BiologyCBSE Class 11 · Cell: The Unit of Life

Cell Theory and the Prokaryotic Cell

Updated 17 October 2026 · Fact-checked

The cell theory states that all living organisms are composed of cells and their products, the cell is the basic unit of life, and all cells arise from pre-existing cells (Virchow). Prokaryotic cells such as bacteria, cyanobacteria and mycoplasma lack a membrane-bound nucleus and organelles; they have a cell envelope, a nucleoid, plasmids and 70S ribosomes.

Understand Cell Theory and Prokaryotic Cells

Cell theory. In 1838 Matthias Schleiden, a German botanist, found that all plants are composed of cells; Theodor Schwann, a German zoologist, in 1839 found that cell membranes were present in animal cells and that plant cell walls were unique to plants, and proposed that the bodies of animals and plants are composed of cells and their products. Rudolf Virchow (1855) stated *omnis cellula-e cellula*, that new cells arise from pre-existing cells. Cell theory: (i) all living organisms are composed of cells and products of cells; (ii) all cells arise from pre-existing cells.

Cell size. *Mycoplasma* (PPLO) about 0.3 µm; bacteria 3 to 5 µm; typical eukaryotic cells 10 to 20 µm. (1 µm = 10⁻⁶ m.) Shapes vary: disc-like, polygonal, columnar, cuboid, thread-like.

Prokaryotic cells. Bacteria, blue-green algae, mycoplasma and PPLO. Generally smaller and multiply faster. Common features: cell envelope, plasma membrane, cytoplasm, ribosomes, nucleoid, no membrane-bound organelles except ribosomes.

Cell envelope. A tightly bound three-layered structure: outermost glycocalyx, then the cell wall, then the plasma membrane. The glycocalyx differs: a loose sheath is a slime layer, a thick tough one is a capsule. Based on the cell envelope's stain response, bacteria are Gram positive (take the Gram stain) or Gram negative (do not). The cell wall determines shape and gives strength. The plasma membrane is semipermeable.

Mesosomes. Infoldings of the plasma membrane in the form of vesicles, tubules and lamellae, which help in cell wall formation, DNA replication and distribution to daughter cells, respiration and secretion, and increase the surface area of the plasma membrane and enzymatic content.

Genetic material. The nucleoid has no nuclear membrane; single circular chromosome. Many bacteria carry small, circular extra-chromosomal DNA, plasmids, giving traits like antibiotic resistance, useful in genetic engineering.

Ribosomes. Free in the cytoplasm; 70S (50S + 30S); several ribosomes may attach to a single mRNA, forming a polysome. Protein synthesis occurs on them.

Inclusion bodies. Reserve material in the cytoplasm, not membrane-bound: phosphate granules, cyanophycean granules, glycogen granules. Gas vacuoles in blue-green and purple and green photosynthetic bacteria.

Motility structures. Flagella: filament, hook and basal body. Pili and fimbriae: surface structures; pili are elongated tubular, fimbriae are small bristle-like fibres that help attach to rock in streams or to host tissues.

Table: prokaryote versus eukaryote. Nucleus: nucleoid versus true nucleus. Ribosomes: 70S versus 80S. Organelles: only ribosomes versus many.

Draw and label: a bacterial cell with capsule, cell wall, plasma membrane, nucleoid, plasmid, ribosomes, mesosome, flagellum.

Cell Theory and Prokaryotic Cells: key facts

Cell theory: All organisms made of cells | cells from pre-existing cells (Virchow 1855); Schleiden 1838, Schwann 1839
Sizes: $M y co pl a s ma 0.3µ m ∣ ba c t er ia 3 - 5µ m ∣ e u k a r y o t i c 10 - 20µ m$; Typical
Envelope: Glycocalyx
cell wall
plasma membrane; Three layers
Ribosomes: $P r o k a r y o t i c 70 S = 50 S + 30 S$; Svedberg units are not additive in everyday sense

How to answer Cell Theory and Prokaryotic Cells questions

Questions ask for scientists, the postulates, structures or comparisons.

1Name the scientist and year for each step of the cell theory.
2For prokaryotic structures, state function with the structure.
3Give sizes with the right units.
4Compare with the eukaryotic cell using a table.
5Mention exceptions (viruses are not cellular).

Cell Theory and Prokaryotic Cells 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.

Antibiotic-resistant bacteria

Suppose a hospital finds bacteria that survive a common antibiotic, and the resistance passes to other bacteria.

Which cell structure may carry the resistance gene and how is it significant in biotechnology?

Try it first, then show the solutionSolution

Plasmids are small circular extra-chromosomal DNA.
Resistance genes on plasmids can pass between bacteria.
Plasmids are used as vectors in genetic engineering.

Answer: Plasmids carry resistance genes and are used as vectors.

Memory hook: Plasmid = passport for genes.

Tricks to remember Cell Theory and Prokaryotic Cells

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

SSV
Schleiden plants, Schwann animals, Virchow cells from cells.
Use it when: Cell theory
G-W-M
Glycocalyx, Wall, Membrane: outside to in.
Use it when: Envelope
70 vs 80
Prokaryote 70S, eukaryote 80S.
Use it when: Ribosomes

Cell Theory and Prokaryotic Cells: solved examples

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

Example 1: Cell theory credit · 2 marks · Very short answerBoard level

Match the contribution with the scientist: (a) all plants consist of cells (b) animal bodies are cells and their products (c) new cells arise only from old cells.

Try it first, then show the solutionSolution

Schleiden (1838); Schwann (1839) (1 mark).
Virchow (1855), omnis cellula-e cellula (1 mark).

Answer: Schleiden, Schwann, Virchow.

Example 2: Cell envelope · 3 marks · Short answerBoard level

A bacterium is stained and its outermost coat is seen to be thick and tough. Name the three layers of the envelope from outside to inside, and identify this coat.

Try it first, then show the solutionSolution

Glycocalyx, cell wall, plasma membrane from outside (1 mark).
The coat is the glycocalyx, which is outermost (1 mark).
A thick, tough glycocalyx is a capsule, whereas a loose one is a slime layer (1 mark).

Answer: Three-layered envelope; this coat is a capsule.

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

What are mesosomes? State three functions.

Try it first, then show the solutionSolution

Infoldings of the plasma membrane as vesicles, tubules or lamellae (1 mark).
Help in cell wall formation, DNA replication and distribution (1 mark).
Help in respiration and secretion, increasing surface area and enzymatic content (1 mark).

Answer: Membrane infoldings aiding replication, respiration, secretion.

Example 4: Compare cells · 4 marks · Short answerHarder board level

Compare prokaryotic and eukaryotic cells on nucleus, ribosomes, DNA form, organelles.

Try it first, then show the solutionSolution

Nucleus: nucleoid without membrane versus true nucleus (1 mark).
Ribosomes: 70S versus 80S (1 mark).
DNA: single circular versus linear chromosomes (1 mark).
Organelles: none membrane-bound versus many (1 mark).

Answer: Four differences.

Example 5: Virchow's principle · 4 marks · Short answerHOTS / competency

A student says viruses refute the cell theory because they reproduce but are not cells. Evaluate the claim and explain how cell theory handles viruses.

Try it first, then show the solutionSolution

Cell theory concerns cellular organisms (1 mark).
Viruses cannot reproduce on their own; they depend on host cell machinery (1 mark).
Their reproduction occurs inside a host cell, so it does not contradict that cells arise from pre-existing cells (1 mark).
Viruses are generally considered non-cellular and outside the scope of the theory (1 mark).

Answer: Viruses depend on host cells; theory covers cellular life.

Where students lose marks in Cell Theory and Prokaryotic Cells

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

Mistake: Cell theory by Schwann alone
Credit confusion.
Wrong working
Schwann stated all cells arise from cells.
Right working
Virchow stated it.
Do this: Virchow 1855.
Usually costs: 1 mark.
Mistake: 80S ribosomes in bacteria
Types mixed.
Wrong working
Bacterial ribosomes are 80S.
Right working
Bacterial ribosomes are 70S.
Do this: Prokaryote = 70S.
Usually costs: 1 mark.
Mistake: Nucleus in bacteria
Nuclear membrane assumed.
Wrong working
Bacteria have a true nucleus.
Right working
Bacteria have a nucleoid.
Do this: No membrane.
Usually costs: 1 mark.

Exam tips

Write year with scientist.
Define each prokaryotic structure with a function.
Use micrometre symbol correctly.
Mention Gram staining for envelope.

Practice questions on Cell Theory and Prokaryotic Cells

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
The size of Mycoplasma is about
(a) 0.3 µm (b) 3 µm (c) 10 µm (d) 20 µm
Hint
Smallest.
Answer
(a).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Bacteria with a thick capsule are often more resistant to engulfing by white cells.
Reason (R): The capsule is a thick glycocalyx outside the cell wall.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Capsule is outermost.
Answer
(b).
Q3 · 2 marks · Very short answerBoard level
What are plasmids and why are they useful in biotechnology?
Hint
Small circular DNA.
Answer
Small circular extra-chromosomal DNA in bacteria, giving traits like antibiotic resistance; used as vectors to carry genes.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a microscope shows a cell of 4 µm with no nucleus, a looped DNA, many 70S ribosomes in a chain, and a long whip-like tail.
(a) Name the cell type. (1)
(b) Name the ribosome chain. (1)
(c) Name the parts of the tail. (2)
Hint
Bacterium.
Answer
(a) A bacterium (prokaryote). (b) A polysome (many ribosomes on one mRNA). (c) Filament, hook and basal body of the flagellum.
Q5 · 4 marks · Long answerHOTS / competency
Suppose a bacterium loses its mesosomes. List three processes likely to suffer and explain one consequence for population growth.
Hint
Replication and respiration.
Answer
Cell wall formation, DNA replication and its distribution, and respiration and secretion would suffer. Without proper DNA replication and distribution, cells cannot divide effectively, so the population would grow more slowly or stop.

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 gave omnis cellula-e cellula?Show answer
Virchow.
2.What are pili?Show answer
Elongated tubular surface structures.
3.What is a nucleoid?Show answer
Region of DNA without a membrane.
4.Size of bacteria?Show answer
3 to 5 µm.
5.What are inclusion bodies?Show answer
Reserve granules in the cytoplasm.
6.Ribosome type in bacteria?Show answer
70S.

Cell Theory and Prokaryotic Cells: frequently asked questions

What is the cell theory?

All organisms are cells; cells come from cells.

What is a prokaryote?

An organism without a membrane-bound nucleus.

What is the glycocalyx?

The outermost layer of the cell envelope.

What is Gram staining?

Differential staining based on the envelope.

What is a polysome?

Many ribosomes on one mRNA.