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Plants, animals and microorganisms need to respire

  • Active transport
  • Secretion (large molecules released by cells by exocytosis)
  • Endocytosis (bulk movement of larger molecules into cell)
  • Metabolic reactions (synthesis of small to large molecules)
  • Replication of DNA and synthesis of organelles (before cell division)
  • Movement
  • Activation of chemicals
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Describe the structure of ATP

An Adenosine group attached to a Ribose sugar and three phosphate molecules


State that ATP provides the immediate source of energy for biological processes.

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Explain the importance of coenzymes in respiration

Coenzymes aid in the oxidation and reduction of reactions.

NAD combines with the H atoms and takes them to the mitochondrial membrane where they can be later split into H ions and electrons for the ETC. - it is used in glycolysis, the Krebs cycle and anaerobic respiration

Coenzyme A carries acetate groups either from the link reaction, or that have been made from fatty acids or amino acids onto the Krebs cycle

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Outline the process of glycolysis

1.) ATP molecule is hydrolysed and the phosphate is attached to the glucose molecule at C-6

2.) Glucose 6 Phosphate is turned into fructose 6 phosphate

3.) Another ATP hydrolysed, and the phosphate attached to C-1

4.) The hexose sugar is activated by the energy release from the hydrolysed ATP molecules. - it now cannot leave the cell - it is known as Hexose-1, 6-bisphosphate

5.) It is split into 2 molecules of Triose phosphate

6.) 2 H atoms removed from each Triose Phosphate, which involved dehydrogenase enzymes

7.) NAD combines with the H atoms to form red. NAD

8.) 2 molecules of ATP are formed - substrate level phosphorylation

9.) 4 enzyme-catalysed reactions convert each triose phosphate molecule into a molecule of pyruvate

10.) 2 more ATP are formed, so there is net gain of 2 ATP

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Process of glycolysis

Glycolysis takes place in cytoplasm


  • phosphorylation of glucose to hexose bisphosphate
  • splitting of hexose bisphosphate to 2 triose phosphate molecules
  • further oxidation to pyruvate
  • Producing small yield of ATP and red.NAD
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State that...

During aerobic respiration in animals, pyruvate is actively transported into mitochondria

Oxygen is the final electron acceptor in aerobic respiration


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Structure of mitochondria enables to carry out fun

The Matrix:


  • enzymes that catalyse the stages of aerobic respiration
  • molecules of coenzyme NAD
  • oxacloatate- 4carbon compound that accepts acetate from the link reaction
  • mitochondrial DNA, some codes for the mitochondrial enzymes and proteins
  • mitochondrial ribosome, where proteins are assembled

The Inner Membrane:

  • different lipid composition than the outer layer - impermeable to most small ions (incl. H ions)
  • folded into many cristae to give large SA
  • has many electron carriers and ATP synthase enzymes embedded in it
  • high protein-to-phospholipid ratio
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Structure of mitochondria enables to carry out fun

Outer Membrane:

  • Contains proteins, some form channels or carriers that allow passage of molecules (e.g pyruvate)
  • other proteins in the membrane are enzymes

Electron Transport Chain:

  • contains 100s of oxidoreductase enzymes- involved in oxidation and reduction reactions
  • some also has a co-enzyme that pumps protons from the matrix to the intermembrane space
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Link Reaction

The Link reaction takes place in the mitochondrial matrix 

  • Pyruvate dehydrogenase removes H atoms from pyruvate
  • Pyruvate decarboxylase removes a carboxyl group, which eventually becomes CO2, from pyruvate
  • NAD accepts the H atoms
  • CoA accepts the acetate to become Acetyl CoA, which then travels to the Krebs Cycle

Acetate is combined with coezyme A to be carried to the next stage

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Electron Transport Chain

  • Each electron carrier is an oxireductase enzyme as they are involved in oxidation and reduction reactions
  • Each associated with a cofactor (non-protein groups) They are haem groups and contain an iron atom
  • The cofactors can accept and donate electrons because the iron atom can become reduced (Fe2+) by accepting, and oxidised (Fe3+) by donating an electron to the next electron carrier

Oxidation is Loss. Reduction is Gain

  • Some of the electron carriers have a coenzyme that pumps protons from the matrix to the intermembrane space
  • The inner membrane is impermerable to small ions and so protons accumulate in the intermembrane space, building up a proton gradient - source of potential energy
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Krebs Cycle

The Krebs cycle takes place in the mitochondrial matrix

1.) Acetate is offloaded from CoA and joins with Oxalacetate to form citrate

2.) Citrate is decarboxylated and dehydrogenated to form a 5C compound  (a.) the H atoms are accepted by NAD- taken to the ETC    (b.) The Carboxyl group becomes CO2

3.) The 5C compound is decarboxylated and dehydrogenated to form a 4C compound

4.) The 4C compound is changed into another 4C compound and a molecule of ATP is phosphorylated

5.) The 2nd 4C compound is changed to a 3rd 4C compound and a pair of H atoms are removed- reducing FAD

6.) The 3rd 4C compound is further dehydrogenated to regenerate oxaloacetate

During the Krebs cycle, decarboxylation and dehydrogenation occur, NAD and FAD are reduced and substrate phosphorylation occurs

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Link Reaction and Krebs Cycle


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Oxidative phosphorylation

Final stage of respiration involves electron carriers (embedded in the mitochondrial membrane)

Oxidative phosphorylation is the formation of ATP by the addition of an inorganic phosphate to ADP in the presence of Oxygen

  • As protons flow through ATPsynthase they drive the rotation part of the enzyme and join ADP to Pi to make ATP
  • The electrons are passed from the final electron carrier to molecular Oxygen (the final electron acceptor)
  • H ions also join, so Oxygen is reduced to water
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1.) Reduced NAD and FAD donate Hydrogens which are split into protons and electrons, to the electron carriers

2.) The protons are pumped across the inner mitochondrial membrane using only energy released from the passing of electrons down the ETC

3.) This builds up a proton gradient, also a pH gradient and an electrochemical gradient

4.) Potential energy builds up

5.) The H ions cannot diffuse through the lipid part of the inner membrane, but can diffuse through ATP synthase- an ion channel in the membrane

The flow of Hydrogen ions is chemiosmosis

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Experimental evidence for Chemiosmosis

  • Researchers isolated mitochondria and treated them- placing in a solution with very low WP
  • The outer membrane ruptured- relasing contents of intermembrane space
  • treated mitoblasts with strong detergent they release contents of matrix
  • Could identify enzymes in mitochondria and work out that the link reaction and the Krebs cycle occured in the matrix, whilst the enzymes for the ETC were embedded in the mitochondrial membrane
  • Electron transfer in mitoblasts did not produce ATP, so concluded that the intermembrane space was also involved
  • ATP was not made if the mushroom-shaped parts of the stalked particles were removed from the i-membrane of the intact mitochondria
  • ATP was not made in the presence of oligomycin- antibiotic now known to block the flow of protons through the ion channel part of ATP synthase
  • In intact mitochondria:
    • potential difference across membrane was -200V (more negative on matrix side than i-membrane space)
    • the pH of i-membrane space was lower than that of matrix
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Yield of ATP

The maximum yield of ATP is rarely reached as:

  • Some Hydrogens leak across the mitochondrial membrane - (less protons to generate the proton motive force)
  • Some ATP is used to actively transport pyruvate into the mitochondria
  • Some ATP is used to bring Hydrogen from red.NAD made during glycolysis, into the cytoplasm, into the mitochondria
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Anaerobic produces lower yield of ATP than aerobic

Only glycolysis occurs

The ETC does not occur- there is no Oxygen for the final acceptor

The Kreb cycle stops- there is no NAD - they are all reduced

Prevents the Link reaction from occurring

Anaerobic respiration takes the pyruvate- reduces it and therefore frees up the NAD - glycolysis continues, producing 2 molecules of ATP per glucose molecule repired

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Anaerobic respiration- yeast and mammals


1.) Pyruvate combines with a H, provided by red.NAD, forming lactate and oxidised NAD

2.) Involves the enzyme lactate dehydrogenase- the lactate pathway

3.) Oxidised NAD can go back to accpeting H from glucose- glycolysis can continue


1.) Pyruvate converted to ethanal wich involves decarboxalation as CO2 is released

2.) Ethanal combines with H from red.NAD to form ethanol (catalysed by alcohol dehydrogenase)

3.) Oxidised NAD can go back to accept H from glucose- glycolysis can continue

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Respiratory substrates

Respiratory substrate: an organic substance that can be used for respiration

The difference in relative energy values of carb, lipid and protein repiratory substrates:

Higher the number of H atoms per mole, higher the relative energy value- as more NAD molecules can be reduced and used in the ETC

Lipids: 0.7

Proteins: 0.9

Carbohydrates: 1.0

Respiratory Quotient: amount of potential energy is calculated

RQ = Carbon dioxide / Oxygen used

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