Cellular Respiration and Energy

This topic sits just above where most General Science questions land, so the test asks for the outline rather than the chemistry. Learn the three stages, where each happens, and what goes in and out, and you have what the subtest wants.

The overall equation

Glucose + oxygen produces carbon dioxide + water + energy (ATP).

In symbols: C6H12O6 + 6 O2 gives 6 CO2 + 6 H2O + ATP.

This is photosynthesis run backward, and that is worth noticing, because the two equations are each other's mirror and the test asks you to tell them apart.

Plants do both. They make glucose in the chloroplasts and then release its energy in the mitochondria, exactly as an animal does. That is the most missed fact in this pair of topics.

ATP

Adenosine triphosphate is the molecule cells use to carry energy where it is needed.

Energy is stored in the bond holding the third phosphate. Breaking that bond turns ATP into ADP and releases usable energy. Respiration does the reverse, attaching a phosphate back onto ADP to recharge it.

Think of it as a rechargeable battery rather than a fuel: it is spent and restored continuously rather than consumed once.

The three stages

Enlarge

Zoom in

1. Glycolysis

Where: the cytoplasm. Oxygen needed: no. Input: one glucose molecule. Output: two pyruvate molecules, and a net gain of 2 ATP.

Glucose is split in half. This is the only stage that runs without oxygen, which is why it is also the first stage of fermentation.

Between stages 1 and 2: pyruvate oxidation. Each pyruvate is carried into the mitochondrion and converted to a molecule called acetyl-CoA, giving off carbon dioxide as it goes. Acetyl-CoA is what the Krebs cycle actually runs on. It is a short step, and it is asked by name.

2. The Krebs cycle

Also called the citric acid cycle.

Where: the mitochondrial matrix. Oxygen needed: yes, indirectly. Input: the products of glycolysis, after conversion. Output: carbon dioxide, a small amount of ATP, and electron carriers.

The electron carriers are NADH and FADH2. Their job is to collect high-energy electrons here and deliver them to stage 3, which is where the energy is actually cashed in. From one glucose, the Krebs cycle loads 6 NADH.

This is where the carbon dioxide you exhale is produced. That is a useful hook, because it is the stage's most concrete output.

3. The electron transport chain

Where: the inner mitochondrial membrane. Oxygen needed: yes, directly. Output: the great majority of the ATP, and water.

Oxygen's role is to accept electrons at the end of the chain, and in doing so it combines with hydrogen to form water. That is why you breathe.

Without oxygen the chain backs up and stops, which halts the Krebs cycle too, leaving only glycolysis.

The total

About 36 to 38 ATP per glucose molecule across all three stages, against 2 from glycolysis alone.

That ratio is the point of the topic. Aerobic respiration yields roughly eighteen times as much energy as the anaerobic route, which is why organisms that can use oxygen do.

Without oxygen: fermentation

When oxygen runs out, glycolysis continues and its products are dealt with differently.

In animals and in human muscle: lactic acid fermentation. Glucose becomes lactic acid, yielding only the 2 ATP from glycolysis. This is what happens in muscles working harder than the blood can supply oxygen for.

In yeast and some bacteria: alcoholic fermentation. Glucose becomes ethanol and carbon dioxide. The carbon dioxide is what makes bread rise and the ethanol is what brewing is for.

What you can skip

The textbook chapter on respiration runs far deeper than the questions do. Across all 2,104 General Science questions in our bank, none mentions:

• The names of the Krebs cycle's intermediates. Citrate, oxaloacetate and the rest appear nowhere. Know what goes in (acetyl-CoA), what comes out (carbon dioxide, NADH, FADH2, a little ATP) and where it happens.
• How the ATP is physically made. ATP synthase, chemiosmosis and the proton gradient are not asked. "The electron transport chain makes most of the ATP" is the whole of it.
• Oxidative against substrate-level phosphorylation. Not asked.

What is asked, over and over, is where each stage happens, whether it needs oxygen, and what it makes. The table above is the topic.

Where people lose points

Saying respiration creates energy. It releases and transfers it.

Thinking plants do not respire. They do, constantly, including at night when photosynthesis has stopped.

Confusing cellular respiration with breathing. Breathing moves air; cellular respiration is a chemical process inside cells. The test uses the word for the second.

Putting glycolysis in the mitochondria. It is in the cytoplasm.

Saying oxygen is used to break down glucose. Oxygen accepts electrons at the end of the chain; glucose is broken down in the earlier stages.

Forgetting that fermentation still produces some ATP. It produces 2, not none.

Work one in under a minute

A runner sprints until their legs burn. Which process is producing the majority of their ATP at that moment, and what is the byproduct?

Sprinting outruns the oxygen supply, so the muscles are working anaerobically. That means glycolysis followed by lactic acid fermentation, and the byproduct is lactic acid.

Note the cost: 2 ATP per glucose instead of about 36, which is exactly why a sprint cannot be sustained.

Where this leads

Respiration and photosynthesis are one pair, and the mitochondria that run it are the organelle the cell lesson introduced.