All living cells require energy, and this energy is provided by respiration.

glucose + oxygen -> carbon dioxide + water (+ energy)

What form is this energy in?


the energy provided by cellular respiration is in the form of chemical energy - stored in a molecule called ATP

(adenosine triphosphate).


What does cellular respiration do?


Well, in essence cellular respiration simply converts chemical energy which is stored in glucose and converts it into chemical energy which is stored in ATP.

Any process in a cell that requires energy (e.g. muscle contraction and active transport) use ATP as the source of energy.


So any  processes that require energy, will all utilise ATPase enzymes, i.e. ATP hydrolase, which catalyses the breakdown (hydrolysis) of ATP into ADP + Pi the chemical energy released can be utilised. 


So, as you have learned by know, ATP molecules in a cell are constantly being cycled between ATP and ADP + Pi in the ADP-ATP cycle.

Aerobic and Anaerobic Respiration:

Respiration is not a single reaction, but consists of about 30 individual reaction steps. For now we

can usefully break respiration into just two parts: anaerobic and aerobic.

The first part of respiration is simply the breakdown of glucose to a compound called pyruvate. This doesn’t require oxygen, so is described as anaerobic respiration (without air).


It is also called glycolysis and it takes place in the cytoplasm of cells.

It only produces 2 molecules of ATP per molecule of glucose.


Normally pyruvate goes straight on to the aerobic part, but if there is no oxygen it is converted to lactate (or lactic acid) instead. Lactate stores a lot of energy, but it isn’t wasted: when oxygen is available it is converted back to pyruvate, which is then used in the aerobic part of respiration.

The second part of respiration is the complete oxidation of pyruvate to carbon dioxide and water. Oxygen is needed for this, so it is described as aerobic respiration (with air).


It takes place in the mitochondria of cells and produces far more ATP: 34 molecules of ATP per molecule of glucose.


Fats (mainly triglycerides) can also be used in aerobic respiration (but not anaerobic) to produce ATP.

A Level Biology - 2 types of Cellular Respiration - 

Anaerobic Respiration


Check Where this Lesson fits into your Exam Specification!

A Level Biology:

2 types of Cellular Respiration - Anaerobic Respiration. Key Points.

00:00 Introduction

00:40 Learning Outcomes

01:16 Two Types of Respiration

02:04 Anaerobic Respiration

02:43 Alcoholic (Ethanol) fermentation

03:41 Lactate (lactic acid) fermentation

04:41 Summary

Cellular respiration - Anaerobic respiration Revision Notes with Knowledge Check Questions and Answers PDF for A- Level Biology
Check Your Exam Specification

★ AQA A Level Biology Specification Reference: - 3.5.2 Respiration (A-level only) Respiration produces ATP. Glycolysis is the first stage of anaerobic and aerobic respiration. It occurs in the cytoplasm and is an anaerobic process. If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD. The oxidised NAD produced in this way can be used in further glycolysis.


★ CIE A Level Biology Specification Reference: - 12.2 Respiration: Respiration is the process whereby energy from complex organic molecules is transferred to ATP. Some organisms and some tissues are able to respire in both aerobic and anaerobic conditions. When yeast and plants respire under anaerobic conditions, they produce ethanol and carbon dioxide as end-products; mammalian muscle tissue produces lactate when oxygen is in short supply. j) distinguish between respiration in aerobic and anaerobic conditions in mammalian tissue and in yeast cells, contrasting the relative energy released by each (a detailed account of the total yield of ATP from the aerobic respiration of glucose is not required). k) explain the production of a small yield of ATP from respiration in anaerobic conditions in yeast and in mammalian muscle tissue.


★ Edexcel A Level Biology (Biology A – Salters-Nuffield) Specification Reference: - Topic 7: Run for your Life: 7.4. Understand the roles of glycolysis in aerobic and anaerobic respiration, including the phosphorylation of hexoses, the production of ATP, reduced coenzyme, pyruvate and lactate (details of intermediate stages and compounds are not required). 7.7 Understand what happens to lactate after a period of anaerobic respiration in animals.


★ Edexcel A Level Biology (Biology B) Specification Reference: - Topic 5: Energy for Biological Processes. 5.5 Anaerobic respiration. Know that anaerobic respiration is the partial breakdown of hexoses (glucose) to produce a limited yield of ATP in the absence of oxygen. ii Understand the difference in ATP yields from one molecule of hexose sugar in aerobic conditions compared with anaerobic conditions. iii Understand how lactate as a by-product of anaerobic respiration affects mammalian muscle contraction. iv Understand how anaerobic respiration in plants results in ethanol formation.


★ OCR A Level Biology (Biology A) Specification Reference: - 5.2.2 Respiration. Respiration is the process whereby energy stored in complex organic molecules is transferred to ATP. ATP provides the immediate source of energy for biological processes. (i) the process of anaerobic respiration in Eukaryotes. To include anaerobic respiration in mammals and yeast and the benefits of being able to respire Anaerobically. Why anaerobic respiration produces a much lower yield of ATP than aerobic respiration.

★ OCR A Level Biology (Biology B) Specification Reference: - 4.1.1 Cellular respiration. An outline of the process of anaerobic respiration in muscle cells and in yeast. To include an appreciation that anaerobic respiration produces a much lower yield of ATP than aerobic respiration.


★ WJEC A Level Biology Specification Reference: - 2.1 component 1. Energy for Life (3) Respiration releases chemical energy in biological processes. (e) The breakdown of glucose under anaerobic conditions.