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BSC 2010C: Principles of Biology I

This library guide supports the FSCJ course BSC 2010C Principles of Biology.

Video Cell Respiration



Respiration is a catabolic reaction that breaks down glucose to release energy ( ATP ). Energy is stored in the cell as ATP or NADH.

Glucose + Oxygen -----> Carbon dioxide + Water + Energy.
C6H12O6 + 6 O2 ---------> 6 CO2 + 6 H2O.

Aerobic respiration : uses oxygen.
Anaerobic respiration : does not use oxygen.

Aerobic respiration

Aerobic respiration is divided into three main stages: Glycolysis, Citric acid cycle and Electron transport chain.

Glucose ( 6 carbon atoms) is split into 2 molecules of glyceraldehyde phosphate (3 carbon each), then these are turned into pyruvate (3 carbons each). This produces 2 ATP and 2 NADH. Glycolysis takes place in the cytoplasm.

Citric acid cycle (or Krebs cycle) 

The main chemicals are Acetyl CoA (2 carbons), citrate (6C), and oxaloacetate (4C). Diagram. This produces 2 ATP, 6 NADH and 2 FADH2. The citric acid cycle takes place inside the mitochondria, and it produces the CO2 that you breath out.

Electron transport chain

This stage produces most of the energy ( 32 ATP molecules, compared to only 2 ATP for glycolysis and 2 ATP for citric acid cycle). This stage converts the NADH and FADH2 into ATP. The electron transport chain takes place in the mitochondria.

The electron transport chain works as a proton pump: it pumps hydrogen ions (protons) through the membrane, and only allows them back through a protein (ATP synthase) which makes ATP. The electron transport chain uses oxygen to accept electrons at the end of the chain (the electrons combine with hydrogen ions and oxygen to produce water molecules).

Summary: the three stages of Aerobic Respiration


Citric acid cycle

Electron transport chain




Breaks down Glucose to Pyruvate

Turns Pyruvate into CO2

Converts NADH and FADH2 into ATP



32 ATP

Catabolism of food Cells can use several organic molecules in respiration: not just glucose.
Carbohydrates are broken down using all three stages of respiration (glycolysis, citric acid cycle and the electron transport chain).

Lipids are broken down to glyceraldehyde, so they go through part of glycolysis, then the citric acid cycle and the electron transport chain.

Proteins are deaminated (the amino group is removed), and then converted in Acetyl CoA, which is used in the citric acid cycle, and then the electron transport chain (so they skip glycolysis).

Anaerobic respiration

Anaerobic respiration does not use oxygen. Only glycolysis can occur. Single celled organisms eg bacteria and yeast can survive anaerobically. Large animals (eg humans) build up an oxygen debt when anaerobic.

During anaerobic respiration, pyruvate builds up and is converted to :

  • lactate ( lactic acid ) in animals. 
  • ethanol ( alcohol) in plants.

"Beer is proof that God loves us and wants us to be happy". - Benjamin Franklin

Last edited September 2014, by David Byres,