Quick questions on Cell signalling and apoptosis: VCE Biology Unit 3

1. Stimulus. A change in the internal or external environment of the organism (for example, blood glucose rising, an immune trigger, a developmental cue). 2. Reception. The signalling molecule binds a specific receptor on or inside the target cell.

Signalling molecules include hormones (insulin, adrenaline, oestrogen, testosterone), neurotransmitters (acetylcholine, dopamine), cytokines (in immune responses) and growth factors. They fall into two broad classes based on solubility.

A receptor is a protein with a binding site specific to one signalling molecule (or a small family of related molecules). Binding is reversible and complementary in shape and chemistry, like an enzyme-substrate fit.

Signal transduction is the chain of events between receptor binding and the cellular response. A surface receptor cannot directly change cytoplasmic enzymes or gene expression, so it triggers a cascade of intracellular messengers.

Apoptosis is a controlled, programmed sequence of events that dismantles a cell from within. It is triggered when receptors detect signals such as DNA damage, viral infection, withdrawal of growth factors, or developmental cues.

Examples: peptide and protein hormones (insulin, glucagon, adrenaline), most neurotransmitters. They cannot cross the phospholipid bilayer, so they bind surface receptors on the plasma membrane.

Examples: steroid hormones (oestrogen, testosterone, cortisol), thyroid hormone, vitamin D. They diffuse straight through the membrane and bind intracellular receptors in the cytosol or nucleus. The activated receptor-hormone complex usually acts as a transcription factor, changing which genes are expressed.

Apoptosis sculpts tissues during embryonic development. The webbing between fingers and toes in the human embryo is removed by apoptosis. Tadpole tails resorb in the same way.

Steroids are hydrophobic and pass through the membrane. They bind intracellular receptors.

Reception and response are not enough; the transduction cascade is what amplifies and shapes the response. Markers want all four steps of the stimulus-response sequence.

That is necrosis. Apoptosis is orderly: the cell shrinks, packages itself into apoptotic bodies, and is engulfed without releasing contents.

Apoptosis is essential. Most problems come from too little (cancer) or too much (neurodegeneration) apoptosis, not from apoptosis itself.

Necrosis causes inflammation; apoptosis does not.