11.1. Sensing the Conditions Flashcards
Organisms sense things…
Microbial cells must respond to and adapt to environmental changes.
Cells in animals must respond to hormonal and other signals to maintain homeostasis.
Animals and plants must respond to external signals (fight or flight).
Sensing pathway…
Receive the signal –> transmit this to the appropriate part of the body, through signal transduction –> to induce a biological response.
Protein kinase receptors…
Catalyse the phosphorylation of themselves and / or other proteins.
For example, the insulin receptor phosphorylates itself and other insulin response substrates, which initiates the insertion of glucose transporters into the plasma membrane.
Ion channel receptors…
Channel proteins that are not always open but allow specific ions in and out of the cell depending on whether they are activated by correct signals.
Signals can be chemical (hormonal), sensory (light) or electrical charge differences.
For example, the acetylcholine receptor on muscle cells is a gated ion channel.
G-protein linked receptors…
Ligands binding to these receptors changes the shape of the cytoplasmic region (inside the cell) which binds to a G protein.
This activates a G protein, a highly mobile membrane protein (that are bound to GDP or GTP) with three subunits.
Upon activation, the GTP-subunit separates from the G protein and binds to a membrane-associated enzyme.
This binding initiates an excitatory or inhibitory response:
- Excitatory: epinephrine binds to G protein-linked receptors in the heart muscle, which activates an enzyme to produce cyclic AMP.
- Inhibitory: inhibition occurs with the same hormone, but the enzyme that produces cyclic AMP is inhibited.
Cytoplasmic receptors…
The steroid hormone, ligand, enters the cell and forms a complex with a cytoplasmic receptor.
The receptor dissociates from its chaperone where it enters the nucleus and acts as a transcription factor.
Direct signal transduction…
Results from the action of the receptor itself on effector proteins.
For example, the protein kinase cascade, where the phosphorylation of one effector triggers the phosphorylation of another and so on. This has a large response as it is amplified.
The information that arrived at the plasma membrane is then communicated to the nucleus.
This is usually a large amount of information.
Indirect signal transduction…
Uses secondary messengers to amplify the interaction between receptors binding and cellular reaction.
cAMP and cGMP:
- The enzyme that catalyses the formation of cAMP from ATP.
- Located on the cytoplasmic side of the plasma membrane.
- cAMP has two targets (binding to protein kinases and causing a cascade / binding to ion channels and opening them).
Lipids and Ca2+:
- Derived from phospholipids in the plasma membrane, hydrolysed by phospholipases.
- Secondary messengers from PIP2:
- The hydrophobic part is diaclyglycerol (embedded in the membrane).
- The hydrophillic part is inositol triphosphate (projecting into the cytoplasm).
- Low Ca2+ concentrations in the cytoplasm is maintained by active transport proteins at plasma and ER membranes.
- Many signals cause Ca2+ to open.
Nitric oxide:
- Discovered in studies on acetylcholine effects on smooth tissue.
- Acetylcholine stimulates the IP3/DAG signal transduction pathway to cause an influx of Ca2+, which leads to a second messenger, cyclic GMP.
- Furthermore, acetylcholine stimulates the IP3/DAG pathway of endothelial cells that line blood vessels to cause an influx of Ca2+, which activates the NO synthase that catalyses the production of NO for arginine.
- NO diffuses to nearby smooth muscle tissue where it stimulates the synthesis of cAMP.
Responses…
Fertilisation of an egg triggers a wave of calcium ions that stimulate the initiation of development.
A rapid increase in intracellular cAMP concentration is observed in a nerve cell within 20 seconds of exposure to serotonin.
Sense of smell:
- Dependent upon G linked-receptors in the top of the nasal cavity.
- The influx of Na+ and Ca2+ stimulates nerves to send signals to the brain.
