BiologyISC Class 11 · Biological Classification

Kingdom Monera: Bacteria, Archaebacteria, Cyanobacteria and Mycoplasma

Updated 17 October 2026 · Fact-checked

Kingdom Monera has prokaryotic, mostly unicellular organisms: bacteria, archaebacteria, cyanobacteria and mycoplasma. They lack a membrane-bound nucleus and organelles. Bacteria are classified by shape (coccus, bacillus, vibrio, spirillum), Gram reaction and nutrition, and multiply rapidly by binary fission.

Match each shape name to its outline, then compare the thick wall that keeps the purple stain with the thin wall that turns pink.

Illustration drawn for QuestionBuddy.

Understand Kingdom Monera: Bacteria and Archaebacteria

Bacteria. The most abundant microorganisms. Shapes: spherical (coccus), rod-shaped (bacillus), comma-shaped (vibrio) and spiral (spirillum). The cell wall is made of peptidoglycan. They may be motile with flagella.

Gram staining. Gram-positive bacteria have a thick peptidoglycan layer, retain the crystal violet stain and look purple. Gram-negative bacteria have a thin peptidoglycan layer and an outer lipid-rich membrane; they lose the violet stain and take the pink counter-stain (safranin).

Nutrition. Autotrophic bacteria are photosynthetic (use light) or chemosynthetic (oxidise inorganic substances such as nitrates, nitrites and ammonia); most are heterotrophic, acting as decomposers or parasites.

Reproduction. Mainly by binary fission. Under unfavourable conditions some form endospores. In some, DNA is transferred by conjugation (cell contact), transformation (uptake of free DNA) or transduction (through a bacteriophage). Under ideal conditions a bacterium may divide about every 20 minutes, so numbers grow as N = N₀ × 2ⁿ for n generations.

Archaebacteria. Live in extreme habitats and lack peptidoglycan in the cell wall. Halophiles live in salty areas; thermoacidophiles in hot, acidic springs; methanogens in marshes and in the gut of ruminants, producing methane (biogas from cattle dung).

Cyanobacteria. Photosynthetic prokaryotes (blue-green algae), unicellular or filamentous, with chlorophyll a. Some, such as Nostoc and Anabaena, fix atmospheric nitrogen in specialised cells called heterocysts.

Mycoplasma. The smallest living cells, lacking a cell wall, able to survive without oxygen, and pathogenic in plants and animals.

Importance. Curd formation (Lactobacillus), antibiotics (Streptomyces), nitrogen fixation (Rhizobium) and diseases such as cholera, typhoid, tetanus and citrus canker.

Kingdom Monera: Bacteria and Archaebacteria: key facts

Shapes of bacteria: $C occ u s : s p h er i c a l ∣ B a c i l l u s : r o d ∣ V ib r i o : co mma ∣ S p i r i l l u m : s p i r a l$; Cell wall of peptidoglycan.
Gram reaction: Gram positive: thick peptidoglycan, purple | Gram negative: thin peptidoglycan with outer membrane, pink; Safranin is the counter-stain.
Archaebacteria: $H a l o p hi l es ∣ T h er m o a c i d o p hi l es ∣ M e t han o g e n s$; Salt, hot acid springs, marshes and rumen.
Bacterial growth: $N = N_{0} \times 2^{n}$; n = number of generations.

Kingdom Monera: Bacteria and Archaebacteria 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.

Spoilage of milk

Suppose a single bacterium falls into milk left in a warm kitchen and divides every 20 minutes.

Find the number of bacteria after 3 hours, and after how long the population first exceeds 1 million.

Try it first, then show the solutionSolution

3 hours = 180 minutes = 9 generations, so N = 2⁹ = 512.
For N > 10⁶ we need 2ⁿ > 10⁶; 2²⁰ = 1 048 576, so n = 20.
Time = 20 × 20 = 400 minutes = 6 hours 40 minutes.

Answer: 512 bacteria after 3 hours; more than a million after 6 h 40 min.

Memory hook: Population doubles every generation.

Tricks to remember Kingdom Monera: Bacteria and Archaebacteria

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

Purple is positive
Gram-positive cells keep the crystal violet and look purple.
Use it when: Gram staining questions.
Hetero-cyst, N-fix
Heterocysts of Nostoc and Anabaena fix nitrogen.
Use it when: Cyanobacteria questions.

Kingdom Monera: Bacteria and Archaebacteria: solved examples

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

Example 1: Cell wall of bacteria · 1 mark · MCQBoard level

The cell wall of most bacteria is made of:
(a) cellulose
(b) chitin
(c) peptidoglycan
(d) pectin

Try it first, then show the solutionSolution

Bacterial cell walls contain peptidoglycan; archaebacteria lack it.

Answer: (c) peptidoglycan

Example 2: Gram-positive and Gram-negative · 3 marks · Short answerBoard level

Distinguish between Gram-positive and Gram-negative bacteria on the basis of cell wall and staining.

Try it first, then show the solutionSolution

Gram-positive: thick peptidoglycan layer, retains crystal violet, appears purple. (1.5 marks)
Gram-negative: thin peptidoglycan layer with an outer lipid-rich membrane, loses crystal violet and takes the pink safranin counter-stain. (1.5 marks)

Answer: Thick wall, purple (positive) versus thin wall with outer membrane, pink (negative).

Example 3: Bacterial growth · 3 marks · Short answerHarder board level

A bacterial culture starts with 100 cells and the generation time is 20 minutes. Find the number of cells after 2 hours. How long does a single cell take to give more than 1 million cells?

Try it first, then show the solutionSolution

2 hours = 120 minutes = 6 generations. (1 mark)
N = 100 × 2⁶ = 6400 cells. (1 mark)
For one cell, 2²⁰ = 1 048 576 > 10⁶, so 20 generations = 400 minutes ≈ 6.7 hours. (1 mark)

Answer: 6400 cells; about 6 h 40 min.

Example 4: Archaebacteria · 4 marks · Short answerHarder board level

Name the three groups of archaebacteria with their habitats and explain how their cell wall differs from that of eubacteria. Give one use of methanogens.

Try it first, then show the solutionSolution

Halophiles live in extremely salty areas; thermoacidophiles in hot acidic springs; methanogens in marshy areas and in the guts of ruminants. (1.5 marks)
Their cell wall lacks peptidoglycan (unlike eubacteria), which helps them survive harsh conditions. (1 mark)
Methanogens produce methane, the main component of biogas from cattle dung. (1 mark)
They are prokaryotes of the kingdom Monera. (0.5 mark)

Answer: Halophiles, thermoacidophiles and methanogens; no peptidoglycan; methanogens produce biogas.

Example 5: Endospores and gene transfer · 5 marks · Long answerHOTS / competency

A hospital finds that boiling for a few minutes does not remove a contaminant from a culture medium, but autoclaving does. Identify the likely reason, explain how the organism reproduces when conditions are good, and describe three ways bacteria exchange genetic material.

Try it first, then show the solutionSolution

Some bacteria form endospores under unfavourable conditions; endospores are heat-resistant and survive boiling, while autoclaving (high temperature and pressure) kills them. (1.5 marks)
In favourable conditions bacteria multiply by binary fission. (1 mark)
Conjugation: transfer of DNA between two cells in contact. (0.75 mark)
Transformation: uptake of free DNA from the surroundings. (0.75 mark)
Transduction: transfer of DNA from one bacterium to another by a bacteriophage. (1 mark)

Answer: Endospores resist boiling; binary fission; conjugation, transformation and transduction.

Where students lose marks in Kingdom Monera: Bacteria and Archaebacteria

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

Mistake: Confusing Gram-positive and Gram-negative colour
Writing that Gram-positive bacteria appear pink.
Do this: Gram-positive: purple; Gram-negative: pink.
Usually costs: 1 mark.
Mistake: Counting generations wrongly
Using the number of minutes instead of generations in N₀ × 2ⁿ.
Do this: Divide the total time by the generation time first.
Usually costs: 1 to 2 marks.
Mistake: Saying archaebacteria are eukaryotes
Placing them outside the prokaryotes.
Do this: They are prokaryotes without peptidoglycan in the cell wall.
Usually costs: 1 mark.

Exam tips

Draw and label the four shapes of bacteria.
For growth numericals, find n = time/generation time, then 2ⁿ.
Link each archaebacterial group to its habitat.
Give named examples for importance.

Practice questions on Kingdom Monera: Bacteria and Archaebacteria

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
Heterocysts in Nostoc are meant for:
(a) photosynthesis
(b) nitrogen fixation
(c) reproduction
(d) storing food
Hint
They are thick-walled special cells.
Answer
(b) nitrogen fixation
Q2 · 2 marks · Short answerBoard level
Why are mycoplasmas unique among prokaryotes?
Hint
Think about the cell wall and size.
Answer
They lack a cell wall, are the smallest living cells known, and can survive without oxygen; they can be pathogenic.
Q3 · 3 marks · Short answerHarder board level
A bacterium divides every 30 minutes. How many bacteria arise from one cell in 5 hours?
Hint
Count generations first.
Answer
5 hours = 300 minutes = 10 generations; N = 2¹⁰ = 1024.
Q4 · 3 marks · Short answerHarder board level
Differentiate between chemosynthetic and photosynthetic autotrophic bacteria with examples of the energy source.
Hint
Think about the source of energy.
Answer
Photosynthetic autotrophs (cyanobacteria) use light energy. Chemosynthetic autotrophs oxidise inorganic substances such as ammonia, nitrites and nitrates to get energy.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Archaebacteria can survive in extreme habitats. Reason (R): Their cell wall lacks peptidoglycan and has a different chemical structure. 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
Their special cell wall structure helps them tolerate harsh conditions.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a village biogas plant uses cattle dung, and the curd in a nearby dairy is set by bacteria.
(i) Name the group of archaebacteria producing the biogas. (1 mark)
(ii) Name the main gas produced. (1 mark)
(iii) Name the bacterium that converts milk to curd. (1 mark)
(iv) Name one disease caused by bacteria. (1 mark)
Hint
Recall the examples from the importance of bacteria.
Answer
(i) Methanogens. (ii) Methane. (iii) Lactobacillus. (iv) Cholera, typhoid or tetanus.

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.Shapes of bacteria?Show answer
Coccus, bacillus, vibrio, spirillum.
2.What do heterocysts do?Show answer
Fix atmospheric nitrogen.
3.Cell wall in archaebacteria?Show answer
Lacks peptidoglycan.
4.Smallest living cells?Show answer
Mycoplasma.

Kingdom Monera: Bacteria and Archaebacteria: frequently asked questions

What is the difference between conjugation and transduction?

In conjugation DNA passes directly between two bacteria that are in contact. In transduction a bacteriophage carries bacterial DNA from one cell to another.

Are all bacteria harmful?

No. Many are useful, for example Lactobacillus in curd, Rhizobium in nitrogen fixation and Streptomyces in antibiotics.

Why does Gram staining matter?

It divides bacteria into two groups with different cell wall structures, which helps in identification and in choosing antibiotics.