Communication and signalling

• extracellular signalling molecules


  molecules which are released from one cell and attach to a specific receptor and target cell as a form of communicatio between cells.
• what effect should a extracellular signalling molecule have?

  should change the conformation of the receptor and thus the behaviour of the target cell in some way.
• specific signals+ specific receptors


  different cell types produce specific signals which can only be detected and responded to by specific receptors.
• examples of extracellular signalling molecules


  steroid hormones, peptide hormones, neurotransmitters
• Hormones


  Secreted by a tissure, such as the endocrine gland, move through the bloodstream to the target organ where it binds to a receptor protein. Can be hydrophobic or hydrophilic
• Hydrophobic hormones


  Thyroxine or steroid
• Hydrophilic hormones


  ADH or Insulin
• Neurotransmitters


  Move through the synapse from one neuron to another. Faster and more specific than hormones.

  Includes nor-adrenaline and acetylcholine.
• location of hydrophobic receptor proteins


  Hydrophobic signalling molecules can pass through the cell membrane by diffusion as this is also hydrophobic. The receptors for this type of molecule are inside the cell in the cytoplasm or nucleus.
• Hydrophilic signalling molecules


  Hydrophilic signalling molecules cannot pass through the membrane and so a receptor on the cell membrane surface is required
• Hydrophobic signals and the control of transcription


  Steroid hormones cause transcription to take place. When a steroid reaches the receptor inside the cell, it causes conformational changes in the receptor which turns ON transcription.
• Examples of steroid hormones


  Oestrogen and Testosterone
• How do hydrophobic signalling molecules work?


  Firstly , steroid hormones bind to specific intracellular receptors in the cytosol or nucleus. From there the hormone receptor complex moves into the nucleus (if not there already) and binds with specific sections of DNA called Hormone Response Elements. This influences the rate of transcription, with each steroid affecting the expression of many genes.
• Why can't hydrophilic signalling molecules pass trough the cell membrane?

  They are not lipid soluble
• Examples of hydrophilic signalling molecules


  Peptide hormones and neurotransmitters
• How does a hydrophilic signalling molecule work?


  Molecules must join to receptor on the outside of the cell membrane. Once bound, a conformational change occurs. The signal is transduced and a response occurs
• Signal transduction


  The signal from the signalling molecule is passed across the membrane and a response happens. Often involves cascades activated by G- proteins and Phosphorylation by kinases enzymes.
• Examples of peptide hormones


  Insulin, Glucagon and ADH
• where are receptors located for peptide hormones?


  Specific receptors are on the target cell surface, making them highly specific.
• G-protein

  Molecules which relay signals from activated receptors. An activated receptor is bound to a signalling molecule.
• Phosphorylation cascade


  Allows more than one intracellular signalling pathway to be activated.
• How does a phosphorylation cascade work?


  A series of events where the kinase enzyme activates the next in the sequence and so on.

  Result in the phosphorylation of many proteins as a result of one signalling event.
• Diabetes mellitus


  caused by a deficiency in the effect of insulin and results in the loss of control of blood glucose level.
• what peptide hormones control blood glucose levels?

  insulin and glucagon
• what organ detects an increase in blood glucose?


  The pancreas
• What happens as blood glucose increase is detected?


  insulin is released and binds to an insulin receptor on the membrane of fat and muscle cells. This insulin receptor is kinase linked. The signal is transduced across the membrane and a series of phosphorylation events trigger the recruitment of GLUT4 glucose transporters in the cell membrane.
• what happens after the recruitment of GLUT4 transporters?


  Glucose is allowed to enter te cell, lowering it's concentration in the blood. The glucose in the cell is metabolised
• Type 1 diabetes


  Failure to produce insulin
• Type 2 diabetes


  loss of receptor function
• Type 1 diabetes treatment 


  Treated with insulin injections. Irreversible and often genetic
• Type 2 diabetes treatment


  Exercise and losing weight can reduce the impact of type 2 diabetes as it triggers the recruitment of GLUT 4 through other metabolic pathways. Exercising improves the uptake of glucose to fat and muscle cells.
• nerve cells and potassium ions


  in nerve cells the cell membrane has potassium channels which let some of the potassium ions leak back out of the cell. this results in a net positive charge outside the cell and a net negative charge inside the cell
• resting potential


  the imbalance in electrical charge across the neuron membrane
• when can a nerve impulse be passed along?

  when the resting potential is depolarised - changed from negative inside to neutral
• How are nerves triggered?


  Depolarisation of the resting potential is triggered by neurotransmitters at a synapse. The neurotransmitter diffuses across the synapse and binds to a transmembrane receptor protein on the surface of the next neuron. This receptor is a ligand-gated sodium ion channel so a conformational change occurs making the channel open and allowing sodium ions to flow through. If many ions move through then the voltage across the membrane becomes depolarised. This causes neighbouring voltage-gated sodium ion channels to open leading to a wave of depolarisation through the ns
• How are nerves reset?- repolarisation


  When the voltage inside the cell reaches a critically high level the voltage-gated sodium channels close and voltage-gated potassium channels open.
• what happens when action potential reaches the end of the neuron?


  it causes vesicles to fuse with the membrane and this releases the neurotransmitter, stimulating a response in connecting cell.