Osmosis

Water moves toward the side with more dissolved substance, such as salt or sugar. The membrane lets water through but holds back most of the dissolved particles. In a plant cell, the cell membrane does this job. The cell wall lets almost everything through, so it is freely permeable.

Two words give the direction. Endosmosis is water moving into a cell. Exosmosis is water moving out. Diffusion is a wider idea: any substance spreading from where there is more of it to where there is less. It needs no membrane.

A small example

Sprinkle salt on cucumber slices. The salty water outside is stronger than the cell sap. So water leaves the cells by exosmosis and collects in the bowl. A limp strip of potato in plain water does the opposite. It turns firm, because water enters its cells by endosmosis.

Where marks go

The ICSE 2027 specimen paper uses this cucumber salad as a one-mark question. Its answer is exosmosis, not endosmosis. So say which way the water moved.

The 2024 ICSE Biology paper asked when osmosis takes place. The answer was "a selectively permeable membrane". CISCE's report on the 2024 ICSE Biology paper asks teachers to separate four kinds of membrane: semi-permeable, selectively permeable, freely permeable and impermeable. Name the exact kind in your answer.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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