Respiration (AQA A-Level Biology): Revision Notes

Glycolysis

Glycolysis is the metabolic pathway that breaks down glucose to release energy for cellular processes. This process represents the first stage of cellular respiration and occurs in the cytoplasm of all living cells.

Role in cellular respiration

Cells cannot use glucose directly as an energy source. Instead, they rely on ATP as their immediate energy currency. The breakdown of glucose during cellular respiration produces ATP through a series of enzyme-controlled reactions.

There are two main types of cellular respiration:

1. Aerobic respiration - requires oxygen and generates carbon dioxide, water, and large amounts of ATP
2. Anaerobic respiration - occurs without oxygen, producing lactate (in animals) or ethanol and carbon dioxide (in plants and fungi), but yielding much less ATP

Aerobic respiration consists of four sequential stages:

1. Glycolysis - glucose breakdown to pyruvate
2. Link reaction - pyruvate conversion to acetyl coenzyme A
3. Krebs cycle - cyclical oxidation reactions producing ATP and reduced coenzymes
4. Oxidative phosphorylation - ATP synthesis using electrons from reduced coenzymes

Glycolysis serves as the initial stage for both aerobic and anaerobic pathways, making it a universal metabolic process.

The four stages of glycolysis

Glycolysis converts one glucose molecule (6-carbon sugar) into two pyruvate molecules (3-carbon compounds). This transformation occurs through four distinct stages:

Stage 1: Phosphorylation of glucose

Glucose must be activated before it can be metabolised effectively. This activation involves adding two phosphate groups to the glucose molecule through phosphorylation.

The phosphate groups come from the hydrolysis of two ATP molecules into ADP. This process provides the activation energy needed to make glucose more reactive and enables the subsequent enzyme-controlled reactions to proceed.

Stage 2: Splitting of phosphorylated glucose

The phosphorylated glucose molecule is then divided into two separate 3-carbon molecules called triose phosphate. Each of these molecules will continue through the remaining stages of glycolysis independently.

Stage 3: Oxidation of triose phosphate

During this stage, hydrogen atoms are removed from each triose phosphate molecule through oxidation. These hydrogen atoms are transferred to the coenzyme NAD, forming reduced NAD (NADH). This step is crucial because reduced NAD carries high-energy electrons that can be used to generate additional ATP in later stages of respiration.

Stage 4: ATP production

The final stage involves enzyme-controlled reactions that convert each triose phosphate molecule into pyruvate. During this conversion, ADP molecules are phosphorylated to regenerate ATP. Since two triose phosphate molecules are processed (from the original split glucose), this stage produces multiple ATP molecules.

C = number of carbon atoms in the molecule involved

Energy yields from glycolysis

The net energy yield from one glucose molecule undergoing glycolysis includes:

• Two molecules of ATP (net gain) - Four ATP molecules are produced, but two were consumed during the initial phosphorylation, resulting in a net gain of two
• Two molecules of reduced NAD - These carry electrons with the potential to generate additional ATP during oxidative phosphorylation
• Two molecules of pyruvate - These can be further metabolised through aerobic or anaerobic pathways

The yield calculations must account for the fact that each glucose molecule produces two triose phosphate molecules, so the gross yields are doubled from what each triose phosphate generates individually.

Biological significance of glycolysis

Glycolysis is a key metabolic pathway in every living organism.

• Cellular location: The enzymes responsible for glycolysis are located in the cytoplasm, meaning the process does not require specialised organelles or membrane structures.
• Oxygen independence: Glycolysis does not require oxygen and can take place whether or not it is present (anaerobic respiration). However, anaerobic respiration yields only a small amount of the potential energy stored in the pyruvate molecule.