pastpaperbd/ Biology/ Notes/ Respiration
T5

Respiration

Respiration is the process by which cells release energy from organic molecules, primarily glucose, to synthesise ATP (adenosine triphosphate). ATP is the universal energy currency of the cell - it is used to drive all energy-requiring processes, from active transport and muscle contraction to biosynthesis and nerve impulse transmission. AQA's definition of respiration focuses on ATP synthesis, not simply "releasing energy," so always frame it in terms of ATP.

Aerobic respiration uses oxygen and completely oxidises glucose to CO₂ and water, yielding a large amount of ATP. Anaerobic respiration occurs without oxygen and is far less efficient, yielding only 2 ATP per glucose. The two pathways share the first stage (glycolysis) but diverge after pyruvate.

ATP

ATP is a nucleotide consisting of adenine, ribose, and three phosphate groups. Energy is stored in the bonds between the phosphate groups - specifically the terminal phosphate bond. When ATP is hydrolysed to ADP + Pᵢ by ATP hydrolase, approximately 30.5 kJ mol⁻¹ is released. This reaction is coupled to energy-requiring processes in the cell.

ATP is resynthesised from ADP and Pᵢ by ATP synthase, driven by the movement of protons (H⁺) down their electrochemical gradient through the enzyme - a process called chemiosmosis. This occurs in the inner mitochondrial membrane during oxidative phosphorylation.

ATP is the immediate energy source for all cellular work - not glucose. Glucose cannot directly power reactions; it must first be broken down to generate ATP.

Coenzymes: NAD and FAD

Respiration relies on two key coenzymes to carry hydrogen (as protons + electrons) between the stages:

  • NAD (nicotinamide adenine dinucleotide) - accepts 2H (2 electrons + 2 protons) to become reduced NAD (NADH). NAD is used in glycolysis, the link reaction, and the Krebs cycle.
  • FAD (flavin adenine dinucleotide) - similarly accepts 2H to become reduced FAD (FADH₂). FAD is used specifically in the Krebs cycle (at the succinate → fumarate step).

These reduced coenzymes then donate their electrons to the electron transport chain, driving ATP synthesis. Without NAD and FAD being continually re-oxidised (regenerated), the stages of respiration would grind to a halt.

Stage 1 - Glycolysis

Glycolysis occurs in the cytoplasm and does not require oxygen. It is the universal first stage of respiration in virtually all living organisms, reflecting its ancient evolutionary origin.

Glucose (a 6-carbon molecule) is broken down to two molecules of pyruvate (a 3-carbon molecule) in a series of enzyme-controlled reactions.

The net products per glucose molecule are:

  • 2 ATP (net - 4 are produced but 2 are used in the energy investment phase)
  • 2 reduced NAD
  • 2 pyruvate

The process has two phases:

  1. Energy investment phase - 2 ATP are used to phosphorylate glucose, making it more reactive (forming glucose-6-phosphate, then fructose-1,6-bisphosphate). This destabilises the 6-carbon molecule, allowing it to be split.
  2. Energy pay-off phase - the 6-carbon intermediate is split into two triose phosphate (3-carbon) molecules, which are oxidised. This generates 4 ATP (by substrate-level phosphorylation) and 2 reduced NAD.

Substrate-level phosphorylation means ATP is made directly by transferring a phosphate group from a phosphorylated intermediate to ADP - this is distinct from oxidative phosphorylation and does not require the electron transport chain.

Stage 2 - Link Reaction

The link reaction occurs in the mitochondrial matrix and connects glycolysis to the Krebs cycle. It is not technically part of the Krebs cycle but is always covered alongside it.

Per pyruvate molecule:

  • Pyruvate is decarboxylated (CO₂ removed) to form a 2-carbon acetyl group
  • The acetyl group is oxidised, reducing NAD to reduced NAD
  • The acetyl group combines with coenzyme A (CoA) to form acetyl-CoA

Since there are 2 pyruvate per glucose, per glucose the link reaction produces:

  • 2 CO₂
  • 2 reduced NAD
  • 2 acetyl-CoA

Stage 3 - Krebs Cycle

The Krebs cycle (also called the citric acid cycle) takes place in the mitochondrial matrix. Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), which is then progressively broken down through a series of oxidation and decarboxylation reactions until oxaloacetate is regenerated - completing the cycle.

Per turn of the cycle (= per acetyl-CoA = per pyruvate):

  • 2 CO₂ released (by decarboxylation)
  • 3 reduced NAD produced
  • 1 reduced FAD produced
  • 1 ATP produced (by substrate-level phosphorylation)

Since 2 acetyl-CoA enter per glucose, per glucose the Krebs cycle produces:

  • 4 CO₂
  • 6 reduced NAD
  • 2 reduced FAD
  • 2 ATP

The cycle regenerates oxaloacetate at the end of each turn, allowing it to continue accepting acetyl-CoA. All the carbon from glucose has now been released as CO₂ - the remaining energy is held in the reduced coenzymes, which pass it to the electron transport chain.

Stage 4 - Oxidative Phosphorylation

Oxidative phosphorylation takes place on the inner mitochondrial membrane and is responsible for the vast majority of ATP produced in aerobic respiration.

The reduced NAD and reduced FAD from the earlier stages donate their electrons to protein complexes (electron carriers) embedded in the inner mitochondrial membrane, collectively called the electron transport chain (ETC). As electrons pass along the chain, they lose energy, which is used to pump H⁺ ions (protons) from the matrix into the intermembrane space, creating an electrochemical gradient.

The protons then flow back into the matrix down their concentration gradient through the enzyme ATP synthase (also called the F₁F₀-ATPase). This flow of protons drives the rotation of ATP synthase, catalysing the synthesis of ATP from ADP + Pᵢ. This coupling of proton flow to ATP synthesis is called chemiosmosis, and the proton gradient across the inner mitochondrial membrane is called the proton-motive force.

At the end of the electron transport chain, the electrons are passed to oxygen, which acts as the final electron acceptor. Oxygen combines with the electrons and H⁺ ions to form water:

½O₂ + 2H⁺ + 2e⁻ → H₂O

This is why oxygen is essential for aerobic respiration - without it, the ETC backs up, electrons cannot flow, protons cannot be pumped, and ATP synthesis via chemiosmosis stops.

ATP yield:

  • Each reduced NAD yields approximately 2.5 ATP
  • Each reduced FAD yields approximately 1.5 ATP

Per glucose (approximate totals):

  • Glycolysis: 2 ATP + 2 NADH → ~7 ATP total
  • Link reaction: 2 NADH → ~5 ATP
  • Krebs cycle: 2 ATP + 6 NADH + 2 FADH₂ → ~22 ATP total
  • Overall: ~30 - 32 ATP per glucose (AQA accepts ~32)

Note: older textbooks quote 36 - 38 ATP using a P/O ratio of 3 for NADH. AQA uses the more accurate chemiosmotic values (~2.5 per NADH, ~1.5 per FADH₂), giving ~30 - 32.

Anaerobic Respiration`

When oxygen is absent or insufficient, the electron transport chain cannot function (no final electron acceptor). Reduced NAD accumulates and NAD becomes unavailable, meaning glycolysis - which requires NAD - would also stop.

To regenerate NAD and keep glycolysis running, organisms use anaerobic pathways. These do not produce additional ATP beyond glycolysis's 2 ATP but allow ATP production to continue in the absence of oxygen.

In animals and some bacteria - lactate fermentation:

  • Pyruvate is reduced to lactate by the enzyme lactate dehydrogenase
  • Reduced NAD is oxidised back to NAD in the process
  • Lactate accumulates in muscle tissue during intense exercise

In yeast and plants - alcoholic fermentation:

  • Pyruvate is first decarboxylated to ethanal (acetaldehyde) by pyruvate decarboxylase (CO₂ released)
  • Ethanal is then reduced to ethanol by ethanol dehydrogenase
  • Reduced NAD is oxidised back to NAD in the process
  • Ethanol and CO₂ are the end products - the basis of brewing and baking

In both cases, the key point is NAD regeneration, not the specific products. AQA mark schemes will always credit "to regenerate NAD / so glycolysis can continue."

The Mitochondrion and Adaptations for Respiration

The structure of the mitochondrion is closely linked to its function (see Cell Structure for organelle detail):

  • Double membrane - the outer membrane is freely permeable; the inner membrane is highly folded into cristae, greatly increasing surface area for the ETC and ATP synthase
  • Matrix - contains the enzymes for the Krebs cycle and link reaction, as well as mitochondrial DNA and ribosomes (evidence for endosymbiotic origin)
  • Intermembrane space - the narrow space between the two membranes where H⁺ accumulates, building the proton gradient
  • Cristae - the folds of the inner membrane where ATP synthase and electron carriers are located; cells with high energy demands (e.g. muscle, liver) have more densely packed cristae

AQA Exam Tips

  • Don't say respiration releases energy from glucose - say it synthesises ATP using energy from the oxidation of glucose. The mark scheme distinguishes this.
  • Location matters: glycolysis = cytoplasm; link reaction and Krebs = matrix; oxidative phosphorylation = inner mitochondrial membrane. AQA frequently asks for locations.
  • Chemiosmosis: know that protons are pumped into the intermembrane space, flow back through ATP synthase, and that oxygen is the final electron acceptor. These three steps are all commonly asked.
  • NAD regeneration in anaerobic respiration: the purpose is always to regenerate NAD so glycolysis can continue. This is what AQA wants, not just "to continue producing ATP."
  • Substrate-level vs oxidative phosphorylation: be able to distinguish - substrate-level is direct phosphate transfer from an intermediate; oxidative uses the ETC and proton gradient.
  • Decarboxylation and oxidation: in the link reaction and Krebs cycle, AQA often asks you to state what type of reaction is occurring. Decarboxylation (CO₂ removal) and oxidation (H removal, reducing a coenzyme) are the two key terms.
  • RQ (respiratory quotient): RQ = CO₂ produced / O₂ consumed. For glucose (carbohydrate) RQ = 1.0; for lipids RQ ≈ 0.7; for protein RQ ≈ 0.9. A value below 1 suggests lipid use; above 1 suggests anaerobic respiration is also occurring.

Summary

  • Respiration = synthesis of ATP from oxidation of organic molecules (primarily glucose). ATP is the immediate energy currency - not glucose itself.
  • ATP: adenine + ribose + 3 phosphates. Hydrolysed by ATP hydrolase (ADP + Pᵢ + energy); re-synthesised by ATP synthase via chemiosmosis.
  • Coenzymes: NAD (accepts 2H → reduced NAD / NADH); FAD (accepts 2H → reduced FAD / FADH₂). Carry hydrogen to the ETC.
  • Glycolysis (cytoplasm): glucose (6C) → 2 pyruvate (3C). Net: 2 ATP + 2 reduced NAD. No oxygen required.
  • Link reaction (matrix): pyruvate → acetyl-CoA (2C) + CO₂. Per pyruvate: 1 CO₂ + 1 reduced NAD + acetyl-CoA. Per glucose: 2 CO₂ + 2 reduced NAD.
  • Krebs cycle (matrix): acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C) → ... → oxaloacetate regenerated. Per turn: 2 CO₂ + 3 reduced NAD + 1 reduced FAD + 1 ATP. Per glucose: 4 CO₂ + 6 reduced NAD + 2 reduced FAD + 2 ATP.
  • Oxidative phosphorylation (inner mitochondrial membrane): reduced NAD and FAD donate electrons to ETC → H⁺ pumped into intermembrane space → flow back through ATP synthase (chemiosmosis) → ATP. O₂ is the final electron acceptor (→ H₂O). Per glucose: ~30 - 32 ATP.
  • Anaerobic respiration: no O₂ → ETC cannot function → reduced NAD accumulates → NAD regenerated by reducing pyruvate. In animals: pyruvate → lactate. In yeast: pyruvate → ethanal + CO₂ → ethanol. Purpose: regenerate NAD so glycolysis continues (only 2 ATP net).
  • RQ: glucose = 1.0; lipid ≈ 0.7; protein ≈ 0.9