C2.1 Chemical Signalling

Chemical signalling

The process by which cells, tissues and organisms communicate with each other through the use of signalling molecules

Ligands

Signalling molecules

Stages of chemical signalling

Synthesis and release of a ligand from a signalling cell

Ligands travel to reach the target cells

Ligands bind to specific receptors on the surface of the target cell

Causes a conformational change in the receptor, initiating a cascade of biochemical reactions

The activated signalling pathways lead to specific cellular responses

Ligand is either degraded or removed from the extracellular space

Quorum sensing

A process of cell to cell communication that allows bacteria to share information about cell density and adjust gene expression accordingly

Autoinducers

A signalling molecule produced and used by bacteria participating in quorum sensing

Categories of signalling molecules in animals

Hormones, neurotransmitters, cytokines, calcium ions

Hormones

They are produced and secreted by glands and transported in the bloodstream to reach specific target cells. Widespread effects because target cells can be found in many places around the body

Neurotransmitters

Stored in vesicles and are secreted by the presynaptic neuron. They travel through the synaptic cleft to be received by the postsynaptic neuron. Excess ones are rapidly broken down by enzymes or reabsorbed by the presynaptic neuron. Localized and specific effects due to short distance

Cytokines

Small proteins that act on the single cell that produced them, or nearby ones. They can’t enter the cell membrane and work by binding to transmembrane receptors. Involved in immune responses and inflammation

Calcium ions

They are involved in chemical signalling within muscle fibers and neurons

3 categories of hormones

Amines, peptides, and steroids

Mechanisms of hormone action

Either activate second messengers, which either activate or deactivate enzymes in the cell, or activate genes and cause them to be expressed or switched off

Mechanism of action of neurotransmitters

When a neuron is stimulated, the vesicles fuse with the plasma membrane

Vesicles release neurotransmitters into the synaptic cleft

Neurotransmitters diffuse across the synapse and bind to the receptors on the target cells

Localised effects

Restricted to the immediate vicinity of the site of release of the signalling molecule

Types of neurotransmitters

Amino acids, peptides, amines, and nitrous oxide

Second messengers

Small molecules generated intracellularly in response to the binding of a ligand to the cell surface

Intracellular receptors

Located inside of target cells. They may be in the cytoplasm (Type 1) or the nucleus (Type 2). Made from hydrophilic amino acids

3 categories of transmembrane receptors

Enzyme-linked receptors, G-protein-linked receptors, and ion channel-linked receptors

Transmembrane receptors (cell surface receptors)

Proteins found within the plasma membrane

Hydrophilic and/or polar ligands

Cannot pass through the cell membrane

Hydrophobic and/or non-polar ligands

Can pass through the cell membrane

Signal transduction pathway

A sequence of biochemical reactions that occur inside a cell in response to the binding of a ligand to its receptor on its cell surface

Signal amplification

A process in which a small amount of signal or stimulus is capable of triggering a larger response

Signal transduction leads to…

Change in cellular behaviour or gene expression

Binding of a neurotransmitter

Causes the opening and/or closing of ion channels

G proteins

Proteins that act as molecular switches inside cells, involved in transmitting signals from stimuli outside a cell to its interior. They are bound to G protein-coupled receptors

G protein-coupled receptors (GPCR)

Transmembrane proteins that are associated with a G protein on the cytoplasmic side. Upon activation, the G protein dissociates into two parts: alpha subunit and beta-gamma dimer. The alpha subunit stimulates adenylyl cyclase to synthesise cAMP

Epinephrine (adrenaline)

Increases heart rate and blood pressure and boosts the energy supply for a short time

cAMP

A second messenger used for intracellular signal induction. They relay signals by either activating or inhibiting certain intracellular proteins and enzymes causing a cellular response

Kinase

An enzyme that adds a phosphate group from ATP to a specific molecule, a process known as phosphorylation

Tyrosine kinase receptors

A type of enzyme-linked receptor

Mechanism of tyrosine kinase receptors

The ligand binds to the extracellular domain of 2 nearby tyrosine kinase receptors, causing them to bond together

The tyrosine residues on the intracellular domain of the receptors are phosphorylated, triggering the assembly of an intercellular signalling complex

The phosphorylated tyrosine residues act as binding sites for signalling proteins, which pass the signals onto target proteins to achieve a specific cellular response

Oestradiol

Has a broad range of effects in the ovary and uterus. It also acts on the brain, regulating the release of reproductive hormones

Progesterone

Maintains the uterine lining so that it can support a developing foetus, endometrium

Positive feedback

Results in the amplification of cell signals, for example blood clotting

Negative feedback

Results in the stabilisation of the system, for example the control of blood sugar by insulin

Amines

Cannot pass through the cell membrane. Examples include epinephrine and dopamine

Peptides

Cannot pass through the cell membrane. Examples include insulin and glucagon

Steroids

Can pass through the cell membrane. Examples include oestradiol and progesterone