BiologyCBSE Class 11 · Cell: The Unit of Life

Mitochondria, Plastids and Ribosomes

Updated 17 October 2026 · Fact-checked

Mitochondria are double-membrane organelles whose inner membrane folds into cristae; they make ATP and are called the powerhouses. Plastids are double-membrane organelles in plants and algae: chloroplasts, chromoplasts and leucoplasts. Mitochondria and chloroplasts contain circular DNA and 70S ribosomes (semi-autonomous). Eukaryotic ribosomes are 80S (60S + 40S).

Understand Mitochondria, Plastids and Ribosomes

Mitochondria. Seen under the electron microscope; typically sausage-shaped or cylindrical, diameter 0.2 to 1.0 µm (average 0.5 µm) and length 1.0 to 4.1 µm. Each is double-membrane bound: the outer membrane is smooth and permeable to small molecules; the inner membrane forms folds, cristae, that increase the surface area. The two membranes have their own specific enzymes. The matrix is the space inside the inner membrane; it is rich in enzymes (of the Krebs cycle), contains a single circular DNA, a few RNA molecules, 70S ribosomes and the components for protein synthesis. Mitochondria divide by fission. Function: aerobic respiration, production of ATP, hence the name powerhouse.

Plastids. Found in plant cells and euglenoids; large organelles easily observed under the microscope. Types:

Chloroplasts: contain chlorophyll and carotenoid pigments; trap light energy for photosynthesis; green.
Chromoplasts: contain fat-soluble carotenoid pigments such as carotene and xanthophylls, giving yellow, orange or red colours (flower petals, fruits).
Leucoplasts: colourless, store nutrients: amyloplasts (starch, e.g. potato), elaioplasts (oils and fats), aleuroplasts (proteins).

Chloroplast structure. Length 5 to 10 µm, width 2 to 4 µm; lens-, oval-, spherical-, discoid- or ribbon-shaped; one to several per cell (Chlamydomonas has one, mesophyll cells have 20 to 40). Double membrane: outer permeable, inner less permeable. The space limited by the inner membrane is the stroma; it contains enzymes of carbohydrate and protein synthesis, circular DNA, 70S ribosomes. Inside are flattened membranous sacs, thylakoids, arranged in stacks called grana (singular granum), joined by stroma lamellae. Thylakoid membranes carry chlorophyll pigments.

Semi-autonomous organelles. Mitochondria and chloroplasts contain their own DNA and 70S ribosomes and can make some proteins, but depend on the nucleus for others.

Ribosomes. Granular, non-membrane-bound structures of RNA and proteins, first observed by George Palade (1953) under the electron microscope. Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic 70S (50S + 30S). S stands for the Svedberg unit, the sedimentation coefficient, an index of sedimentation rate in centrifugation. S values depend on size and shape, so they are not additive: 60S + 40S combine to give 80S (not 100S), and 50S + 30S give 70S. Function: protein synthesis.

Table: mitochondria versus chloroplast. Pigments: absent versus present. Inner folds: cristae versus thylakoids. Function: respiration versus photosynthesis. Both: double membrane, DNA, 70S ribosomes.

Draw and label: a mitochondrion and a chloroplast. Checklist: outer and inner membrane, cristae, matrix; stroma, thylakoid, granum, stroma lamellae.

Mitochondria, Plastids and Ribosomes: key facts

Ribosomes: $E u k a r y o t i c 80 S = 60 S + 40 S ∣ p r o k a r y o t i c 70 S = 50 S + 30 S$; Svedberg unit
Mitochondrion: Double membrane
cristae
matrix
circular DNA
70S; ATP production
Chloroplast: Double membrane
stroma
thylakoids
grana
stroma lamellae; Photosynthesis
Plastid types: $C h l or o pl a s t (g r ee n) ∣ c h r o m o pl a s t (y e l l o w - r e d) ∣ l e u co pl a s t (co l o u r l ess : am y l o -, e l ai o -, a l e u r o -)$; Three types

How to answer Mitochondria, Plastids and Ribosomes questions

Questions give a structure and ask for its function, or compare organelles.

1Say which membrane and which space the structure belongs to.
2Link each internal fold to its function (cristae to ATP; thylakoids to light reactions).
3Name pigments and the colour for each plastid.
4State that both organelles have DNA and 70S ribosomes.
5Use S as the Svedberg unit, not a size.

Mitochondria, Plastids and Ribosomes 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.

Ripe tomato

Suppose a green tomato turns red as it ripens and a potato tuber stays pale but is rich in starch.

Name the plastid in each case.

Try it first, then show the solutionSolution

Green to red: chloroplasts convert into chromoplasts rich in carotenoids.
Potato tuber stores starch in colourless amyloplasts, a leucoplast.
Both are plastids with a different pigment or storage role.

Answer: Chromoplast in ripe tomato; amyloplast in potato.

Memory hook: Colour or store: the plastid changes role.

Tricks to remember Mitochondria, Plastids and Ribosomes

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

Cristae = Create ATP
Cristae house ATP-making machinery.
Use it when: Mitochondria
CCL
Chloro (green), Chromo (colour), Leuco (white).
Use it when: Plastids
70 inside, 80 outside
Organelle ribosomes 70S; cytoplasmic 80S.
Use it when: Ribosomes

Mitochondria, Plastids and Ribosomes: solved examples

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

Example 1: Mitochondrial structure · 3 marks · Short answerBoard level

Describe the structure of a mitochondrion and the role of cristae.

Try it first, then show the solutionSolution

Double membrane: smooth outer membrane and inner membrane (1 mark).
Inner membrane folded into cristae to increase surface area (1 mark).
Matrix contains enzymes, circular DNA, 70S ribosomes (1 mark).

Answer: Double membrane, cristae, matrix.

Example 2: Plastids · 3 marks · Short answerBoard level

Name the three types of plastids and one function of each.

Try it first, then show the solutionSolution

Chloroplasts trap light for photosynthesis (1 mark).
Chromoplasts give yellow, orange or red colour (1 mark).
Leucoplasts store nutrients (starch, oil, protein) (1 mark).

Answer: Chloro-, chromo-, leucoplasts.

Example 3: Chloroplast parts · 3 marks · Short answerHarder board level

Label the parts of a chloroplast and state what each contains or does.

Try it first, then show the solutionSolution

Stroma: enzymes for carbohydrate synthesis, DNA, ribosomes (1 mark).
Thylakoids stacked as grana carry chlorophyll (1 mark).
Stroma lamellae connect grana (1 mark).

Answer: Stroma, grana, stroma lamellae.

Example 4: Semi-autonomous · 3 marks · Short answerHarder board level

Why are mitochondria and chloroplasts called semi-autonomous?

Try it first, then show the solutionSolution

They have their own circular DNA and 70S ribosomes (1 mark).
They can synthesise some of their own proteins and divide by fission (1 mark).
Many other proteins are made from nuclear genes, so they are not fully independent (1 mark).

Answer: Own DNA and ribosomes, yet dependent on nucleus.

Example 5: Ripening fruit · 4 marks · Short answerHOTS / competency

A green fruit turns red on ripening. Name the change in plastid and pigment, explain its function and why the red colour is useful to the plant.

Try it first, then show the solutionSolution

Chloroplasts become chromoplasts (1 mark).
Chlorophyll is lost; carotenoid pigments become visible (1 mark).
Bright colour attracts animals (1 mark).
Animals eat the fruit and disperse the seeds (1 mark).

Answer: Chloroplast to chromoplast; colour helps dispersal.

Where students lose marks in Mitochondria, Plastids and Ribosomes

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

Mistake: Mitochondria in prokaryotes
Prokaryote confusion.
Wrong working
Bacteria have mitochondria.
Right working
Bacteria lack mitochondria; they have mesosomes.
Do this: Eukaryote organelle.
Usually costs: 1 mark.
Mistake: Thylakoid in mitochondria
Folds mixed.
Wrong working
Mitochondria have thylakoids.
Right working
Cristae in mitochondria; thylakoids in chloroplasts.
Do this: Cristae vs thylakoids.
Usually costs: 1 mark.
Mistake: S as mass
Unit misread.
Wrong working
S is the size in nm.
Right working
S is the Svedberg sedimentation unit.
Do this: Sedimentation.
Usually costs: 1 mark.

Exam tips

Draw both organelles for comparison.
Give size where asked.
Name enzymes in matrix and stroma by function.
Mention double membrane in both.

Practice questions on Mitochondria, Plastids and Ribosomes

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 site of ATP formation in a cell is
(a) Golgi body (b) mitochondrion (c) lysosome (d) ribosome
Hint
Powerhouse.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Chloroplasts and mitochondria are called semi-autonomous organelles.
Reason (R): They contain DNA and 70S ribosomes.
(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
Own genetic system.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
A centrifuge separates ribosomes into particles of 80S and 70S. Which belongs to a cell with a nucleus, and what subunits make each up?
Hint
Sedimentation.
Answer
The 80S ribosomes belong to the eukaryotic cell (the one with a nucleus) and are made of 60S + 40S subunits. The 70S ribosomes are prokaryotic and are made of 50S + 30S subunits. S is the Svedberg unit of sedimentation coefficient.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose an electron micrograph shows organelle X with folded inner membrane and organelle Y with stacks of discs in a fluid matrix and green pigment.
(a) Name X and Y. (1)
(b) Name the folds of X and the discs of Y. (1)
(c) State the process each performs. (2)
Hint
Mitochondrion and chloroplast.
Answer
(a) X mitochondrion, Y chloroplast. (b) Cristae; thylakoids in grana. (c) X aerobic respiration (ATP); Y photosynthesis.
Q5 · 4 marks · Long answerHOTS / competency
Suppose a researcher isolates a organelle with a double membrane, circular DNA and 70S ribosomes from a root cell. Which organelles could it be, how would you distinguish them, and why not a nucleus?
Hint
Mitochondria or plastids.
Answer
It could be a mitochondrion or a leucoplast/plastid. Distinguish by cristae and absence of pigments (mitochondrion) versus thylakoids or storage grains (plastid). A nucleus has a double membrane but linear DNA, 80S ribosomes outside and nuclear pores, and no 70S ribosomes.

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.What are cristae?Show answer
Folds of the inner mitochondrial membrane.
2.What are grana?Show answer
Stacks of thylakoids.
3.Who first saw ribosomes?Show answer
George Palade.
4.What is an amyloplast?Show answer
Leucoplast storing starch.
5.Ribosome subunits in eukaryotes?Show answer
60S and 40S.
6.Which pigment is in chromoplasts?Show answer
Carotenoids.

Mitochondria, Plastids and Ribosomes: frequently asked questions

Why are mitochondria called powerhouses?

They make ATP by aerobic respiration.

What is the stroma?

Matrix of the chloroplast.

What are leucoplasts?

Colourless plastids that store food.

What is the S unit?

Svedberg unit of sedimentation.

Are plastids found in animals?

No.