Signalling in unicellular organisms
- Prokaryotes respond to changes in environment
- These changes are detected at the cell surface
- Information is relayed inside the cell
- Expression of genes enable the cell to cope with new environment
Signalling in multicellular organisms
- Control of gene expression enables cells to carry out specialised functions
- Cooperation and coordination ensure correct function of whole organism and prevent uncontrolled proliferation
- Signalling can occur over a range of distances
What are the four basic tissue types?
- Endothelial
- Connective
- Muscle* (skeletal, smooth and cardiac)
- Nervous*
- Electrically excitable
Dictostelium aggregation
- An amoeba that is essentially a bag of nuclei
- Can exist as a single cellular or multicellular organism
- The multicellular form is generated when the cells are starved
Cell-to-cell communication by extracellular signalling
- Synthesis of signalling molecule by signalling cell
- Release of signal by signalling cell
- Transport of signal to target cell
- Detection of signal by a specific receptor protein
- Change in cellular behaviour triggered by receptor-signal complex
- Removal of the signal to terminate cellular response
Plasma membrane
- Defines the interface between the cell and the environment
- Huge distance for a signal to cross
- Hydrophobic signals are able to pass through without a carrier, but need carriers outside of the membrane
Lipid rafts
- More solid lipid micro-environments on the cell surface containing proteins with specific post translational modifications
- These areas can favour specific protein interactions and activate signalling cascades
Gap junctions
- Allow direct communication between adjacent cells
- Exist where plasma membranes are close together and connexion proteins form tubes; allows the passage of small molecules such as cAMP and Ca2+
What is signal transduction in multicellular organisms?
The process by which extracellular signals bring about their characteristic effects inside the cells; information is converted from one form to another
Intercellular
Between cells
Intracellular
Within cells
What are ligands (signals/first messengers)?
- Hormones, growth factors, neurotransmitters, etc.
- Bind/activate specific receptors, either within or on the surface of target cells
What happens to the ligand after binding?
It is unchanged by the interaction, only the receptor protein is changed
What do receptor proteins do?
Bind to and interact with physiologically active substances to relay signals across the membrane or into the nucleus
What do receptor-ligand complexes regulate?
- Cellular metabolism
- Enzyme activity
- Nuclear activity leading to the transcription of specific genes
- Cell development/differentiation/division
- Changes in cytoskeleton
What does generating an intracellular response usually involve?
The release of a second messenger within the target cell
Endocrine signals
- Long-distance signals, e.g. hormones via blood, transpiration stream
- These signals are transported
Paracrine signals
- Short-distance signals, e.g. growth factors, neurotransmitters
- These signals diffuse, and their concentration may generate different responses
Autocrine signalling
Essentially a type of signalling where the signalling cell is also the target cell
Signalling by plasma membrane-attached proteins
- This type of signalling occurs during development, and only affects adjacent cells which are very close by
- One cell has a particular molecule on its surface, and the other has a receptor for the molecule
What does delta do?
Delta chooses particular cells to become neurones; it is perceived by adjacent cell receptors called notch, which when bound to delta, prevent other cells from producing delta and thus becoming neurones
