BiologyISC Class 11 · Cell: The Unit of Life

Cell Organelles and the Nucleus: Structure, Function and Comparisons

Updated 17 October 2026 · Fact-checked

Eukaryotic cells have membrane-bound organelles: the endomembrane system (ER, Golgi, lysosomes, vacuoles), semi-autonomous organelles (mitochondria and plastids with their own DNA and 70S ribosomes), ribosomes, microbodies, the cytoskeleton, centrioles and the nucleus. Prokaryotic cells lack a nucleus and membrane-bound organelles.

Understand Cell Organelles, Nucleus and Cell Comparisons

Endomembrane system. *ER:* a network of tubules; rough ER (ribosomes on the surface: protein synthesis and secretion) and smooth ER (lipid synthesis; steroid hormones). *Golgi complex* (Camillo Golgi): stacked flattened cisternae; modifies, packages and dispatches materials; makes lysosomes; forms the cell plate. *Lysosomes:* membrane-bound vesicles with hydrolytic enzymes (acidic pH); digest intracellular material; "suicide bags". *Vacuoles:* in plants the large central vacuole is bounded by tonoplast and holds sap, water and wastes.

Mitochondria. Double-membrane organelles; the inner membrane has folds, cristae, which increase the area for ATP synthesis; the matrix contains enzymes, a circular DNA and 70S ribosomes; they divide by fission, called the powerhouses of the cell.

Plastids. In plants: chloroplasts (green, with chlorophyll; grana of thylakoids, stroma lamellae, stroma containing DNA, 70S ribosomes and enzymes for the dark reaction), chromoplasts (coloured, carotenoids), and leucoplasts (colourless; amyloplasts store starch, elaioplasts fats, aleuroplasts proteins).

Ribosomes. Non-membrane-bound, made of rRNA and proteins, site of protein synthesis: 70S in prokaryotes (50S + 30S subunits) and 80S in eukaryotes (60S + 40S). S is the Svedberg unit of sedimentation.

Cytoskeleton and others. Microtubules, microfilaments and intermediate filaments give shape and movement. Cilia and flagella have a 9 + 2 arrangement of microtubules. The centrosome has two centrioles (9 triplets, cartwheel) and forms spindle fibres. Microbodies include peroxisomes and glyoxysomes.

Nucleus. Nuclear envelope with nuclear pores, nucleoplasm, nucleolus (rRNA synthesis) and chromatin (DNA + histone proteins). Chromosomes have a centromere with kinetochores; by its position they are metacentric (middle), sub-metacentric, acrocentric (near one end) and telocentric (at the end).

Prokaryotic and eukaryotic cells. Prokaryotes: no true nucleus (nucleoid), no membrane-bound organelles, 70S ribosomes, usually small (bacteria and cyanobacteria). Eukaryotes: true nucleus, membrane-bound organelles, 80S ribosomes (plus 70S in mitochondria and chloroplasts).

Cell Organelles, Nucleus and Cell Comparisons: key facts

Ribosomes: $P r o k a r y o t es : 70 S = 50 S + 30 S ∣ E u k a r y o t es : 80 S = 60 S + 40 S$; S = Svedberg unit.
Semi-autonomous organelles: $M i t oc h o n d r ia ∣ C h l or o pl a s t s$; Have own DNA and 70S ribosomes; double membrane.
Chromosome types: $M e t a ce n t r i c ∣ S u b - m e t a ce n t r i c ∣ A cr oce n t r i c ∣ T e l oce n t r i c$; By the position of the centromere.
Cilia and flagella: 9 + 2 arrangement of microtubules; Centriole: 9 triplets, cartwheel.

Cell Organelles, Nucleus and Cell Comparisons 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.

A muscle cell and a pancreatic cell

Suppose a student examines electron micrographs of a muscle cell and a pancreatic enzyme-secreting cell.

Predict which organelles are abundant in each.

Try it first, then show the solutionSolution

A muscle cell needs a lot of ATP, so it has many mitochondria with cristae.
A pancreatic enzyme-secreting cell makes proteins for export, so it has abundant rough ER and Golgi complexes.
The Golgi packages the enzymes into secretory vesicles.

Answer: Many mitochondria in muscle; rough ER and Golgi in the pancreatic cell.

Memory hook: Function decides the abundance of organelles.

Tricks to remember Cell Organelles, Nucleus and Cell Comparisons

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

Powerhouse and kitchen
Mitochondria produce ATP (power); chloroplasts make food (kitchen).
Use it when: Function of organelles.
70S in prokaryotes and in organelles
Mitochondria and chloroplasts carry bacterial-type ribosomes.
Use it when: Origin of organelles questions.

Cell Organelles, Nucleus and Cell Comparisons: solved examples

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

Example 1: Site of ATP production · 1 mark · MCQBoard level

The cristae of mitochondria are the sites of:
(a) protein synthesis
(b) ATP synthesis
(c) DNA replication
(d) lipid synthesis

Try it first, then show the solutionSolution

The inner mitochondrial membrane folds (cristae) carry the electron transport chain and ATP synthase.

Answer: (b) ATP synthesis

Example 2: Prokaryotic and eukaryotic cells · 3 marks · Short answerBoard level

A bacterium and an onion root-tip cell are viewed under an electron microscope. Contrast the two cells on three structural points.

Try it first, then show the solutionSolution

Nucleus: absent (nucleoid) in prokaryotes; true nucleus with envelope in eukaryotes. (1 mark)
Organelles: no membrane-bound organelles in prokaryotes; present in eukaryotes. (1 mark)
Ribosomes: 70S in prokaryotes; 80S in eukaryotes. (1 mark)

Answer: Nucleus, membrane-bound organelles and ribosome type.

Example 3: Endomembrane system · 4 marks · Short answerHarder board level

Name the organelles of the endomembrane system and give one function of each.

Try it first, then show the solutionSolution

Endoplasmic reticulum: protein synthesis (rough ER) and lipid synthesis (smooth ER). (1 mark)
Golgi complex: packaging and secretion. (1 mark)
Lysosomes: intracellular digestion with hydrolytic enzymes. (1 mark)
Vacuoles: storage, osmotic regulation (tonoplast in plants). (1 mark)

Answer: ER, Golgi, lysosomes and vacuoles.

Example 4: Semi-autonomous organelles · 4 marks · Short answerHarder board level

Why are mitochondria and chloroplasts called semi-autonomous organelles? Give four features common to them and one difference.

Try it first, then show the solutionSolution

They have their own circular DNA and 70S ribosomes and can make some of their own proteins; they divide by fission, but depend on the nucleus for many proteins. (1.5 marks)
Common features: double membrane, own DNA, 70S ribosomes, energy conversion. (1.5 marks)
Difference: mitochondria make ATP by respiration; chloroplasts trap light and make food. (1 mark)

Answer: Own DNA and 70S ribosomes but partly dependent on the nucleus.

Example 5: Chromosomes and nucleus · 5 marks · Long answerHOTS / competency

Name the types of chromosomes based on the position of the centromere with a sketch description of each. State the roles of the nucleolus and the nuclear pores. A cell has 46 chromosomes: how many chromatids are present just before cell division?

Try it first, then show the solutionSolution

Metacentric: centromere in the middle, two equal arms; sub-metacentric: slightly away from the middle; acrocentric: near one end; telocentric: at the end. (2 marks)
The nucleolus is the site of rRNA synthesis and ribosome assembly. (1 mark)
Nuclear pores allow the movement of RNA and proteins between the nucleus and the cytoplasm. (1 mark)
Each of the 46 chromosomes has replicated to give two chromatids: 92 chromatids. (1 mark)

Answer: Four types by centromere position; nucleolus makes rRNA; 92 chromatids.

Where students lose marks in Cell Organelles, Nucleus and Cell Comparisons

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

Mistake: Writing 80S for mitochondrial ribosomes
Using 80S for all ribosomes.
Do this: Mitochondria and chloroplasts have 70S ribosomes.
Usually costs: 1 mark.
Mistake: Mixing up cristae and grana
Giving cristae for chloroplasts.
Do this: Cristae belong to mitochondria; grana belong to chloroplasts.
Usually costs: 1 mark.
Mistake: Naming the centromere position wrongly
Calling a chromosome with the centromere near one end metacentric.
Do this: That is acrocentric; metacentric has the centromere in the middle.
Usually costs: 1 mark.

Exam tips

Draw a labelled diagram of mitochondrion and chloroplast.
Use a comparison table for prokaryotic and eukaryotic cells.
Mention the DNA and ribosomes for semi-autonomous organelles.
Give function with each organelle name.

Practice questions on Cell Organelles, Nucleus and Cell Comparisons

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 organelle is called the "suicide bag"?
(a) Golgi
(b) lysosome
(c) ribosome
(d) peroxisome
Hint
It contains hydrolytic enzymes.
Answer
(b) lysosome
Q2 · 2 marks · Short answerBoard level
What is the 9 + 2 arrangement?
Hint
Think of cilia and flagella.
Answer
The arrangement of nine doublets of microtubules around two central single microtubules in the axoneme of cilia and flagella.
Q3 · 3 marks · Short answerHarder board level
State the functions of the rough ER and the smooth ER.
Hint
One has ribosomes.
Answer
Rough ER has ribosomes on its surface and is involved in protein synthesis and secretion. Smooth ER lacks ribosomes and synthesises lipids and steroid hormones.
Q4 · 3 marks · Short answerHarder board level
Compare chloroplasts and mitochondria in structure.
Hint
Think of membranes and internal compartments.
Answer
Both have a double membrane, matrix or stroma, DNA and 70S ribosomes. Mitochondria have an inner membrane folded into cristae; chloroplasts have thylakoids stacked into grana in the stroma.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Ribosomes of mitochondria resemble those of bacteria. Reason (R): Mitochondria have 70S ribosomes. 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
Recall the ribosome type.
Answer
(a) Both are true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a cell is observed with a green organelle having grana, a double-membrane organelle with cristae, and a network of tubules studded with ribosomes.
(i) Name the three organelles. (1 mark)
(ii) State one function of each. (1 mark)
(iii) Which two have DNA? (1 mark)
(iv) Which type of cell is this? (1 mark)
Hint
The green organelle is present only in plants.
Answer
(i) Chloroplast, mitochondrion and rough ER. (ii) Photosynthesis; ATP production; protein synthesis. (iii) Chloroplast and mitochondrion. (iv) A plant (eukaryotic) cell.

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.Ribosomes in eukaryotes?Show answer
80S.
2.Site of protein synthesis?Show answer
Ribosomes.
3.Powerhouse of the cell?Show answer
Mitochondrion.
4.Chromosome with the central centromere?Show answer
Metacentric.

Cell Organelles, Nucleus and Cell Comparisons: frequently asked questions

What is the difference between cristae and grana?

Cristae are infoldings of the inner membrane of the mitochondria; grana are stacks of thylakoids in chloroplasts.

What is the role of the Golgi complex?

It packages, modifies and dispatches materials synthesised in the ER and forms lysosomes.

Why are lysosomes called suicide bags?

If they burst, their enzymes digest the cell itself.