Respiration - Processes

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Glycolysis

  • Takes place in the cytoplasm
  • Starch, disaccharide or glycogen are hydrolysed to glucose
  • Glucose is phosphorylated by an organic phosphate (gained from hydrolising an ATP molecule, this is catalysed by ATPase) to make glucose-6-phosphate.
  • Glucose-6-phosphate undergoes isomeric change, catalysed by isomerase enzyme, to create fructose-1-phosphate
  • Fructose-1-phosphate is phosphorylated by an organic phosphate (gained from hydrolising an ATP molecule, this is catalysed by ATPase) to hexose-1,6-bisphosphate
  • Hexose-1,6-bisphosphate goes through lysis to produce 2 molecules of triose phosphate
  • Each triose phosphate molecule is oxidised to make 2 intermediate compounds; this is catalysed by dehydrogenase. One hydrogen atom is removed from each triose phosphate molecule, these two hydrogen are then accpeted by a NAD molecule to make reduced NAD
  • The two molecules of intermediate compund are then condensed to two molecules of pyruvate
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Why plants, animals and microorganisms respire

  • Cells obtain energy by metabolic pathways known as respiration.
  • Respiration releases chemical potential energy from glucose and ther energy-containing organic molecules
  • To drive metabolic reactions:
    • Anabolic - synthesis
    • Catabolic - break down
  • Every living cell must be able to move substances across its membrane against their concentration gradients, by active transport
  • Plants obtain their energy from light energy, via photosynthesis
  • Animals obtain their energy from chemical potential energy
  • Light energy is transferred to chemical potential energy (Plants -> via photosynthesis -> animals)
  • Chemical potential energy is available to animals in organic molecules
  • When organic molecules are broken down, they release their energy -> 'to do work'
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ATP

  • Adenosine Triphosphate
  • A phosphorylated nucleotide.
  • When one phosphate group is removed from each molecule in one mole of ATP, 30.5kJ of energy is released. This is a hydrolysis reaction, and is catalysed by enzymes called ATPases.
  • ATP is the intermediary between the energy yielding reactions of the cell (respiration), and the energy requiring reactions of the cell (work).
  • ATP is an energy donor, not a store.
  • If 40kJ/mol were released from respiration, it would be sufficient to make 1 molecule of ATP. The remaining energy is converted to thermal energy and is given out as heat.
  • If 'work' required 80kJ/mol to complete, then atleast 3 molecules of ATP would need to be hydrolysed and 'spare' molecules are given off as heat.
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Mitochondria

  • The matrix is where the Link Reaction and Krebs Cycle take place, it contains:
    • Enzymes that catalyse these stages
    • NAD molecules
    • Oxaloacetate
    • Mitochondrial DNA that codes for mitochindrial proteins
    • Mitochondrial ribosomes
  • Outer Membrane:
    • Phospholipids with proteins forming channels allowing pyruvate through
    • Proteins that are enzymes are also contained here
  • Inner Membrane:
    • Different lipid composition from outer membrane
    • Impermeable to most small ions, including hydrogen ions (protons)
    • Folded into cristae to give large surface area, allowing more enzymes to attach on to carry out respiration, speeding up the rate of respiration
    • Electron carriers and ATP synthase embedded into it
  • If there is plenty of oxygen available in the cell then aerobic respiration can take place, and so pyruvate is moved into the mitochondrion by active transport.
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Link Reaction

  • Takes place in the matrix of the mitochondria
  • Provides the link between the two main series of reactions in aerobic respiration - glycolysis and the Krebs cycle.
  • The 2 molecules of Pyruvate (3C) are decarboxylated and two molecules of carbon dioxide are produced, this is catalysed by a decarboxylase enzyme.
  •  They are also dehydrogenated, the 2 hydrogen molecules are accepted by NAD molecules to create reduced NAD, this is catalysed by a dehydrogenase enzyme.
  • The remainder of the Pyruvate creates 2 molecules of Acetate (2C)
  • This then combines with Coenzyme A and so 2 molecules of Acetyl Coenzyme A (2C) are created.
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Krebs Cycle

  • Takes place in the matrix of the mitochondria
  • The acetyl coA (2C) made in the link reaction combines with oxaloacetate (4C). Coenzyme A is released to go back to combine with more pyruvate, and Citrate (6C) is produced.
  • The citrate (6C) is the decarboxylated and dehydrogenated, releasing carbon dioxide and reduced NAD, to form a 5C compound.
  • This 5C compund is then decarboxylated and dehydrogenated, releasing carbon dioxide and reduced NAD, to form a 4C compund. A molecule of ATP is also created from a molecule of ADP and Pi, this is catalysed by ATPase.
  • This 4C compund is then dehydrogenated, releasing a molecule of reduced FAD, to form a new 4C compound.
  • This new 4C comund is then dehydrogenated, releasing a molecule of reduced NAD, to form oxaloacetate.
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Oxidative Phosphorylation

  • Occurs in the Cristae
  • Reduced NAD and reduced FAD, which were produced in Glycolysis, Link and Krebs, are reoxidised and donate their hydrogen atoms to the electron transport chain. The NAD and FAD then go back to the Krebs cycle so that they can accept more hydrogens
  • The hydroge atoms enter the chain and then split to form protons (H+) and electrons (e-)
  • The hydrogen ions remain in the matrix and the electrons are transferred through a series of electron carriers on the inner membrane.
  • The carriers are at different energy levels and as an electron carrier accepts an electron it is reduced.
  • As the reduced carrier passes the electron to the next carrier,it becomes oxidised and then the next electron carrier becomes reduced.
  • A series of redox reactions occur as the electrons move down the electron carrier chain.
  • as the electrons are passed through the carriers, energy is released to make ATP molecules.
  • When the electrons rejoin the hydrogen ions, they donate their hydrogen ions to molecular oxygen, which is then reduced to form water.
  • Potentially three molecules of ATP are made for each molecule of reduced NAD that is fed into the chain, and two molecules of ATP are made for each molecule of reduced RAD that is fed into the chain
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Role of Coenzymes

  • Nicotinamide adenine dinucleotide.

    •  It accepts two hydrogen atoms to become reduced NAD
    • This occurs in glycolysis, the link reaction and the Krebs cycle

    • They move to oxidative phosphorylation (the last step in photosynthesis) where the hydrogen atoms are released as hydrogen ions and electrons

    Assists dehydrogenase enzymes carry out oxidation reaction

  • Coenzyme A

    • Abbreviated CoA

    • Instead of carrying hydrogens, CoA carries ethanoate/acetate groups (CH

      3COO-)

    • This occurs when pyruvate from glycolysis is converted to acetate in the link reaction and must be transported to the Krebs cycle

        
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Chemiosmosis

  • The hydrogen ions that are released in oxidative phosphorylation are pumped across from the matrix to the intermembrane space.
  • The energy for this is the energy released from the electron transport chain
  • This leads to a higher concentration of hydrogen ions in the intermembrane space than in the matrix and so a hydrogen ion concentration gradient is established across the inner membrane
  • An electrochemical gradient is also established as the hydrogen ion concentration affects the pH.
  • The hydrogen ions then diffuse back into the matrix through protein channels
  • Each protein is associated with an ATP synthase enzyme that catalyses the formation of an ATP molecule.
  • As the hydrogen ions pass through their carriers, the energy they have gained by being actively transported  is released to make one molecule of ATP
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Anaerobic Respiration

  • Glycolysis is the only process that can occur
  • Takes place with the absence if molecular oxygen
  • In animals:
    • Pyruvate is the hydrogen acceptor
    • 2 molecules of Pyruvate are reduced to form 2 molecules of Lactate
  • In plants:
    • 2 molecules of Pyruvate are decarboxylate to form 2 molecules of Ethanal
    • The 2 mlecules of Ethanal are then reduced to form 2 molecules of Ethanol
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