ATP and Respiration

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Energy and ATP

Energy: the ability to do work

  • it takes a variety of forms, e.g light, heat, sound, electrical, magnetic, chemical
  • it chan be changed from one form to another
  • it cannot be created or destroyed
  • it is measure in joules (J)

What do we need energy for?

  • metabolic reactions
  • movement
  • active transport
  • maintenance, repair and division of cells and organells
  • production of susbstances, e.g enzymes and hormones
  • maintenance of body temperature

1. Light energy from the sun is converted by plants into chemical energy during photosynthesis

2. This chemical energy is converted into ATP during respiration

3. ATP is used by cells to perform work

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The synthesis of ATP from ADP occurs in three ways:

  • photophosphorylation
  • oxidative phosphorylation
  • substrate-level phosphorylation
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Aerobic respiration requires oxygen and produces carbon dioxidem water and lots of ATP

Anaerobic respiration takes place in the absence of oxygen and produces lactate in animals or ethanol and co2 in plants. In both cases, only little ATP is produced

Stages of AEROBIC respiration

  • Glycolysis: the splitting of the 6-carbon glucose molecule into two 3-carbon pyruvate molecules
  • Link Reaction: the conversion of the 3-carbon pyruvate molecule into carbon dioxide and a 2-carbon molecule called acetylcoenzyme A
  • Krebs Cycle: introduction of acetylcoenzyme A into a cycle of oxidative-reduction reactions that yield some ATP and a large number of electrons
  • Electron Transport Chain: the use of elecrtons produced in the Krebs Cycle to synthesise ATP with water produced as a by-product
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  • Activation of Glucose by Phosphorylation

Glucose must first be made more reactive by the addition of two phosphate molecules (phosphorylation) that come from the hydrolysis of two ATP molecules. This provides the energy to activate glucose

  • Splitting of the Phosphorylated Glucose

Each glucose molecule is split into two 3-carbon molecules called Triose Phosphate

  • Oxidation of Triose Phosphate

Hydrogen is removed from each of the two TP molecules and transferred to a hydrogen carrier molecule known as NAD to form reduced NAD

  • Production of ATP

Enzyme controlled reactions convert each TP into a 3-carbon molecule called Pyruvate. During this, two molecules of ATP are regenerated from ADP

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Link Reaction

The pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the matrix of the mitochondria. Here pyruvate undergoes a series of reactions:

  • the pyruvate is oxidised by removing hydrogen. This hydrogen is accepted by NAD to form reduced NAD, which is later used to form ATP
  • the 2-carbon molecule, called an acetyl group, combines with coenzyme A to produce acetylcoenzyme A
  • a carbon dioxide molecule is formed from each pyruvate

Pyruvate + NAD + CoA = acetyl CoA + reduced NAD + Co2

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Krebs Cycle

The Krebs Cycle is a series of oxidation-reduction reactions that take place in the matrix of the mitochondria. 

  • The 2-carbon acetylcoenzyme A combines with a 4-carbon molecule to produce a 6-carbon molecule
  • This 6-carbon molecule loses carbon dioxide and hydrogens to give a 4-carbon molecule and a single molecule of ATP produced from substrate-level phosphorylation
  • The 4-carbon molecule can now combine with a new molecule of acetylcoenzyme A to begin the cycle again

For each molecule of the Pyruvate, the Link Reaction and the Krebs Cycle produce:

  • reduced coenzymes NAD and FAD. These have the potential to produce ATP molecules
  • one molecule of ATP
  • three molecules of CO2

As two pyruvate molecules are produced from glycolysis, the yield from one single glucose molecule is double this.

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Coenzymes are molecules that some enzymes require in order to function. Coenzymes play a major role in photosynthesis and respiration where they carry hydrogen atoms from one molecule to another.

  • NAD, important throughout respiration
  • FAD, important in the Krebs Cycle
  • NADP, important in photosynthesis
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Electron Transport Chain

  • Hydrogen atoms produced in Glycolysis and the Krebs Cycle combine with coenzymes NAD and FAD that are attatched to the cristae of the mitochondria
  • The reduced NAD and FAD donate the electrons of the hydrogen atoms to the first molecule in the electron transport chain
  • This releases the protons from the hydrogen atoms, which are then actively transported across the inner mitochondrial membrane
  • The electrons pass along a chain of electron transport carrier molecules in a series of oxidation-reduction reactions. The electrons lose energy as they pass down the chain, some of this is used to combine ADP and Pi to make ATP. The remaining energy is released in the form of heat
  • The protons accumulate in the space between the two mitochondrials memebranes before they diffuse back into the matrix through protein channels
  • At the end of the chain, the electrons combine with these protons and oxygen to form water. Oxygen is therefore the final acceptor of electrons in the electron transport chain.
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Anaerobic Respiration


The pyruvate molecule from glycolysis loses a molecule of carbon dioxide and accepts hydrogen from reduced NAD to produce ethanol.

pyruvate + reduced NAD = ethanol + carbon dioxide + NAD


Each pyruvate molecule takes uo the tywo hydrogen atoms from the reduced NAD from glycolysis to form lactate.

pyruvate + reduced NAD = lactate + NAD


  • substrate-level phosphorylation in Glycolysis and the Krebs Cycle. This is the direct liking of ADP and Pi to form ATP
  • oxidative phosphorylation in the ETC. Indirect linking of ADP and Pi to produce ATP using the hydrogen atoms from G and KC that are carried on NAD and FAD.
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Anaerobic Respiration


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