Welcome to Cellular Respiration!

Hello and welcome to one of the most important chapters in A2 Biology! Respiration is the fundamental process that powers every single living cell. Whenever you move a muscle, send a nerve impulse, or build new proteins, your body is using energy released through respiration.

Don't worry if this topic feels a bit overwhelming with all the chemical names and pathways at first. We will break everything down into bite-sized, logical steps, use helpful everyday analogies, and highlight the exact points examiners look for in your CCEA A2 Unit 2 exam.

The Overall Equation for Aerobic Respiration:
\(\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 30\text{–}32\text{ ATP}\) (with a theoretical maximum of \(\approx 38\text{ ATP}\)).

1. ATP: The Universal Energy Currency

Think of glucose like a large-denomination banknote stored in a bank vault: it holds plenty of value, but a vending machine won't accept it. ATP (Adenosine Triphosphate) is the convenient, ready-to-use pocket change that your cells can spend instantly on metabolic work.

Structure of ATP

An ATP molecule is made of three key components:
1. A purine nitrogenous base called adenine.
2. A 5-carbon pentose sugar called ribose.
3. A chain of three phosphate groups joined together by high-energy phosphoanhydride bonds.

Why is ATP the "Universal Energy Currency"?

ATP is uniquely suited for cellular work because:
• Immediate energy release: Energy is released in a single-step hydrolysis reaction catalyzed by the enzyme ATPase:
\(\text{ATP} + \text{H}_2\text{O} \xrightarrow{\text{ATPase}} \text{ADP} + \text{P}_i + \text{Energy}\)
• Manageable quantities: Hydrolysis releases approximately \(30.6\text{ kJ mol}^{-1}\) of energy under standard physiological conditions—enough to drive cellular reactions without wasting excess energy as damaging heat.
• Soluble and easily diffused: It moves quickly throughout the cytoplasm to wherever energy is needed.
• Rapidly recycled: ATP is rapidly re-synthesized from ADP and inorganic phosphate (\(\text{P}_i\)) using the enzyme ATP synthase:
\(\text{ADP} + \text{P}_i + \text{Energy} \xrightarrow{\text{ATP synthase}} \text{ATP} + \text{H}_2\text{O}\)

Note: ATP cannot be stored in large amounts because it is unstable. Cells must synthesize it continuously to meet their immediate metabolic demands.

Key Takeaway: ATP is hydrolyzed by ATPase to release \(30.6\text{ kJ mol}^{-1}\) instantly, and is reformed by ATP synthase during respiration.

2. The Four Stages of Aerobic Respiration

Aerobic respiration takes place in four distinct, consecutive stages:
1. Glycolysis (in the cytoplasm)
2. The Link Reaction (in the mitochondrial matrix)
3. The Krebs Cycle (in the mitochondrial matrix)
4. Oxidative Phosphorylation & the Electron Transport Chain (on the inner mitochondrial membrane / cristae)

Stage 1: Glycolysis

Location: Cytoplasm (cytosol).
Oxygen Requirement: None! Glycolysis is completely anaerobic and occurs in both aerobic and anaerobic respiration.

Glycolysis splits one \(6\text{C}\) glucose molecule into two \(3\text{C}\) pyruvate molecules through three main steps:

Step 1: Phosphorylation
Glucose (\(6\text{C}\)) is relatively unreactive. To kickstart the process, \(2\text{ ATP}\) molecules are hydrolyzed to add two phosphate groups to glucose, forming Hexose Bisphosphate (Fructose-1,6-bisphosphate). This lowers the activation energy and traps the sugar inside the cell.

Step 2: Lysis
The unstable hexose bisphosphate (\(6\text{C}\)) is split directly into two molecules of Triose Phosphate (\(\text{TP}\), \(3\text{C}\)).

Step 3: Dehydrogenation (Oxidation) & ATP Formation
Each \(\text{TP}\) molecule is oxidized by removing hydrogen atoms (dehydrogenation). The coenzyme \(\text{NAD}^+\) accepts these hydrogen atoms to become reduced NAD (\(\text{NADH} + \text{H}^+\)). In the process, \(4\text{ ATP}\) molecules are produced directly via substrate-level phosphorylation, converting the two \(\text{TP}\) molecules into two molecules of Pyruvate (\(3\text{C}\)).

Glycolysis Yield per Glucose Molecule (\(6\text{C}\)):
• Gross ATP produced: \(4\text{ ATP}\)
• Net ATP yield: \(2\text{ ATP}\) (because \(2\text{ ATP}\) were invested at the start!)
• Reduced NAD: \(2\text{ NADH}\)
• Pyruvate: \(2\text{ molecules}\) (\(3\text{C}\))

Common Examiner Trap: Always read the question carefully! If asked for the gross yield, write \(4\text{ ATP}\); if asked for the net yield, write \(2\text{ ATP}\).

Stage 2: The Link Reaction

Location: Mitochondrial matrix.
If oxygen is present, pyruvate is actively transported from the cytoplasm across the outer and inner mitochondrial membranes into the matrix.

The Link Reaction converts each pyruvate (\(3\text{C}\)) into an acetyl group (\(2\text{C}\)) via oxidative decarboxylation:
1. Decarboxylation: A carbon atom is removed and released as a molecule of carbon dioxide (\(\text{CO}_2\)).
2. Dehydrogenation: Two hydrogen atoms are removed and accepted by \(\text{NAD}^+\) to form reduced NAD (\(\text{NADH}\)).
3. Combination with Coenzyme A: The remaining \(2\text{C}\) acetyl group combines with Coenzyme A (CoA) to form Acetyl Coenzyme A (Acetyl-CoA, \(2\text{C}\)).

Link Reaction Yield per Glucose Molecule (Remember: \(1\text{ glucose} = 2\text{ pyruvates}\)):
• \(2\text{ Acetyl-CoA}\) (\(2\text{C}\))
• \(2\text{ CO}_2\)
• \(2\text{ NADH}\)
• \(0\text{ ATP}\) (no ATP is directly made in this stage!)

Stage 3: The Krebs Cycle (Citric Acid Cycle)

Location: Mitochondrial matrix.
The Krebs cycle is a continuous cyclic series of enzyme-controlled redox reactions that fully oxidizes the acetyl groups.

Here is the cycle step-by-step:
1. Formation of Citrate (\(6\text{C}\)): Acetyl-CoA (\(2\text{C}\)) delivers its acetyl group to a \(4\text{C}\) acceptor molecule called Oxaloacetate, forming a \(6\text{C}\) molecule called Citrate (citric acid). Coenzyme A is released and recycled back to the Link Reaction.
2. Conversion to a \(5\text{C}\) Intermediate: Citrate undergoes oxidative decarboxylation. It loses one carbon as \(\text{CO}_2\) and two hydrogens to reduce \(\text{NAD}^+\) to \(\text{NADH}\), forming a \(5\text{C}\) compound (\(\alpha\)-ketoglutarate).
3. Substrate-Level Phosphorylation & Further Decarboxylation: The \(5\text{C}\) compound undergoes another oxidative decarboxylation, releasing a second \(\text{CO}_2\) and producing another \(\text{NADH}\). During this step, \(1\text{ ATP}\) is directly produced via substrate-level phosphorylation.
4. Regeneration of Oxaloacetate (\(4\text{C}\)): A series of oxidation reactions yields \(1\text{ FADH}_2\) (reduced FAD) and \(1\text{ NADH}\), regenerating the original \(4\text{C}\) oxaloacetate to accept another acetyl group.

Krebs Cycle Yield per Glucose Molecule (2 turns of the cycle):
• \(4\text{ CO}_2\)
• \(6\text{ NADH}\)
• \(2\text{ FADH}_2\)
• \(2\text{ ATP}\) (via substrate-level phosphorylation)

Memory Trick: Remember that 1 glucose splits into 2 pyruvates, so the Link Reaction happens twice and the Krebs Cycle turns twice per glucose molecule!

Stage 4: Oxidative Phosphorylation & The Electron Transport Chain (ETC)

Location: Inner mitochondrial membrane (cristae).
This is the grand finale of aerobic respiration where the majority of ATP is produced using the Chemiosmotic Theory.

Step-by-Step Mechanism:
1. Delivery of Hydrogen: Reduced coenzymes (\(\text{NADH}\) and \(\text{FADH}_2\)) arrive at the inner mitochondrial membrane and donate their electrons and hydrogen ions to electron carrier protein complexes, regenerating \(\text{NAD}^+\) and \(\text{FAD}\).
2. Electron Flow: High-energy electrons pass along a chain of electron carriers (the Electron Transport Chain) via a series of redox reactions at progressively lower energy levels.
3. Proton Pumping: The energy released from electron transport is used to pump protons (\(\text{H}^+\)) from the matrix across the inner membrane into the intermembrane space.
4. Electrochemical Gradient: This pumping creates a high concentration of protons in the narrow intermembrane space, setting up a steep proton / electrochemical gradient (and lower pH).
5. Chemiosmosis: Because the phospholipid bilayer is impermeable to ions, protons can only diffuse back into the matrix through channel proteins associated with ATP synthase. As protons flow down their gradient, the proton motive force drives the phosphorylation of ADP:
\(\text{ADP} + \text{P}_i \rightarrow \text{ATP}\)
6. The Terminal Electron Acceptor: Oxygen (\(\text{O}_2\)) acts as the final electron and proton acceptor at the end of the chain, combining with electrons and protons to form water:
\(4\text{H}^+ + 4e^- + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\)

Why is Oxygen Essential? If oxygen is not present to accept electrons at the end of the chain, electrons back up along the transport chain. \(\text{NADH}\) and \(\text{FADH}_2\) can no longer be reoxidized to \(\text{NAD}^+\) and \(\text{FAD}\), which causes the Krebs Cycle and Link Reaction to grind to a halt!

Key Takeaway: Oxidative phosphorylation uses energy from electron transfer along the ETC to pump protons into the intermembrane space. The diffusion of protons through ATP synthase generates ATP (chemiosmosis), while oxygen acts as the final electron acceptor forming water.

3. Anaerobic Respiration (Fermentation Pathways)

When oxygen is absent or in short supply (hypoxic or anoxic conditions), the ETC stops running. Without oxygen, the cell cannot reoxidize \(\text{NADH}\) at the inner membrane.

To keep generating at least some ATP, cells switch to anaerobic pathways in the cytoplasm. Their sole purpose is to reoxidize \(\text{NADH} \rightarrow \text{NAD}^+\) so that glycolysis can continue to produce a net yield of \(2\text{ ATP}\) per glucose.

1. The Lactate Pathway (Mammals & Strenuous Muscle Contraction)

In mammalian muscle cells working under oxygen deficit:
• Pyruvate accepts hydrogen atoms directly from \(\text{NADH}\), catalyzed by the enzyme lactate dehydrogenase:
\(\text{Pyruvate} + \text{NADH} + \text{H}^+ \xrightarrow{\text{Lactate Dehydrogenase}} \text{Lactate} + \text{NAD}^+\)
• Reversibility: Lactate is not broken down in the muscle. It is transported in the bloodstream to the liver, where it is converted back into pyruvate and glucose once oxygen is available (paying off the "oxygen debt").

2. Ethanolic Fermentation (Yeast & Higher Plant Tissues)

In yeast cells and flooded plant roots:
• Step 1: Pyruvate (\(3\text{C}\)) is decarboxylated by pyruvate decarboxylase to form ethanal (\(2\text{C}\)) and carbon dioxide (\(\text{CO}_2\)):
\(\text{Pyruvate } (3\text{C}) \xrightarrow{\text{Pyruvate decarboxylase}} \text{Ethanal } (2\text{C}) + \text{CO}_2\)
• Step 2: Ethanal accepts hydrogen atoms from \(\text{NADH}\), catalyzed by alcohol dehydrogenase, forming ethanol (\(2\text{C}\)) and regenerating \(\text{NAD}^+\):
\(\text{Ethanal} + \text{NADH} + \text{H}^+ \xrightarrow{\text{Alcohol dehydrogenase}} \text{Ethanol } (2\text{C}) + \text{NAD}^+\)
• Irreversibility: This pathway is irreversible. If ethanol accumulates to high concentrations, it becomes toxic to yeast cells.

Key Takeaway: Anaerobic respiration yields only \(2\text{ ATP}\) per glucose (from glycolysis). Its critical role is reoxidizing \(\text{NADH}\) to \(\text{NAD}^+\) so glycolysis does not shut down.

4. Alternative Respiratory Substrates & Respiratory Quotients (RQ)

While glucose is the primary fuel, cells can break down other organic molecules to generate ATP.

Lipids (Triglycerides)

• Hydrolyzed into glycerol and fatty acids.
• Glycerol is phosphorylated and converted to triose phosphate (\(\text{TP}\)), entering glycolysis.
• Fatty acid chains are broken down into \(2\text{C}\) acetyl fragments via beta-oxidation, which enter the Krebs cycle as Acetyl-CoA.
• Energy Yield: Lipids provide more than double the energy per gram compared to carbohydrates. Why? Fatty acids have a much higher proportion of hydrogen atoms per gram, generating more reduced coenzymes and a larger proton gradient during oxidative phosphorylation!

Proteins

• Hydrolyzed into amino acids.
• Amino acids undergo deamination in the liver (the amine group is removed and converted to urea).
• The remaining carbon skeletons enter glycolysis or the Krebs cycle at different intermediate stages depending on their carbon chain length.

The Respiratory Quotient (RQ)

The Respiratory Quotient indicates which respiratory substrate an organism is using and whether respiration is aerobic or anaerobic.

The RQ Formula:
\(\text{RQ} = \frac{\text{Volume or moles of }\text{CO}_2\text{ produced}}{\text{Volume or moles of }\text{O}_2\text{ consumed}}\)

Standard RQ Values to Memorize:
• Carbohydrates (Glucose): \(\text{RQ} = 1.0\)
Proof: \(\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} \implies 6/6 = 1.0\)
• Lipids (Fatty acids): \(\text{RQ} \approx 0.7\) (more \(\text{O}_2\) is needed to oxidize the hydrogen-rich chains)
• Proteins: \(\text{RQ} \approx 0.8\text{–}0.9\)
• Anaerobic Respiration in Yeast: \(\text{RQ} = \infty\) (or \(> 1.0\), because \(\text{CO}_2\) is produced while zero \(\text{O}_2\) is consumed: \(\text{CO}_2 / 0 = \infty\))

Key Takeaway: \(\text{RQ} = 1.0\) indicates carbohydrate use, \(\text{RQ} \approx 0.7\) indicates lipid use, and \(\text{RQ} > 1.0\) or \(\infty\) indicates anaerobic respiration in yeast.

5. Respirometry & Practical Skills

A respirometer measures the rate of oxygen consumption of respiring living organisms (such as germinating seeds or blowfly larvae).

How a Respirometer Works:

1. Living organisms are placed in a sealed boiling tube connected to a manometer tube containing colored liquid.
2. Potassium hydroxide (\(\text{KOH}\)) or sodium hydroxide (\(\text{NaOH}\)) pellets/solution are placed in the bottom of the tube to absorb all \(\text{CO}_2\) produced by the organisms.
3. As the organisms respire, they absorb \(\text{O}_2\) and release \(\text{CO}_2\). Because the \(\text{CO}_2\) is absorbed by \(\text{KOH}\), there is a net decrease in air volume and pressure inside the experimental tube.
4. This drop in pressure causes the manometer fluid to move towards the organism tube.
5. The distance moved by the fluid directly represents the volume of \(\text{O}_2\) consumed.

Calculating Rate of Respiration:
\(\text{Volume of }\text{O}_2 = \pi r^2 \times \text{distance moved}\)
\(\text{Rate of Respiration} = \frac{\pi r^2 \times \text{distance moved}}{\text{time taken}}\)
(where \(r\) is the internal radius of the capillary lumen).

Essential Experimental Controls:

• Thermobarometer / Inert Control: A second identical tube containing non-respiring glass beads of equal volume is set up. This controls for changes in environmental temperature and atmospheric pressure that could expand or contract the air inside the apparatus.
• Water Bath: The apparatus is kept in a thermostatically controlled water bath to maintain a constant temperature.

Measuring RQ Using a Respirometer:

To calculate RQ, two experimental runs are conducted:
• Run 1 (With \(\text{KOH}\)): Measures the volume of \(\text{O}_2\) consumed alone (since \(\text{CO}_2\) is absorbed).
• Run 2 (Without \(\text{KOH}\) / With Water): Measures the net difference between \(\text{O}_2\) consumed and \(\text{CO}_2\) produced.
• The volume of \(\text{CO}_2\) produced is calculated by comparing both runs, allowing \(\text{RQ} = \frac{\text{CO}_2\text{ produced}}{\text{O}_2\text{ consumed}}\) to be calculated.

6. Common Exam Pitfalls & Mistakes to Avoid

• Confusing Phosphorylation Types: Substrate-level phosphorylation is the direct transfer of a phosphate group to ADP by an enzyme (occurs in Glycolysis and Krebs cycle). Oxidative phosphorylation is driven by the proton gradient established by the electron transport chain in the cristae.
• Forgetting to Double Krebs Products: Always remember that one glucose produces two pyruvates, so multiply the products of the Link reaction and Krebs cycle by two when calculating yields per glucose molecule.
• Misunderstanding the Role of Oxygen: Oxygen does not react directly with glucose, nor is it involved in Glycolysis or the Link reaction. Oxygen is strictly the terminal electron and proton acceptor at the end of the ETC.
• Respirometer Controls: In practical questions, always mention using inert glass beads of equal volume and a temperature-controlled water bath to ensure valid, reliable results.

Quick Summary Checklist:
• Glycolysis occurs in cytoplasm: Net \(2\text{ ATP}\), \(2\text{ NADH}\), \(2\text{ pyruvate}\).
• Link Reaction occurs in matrix: \(2\text{ Acetyl-CoA}\), \(2\text{ CO}_2\), \(2\text{ NADH}\).
• Krebs Cycle occurs in matrix: \(4\text{ CO}_2\), \(6\text{ NADH}\), \(2\text{ FADH}_2\), \(2\text{ ATP}\).
• Oxidative phosphorylation occurs on cristae: Chemiosmosis via ATP synthase, \(4\text{H}^+ + 4e^- + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\).
• Anaerobic respiration regenerates \(\text{NAD}^+\) (Lactate in mammals; Ethanol + \(\text{CO}_2\) in yeast).
• \(\text{RQ} = \text{CO}_2\text{ produced} / \text{O}_2\text{ consumed}\) (Carb = 1.0, Lipid \(\approx 0.7\), Protein \(\approx 0.8\text{–}0.9\)).