Cell Communication

The Cellular “Internet”

Within multicellular organisms, cells must communicate with one another to coordinate their activities

A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response

Signal transduction pathways are very similar in all organisms, even organisms as different as unicellular yeasts and multicellular mammals

Local (Short-Distance) SignalingCells may communicate by direct contact

Plasmodesmata in plant cells Gap junctions in animal cells

Animal cells can also use cell-cell recognition Membrane-bound surface molecules can interact and

communicate

Local (Short-Distance) Signaling Messenger molecules can also be secreted by the signaling cell Paracrine signaling:

One cell secretes (releases) molecules that act on nearby “target” cells Example: growth factors

Synaptic Signaling: Nerve cells release chemical messengers (neurotransmitters) that

stimulate the target cell

Long-Distance Signaling

Endocrine (hormone) signalingSpecialized cells

release hormone molecules, which travel (usually by diffusion through cells or through the circulatory system) to target cells elsewhere in the organism

Hormones in Plants

In plants hormones can travel in vessels or diffuse through the air as a gas

Ethylene = gas that helps regulate growth and promotes ripening

The Three Stages of Cell Signaling

There are 3 stages at the “receiving end” of a cellular conversation:

1. Reception2. Transduction3. Response

Stage 1: ReceptionThe target cell “detects” that there is a signal molecule

coming from outside the cell The signal is detected when it binds to a protein on the cell’s

surface or inside the cell The signal molecule “searches out” specific receptor proteins

The signal molecule is a ligand• It is a molecule that specifically binds to another one (think enzymes!)

Stage 2: Transduction

This stage converts the signal into a form that can bring about a specific cellular responseOne signal-activated receptor activates another

protein, which activates another molecule, etc., etc.

These act as relay moleculesOften the message is transferred using protein

kinases, which transfer phosphate groups from ATP molecules to proteins

Stage 2: Transduction

Stage 3: Response

The signal that was passed through the signal transduction pathway triggers a specific cellular response Examples: enzyme

action, cytoskeleton rearrangement, activation of genes, etc., etc.

Diagram example: transcription of mRNA

The Specificity of Cell Signaling

The particular proteins that a cell possesses determine which signal molecules it will respond to and how it will respond to them

Liver cells and heart cells, for example, do not respond in the same way to epinephrine because they have different collections of proteins