1. Cell Signaling (Lecture 1)

2. Introduction Cells can not live in an isolated environment.
Prokaryotes communicate with each other, other organisms and surrounding environment.
Eukaryotes e.g yeasts, slime molds, and protozoans mate, differentiate and respond to the environment by secreting pheromones
Cells are able to receive and process signals.
Individual cells receive many signals simultaneously, and they then integrate the information they receive into a unified action plan.
They also send out messages to other cells both near and far.

3. What kind of signals do cells receive?
Most cell signals are chemical in nature.
Prokaryotic organisms have sensors that detect nutrients and help them navigate toward food sources.
In multicellular organisms, growth factors, hormones, neurotransmitters, and extracellular matrix components are some of the many types of chemical signals cells use.
These substances can exert their effects locally, or they might travel over long distances.
Some cells also respond to mechanical stimuli. For example sensory cells in skin and ear.

4. Signaling in plants and animals
In plants and animals, extra cellular signaling molecules control
Metabolism
Growth and differentiation of tissues
Synthesis and secretion of proteins
Composition of intracellular and extracellular fluids

5. General principle signaling
Synthesis of signaling molecules by the signaling cells
Release of signaling molecules
Transport of the signal to the target cell
Detection of a signal by a specific receptor protein present on the target cell
A change in cellular metabolism, function or development triggered by the receptor- signal complex
Removal of the signal, which often terminate the cellular response

6. 1 3 2
Signaling cell 4 5 6

7. Cellular responses due to cell signaling
Changes in the activity or function of specific enzymes and other proteins present in the cells
Changes in the amount of protein produced by a cell e.g. modification of transcription factors that stimulate or repress gene expression

8. Types of signaling

9. Receptor protein exhibit ligand binding effect
Receptor present on Plasma or nuclear membrane has ligand binding sites
Signaling molecules (hormones, pheromones or neurotransmitters) act as ligands
Confirmational change occurs in the receptor that initiate a sequence of chemical reactions
Receptor proteins are specific for each horomone

10. Different cells have different sets of receptor for the same ligand and each of which induces a different response
Different cells respond in a variety of way to the same ligand (e.g. acetylcholine)
Different ligands can induce the same cellular response in some cells (glucagon/epinephrine)
In most receptor-ligand system, the ligand do not have any function except to bind to receptor
Upon binding it changes the properties of receptor which then produce signals to the cell that a specific product is present
Target cells often degrade or modify the ligand to terminate or modify their response

11. Types of receptors

12. Classification of hormones
Lipophillic Hormones with intracellular receptors e.g steroid, thyroxine, retinoic acid
Hydrophillic with cell-surface receptors e.g peptide hormones (insulin growth factor and glucagon), small charge molecules (epinephrine and histamine)
Lipophillic with cell surface receptor e.g. prostaglandins