1.4 Communication and Signalling

Receptor molecules of target cells

Proteins with a binding site for a specific signal molecule

Binding of signal molecule to its specific receptor

changes the conformation of the receptor, which initiates a response within the cell

Signalling in a multicellular organism

Communication between cells uses extracellular signalling molecules. Different cell types produce specific signals that can only be detected and responded to by cells with the specific receptor. Different cell types may show a tissue-specific response to the same signal

Hydrophobic signalling molecules

can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors

Transcription factors

The receptors for hydrophobic signalling molecules They are proteins that when bound to DNA can either stimulate or inhibit initiation of transcription.

Examples of hydrophobic signalling molecules

The steroid hormones oestrogen and testosterone. Steroid hormones bind to specific receptors in the cytosol or the nucleus forming the hormone receptor complex.

The hormone-receptor complex

This moves to the nucleus where it binds to specific sites on DNA and affects gene expression

Hormone response elements (HREs).

The hormone-receptor complex binds to these specific DNA sequences. Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.

Hydrophilic signalling molecules

These bind to transmembrane receptors and do not enter the cytosol

Examples of hydrophilic extracellular signalling molecules

Peptide hormones and neurotransmitters

Signal transduction

Transmembrane receptors change conformation when the ligand binds to the extracellular face; the signal molecule does not enter the cell, but the signal is transduced across the plasma membrane

G Proteins

Relay signals from activated receptors (receptors that have bound a signalling molecule) to target proteins such as enzymes and ion channels.

Phosphorylation cascades

A series of events with one kinase activating the next in the sequence and so on. These can result in the phosphorylation of many proteins as a result of the original signalling event.

Binding of insulin to its receptor

Causes a conformational change that triggers phosphorylation of the receptor. This starts a phosphorylation cascade inside the cell, which eventually leads to GLUT4-containing vesicles being transported to the cell membrane.

Diabetes Mellitus Type 1

Caused by a failure to produce insulin

Diabetes Mellitus Type 2

Caused by loss of receptor function. Exercise also triggers recruitment of GLUT4, so can improve uptake of glucose to fat and muscle cells in subjects with type.

Resting Membrane Potential

A state where there is no net flow of ions across the membrane

Action potential

A wave of electrical excitation along a neuron's plasma membrane

Binding of a neurotransmitter

Triggers the opening of ligand-gated ion channels at a synapse.

Depolarisation of the plasma membrane

If sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, the opening of voltage-gated sodium channels is triggered and sodium ions enter the cell down their electrochemical gradient. This leads to a rapid and large change in the membrane potential.

Repolarisation of the plasma membrane

A short time after opening, the sodium channels become inactivated. Voltage-gated potassium channels then open to allow potassium ions to move out of the cell to restore the resting membrane potential.

Depolarisation of a patch of membrane

causes neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened

When the action potential reaches the end of the neuron

Vesicles containing neurotransmitter fuse with the membrane — this releases neurotransmitter, which stimulates a response in a connecting cell

Importance of the sodium potassium pump

Following repolarisation the sodium and potassium ion concentration gradients are reduced. The sodium-potassium pump restores the sodium and potassium ions back to resting potential levels.

The retina

The area within the eye that detects light and contains two types of photoreceptor cells: rods and cones

Rods

Function in dim light but do not allow colour perception

Cones

Responsible for colour vision and only function in bright light.

Retinal

The light sensitive molecule in animals. It combines with a membrane protein called opsin to form rhodopsin.

Opsin

A membrane protein that combines with retinal to form rhodopsin

Photoexcited rhodopsin

Rhodopsin changes conformation to photoexcited rhodopsin when retinal absorbs a photon of light. This causes a cascade of proteins amplifying the signal.

Transducin

A G protein which is activated by photoexcited rhodopsin. A single photoexcited rhodopsin activates hundreds of molecules of G-protein. Each activated G-protein activates one molecule of PDE (phosphodiesterase)

Phosphodiesterase (PDE)

Catalyses the hydrolysis of a molecule called cyclic GMP (cGMP). Each active PDE molecule breaks down thousands of cGMP molecules per second.

Ion channels in rod cells

The reduction in cGMP concentration as a result of its hydrolysis affects the function of ion channels in the membrane of rod cells. Ion channels close which triggers nerve impulses in neurons in the retina

In low light intensities

A very high degree of amplification results in rod cells being able to respond

Opsin in cone cells

Different forms of opsin combine with retinal to give different photoreceptor proteins, each with a maximal sensitivity to specific wavelengths: red, green, blue or UV