IB Biology Sub-topic C2.1 Notes

Receptors

You now know about cell origin, structure, function, and differentiation. The next step is to understand how cells are able to generate responses to environmental factors or other cells via receptors. These are defined as protein with binding sites for specific signalling chemicals, called ligands. Note that receptors are specific to their particular ligand to ensure efficient and appropriate responses, preventing unnecessary energy expenditure. There are two types of receptors you are expected to be aware of:

1. Intracellular - found within the cell’s cytoplasm or nucleus. These receptors bind hydrophobic ligands able to cross the cell membrane.
2. Extracellular (transmembrane) - found spanning the cell membrane with regions both inside and outside the cell, these receptors bind hydrophilic ligands that cannot cross the cell membrane.

There are number of different types of intracellular and transmembrane receptors. Regardless of the receptor type, the basic process of signal transduction is as follows:

1. A ligand binds to a receptor on the plasma membrane.
2. This causes a change in receptor conformation, which starts the signal transduction process.
3. During this, a chemical cascade produces second messenger chemicals that head to a target site.
4. There, they activate a particular process that generates a cellular response.

Note that some intracellular receptors are present at the target site and can directly activate a cellular response.

Quorum sensing

An example you need to be aware of is quorum sensing in  bacteria. This is a process by which bacteria communicate cell density to one another.

1. Quorum sensing bacteria secrete ligands called autoinducers into the extracellular matrix (ECM).
2. These can diffuse back into the cell and neighboring cells.
3. When there is a high cell density, there a high extracellular autoinducer concentration, causing a high diffusion into cells.
4. If autoinducers are above a threshold concentration, they binds to internal receptors to trigger a change in gene expression.
5. This results in a coordinated response of the cells to regulate motility, virulence, and symbiosis.

An example of quorum sensing occurs in the symbiotic relationship between the Hawaiian Bobtail squid and Vibrio fischeria, a bioluminescent bacteria. The hatchling squid collect the bacteria in their light organs and provide them with glucose and amino acids. In turn, the squid uses the bacteria for camouflage via counter-illumination, triggered by quorum sensing.