AP Bio Unit 4- Cell Signaling and Homeostasis

Paracrine signaling

secretory cells release local regulators (ie growth factors) via exocytosis to an adjacent cell

Synaptic signaling

Neurons secrete neurotransmitters (signaling molecules) that diffuse across the synaptic cleft (space between the nerve cell and target cell) and bind to receptors, causing a chemical change inside the receiving cell

Autocrine signaling

●a cell signals to itself

What do animal cells use

endocrine signaling: Specialized cells release hormones into the circulatory system where they reach target cells

Local regulators

a secreting cell will release chemical messages (local regulators/ligands) that travel a short distance through the extracellular fluid

Cell signalling

1. Reception: Ligand binds to receptor

2. Transduction: Signal is converted

3. Response: A cell process is altered

Reception

the detection and receiving of a ligand by a receptor in the target cell

All receptors have an area that interacts with the ligand and an area that transmits a signal to another protein

Plasma membrane receptors

Most common type of receptor involved in signal pathways

Bind to ligands that are:

Polar, water-soluble

Large

Intracellular Receptors

Found in the cytoplasm or nucleus of target cell

Bind to ligands that can pass through the plasma membrane

Ie hydrophobic molecules

Steroid and thyroid hormones

Gasses like nitric oxide

Transduction

the conversion of an extracellular signal to an intracellular signal that will bring about a cellular response

regulates protein activity through:

Phosphorylation by the enzyme protein kinase

Relays signal inside cell

Dephosphorylation by the enzyme protein phosphatase

Shuts off pathways

Secondary messengers

small, non-protein molecules and ions help relay the message and amplify the response

Cyclic AMP (cAMP) is a common second messenger

Response

the final molecule in the signaling pathway converts the signal to a response that will alter a cellular process

Ion Channels

Located in the plasma membrane

Important in the nervous system

Receptors that act as a “gate” for ions

When a ligand binds to the receptor, the “gate” opens or closes allowing the diffusion of specific ions

Initiates a series of events that lead to a cellular response

Protein Phosphorylation and Dephosphorylation

Series of protein kinases – enzyme that transfer phosphates. Below, series of activated protein kinases take a phosphate group from ATP and transfer it to another molecule (e.g., next protein kinase, and final protein involved in response)

Signal Amplification

•Occurs during the transduction step.

•Figure shows a single ligand generating 10⁸ molecules as the response

GPCRs

●Largest category of cell surface receptors

●Important in animal sensory systems

●Binds to a G protein that can bind to GTP, which is an energy molecule similar to ATP

Negative Feedback

The most common feedback mechanism

This type of feedback reduces the effect of the stimulus

Sweat

Blood sugar

Breathing rate

Positive Feedback

This type of feedback increases the effect of a stimulus

Examples:

Child labor

Blood clotting

Fruit ripening

Stimulus

a variable that will cause a response

Receptor

sensory organs that detect a stimulus. This information is sent to the control center (brain)

Effector

muscle or gland that will respond

Response

changes (decreases or increases) the effect of the stimulus