Biology · Cell: The Unit of Life · NEET
Both are passive (no ATP) and both move a substance from high to low concentration. The difference is WHAT moves. Diffusion = movement of any solute (like a neutral molecule) down its concentration gradient. Osmosis = movement of WATER only, across the membrane, from higher to lower water concentration. NCERT says it directly: 'Movement of water by diffusion is called osmosis.' So osmosis is a special case of diffusion where the moving particle is water.
Osmosis is PASSIVE transport. Water moves on its own from higher concentration to lower concentration and the cell spends no ATP energy. Only ACTIVE transport (like the Na+/K+ pump) uses ATP, and that moves molecules AGAINST the gradient (low to high). Since osmosis goes with the gradient, it is passive. This is a very common NEET trap.
This is a direct NCERT question. Neutral (uncharged) solutes move by simple diffusion. They pass across the membrane along the concentration gradient — from higher concentration to lower — without needing any energy or any carrier protein, because they are not blocked by the nonpolar lipid bilayer.
The membrane's interior is a nonpolar lipid bilayer (the fatty tails). Neutral, nonpolar solutes dissolve in this oily layer and slip through by simple diffusion. But polar molecules and ions cannot pass through the nonpolar lipid bilayer. So NCERT states they require a carrier protein of the membrane to facilitate their transport. This carrier-assisted passive movement is called facilitated diffusion — still no ATP is used.
No. Simple diffusion needs zero energy from the cell. NCERT calls it passive transport: 'Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.' The energy that drives it comes from the concentration gradient itself, not from ATP. Only movement AGAINST the gradient (active transport) needs ATP.
Facilitated diffusion is when a polar molecule or ion moves DOWN its concentration gradient with the help of a carrier or channel protein. It is PASSIVE because it still follows the gradient (high to low) and uses no ATP. The protein only helps the polar molecule cross the nonpolar bilayer — it does not push it against the gradient.
Movement and accumulation of ions across a membrane against their concentration gradient can be explained by
The main difference between active and passive transport across cell membrane is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Osmosis. This is the exact NCERT line. Osmosis is the diffusion of water across a selectively permeable membrane from a region of higher water concentration to lower water concentration.
Selectively permeable. It lets some molecules pass freely (small, neutral, nonpolar ones by diffusion) while polar molecules and ions need carrier proteins. This selective control is one of the most important functions of the plasma membrane.
The Na+/K+ Pump. It moves sodium and potassium ions against their concentration gradients and uses ATP, which is why it is active transport and not diffusion.
NEET repeatedly tests whether osmosis/diffusion are passive or active. Remembering that osmosis = passive diffusion of water (no ATP, down the gradient) helps you instantly reject wrong options that call it active transport.
