Topic 3.5 Notes – Cellular Respiration

1. What Cellular Respiration Is

At its core, cellular respiration is energy transfer. The chemical energy in glucose is not used directly. Instead, electrons are removed from glucose and passed through a series of carriers to eventually make ATP, the cell’s usable energy currency.

Big picture flow:

Glucose → NADH/FADH₂ → Electron Transport Chain → Proton Gradient → ATP

Key features you need to know:

• Occurs in all organisms
  • Eukaryotes use mitochondria
  • Prokaryotes use their plasma membrane
• Involves oxidation-reduction (redox) reactions
  • Glucose is oxidized
  • Electron carriers like NAD⁺ and FAD are reduced
• In aerobic respiration, oxygen is the terminal electron acceptor, forming water

ATP yield per glucose:

• Aerobic respiration: about 30-36 ATP
• Fermentation: 2 ATP (from glycolysis only)

That huge difference in ATP output is why oxygen matters so much.

2. The Stages of Cellular Respiration

Each stage extracts more energy and passes electrons forward.

a. Glycolysis (Cytoplasm)

Glycolysis splits one 6‑carbon glucose into two 3‑carbon pyruvate molecules.

Per glucose:

• 2 ATP (net)
• 2 NADH
• 2 pyruvate

Important details:

• Does not require oxygen
• NAD⁺ is reduced to NADH
• Happens in the cytosol of all cells

This is the only ATP you get if oxygen is absent.

b. Pyruvate Oxidation

Pyruvate moves into the mitochondrion (in eukaryotes).

Each pyruvate:

• Is converted to acetyl CoA
• Releases CO₂
• Produces NADH

This step links glycolysis to the Krebs cycle.

c. Krebs Cycle (Mitochondrial Matrix)

The Krebs cycle fully oxidizes acetyl CoA.

Per glucose (2 turns of the cycle):

• 6 NADH
• 2 FADH₂
• 2 ATP
• 4 CO₂

Most of the energy is now stored in NADH and FADH₂, not ATP yet. That sets up the most important stage.

d. Electron Transport Chain and Oxidative Phosphorylation

This happens in the inner mitochondrial membrane and produces most ATP.

Electron flow

• NADH and FADH₂ donate electrons
• Electrons move through protein complexes via redox reactions
• Oxygen accepts the electrons at the end → forms H₂O

Proton gradient formation

As electrons move, protons (H⁺) are pumped:

• From matrix → intermembrane space
• Intermembrane space = high H⁺
• Matrix = low H⁺, higher pH

Chemiosmosis

Protons flow back through ATP synthase, driving:

$$\text{ADP} + \text{Pi} \rightarrow \text{ATP}$$

This ATP production powered by an electron-driven gradient is called oxidative phosphorylation.

Students often mix this up:

• Electron transport builds the gradient
• Chemiosmosis uses the gradient to make ATP

They are connected but not the same thing.

3. Mitochondrial Structure and Why It Matters

Structure directly enables function here. As you review the diagram, connect each labeled region to the step of cellular respiration it supports.

Structure of a mitochondrion

Outer membrane

• Encloses the organelle

Inner membrane

• Contains ETC proteins and ATP synthase
• Folded into cristae
• More folds = more surface area = more ATP production

Intermembrane space

• Stores pumped H⁺
• High proton concentration

Matrix

• Contains enzymes for Krebs cycle
• Lower H⁺ concentration (higher pH)

Compartmentalization allows a proton gradient to exist. Without separate spaces, chemiosmosis would not work.

In prokaryotes, there is no mitochondrion. The ETC is embedded in the plasma membrane, and the proton gradient forms across it.

4. Fermentation and Anaerobic Respiration

If oxygen is unavailable, the ETC stops. That creates a problem.

Glycolysis requires NAD⁺. Without oxygen, NADH cannot unload electrons to the ETC. Cells must regenerate NAD⁺ another way.

Lactic acid fermentation

• Pyruvate + NADH → Lactate + NAD⁺
• Occurs in muscle cells during low O₂
• Still only 2 ATP per glucose

Alcoholic fermentation

• Pyruvate → Ethanol + CO₂
• Regenerates NAD⁺
• Used by yeast

Fermentation keeps glycolysis running but does not increase ATP yield.

5. Key Concepts About Electron Transport and Heat

Redox basics

• Oxidation = loss of electrons
• Reduction = gain of electrons
• Oxygen is reduced at the end of the ETC

Anaerobic prokaryotes may use other molecules as terminal electron acceptors, but you do not need to memorize specific ones for AP.

Heat generation

If the proton gradient is dissipated without ATP synthase making ATP, the energy is released as heat.
Endotherms use this mechanism for thermoregulation.