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Leaving Cert Biology Revision: Respiration


Aerobic Respiration

  1. Occurs in the presence of oxygen.

  2. Glucose is completely broken down into carbon dioxide and water, releasing a large amount of ATP.

  3. Takes place in the mitochondria of plant cells.

  4. Produces approximately 36-38 molecules of ATP per glucose molecule.

Anaerobic Respiration

  1. Occurs in the absence of oxygen.

  2. Glucose is partially broken down into compounds such as ethanol or lactic acid, releasing a small amount of ATP.

  3. Common in plant roots under waterlogged conditions.

  4. Produces only 2 molecules of ATP per glucose molecule.

ATP (Adenosine Triphosphate)


  •   Consists of an adenine base, a ribose sugar, and three phosphate groups.

  •   High-energy bonds between phosphate groups store energy.


  • Serves as the primary energy currency of the cell.

  • Provides energy for cellular processes like muscle contraction, active transport, and biosynthesis.

  • ATP hydrolysis releases energy, converting ATP to ADP (adenosine diphosphate) and inorganic phosphate.


  • ATP is constantly regenerated in cells through processes like cellular respiration and photosynthesis.

  •  ADP is phosphorylated back into ATP using energy from cellular reactions.


Light Reactions

  • Occur in the thylakoid membranes of chloroplasts.

  • Light energy is absorbed by chlorophyll, exciting electrons and generating ATP and NADPH.

Calvin Cycle (Dark Reactions)

  • Occur in the stroma of chloroplasts.

  • ATP and NADPH produced in the light reactions are used to convert carbon dioxide into glucose.


  •   Glucose is the primary product of photosynthesis.

  •   Oxygen is released as a byproduct.

Cellular Respiration


  • Occurs in the cytoplasm.

  • Glucose is broken down into two pyruvate molecules, generating a small amount of ATP and NADH.

Krebs Cycle (Citric Acid Cycle)

  • Occurs in the mitochondrial matrix.

  • Acetyl-CoA produced from pyruvate is further broken down, releasing CO2 and producing ATP, NADH, and FADH2.

Electron Transport Chain (ETC)

  • Occurs in the inner mitochondrial membrane.

  • NADH and FADH2 donate electrons to the ETC, generating a proton gradient across the membrane.

  • ATP synthase uses the proton gradient to produce ATP through chemiosmosis.

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