7.1 Cell Respiration

Topic 7 Cell Respiration & Photosynthesis

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