP (2 per glucose). Anaerobic respiration is therefore less efficient, but it is fast.

Lactate builds up in muscles during intense exercise and lowers pH. After exercise it is transported in the blood to the liver. There it is converted back to pyruvate, which is either oxidised in aerobic respiration or converted to glucose and glycogen.

This needs oxygen, which is why breathing stays heavy after exercise. This extra oxygen is the oxygen debt.

A redox indicator such as methylene blue (blue when oxidised, colourless when reduced) can show the rate of respiration in yeast. Hydrogen released in respiration, carried by reduced NAD, reduces the dye, so it loses its colour.

• Mix yeast suspension, glucose and the indicator, and record the time to turn colourless; a shorter time means a faster rate.
• Cover with a layer of oil to keep oxygen out, as oxygen would reoxidise the dye.
• Vary one factor, such as temperature or glucose concentration, and control the others (volume and concentration of yeast, indicator, glucose).
• Use a control of boiled and cooled yeast: the enzymes are denatured, so the colour should not change.
• Rate can be calculated as 1 ÷ time.