C2.1 Chemical signallingIB Biology HL: Revision notes

A receptor is a protein with a binding site for a specific signalling chemical, called the ligand. Binding changes the receptor's shape and triggers a response.

In quorum sensing, bacteria release an autoinducer. Its concentration rises with population density; above a threshold, enough binds receptors to change gene expression. In Vibrio fischeri, this switches on bioluminescence only when the population is dense, for example in the light organ of the bobtail squid.

• Hormones: secreted by glands into the blood, act on distant target cells (e.g. insulin).
• Neurotransmitters: released by neurons into a synaptic gap, act on the adjacent cell (e.g. acetylcholine).
• Cytokines: small proteins released by cells, often of the immune system, acting on nearby cells (e.g. interleukins).
• Calcium ions: act as a signal inside cells (e.g. triggering muscle contraction).

Hormones have distant effects via the blood; neurotransmitters have localised effects across a synapse.

Many different substances are used as signals so that each can be recognised by its own receptor and many separate messages can be carried at once. Hormones include amines (e.g. adrenaline, thyroxine), proteins (e.g. insulin) and steroids (e.g. oestradiol, testosterone). Neurotransmitters include amino acids (e.g. glutamate), peptides (e.g. endorphins), amines (e.g. dopamine) and nitric oxide.

Transmembrane receptors span the plasma membrane: hydrophobic amino acids sit in the membrane, hydrophilic ones face the water on each side. Their ligands are hydrophilic and stay outside the cell.

Intracellular receptors are in the cytoplasm or nucleus. Their ligands (e.g. steroids) are hydrophobic and cross the membrane.

In every case, binding starts a signal transduction pathway: a sequence of responses within the cell.

Acetylcholine receptor: binding opens an ion channel in the receptor; positive ions diffuse in, changing the membrane potential.

G protein-coupled receptors (there are many in humans): adrenaline binds, the receptor activates a G protein, which activates an enzyme that makes cyclic AMP (cAMP), the second messenger. In liver cells cAMP leads to glycogen breakdown.

Tyrosine kinase receptors: insulin binds, causing phosphorylation of tyrosine inside the cell; a sequence of reactions moves vesicles containing glucose transporters to the membrane.

Oestradiol, progesterone and testosterone bind to intracellular receptors, activating them. The activated receptor binds to specific DNA sequences and promotes transcription.

• Oestradiol acts on cells of the hypothalamus that secrete GnRH.
• Progesterone acts on cells of the endometrium, maintaining the thickened lining.

Negative feedback: the result of a pathway reduces the signal, keeping a variable stable (e.g. falling blood glucose reduces insulin secretion).

Positive feedback: the result increases the signal, amplifying change (e.g. V. fischeri autoinducer switches on genes that make more autoinducer; rising oestradiol stimulates the surge before ovulation).