Cellular Respiration: Study Guide (Biology 3.5)

Why cells respire

Every cell needs a steady supply of ATP to run active transport, build molecules, move and divide. ATP is not stored in large amounts; a cell remakes it from ADP and inorganic phosphate (Pᵢ) all the time. The energy to do this comes from food molecules such as carbohydrates, fats and proteins. Cellular respiration is the set of enzyme-catalysed reactions that releases that energy in small, controlled steps and uses it to make ATP.

Respiration, and its oxygen-free relative fermentation, are found in all forms of life: bacteria, archaea, fungi, plants and animals. That is strong evidence that these pathways are very old and were inherited from a common ancestor.

For glucose, the overall change in aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy released: part captured as ATP, the rest lost as heat)

The equation hides many steps. You do not need to memorise the steps, the intermediate molecules or the enzyme names. You do need to know where each stage happens, what goes in and out, and how the stages are linked.

Oxidation, reduction and electron carriers

Energy is released when electrons move from food molecules toward oxygen, which attracts electrons strongly. Two words describe this:

• Oxidation is the loss of electrons (often with hydrogen). Glucose is oxidised to CO₂.
• Reduction is the gain of electrons. Oxygen is reduced to water.

Cells do not pass electrons straight from glucose to oxygen: that would release the energy in one burst, mostly as heat. Instead, electrons are picked up by coenzymes that act as carriers:

• NAD⁺ accepts electrons (and a proton) to become NADH.
• FAD accepts electrons to become FADH₂.

Think of NADH and FADH₂ as charged batteries. They carry electrons from the early stages to the electron transport chain, where the energy is used.

The four stages and where they happen

Stage 1: glycolysis (cytosol)

Glycolysis happens in the cytosol of every cell. It splits one six-carbon glucose into two three-carbon pyruvate molecules. Along the way it makes ATP from ADP and Pᵢ, and reduces NAD⁺ to NADH. Per glucose the net gain is 2 ATP and 2 NADH. Glycolysis needs no oxygen and no mitochondria, which is one reason it is thought to be the oldest part of the pathway.

Stages 2 and 3: pyruvate oxidation and the Krebs cycle (mitochondrial matrix)

If oxygen is available, pyruvate is transported from the cytosol into the mitochondrion. In the matrix (the fluid inside the inner membrane):

• each pyruvate is oxidised: one carbon leaves as CO₂ and NAD⁺ is reduced to NADH;
• the remaining two-carbon unit enters the Krebs cycle (also called the citric acid cycle), a loop of reactions that releases the rest of the carbon as CO₂, reduces more NAD⁺ to NADH and FAD to FADH₂, and makes a small amount of ATP.

Background, not for memorising: per glucose, stages 2 and 3 release all 6 carbons as 6 CO₂ (2 in pyruvate oxidation, 4 in the cycle). Together with glycolysis they produce about 10 NADH and 2 FADH₂ but only 4 ATP directly. Most of glucose’s energy is now held in those reduced carriers, not in ATP.

Stage 4: electron transport and oxidative phosphorylation (inner membrane)

This is where most ATP is made. Figure 2 shows how.

1. Electron delivery. NADH and FADH₂ give their electrons to the electron transport chain (ETC), a series of proteins in the inner mitochondrial membrane. This oxidises them back to NAD⁺ and FAD, which return to glycolysis and the Krebs cycle.
2. Redox series. Each carrier is reduced when it accepts electrons and oxidised when it passes them on. Each acceptor holds electrons more strongly than the one before, so energy is released in small steps.
3. Final acceptor. At the end, oxygen accepts the electrons and combines with protons to form water. This is the only place oxygen is used. Aerobic prokaryotes also use oxygen; anaerobic prokaryotes use another acceptor, such as nitrate or sulfate.
4. Proton pumping. The energy released moves protons (H⁺) from the matrix into the intermembrane space. This builds an electrochemical gradient: high H⁺ concentration (lower pH) outside the inner membrane, low H⁺ concentration (higher pH) inside, in the matrix.
5. Chemiosmosis. The membrane is almost impermeable to H⁺, so protons can only flow back into the matrix through ATP synthase. Their flow turns part of the enzyme and drives ADP + Pᵢ → ATP. Making ATP this way, powered by electron transport to oxygen, is oxidative phosphorylation.

Structure and function. The inner membrane is folded into cristae. Folding gives more surface area for ETC proteins and ATP synthase, so more ATP can be made per mitochondrion. Cells with high energy demand, such as muscle cells, tend to have many mitochondria with densely packed cristae.

Prokaryotes have no mitochondria. Their ETC and ATP synthase sit in the plasma membrane, and protons are pumped out of the cell across it. The chemiosmotic principle is the same.

Uncoupling: making heat instead of ATP

Normally electron transport and ATP synthesis are coupled: the ETC can only run as fast as protons flow back through ATP synthase. If protons leak back across the inner membrane by another route, the gradient is used up without making ATP. The ETC then runs fast, oxygen use rises, and the energy is released as heat. This is called uncoupling (or decoupling) of oxidative phosphorylation from electron transport.

Some endotherms use this on purpose. Brown adipose tissue (brown fat) in newborn humans and in many small and hibernating mammals contains an uncoupling protein that lets protons back into the matrix. The heat helps keep body temperature up in the cold.

Fermentation: glycolysis without oxygen

What happens if oxygen runs out? The ETC stops, because there is nothing to accept electrons at the end. NADH can no longer unload, so the cell’s small supply of NAD⁺ is used up. Without NAD⁺, glycolysis would stop too.

Fermentation solves this. NADH passes its electrons to pyruvate (or a molecule made from it), regenerating NAD⁺ so that glycolysis can keep running:

Type	Example organisms	Product(s) from pyruvate
Lactic acid fermentation	many bacteria; human muscle cells during intense exercise	lactic acid (lactate)
Alcohol fermentation	yeast and some bacteria	ethanol and CO₂

Fermentation itself makes no extra ATP. The cell gets only the 2 ATP per glucose from glycolysis, compared with roughly 30 from full aerobic respiration. Most of the energy stays locked in the lactic acid or ethanol. That is why fermenting cells use glucose so quickly.

Respiration and photosynthesis compared

Feature	Respiration (mitochondria)	Photosynthesis (chloroplasts, Topic 3.4)
ETC location	inner mitochondrial membrane	thylakoid membrane
Electrons come from	NADH and FADH₂ (from food)	water, boosted by light
Final electron acceptor	O₂ (forms water)	NADP⁺ (forms NADPH)
Side with high H⁺	intermembrane space	inside the thylakoid
ATP made by	ATP synthase: oxidative phosphorylation	ATP synthase: photophosphorylation

Worked example 1: graphing respirometer data

Question. A student measures oxygen uptake by woodlice (small land crustaceans) in a respirometer at 10 °C and at 25 °C. Each tube holds 2.5 g of woodlice. A control tube of glass beads was used to correct the readings for changes in air pressure and temperature. Corrected data (fictional):

Time / min	0	5	10	15	20	25
O₂ used at 25 °C / mL	0.00	0.14	0.27	0.41	0.55	0.68
O₂ used at 10 °C / mL	0.00	0.05	0.11	0.16	0.21	0.27

(a) Construct an appropriate graph. (b) Calculate the rate at each temperature in mL g⁻¹ h⁻¹. (c) Explain the difference.

(a) Building the graph. Check each component:

1. Type: both variables are continuous and measured over time, so a line graph (x–y plot) is right, not a bar chart.
2. Axes: independent variable (time) on x; dependent variable (O₂ used) on y. Labels include units: “Time / min” and “Volume of O₂ used / mL”.
3. Scaling: even steps that use most of the grid (here 0–25 min and 0–0.7 mL).
4. Plotting: each point placed accurately, with a different marker for each temperature.
5. Legend naming each series, and trend lines: the points lie close to straight lines, so a best-fit straight line through each set is appropriate.

(b) Rates. The lines are straight, so the gradient equals total change ÷ time (a best-fit gradient gives the same answer to 2 significant figures).

1. 25 °C: 0.68 mL ÷ 25 min = 0.0272 mL min⁻¹. × 60 = 1.632 mL h⁻¹. ÷ 2.5 g = 0.65 mL g⁻¹ h⁻¹.
2. 10 °C: 0.27 mL ÷ 25 min = 0.0108 mL min⁻¹. × 60 = 0.648 mL h⁻¹. ÷ 2.5 g = 0.26 mL g⁻¹ h⁻¹.
3. Ratio: 0.653 ÷ 0.259 ≈ 2.5, so the rate is about two and a half times higher at 25 °C.

Check the units. mL ÷ min × (min per h) ÷ g gives mL g⁻¹ h⁻¹, a rate per gram of animal, so groups of different mass could be compared fairly.

(c) Explanation. Woodlice are ectotherms: their body temperature follows their surroundings. Respiration is a series of enzyme-catalysed reactions. At 25 °C, molecules move faster and collide with active sites more often, so glycolysis, the Krebs cycle and the ETC all run faster, and oxygen, the final electron acceptor, is used up faster. The conclusion applies to this range of temperatures only; at much higher temperatures enzymes would denature and the rate would fall.

Worked example 2: predicting the effect of blocking the ETC

Question. A fictional poison, compound K, binds to the last carrier of the ETC so that electrons cannot pass to oxygen. Cultured muscle cells are measured before and after adding K (rates per million cells, nmol min⁻¹; fictional data):

Measurement	Before K	After K
O₂ consumption	12.0	0.6
Lactate released	4.0	40
ATP production	64	43

(a) Explain the change in O₂ consumption and in ATP production. (b) Explain the change in lactate. (c) Using the model values of 6 O₂ and about 30 ATP per glucose for aerobic respiration, and 2 lactate and 2 ATP per glucose for fermentation, show that the ATP figures are consistent and find how glucose use changes.

(a) With the last carrier blocked, electrons cannot reach O₂, so O₂ consumption falls by (0.6 − 12.0) ÷ 12.0 × 100 = −95%. Electrons back up along the chain, so no more protons are pumped. The proton gradient runs down, ATP synthase stops, and oxidative phosphorylation stops. ATP production falls by (43 − 64) ÷ 64 × 100 = −33%.

(b) NADH can no longer give its electrons to the ETC, so NAD⁺ runs short. The cells switch to lactic acid fermentation: NADH reduces pyruvate to lactate, regenerating NAD⁺ so that glycolysis continues. Lactate output rises tenfold (40 ÷ 4.0 = 10).

(c) Before K: aerobic glucose = 12.0 ÷ 6 = 2.0; fermented glucose = 4.0 ÷ 2 = 2.0. ATP = 2.0 × 30 + 2.0 × 2 = 64 ✓. Glucose use = 4.0 nmol min⁻¹. After K: aerobic glucose = 0.6 ÷ 6 = 0.1; fermented glucose = 40 ÷ 2 = 20. ATP = 0.1 × 30 + 20 × 2 = 43 ✓. Glucose use = 20.1 nmol min⁻¹.

So the cells use about 5 times as much glucose (20.1 ÷ 4.0 ≈ 5.0) yet make a third less ATP. Fermentation keeps the cells alive for a while, but it is a costly back-up.

Interpretation. The prediction rests on a chain of cause and effect: block electron transfer → no proton pumping → no gradient → no oxidative phosphorylation → NADH builds up → fermentation. In an exam, write every link; marks are usually lost by jumping from “ETC blocked” straight to “less ATP”.

Common misconceptions

• “Plants photosynthesise; animals respire.” Plant cells respire all the time. Respiration happens in all living things.
• “Respiration means breathing.” Breathing moves air in and out of lungs. Cellular respiration is a chemical process in cells.
• “Oxygen is used in the Krebs cycle.” Oxygen is used only at the end of the ETC. The Krebs cycle stops without oxygen only because NAD⁺ and FAD are not regenerated.
• “The CO₂ we breathe out comes from the oxygen we breathe in.” The carbon and oxygen in CO₂ come from the food molecules (and water). Inhaled O₂ ends up in water.
• “The ETC makes ATP.” The ETC pumps protons. ATP synthase makes the ATP, driven by protons flowing back.
• “Protons are pumped into the matrix.” They are pumped out of the matrix into the intermembrane space. The matrix has the higher pH.
• “Fermentation produces extra ATP.” It produces none; it only regenerates NAD⁺ so glycolysis can keep making 2 ATP per glucose.
• “Anaerobic respiration and fermentation are the same thing.” Anaerobic respiration uses an ETC with a non-oxygen final acceptor; fermentation has no ETC.
• “Uncoupling is always harmful.” Brown fat uncouples on purpose to produce heat.

Where this leads

Respiration completes Unit 3: compare it carefully with Topic 3.4, photosynthesis, which uses the same kind of chemiosmotic machinery but captures energy from light instead of releasing it from food. The ATP made here powers the signalling and cell division in Unit 4, starting with Topic 4.1, cell communication. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.