Respiration

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Mitochondria

Adaptations for respiration

Mitochondria are adapted to their function in the following ways:

  • The inner mitochondrial membrance is folded into cristae, which increases the membrance's surface area to maximise respiration
  • There are lots of ATP synthase molecules in the inner mitochondrial membrane to produce lots of ATP in the final stage.
  • The mitochondrial matrix contains all the reactants and enyzmes needed for the Krebs cycle to take place.

Coenzymes

A coenzyme is a molecule that aids the function of an enzyme by transferring a chemical group from one molecule to another. Coenzymes used in respiration include NAD, coenzyme A and FAD. NAD and FAD transfer hydrogen from one molecule to another. This means they can reduce or oxidise a molecule. Coenzme A transfers acetate between molecules.

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Stage 1- Glycolysis

1) Phosphorylation

Glucose is phosphrylated by adding a phosphate from a molecule of ATP. This creates one molecule of hexose phosphate and a molecule of ADP. Hexose phosphate is phosphorylated by ATP to form hexose bisphoshate and another molecule of ADP. Then, hexose bisphosphate is split up into 2 molecules of triose phosphate.

2) Oxidation

Triose phophate is oxidised, forming 2 molecules of pyruvate. NAD collects the hydrogen ions, forming two NADH2. Four ATP are produced, but two were used up in stage one, so there is a net gain of two ATP.

The products of glycolysis

  • 2 reduced NAD (NADH2)- To oxidative phosphorylation
  • 2 pyruvate- To the link reaction
  • Net gain of 2 ATP- used for energy
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Stage 2- The Link reaction

Pyruvate is actively transported into the matrix of the mitochondria. The link reaction converts pyruvate to acetyl coenzyme A. Pyruvate is decarboxylated, so one carbon atom is removed from pyruvate in the form of carbon dioxide. NAD is reduced- it collects hydrogen from pyruvate, changing into acetate is combined with conenzyme A (CoA) to form acetyl conenzyme A (acetyl CoA). No ATP is produced in this reaction.

The products of link reaction

Two pyruvate molecules are made for every glucose molecule that enters glycolysis. This means the link reaction and the third stage (Krebs Cycle) happen twice for every glucose molecule.

  • 2 Acetyl Co A- To the Krebs cycle
  • 2 Carbon Dioxide- Released as a waste product
  • 2 Reduced NAD- To oxidative phosphorylation
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Stage 3- The Krebs cycle

1) Formation of citrate
   Acetyl CoA from the link reaction combines with oxaloacetate to form citrate. CoA goes back to the link reaction to be used again.

2) Formation of a 5-carbon compound
   The 6C citrate molecule is converted to a 5C molecule. Decarboxylation occurs, where carbon dioxide is removed. Dehydrogenation also occurs. The hydrogen is used to produce NAD and NADH2.

3) Regeneration of oxaloacetate
   The 5C molecules is then converted to a 4C molecule. Decarboxylation and dehydrogenation occur, producing by the direct transfer of a phosphate group from an intermediate compound to ADP. When a phosphate group is directly transferred from one molecule to anothers it's called substrate-level phosphorylation. Citrate has now been converted into oxaloacetate.

  • 1 CoA- reused in the next link reaction
  • Oxaloacetate- Regenerated for the use in the next Krebs cycle
  • 2 Carbon dioxide- Released as a waste product
  • 1 ATP- Used for energy, 3 NADH2 and 1 NAD
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Stage 3- The Krebs cycle

1) Formation of citrate
   Acetyl CoA from the link reaction combines with oxaloacetate to form citrate. CoA goes back to the link reaction to be used again.

2) Formation of a 5-carbon compound
   The 6C citrate molecule is converted to a 5C molecule. Decarboxylation occurs, where carbon dioxide is removed. Dehydrogenation also occurs. The hydrogen is used to produce NAD and NADH2.

3) Regeneration of oxaloacetate
   The 5C molecules is then converted to a 4C molecule. Decarboxylation and dehydrogenation occur, producing by the direct transfer of a phosphate group from an intermediate compound to ADP. When a phosphate group is directly transferred from one molecule to anothers it's called substrate-level phosphorylation. Citrate has now been converted into oxaloacetate.

  • 1 CoA- reused in the next link reaction
  • Oxaloacetate- Regenerated for the use in the next Krebs cycle
  • 2 Carbon dioxide- Released as a waste product
  • 1 ATP- Used for energy, 3 NADH2 and 1 NAD
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Stage 4- Oxidative Phosphorylation

1) Hydrogen atoms are released from NADH2 and FADH2 as they are oxidised to NAD and FAD. The hydrogen atoms are split into protons and electrons.

2) The electrons move along the electron transport chain, losing energy at each carrier.

3) This energy is used by the electron carriers to pump protons from the mitochondrial matrix into the intermembrane space.

4) The concentration of protons is now higher in the intermembrane space than in the mitochondrial matrix- this forms an electrochemical gradient.

5) Protons move down the electrochemical gradient, back into the mitochondrial matrix, via ATP synthase.

6) This movement drives the synthesis of ATP from ADP and inorganic phosphate. The movement of hydrogen ions across a membrane, which generates ATP, is called chemiosmosis.

7) In the mitochondrial matrix, at the end of the transport chain, the protons, electrons and oxygen combine to form water. Oxygen is the final electron acceptor.

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Actual yield of ATP

Aerobic respiration can make 32 ATP per glucose molecule.

The actual yield is lower because:

  • Some of the NADH2 formed during the first three stages of aerobic respiration is used in other reduction reactions in the cell instead of in oxidative phosphorylation.
  • Some ATP is used up by actively transporting substances into the mitochondira during respiration, e.g. pyruvate, ADP and phosphate.
  • The inner mitochondrial membrane is leaky- some protons may leak into the matric without passing through ATP synthase and without making ATP.
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Anaerobic Respiration

Anaerobic respiration is a type of respiration that doesn't use oxygen. Its starts with glycolysis, but it doesn't involve the link reaction, Krebs cycle or oxidative phosphorylation. There are two types of anaerobic respiration- alcogolic fermentation and lactate fermentation. They both take place in the cytoplasm, they both produce two ATP molecules per molecule of glucose and they both start with glycolysis.

Lactate fermentation
   Lactate fermentation occurs in mammals and produces lactate. NADH2 transfers hydrogen to pyruvate to form lacate and NAD. NAD can be reused in glycolysis. The production of lactate regenerates NAD. This means glycolysis can continue even when oxygen is in short supply, so a small amount of ATP can still be produced.

Alcoholic fermentation
   Alcoholic fermentation occurs in yeast cells and produces ethanol. CO2 is removed from pyruvate to form ethanal. NADH2 transfers hydrogen to ethanal to form ethanol and NAD. NAD can then be reused in glycolysis. The producetion of ethanol also regenerated NAD so glycolysis can continue even when oxygen is in short supply.

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Respiratory Substrates

When an organism respires a specific respiratory substrate, the RQ can be worked out. The RQ is the volume of Carbon Dioxide produced when that substrate is respired, divided by the volume of oxygen consumed, in a set period of time.

Uses of the RQ

You can work out the RQ for a whole organism as well as a particular substrate. The RQ for a whole organism is an average of all the RQ for all the different molecules the organism is respiring.

Carbohydrates- RQ of 1

Lipids- RQ of 0.7

Proteins- RQ of 0.9

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