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8.1.1 Forms of oxidation and reduction.

In respiration the oxidation of organic compounds is coupled to the reduction of ADP to ATP.
The oxidation of ATP is then coupled to biological processes such as muscle contraction of protein synthesis.

Oxidation: often associated with the release of energy

Reduction: often associated with the gain of energy

8.1.2 Glycolysis.

Location: Cytoplasm of all cells

Outline: Oxidation of Glucose (6 carbons) to two Pyruvate (3 carbons) is coupled to the reduction of ADP to ATP

In the following models the hydrogen and oxygen are not shown. The models show the number of carbons in each molecule not the structural formula.

The first stage actually begins by phosphorylating glucose to a hexose diphosphate.

The phosphate groups allow a stronger interaction between the hexose and its enzyme.

Click the image to the right to see the effects on the energy content of the molecule.

This stage involves the breaking of the hexose diphosphate into two triose phosphate molecules.

The triose phosphate is an intermediate in many biochemical reactions.

The phosphate group allows the sugar to form stronger interaction with the next enzyme in the pathway.

This is the main oxidative stage of glycolysis which results in the formation of ATP and NADH + H⁺

Each Triose phosphate is oxidised to a 3 carbon molecule called Pyruvate

Each TP has hydrogen removed (oxidation) to reduce one NAD⁺ to NADH

Each TP adds a phosphate to Adenosine Diphosphate reducing this to ATP (substrate level phosphorylation)
Note that each Triose phosphate releases enough energy for the formation of two ATP

Summary of glycolysis:

Remember in the examination you will come across the names of the molecules and stages rather than these model diagrams. So make sure you learn the terminology.

Glycolysis takes place in the cytoplasm of the cell.
It does not require oxygen.
The hexose sugar (glucose) is converted into two 3C atoms compounds called pyruvate.
Two ATP are consumed but four are produced making a net gain of 2 ATP
Two NADH + H⁺ are produced which will yield more ATP when they are transferred to the mitochondria and oxidative phosphorylation.
Yield: 2 Pyruvate + 2 ATP + 2NADH + 2H⁺

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8.1.3 Structure of the mitochondria.

Location of aerobic respiration

Pyruvate, the product of glycolysis can be further oxidised here to release more energy.

Mitochondria are only found in eukaryotic cells.

Cells that need a lot of energy will have many mitochondria ( liver cell) or can develop them under training (muscles cells).

There is a double membrane.
The inner membrane is folded to form 'cristae'.
There is a space between the two membranes which is important for creating a place to concentrate H⁺ (see 8.1.6 )
The inner space is called the matrix.
Mitochondria contain some of their own DNA (mDNA).

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8.1.4 Aerobic respiration.

Stages in the Aerobic respiration:

Link Reaction: Pyruvate is transported into the matrix of the mitochondria

Krebs cycle: carbon fragments (C2) are progressively decarboxylated to yield ATP and reduced coenzymes

Electron Transport System: reduced coenzymes are used to generate more ATP (see 8.1.5).

Link Reaction: Pyruvate(3C) is transported to the matrix of the mitochondria

A large Co-enzyme A joins with the 3 carbon fragment pyruvate.

Pyruvate is decarboxylated removing a single carbon as carbon dioxide.

The remaining fragment is an Acetyl group and temporarily forms Acetyl CoA.

NAD⁺ is reduced to NADH + H^+.

Acetyl (2C) is already transported into the matrix.

Krebs Cycle: oxidative decarboxylation of the C2 Acetyl group (CH₃CO). This cycle has been broken down into 4 steps. The carbons from the original glucose molecule are shown in purple and those of mitochondria molecules in blue.

Acetyl CoA joins with the C4(acceptor)group

CoA is released to transport more pyruvate into the matrix

A C6 fragment is formed (citric acid)

C6 (Citric Acid) is oxidatively decarboxylated.

A C5 group is formed.

The Carbon is given off as Carbon Dioxide

NAD⁺ is reduced to NADH + H⁺

The C5 fragment is oxidised and decarboxylated further to a C4 compound.

Again the carbon removed forms carbon dioxide.

NAD⁺ is further reduced to NADH + H⁺.

The final stage in the cycle has the C4 acceptor regenerated.

There is a reduction of NAD⁺ to NADH + H⁺.

FAD (Coenzyme)is reduced to FADH₂ .

ADP is reduced to ATP

The krebs cycle is an example of the metabolic cycles mentioned in section 7.6.1 . Each step in the cycle requires enzymes to reduce the activation energy. The reactions all take place in the matrix of the mitochondria and are usually represented as a circular diagram. Try to overcome the idea that the molecules are going in a circle but more that this reaction is taking place within the confined space of the matrix where each intermediate becomes the substrate for the next step.

(a) Pyruvate (3C)

(b) Link reaction

(d) Recycling of CoA

(e) Decarboxylation C6 to C5 and the reduction of NAD

(f) Decarboxylation C5 to C4 and the reduction of NAD

(g) C4 to C4 with the reduction of coenzymes FAD and NAD. ATP is made directly.

(h) C4 to C4 acceptor

This cycle follows one acetyl group.

Each glucose that enters glycolysis will produce 2 acetyl groups.

8.1.5 Oxidative phosphorylation in terms of chemiosmosis.

On the inner membrane of the mitochondria (Cristae) there are membrane proteins.

The oxidation of reduced coenzymes (NADH + H⁺and FADH₂) allows these membrane proteins to pump protons (H⁺) into the space between the outer and inner mitochondrial membranes.
The electrons released from the reduced coenzyme flows along the electron transfer chain of proteins.

These H⁺form a high concentration (low pH) within this space. They diffuse back to the matrix through a channel in a membrane protein called an ATP synthetase.

This flow of H⁺through the ATP synthetase drives an enzyme reaction that brings about the phosphorylation of ADP to ATP.

The following sequence of diagrams breaks down the process of oxidative phosphorylation into a number of stages. There are a number of membrane proteins involved in this process. Only a few of these proteins are shown and then only to allow specific reference to the diagrams. The overall process is shown but it is not realistic at this level to 'balance' the chemistry.

Oxidative Phosphorylation coupled to the synthesis of ATP.

The membrane shown is only the inner mitochondrial membrane folded into the cristae.

The NADH is oxidised and the reduced proteins transport H⁺from the matrix into the space between both mitochondrial membranes.

There is electron transfer down the chain of proteins in a series of oxidation and reductions.

For each NADH, 3 Moles of H⁺ are pumped into the space.

Oxygen supplied by the respiratory/ circulatory system acts as the final H⁺acceptor forming water.

The FADH₂is oxidised and the reduced membrane proteins pump H⁺into the space between the mitochondrial membranes.

The H⁺diffuse back to the matrix driving the ATP Synthetase to produce ATP.

One FADH₂ produces two moles of hydrogen ions.

Again, the H⁺ are accepted by oxygen to form water.

A concentration gradient has been created between the high concentration of H⁺ between the mitochondrial membranes and the lower concentration in the matrix.