AH Cell Signalling

Introduction

Elaborate, complex, intracellular and intercellular

Animal cells complex system of proteins e.g.•Receptor proteins (surface and intracellular)•Protein kinases•Protein phosphatases•GTP-binding proteins (G-proteins)

Signal molecules include proteins, nucleotides, steroidhormones, fatty acid derivatives, dissolved gas.

Target cell, receptor, response

AH Cell Signalling

Direct communication

AnimalTight – membranes fused preventing leakageDesmosome – filaments protrude giving linkages between cell sheetsGap – cytoplasmic channels so molecules canflow between cells

PlantPlasmodesmata – channels in adjacent cell walls with plasma membrane lining.Connection of cytoplasm

Junctions

AH Cell Signalling

Stages of cell signalling

Reception – transduction – response

Signal reception

•Specific•target protein(receptor)•Signal molecule called ligand•Change in shape of receptor protein•Local or distant effects•3 main types of membrane receptors enzyme linked, G-protein linked and Ion-channel linked.

AH Cell Signalling

Signal transduction (transfer of signal within cell)

•Staged in signal transduction pathway•Allows amplification•Activates large numbers of molecules•Cascade of molecular events

Molecules involved

Protein kinases - phosphorylate proteins, cascade ATP into ADP to activate proteins.

Cyclic AMP – second messenger small non protein moleculesCa2+ - second messengers increase in ions causes response

AH Cell Signalling

Response

Cell may respond by:

•opening/closing of ion channels in membrane

•rearranging cytoskeleton

•synthesis or degradation of enzymes orother proteins

•change in gene expression

AH Cell Signalling

Extracellular hydrophobic signalling

Signals can diffuse across membrane

Examples include hormones e.g. thyroxine, sex hormonesand cortisol

Activate gene regulatory proteins

Occur in animals and plantsEcdysone – promotes moulting in arthropodsThyroxine – increases metabolismCortisol – increases release of sugarsSex hormones – stimulate secondary sexual characteristics etc.

Slower than electrical signals but may persist for hrs/days

AH Cell Signalling

Extracellular hydrophillic cell signalling

Act on surface of plasma membrane to induce a response inside cells.

3 main types of receptor:

•enzyme linked receptors

•G-protein-linked receptors

•ion-channel-linked receptors

AH Cell Signalling

Receptor proteins

Enzyme linkedGive enzyme linked response (kinase) on cytoplasmicside, activating intracellular proteins

G-protein linked receptorsActivate a GTP-binding protein (G-protein) made of seven alpha helices.G-protein acts as switch for enzymes in the celldepending on binding of guanine nucletidesGDP (gives inactive form) or GTP (gives active form)

Ion-channel linked receptorsTransmembrane proteins open with a signal moleculeAllows rapid ion movement e.g. Synapses

AH Cell Signalling

Cell signalling in plants

Auxins (IAA) – promotes elongation of shoots

Cytokinins – stimulate cytokinesis

Gibberellins – stimulate stem elongation and germination

Abscisic acid (ABA) – promotes abscission

Ethylene – gas that inhibits fruit ripening

Include:

AH Cell Signalling

Stages in plant cell signalling

Reception – transduction - responses

Signals involve light, gravity, touch, water

Responses include tropisms, general growth, flower/fruit production, leaf/fruit abscission