BiologyISC Class 11 · Cell: The Unit of Life

Cell Theory, the Fluid Mosaic Model of the Plasma Membrane and Transport

Updated 17 October 2026 · Fact-checked

The cell theory states that all organisms are made of cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells. The plasma membrane is a fluid mosaic of a phospholipid bilayer with proteins; substances cross it by passive transport (diffusion, osmosis) or active transport against a gradient using energy.

Understand Cell Theory, Plasma Membrane and Transport

Cell theory. Cells were first seen by Robert Hooke (1665) in cork. Anton von Leeuwenhoek saw living cells; Robert Brown described the nucleus. Schleiden (botanist) and Schwann (zoologist) proposed that the bodies of plants and animals are made of cells and their products (1838 to 1839). Rudolf Virchow (1855) added omnis cellula-e cellula: new cells arise from pre-existing cells. The modern theory: (1) all living organisms are composed of cells and their products; (2) the cell is the structural and functional unit of life; (3) all cells arise from pre-existing cells.

Cell size. Mycoplasma about 0.3 µm; bacteria 3 to 5 µm; human RBC about 7 µm in diameter; the largest isolated single cell is the egg of an ostrich; the longest cells are nerve cells. 1 µm = 10⁻⁶ m = 10³ nm. Magnification = size of image / actual size.

Plasma membrane (fluid mosaic model, Singer and Nicolson, 1972). A quasi-fluid bilayer of phospholipids (hydrophilic heads outward, hydrophobic tails inward), with proteins floating in it: integral proteins span the membrane, peripheral proteins lie on the surface. Cholesterol in animal membranes adds stability. The fluidity allows lateral movement of proteins, which helps in cell growth, endocytosis, secretion and cell division.

Transport. *Passive* (no energy, along a gradient): simple diffusion of small non-polar molecules (O₂, CO₂), facilitated diffusion through carrier or channel proteins (glucose, ions), and osmosis (diffusion of water across a selectively permeable membrane). *Active* (needs ATP, against a gradient): the Na⁺–K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP.

Cell wall. A non-living rigid covering outside the plasma membrane in plants, fungi and bacteria. In plants it is made of cellulose, hemicellulose, pectins and proteins; the middle lamella (calcium pectate) glues cells; secondary walls form in mature cells; plasmodesmata connect adjacent cells. In algae it is cellulose, galactans, mannans and minerals; in fungi chitin; in bacteria peptidoglycan.

Cell Theory, Plasma Membrane and Transport: key facts

Cell theory: Cells are the basic unit
All organisms are made of cells
New cells arise from old cells; Schleiden, Schwann, Virchow.
Membrane: Phospholipid bilayer + integral proteins + peripheral proteins + cholesterol; Singer and Nicolson, 1972.
Transport: Passive: diffusion, facilitated diffusion, osmosis | Active: pumps using ATP; Active against the concentration gradient.
Magnification: $M = s i z eo f ima g e / a c t u a l s i z e$; Convert to the same units first.

Cell Theory, Plasma Membrane and Transport in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Magnification of a cell under a microscope

Suppose a student measures a cheek cell as 60 mm long in a drawing made from a microscope and knows its actual length is 0.05 mm.

Find the magnification and express the actual size in micrometres.

Try it first, then show the solutionSolution

Magnification = image size/actual size = 60/0.05 = 1200.
Actual size = 0.05 mm = 0.05 × 1000 µm = 50 µm.
The cell is therefore drawn 1200 times larger than it is.

Answer: Magnification 1200; actual length 50 µm.

Memory hook: Use the same unit for the image and the object.

Tricks to remember Cell Theory, Plasma Membrane and Transport

Short memory aids. Say each one aloud once, then use it on a question the same day.

Heads out, tails in
The phospholipid heads are hydrophilic and face water; the tails face each other inside.
Use it when: Describing the membrane.
Active needs ATP
Moving against a gradient needs energy and a carrier.
Use it when: Distinguishing transport types.

Cell Theory, Plasma Membrane and Transport: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Cell theory · 1 mark · MCQBoard level

The statement "all cells arise from pre-existing cells" was proposed by:
(a) Schleiden
(b) Schwann
(c) Virchow
(d) Robert Brown

Try it first, then show the solutionSolution

Rudolf Virchow (1855) extended the cell theory with omnis cellula-e cellula.

Answer: (c) Virchow

Example 2: Fluid mosaic model · 3 marks · Short answerBoard level

Describe the fluid mosaic model of the plasma membrane and state why it is called "fluid".

Try it first, then show the solutionSolution

The membrane is a bilayer of phospholipids with hydrophilic heads outward and hydrophobic tails inside. (1 mark)
Integral proteins span the bilayer and peripheral proteins lie on the surface, like a mosaic. (1 mark)
It is fluid because lipids and proteins can move laterally within the bilayer. (1 mark)

Answer: Phospholipid bilayer with floating proteins that move laterally.

Example 3: Magnification · 3 marks · Short answerHarder board level

A cell has an actual diameter of 20 µm. Find its image size in millimetres if the magnification of a microscope is 500, and find the magnification needed to see a 7 µm RBC as 14 mm.

Try it first, then show the solutionSolution

Image size = 500 × 20 µm = 10 000 µm = 10 mm. (1.5 marks)
For the RBC: M = 14 mm/7 µm = 14 000 µm/7 µm = 2000. (1.5 marks)

Answer: 10 mm; magnification 2000.

Example 4: Transport across the membrane · 4 marks · Short answerHarder board level

Distinguish between passive transport and active transport. Give two examples of passive transport and explain how the Na⁺–K⁺ pump works.

Try it first, then show the solutionSolution

Passive: along a concentration gradient, no energy needed; active: against the gradient and needs ATP. (1.5 marks)
Passive examples: simple diffusion of O₂ and CO₂, facilitated diffusion of glucose, osmosis of water. (1 mark)
The Na⁺–K⁺ pump uses one ATP to move 3 Na⁺ out and 2 K⁺ in, against their gradients. (1.5 marks)

Answer: Passive: no energy; active: ATP; Na⁺–K⁺ pump moves 3 Na⁺ out and 2 K⁺ in.

Example 5: Cell wall and cell size · 5 marks · Long answerHOTS / competency

A cell has a wall with a middle lamella of calcium pectate and plasmodesmata. Name the cell type and the main component of its wall. State the composition of the cell wall in bacteria and fungi, and explain why the cell wall is not part of the living protoplasm. If a plant cell is 50 µm long, how many such cells would fit across a 1 mm leaf section?

Try it first, then show the solutionSolution

A plant cell; the wall is mainly cellulose with hemicellulose, pectins and proteins. (1 mark)
Bacterial wall: peptidoglycan; fungal wall: chitin. (1 mark)
The cell wall is a non-living secreted covering outside the plasma membrane, so it is not part of the protoplasm. (1 mark)
1 mm = 1000 µm, so the number of cells = 1000/50 = 20. (1 mark)
Plasmodesmata connect the cytoplasm of adjacent cells for communication and transport. (1 mark)

Answer: Plant cell, cellulose; peptidoglycan and chitin in bacteria and fungi; 20 cells.

Where students lose marks in Cell Theory, Plasma Membrane and Transport

The slips examiners see most often, and what to write instead.

Mistake: Writing 1 µm as 10⁻³ m
A wrong power of ten in size questions.
Do this: 1 µm = 10⁻⁶ m; 1 mm = 10³ µm.
Usually costs: 1 mark.
Mistake: Saying the membrane is rigid
Describing a fixed structure.
Do this: The fluid mosaic model emphasises its fluid nature.
Usually costs: 1 mark.
Mistake: Confusing passive and active transport
Saying that active transport follows the gradient.
Do this: Active transport is against the gradient and uses ATP.
Usually costs: 1 to 2 marks.

Exam tips

Draw and label the fluid mosaic model.
State the three postulates of the cell theory.
In numericals, convert to the same units before dividing.
Name examples for each type of transport.

Practice questions on Cell Theory, Plasma Membrane and Transport

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is a feature of the plasma membrane?
(a) it is rigid
(b) it has a phospholipid bilayer
(c) it is made of cellulose
(d) it is non-living
Hint
It is semi-fluid.
Answer
(b) it has a phospholipid bilayer
Q2 · 2 marks · Short answerBoard level
Who gave the fluid mosaic model and in which year?
Hint
Two scientists in the 1970s.
Answer
Singer and Nicolson, in 1972.
Q3 · 3 marks · Short answerHarder board level
Differentiate between diffusion and osmosis.
Hint
One involves water only.
Answer
Diffusion is the movement of molecules of any substance from a region of high to low concentration. Osmosis is the diffusion of water across a selectively permeable membrane from a region of high water potential to one of low water potential.
Q4 · 3 marks · Short answerHarder board level
Name the cell wall components of plants, fungi and bacteria and one function of the cell wall.
Hint
Match the group with the chemical.
Answer
Plants: cellulose, hemicellulose, pectins; fungi: chitin; bacteria: peptidoglycan. The cell wall gives shape, protects against mechanical damage and prevents bursting in a hypotonic solution.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Cholesterol is present in animal cell membranes. Reason (R): It helps to maintain the fluidity and stability of the membrane. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Think of its role in the bilayer.
Answer
(a) Both are true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a nerve cell and a red blood cell are compared.
(i) Which one is among the longest cells? (1 mark)
(ii) Give the approximate diameter of a human RBC. (1 mark)
(iii) Name the model of the cell membrane. (1 mark)
(iv) Name the pump that maintains the Na⁺ gradient. (1 mark)
Hint
Recall cell sizes.
Answer
(i) The nerve cell. (ii) About 7 µm. (iii) The fluid mosaic model. (iv) The Na⁺–K⁺ pump.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Who coined omnis cellula-e cellula?Show answer
Rudolf Virchow.
2.Cell wall of fungi?Show answer
Chitin.
3.Largest single cell?Show answer
Egg of an ostrich.
4.Pump moving Na⁺ out?Show answer
Na⁺–K⁺ pump.

Cell Theory, Plasma Membrane and Transport: frequently asked questions

Why is the plasma membrane called selectively permeable?

It allows some substances to pass more easily than others, depending on size, charge and lipid solubility.

Is the cell wall present in animal cells?

No. It is present in plants, fungi, bacteria and algae.

What is the importance of the middle lamella?

It cements neighbouring cells together; it is made of calcium pectate.