Glucose is changed to fructose 6-phosphate.
Another ATP molecule is hydrolysed to release another 30.6Kj of energy.
This released phosphate group joins to the fructose 6-phosphate.
This activates the hexose sugar so it is now fructose 1,6-bisphosphate.
It now becomes a phosphorylated sugar known as hexose 1,6-bisphosphate.
Two molecules have been used for each molecule of glucose
Splitting of hexose bisphosphate
Each molecule is split into two molecules of triose phosphate which is a three carbon sugar.
Oxidation of triose phosphate
This is an aerobic anaerobic.
From each triose phosphate two hydrogen atoms are removed, which also involves dehydrogenase enzymes.
NAD is a hydrogen acceptor that combines with the hydrogen atoms to make Reduced NAD. This means that there are two molecules of reduced NAD for each molecule of glucose.
Two molecules of ATP are formed via substrate-level phosphorylation.
Conversion of triose phosphate to pyruvate
Four enzyme catalysed reactions convert triose phosphate to pyruvate.
Another two molecules of ATP are made by the phosphorylation of ADP.
In total two molecules of ATP (net gain) have been made, two molecules of reduced NAD and two molecules of pyruvate.
g. State that, during aerobic respiration in animals, pyruvate is actively transported into mitochondria
The two molecules of pyruvate that have been made in glycolysis are actively transported into the mitochondrial matrix for the link reaction.
h. Explain how the structure of the mitochondria enables them to carry out their function
It is usually between 0.5-1.0μm in diameter and 2-5μm in length.
They can move about within in cells by the cytoskeleton.
Matrix
This is where the link reaction and the Krebs cycle take place.
Here there are the enzymes needed for the reactions.
There are molecules of NAD for the hydrogen molecules to attach to.
There is oxaloacetate which accepts acetate from the link reaction.
There is mitochondrial DNA which codes for some of the proteins and mitochondrial enzymes that may be needed.
There are mitochondrial ribosomes for the assemblage of proteins.
Outer membrane
It contains proteins some of which will form channels or carriers to allow the passage of molecules.
There are also enzymes here.
Inner membrane
It is impermeable to most small ions including hydrogen. This causes a proton gradient which is a source of potential energy.
It is folded into cristae to give a large surface area.
It has many electron carriers and ATP synthase embedded into it.
The electron carriers allow the movement of protons which would be unable to get through the membrane.
The ATP synthase is involved in chemiosmosis.
i. State that the link reaction takes place in the mitochondrial matrix
The link reaction takes place in the mitochondrial matrix because this is where the enzymes needed are.
j. Outline the link reaction with reference to the decarboxylation of pyruvate to acetate and the reduction of NAD
Pyruvate dehydrogenase removes the hydrogen atoms from pyruvate.
Pyruvate decarboxylase then removes a carboxyl group from pyruvate which will be turned into carbon dioxide.
NAD accepts the hydrogen atoms and forms Reduced NAD.
No ATP is produced.
The reduced NAD takes a pair of hydrogen atoms each to be used in Oxidative Phosphorylation.
In total, 2 reduced NADs and 2 Carbon dioxide molecules are made.
k. Explain that acetate is combined with coenzyme A to be carried to the next stage
Coenzyme A (CoA) accepts acetate and form acetyl coenzyme A which carries the acetate to the Krebs cycle.
l. State that the Krebs cycle takes place in the mitochondrial matrix
The Krebs cycle takes place in the mitochondrial matrix like the link reaction.
m. Outline the Krebs cycle with reference to the formation of citrate from acetate and oxaloacetate and the reconversion of citrate to oxaloacetate and n. Explain that during the Krebs cycle decarboxylation and dehydrogenation occur, NAD and FAD are reduced and substrate phosphorylation takes place
Krebs cycle
Acetate is removed from the coenzyme a and joins with oxyaloacetate to form citrate.
Citrate is decarboxylated (removing one carbon dioxide molecule) and dehydrogenated (removing a pair of hydrogen atoms) to form a five carbon compound.
The pair of hydrogen atoms are accepted by NAD which makes Reduced NAD.
The 5 carbon compound is decarboxylated and dehydrogenated to form a 4 carbon compound and another molecule of Reduced NAD is made.
The 4 carbon compound is changed to another 4 carbon compound.
Substrate level phosphorylation takes place where a molecule of ATP is produced by the phosphorylation of ADP.
The new 4 carbon compound is changed into another 4 carbon compound. A pair of hydrogen atoms are removed and accepted by FAD to make Reduced FAD.
This new 4 carbon compound is then dehydrogenated and makes oxaloacetate again. Another molecule of Reduced NAD is made.
This happens once for every molecule of acetate so happens twice for every molecule of glucose.
In total 6 reduced NADs, 2 reduced FADs, 4 carbon dioxide and 2 ATPs are made.
o. Outline the process of oxidative phosphorylation with reference to the roles of electron carriers, oxygen and the mitochondrial cristae
Oxidative phosphorylation
This is the formation of ATP by the addition of inorganic phosphate to ADP in the presence of oxygen.
Protons flow through an ATP synthase enzyme which is implanted into the inner mitochondrial membrane.
This movement drives the rotation of the enzyme and joins ADP and a Phosphate to form ATP.
The electrons are passed through the last electron carrier in the chain to molecular oxygen which is the final acceptor.
Hydrogen ions join which causes the reduction of oxygen to water.
p. Outline the process of chemiosmosis with reference to the electron transport chain, proton gradients and ATPsynthase
Chemiosmosis
Reduced NAP and Reduced FAD are reoxidised when they release the hydrogen atoms.
The hydrogen atoms are split into protons and electrons.
The first electron carrier accepts the electrons from reduced NAD. The electron carrier is known as protein complex I and has an enzyme calle NADH-coenzyme Q reductase otherwise known as NADH dehydrogenase.
The electrons flow down the electron transport chain releasing energy which is used by the coenzymes associated with some of the electron carriers.
The energy pumps the protons across the intermembrane space.
So a proton gradient, a pH gradient and a electrochemical gradient build up.
Potential energy builds up in the intermembrane space.
The hydrogen ions cannot diffuse through the lipid inner membrane but it can diffuse through channels in it. The channels are associated with ATP synthase.
The flow of the hydrogen ions is chemiosmosis.
Then Oxidative phosphorylation takes place.
q. State that oxygen is the final electron acceptor in aerobic respiration
In oxidative phosphorylation the electrons are accepted by oxygen.
r. Evaluate the experimental evidence for the theory of chemiosmosis
Peter Mitchell’s Chemiosmosis theory
He saw that there was a build-up of hydrogen ions on one side of the membrane and that it would be a source of potential energy. Also he thought that the movement of ions down an electrochemical gradient could provide the energy needed for the formation of ATP.
He thought that the inner mitochondrial membrane was an energy-transducing membrane.
He though that the energy from the transfer of electrons pumped hydrogen ions from the matrix to the intermembrane space and the protons flowed through protein channels attached to enzymes. He thought that the proton motive force drove the formation of ATP.
It was radically different from previous ideas of a high-energy intermediate compound.
Since then there have been more discoveries such as the stalked particles are ATP synthase enzymes and have discovered how they function. Now it is known some of the complexes in the electron transport chain have coenzymes.
Some people have put separated mitochondria in solutions of low water potential so the outer membrane is ruptured releasing the contents of the intermembrane space. This allowed them to see where various enzymes are and allowed them to work out where the parts of respiration take place.
The more studies are replicated and the more people that come up with the same conclusion the more reliable the evidence is.
s. Explain why the theoretical maximum yield of ATP per molecule of glucose is rarely, if ever, achieved in aerobic respiration
The 10 molecules of reduced NAD theoretically could produce 26 molecules of ATP. So for each molecule of reduced NAD up to 2.6 molecules of ATP should be made.
The total yield of ATP including the ATP from glycolysis and Krebs cycle should be about 30 per molecule of glucose.
But this is rarely achieved because:
Some protons may leak across the mitochondrial membrane so reducing the number of protons to generate the proton motive force.
Some ATP produced is used to actively transport pyruvate into the mitochondria.
Some ATP is used to bring hydrogen from reduced NAD made in glycolysis into the cytoplasm of the mitochondria.
t. Explain why anaerobic respiration produces a much lower yield of ATP than aerobic respiration
There is no final electron acceptor in oxidative phosphorylation if there is no oxygen.
If there is no oxygen then the electron transport chain cannot happen so the Krebs cycle and the link reaction also cannot happen.
So only glycolysis can happen and only ATP can be produced via glycolysis.
Reduced NAD generated by the oxidation of glucose has to be reoxidised so that glycolysis can keep happening.
u. Compare and contrast anaerobic respiration in mammals and in yeast
v. Define the term respiratory substrate
Respiratory substrate- An organic substance that can be used for respiration.
w. Explain the difference in relative energy values of carbohydrate, lipid and protein respiratory substrates
