Elaborate, complex, intracellular and intercellular

1
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,
steroid hormones, fatty acid derivatives,
dissolved gas.
Target cell, receptor, response
2
Direct communication
Junctions
Animal Tight membranes fused preventing
leakage Desmosome filaments protrude giving
linkages between cell sheets Gap cytoplasmic
channels so molecules can flow between cells
Plant Plasmodesmata channels in adjacent cell
walls with plasma membrane lining. Connection of
cytoplasm
3
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.

4
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
molecules
Ca2 - second messengers increase in ions causes
response
5
Response

• Cell may respond by
• opening/closing of ion channels in membrane
• rearranging cytoskeleton
• synthesis or degradation of enzymes or
• other proteins
• change in gene expression

6
Extracellular hydrophobic signalling
Signals can diffuse across membrane
Examples include hormones e.g. thyroxine, sex
hormones and cortisol
Activate gene regulatory proteins
Occur in animals and plants Ecdysone promotes
moulting in arthropods Thyroxine increases
metabolism Cortisol increases release of
sugars Sex hormones stimulate secondary sexual
characteristics etc.
Slower than electrical signals but may persist
for hrs/days
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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

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Receptor proteins
Enzyme linked Give enzyme linked response
(kinase) on cytoplasmic side, activating
intracellular proteins
G-protein linked receptors Activate a GTP-binding
protein (G-protein) made of seven alpha
helices. G-protein acts as switch for enzymes in
the cell depending on binding of guanine
nucletides GDP (gives inactive form) or GTP
(gives active form)
Ion-channel linked receptors Transmembrane
proteins open with a signal molecule Allows
rapid ion movement e.g. Synapses
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Cell signalling in plants
Include
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
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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