Salters-Nuffield A level Biology Student Book 2 ActiveBook
Full and complete notes from Topic 7 of A-Level Biology A (Salters-Nuffield). Presented in a concise manner with all necessary diagrams given and all specification points covered. Achieved a high A* using solely these notes.
TOPIC 7 – Run For Your Life
Aerobic Respiration Background:
• Aerobic respiration involves generating energy via the splitting of glucose into
CO2 as a waste product, and H2, which reduced oxygen to produce H2O. It
produces 36/38 ATP molecules per cycle.
✓ Aerobic respiration is a series of chemical reactions, and is therefore a
metabolic pathway.
C6H12O6 + 6O2 → 6CO2 + 6H2O
• The energy released via respiration, is used to phosphorylate ADP into ATP.
• ATP is a currency of energy that carries it from energy-yielding reactions to
energy-requiring reactions.
✓ It is created via the phosphorylation of ADP using an inorganic
phosphate (Pi). There are 2 types of phosphorylation:
▪ Substrate-Level: The phosphate for the formation of ATP
comes from a substrate such as intermediary compounds in
glycolysis. Happens everywhere apart from ETP.
▪ Oxidative: Redox reactions along the ETP ending with oxygen
as the final acceptor, creates energy via chemiosmosis which
drives ATP synthase to phosphorylate ADP + Pi.
• There are 4 stages to aerobic respiration:
✓ Glycolysis
✓ Link Reaction
✓ Krebs Cycle
✓ Oxidative Phosphorylation
▪ The first three stages are a series of reactions,
and the products of these stages are used in
the final stage to produce ATP.
▪ Glycolysis occurs in the cytoplasm
of a cell, whereas the remainder of
the stages occur within
mitochondria.
• Glucose is a respiratory substrate meaning it can
be respired in an enzymatic reaction.
COENZYMES:
• Coenzymes play a significant role in respiration. There are 3 main ones in
aerobic respiration:
✓ NAD & FAD transfer hydrogen from one molecule to another. They are
therefore involved in REDOX reactions, which form the ETC.
✓ Coenzyme A transfers the intermediary carbon compound to acetyl
CoA, which is then used in the Krebs Cycle.
,Biology A2 Nikhil 13Y Topic 7 Full Notes
Glycolysis:
• This is the first step in respiration and is anaerobic. It produces 2 molecules of
pyruvate (3C), from one molecule of glucose (6C).
• It occurs in the cytoplasm of cells. It DOES NOT require oxygen.
STEPS:
1. The initial glucose molecule is phosphorylated (substrate-level) using 2
molecules of inorganic phosphate from 2 molecules of ATP.
✓ This creates two molecules of triose phosphate and 2 molecules of
ADP.
2. The triose phosphates are then oxidised lose hydrogen (protons). This
results in 2 molecules of pyruvate.
✓ The coenzymes NAD is reduced, and gains these 2 hydrogen ions.
This results in 2 molecules of reduced NAD.
▪ These reduced NADs are used for oxidative phosphorylation.
3. This then releases 4 molecules of ATP, so there is a net gain of 2.
• The pyruvate then moves into the mitochondrial matrix for the link reaction.
The Link Reaction:
• The link reaction occurs in the mitochondrial matrix, as the enzymes needed
for this reaction are located there. No ATP is produced.
STEPS:
1. Pyruvate is decarboxylated, meaning a carbon is removed. This results in a
by-product of CO2.
2. NAD is reduced as it gains a hydrogen ion, resulting in reduced NAD.
3. This forms a 2C compound, acetate, which combines with coenzyme acetyl
CoA to be used in the Krebs Cycle.
• Glycolysis produces 2 molecules of pyruvate, therefore
per one glucose molecule, the link reaction will occur
twice. This also means that 2 CO2 molecules are made,
and 2 molecules of acetyl CoA are produced. So Krebs
Cycle also occurs twice per one molecule of glucose.
Krebs Cycle:
• The Krebs Cycle is dominated by REDOX reactions, for
which each reaction has a specific intracellular enzyme catalysing it.
✓ These enzymes are found in the mitochondrial matrix.
STEPS:
1. Acetyl CoA from the Link Reaction, combines with an intermediate to form
citrate (6C). The coenzyme A is then recycled to the Link Reaction.
2. Citrate (6C) is then converted into a 5C intermediate compound, via
decarboxylation. This produces CO2 as a by-product.
✓ NAD is reduced to produce reduced NAD.
, Biology A2 Nikhil 13Y Topic 7 Full Notes
3. The 5C compound is converted to a 4C intermediate compound.
Decarboxylation occurs, producing a by-product of CO2.
✓ 2 NAD and 1 FAD are reduced to 2 reduced NAD and reduced FAD.
✓ These reduced coenzymes are used in oxidative phosphorylation.
4. During this 5C → 4C reaction, ADP is phosphorylated by a phosphate group
to produce a molecule of ATP.
• Bear in mind for one molecule of glucose, the Krebs Cycle occurs twice.
Oxidative Phosphorylation:
• This process uses electrons, which are the energy carriers, that are
transported by coenzymes. This is why the previous stages are important to
produce reduced NAD and reduced FAD.
• Oxidative phosphorylation occurs across the mitochondrial membrane.
STEPS:
1. Reduced NAD and reduced FAD are oxidised (loss of hydrogen) to release
hydrogen ATOMS. NAD and FAD coenzymes result, which are regenerated
for use in the Krebs Cycle.
✓ These atoms split into hydrogen ions (H+) and electrons (e-).
2. The electrons move down the electron transport chain. This is made up of
electron carriers. The electrons lose energy as they move down the chain.
3. This lost energy is used to pump protons (H+) from the mitochondrial
matrix → intermembrane space.
✓ The intermembrane space is the space between the inner and outer
membrane of a mitochondrion.
4. The concentration of protons (H+) increases in the intermembrane space, and
an electrochemical gradient forms between the intermembrane space and
the mitochondrial matrix.
5. Protons (H+) move down the electrochemical gradient into the matrix, via
the enzyme, ATP synthase. This movement drives the phosphorylation of
ADP + Pi → ATP. This movement and ATP generation is called
CHEMIOSMOSIS.
6. Oxygen is the final electron acceptor. In the mitochondrial matrix, at the end
of the ETC, protons (H+), electrons and oxygen combine to form water as a
by-product.
• Oxidative phosphorylation produces many ATP, as 3 ATP are produced from
each molecule of reduced NAD, and 2 ATP from each reduced FAD. In
aerobic respiration, 38 ATP are produced.
• The Krebs Cycle is dependent on oxidative phosphorylation, as the
coenzymes are oxidised and regenerated here, so if this stops then so does
the cycle.
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