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CSEC Biology: Respiration

When you hear the word respiration, you may be tempted to think about breathing. The two processes are very closely related, however, respiration is a completely different process from breathing (properly referred to as ventilation). Respiration is the chemical process by which energy is released from organic compounds (food) by living cells.

The digestive system breaks down food into simpler molecules that cells can use. The cells then break down these molecules to release energy.

We can't discuss respiration and energy without first understanding the role of ATP (Adenosine Triphosphate). ATP is a sort of 'energy currency' for cells.

Most of the energy released during respiration is used to form ATP (an energy carrier molecule) to store the energy (some of the energy is released as heat). This molecule is formed by using energy released during respiration to combine Adenosine Diphosphate (ADP) with a phosphate group in the cell. Of course, this energy can be released again whenever it is needed.

We can express this through a reversible reaction like this:

ADP + phosphate group + energy ⇌ ATP

The energy stored in ATP can be used to drive several energy-requiring processes, such as moving materials across cell membranes, manufacturing cell proteins, cell division and special functions in specialized cells, like muscle contraction and the transmission of nerve impulses.

There are two types of respiration: Aerobic respiration and anaerobic respiration.

Aerobic Respiration

This respiration occurs in the mitochondria of cells and requires oxygen. Most cells carry out aerobic respiration. The reactants are the organic compound (usually glucose) and oxygen, and the products are always carbon dioxide, water, and around 38 ATP molecules. This is a complete oxidation of glucose.

glucose + oxygen → carbon dioxide + water + energy

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP

There are four stages of aerobic respiration, glycolysis, the link reaction, the Krebs Cycle and oxidative phosphorylation. You don't have to know the deeper working of each though. (You can read up on that here if you so desire)

Anaerobic Respiration

This form of respiration occurs in the cytoplasm of cells and occurs without the presence of oxygen. The sole reactant (or substrate, since the reaction is catalyzed by enzymes in the cytoplasm) of the reaction is the organic compound (glucose), and the products will usually be carbon dioxide, considerably less energy than what is released in aerobic respiration (about 2 ATP), and another organic compound. An organic compound remains as a product because the reactant (glucose) isn't fully broken down in anaerobic respiration. It is an incomplete oxidation of the glucose.

The organic compound produced in anaerobic respiration is different based on each organism.

Anaerobic Respiration in Yeast Cells (Fermentation)

In fermentation, the products are ethanol, carbon dioxide and 2 ATP molecules per glucose molecule.

glucose → ethanol + carbon dioxide + energy

C₆H₁₂O₆ → 2C₂H₅OH + CO₂ + 2ATP

Anaerobic Respiration in Muscle Cells (Lactic Acid Fermentation)

Whenever you engage in strenuous exercise, your muscle cells use up large quantities of oxygen. Sometimes, the oxygen supply to these cells will become to low to satisfy the demands of aerobic respiration, causing the cells to begin respiring anaerobically. This will produce lactic acid and 2 ATP molecules (no carbon dioxide).

glucose → lactic acid + energy

C₆H₁₂O₆ → C₃H₆O₃ + 2ATP

This lactic acid will begin to build up in the muscle cells and cause them to stop contracting. If you have ever tried doing too many push-ups at once and felt your arms start burning and collapsing helplessly, then you have felt lactic acid buildup. This lactic acid accumulation in the muscle tissue reduces the pH, making it more acidic and producing the stinging feeling in muscles when exercising. This further inhibits anaerobic respiration, inducing fatigue. The muscle cells are said to have incurred an oxygen debt. You see, the lactic acid formed cannot remain there, as it is not the completely oxidized form (i.e. it hasn't been completely broken down). So, after exercise, lactic acid must be respired aerobically using oxygen inhaled to repay the oxygen debt.

Anaerobic Respiration in Certain Bacteria

Certain bacteria break down organic waste to produce energy without the presence of oxygen to produce biogas. This is a mixture of methane, carbon dioxide and trace amounts of contaminant gases like hydrogen sulfide.

You can read a very complex explanation of anaerobic respiration in bacteria here. (Not necessary for CSEC)

Differences Between Aerobic and Anaerobic Respiration:


Aerobic respiration occurs in the presence of oxygen.

Anaerobic respiration occurs in the absence of oxygen.

Type of Plants and Animals

Aerobic respiration is found in all higher plants and animals.

Anaerobic respiration is usually found in microorganisms, but rarely in higher organisms.

Localization inside Cell

Aerobic respiration occurs in the mitochondria.

Anaerobic respiration occurs only in the cytoplasm.

Permanent/Temporary Nature

Aerobic respiration occurs continuously in the presence of oxygen gas.

Anaerobic respiration occurs continuously in some microorganisms (obligate anaerobes). But in higher animals, it occurs in the absence of oxygen.


Aerobic respiration generates 38 ATPs per glucose molecule.

Anaerobic respiration generates 2 ATPs per glucose molecule.

End Products

End products in the aerobic respiration are carbon dioxide and water.

End products of the fermentation in yeast are ethanol and carbon dioxide. In animals, the end product is lactic acid. Bacteria produce methane and hydrogen sulfide as end products.


Substrate is oxidized completely into carbon dioxide and water during aerobic respiration.

Substrate is incompletely oxidized during anaerobic respiration.

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