notes taken from CGP A2 Biology Revision guide specific to the OCR exam board.

1. Respiration
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2. Aerobic Respiration
• Four Stages in Aerobic Respiration
– Glycolysis Series of reactions;
products from these
– Link reaction reactions are used in
final stage – to
produce ATP
– Krebs cycle Last three occur in
the mitochondria –
(Glycolysis in the
– Oxidative phosphorylation cytoplasm)
Cristae
Inner membrane
Matrix
Outer membrane

3. Stage one, Glycolysis
Glycolyisis makes Pyruvate Glycolysis
from Glucose
• Glycolysis involves splitting
1 molecule of glucose (6C) Link reaction
into 2 smaller molecules of
pyruvate (3C).
• Occurs in the cytoplasm Krebs cycle
• The first stage of both
aerobic and anaerobic Oxidative
and doesn’t need oxygen phosphorylation
to take place – anaerobic
process

4. 2 Stages of Glycolysis
Stage 1: Phosporylation glucose (6C)
1) Glucose is 2ATP
phosphorylated by adding
2 phosphates from 2 2ADP
molecules of ATP hexose biphosphate (6C)
2) This creates 1 molecule of
hexose bisphosphate and 2x triose phosphate (3C)
2 molecules of ADP
4ADP+4P 2H+ 2NAD
3) Then, hexose
bisphosphate is split up 4ATP 2reducedNAD
into 2 molecules of triose
phosphate. 2x pyruvate (3C)

5. 2 Stages of Glycolysis
Stage 2: Oxidation glucose (6C)
• Triose phosphate is 2ATP
oxidised, forming 2
molecules of pyruvate. 2ADP
• NAD collects the hydrogen hexose biphosphate (6C)
ions, forming 2 reduced
NAD 2x triose phosphate (3C)
• 4 ATP are produced, but 2 4ADP+4P 2H+ 2NAD
were used up in stage one
so there’s a net gain of 2 4ATP 2reducedNAD
ATP.
2x pyruvate 3C

6. Stage 2, the link reaction
Link reaction occurs in
mitochondrial matrix pyruvate (3C)
• Pyruvate is decarboxylated – CO2 (1C)
one carbon atom is removed
from pyruvate in the form of CO2 NAD
• NAD is reduced – it collects reduced NAD
hydrogen from pyruvate,
changing pyruvate to acetate acetate (2C)
• Acetate is combined with coenzyme A (CoA)
coenzyme A (CoA) to form
acetyl coenzyme A (acetyl
CoA)
• No ATP is produced in this
acetyl CoA (2C)
reaction

7. Link reaction
The Link Reaction occurs twice for
every glucose molecule Glycolysis
• Two molecules of acetyl
coenzyme A go into Krebs
cycle Link reaction
• Two CO2 molecules are
released as a waste product of
respiration Krebs cycle
• Two molecules of reduced
NAD are formed and go to the Oxidative
last stage (oxidative phosphorylation
phosphorylation)

8. Mitochondria
Mitochondria are adapted to their function:
• Inner membrane is folded into cristae, which increases membrane’s surface area to
maximise respiration
• There are lots of ATP Synthase molecules in the inner membrane to produce lots of
ATP in the final stage of respiration
• The matrix contains all the reactants and enzymes needed for the Krebs cycle to
take place.
Cristae – increase Inner membrane –
surface area to lots of ATP
maximise synthase molecules
respiration in here to produce
lots of ATP in final
stage
Matrix – contains
all the reactants Outer membrane
and enzymes
needed for the
Krebs cycle

9. Stage 3, the Krebs Cycle
The Krebs cycle produces Reduced Coenzymes and ATP
1)
– Acetyl CoA from the link reaction combines with oxaloacetate to
form citrate
– Coenzyme A goes back to the link reaction to be used again
Acetyl CoA (2C)
CoA
Oxaloacetate (4C) Citrate (6C)
CO2 (1C)
reduced NAD
NAD
reduced FAD NAD
FAD
ATP reduced NAD
ADP + Pi
5C compound
reduced NAD

10. Stage 3, the Krebs Cycle
2) Acetyl CoA (2C)
– the 6C citrate CoA
molecule is converted to
a 5C molecule Oxaloacetate (4C) Citrate (6C)
– Decarboxylation CO2 (1C)
occurs where CO2 is reduced NAD
NAD
removed reduced FAD
FAD
NAD
– Dehydrogenation also ATP reduced NAD
occurs where hydrogen ADP + Pi
reduced NAD
5C compound
is removed NAD
CO2 (1C)
– The hydrogen is used
to produce reduced
NAD from NAD

11. Stage 3, the Krebs Cycle
3) Acetyl CoA (2C)
– The 5C molecule is then CoA
converted to a 4C molecule (there
are some intermediate compounds
formed during this conversion) Oxaloacetate (4C) Citrate (6C)
– Decarboxylation and
dehydrogenation occur, producing CO2 (1C)
one molecule of reduced FAD reduced NAD
and two of reduced NAD NAD
– ATP is produced by the direct reduced FAD
FAD
NAD
transfer of a phosphate group ATP reduced NAD
from an intermediate compound ADP + Pi
5C compound
to ADP reduced NAD
NAD
– When a phosphate group is CO2 (1C)
directly transferred from one
molecule to another it’s called
substrate-level phosphorylation.
– Citrate has now been converted
to oxaloacetate.

12. Stage 3, the Krebs cycle
• The Krebs cycle involves a
Glycolysis
series of oxidation-
reduction reactions which
takes place in the matrix of Link reaction
the mitochondria.
• The cycle happens once Krebs cycle
for every pyruvate
molecule, it goes round Oxidative
twice for every glucose phosphorylation
molecule

13. Products of the Krebs cycle
• Some of the products of
the Krebs cycle are used
in Oxidative
Phosphorylation
Products from one Krebs cycle Where it goes
1 x coenzyme A Reused in next link reaction
Oxaloacetate Regenerated for use in the next Krebs cycle
2 x CO2 Released as a waste product
1 ATP Where it goes
3 reduced NAD Reused in next link reaction
1 reduced FAD Regenerated for use in the next Krebs cycle

14. Stage 4, Oxidative Phosphorylation
• Produced lots of ATP
• Process where the energy Glycolysis
carried by electrons from
reduced coenzymes (reduced
NAD and reduced FAD) is used Link reaction
to make ATP. (The whole point
of the previous stages is to make
reduced NAD and reduced FAD Krebs cycle
for the final stage)
• (Oxidative phosphorylation Oxidative
involves 2 processes – electron phosphorylation
transport chain and
chemieosmosis.

15. Stage 4, Oxidative Phosphorylation
Protons are pumped across the inner Mitochondrial Membrane.
• Hydrogen atoms are released from reduced NAD and reduced FAD as they’re
oxidised to NAD and FAD – H atoms split into protons and electrons (H+ and e-)
• e- move along electron transport chain (made up of 3 electron carriers) losing
energy at each carrier.
• This energy is used by electron carriers to pump protons from the mitochondrial
matrix into the intermembrane space (space between the inner and outer
mitochondrial membranes)
• Concentration of protons is now higher in the intermembrane space than the
mitochondrial matrix - forms an electrochemical gradient
• Protons move down the electrochemical gradient back into the matrix via ATP
synthase - this movement drives the synthesis of ATP from ADP and inorganic
phosphate.
• Movement of H+ across a membrane, generates ATP = CHEMIOSMOSIS
• In matrix, at ed of transport chain the protons, electrons and O2 combine with water
Oxygen is said to be the final electron acceptor

16. Stage 4, Oxidative Phosphorylation
outer mitochondrial membrane
high H+ concentration
H+ H+ H+ H+ H+
H+ H+
innermembrane space H+
H+ H+
inner mitochondrial membrane
matrix of mitochondrion ADP +
Pi
2e-
reduced NAD 2H
2e- ATP
H2O
NAD H+
H+ H+ H+
2H+ ATP
H+ syntase
low H+ concentration ½ O2 + 2H-

17. one mole of glucose makes 38 atp
ELECTRON TRANSPRORT CHAIN
GYCOLYSIS
2 ATP
ALL TOGETHER
2 NADH
4 ATP
LINK REACTION
1 NADH x 2  2NADH 1 NADH = 3 ATP 30 ATP
10 NADH
1 FADH2 = 2 ATP
KREBS CYCLE 2 FADH2 4 ATP
1 ATP x2  2ATP
3 NADH x2  6NADH
1 FADH2 x2  2 FADH2