Signal Transduction AP Biology Unit 3

Cell to Cell Communication Can occur in both eukaryotes and prokaryotes How? –Mostly through chemical signals –Can also occur by cell to cell contact Why? –To trigger specific reactions in the cell –May cause growth, the production of specific molecules, mating, other actions (like muscle contraction) Image taken without permission from

Signal Transduction The entire process from a cell’s detection of a chemical signal to the final response of the cell (to the signal) Ex. Testosterone entering a cell to trigger genes that control male characteristics

Local vs. long distance signalling Cells can send chemical signals –Locally = to nearby cells –Over long distances = through the bloodstream to cells far far away

3 Stages of Cell Signalling Reception –Chemical signal (ligand) binds to the receptor molecule (on cell surface or inside cell) Transduction –Reaction or series of reactions (cascade) leading to the final response Response –The specific cellular response

Receptors Chemical signals can be small chemicals, proteins, steroid hormones, etc. Each protein receptor binds to a specific ligand to trigger an signal transduction pathway

Receptors (cont.) When the signal molecule binds, it causes a change in the receptor protein’s shape  triggers the transduction pathway Ex. Receptor tyrosine kinase

Second Messengers Small, water-soluble molecules that are not proteins that are a part of the transduction pathway Often important to the functioning of the cascade Ex. cAMP

Types of Responses Molecularly, possible responses include: –Ion channels opened –Enzyme activities are changed (enzymes activated or inhibited) –Certain genes are transcribed Changes to/problems with signal transduction pathways can affect the cellular response.

Common Aspects of Pathways Many pathways include: –Protein Modifications (individual steps change proteins to trigger subsequent steps) –Phosphorylation cascades (proteins gain a phosphate group to activate them and trigger the next step)

Example Epinephrine (aka adrenaline) is a hormone associated with the “fight or flight” response When epinephrine is received by liver or muscle cells, it triggers a series of reactions in the cell. Final response: breakdown stored glycogen into monosaccharides

Questions Why would this happen? –When your body is under stress, it needs energy –Carbohydrates (monosaccharides) the body’s source of immediate energy. Why control this process? –Allows you to only release the energy when needed

Pathways are complex In most cases, it is more than on/off –Not just one step between reception and response. –Usually many different molecules and steps are involved. –Having many steps also provides more opportunities to control the process.

Amplification of Signal In a cascade of steps, each step activates more molecules than the step before it Allows for just a few signal molecules to create a large-scale response in a cell.

Specificity of Response Only some cells respond to certain signals The same signal molecule can trigger different responses in different types of cells –Exact response depends on the proteins present in the cell –Ex: epinephrine stimulates liver cells to break down glycogen, causes heart cells to contract