The stages of cellular respiration (glycolysis, the citric acid cycle and the electron transport chain), the role of ATP, NAD and dehydrogenase enzymes, the net energy yield of glycolysis, and the use of alternative respiratory substrates and fermentation in the absence of oxygen.

What this key area is asking

The SQA wants you to describe the three stages of cellular respiration, explain the roles of ATP, NAD and dehydrogenase enzymes, state the net energy yield of glycolysis, and describe how alternative substrates and fermentation pathways are used when oxygen is in short supply.

ATP, NAD and dehydrogenases

Dehydrogenase enzymes remove hydrogen ions and electrons from respiratory substrates and pass them to the hydrogen carrier NAD, forming NADH. NADH delivers the hydrogen to the electron transport chain. This handover is the key to respiration: the energy in glucose is gradually released by stripping hydrogen from intermediate molecules and using it later to drive ATP synthesis.

Glycolysis

During glycolysis, dehydrogenase enzymes also remove hydrogen from intermediate molecules, so NADH is produced as well as the net two ATP. Because glycolysis needs no oxygen, it is the only stage that can continue when oxygen is short.

The citric acid cycle

In the presence of oxygen, pyruvate moves into the matrix of the mitochondrion and is broken down. The citric acid cycle then:

• releases carbon dioxide as a waste product,
• removes hydrogen (collected by NAD as NADH),
• and generates a small amount of ATP directly.

The main purpose of the cycle is not to make ATP directly but to strip hydrogen from the respiratory substrate and load it onto NAD, ready for the electron transport chain.

The electron transport chain

As electrons pass along the chain, hydrogen ions are pumped across the membrane, building up a concentration gradient. Their flow back through the enzyme ATP synthase drives ATP production. Oxygen is the final acceptor of the electrons and hydrogen, forming water. This is why oxygen is essential: without it the chain backs up, NADH cannot be unloaded, and the whole process of aerobic respiration grinds to a halt.

Respiration without oxygen

If oxygen is absent, only glycolysis can run, so fermentation regenerates NAD so glycolysis can continue:

• In animal cells, pyruvate is converted to lactate.
• In plant cells and yeast, pyruvate is converted to ethanol and carbon dioxide.

The point of fermentation is to regenerate NAD from NADH so that glycolysis can keep making its small amount of ATP. Fermentation yields only the two ATP from glycolysis, far less than aerobic respiration. Alternative substrates such as fats and proteins can also be respired by entering the pathways at different points; fats yield more energy per gram than carbohydrates.

Examples in context

Example 1. Lactate build-up during sprinting. During a 400 metre sprint, an athlete's muscles cannot get enough oxygen to meet the demand for ATP, so they respire anaerobically. Pyruvate is converted to lactate, allowing glycolysis to keep producing a small amount of ATP. The lactate causes the burning sensation and fatigue in the muscles, and afterwards the athlete breathes heavily to supply the oxygen needed to break the lactate back down. This shows how fermentation buys time when oxygen runs short.

Example 2. Yeast fermentation in baking and brewing. In bread-making and brewing, yeast respires anaerobically, converting sugars to ethanol and carbon dioxide. The carbon dioxide makes bread dough rise, while the ethanol is the alcohol in beer and wine. Industrially, the same fermentation is carried out in large fermenters, showing how an understanding of respiration without oxygen underpins major food industries.

Try this

Q1. State the net ATP yield of glycolysis and where it occurs. [2 marks]

• Cue. A net gain of two ATP, in the cytoplasm.

Q2. Explain why much more ATP is produced in aerobic than in anaerobic respiration. [2 marks]

• Cue. With oxygen the citric acid cycle and electron transport chain also run, generating most of the ATP; without oxygen only glycolysis operates.