Revise Cell respiration for IB Biology (C1.2)

Cell respiration is the controlled release of energy from organic molecules to produce ATP. The energy in glucose is not used directly; instead it is transferred to ATP (adenosine triphosphate), the cell's immediate energy currency. ATP stores energy in the bond to its terminal phosphate group. When a cell needs energy, that bond is hydrolysed — ATP becomes ADP plus an inorganic phosphate — releasing a small, usable quantity of energy that can be coupled straight to work. Respiration then re-attaches a phosphate to ADP to remake ATP, so the two forms are continuously recycled. Because the packets are small and uniform, ATP can power almost any cellular process on demand.

All respiration of glucose begins the same way, with glycolysis in the cytoplasm. What happens next depends on whether oxygen is available. Without oxygen (anaerobic respiration), the cell can only run glycolysis and then convert the product to keep glycolysis going; the yield is a small net 2 ATP per glucose. With oxygen (aerobic respiration), the products of glycolysis are taken into the mitochondria and fully oxidised, releasing far more energy and yielding roughly 30–38 ATP per glucose. The same starting molecule therefore gives a very different payoff depending on the presence of one gas. This contrast — same start, oxygen requirement, location, products and yield — is the backbone of most C1.2 exam questions.

Glycolysis takes place in the cytoplasm and does not require oxygen. In outline, one molecule of glucose (a 6-carbon sugar) is split and rearranged through a sequence of enzyme-catalysed steps into two molecules of pyruvate (each 3 carbons). A little ATP is invested at the start to get the process going, but more is generated later, leaving a small net gain of 2 ATP per glucose. Energised electrons are also removed and picked up by carrier molecules. Because glycolysis needs no oxygen and no mitochondria, it is the common foundation of both anaerobic and aerobic respiration — the difference between the two pathways lies entirely in what happens to the pyruvate afterwards.

When oxygen is unavailable, pyruvate cannot enter the mitochondria to be oxidised. Instead it is converted into another molecule, and the point of that conversion is to regenerate the electron carrier that glycolysis needs so that glycolysis — and its small ATP supply — can keep running. In animals (including humans), pyruvate is converted to lactate. This lets muscles keep contracting during short bursts of intense exercise when oxygen delivery cannot keep up, though lactate accumulates. In yeast and some plant tissues, pyruvate is instead converted to ethanol and carbon dioxide, a process called alcoholic fermentation. Humans exploit this directly: the ethanol in brewing and the carbon dioxide that makes bread dough rise both come from yeast respiring anaerobically. In every case the ATP yield stays at the small net 2 per glucose from glycolysis alone.

If oxygen is present, pyruvate is transported into the mitochondria and respiration continues through three overview stages, extracting far more energy than glycolysis alone. First the LINK REACTION (in the matrix): each pyruvate is oxidised and loses a carbon as CO2, forming a 2-carbon acetyl group. Next the KREBS CYCLE (also in the matrix): the acetyl group is fully oxidised, releasing the remaining carbons as CO2, making a little ATP directly, and — most importantly — loading energised electrons onto carrier molecules. Finally the ELECTRON TRANSPORT CHAIN (on the inner mitochondrial membrane): those energised electrons are passed along a series of carriers, and the energy released as they move is used to generate the BULK of the cell's ATP. At the very end of the chain, OXYGEN acts as the final (terminal) electron acceptor, combining with the spent electrons and hydrogen ions to form water. This is why oxygen is essential: without it the chain has nowhere to offload electrons, so it backs up and stops, and the large mitochondrial ATP yield is lost. Fully oxidising one glucose aerobically yields roughly 30–38 ATP.

The table below draws together the features that exam questions most often ask you to compare. Notice that the two pathways are not opposites from the start — they share glycolysis — and diverge only in whether oxygen is used, where the process continues, what is produced and how much ATP results.

The rate of respiration is a direct measure of how fast an organism is releasing energy, and it can be measured with a respirometer. A living organism (such as germinating seeds or small invertebrates) is sealed in a tube together with an alkali such as potassium hydroxide (KOH), which absorbs the carbon dioxide the organism produces. As the organism consumes oxygen and its CO2 is absorbed, the total gas volume in the tube falls, lowering the pressure. This pressure drop draws a drop of coloured fluid along an attached capillary tube; the distance it moves over a set time indicates the volume of oxygen consumed, and hence the rate of respiration. A control tube containing non-respiring objects of the same volume corrects for changes in atmospheric pressure and temperature. Note that for an animal respiring purely anaerobically the respirometer reads zero, because lactate fermentation consumes no oxygen.

$\text{Volume of } O_2 \text{ consumed} = \pi r^2 l \qquad \text{Rate} = \frac{\text{volume of } O_2}{\text{time} \times \text{mass}}$

Comparison questions in C1.2 are marked ANALYTICALLY: each distinct valid comparative point is worth one mark, up to the total available. A comparison mark requires you to link the two pathways in one statement (a similarity or a difference), not to describe them separately. Answer marks (A) credit a correct point; error-carried-forward (ECF) means a genuine comparison still scores even if a related earlier value was wrong; and equivalent correct wording is accepted. Study how each mark below is tied to a specific comparative idea.

Cell respiration is one half of the energy story of life; photosynthesis is the other, and the two are studied together as complementary energy conversions. The ATP made here powers every active process you meet elsewhere in the course — active transport across membranes, muscle contraction, protein synthesis and DNA replication. At HL the overview stages sketched here (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation on the electron transport chain) are developed in full molecular detail, but the framework is the same one you have built in this lesson: release energy in controlled steps, capture it as ATP, and let oxygen — when available — unlock the large mitochondrial yield.