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Cell Membrane

The cell membrane, also called plasma membrane, is a thin selectively permeable boundary that controls movement of substances into and out of the cell.

In one line: Diffusion is for particles; osmosis is water through a membrane.

Practice This Concept

Understand it

The membrane is mainly made of lipids and proteins. It allows some substances to pass more easily than others. Diffusion moves particles from higher to lower concentration, osmosis is diffusion of water through a selectively permeable membrane, and active transport uses energy to move substances against a concentration gradient.

Example

Raisins swell in water because water enters by osmosis through their outer membrane.

Rule to remember

Diffusion: higher concentration to lower concentration.

Osmosis: water movement through a selectively permeable membrane.

Active transport: movement using energy, often against the gradient.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If a cell is placed in very salty water, water moves out of the cell by osmosis because the outside solution is more concentrated.

Method you can reuse

Identify the substance moving, compare concentration inside and outside, decide the direction, then name diffusion, osmosis, or active transport.

Sketch it in your answer

Draw a membrane with arrows showing diffusion, osmosis, and active transport in different directions.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is the plasma membrane called selectively permeable?

Think first, then show answer

It is called selectively permeable because it allows only certain substances to enter or leave the cell while restricting others according to the cell's needs.

Common wrong answer

Saying water always enters a cell.

Why it loses marks: Water movement depends on the relative concentration of solutions.

Don't mix this up

Students often write that osmosis is movement of any substance, but osmosis is specifically movement of water through a selectively permeable membrane.

Exam tip

In membrane transport answers, mention the direction of movement and whether energy is needed.

Cell Membrane diagram

Open full screen to read every label clearly.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask what happens to cells in hypotonic, isotonic, or hypertonic solutions, or why the membrane is selectively permeable.

1
Enough for 1 mark
The cell membrane, also called plasma membrane, is a thin selectively permeable boundary that controls movement of substances into and out of the cell.
2
Add for 2 marks
Example: Raisins swell in water because water enters by osmosis through their outer membrane.
3
Add for 3 marks
The membrane is mainly made of lipids and proteins.

MCQ Quiz

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