International A Level Biology / Cellular respiration

Glycolysis revision notes

Edexcel International A Level Biology (YBI11), chapter 20: Cellular respiration, specification point 7.2. Free lesson.

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Glycolysis is the first stage of respiration. It splits one glucose molecule into two pyruvate molecules in the cytoplasm, without needing oxygen, and provides a net yield of 2 ATP.

How glucose is split

Glycolysis means sugar splitting. It takes place in the cytoplasm and does not use oxygen. A six-carbon glucose molecule is changed into two three-carbon pyruvate molecules. Phosphorylation adds phosphate groups. Two ATP each give up one phosphate, forming hexose bisphosphate. The added phosphates make the sugar more reactive and easier to split. Hexose bisphosphate splits into two triose phosphate molecules. Each is a three-carbon sugar with one phosphate group. Each triose phosphate is oxidised: hydrogen is removed and accepted by NAD, which becomes reduced NAD. Each triose…

How glycolysis makes ATP

Substrate-level phosphorylation makes ATP when an enzyme transfers a phosphate group directly from a substrate molecule to ADP. It needs neither oxygen nor an electron transport chain, so it can happen in anaerobic respiration too. This is different from oxidative phosphorylation, which makes most ATP in mitochondria. Each of the two triose phosphate molecules produces 2 ATP, so 4 ATP are made altogether. The first step used 2 ATP. Subtract these from the 4 ATP made to find the net yield: 2 ATP per glucose.

How many reduced NAD and ATP are produced?

One glucose forms two triose phosphate molecules. Count the products from each one, then combine them. The ATP used at the start must be subtracted from the ATP made.

What changes when oxygen is absent?

Glycolysis is the same with or without oxygen. What changes is the fate of its products. In both routes, NAD must be regenerated from reduced NAD. Otherwise glycolysis cannot continue. With oxygen, pyruvate enters the mitochondrion for the link reaction. Reduced NAD passes its hydrogen to the electron transport chain, and more ATP can be made after glycolysis. Without oxygen, pyruvate stays in the cytoplasm and becomes lactate in animals. Reduced NAD passes its hydrogen to pyruvate, regenerating NAD. Without oxygen, the only ATP available is the 2 ATP net yield from glycolysis.

Where glycolysis happens

Glycolysis happens in the cytoplasm, not in a mitochondrion. Glucose does not enter a mitochondrion. Pyruvate, the product of glycolysis, enters for the next stage when oxygen is available. The link reaction and Krebs cycle happen in the mitochondrial matrix.