8.1 Cell Respiration
Topic 8 Cell Respiration & Photosynthesis

Cell Respiration
 8.1.1 State that oxidation involves the loss of electrons
from an element, whereas reduction involves a gain
frequently involves gaining oxygen or losing hydrogen,
whereas reduction frequently involves losing oxygen or
gaining hydrogen.
 8.1.2 Outline the process of glycolysis, including
phosphorylation, lysis, oxidation and ATP formation.
 In the cytoplasm, one hexose sugar is converted into two
three-carbon atom compounds (pyruvate) with a net gain
of two ATP and two NADH + H+
.
 8.1.3 Draw and label a diagram showing the structure of a
mitochondrion as seen in electron micrographs.

 8.1.4 Explain aerobic respiration, including the link
reaction, the Krebs cycle, the role of NADH + H+
, the
electron transport chain and the role of oxygen.
 In aerobic respiration (in mitochondria in eukaryotes),
each pyruvate is decarboxylated (CO2 removed). The
remaining two-carbon molecule (acetyl group) reacts with
reduced coenzyme A, and, at the same time, one NADH +
H+ is formed. This is known as the link reaction.

 In the Krebs cycle, each acetyl group (CH3CO) formed in
the link reaction yields two CO2. The names of the
intermediate compounds in the cycle are not required.
Thus it would be acceptable to note:

 8.1.5 Explain oxidative phosphorylation in terms of
chemiosmosis.
 8.1.6 Explain the relationship between the structure of the
mitochondrion and its function.
 (Limit this to cristae forming a large surface area for the
electron transport chain, the small space between inner
and outer membranes for accumulation of protons, and
the fluid matrix containing enzymes of the Krebs cycle.)

Oxidation & Reduction
 Cell Respiration involves many oxidation and reduction
reactions.
 Oxidation and Reduction occur together;
As one reactant is oxidised the other is reduced.
 Remember: OIL RIG
 Oxidation Is Loss (of electrons or hydrogen)
 Reduction Is Gain (of electrons or hydrogen)

A Comparison of Oxidation and Reduction
 Oxidation Reactions
 Addition of oxygen
atoms to a substance.
 Removal of
hydrogen atoms
from a substance.
 Loss of electrons
from a substance
 Reduction Reactions
 Removal of oxygen
atoms from a
substance.
 Addition of hydrogen
atoms to a substance.
 Gain of electrons
from a substance

Hydrogen Carriers
 NAD+
(nicotinamide adenine dinucleotide)
is the most commonly used hydrogen
carrier.
 When a substrate is oxidised by the
removal of 2 hydrogen atoms, NAD+
accepts the electrons from both atoms and
proton from one:
NAD+
+ 2H  NADH + H+

Hydrogen Carriers
 Other hydrogen acceptors include:
Oxidised State Reduced State
NAD +
NADH + H+
NADP +
NADPH + H+
FAD FADH2

Diphosphate & Bisphosphate
 Both of these terms mean that the molecule has 2
phosphate groups attached to it.
 Di means two phosphates are attached to each other.
 Bis means that the two phosphates are attached to
different parts of the molecule.

Glycolysis
 The first step in Cellular Respiration is Glycolysis.
 Glycolysis can occur in the absence of oxygen,
anaerobically.
 Thus glycolysis is the first step for both anaerobic or
aerobic respiration.
 A summary of Glycolysis:
 Occurs in the cytoplasm
 One glucose molecule is converted into two pyruvate
molecules.
 Two ATP molecules are used but 4 are produced, so there is a
net yield of 2 ATP.
 2NAD+
are converted into two NADH + H+

Glycolysis
Ref: IB Biology Higher Level, OSCRef: Biology for the IB Diploma, Allott

Glycolysis
 There are 4 main stages to Glycolysis:
1. 2 phosphates are added to a molecule of glucose to form glucose
bisphosphate. Adding a phosphate groups is called phosphorylation.
2 ATP molecules provide the phosphate groups.
2. Glucose bisphosphate is split into 2, 3 carbon molecules called
triose phosphate. Splitting molecules is called Lysis.
3. 2 hydrogen atoms are removed from each trios phosphate molecule
(NAD+
is the hydrogen acceptor). This is an oxidation reaction. The
energy released is used to add another phosphate group to each
triose phosphate molecule.
4. Pyruvate is formed by removing the two phosphate groups and by
passing them to ADP. This results in ATP formation. This is called
substrate level phosphorylation.

Mitochondrion Structure
Ref: Biology for the IB Diploma, Allott

Mitochondrion Structure
Ref: Biology for the IB Diploma, Allott

Cellular Respiration
 Cellular Respiration can proceed along two paths:
 Aerobic Respiration
 Involving the use of oxygen
 Anaerobic Respiration
 Without Oxygen

Aerobic Respiration
 Aerobic Respiration can be divided up into a number of
stages:
 Glycolysis
 The Link Reaction
 The Krebs Cycle
 The Electron Transport chain
 Glycolysis occurs in the cytoplasm, whereas the rest
occur in the Mitochondrion.

The Link Reaction
 In the Link Reaction, Pyruvate, the end product of
glycolysis, moves to the mitochondrion.
 Enzymes in the matrix of the mitochondrion remove
hydrogen and carbon dioxide.
 The hydrogen is accepted by NAD+
(oxidation).
 The removal of carbon dioxide is called decarboxylation.
 The whole conversion is called oxidative
decarboxylation.
 The product of this is an acetyl group, which is accepted
by an enzyme, coenzyme A, (Co A).
 This forms Acetyl Co A

The Link Reaction
Ref: Biology for the IB Diploma, Allott

The Krebs Cycle
 The Krebs Cycle occurs in a number of stages:
1. The acetyl CoA enters the cycle and is joined to a 4 carbon compound
(C4) oxoaloacetate.The Co A is released and recycled within the
mitochondrion. This forms a 6 carbon compound, citrate (C6).
2. Citrate (C6) is converted to a 5 carbon compound (C5). Carbon dioxide
is released (decarboxylation) and NAD+
accepts 2 hydrogen atoms
(oxidation).
3. The C5 compound is converted to a C4 compound. Carbon dioxide is
released (decarboxylation) and NAD+
accepts 2 hydrogen atoms
(oxidation).
4. The final stage involves the synthesis of ATP from ADP (substrate
level phosphorylation)and two oxidation reactions:
NAD+
+ 2H  NADH + H+
FAD + 2H  FADH2

Ref: Biology for the IB Diploma, Allott

The Krebs Cycle
 One turn of the Krebs Cycle yields:
 2 CO2
 3 NADH + H+
 1 FADH2
 1 ATP (by substrate level phosphorylation)
 Remember that there are 2 turns of the Krebs cycle for
each glucose molecule.

The Electron Transport Chain
 The last step of aerobic respiration is the electron transport chain.
 The ETC passes 2 electrons from NADH or FADH2 from one
electron carrier to another (these electron carriers are found in the
inner membrane of the mitochondrion) by a series of
oxidation/reduction reactions.
 The hydrogens are pumped across the membrane to the thin inter-
membrane space by the energy released from the electrons.
 The final acceptor of the electrons is oxygen, which uses them to
combine with hydrogen to form water. This occurs in the matrix of
the mitochondrion.
 Cytochrome oxidase catalyses this last reaction. Some metabolic
poisons, such as cyanide, inhibit the action of this enzyme, with
potentially fatal results

The Electron Transport Chain
Ref: Biology for the IB Diploma, Allott

Oxidative phosphorylation & Chemiosmosis
 In 1961, Peter Mitchell, a British biochemist, presented what is termed
the Chemiosmotic Hypothesis of ATP production. He received a Nobel
prize for this work in 1978.
 How it works:
 As electrons are passed down the ETC, protons are being pumped into the
inter-membrane space.
 This forms a concentration gradient, which is a store of potential energy.
 ATP synthase, located in the inner mitochondrial membrane, transports
the protons back across the membrane, down the concentration gradient.
 As the protons pass across the membrane they release energy and this
energy is used by ATP synthase to produce ATP from ADP and a
phosphate group.
 As the ATP produced relies on the energy released by oxidation, it is
called oxidative phosphorylation.

Ref: IB Biology Higher Level, OSC

Energy Produced by Aerobic Respiration
 The net result of the ETC is that:
 1 NADH + H+
supplies enough energy to produce 3 ATP,
 1 FADH2 supplies enough energy to produce 2 ATP.
 Thus the net production of energy from aerobic
respiration from ONE glucose molecule is:
Stage ATP
Glycolysis 2 ATP used at the start
2 NADH + H+
Substrate level phosphorylation
-2 ATP
6 ATP
4 ATP
Link Reaction 2 NADH + H+
6 ATP
Krebs cycle Substrate level phosphorylation
6 NADH + H+
2FADH2
2 ATP
18 ATP
4 ATP
Net Yield of ATP 38 ATP

Mitochondrion Structure
 The structure of the mitochondrion should now make
more sense.
 The large amount of infolding of the inner membrane,
forming cristae, provide a large surface area for the
electron transport chain.
 The small space between the inner and outer membranes
allow for the accumulation of protons (hydrogen atoms),
driving ATP synthesis.
 The fluid filled matrix contains enzymes needed for the
Krebs cycle.

The Central Role of Acetyl CoA
 Acetyl groups are the substrate used in the Krebs cycle.
 CoA is a carrier molecule which brings the acetyl groups
into the Krebs cycle.
 Acetyl CoA is formed in both carbohydrate and fat
metabolism.
 Carbohydrates are converted into pyruvate and then to acetyl
CoA in the link reaction.
 Fats are broken down into fatty acids and glycerol. The fatty
acids are then broken down into 2 Carbon fragments and
oxidised to form acetyl CoA.

Ref: Biology for the IB Diploma, Allott

IBO guide:
 8.1.1 State that oxidation involves the loss of electrons
from an element, whereas reduction involves a gain
frequently involves gaining oxygen or losing hydrogen,
whereas reduction frequently involves losing oxygen or
gaining hydrogen.
 8.1.2 Outline the process of glycolysis, including
phosphorylation, lysis, oxidation and ATP formation.
 In the cytoplasm, one hexose sugar is converted into two
three-carbon atom compounds (pyruvate) with a net gain
of two ATP and two NADH + H+
.
 8.1.3 Draw and label a diagram showing the structure of a
mitochondrion as seen in electron micrographs.

 8.1.4 Explain aerobic respiration, including the link
reaction, the Krebs cycle, the role of NADH + H+
, the
electron transport chain and the role of oxygen.
 In aerobic respiration (in mitochondria in eukaryotes),
each pyruvate is decarboxylated (CO2 removed). The
remaining two-carbon molecule (acetyl group) reacts with
reduced coenzyme A, and, at the same time, one NADH +
H+ is formed. This is known as the link reaction.

 In the Krebs cycle, each acetyl group (CH3CO) formed in
the link reaction yields two CO2. The names of the
intermediate compounds in the cycle are not required.
Thus it would be acceptable to note:

 8.1.5 Explain oxidative phosphorylation in terms of
chemiosmosis.
 8.1.6 Explain the relationship between the structure of the
mitochondrion and its function.
 (Limit this to cristae forming a large surface area for the
electron transport chain, the small space between inner
and outer membranes for accumulation of protons, and
the fluid matrix containing enzymes of the Krebs cycle.)