Cell signalling and an overview of metabolism

Cells talk to each other with chemical signals. A signal molecule (a ligand) affects a cell only if that cell has the matching receptor, on its surface, in its cytoplasm or in its nucleus. The fit is highly specific. When the ligand binds, the receptor changes shape and starts a chain of messages (a relay) inside the cell.

Signals are grouped by how far they travel:

• Paracrine: to nearby cells, as at the junctions between nerve cells.
• Autocrine: a cell responds to its own signal, as cancer cells do with growth factors they make themselves.
• Endocrine: hormones carried in the blood to distant cells.

Metabolism is all the chemical reactions in a cell. Phototrophs get energy from sunlight; chemotrophs get it by oxidising chemicals. Anabolic pathways build large molecules and use energy; catabolic pathways break molecules down and release energy, captured as ATP and reducing power (like NADH).

How the main fuels are handled:

• Glucose is broken to pyruvate in glycolysis. With oxygen, pyruvate goes on to acetyl CoA and the citric acid cycle, ending as CO₂ and water. Without oxygen, it becomes lactate. Glycolysis also branches off to make glycogen, fats and amino acids, and to the pentose phosphate pathway, which supplies NADPH and ribose (for nucleotides). In starvation, the body makes new glucose from lactate, amino acids and glycerol: gluconeogenesis.
• Fatty acids are broken into acetyl CoA by beta-oxidation, or stored as triacylglycerol. Acetyl CoA can also be used to make cholesterol or ketone bodies.
• Amino acids build proteins. Transamination moves amino groups between molecules to make non-essential amino acids; deamination removes nitrogen, which leaves the body as urea.