Biology – Fluid mosaic membranes | e-Consult

Cell surface receptors are integral membrane proteins that bind to specific signaling molecules (ligands), initiating a signal transduction pathway within the cell. This process is crucial for cell communication and regulation of cellular activities.

Example 1: Insulin Receptor. Insulin, a hormone released by the pancreas, binds to the insulin receptor on target cells (e.g., liver cells). This binding triggers a conformational change in the receptor, activating intracellular signalling pathways that lead to increased glucose uptake from the blood. This is a classic example of hormone-receptor interaction and cell signalling.

Example 2: Growth Factor Receptors. Growth factors bind to specific receptors on cell surfaces, stimulating cell growth and division. These receptors often trigger intracellular signalling cascades involving kinases and transcription factors.

Carrier proteins facilitate the transport of specific molecules across the cell membrane down their concentration gradient. They bind to the transported molecule and undergo a conformational change to move it across the membrane.

Example 1: Glucose Transporter (GLUT). GLUT proteins facilitate the transport of glucose across the cell membrane. Different GLUT isoforms have different affinities for glucose and different tissue distributions. GLUT4, for example, is insulin-regulated and is found in muscle and adipose tissue, allowing for increased glucose uptake in response to insulin.

Example 2: Sodium-Potassium Pump. This is a specific type of carrier protein that actively transports sodium ions out of the cell and potassium ions into the cell, maintaining the electrochemical gradient essential for nerve impulse transmission and muscle contraction. This process requires energy in the form of ATP.

In summary, cell surface receptors are essential for receiving and responding to external signals, while carrier proteins are crucial for the regulated transport of specific molecules across the cell membrane.