Question

Cell membranes are selectively permeable, meaning they let some things in but keep others out. Why might a cell want to do this? What kinds of things would a cell want to keep out? bio 181 book Kenneth Saladin 8e

Answer 1

• The plasma membrane or cell membrane is a semi-permeable or selectively-permeable membrane bounding a cell.
• It comprises of phospholipids bilayer with proteins within.
• The fluid mosaic model best explains the structure of the plasma membrane in terms of the fluid nature of the membrane.
• Cell membrane or plasma membrane is composed of a lipid bilayer, with hydrophobic tails within the membrane and hydrophilic heads facing outside.
• The solubility of the molecule in the lipid bilayer of the plasma membrane determines the rate of penetration.
• This creates a partition coefficient for the molecules, with the plasma membrane as a phase and the phase in which the molecule is itself dissolve (like water, being small molecule).
• The higher is the solubility of a molecule in lipid, the faster is the penetration rate of the molecule across lipid bilayer of the cell membrane (Overton’s rule). Like oxygen.
• Membrane receptor proteins facilitate the binding of ligand molecules, in the form of hormones, neurotransmitters, etc., to bind to them and trigger specific effector activities in the cells.
• These ligands, on binding to receptors, (sometimes through second messengers) initiate a signal transduction pathway.
• Thus, only molecules that are required for survival of the cells are selectively taken in. Otherwise, the unwanted substances may create a pressure in cell causing it to burst or get damaged.
• The unnecessary molecules or ions may create signal that trigger unnecessary cellular metabolic pathways that cause energy waste in cell.
• Poisonous and toxic substances are also prevented from entering the cell due to selective permeability.