Photosynthesis, respiration and fermentation

Follow matter and energy separately

Cells require a continuing supply of usable energy. ATP transfers energy between processes: its hydrolysis can be coupled to cellular work such as active transport or synthesis. ATP is continually regenerated from ADP and phosphate. Energy is not a substance that appears from nothing when ATP forms. Identify the energy source and the process that captures part of it. Carbon atoms and electrons can be followed through pathways, while some energy is dispersed as heat at each transfer.

Oxidation involves loss of electrons, and reduction involves gain. These events occur together because an electron donor needs an acceptor. Carriers such as NADH move reducing power between reactions. The related oxidised form, NAD+, must be regenerated for pathways that require it to continue. A useful pathway comparison asks four questions: where does it happen, what enters, what leaves, and how are ATP and electron carriers involved? This framework is more reliable than memorising a list of names without connections.

Begin glucose breakdown in the cytosol

Glycolysis occurs in the cytosol and converts one six-carbon glucose molecule into two three-carbon pyruvate molecules. An early investment of ATP is followed by ATP production, giving a net yield of two ATP per glucose in the standard pathway. NAD+ is reduced to NADH. Glycolysis does not use oxygen directly. It can therefore operate when oxygen is unavailable, provided that the cell has a way to regenerate NAD+ and has the required substrates and enzymes.

In aerobic respiration, pyruvate-derived carbon enters further oxidation reactions. In eukaryotic cells, pyruvate oxidation and the citric acid cycle occur mainly in the mitochondrial matrix. Carbon dioxide is released as carbon compounds are oxidised. Reduced electron carriers deliver electrons to an electron transport chain in the inner mitochondrial membrane. Electron transport helps establish a proton gradient, and ATP synthase uses movement down that gradient to support ATP production.

Oxygen is the final electron acceptor in aerobic mitochondrial respiration and is reduced to water. Its role is not to become the carbon in carbon dioxide. Much ATP production depends on the electron transport chain and proton gradient rather than being made directly in glycolysis or the citric acid cycle. Exact total ATP yield depends on cellular conditions and accounting assumptions, so use a stated yield when a calculation supplies one. The robust comparison is that complete aerobic oxidation usually supplies much more ATP per glucose than fermentation.

Regenerate a carrier when oxygen is unavailable

Fermentation transfers electrons from NADH to an organic acceptor, regenerating NAD+. In lactate fermentation, pyruvate is reduced to lactate. In alcoholic fermentation, carbon dioxide is released and ethanol is formed through a pathway involving acetaldehyde. The fermentation steps themselves do not add a large ATP yield to glycolysis. Their essential contribution is allowing glycolysis to continue by restoring its oxidised electron carrier. Do not confuse producing lactate with a complete breakdown of glucose to carbon dioxide and water.

Connect light capture with carbon fixation

Photosynthesis uses light energy to support the synthesis of organic compounds. In plant chloroplasts, light-dependent reactions occur at thylakoid membranes. Water supplies electrons, oxygen is released, and ATP and NADPH are produced. The oxygen released by these reactions comes from water. Carbon dioxide provides carbon for organic compounds through carbon-fixation reactions in the stroma. These reactions use ATP and reducing power produced by the light-dependent reactions.

Plants also perform cellular respiration, including in cells that photosynthesise. Photosynthesis and respiration are connected through matter and energy flows, but they are not simply the same reaction running backwards through identical enzymes. At a given moment, a leaf's net gas exchange depends on the relative rates of both processes. In darkness, photosynthetic oxygen production stops while respiration can continue. A measured decrease in oxygen around a leaf therefore need not mean that the leaf has stopped being a plant or that it lacks chloroplasts; interpret the conditions and the net balance.