Biology: Chapter 4: Cell surface membrane: Cell signalling

Biology: Chapter 4: Cell surface membrane: Cell signalling

• Getting a message from one place to another.
• Complex range of signalling pathways which coordinate activities of the cell so they respond appropriately to the environment, even if the cells are large distances apart in the body.
  
• Signalling pathway includes:

1. Receptor receiving a stimulus or signal
2. Transduction: Converting the signal to a message that is readable 
3. Transmission of message/signal to effector (target)
4. The effector making an appropriate response

• Distances can be short (diffusion within a cell) or long (transport through blood)
  -Endocrine signalling: Signalling over large distances, often through circulatory system
  -Paracrine signalling: Signalling occurring between cells close together either through extracellular fluid or directly between cells
  -Autocrine signalling: Cell stimulates response within itself by releasing signals for it's own receptors
• 
  
• Many components and different mechanisms along the route
• Signalling includes both electrical (nerves) and chemical (hormones).
• Stimuli can be from inside (hormones) as well as outside (light).

• The cell surface membrane is a important component of most signalling pathways because it controls what molecules move in and out of the cell.
• In a typical signalling pathway, molecules must cross or interact with the cell surface membrane.
• Hydrophobic signalling molecules eg. steroid hormones can diffuse directly through the cell surface membrane and bind to receptors in the cytoplasm or nucleus.
• Signalling molecules are usually water-soluble.
  

Typical signalling pathway (for water soluble signalling molecules):

1. Signal arrives at a protein receptor in the cell surface membrane.
   Receptor is a specific shape which recognizes the signal. Only cells with this receptor can recognize the signal.
2. Signal changes the shape of the receptor, and since this spans the membrane, the message is in effect passed to the inside of the cell (signal transduction).
   Changing the shape of the reactor allows it to interact with the next component of the pathway, so the message gets transmitted.
3. The next component is usually a G-protein - a small molecule which diffuses through the cell relaying the message, like a switch to bring about the release of a 'second messenger'.
   G-proteins got their name because the switch mechanism involves binding to GTP molecules - similar to ATP, but with guanine instead of adenine.
4. After 1 receptor molecule is stimulated, many second messenger molecules can be made in response - amplification, a key feature of signalling.
   Second messenger usually activates an enzyme, which in turn activates further enzymes, increasing amplification at each stage.
   Signalling cascade: Amplification triggered by G-protein.
5. Response is produced.

Diagram of how cell signalling works.

Other ways receptors alter activity of the cell:

• Opening an ion channel, resulting in change of membrane potential
• Acting directly as a membrane-bound enzyme
• Act as a intracellular (inside the cell) receptor when initial signal passes through