OCR A2 Biology - Respiration

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Structure of Mitochondria

The mitochondria -  it is usually rod shaped and can be between 0.5 - 1.0 um in diameter and 2-5 um long. There are more mitochondria in areas that are metabolically active and so have greater energy requirements

The outer and inner membrane - the outer membrane is smooth and contains protein channels and carriers so that molecules can passs through into the mitochondria e.g. Pyruvate. The inner membrane is folded into cristae and together with the outer membrane it forms the envelope. The cristae provide a large surface area for the electron carriers, and ATP synthase. It is the site of oxidative phosphorylation.

The matrix -  this is a fluid filled space enclosed by the inner membrane. It is the site of the lin reaction and the Krebs cycle. It contains the many enzymes needed for the two reactions, molecules of NAD and mitochondrial ribosomes and DNA used to code for and synthesise proteins

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Glycolysis is a metabolic pathway where each glucose molecule is broken down to two molecules of pyruvate. It occurs in  the cytoplasm of all living cells and can take place without the presence of oxygen.

  • Glucose is phosphorylated using one ATP molecule. Glucose-6-phosphate is then changeed to Fructose-6-phosphate. This is phosphorylated further by ATP to produce the activated hexose sugar: Hexose-1,6-bisphosphate.
  • Each molecule of Hexose-1,6-bisphoate is broken down into two molecules of triose phosphate
  • Two hydrogen atoms from triose phosphate are removed and are accepted by NAD which becomes reduced NAD. Each Triose Phosphate is converted into Pyruvate during which 2 molecules of ATP are formed

From each molecule of glucose: two ATP are formed, two molecules of reduced NAD are formed and two molecules of Pyruvate are formed.

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The Link Reaction

The link reaction occurs in the Matrix. Pyruvate made during glycolysis is transported here across the inner and outer membranes. No ATP is made.

  • Pyruvate is decarboxylated by the enzyme pyruvate decarboxylase. A carboxyl group is removed which eventually becomes carbon dioxide
  • Pyruvate is then dehydrogenated by the enzyme pyruvate dehydrogenase. The hydrogen atoms are picked up by NAD which becomes reduced
  • Pyruvate is then converted into acetate which is combined with co-enzyme A (coA) to form acetyl coA. The function of the coA is carry acetate to the Krebs cycle. 
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The Krebs Cylcle

The Krebs cycle also takes place in the matrix and it uses the acetate produced during the link reaction. During this cycle, one molecule of ATP is produced by substrate-level phosphorylation. 

  •  The acetate is offloaded by coA (which is then free to collect more from the link reaction) and combines with oxaloacetate (4C) to form Citrate (6C)
  • Citrate is decarboxylated and dehydrogenated to form a 5C intermediate. A molecule of reduced NAD is formed as it accepts the hydrogen atoms.  The 5C compound is then decarboxylated and dehydrogenated to form a 4C compound - reduced NAD is formed again
  • The 4C compound is converted into another 4C compound, during which a molecule of ATP is formed by joining ADP and Pi. The second 4C compound is dehydrogenated to form a third 4C compound during which FAD accepts the hydrogen atoms and is reduced
  •  The third 4C compound is further dehydrogenated and regenerates oxaloacetate
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Oxidative Phosphorylation

This is the final stage of respiration and it occurs on the cristae as it involved electron carriers embedded in the membrane.

  • Reduced NAD and FAD molecles donate the hydrogen atoms and so become reoxidised and are free to accept more hydrogen atoms. The hydrogen atoms are then split up into protons and electrons
  • The electrons are passed through a series of electron carriers in a series of redox reactions and they release energy. This energy is used to pump protons across to the intermembrane space
  • The concentration of protons build up and so it becomes a source of potential energy. A proton gradient is created so the protons diffuse down this proton gradient back into the matrix through ATP synthase enzymes
  • As protons flow through an ATP synthase enzymes, they drive the formation of ATP from ADP Aand Pi. The electrons are passed from the last electron carrier to oxygen which is the final electron acceptor and water is formed
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Anaerobic Respiration

Lactate fermentation

  • This takes place in mammals when the demand for ATP is high
  • Pyruvate is the hydrogen acceptor and accepts hydrogen atoms from reduced NAD which is then reoxidised and able to pick up more hydrogen from glyolysis
  • The enxyme Lactate dehydrogenase catalyses the oxidation of reduced NAD and the reduction of pyruvate to lactate
  • The build up of lacate causes a reduction in blood pH and reduces enzyme activity 

Alcoholic fermenation

  • This takes place in yeast cells
  • Pyruvate is decarboxylated by the enzyme pyruvate decarboxylase to form ethanal.
  • Ethanal is the hydrogen acceptor and accepts hydrogen atoms from reduced NAD which becomes reoxidised so it can accept more hydrogen atoms from glycolysis
  • Ethanal is reduced to ethanol and this is catalysed by the enzyme ethanol dehydrogenase
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