Class 9 Science · Original EZT practice
Cell: The Building Block of Life (Class 9 Science Chapter 2): summary and practice
Every living thing, from a blade of grass to you, is built from cells. This chapter opens biology: how scientists first saw cells, the selectively permeable cell membrane and osmosis, the cell wall, the nucleus and other organelles, and how cells grow and divide. All questions are original EZT tutor practice following the chapter's own section order.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 22 October 2026:
1. Discovering and studying the cell
Cells are far too small for the unaided eye - Robert Hooke first observed a cell in 1665 with a microscope he designed himself, and microscopes remain the basic tool for studying them.
2. Cell membrane and osmosis
The cell membrane is the thin, selectively permeable boundary of every cell - it lets some substances pass and blocks others. Water crosses it by osmosis, moving towards the side with less water (more solute).
3. Cell wall and the cell interior
Plant cells add a tough cell wall outside the membrane for shape and support. Inside, the nucleus controls the cell's activities and carries the genetic material, while organelles work as a coordinated system.
4. How cells grow and divide
Cells grow only to a limited size, then divide into two. Organisms grow and replace worn-out cells - like skin and blood cells - through repeated cell division.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Recall the discovery
Choose the method: Match the scientist, year and instrument.
EZT practice question
Who first observed a cell, and when?
Show the worked solution
- Cells are too small for the unaided eye, so the discovery waited for the microscope.
- Robert Hooke designed his own microscope.
- With it, in 1665, he became the first person to observe a cell.
Answer: Robert Hooke, in 1665, with a self-designed microscope.
Check: The microscope remains the basic tool for studying cells - magnification reveals structure the eye cannot resolve.
Watch for this: Do not credit the discovery to the eye alone - no microscope, no cell.
Now try it yourself
Why do we need a microscope to study cells?
Check your answer
Most cells are far smaller than the eye can see.
Their structure appears only under magnification.
Explain selective permeability
Choose the method: The membrane passes some substances and blocks others.
EZT practice question
What does a selectively permeable membrane do?
Show the worked solution
- The cell membrane is the thin boundary of every cell.
- It protects the contents and controls what enters and leaves.
- Some substances pass through; others are blocked - that selectivity keeps the cell's interior balanced.
Answer: It allows some substances to pass through while blocking others.
Check: Every cell has this membrane - it is the universal feature, present with or without a cell wall.
Watch for this: Do not call it fully permeable - free passage for everything would dissolve the cell's identity.
Now try it yourself
Does an animal cell have a cell membrane?
Check your answer
Yes.
The membrane is universal; the wall is not.
Apply osmosis
Choose the method: Water moves towards the side with less water (more solute).
EZT practice question
A potato piece is placed in a concentrated salt solution. After some time it weighs less. Why?
Show the worked solution
- The cell membrane lets water pass but not the salt.
- The salt solution has less water (more solute) than the potato cells.
- Water moves out of the potato by osmosis - so it loses water and weighs less.
Answer: Water leaves the potato cells by osmosis towards the concentrated solution, so it weighs less.
Check: Reverse it: in plain water the potato gains weight, because then its cells hold more solute than the surroundings and water flows in.
Watch for this: Osmosis moves WATER, not salt - the salt stays put across the membrane.
Now try it yourself
Define osmosis.
Check your answer
Water moving across a selectively permeable membrane from a dilute to a concentrated side.
Until concentrations balance.
Compare plant and animal cells
Choose the method: Look for the wall and the large vacuoles.
EZT practice question
Name two features present in a plant cell but absent in a typical animal cell.
Show the worked solution
- Plant cells carry a tough cell wall outside the membrane.
- They also carry large vacuoles for storage and support.
- Typical animal cells have neither - only the membrane.
Answer: The cell wall and large vacuoles.
Check: The wall is why plants stand upright without a skeleton - it gives each cell shape, strength and support.
Watch for this: The membrane is NOT a difference - both cell types have one; the wall is the extra.
Now try it yourself
Why is the cell wall important for plants?
Check your answer
It gives shape, strength and support.
That replaces a skeleton's job at cell level.
State the nucleus's job
Choose the method: Control centre plus genetic material.
EZT practice question
What is the function of the nucleus?
Show the worked solution
- Inside the cell, organelles work as a coordinated system.
- The nucleus directs that system.
- It controls the cell's activities and carries the genetic material.
Answer: It controls the cell's activities and carries the genetic material.
Check: Genetic material passing on during division is how each new cell knows what to be - the nucleus is central to both control and inheritance.
Watch for this: Do not confuse control with energy - the nucleus coordinates, other organelles power.
Now try it yourself
Which part of the cell carries the genetic material?
Check your answer
The nucleus.
That is one of its two core jobs.
Explain why cells divide
Choose the method: Growth has a size limit; division continues growth.
EZT practice question
Why do cells divide instead of growing indefinitely?
Show the worked solution
- A cell can only manage its contents up to a limited size.
- Beyond that, growth stops being efficient.
- So the cell divides into two smaller cells - which also grows the organism and repairs worn-out tissue.
Answer: Growth is limited, so cells divide - enabling growth and repair.
Check: Adults still need division: skin and blood cells wear out and are replaced by repeated cell division throughout life.
Watch for this: An organism does NOT grow by cells swelling without limit - it grows by cell numbers increasing.
Now try it yourself
Name one reason an adult body still needs cell division.
Check your answer
Replacing worn-out cells like skin or blood.
Repair never stops.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Cell?
Show answer and reason
The basic structural and functional unit of life.
Every organism is made of one or more cells.
2. Selectively permeable membrane?
Show answer and reason
Passes some substances, blocks others.
It controls what enters and leaves the cell.
3. Osmosis?
Show answer and reason
Water moving to the side with less water (more solute).
Across a selectively permeable membrane.
4. Cell wall?
Show answer and reason
Tough outer covering of plant cells.
Gives shape, strength and support.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- Who first observed a cell, and in which year?
- A peeled potato in plain water - does its weight rise or fall, and why?
- Name two features a plant cell has but a typical animal cell lacks.
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Questions about this practice page
- What is a cell?
- The basic structural and functional unit of life - every living organism is made of one or more cells.
- What is osmosis?
- The movement of water across a selectively permeable membrane from a dilute solution to a concentrated one, until the concentrations balance.
- What is the difference between the cell membrane and the cell wall?
- The membrane is the thin, selectively permeable boundary every cell has. The wall is an additional tough outer covering found in plant cells, giving shape and support.
- Who discovered the cell?
- Robert Hooke first observed a cell in 1665, using a microscope he designed himself.
- Are these the NCERT exercise questions for Class 9 Science Chapter 2?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.