Cell signalling
Cell signalling is a fundamental biological process by which cells detect and respond to external stimuli, including communication between cells. It is crucial for living organisms to control and coordinate their bodies and respond appropriately to their environments.
Key Concepts and Stages of Cell Signalling:
Purpose of Signalling:
Allows multicellular organisms to control and coordinate their bodies and respond to their environments.
Ensures that cells function efficiently by maintaining constant conditions within the internal environment, such as pH, temperature, and water potential, ensuring enzymes function at a constant rate.
Coordinates cell activities, even if cells are far apart in the body.
Enables communication between different parts of an organism, which is essential for survival.
Basic Stages of a Cell Signalling Pathway:
Receiving a stimulus/signal: A stimulus causes cells to secrete a specific chemical, known as a ligand. Ligands can be hormones or growth factors that instruct other cells to divide or alter their activity.
Transmission of the message: The ligand is transported to the target cells, often by diffusion for nearby cells or through transport fluids like blood for distant cells.
Binding to receptors: Ligands bind to specific receptor molecules, which are typically proteins located in the cell surface membrane of target cells.
Transduction: The binding of the ligand causes a change in the shape of the receptor protein (conformational change). Since the receptor spans the membrane, this passes the message to the inside of the cell. This conversion of the original signal to a transmittable message is called transduction.
Internal response: The altered receptor interacts with other components, initiating a series of chemical reactions inside the cell, often involving a signalling cascade. This eventually leads to a specific cellular response, such as metabolic change, secretion, transcription, or movement.
Molecules Involved and Their Roles:
Cell Surface Membrane: It is a critical component of most signalling pathways because it acts as a barrier, controlling which molecules can move between the external and internal environments of the cell.
Ligands / Signalling Molecules:
Chemicals secreted by cells (e.g., hormones, neurotransmitters, growth factors) that bind specifically to receptors.
Hydrophobic ligands (e.g., steroid hormones like oestrogen, progesterone, testosterone) can diffuse directly across the cell surface membrane and bind to receptors in the cytoplasm or nucleus.
Water-soluble ligands (e.g., protein hormones like insulin, glucagon, adrenaline, ADH, and most neurotransmitters) cannot pass through the phospholipid bilayer and must bind to cell surface receptors.
Receptor Proteins:
Specific proteins on the cell surface membrane or within the cell that bind to specific ligands.
Different cells have different receptors depending on their function, ensuring specific responses.
Some receptors can also be cells themselves, specialized to detect stimuli.
G Proteins: Often act as a switch, activated by the ligand-receptor binding, to trigger the release of a second messenger.
Second Messengers:
Small, water-soluble, non-protein molecules (e.g., cyclic AMP (cAMP) and calcium ions (Ca2+)) that are produced inside the cell in response to the initial signal.
Amplify (magnify) the original signal by diffusing quickly through the cytosol and activating many downstream enzymes.
Enzyme Cascades: A series of reactions initiated by second messengers, where one activated enzyme activates many others, leading to further amplification and the final cellular response.
Protein Kinases: Enzymes often activated by second messengers (like cAMP) that then activate other enzymes by adding phosphate groups (phosphorylation).
Transcription Factors: Can be activated during signaling cascades, influencing the transcription of genes in the cell nucleus.
Examples of Cell Signalling:
Hormonal Control:
Insulin: Binds to receptors on liver, muscle, and adipose cells, increasing glucose uptake by promoting the movement of glucose transporter proteins (channel proteins) from vesicles to the cell membrane.
Glucagon/Adrenaline: Bind to specific receptors on liver cell membranes, activating adenylyl cyclase to produce cAMP (a second messenger). cAMP then activates a cascade of enzymes (protein kinase A), leading to the breakdown of glycogen into glucose (glycogenolysis).
ADH (Antidiuretic Hormone): Binds to receptors on collecting ducts, activating a cascade involving cAMP that leads to the phosphorylation and increased permeability of aquaporin molecules, allowing more water to be reabsorbed.
Oestrogen: Being lipid-soluble, it passes through the cell surface membrane and binds to intracellular receptors (transcription factors) in the nucleus to directly control gene expression.
Nervous Communication:
Neurotransmitters (e.g., acetylcholine, dopamine, noradrenaline) are released from a presynaptic neurone and diffuse across the synaptic cleft to bind to specific receptors on the postsynaptic membrane. This binding can trigger an action potential in the postsynaptic neurone by opening ion channels (e.g., sodium ion channels).
Receptor cells: Sensory receptors (e.g., in taste buds, Pacinian corpuscles) detect stimuli and cause ion channels (e.g., sodium channels) to open, leading to depolarization and potentially triggering an action potential.
Voltage-gated ion channels: Proteins embedded in the membrane of neurones and muscle cells that open or close in response to changes in electrical potential, facilitating the rapid movement of ions during action potentials.
Immune Response: Glycolipids and glycoproteins act as cell markers or antigens for cell-to-cell recognition and adhesion, which is important for immune responses. Lymphocytes detect foreign antigens on other cells through direct cell-to-cell contact signalling.
Plant Hormones: Plant growth regulators like auxins, gibberellins, and abscisic acid also function via cell signalling pathways, affecting growth and development. For example, abscisic acid can trigger stomatal closure by causing the release of chloride ions as second messengers.
Cell signalling ensures the cell's dynamic and coordinated response to its environment, regulating processes from metabolism to cell division and immune responses.
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