Respiration - AL only (3.5.2)

Respiration is a process carried out by cells to release energy for cellular processes such as active transport and muscle contraction.

Cells break down glucose using oxygen, producing water and carbon dioxide.

The energy released is used to produce ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and a phosphate ion.

Coenzymes NAD and FAD can be represented in different ways. For example, coenzyme NAD can also be written as NAD⁺, whilst reduced NAD can be shown as NADH or NADH⁺ + H⁺.

The coenzymes and their reduced forms in the following notes are shown in the same way as in the exam board specification.

Glycolysis is the first step of respiration and is an anaerobic process that occurs in the cytoplasm of the cell.

• Glucose undergoes phosphorylation, using 2 × ATP, which produces glucose phosphate and 2 × ADP.
• Glucose phosphate is then split, producing 2 × triose phosphate.
• Each triose phosphate is oxidised, which converts the coenzyme NAD to reduced NAD. This process also produces ATP by using each triose phosphate to convert 2 × ADP to 2 × ATP by removing 1 × phosphate from each triose phosphate and using an additional phosphate group.
• A 3C pyruvate is produced by each molecule of triose phosphate.

Per molecule of glucose, glycolysis uses 2 × ATP in the first stage of phosphorylation, but later 4 × ATP are produced in the last step. Overall, glycolysis produces 2 × ATP.

The link reaction occurs in the mitochondrial matrix.

Pyruvate moves into the mitochondria and is oxidised to make acetate. This involves removing carbon dioxide and also converts NAD to reduced NAD.

Acetate combines with coenzyme A (CoA) to make acetyl coenzyme A (acetyl CoA).

Overall reaction:

The Krebs cycle occurs in the mitochondrial matrix.

• Acetyl CoA transfers the acetate (2C) to a 4C compound, which makes a 6C compound.
• This 6C compound is involved in a series of reactions where 2 × carbon dioxide are produced. Reduced NAD, reduced FAD and 1 × ATP are also made in this process.
• These reactions reform the 4C compound.

Overall yield: Reduced NAD, reduced FAD, 2 × 1 × ATP.

NAD and FAD are coenzymes involved in cellular processes including respiration. NADP is a coenzyme involved in photosynthesis.

NAD and FAD transport hydrogen ions and electrons and are converted into their reduced forms in the process.

Dehydrogenase enzymes can remove the hydrogen ions and electrons from the reduced coenzymes to transfer to other molecules during reactions (e.g., oxidative phosphorylation).

Oxidative phosphorylation occurs in the cristae of the mitochondria. This process uses reduced NAD, reduced FAD and oxygen.

A series of reactions takes place, which produce water and ATP.

The hydrogen ions and electrons from the reduced NAD and reduced FAD are delivered to the electron transport chain for oxidative phosphorylation.

The electrons flow along the four complexes which are embedded in the inner mitochondrial membrane. Each complex is reduced when the electrons are received, and then oxidised when the electrons are passed to the next complex.

The energy released during this process is used to create a hydrogen ion gradient within the mitochondria.

The hydrogen ions released from the reduced coenzymes are used to create a hydrogen ion gradient within the mitochondria.

The concentration of hydrogen ions is higher in the intermembrane space than in the mitochondrial matrix. This creates a steep electrochemical gradient within the mitochondria. The intermembrane space is more positively charged than the matrix.

Hydrogen ions flow down the electrochemical gradient into the mitochondrial matrix, through ATP synthase. As this happens the ATP synthase changes shape allowing for ATP synthesis from ADP and P_i. Oxygen is the final electron and hydrogen ion acceptor, producing water.

This process is sometimes referred to as the chemiosmotic theory.

Mammals carry out lactate fermentation.

Lactate fermentation produces less ATP than aerobic respiration, providing only enough for essential processes for a short period of time.

During lactate fermentation, reduced NAD is converted back into NAD, so glycolysis can continue. This process also converts pyruvate to lactic acid:

Afterwards, when enough oxygen is present, lactate is converted back into pyruvate or can be converted to glycogen in the liver.

Alcoholic fermentation occurs in yeast, some bacteria and occasionally plant roots. It is used in processes such as brewing and baking.

This process converts reduced NAD back into NAD, allowing for glycolysis to continue.

Aerobic respiration produces more ATP than anaerobic respiration.

During anaerobic respiration, ATP is only produced during glycolysis which yields 2 × ATP per molecule of glucose.

The link reaction, Krebs cycle and electron transport chain are not able to take place under anaerobic conditions.

Glucose can be broken down to produce ATP through the process of respiration.

Triglycerides, alcohol and proteins can also act as respiratory substrates. This occurs through a different process than aerobic respiration as additional stages are required before respiration can occur.

Different quantities of ATP are produced by the different respiratory substrates.

To be used as a respiratory substrate, triglycerides are first hydrolysed to produce 1 × glycerol and 3 × fatty acid molecules.

Glycerol is converted to triose phosphate, which can then be used in oxidative decarboxylation (link reaction), the Krebs cycle and oxidative phosphorylation.

Fatty acids are gradually converted to acetylcoenzyme A (acetyl CoA), which is then used in the Krebs cycle and oxidative phosphorylation.