(a) outline why plants, animals and microorganisms need to respire, with reference to active transport and metabolic reactions

Respiration– the process whereby energy stored in complex organic molecules (carbohydrates, fats and proteins) is used to make ATP, occurring in living cells.


All living organisms need energy to drive their biological processes in order to survive. Metabolic reactions that need energy include:

  • Active Transport – moving ions and molecules across a membrane against a concentration gradient.
  • Secretion– large molecules made in some cells are exported by exocytosis.
  • Anabolism– synthesis of large molecules from smaller ones, e.g. proteins from amino acids, steroids into cholesterol and cellulose from β-glucose.
  • Replication of DNA and synthesis of organelles before a cell divides.
  • Endocytosis– bulk movement of large molecules into cells.
  • Movement– movement of bacterial flagella, eukaryotic cilia and undulipodia, muscle contraction and microtubule motors that move organelles around inside cells.
  • Activation of chemicals– glucose is phosphorylated at the beginning of respiration so that it is more unstable and can be broken down to release energy.


(b) describe, with the aid of diagrams, the structure of ATP

ATP stands for adenosine triphosphate and is a phosphorylated nucleotide. Each molecule consists of adenine, ribose and three phosphates.


(c) state that ATP provides the immediate source of energy for biological processes

ATP can be hydrolysed to ADP and Pi (inorganic phosphate), releasing 30.6 kJ energy per mol. So, energy is immediately available to cells in small, manageable amounts that will not damage the cell(enzymes and proteins can denature or membranes could become too fluid if too much energy is released), so it’s easier to harness the energy and it will not be wasted. The energy released from ATP hydrolysis is an immediate source of energy for biological processes, such as DNA replication and protein synthesis.

ATP is the ‘universal energy carrier’:

  • Found in all living cells.
  • Small and soluble – can move around the cell.
  • High energy bonds between phosphates – breaks down to release energy where required.
  • Produced where energy is released.

Immediate source of energy

Anabolic reactionsbuilding larger molecules from smaller molecules (hydrolysis).

Catabolic reactionsbreaking larger molecules to form smaller molecules (condensation).

Catabolic reactions release energy that the building of ATP uses. The hydrolysis of ATP releases energy that other anabolic reactions could use.

(d) explain the importance of coenzymes in respiration, with reference to NAD and coenzyme A

The Stages of Respiration:

  • Glycolysis– occurs in the cytoplasm which can take place in aerobic or anaerobic conditions. Glucose is broken down to two molecules of pyruvate.
  • The link reaction– occurs in the matrix of the mitochondria. Pyruvate is dehydrogenated and decarboxylated `and converted to acetate.
  • Krebs cycle– occurs in the matrix of the mitochondria. Acetate is dehydrogenated and decarboxylated.
  • Oxidative phosphorylation– occurs on the folded inner membrane (cristae) of mitochondria. This is where ADP is phosphorylated to ATP.

Coenzymes are needed to help enzymes carry out oxidation reactions, where hydrogen atoms are removed from substrate molecules in respiration. The hydrogen atoms are combined with coenzymes, so that they can be carried and can later be split into hydrogen ions and electrons, to the inner mitochondrial membranes where they will be involved in oxidative phosphorylation.

(e) state that glycolysis takes place in the cytoplasm

Glycolysis is a very ancient biochemical pathway, occurring in the cytoplasm of all living cells (prokaryotic and eukaryotic) that respire. It is an anaerobic metabolic pathway.

(f) outline the process of glycolysis beginning with the phosphorylation of glucose to hexose bisphosphate, splitting of hexose bisphosphate into two triose phosphate molecules and further oxidation to pyruvate, producing a small yield of ATP and reduced NAD

  1. One ATP molecule is hydrolysed and the phosphate group released is attached to the glucose molecule at carbon 6, called phosphorylation. The energy from the hydrolysed ATP molecule activates the hexose sugar and prevents it from being transported out of the cell.
  2. Glucose 6-phosphate is rearranged, using the enzyme isomerase, into fructose 6-phosphate.
  3. Phosphorylation occurs again forming hexose 1,6-bisphosphate.
  4. The hexose 1,6-bisphosphate splits into two molecules of triose phosphate.
  5. Each triose phosphate is oxidised, removing hydrogen atoms using dehydrogenaseenzymes.
  6. The coenzyme NADaccepts the hydrogen atoms and becomes reduced NAD.
  7. Two molecules of ATP are formed, calledsubstrate-level phosphorylation (the formation of ATP directly during glycolysis and the Krebs cycle only).
  8. The triose phosphate molecules are converted to pyruvate, which is actively transportedto the mitochondrial matrix. In the process, another two molecules of ADP are phosphorylatedto make twomolecules of ATP.

(g) state that, during aerobic respiration in animals, pyruvate is actively transported into mitochondria

During aerobic respiration in animals, the triose phosphate molecules are converted into pyruvate and are activelytransported into mitochondria.

(h)explain, with the aid of diagrams and electron micrographs how the structure of mitochondria enables them to carry out their functions

How does the structure of mitochondria enable them to carry out their functions?

(i) state that the link reaction takes place in the mitochondrial matrix

Pyruvate that is produced during glycolysis is transported across the inner and outer membrane to the mitochondrial matrix where the link reaction takes place.

(j) outline the link reaction, with reference to decarboxylation oh pyruvate to acetate and the reduction of NAD

  1. The pyruvate molecule is decarboxylated by the enzyme pyruvate decarboxylase, removing a carboxyl group which eventually becomes carbon dioxide.
  2. The pyruvate molecule is also dehydrogenated by the enzyme pyruvate dehydrogenase, removing hydrogenatoms forming acetate.
  3. The hydrogen atoms are accepted by the coenzyme NAD, becoming reduced NAD.
  4. The acetate combines with coenzyme A forming acetyl CoA.

2 pyruvate + 2NAD+ + 2CoA → 2CO2 + 2NADH + 2 acetyl CoA

(k) explain that acetate is combined with coenzyme A to be carried to the next stage

Coenzyme A (CoA) accepts acetate to become acetyl coenzyme A. The function of CoA is to carry 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. It produces one molecule of ATP by subtrate-level phosphorylation and reduces three molecules of NAD and one molecule of FAD.

(m) outline the Krebs cycle, with reference to the formation of citrate from acetate and oxaloacetate and the reconversion of citrate to oxaloacetate (names of intermediate compounds are not required)

  1. The acetate is offloaded from coenzyme A and joins with oxaloacetate (4C), to form citrate (6C).
  2. Citrate is decarboxylated and dehydrogenated forming a 5-carbon compound. The pair of hydrogen atoms is accepted by the coenzyme NAD (hydrogen acceptor), which becomes reduced NAD.
  3. The 4-carbon compound is decarboxylated and dehydrogenated forming a 4-carbon compound and another molecule of reduced NAD.
  4. The 4-carbon compound is changed into another 4-carbon compound. During this reaction a molecule of ADP is phosphorylated to produce a molecule of ATPsubstrate-level phosphorylation.
  5. The second 4-carbon compound is changed into another 4-carbon compound. It’s dehydrogenated and the coenzyme FAD (hydrogen acceptor) accepts the hydrogen atoms, and becomes reduced FAD.
  6. The third 4-carbon compound is further dehydrogenated and regenerates oxaloacetate and forms another molecule of reduced NAD.

(n) explain that during the Krebs cycle, decarboxylation and dehydrogenation occur, NAD and FAD are reduced and substrate level phosphorylation occurs

(o) outline the process of oxidative phosphorylation, with reference to the roles of electron carriers, oxygen and mitochondrial cristae

  1. NADH is reoxidised to form NAD+ and 2 hydrogen atoms, aided by the enzyme NADH dehydrogenase which is attached to the first electron carrier. The hydrogen atoms split into protons and electrons.

NADH à NAD+ + 2H

2H à2H+ + 2e

  1. The electrons are passed along electron carriers in the electron transport chain and lose energy by doing this.
  2. The energy that was lost in the electron transport chain is used to pump protons into the intermembranespace creating a proton gradient – the protons will want to move back into the matrix from a high concentration to a low concentration.
  1. The H+ ions cannot diffuse through the lipid part of the membrane so they diffuse through protein channels that are associated with ATP synthase, which is linked to the synthesis of ATP. The flow of protons is chemiosmosis.
  2. The flow of protons through the protein channels creates a proton motive forcewhich drives the rotation of the ATP synthase enzyme attached to the protein channel. The rotation causes the phosphorylation of ADP to make ATP.


  1. The electrons are passed from the last electron carrier in the chain to oxygen, which is the final electronacceptor. Hydrogenions also join forming water.

4H+ + 4e + O2à 2H2O


(p) outline the process of chemiosmosis, with reference to the electron transport chain, proton gradients and ATP synthase

Chemiosmosis – the flow of hydrogen ions through a partially permeable membrane, relating the synthesis of ATP. The flow creates a proton motive force that rotates the enzyme ATP synthase, joining ADP and Pi to form ATP.

  1. Electrons are passed along electron carriers in the electron transport chain and lose energy.
  2. The energy is used to pump protons into the intermembrane space, creating a proton gradient between the intermembrane space and the matrix.
  3. The hydrogen ions diffuse through the protein channels, creating a proton motive force which drives the rotation of the ATP synthase enzyme attached to the protein channel. The rotation causes the phosphorylation of ADP to make ATP.

(q) state that oxygen is the final electron acceptor in aerobic respiration

Oxygen is the final electron acceptor in aerobic respiration, which joins to hydrogen to form water.

4H+ + 4e + O2à 2H2O

(r) evaluate the experimental evidence for the theory of chemiosmosis

Chemiosmosis is the flow of hydrogen ions (protons) through a partially permeable membrane, relating to the synthesis of ATP. The flow of the hydrogen ions create a proton motive force. This rotates the enzyme ATP synthase joining ADP and Pi to form ATP.

(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 NAD can theoretically produce 26 molecules of ATP during oxidative phosphorylation (each NAD molecule can make 2.6 molecules of ATP). Together with the 4 ATP made doing glycolysis and the Krebs cycle, the total yield of ATP molecules, per molecule of glucose respired, should be 30. However this is rarely achieved for the following reasons:

  • Some protons leak across the mitochondrial membrane, 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 for the shuttle to bring hydrogen from reduced NAD made during glycolysis, in the cytoplasm, into the mitochondria.

(t) explain why anaerobic respiration produces a much lower yield of ATP than aerobic respiration.

Anaerobic respirationis the process where ATP is produced by substrate-level phosphorylation during glycolysis in the absence of oxygen, in the cytoplasm of eukaryotic cells.

As anaerobic respiration occurs in the absence of oxygen, the electron transport chain cannot happen so the link reaction, Krebs cycle and oxidative phosphorylation cannot happen. Therefore only glycolysis can happen and only ATP can be produced via glycolysis. The reducedNAD has to be reoxidised so that it can keep accepting hydrogen atoms in glycolysis. There are two ways that NAD can be reoxidised – lactate fermentation and alcohol fermentation.

(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.

The more protons, the more ATP produced as most ATP is formed from the flow of protons through channel proteins during chemiosmosis. Therefore the more hydrogen atoms there are in a molecule of respiratory substrate, the more ATP can be generated when that substrate is respired. It also follows that if there are more hydrogen atoms per mole of respiratory substrate, then more oxygen is needed to respire that substance.

(w) explain the difference in relative energy values of carbohydrate, lipid and protein respiratory substrates