Grade 11-12 (age 16-18)

high school Cellular Respiration

Cellular respiration breaks glucose down a little at a time over several steps and stores the energy released in ATP. In glycolysis, in the cytoplasm, glucose splits into two pyruvate molecules; with oxygen present, pyruvate enters the mitochondria and passes through the TCA cycle and the electron transport chain, giving up to about 32 ATP per glucose. Without oxygen the electron transport chain stops and only glycolysis keeps running in fermentation, yielding just 2 ATP, and human muscle cells make lactic acid. In the diagram you can lower the oxygen supply from 100% to 0% and see the ATP from the TCA cycle and the electron transport chain fall together as pyruvate is diverted to lactic acid.

Burning Glucose Slowly for Energy

🔥 Respiration Is Slow Burning

①When wood burns, its energy scatters all at once as heat and light

②A cell oxidizes glucose a little at a time over many steps and stores part of the energy released in ATP

③The end products, carbon dioxide and water, are the same as in burning, but enzymes take the reaction one step at a time, so the cell is not damaged

The Three Stages of Cellular Respiration

Oxygen supply100%

💡 Lowering the Oxygen

①With plenty of oxygen, glycolysis gives 2, the TCA cycle 2 and oxidative phosphorylation about 28, up to about 32 ATP per glucose

②As oxygen drops, the electron transport chain runs short of its final electron acceptor, and it slows down together with the TCA cycle

③Leftover pyruvate is turned into lactic acid (lactic acid fermentation), which regenerates NAD⁺ so that glycolysis can keep going

④With no oxygen at all, only the 2 ATP from glycolysis remain, so the cell has to use almost 16 times as much glucose for the same energy

Key Reactions

Overall Equation of Cellular Respiration

C₆H₁₂O₆ + 6O₂ + 6H₂O → 6CO₂ + 12H₂O + energy (up to about 32 ATP)

counted at about 2.5 ATP per NADH and 1.5 ATP per FADH₂ (older textbooks count 3 and 2 and give 38)

Stage comparison

Comparing the Three Stages

Stage	Location	Substrate	Product	ATP
Glycolysis	cytoplasm	glucose	2 pyruvate, 2 NADH	2
TCA cycle	mitochondrial matrix	pyruvate → acetyl-CoA	CO₂, NADH, FADH₂	2
Electron transport chain (oxidative phosphorylation)	inner mitochondrial membrane	NADH, FADH₂, O₂	H₂O	about 28

Fermentation: ATP Without Oxygen

Fermentation

Alcoholic and Lactic Acid Fermentation

Item	Alcoholic	Lactic acid
Organisms	yeast	lactic acid bacteria, human muscle cells
End products	ethanol + CO₂	lactic acid
ATP	2 per glucose	2 per glucose
Uses	bread, alcoholic drinks	yogurt, kimchi (lactic acid bacteria)

Worked Examples and Exam Practice

Example 1

With plenty of oxygen, what is the maximum number of ATP molecules made when one glucose molecule is completely broken down? (Count 2.5 ATP per NADH and 1.5 ATP per FADH₂.)

Glycolysis makes 2 ATP and the TCA cycle 2 ATP by substrate-level phosphorylation.

One glucose yields 10 NADH and 2 FADH₂, so oxidative phosphorylation makes 10 × 2.5 + 2 × 1.5 = 28 ATP.

2 + 2 + 28 = 32

▸ up to about 32 ATP

Most of the ATP (28 molecules) comes from oxidative phosphorylation through the electron transport chain. That is why lowering the oxygen to 0% in the diagram switches off the electron transport box and leaves only the 2 ATP from glycolysis.

Example 2

In human muscle cells short of oxygen during hard exercise, name the fermentation that takes place, its end product and the number of ATP gained per glucose.

Human muscle cells short of oxygen carry out lactic acid fermentation.

Glucose becomes pyruvate through glycolysis, and pyruvate takes electrons from NADH and becomes lactic acid.

▸ lactic acid fermentation; end product lactic acid; 2 ATP

In fermentation only glycolysis makes ATP, so there are just 2 ATP per glucose. Turning pyruvate into lactic acid makes no ATP, but it restores NAD⁺ so that glycolysis does not stop.

CSAT-style problem

Which statement about human cellular respiration is correct?

①Glycolysis takes place in the mitochondrial matrix.

②CO₂ is released in the TCA cycle.

③The electron transport chain runs even without oxygen.

④Glycolysis makes the most ATP.

⑤Lactic acid fermentation takes place in the mitochondria.

▸ ② CO₂ is released in the TCA cycle.

In the TCA cycle (mitochondrial matrix), carbon is oxidized and released as CO₂, and NADH and FADH₂ are made.

Glycolysis takes place in the cytoplasm (①), the electron transport chain needs O₂ (③), the most ATP comes from oxidative phosphorylation through the electron transport chain (④), and fermentation takes place in the cytoplasm (⑤).