C2.1

Receptors as proteins

Cell receptors are proteins with specific binding sites for signalling chemicals.

Ligand

A signalling chemical that binds to a specific receptor protein.

Receptor-ligand specificity

Ligands bind only to receptors with complementary shape and chemical properties.

Role of receptors in signalling

Receptors detect ligands and initiate a cellular response through signal transduction.

Quorum sensing

A form of cell signalling in bacteria where gene expression changes in response to population density.

Purpose of quorum sensing

Allows bacterial populations to coordinate behaviour once a threshold population size is reached.

Mechanism of quorum sensing

Bacteria release ligands that bind to receptors; when enough receptors are occupied, gene expression changes.

Vibrio fischeri habitat

A marine bacterium that forms a mutualistic relationship with the bobtail squid.

Role of Vibrio fischeri bioluminescence

The bacteria emit light that camouflages the squid against the bright ocean surface.

Autoinducer in Vibrio fischeri

A ligand released by bacteria that increases in concentration as population density rises.

LuxR receptor

The cytoplasmic receptor in Vibrio fischeri that binds the autoinducer.

Threshold effect in Vibrio fischeri

When enough autoinducer-LuxR complexes form, transcription of luciferase genes occurs.

Luciferase enzyme

Catalyses an oxidation reaction that releases energy as light in bioluminescence.

Hormones definition

Chemical messengers produced by endocrine glands and transported in blood to target cells.

Transport of hormones

Hormones travel long distances through the bloodstream to reach target cells.

Hormone specificity

Hormones only affect cells with specific receptors for that hormone.

Neurotransmitters definition

Chemicals that transmit signals across a synaptic cleft between neurones.

Distance of neurotransmitter action

Neurotransmitters act over very short distances across synapses.

Cytokines definition

Proteins released by cells that affect the activity and gene expression of other cells.

Role of cytokines

Cytokines regulate immune responses, cell growth, and cell proliferation.

Calcium ions as signalling molecules

Ca²⁺ ions act in signalling pathways and can function as second messengers.

Calcium ions in muscle contraction

Ca²⁺ influx triggers protein shape changes that cause muscle fibres to contract.

Calcium ions in synaptic transmission

Ca²⁺ entry into presynaptic knobs triggers neurotransmitter release.

Chemical diversity of hormones

Hormones include amines, proteins (peptides), and steroids.

Reason for chemical diversity

Different chemical properties allow signalling molecules to bind receptors and move effectively.

Amines and peptide hormones

Amines and proteins are hydrophilic and bind to cell surface receptors.

Steroid hormones

Steroids are hydrophobic and diffuse through membranes to bind intracellular receptors.

Examples of steroid hormones

Oestradiol, progesterone, and testosterone.

Chemical diversity of neurotransmitters

Neurotransmitters include amino acids, peptides, amines, and gases.

Nitrous oxide as neurotransmitter

A gaseous neurotransmitter that diffuses freely across membranes.

Localised signalling

Neurotransmitters have localised effects at synapses.

Distant signalling

Hormones have distant effects as they travel in the blood.

Transmembrane receptors

Receptors that span the plasma membrane with extracellular and intracellular regions.

Amino acid distribution in transmembrane receptors

Hydrophilic regions face aqueous environments, hydrophobic regions interact with membrane lipids.

Ligand behaviour with transmembrane receptors

Ligands bind externally and do not enter the cell.

Intracellular receptors

Receptors located in the cytoplasm or nucleus that bind hydrophobic ligands.

Ligand behaviour with intracellular receptors

Ligands diffuse through the membrane and bind inside the cell.

Signal transduction pathway

A sequence of events triggered by ligand binding that leads to a cellular response.

Initiation of signal transduction

Ligand binding causes a receptor shape change that starts intracellular signalling.

Acetylcholine as neurotransmitter

ACh is a neurotransmitter used at cholinergic synapses.

Acetylcholine receptor

A ligand-gated sodium ion channel in the postsynaptic membrane.

Effect of ACh binding

Binding opens Na⁺ channels, allowing sodium ions to enter the cell.

Change in membrane potential

Sodium ion influx reverses membrane charge and may trigger an action potential.

G-protein-coupled receptor (GPCR)

A transmembrane receptor that activates a G protein inside the cell.

G protein activation

GDP is replaced by GTP, activating the G protein.

Function of activated G proteins

They interact with enzymes or ion channels to generate second messengers.

Prevalence of GPCRs

GPCRs form a large and diverse receptor family in humans.

Epinephrine alternate name

Epinephrine is also known as adrenaline.

Epinephrine receptor type

Epinephrine binds to a GPCR on the cell surface.

Second messenger model

A signalling mechanism where an intracellular messenger relays the signal.

Role of cAMP

cAMP acts as a second messenger that activates protein kinase A.

Effect of epinephrine signalling

Leads to glycogen breakdown and increased blood glucose.

Amplification in epinephrine pathway

One epinephrine molecule leads to activation of many enzymes.

Naming conventions in science

Adrenaline and epinephrine reflect international scientific cooperation.

Receptor tyrosine kinases (RTKs)

Transmembrane receptors that phosphorylate tyrosine residues inside cells.

Insulin receptor type

Insulin binds to a receptor tyrosine kinase.

Effect of insulin binding

Triggers phosphorylation and relay proteins.

Final effect of insulin signalling

Vesicles with glucose transporters move to the plasma membrane.

Intracellular steroid hormone receptors

Activated receptors bind to DNA and regulate gene transcription.

Oestradiol receptor mechanism

Oestradiol-receptor complexes bind to promoter regions of genes.

Progesterone receptor mechanism

Progesterone-receptor complexes activate gene transcription in target cells.

Effect of oestradiol on hypothalamus

Oestradiol regulates secretion of gonadotropin-releasing hormone.

Effect of progesterone on endometrium

Progesterone promotes gene expression that maintains the endometrial lining.

Negative feedback regulation

A response reverses a change to restore normal conditions.

Example of negative feedback

Blood glucose or body temperature regulation.

Positive feedback regulation

A response amplifies the original stimulus.

Example of positive feedback

Oxytocin release during labour increases uterine contractions.

Purpose of positive feedback

Rapid completion of a specific process.