1. SIGNALLING MOLECULES Hormones

2. Signal=Ligand

3. Signaling Molecules
The endocrine and nervous system transmit messages by releasing signaling molecules that act by attaching to specific receptors .(enzyme- like protein molecules) in the membranes of target cells.
A target cell is one that has the appropriate receptor for a particular signalling molecule .
Some target cells may be widespread throughout the body (as in the target cells for thyroxine) or may be located in a particular place (as in the hormones released from the hypothalamus that act on certain cells in the pituitary gland).

4. Chemical nature of hormones
The chemical nature of a hormone influences the way in which it interacts with it target cells.
There are three different kinds of hormones based on their chemical structures
_ amino acid hormones
_ peptide and protein hormones
_ steroid hormones

6. Hydrophobic = Lipophillic Hydrophillic = Lipophobic

7. Types of hormones Hydrophilic
Amino acid & peptide
- Soluble in water - Unable to pass through membranes
- Require 2nd messenger molecule to transfer messages - Dissolves into the blood to reach target cell
Hydrophobic Steroid Hormones - Insoluble in water - Able to pass through membranes - Do not require second messenger hormone - Require a carrier protein to reach target cell

8. Hydrophobic & hydrophillic signals
Hydrophillic signalling molecules
Neurotransmitters
Growth Factor Hormones
Insulin
Hydrophobic signalling molecules/Lipophilic
Steroid Hormones
Thyroid Hormones

9. Where are the receptors for hydrophobic signalling molecules?
These hormones are able to pass directly through the plasma membrane and bind their specific receptor (intracellular) within the cytoplasm of the target cell. These molecules require a carrier protein for transport in the blood as it is water based.
Steroids
Cortisol
Testosterone
Oestrogen
Progesterone
Eicosanoids
(derived from fatty acids)
Prostoglandins
Lipid soluble hormones leave the secretory cell by simple diffusion
Target cell
Intracellular receptor
Insoluble in blood plasma so needs water-soluble carrier molecules to transport them around the body.
Once synthesized, steroid hormones pass into the bloodstream; they are not stored by cells, and the rate of synthesis controls them.

10. Hydrophobic signals bind to intracellular receptors
Steroid hormones regulate gene expression. 2.
1. Steroid Hormone moves across the plasma membrane. 1.
Ligand-Receptor
complex
2. Signal binds with intracellular protein receptor. 3.
3. The ligand-receptor complex moves into nucleus to bind with DNA to regulate gene expression.
4. Proteins are the products of gene expression.
Transcription 4.
Translation

11. Receptors for hydrophillic signalling molecules
Small Peptides
Vasopressin
Protein hormones made up of chains of amino acids. Some with carbohydrate side chains.
Insulin
Glucagon
Growth Hormone
Luteinizing hormone
Neurotransmitters
Hormones derived from specific amino acid molecules; tyrosine and tryptophan
Adrenaline
Dopamine
Water soluble hormones are unable to pass through the cell membrane. As a result they bind with specific receptors on the surface of cell membrane of the target cell. They are all amino acid based molecules but vary in the number of amino acid residues.
Water soluble hormones are stored in vesicles and secreted from the cell by exocytosis
Secretory Cell
Soluble in blood plasma
Receptor on plasma membrane
Target Cell

12. Types of plasma membrane bound receptors (activated by hydrophilic hormones)
Ion Channel Receptor
G protein-coupled Receptor
Tyrosine kinase linked Receptor
Milliseconds
Seconds
Can take hours
G-Protein & Tyrosine Kinase receptors stimulate other molecules (secondary messengers) in the cytoplasm in what is called the ‘signal transduction pathway’
A common secondary messenger is called cAMP – cyclic adenosine monophosphate
Causes a shape change that opens the channel so ions can enter or leave the cell.
Eg. Acetylcholine at nerve/muscle junctions.
activates a G protein which activates or inhibits an enzyme or modulates an ion channel that generates secondary messengers.
Eg. Receptor for glucagon
causes a shape change that activates enzymes within the cell
Eg. Human Growth Factor

13. Signal Transduction Fatty Acid based Hormones (lipid solu ble)
Protein based Hormones (water soluble)

14. The Signal Transduction Pathway1.
The Signal Transduction Pathway 2.
Applying the stimulus response model.
1.Signal binds to receptor molecule.
2. Receptor molecule changes shape of confirmation.
3. Initiates a molecular cascade of secondary messenger molecules to an effector molecule.
4.Effector initiates the cellular response. 3.
Responses
Enzymes activated or deactivated
Genes switched on or off
Protein channels in plasma membrane opened or closed
Vesicles moved to plasma membrane so content can be released by exocytosis 4.

15. How is the signal amplified?
Each successive step in the signal transduction pathway may activate multiple molecules in the next step.
Pathways may branch to produce multiple responses from the initial signal.
This is why small amounts of signalling molecules can produce a significant response or even multiple responses.
Relay runners pass the baton onto more and more runners
This increase in molecules becoming involved in the process is called the molecular cascade

16. Four Cellular Responses
Effector molecules bring about the cellular response.
Activate or inhibit enzyme action.
Regulate the expression of proteins by switching genes on or off.
Move transport vesicles to the plasma membrane to secrete cellular products.
Open or close protein channels.
Once a signalling molecule has delivered the message to a cell and the cell response is initiated, the signalling molecule is degraded by enzymes.