Biology
A-Level
9700
Respiration
Describe the importance of ATP in cells, giving examples of processes in which it is used.
It provides energy. Examples: muscle contraction, protein synthesis, DNA replication, cell movement,
and active transport.
ATP is described as having a universal role as the energy currency in all living organisms. Explain why
it is described in this way.
Energy is released when ATP is hydrolysed. ATP is easily hydrolysed. The energy it releases is used in
processes/ reactions. It has a rapid turnover, and it links catabolic and anabolic reactions. It is found in
most cells/ all organisms. ATP is water soluble, so it is easily moved (within the cell). It links between
energy yielding and energy requiring reactions. ATP is also produced from a variety of reactions.
State precise places where ATP is synthesised in cells.
ETC/ inner mitochondrial membrane/ crista/ stalked particles, grana/ thylakoids/ inner chloroplast
membrane, cytoplasm/ cytosol, mitochondrial matrix.
Explain the role of ATP in active transport of ions and in named anabolic reactions.
ATP provides energy for active transport, during movement of an ion against its concentration gradient.
A carrier/ transport protein (in the membrane) binds to the (specific) ion, and the protein changes
shape. An anabolic reaction is the synthesis of complex substances from simpler ones. Starch/ cellulose/
glycogen is synthesised from monosaccharides/ named monosaccharides/ named sugar, ATP provides
energy for the formation of glycosidic bonds. For a lipid/ triglyceride being synthesised from fatty acids
and glycerol, ATP provides energy for the formation of ester bonds. For polypeptides/ proteins,
synthesised from amino acids, ATP provides energy for the formation of peptide bonds.
State precisely where these two processes occur in a cell:


Substrate level phosphorylation: In the cytoplasm (in glycolysis), and the matrix of the
mitochondria (in Krebs cycle).
Oxidative phosphorylation: In the inner membrane of mitochondria/cristae.
Compare the relative amounts of ATP produced by the SLP and oxidative phosphorylation when a
molecule of glucose is completely oxidised.
Oxidative phosphorylation produces more ATP molecules than substrate level phosphorylation, 32/34
vs. 4/6 per glucose.
Biology
A-Level
9700
Only substrate level phosphorylation is possible in the absence of oxygen. Explain why oxidative
phosphorylation is not possible in the absence of oxygen.
Oxidative phosphorylation requires a proton gradient produced by the ETC. With no oxygen present, the
ETC does not occur/ there is no electron flow. NAD cannot be reformed/ NADH cannot be oxidised, and
there is no oxygen to combine with the final electron/ proton/ oxygen is the final acceptor in the ETC.
Describe two ways in which the structure of the mitochondrion is adapted for oxidative
phosphorylation.
The mitochondrion has a folded inner membrane/ cristae which increases the surface area available. It
also has an intermembrane space, which allows the accumulation of H⁺. The inner membrane is also
impermeable to H⁺, which maintains an H⁺ gradient/ H⁺ only go through channels of the stalked
particles, which are a channel for H⁺/ ATP synthesis. There is a linear arrangement of the ETC on the
inner membrane, which results in a greater efficiency.
Explain how the lack of oxygen will affect the respiratory processes in the mitochondria. References to
processes in the cytoplasm are not required.
There is no oxygen to combine with e¯/ H⁺/ H/ 2H/ proton at the end of the ETC. There is no H⁺ gradient
produced. No ATP is synthesized/ oxidative phosphorylation does not occur. No NAD is regenerated/
NADH is not oxidised. The Krebs cycle stops.
Describe the main structural features of a molecule of ATP.
It is a nucleotide with three phosphate groups connected by ester linkages, and an organic/ nitrogenous
base/ adenine, connected by covalent bonds, as well as a pentose sugar/ ribose.
Explain how ATP is able to transfer energy in cells.
ATP is synthesised from ADP and Pi, it is a soluble molecule which diffuses rapidly/ transported easily.
On hydrolysis/ removal of (third) phosphate, energy is released/ 30.5 kJ (mol¯¹). It is also an
intermediary molecule between energy yielding and energy requiring reactions.
State how ATP is synthesized in mitochondria.
Oxidative phosphorylation takes place. NADH₂ moves to the cristae/ inner membrane and is oxidised to
NAD. Electrons are transferred to electron carriers/ ETC. H⁺ is pumped into the intermembrane space
producing an H⁺ gradient; H⁺ then diffuses through ATP synthase/ stalked particle, which results in ADP
and Pi being phosphorylated into ATP.
State where in the cell glycolysis takes place.
Cytoplasm.
Biology
A-Level
9700
Explain why the hexose is converted to substance hexose biphosphate in glycolysis.
Hexose is rich in energy, but it does not react easily, so phosphorylation activates hexose. It also
maintains the concentration gradient of glucose inside and outside the cell.
Explain why glucose needs to be converted to hexose bisphosphate during glycolysis.
It provides activation energy for it to split.
Briefly describe what happens to pyruvate if yeast is deprived of oxygen.
Pyruvate does not enter the Krebs cycle. It is decarboxylated/ CO₂ is released, to form ethanal. Ethanal
then gets converted to ethanol with the help of ethanol dehydrogenase, by taking hydrogens from
reduced NAD. Thus, reduced NAD does not enter the ETC. This process is irreversible.
Describe what happens to pyruvate in a yeast cell when oxygen is not present.
Pyruvate is decarboxylated and ethanal is produced. Ethanal is then reduced, by reduced NAD, to
Outline the process of anaerobic respiration in mammal cells.
Pyruvate is converted to lactate by reduced NAD in the liver/ muscle cells. This process is reversible.
ethanol, with the help of ethanol dehydrogenase.
Write down the major products of the following processes:



Glycolysis: ATP, pyruvate, and reduced NAD.
Krebs Cycle: ATP, reduced NAD/ reduced FAD, and CO₂.
Oxidative phosphorylation: ATP, water, and NAD/ FAD.
Describe the process of oxidative phosphorylation in the mitochondrion.
Reduced, NAD/ FAD is passed to the ETC, which is on the inner membrane of the mitochondrion/ cristae.
Hydrogen is released (from reduced NAD/ FAD) and is split into electrons and protons. The protons
remain in the matrix, while electrons pass along carriers/ cytochromes via a series of redox reactions
which form an energy gradient. Energy is released allowing protons to be pumped into the
intermembrane space, building a proton gradient. The protons then pass through (protein) channels and
ATP synthase/ stalked particles. ATP is produced. This process is called chemiosmosis. Remaining
electrons are transferred to oxygen with the addition of protons (to oxygen) to form water/ (oxygen is)
reduced to water.
Biology
A-Level
9700
Outline the process of oxidative phosphorylation.
Hydrogens are split into protons and electrons. The electrons pass along the ETC, and energy is released.
The energy released is used to pump protons, from the matrix, to the intermembrane space. The inner
membrane is impermeable to protons. A proton gradient forms. Protons move down their gradient,
through ATP synthase/ ATP synthetase. The enzyme rotates, and ATP is produced.
Explain the roles of NAD in anaerobic respiration in both plants and animals.
NAD becomes reduced during glycolysis in the cytoplasm. In plants, pyruvate is converted to ethanal,
and then ethanal is reduced, by reduced NAD, to form ethanol. In animals, pyruvate is converted to
lactate by reduced NAD in the liver/ muscles, which allows glycolysis to continue.
Explain briefly the effect of an increase in temperature on the rate of respiration.
There will be an increase in the rate of respiration as kinetic energy increases/ more enzyme-substrate
complexes/ enzyme activity increases. (Effects of too high a rise in temperature) Enzymes may be
denatured.
Explain what is meant by the following terms:


Decarboxylation: removal of carbon dioxide/carboxyl group.
Dehydrogenation: removal of hydrogen.
State the number of reduced NAD molecules that are formed in the Krebs cycle from one acetyl group
that enters the cycle from acetyl CoA.
3.
Describe how the production of lactate in muscle tissue differs from anaerobic respiration in yeast.
There is no decarboxylation/carbon dioxide is not removed. Lactate fermentation occurs in a single step,
and the enzyme lactate dehydrogenase is used. The process is reversible.
Explain why ATP is needed at the start of glycolysis.
It is needed to raise the chemical PE of glucose/ provide activation energy.
State the role of NAD in glycolysis.
It removes hydrogen/ is a hydrogen carrier/ coenzyme.
State the part of the mitochondrion in which the electron transport chain is found.
Crista(e)/ inner membrane.
Biology
A-Level
9700
Describe briefly where the electrons that are passed along the electron transport chain come from.
Electron comes from hydrogen atoms from reduced NAD/ reduced FAD. Reduced NAD comes from
dehydrogenation/ oxidation reactions from substances in the Krebs cycle/ link reaction/ glycolysis in the
matrix of the mitochondrion/ cytoplasm.
Describe the role of oxygen in the process of oxidative phosphorylation.
Oxygen is the final electron acceptor/ accepts electron from the last carrier, so the carrier can be
reduced again. This allows electrons to keep flowing along the ETC/ so the ETC can continue to work.
Oxygen combines with H⁺ to form water.
State how many molecules of ATP are produced from one molecule of glucose during glycolysis.
4/ net 2.
Name the two types of reaction that occur during the conversion of pyruvate to acetyl CoA in the link
reaction.
Dehydrogenation, and decarboxylation.
Name the location, in the mitochondrion, of the link reaction.
The matrix.
Describe what happens to the hydrogen released during the link reaction.
It is accepted by NAD, and passed to the ETC for oxidative phosphorylation.