Glossary: Cellular Respiration

This chapter's terms in context

Chapter 7 works through five terms describing how cells release usable energy from food: cellular respiration, aerobic respiration, anaerobic respiration, ATP and fermentation. Cellular respiration is the overall process by which cells break down glucose to release energy, and this term acts as the umbrella under which the other four terms sit as more specific cases. ATP, adenosine triphosphate, is the immediate energy currency of the cell, energy released during respiration is not used directly by the cell but is first captured in the bonds of ATP, which is then broken down elsewhere in the cell to power processes such as muscle contraction, active transport and protein synthesis. Students sometimes write that respiration 'produces energy', which is an imprecise phrasing; the more accurate SPM-level statement is that respiration releases energy stored in glucose and transfers it to ATP.

Aerobic respiration is the breakdown of glucose in the presence of oxygen, completed inside the mitochondrion, producing carbon dioxide, water and a large yield of ATP, the equation glucose plus oxygen yields carbon dioxide, water and energy summarises the overall process, even though the full pathway happens through several linked stages. Anaerobic respiration, by contrast, is the breakdown of glucose without oxygen, and it yields far less ATP per glucose molecule because the glucose is only partially broken down. In animal muscle cells during vigorous exercise, anaerobic respiration converts glucose into lactic acid, a build-up which is associated with the muscle fatigue and cramp experienced during intense activity. In yeast and some plant cells, anaerobic respiration instead produces ethanol (alcohol) and carbon dioxide, a pathway more specifically called fermentation.

Fermentation deserves separate attention because it is not simply a synonym for anaerobic respiration in general; it is the specific type of anaerobic respiration carried out by yeast and certain microorganisms, industrially useful because the carbon dioxide released makes bread dough rise and the ethanol produced is the basis of alcoholic fermentation used in brewing. A frequent exam error is stating that fermentation and lactic acid formation are 'the same process in different organisms', when in fact they are two distinct anaerobic pathways with different end products, ethanol plus carbon dioxide in fermentation, lactic acid alone in animal muscle, and examiners expect the correct end products to be named for the correct organism.

Comparing aerobic and anaerobic respiration side by side, in terms of oxygen requirement, end products and relative ATP yield, is one of the most frequently asked table-style questions built from this chapter, and it links forward directly to the respiratory system chapter, where the demand for oxygen created by aerobic respiration explains why the lungs, gills or tracheae of different organisms are structured the way they are. Because aerobic respiration yields far more ATP per glucose molecule than any anaerobic pathway, an organism or tissue always favours the aerobic route whenever oxygen is available, switching to an anaerobic pathway only as a short-term measure when oxygen supply cannot keep pace with demand.

A frequent Paper 3 task in this chapter asks a candidate to design an experiment measuring carbon dioxide output or oxygen consumption in germinating seeds or in yeast, with the underlying question testing whether respiration is aerobic, anaerobic, or a mixture of both under the stated conditions. Full marks typically require stating a correctly controlled variable such as temperature or the mass of biological material used, alongside a clear statement of which gas is being measured and why its production or consumption indicates one respiration pathway rather than another. A common error is describing limewater turning milky as proof of 'respiration' in general, when the more precise and creditable answer names carbon dioxide specifically as the gas responsible and links it back to the type of respiration being investigated.