Topic 5: Energy for Biological Processes — Edexcel A-Level Biology B

Cellular respiration breaks down glucose to yield ATP, the universal immediate energy carrier powering active transport, anabolism, and movement. Respiration is exothermic, releasing thermal energy as heat that supports body temperature maintenance in endotherms. Aerobic respiration comprises four stages. Glycolysis occurs in the cytoplasm, converting glucose into pyruvate. Pyruvate is actively transported into the mitochondrial matrix, where the link reaction decarboxylates and dehydrogenates it to acetylcoenzyme A. The Krebs cycle in the matrix oxidises acetyl groups, generating reduced NAD and FAD. Oxidative phosphorylation on the inner mitochondrial cristae uses electron transport and chemiosmosis to produce the vast majority of ATP.

Glycolysis is the first stage of respiration, converting monosaccharides such as glucose into pyruvate within the cytoplasm. Initially, hexose molecules are phosphorylated using ATP, which increases their reactivity and traps them inside the cell. This phosphorylated hexose is then broken down to form glycerate 3-phosphate (GP). As the glycolysis pathway progresses, oxidation occurs, resulting in the production of reduced coenzyme (NADH) and ATP. Although the net ATP yield is small, glycolysis is essential because it supplies pyruvate for the link reaction and provides NADH for oxidative phosphorylation.

The link reaction and Krebs cycle occur in the mitochondrial matrix. In the link reaction, pyruvate is decarboxylated and dehydrogenated, producing acetyl coenzyme A, carbon dioxide and reduced NAD. Acetyl coenzyme A then enters the Krebs cycle, where it combines with oxaloacetate to form citrate. Through a series of reactions, two carbon atoms are removed as carbon dioxide, and reduced NAD and ATP are produced. The complete oxidation of one pyruvate molecule therefore yields three carbon dioxide molecules, four reduced NAD molecules and one ATP molecule. Detailed knowledge of intermediate compounds is not required, but you must understand the overall inputs, outputs and location of these processes.

Oxidative phosphorylation occurs in the inner mitochondrial membrane. The electron transport chain accepts electrons from reduced NAD and reduced FAD, passing them along a series of electron carriers. This releases energy used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient. Protons then flow back into the matrix through ATP synthase, driving the synthesis of ATP by chemiosmosis. Oxygen acts as the terminal electron acceptor, combining with electrons and protons to form water. The inner mitochondrial membrane is crucial as it houses the electron transport chain and ATP synthase, and its impermeability maintains the proton gradient.

Anaerobic respiration breaks down hexoses only partially when oxygen is absent, so ATP yield is limited. Glycolysis still converts one glucose to two pyruvate, giving a net gain of 2 ATP and reduced NAD, but the pyruvate is not oxidised fully. In mammals, pyruvate is reduced to lactate; this regenerates NAD so glycolysis can continue, yet lactate accumulation lowers pH and impairs muscle contraction, causing fatigue. In plants and yeast, pyruvate is decarboxylated to ethanal, which is then reduced to ethanol, again regenerating NAD. Aerobic respiration yields far more ATP per hexose because the Krebs cycle and oxidative phosphorylation complete oxidation.

An absorption spectrum is a graph showing how much light a pigment absorbs at each wavelength, measured by passing light through a pigment extract and recording absorbance. An action spectrum is a graph showing the rate of photosynthesis at each wavelength, measured by the rate of oxygen production or carbon dioxide uptake. Comparing the two reveals that the wavelengths most strongly absorbed by chlorophyll are also those that drive the fastest photosynthesis, so absorption causes the action. Chlorophyll a absorbs strongly in the blue-violet region near 430 nm and the red region near 660 nm, with a trough in the green region, which is why leaves reflect green light. Accessory pigments broaden the range of wavelengths captured.

Chloroplasts have an envelope, stroma, grana and lamellae. The thylakoid membranes host the light-dependent stage: cyclic photophosphorylation produces ATP only, while non-cyclic photophosphorylation produces reduced NADP, ATP and oxygen. In the stroma, the light-independent stage fixes carbon dioxide when it combines with 5C ribulose bisphosphate (RuBP) to form glycerate 3-phosphate (GP), catalysed by RUBISCO. Reduced NADP and ATP from the light-dependent stage synthesise glyceraldehyde phosphate (GALP) from GP and regenerate RuBP in the Calvin cycle. GALP is a raw material for monosaccharides, amino acids and other molecules. Photosynthesis is limited by carbon dioxide, light intensity and temperature.

A respirometer measures gas volume or pressure change to estimate respiration rate. For aerobic respiration, organisms such as germinating seeds or invertebrates are placed in a sealed tube with a carbon dioxide absorbent such as soda lime or potassium hydroxide. As oxygen is used, the pressure falls and the manometer fluid or dye moves; the distance moved per unit time indicates rate. For anaerobic respiration, oxygen is excluded and the carbon dioxide produced can be measured. Temperature, substrate type, organism mass and oxygen availability are factors to vary or control. Safe and ethical use means handling organisms humanely, keeping conditions suitable, minimising stress and returning organisms safely.

In Core Practical 10, you use a photosynthometer or similar apparatus with a submerged plant such as Cabomba or Elodea and coloured filters or a prism to vary the wavelength of light reaching the plant. You count oxygen bubbles or collect gas over a set time to estimate the rate of photosynthesis at each wavelength, then plot rate against wavelength to produce an action spectrum. Many plants have several pigments because each pigment absorbs best at different wavelengths: chlorophyll a absorbs strongly in blue-violet and red, chlorophyll b absorbs at slightly different wavelengths, and carotenoids absorb mainly in the blue-violet region. Having a variety of pigments widens the range of wavelengths that can be captured, so photosynthesis can proceed efficiently in different light conditions.

Chromatography separates chloroplast pigments by their differing solubilities in a solvent and affinities for the paper. You extract pigments from a leaf by grinding it with a solvent such as propanone, then spot the extract onto a pencil baseline on chromatography paper. As solvent travels up the paper, each pigment moves at a different rate, producing distinct coloured bands. You identify pigments by comparing band colours and Rf values with reference data. Rf is the distance travelled by the pigment divided by the distance travelled by the solvent front, so it is always less than 1. This practical shows that chloroplasts contain several pigments, including chlorophyll a and b and carotenoids, which absorb different wavelengths of light.