Respiration — OCR A-Level Biology

Cellular respiration is the controlled release of energy from organic molecules inside cells, and the need for it runs through every living process. Cells must synthesise ATP because ATP is the immediate, universal energy currency: it transfers energy in small, usable amounts to reactions such as active transport, protein synthesis, DNA replication and muscle contraction. Respiration also supplies the reduced coenzymes and carbon skeletons that feed biosynthesis, and it maintains the concentration gradients and metabolic conditions that keep cells alive. Without a continuous supply of ATP, pumps fail, gradients collapse and metabolism stops. A useful method is to link each example of cell work to the ATP it consumes, then explain why diffusion alone cannot power that work.

The mitochondrion is the site of the later stages of aerobic respiration, and its structure is adapted to that role. It has a double membrane: a smooth outer membrane and an inner membrane folded into cristae that increase surface area. The inner membrane holds the electron transport chain and ATP synthase, and it encloses the matrix, a gel-like space containing enzymes for the Krebs cycle, mitochondrial DNA and ribosomes. The intermembrane space lies between the two membranes and is where protons accumulate during chemiosmosis. A useful method is to link each structure to its function: cristae to surface area for the electron transport chain, matrix to Krebs cycle enzymes, and the inner membrane to proton movement and ATP synthesis.

Glycolysis is the first stage of respiration and happens in the cytoplasm, not inside a mitochondrion, so it needs no organelle membranes. Glucose (a 6-carbon sugar) is phosphorylated by two ATP molecules, then split into two 3-carbon triose phosphate molecules. Each triose phosphate is oxidised, losing hydrogen that is picked up by NAD to form reduced NAD, and each is converted to pyruvate. Because two triose phosphates are processed, the net yield per glucose is 2 ATP (4 made minus 2 used), 2 reduced NAD and 2 pyruvate. The pathway is anaerobic and reversible in part, and it is the substrate-level phosphorylation step that regenerates ATP. Learn the site, the inputs, the splitting, the oxidation and the net products, and be ready to explain why the net ATP is two rather than four.

The link reaction joins glycolysis to the Krebs cycle and takes place in the mitochondrial matrix. Pyruvate made in the cytoplasm is moved into the mitochondrion. There it is decarboxylated, losing a carbon as carbon dioxide, and dehydrogenated, with the removed hydrogen reducing NAD to reduced NAD. The remaining 2-carbon acetyl group combines with coenzyme A to form acetyl coenzyme A, which then delivers the acetyl group into the Krebs cycle. Per pyruvate the link reaction removes one carbon dioxide and produces one reduced NAD; because glycolysis gives two pyruvate per glucose, the totals per glucose are 2 carbon dioxide and 2 reduced NAD. No ATP is made directly in the link reaction.

The Krebs cycle is a sequence of enzyme-controlled reactions that oxidise acetyl groups, releasing carbon dioxide and reducing coenzymes. It occurs in the mitochondrial matrix, where the enzymes for the cycle are located. Acetyl coenzyme A delivers a two-carbon acetyl group, which joins a four-carbon intermediate to form a six-carbon compound; this is decarboxylated and dehydrogenated in stages, regenerating the four-carbon intermediate so the cycle can repeat. Carbon dioxide is released, and the reduced coenzymes NADH and FADH₂ are produced, along with a small yield of ATP by substrate-level phosphorylation. The reduced coenzymes later supply electrons to the electron transport chain.

Coenzymes are small organic molecules that assist enzymes by carrying chemical groups or electrons between reactions. In respiration they are essential because they accept and donate hydrogen and electrons during oxidation and reduction. NAD accepts hydrogen to become reduced NAD, and FAD similarly becomes reduced FAD during the Krebs cycle; these reduced coenzymes deliver electrons to the electron transport chain, where their oxidation drives ATP synthesis by oxidative phosphorylation. Coenzyme A carries acetyl groups into the Krebs cycle. Without coenzymes, dehydrogenation reactions could not proceed and the yield of ATP would fall, so they link the stages of respiration and enable controlled transfer of reducing power.

Oxidative phosphorylation is the final stage of aerobic respiration, occurring on the inner mitochondrial membrane (the cristae). Reduced coenzymes NADH and FADH₂ deliver electrons to the electron transport chain, a series of electron carrier proteins embedded in the membrane. As electrons pass along the chain, energy is released and used to pump protons (H⁺) from the matrix into the intermembrane space, building a proton gradient. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. The protons then flow back through ATP synthase, driving the phosphorylation of ADP to ATP. The site is the inner membrane; the process links electron transport to ATP synthesis.

The chemiosmotic theory explains how ATP is synthesised using a proton gradient across a membrane. During electron transport, energy pumps protons (H⁺) across the inner mitochondrial membrane from the matrix into the intermembrane space, creating an electrochemical gradient. Protons then diffuse back through ATP synthase, a membrane protein that acts as a channel and enzyme. The flow of protons down their gradient provides the energy to phosphorylate ADP, forming ATP. This coupling of electron transport to ATP synthesis via a proton gradient is chemiosmosis. The theory applies to mitochondria and chloroplasts, where membranes separate proton pools.

Anaerobic respiration in eukaryotes begins with glycolysis in the cytoplasm, where glucose is converted to pyruvate with a net gain of 2 ATP and 2 reduced NAD. Without oxygen, the electron transport chain cannot reoxidise NADH, so NAD must be regenerated for glycolysis to continue. In mammals, pyruvate is reduced to lactate, catalysed by lactate dehydrogenase, using the hydrogen from reduced NAD; this yields no extra ATP. In yeast and some plants, pyruvate is decarboxylated to ethanal, which is then reduced to ethanol, releasing carbon dioxide. The ATP yield is much lower than in aerobic respiration because the Krebs cycle and oxidative phosphorylation do not operate.

Respiration rate in yeast can be measured by the rate of carbon dioxide production or by the rate of oxygen uptake. Under aerobic conditions yeast respires aerobically, taking up oxygen and releasing carbon dioxide; under anaerobic conditions it ferments, releasing carbon dioxide and ethanol without net oxygen uptake. A common method suspends yeast in a glucose solution and collects gas in a syringe or gas jar, or uses a respirometer to measure oxygen uptake; a dye such as hydrogencarbonate indicator or limewater can detect carbon dioxide. Anaerobic conditions are created by sealing the vessel or layering oil over the culture. Rate is calculated by dividing the volume of gas evolved or taken up by the time taken, and controlled variables include temperature, glucose concentration, yeast mass and pH.

Respiratory substrates are oxidised to release energy, but equal masses do not yield equal energy. Lipids store more energy per gram than carbohydrates because they contain proportionally more hydrogen and less oxygen, so more hydrogen atoms are available to be removed and passed to NAD and FAD, feeding oxidative phosphorylation. Proteins release roughly similar energy per gram to carbohydrates but are usually respired only when other substrates are scarce, being deaminated first. A useful comparison is per gram: lipid highest, then protein and carbohydrate close together. Learners should link the energy value to the proportion of hydrogen and to the yield of reduced coenzymes and ATP, not simply memorise a ranking.

The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration: RQ = CO₂ produced ÷ O₂ consumed. It is used to identify which respiratory substrate is being respired. For carbohydrates such as glucose, RQ is about 1.0 because equal volumes of CO₂ are produced and O₂ used. For lipids, RQ is about 0.7 because they contain much hydrogen and little oxygen, so more O₂ is needed to oxidise them. For proteins, RQ is about 0.8 to 0.9. A value between these indicates a mixture of substrates. In an MCQ, calculate RQ from given gas volumes and interpret the value against these reference ranges, remembering that anaerobic respiration can distort results because CO₂ is produced without O₂ uptake.

This practical criterion asks you to design and carry out investigations measuring how the rate of respiration responds to changing factors. Respiration rate can be followed by the rate of oxygen uptake (for example using a respirometer with organisms such as germinating seeds or invertebrates) or by the rate of carbon dioxide release (for example using a dye indicator or a CO₂ sensor). You vary one factor at a time: temperature (using a water bath and allowing equilibration), substrate concentration (for example glucose solutions of different molarity supplied to yeast), or the identity of the respiratory substrate (for example glucose versus a lipid or protein source, or different sugars). Control variables include mass or number of organisms, volume, pH and time. Record readings at intervals, calculate a rate (change divided by time), and repeat to reduce random error.