Higher Biology | Key Area 2.2: Cellular Respiration

Cellular respiration

An enzyme-controlled series of reactions in which a respiration substrate such as glucose is broken down to generate energy in the form of ATP.

In what types of organisms does cellular respiration and its metabolic pathways occur?

- Cellular respiration pathways are present in cells from all 3 domains of life, and occurs when any organic substance is oxidised.

- The metabolic pathways of cellular respiration are of central importance to cells which are connected to other pathways, and yield energy.

What are the three main pathways of cellular respiration?

- Glycolysis

- Citric acid cycle

- Electron transport chain

Glycolysis

The breakdown of a glucose molecule into 2 pyruvate molecules occurring anaerobically in the cytoplasm.

Describe the process of glycolysis.

- 2ATP is expended in energy investment and broken down to 2ADP+Pi for the phosphorylation of glucose and intermediates (gaining of a phosphate group).

- Dehydrogenase removes 4 hydrogen ions (and electrons) from intermediates, and passes them to coenzyme NAD+. 2NAD+ is reduced to become 2NADH+. Hydrogen ions and electrons from NADH+ passed to the electron transport chain.

- Energy investment leads to energy payoff, yielding 4ATP from 4ADP+Pi, with net gain of 2ATP.

- With the absence of oxygen, pyruvate converts to lactic acid.

Citric acid cycle

Occurs in the presence of oxygen, where pyruvate molecules progress after glycolysis to the matrix of the mitochondrion.

Describe the process of the citric acid cycle.

- Pyruvate enters the matrix of the mitochondrion. Carbon is removed in the form of carbon dioxide (waste product), forming an acetyl group (Ac), which then joins with coenzyme A (CoA) to form acetyl coenzyme A (AcCoA).

- Acetyl coenzyme A joins with oxaloacetate, forming citrate. Enzymes remove more carbon (2CO2 released as by-product), hydrogen, and electrons, forming intermediate molecules. 1ATP synthesised from 1ADP+Pi.

- Dehydrogenase removes hydrogen ions and electrons and passes it to coenzyme NAD+. 3NAD+ reduced to 3NADH+. Hydrogen ions and electrons from NADH+ passed to the electron transport chain.

Electron transport chain

A series of carrier proteins attached to the inner mitochondrial membrane, which leads to the synthesis of ATP.

Describe the process of the electron transport chain:

- Coenzyme NADH+ brings hydrogen ions and electrons from glycolysis and the citric acid cycle, and releases them into the transport chain in the inner mitochondrial membrane.

- Flow of high energy electrons transfer energy to the carrier proteins in the membrane, providing energy for them to pump H+ ions across the membrane.

- Flow of ions back through the membrane protein ATP synthase results in the production of ATP from ADP+Pi. 3ATP is made for each NADH+, and 34ATP is synthesised in electron transport.

- Oxygen is the final hydrogen ion and electron acceptor, and combines to form water (waste product).

Fermentation

An alternative pathway of respiration that occurs when there is an absence of oxygen, and so pyruvate cannot enter the citric acid cycle (stays in cytoplasm). It only yields 2 ATP from glycolysis, compared to aerobic respiration’s 38ATP.

There are two types of fermentation:

- Lactate fermentation occurs in animal cells: pyruvate ⇋ lactate (reversible)

- Alcoholic fermentation occurs in plant and yeast cells: pyruvate → ethanol + carbon dioxide (irreversible)

ATP

Adenosine triphosphate is an energy-carrying molecule used in cells since it releases energy very quickly.

Synthesis of ATP

- Energy is released from ATP when the end phosphate is removed, forming ADP, which is a low-energy molecule. Energy is needed to recharge ADP into ATP by adding a phosphate.

- ATP ⇋ ADP + Pi

- These molecules are recycled so a constant stream of energy-rich ATP is available for all metabolic pathways in a cell.

Role of ATP

- Almost all cellular processes need ATP to give a reaction its required energy.

- ATP is used to transfer energy to cellular processes which require energy, and can phosphorylate other molecules.

- ATP provides energy for protein synthesis, DNA replication, active transport, synthetic pathways, muscle contraction, etc.