Cell transport: diffusion, osmosis and active transport

Diffusion is net movement down a concentration gradient. Particles move in many directions, but the overall movement is from higher to lower concentration. This process can occur without a cell membrane and does not require a cellular pump.

Osmosis specifically concerns water moving through a selectively permeable membrane from higher to lower water potential. A membrane that lets water pass but restricts certain solutes can create different conditions on its two sides. Do not define osmosis as movement of all substances through any barrier.

Active transport moves substances against a concentration gradient using cellular energy. It often involves transport proteins in a membrane. It is different from diffusion because moving against the gradient requires an energy input.

Compare them using four headings: what moves, direction of movement, need for a selectively permeable membrane, and need for cellular energy. This is a more reliable method than trying to remember three nearly identical sentences.

Example: oxygen moving from a region of higher concentration into a cell where its concentration is lower can be described as diffusion. Water moving across an appropriate membrane towards lower water potential is osmosis. Mineral ions absorbed against a concentration gradient by a root cell can involve active transport.

Practice:
1. Which process specifically describes water movement through a selectively permeable membrane?
2. Which process can move a solute against its concentration gradient using cellular energy?
3. Does diffusion always require a living cell?
4. Why is a membrane important in the definition of osmosis?

Answers: 1 osmosis; 2 active transport; 3 no; 4 it permits water movement while restricting passage of some substances, supporting the water-potential difference.

When reviewing a question, identify the substance before the direction. Many distractors rely on confusing water with a general solute. Draw a simple diagram of both sides, label the gradient and write one sentence explaining why your chosen process applies.