Chemical signalling — IB Diploma Biology HL

Revision notes

7 short notes, in the order of the specification. Each one in short:

1. Receptors, ligands and quorum sensing

   A receptor is a protein with a binding site for a specific signalling chemical, called a ligand. The ligand and binding site match in shape and chemical properties. In quorum sensing, bacteria such as Vibrio fischeri detect their own population density from the concentration of a signalling chemical and switch on bioluminescence only when the density is high.

2. Types of signalling chemicals

   Animals use four functional categories of signalling chemicals: hormones, neurotransmitters, cytokines and calcium ions. Hormones travel in the blood and have distant effects; neurotransmitters diffuse across a synaptic gap and act locally. Hormones may be amines, proteins or steroids, and neurotransmitters include amino acids, peptides, amines and nitric oxide.

3. Transmembrane and intracellular receptors

   Transmembrane receptors span the plasma membrane and bind ligands that remain outside the cell, such as proteins and amines. Intracellular receptors are in the cytoplasm or nucleus and bind hydrophobic ligands, such as steroids, that pass through the membrane. Ligand binding to either type sets off a sequence of responses in the cell: a signal transduction pathway.

4. Ion channel receptors and G protein-coupled receptors

   Some transmembrane receptors are ion channels: acetylcholine binding opens a channel that lets positively charged ions diffuse into the cell, changing the membrane potential. Others activate G proteins. The epinephrine (adrenaline) receptor activates a G protein, which activates an enzyme that makes cyclic AMP (cAMP), the second messenger that spreads the signal inside the cell.

5. Tyrosine kinase receptors: insulin

   The insulin receptor is a transmembrane receptor with tyrosine kinase activity. When the protein hormone insulin binds outside the cell, tyrosine in the receptor inside the cell is phosphorylated. This starts a sequence of reactions ending with vesicles containing glucose transporters moving to and fusing with the plasma membrane, so the cell absorbs more glucose.

6. Intracellular receptors and steroid hormones

   Steroid hormones such as oestradiol, progesterone and testosterone are hydrophobic, so they pass through the plasma membrane and bind to intracellular receptors. The activated receptor binds to specific DNA sequences and promotes transcription of particular genes. Oestradiol acts on hypothalamus cells that secrete GnRH; progesterone acts on cells of the endometrium.

7. Positive and negative feedback in cell signalling

   Cell signalling pathways are regulated by feedback. In negative feedback the response reduces the original stimulus, so the signal decreases and conditions stay stable, as when insulin lowers blood glucose and so less insulin is secreted. In positive feedback the response increases the stimulus, amplifying the signal, as when high oestradiol late in the follicular phase increases GnRH secretion.