Membrane lipids consist of a hydrophilic (water-loving) head, with two short hydrophobic (water-repelling) fatty acid tails. They are arranged in a bilayer with the heads facing outwards and the tails inwards. They diffuse freely within the layer (lateral diffusion).
Lipid-soluble (hydrophobic / lipophilic) substances get incorporated into the membrane. If a molecule contains both hydrophobic and hydrophilic parts, they can be tethered part in and part out of the membrane.
Transmembrane proteins (the proteins that span cross both layers of membrane) such as integrins and cadherins provide structural and signaling links.
Membrane-spanning proteins like ion channels and receptors allow signals to be transferred across the membrane.
Most of the receptors are from G-protein coupled receptor (GPCR) family with 7 membrane-spanning domains. When a ligand (e.g. neurotransmitter, hormone) attaches to the receptor, the receptor gets activated.
When a molecule (ligand) binds to its receptor, it leads to activation of enzymes to generate second messengers (like substrates for other enzymes or help activate protein kinases or trigger a rise in cytosolic Ca2+)
Protein kinases recognize target proteins by specific amino acid sequences (known as motifs).
Same motif can be seen in different proteins, so a protein kinase will be able to phosphorylate multiple targets and the result depends on cell type.
And a single motif can be recognized by more than one kinase.
Lipid-soluble molecules (such as O2 and CO2) and small molecules (such as water and urea) can easily pass through the cell membranes.
But larger molecules such as glucose and charged molecules (ions) need transporters or ion channels to go from one side of the cell to the other side.
Proteins or larger molecules can enter the cells via endocytosis (engulfed by membrane segments). Upon entering the cell, the engulfed segments from intracellular vesicles.
Ligand gated Vs Voltage gated
When an ion channel gets activated when a substance like a neurotransmitter attaches to it is called as Ligand gated.
The ion channels / receptors at Post synaptic terminal of skeletal muscle (the neuromuscular junction NMJ) get activated due to Acetyl choline release at NMJ, so they are ligand gated.Frequently tested
When an ion channel is activated due to electrical signal (like an action potential / membrane potential), it is called Voltage gated ion channel. Example – Voltage gated Sodium and potassium channels.
The concentration of ions varies from intracellular compartment to the extracellular compartment.
The concentration of Sodium is higher in extracellular compartment and the Potassium content is higher inside the cell.
The concentration gradient will try to push Sodium in and Potassium out.
If this is not controlled, eventually, intra and extra cellular compartments will have equal quantities of ions.
This is where the Sodium pump – Na+-K+-ATPase comes into picture to preserve variable ion concentrations across the cell.
For each 3 Sodium ions pumped out, it’ll pump 2 Potassium ions into the cell. Both these movements are against the concentration gradient. So it’ll use energy via ATPase.
Any transport utilizing energy directly is known as ACTIVE transport.