Cellular Respiration

Cellular respiration breaks down glucose step by step to make ATP. Glycolysis, the TCA cycle, and the electron transport chain yield up to about 38 ATP when oxygen is plentiful. Without enough oxygen only fermentation runs and gives 2 ATP, and lactic acid builds up in muscle. Lower the oxygen slider and watch the electron transport chain stop and total ATP change.

Burning glucose for energy

🔥 Cellular respiration = slow combustion

①Burning wood releases heat and light (combustion)

②Cells also "burn" glucose to make ATP (respiration)

③But step-by-step — otherwise the cell would burn up!

3 Stages of Cellular Respiration

O₂ supply8

💡 Try lowering O₂!

①With enough O₂: 38 ATP (glycolysis 2 + TCA 2 + ETC 34)

②Without O₂: only 2 ATP (glycolysis alone)

③This is why lactic acid builds up during intense exercise (fermentation)

Core Formula

Cellular respiration overall

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (38 ATP)

complete oxidation of 1 glucose yields up to 38 ATP

Stage comparison

Chart3 stages of respiration

Stage	Location	Substrate	Product	ATP
Glycolysis	cytoplasm	glucose	2 pyruvate	2
TCA cycle	matrix	acetyl-CoA	CO₂, NADH, FADH₂	2
Electron transport	inner membrane	NADH, FADH₂	H₂O	34

Fermentation — energy without O₂

Fermentation

ChartAlcoholic vs Lactic acid

Item	Alcoholic	Lactic acid
Organism	yeast	animal muscle cells
Final product	ethanol + CO₂	lactic acid
ATP	2	2
Uses	bread, wine, beer	yogurt, kimchi

Worked Examples and Exam Practice

Example 1

When one glucose molecule is fully broken down with plenty of oxygen, what is the maximum ATP produced? (glycolysis + TCA + electron transport)

Glycolysis yields 2 ATP and the TCA cycle yields 2 ATP.

In the electron transport chain, NADH and FADH₂ are oxidized to yield up to 34 ATP.

2 + 2 + 34 = 38 ATP.

▸ Up to 38 ATP

Most ATP (34) comes from the electron transport chain. Without oxygen, only the 2 ATP from glycolysis remain.

Example 2

In an oxygen-starved muscle cell during intense exercise, name the fermentation, its end product, and the ATP gained.

Animal muscle cells perform lactic acid fermentation when oxygen is scarce.

Glucose passes through glycolysis only to pyruvate, which is reduced to lactic acid.

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

Fermentation runs only glycolysis, so just 2 ATP. A build-up of lactic acid tires the muscle.

CSAT-style

Which statement about human cellular respiration is correct?

①Glycolysis occurs in the mitochondrial matrix

②CO₂ is released in the TCA cycle

③The electron transport chain proceeds without oxygen

④Glycolysis produces the most ATP

⑤Lactic acid fermentation occurs in mitochondria

▸ ② CO₂ is released in the TCA cycle

In the TCA cycle (mitochondrial matrix), carbon is oxidized to release CO₂ while NADH and FADH₂ form.

Glycolysis is in the cytoplasm (①), the ETC needs O₂ (③), most ATP comes from the ETC (④), and fermentation occurs in the cytoplasm (⑤).

Summary

Aerobic respiration

38 ATP

with O₂

Anaerobic fermentation

2 ATP

without O₂

🎯 Exam Points

①Glycolysis: cytoplasm, glucose → pyruvate, 2 ATP (no O₂ needed)

②TCA: matrix, releases CO₂, makes NADH/FADH₂

③ETC: inner membrane (cristae), needs O₂, 34 ATP (max)

④Without O₂ → fermentation (alcoholic or lactic), only glycolysis runs

⑤ETC final electron acceptor = O₂ → H₂O

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Cell Structure

Photosynthesis

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