Cellular respiration (SACE Stage 2 Biology)

What this dot point is asking

You need to explain the purpose of respiration, write its overall equation, compare aerobic and anaerobic pathways, and identify where they occur and what they produce.

Why cells respire

Cells need a constant supply of usable energy for processes like active transport, muscle contraction and synthesis. Cellular respiration transfers the chemical energy in glucose to ATP (adenosine triphosphate), which cells use directly. Respiration is a controlled, stepwise release of energy, not a single burst.

Aerobic respiration

Aerobic respiration uses oxygen and gives the highest ATP yield. Its overall word and balanced equations are:

glucose + oxygen gives carbon dioxide + water + energy (ATP)

It happens in stages:

1. Glycolysis in the cytoplasm splits glucose into two pyruvate molecules, producing a small amount of ATP. This step does not need oxygen.
2. The remaining stages (the Krebs cycle and electron transport chain) occur in the mitochondria and require oxygen. They release most of the ATP.

The large surface area of the inner mitochondrial membrane (cristae) supports these reactions, which is why active cells have many mitochondria.

Anaerobic respiration

When oxygen is unavailable or in short supply, cells use anaerobic respiration. It begins with glycolysis (the only ATP-producing step), so the yield is low, just 2 ATP per glucose. The pathway differs between organisms:

• In animal cells (and human muscle during intense exercise): pyruvate is converted to lactate (lactic acid). Build-up of lactate contributes to muscle fatigue and is later broken down when oxygen returns.
• In yeast and plant cells: pyruvate is converted to ethanol and carbon dioxide in fermentation, used in brewing and baking.

Anaerobic respiration releases less energy because glucose is not fully broken down. In lactate fermentation no carbon dioxide is produced and the products still hold most of glucose's chemical energy; in alcoholic fermentation the carbon dioxide released is what makes bread rise and gives fermented drinks their fizz. In both cases the point of the conversion is not to extract more energy but to regenerate the molecule glycolysis needs to keep running, so a small trickle of ATP can continue without oxygen.

Comparing the two pathways

A clean comparison is what SACE markers reward. Hold the pathways side by side:

• Oxygen: aerobic requires it; anaerobic runs without it.
• Location: aerobic uses the cytoplasm (glycolysis) then the mitochondria; anaerobic uses only the cytoplasm.
• ATP yield: aerobic is high (about $36$ to $38$ ATP per glucose); anaerobic is low ($2$ ATP per glucose).
• Products: aerobic gives carbon dioxide and water; anaerobic gives lactate (animals) or ethanol and carbon dioxide (yeast, plants).
• Completeness: aerobic fully oxidises glucose; anaerobic only partly breaks it down, leaving energy locked in lactate or ethanol.

Why the mitochondrion is built the way it is

The structure-function link is a common short-answer target. The inner mitochondrial membrane is folded into cristae, greatly increasing the surface area available for the electron transport chain. The fluid-filled matrix holds the enzymes of the Krebs cycle. Cells with high energy demands, such as muscle and liver cells, contain large numbers of mitochondria, each with many cristae, so they can sustain a high rate of aerobic ATP production. This is the same surface-area-for-rate logic that runs through the rest of Topic 2.