7. The cell - an open biological system. Intercellular signalling. Cell contacts. Cell receptors.

Question 1

Barrier contacts

Answer 1

Typical for epithelial tissue

Has:
1. Tight junction - Functions:
- Prevents the diffusion of larger molecules;
- Facilitates the passage of smaller molecules and ions;
- Prevents the penetration of cells into the intercellular space

2. Dense bands
3. Dense spots

• Function of 2 and 3 - Prevent the passage of cells through the intercellular spaces of a certain type of epithelium.

Question 2

Adhesion contacts

Answer 2

Types:
1. cell-cell adhesion
- connection between two neighboring cells
- Cell-cell adhesions are united and called desmosome.

2. cell-extracellular matrix adhesion called HEMIDESOSOMES
   - are similar to desmosomes, but connect the cell to the intercellular space.
   - are asymmetric and obsereved in basal part of epithelial cells

Function of adhesion contacts
- cell attachment
- Transmission of signals

Question 3

Communicating junctions

Answer 3

Type
1. **Nexus **(gap junction)
- Structure: connexons (proteins forming small units)
- Function: transfer of small molecules, ions and electric signals
- Distribution: in embryonic cells and in neurons

2. Synapse
   - Function: transmits info via mediators

Question 4

Types of membrane receptors

Answer 4

Membrane receptors are macromolecules on surface of celss that specifially recognize ligands with which they interact.

Ligands - located outside the cell - can be
hormones, mediators, growth factors, drugs,
viruses, etc.

Types
1. Neurotransmitter receptors
- Channel receptors – for acetylcholine, serotonin, glutamate, GABA, and also glycine-gated channel receptors
-> in postsynaptic membrane of chemical synapses
-> transform chemical signal to electrical one
-> connected to psychotropic drugs - treatment of
depression, schizophrenia, insomnia, fear, tension
- non-channel receptors for dopamine, norepinephrine

2. Hormonal receptors - bind hormones that are proteins or amino acid derivatives
   - insulin receptor, for growth factors, for glucagon, adrenaline, histamine, prostaglandins
3. Immunoreceptors - located on the membrane of immune cells
   - bind to cells foreign to the body
4. Endocytosis receptors – for receptor-mediated endocytosis

Question 5

3 Classes of membrane receptors

Answer 5

1. Membrane receptors associated with an ion channel:
   - Directly affect ion channels on the membrane of excitable cells.
   - Example: acetylcholine receptor.
2. G protein-coupled membrane receptors
   - Indirectly regulate activity of other membrane-bound target proteins (enzymes or ion channels).
   - Interaction between receptor and target protein is mediated by the G protein (trimeric GTP-binding regulatory protein).
   - Example: adrenaline receptor.
3. Enzyme-linked membrane receptors:
   - Receptor has a ligand-binding portion in membrane and a cytoplasmic portion with enzymatic activity or is directly linked to an enzyme.
   - Example: tyrosine kinase receptor.

Question 6

Dysfunction of acetylcholine receptor + insulin receptor

Answer 6

Dysfuntion of acetylcholine receptor leads to MYASTHENIA GRAVIS - autoimmune disorder with periods of skeletal muscle weakness

Dysfunction of insulin receptor has been associated with many diseases, including diabetes, cancer and Alzheimer’s disease.