4. Respiration (Higher)

The Chemistry of Respiration

The Chemistry of Respiration Respiration is the process by which chemical energy is released from food by oxidisation. It occurs in every living cell and involves the regeneration of ATP by a series of reactions.

Metabolism is the sum of all chemical processes that occur in an organism.

Respiration is an example of a metabolic pathway.

Glycolysis The process of cell respiration begins with a 6-carbon molecule of glucose being broken down by a series of enzyme-controlled steps to form two 3-carbon molecules of pyruvic acid

Glycolysis

Glycolysis This process of “glucose-splitting� requires energy from two molecules of ATP to trigger it off but later in the process sufficient energy is released to form 4 molecules of ATP giving a net gain of two molecules of ATP Glycolysis occurs in the cytoplasm

Glycolysis

The hydrogen acceptor During glycolysis, hydrogen released becomes temporarily bound to a coenzyme molecule which acts as a hydrogen acceptor and carrier.

This coenzyme is called NAD (nicotinamide adenine dinucleotide).

In its reduced state the coenzyme is called NADH2

The fate of Pyruvic acid Pyruvic acid produced during glycolysis diffuses into the central matrix of the mitochondrion where it is converted into a 2-carbon compound called Acetyl Co-A. This reaction is accompanied by the release of hydrogen which again becomes bound to NAD to form NADH2

Anaerobic respiration if oxygen absent

The Krebs’ Cycle This is the aerobic phase of respiration and occurs in the matrix of the mitochondria. It is also known as the Citric Acid Cyle or the Tricarboxylic acid Cycle.

Each molecule of Acetyl CoA resulting from glycolysis reacts with a 4carbon compound to form 6-carbon citric acid.

Citric acid is gradually converted back to the 4-carbon compound by a cyclic series of enzyme-controlled reactions which result in the removal of carbon and hydrogen from the intermediate carbon compounds.

Using page 25 of Torrance, draw out a diagram combining glycolysis and the Krebs cycle showing the names and number of carbon atoms in each intermediate Answer “Testing Your Knowledge” questions 1 and 2 on page 26 Answer “Testing Your Knowledge” questions 1 and 2 on page 27

The Cytochrome System

In glycolysis and the Krebs’ cycle, there are six points at which hydrogen is released and bound to NAD to form NADH2.

NADH2 transfers the hydrogen to the cytochrome system in the christae of the mitochondria

Transfer of each molecule of hydrogen along the cytochrome system by NADH2 releases sufficient energy to produce three ATP. This process is called oxidative phosphorylation

The complete oxidation of one molecule of glucose yields a total of 38 ATP i.e. 2 ATP from glycolysis, 36 ATP from oxidative phosphorylation The cytochrome system, therefore, produces most ATP (requires

The final hydrogen acceptor in the cytochrome system is oxygen (forming water)

Alternative respiratory substrates Fatty acids are converted to Acetyl CoA in the matrix which enters the Krebs’ Cycle to produce ATP as above. Fat produces double the energy of the same mass of carbohydrate.

Amino acids can also act as respiratory substrates and some of the body’s energy is always derived from excess dietary protein. Tissue protein may be broken down during extreme and prolonged starvation.

Summary Copy and complete table 4.1 on page 30 of Torrance

Measuring the rate of respiration Make notes on the set-up, design features and precautions for a respirometer from pages 28 and 29 of Torrance Under the heading “Applying your Knowledge� answer question 5 page 31 Torrance

Anaerobic respiration Results in a little energy from the partial breakdown of sugar in the absence of oxygen. Only glycolysis can occur therefore only 2 molecules of ATP are produced

Anaerobic respiration in plants:

Anaerobic respiration in animals: