Question

How do molecules move across the plasma membrane?

Answer 1

The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. Integral membrane proteins enable ions and large polar molecules to pass through the membrane by passive or active transport.

Answer 2

Plasma membrane is selectively permeable in nature. Many molecules and solutes move across it by simple diffusion along the concentration gradient (from higher concentration to the lower concentration). Some neutral molecules also move across it by simple diffusion. Water move through biomembrane by osmosis. These are processes which do no require expenditure of energy and therefore transport such processes is called passive transport.

Few ions and molecules are transported through the membrane against their concentration gradient (from higher concentration to lower concentration). Polar molecules cannot pass through non polar lipid bilayer of cell membrane due to repulsion, so they require carrier proteins to pass through. Such processes require utilisation of ATP, that is, energy and therefore transport through such processes is called active transport. Example: Na+/K+ pump.

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