Cell Organelles and Prokaryotic and Eukaryotic Cells (Class 9)
Updated 17 October 2026 · Fact-checked
A eukaryotic cell has a true nucleus and membrane-bound organelles, each with a job: the nucleus holds the DNA, ribosomes make proteins, the rough ER and Golgi apparatus build, sort and package them, mitochondria release energy as ATP, lysosomes break down waste, plastids (in plants) carry out photosynthesis or store materials, and vacuoles store cell sap. A prokaryotic cell, such as a bacterium, has no true nucleus (its DNA lies in a nucleoid) and no membrane-bound organelles, and is much smaller.
Understand Cell Organelles, Prokaryotic and Eukaryotic Cells
Seeing cells. The human eye cannot separate two points closer than about 0.1 mm (100 μm), so most cells need a microscope. School light microscopes use an objective lens such as 10× or 40× together with an eyepiece, and total magnification is the product of the two (for example 40 × 10 = 400×). Magnification = size of the image ÷ actual size, in the same units. Electron microscopes show much finer detail. 1 mm = 1000 μm and 1 μm = 1000 nm.
Prokaryotic and eukaryotic cells. A prokaryotic cell (bacteria) has no true nucleus: its DNA lies in a region called the nucleoid, with no nuclear membrane, and it has no membrane-bound organelles. It is small, typically 1 to 10 μm, and usually unicellular. A eukaryotic cell (plants, animals, fungi) has a well-defined nucleus with a membrane and membrane-bound organelles, and is typically 10 to 100 μm. Eukaryotic cells also have a cytoskeleton of fine fibres that gives shape, helps movement and moves materials inside. Viruses, viroids and prions are acellular and are not cells.
The organelles, in a factory picture. The nucleus holds the coded instructions: a double nuclear membrane with pores, a nucleolus that makes ribosome subunits, and chromatin (DNA with proteins) that coils into chromosomes before division. A functional segment of DNA is a gene. Ribosomes, free or attached to the ER, are the sites of protein synthesis. The endoplasmic reticulum (ER) is a membrane network continuous with the nuclear envelope; rough ER carries ribosomes and makes and secretes proteins, and smooth ER has none and makes and stores fats and hormones. The Golgi apparatus (seen by Camillo Golgi in 1898) is a stack of flat sacs that modifies, sorts and packages proteins and lipids into vesicles. Lysosomes are single-membrane sacs of enzymes that break down waste and worn-out parts. Mitochondria have two membranes, the inner one folded into cristae to increase surface area, and carry out cellular respiration, releasing energy from glucose and storing it as ATP. Plastids are found in plants: chloroplasts contain chlorophyll and do photosynthesis, chromoplasts hold yellow, orange or red pigments, and leucoplasts are colourless and store starch, oils or proteins. A large central vacuole in a mature plant cell holds cell sap and keeps the cell firm; animal cells have small, temporary vacuoles. Mitochondria and plastids have their own DNA and ribosomes, a sign that they share an evolutionary history with bacteria. Mature human red blood cells have no nucleus, which leaves more room for haemoglobin, and they live for about 120 days.
Plant cell versus animal cell. Only plant cells have a cell wall, plastids and a large central vacuole; animal cells have only a cell membrane, and small vacuoles.
Cell size and the square-cube idea. If two cells are similar in shape, a cell k times longer has k squared times the surface area and k cubed times the volume. So a bigger cell has relatively less surface for exchange with its surroundings.
Cell Organelles, Prokaryotic and Eukaryotic Cells: formulas and key facts
- Magnification
- Total magnification of a microscope
- Objective magnification × eyepiece magnification
- Unit conversions
- Organelle and job
Nucleus DNA Ribosome protein synthesis Mitochondrion ATP Lysosome waste Golgi packaging Chloroplast photosynthesis - Scaling a cell
- k times longer gives k² times the surface and k³ times the volume
How to solve Cell Organelles, Prokaryotic and Eukaryotic Cells questions
Use this method for organelle, cell-type and magnification questions.
- 1For identification, list the features given (wall, nucleus, plastids, size) and match each with the organelle or cell type.
- 2For function questions, write the organelle's name and one clear job, using the standard word (for example 'protein synthesis', 'ATP', 'photosynthesis').
- 3For prokaryote versus eukaryote, compare nucleus, organelles and size in a table.
- 4For magnification, convert both lengths to the same unit first (use μm).
- 5Substitute into magnification = image size ÷ actual size, or multiply objective by eyepiece for total magnification.
- 6Check that the answer makes sense: a cell of 25 μm seen at 400× should look about 10 mm long.
Cell Organelles, Prokaryotic and Eukaryotic Cells in real life
Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.
Magnification of a cell on a slide
Suppose a student looks at an onion peel under a microscope with a 40× objective and a 10× eyepiece, and one cell appears to be 12 mm across in a drawing she makes of the field.
(a) Find the total magnification. (b) Find the actual width of the cell in micrometres. (c) Could the unaided eye see this cell as a separate unit, given that the eye separates points no closer than 0.1 mm?
Try it first, then show the solutionSolution
- (a) Total magnification = 40 × 10 = 400×.
- (b) Actual width = 12 mm ÷ 400 = 0.03 mm = 30 μm.
- (c) 30 μm = 0.03 mm, which is less than 0.1 mm, so the unaided eye cannot resolve it.
Answer: (a) 400×; (b) 30 μm; (c) no, 0.03 mm is smaller than 0.1 mm.
Memory hook: Image over actual, always in the same unit.
Why some cells are packed with mitochondria
Suppose a school project compares a cell from a bird's flight muscle and a cell from the outer layer of a leaf's skin, and finds that the muscle cell shows far more mitochondria in sections.
Explain why a cell that does a lot of work should have many mitochondria.
Try it first, then show the solutionSolution
- Mitochondria carry out cellular respiration, which releases energy from glucose and stores it as ATP, the cell's energy currency.
- A working muscle cell uses ATP rapidly to contract, so it needs many mitochondria to supply it.
- A skin cell of a leaf uses much less energy, so it needs fewer mitochondria.
Answer: More work needs more ATP, and mitochondria make ATP.
Memory hook: Busy cell, big power station.
Tricks to remember Cell Organelles, Prokaryotic and Eukaryotic Cells
Short memory aids. Say each one aloud once, then use it on a question the same day.
Cell as a factory
Nucleus = manager's office, ribosome = machine, rough ER = assembly line, Golgi = packing and post office, lysosome = waste disposal, mitochondrion = power plant.
Use it when: Recalling each organelle's job at once.
Pro = no true nucleus
'Pro' stands for primitive, 'eu' for true. Prokaryotes have no true nucleus; eukaryotes have a true nucleus.
Use it when: Prokaryote versus eukaryote questions.
Chloro, chromo, leuco
Chloro = green (photosynthesis), chromo = coloured (petals, fruits), leuco = colourless (stores starch, like in potato).
Use it when: Naming plastids by function.
Cristae are for surface area
The folds of the inner mitochondrial membrane give more surface to carry out respiration.
Use it when: Questions on structure-function in mitochondria.
Cell Organelles, Prokaryotic and Eukaryotic Cells: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Naming organelles from their function · 3 marks · Short answerBoard level
Name the organelle that: (a) is called the powerhouse of the cell and why, (b) is the site of protein synthesis, (c) breaks down worn-out parts and waste.
Try it first, then show the solutionSolution
- (a) Mitochondria: they release energy from glucose in cellular respiration and store it as ATP. (1)
- (b) Ribosomes (free in the cytoplasm or attached to the rough ER). (1)
- (c) Lysosomes: sacs of enzymes that break down waste and old cell parts so the products can be reused. (1)
Answer: (a) mitochondria; (b) ribosomes; (c) lysosomes.
Example 2: Prokaryotic versus eukaryotic cells · 3 marks · Short answerBoard level
Give three points of difference between a prokaryotic and a eukaryotic cell, and state one example of each.
Try it first, then show the solutionSolution
- Prokaryotic: no true nucleus (DNA in a nucleoid); Eukaryotic: well-defined nucleus with a nuclear membrane. (1)
- Prokaryotic: no membrane-bound organelles; Eukaryotic: has membrane-bound organelles such as mitochondria and ER. (1)
- Prokaryotic: small, typically 1 to 10 μm; Eukaryotic: larger, typically 10 to 100 μm. Example: bacterium (prokaryote), onion or human cell (eukaryote). (1)
Answer: Nucleus, organelles and size differ; bacteria are prokaryotic, plant and animal cells are eukaryotic.
Example 3: Magnification, actual size and the limit of the eye · 5 marks · Long answerHarder board level
A student studies slides using a 40× objective and a 10× eyepiece.
(a) Find the total magnification. (1)
(b) A plant cell has an actual length of 25 μm. How long does it appear, in mm? (1)
(c) A bacterium is 2 μm long. How long does it appear, in mm, and how many times longer is the plant cell than the bacterium? (2)
(d) The unaided eye cannot separate points closer than 0.1 mm. Is the 25 μm cell visible to the eye alone? Is its image at 400× visible? (1)
Try it first, then show the solutionSolution
- (a) 40 × 10 = 400×. (1)
- (b) Apparent length = 25 μm × 400 = 10 000 μm = 10 mm (since 1000 μm = 1 mm). (1)
- (c) Bacterium: 2 μm × 400 = 800 μm = 0.8 mm. Ratio of lengths = 25 ÷ 2 = 12.5, so the plant cell is 12.5 times as long. (2)
- (d) 25 μm = 0.025 mm < 0.1 mm, so it cannot be resolved by the unaided eye. The 400× image is 10 mm, well above 0.1 mm, so it can be seen. (1)
Answer: (a) 400×; (b) 10 mm; (c) 0.8 mm and 12.5 times; (d) not visible alone, visible at 400×.
Example 4: Following a secreted protein through the cell · 5 marks · Long answerHarder board level
A gland cell makes a digestive enzyme (a protein) and releases it from the cell.
(a) Name the part of the cell that carries the instructions for the enzyme. (1)
(b) Name the structures that assemble the protein. (1)
(c) Which membrane network, continuous with the nuclear envelope, carries these structures? (1)
(d) Which organelle sorts and packages the protein into vesicles? (1)
(e) Which organelle supplies the energy as ATP, and by what process? (1)
Try it first, then show the solutionSolution
- (a) The nucleus: the DNA (genes) in the chromatin carries the coded instructions. (1)
- (b) Ribosomes, the sites of protein synthesis. (1)
- (c) The rough endoplasmic reticulum, which has ribosomes on it and makes and secretes proteins. (1)
- (d) The Golgi apparatus, a stack of flat sacs that modifies, sorts and packages proteins into vesicles. (1)
- (e) Mitochondria, by cellular respiration releasing energy from glucose and storing it as ATP. (1)
Answer: (a) nucleus; (b) ribosomes; (c) rough ER; (d) Golgi apparatus; (e) mitochondria, by cellular respiration.
Example 5: Three unknown cells under the microscope · 4 marks · Case-basedHOTS / competency
A student records three cells: Cell X is 60 μm long, has a cell wall, a large central vacuole and green organelles. Cell Y is 3 μm long, has no nucleus, and its DNA lies in one region of the cytoplasm. Cell Z is 20 μm long, has a nucleus, no cell wall and no large vacuole.
(a) Classify X, Y and Z as plant, animal or bacterial cells. (1)
(b) Name the green organelle in X and state its function. (1)
(c) Assuming X and Y are similar in shape (cubes), by what factor is X longer than Y and how many times is its volume greater? (1)
(d) What does your answer to (c) suggest about the surface available for exchange per unit volume in the larger cell? (1)
Try it first, then show the solutionSolution
- (a) X is a plant cell (wall, large vacuole, green organelles); Y is a bacterial (prokaryotic) cell (no nucleus, DNA in a nucleoid); Z is an animal cell (nucleus, no wall, no large vacuole). (1)
- (b) The chloroplast, which contains chlorophyll and carries out photosynthesis. (1)
- (c) Length ratio = 60 ÷ 3 = 20. Volume ratio = 20³ = 8000. (1)
- (d) The surface area ratio is 20² = 400, so the surface per unit volume of X is 400 ÷ 8000 = 1/20 of that of Y. The larger cell has relatively much less surface for exchange. (1)
Answer: (a) X plant, Y bacterial, Z animal; (b) chloroplast, photosynthesis; (c) 20 times longer, 8000 times the volume; (d) one-twentieth the surface per unit volume.
Where students lose marks in Cell Organelles, Prokaryotic and Eukaryotic Cells
The slips examiners see most often, and what to write instead.
Mistake: Writing that prokaryotes have no DNA.
Students confuse 'no nucleus' with 'no genetic material'.
Wrong working
A bacterium has no DNA because it has no nucleus.
Right working
A bacterium has DNA in a nucleoid, with no nuclear membrane around it.
Do this: Say 'no true nucleus; DNA in the nucleoid'.
Usually costs: 1 mark.
Mistake: Comparing sizes without converting units.
Lengths are given in μm, mm and nm in one question.
Wrong working
Magnification = 12 mm ÷ 30 μm = 0.4×
Right working
12 mm = 12 000 μm, so 12 000 ÷ 30 = 400×
Do this: Convert everything to μm before dividing.
Usually costs: 2 marks.
Mistake: Writing 'mitochondria make energy'.
Energy cannot be made; it is released and stored.
Wrong working
Mitochondria produce energy.
Right working
Mitochondria release energy from glucose in cellular respiration and store it as ATP.
Do this: Use 'release' and 'ATP'.
Usually costs: ½ to 1 mark.
Mistake: Saying plant cells have no mitochondria.
Plants have chloroplasts, so students think they do not need mitochondria.
Wrong working
Plant cells have chloroplasts instead of mitochondria.
Right working
Plant cells have both chloroplasts and mitochondria; animal cells have mitochondria only.
Do this: Plants also respire.
Usually costs: 1 mark.
Mistake: Mixing rough ER and smooth ER functions.
Both look like membrane networks.
Wrong working
Smooth ER has ribosomes and makes proteins.
Right working
Rough ER has ribosomes and makes proteins; smooth ER has none and makes and stores fats and hormones.
Do this: Rough = ribosomes = protein.
Usually costs: 1 mark.
Exam tips
- In a table of differences, give a point for each of nucleus, organelles and size, and name an example.
- Always write an organelle's function with its technical word, such as 'protein synthesis', 'ATP', 'photosynthesis' or 'packaging'.
- In magnification questions, convert to μm first, then divide image size by actual size.
- A plant cell diagram must show cell wall, cell membrane, large central vacuole, nucleus and chloroplast with straight leader lines.
- Use reasoning words such as 'because it needs more ATP' when asked why an organelle is abundant.
Practice questions on Cell Organelles, Prokaryotic and Eukaryotic Cells
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 2 marks · Very short answerBoard level
Name two structures found in a plant cell but not in an animal cell, and give the function of each.
Hint
One is outside the membrane; one is coloured.
Answer
Cell wall: rigid cellulose layer that gives shape, support and protection (1 mark). Chloroplast (a plastid): contains chlorophyll and carries out photosynthesis (1 mark). A large central vacuole holding cell sap is also accepted.
Q2 · 3 marks · Short answerHarder board level
A cell has an actual length of 30 μm, and in a photograph taken through a microscope it measures 3.6 cm. Find the magnification of the photograph, and the size of the image of a 5 μm bacterium in the same photograph.
Hint
3.6 cm = 36 000 μm.
Answer
3.6 cm = 36 000 μm. Magnification = 36 000 ÷ 30 = 1200× (2 marks). Bacterium image = 5 × 1200 = 6000 μm = 6 mm (1 mark).
Q3 · 2 marks · Assertion-reasonBoard level
Assertion (A): Smooth ER makes and stores fats and hormones. Reason (R): Smooth ER has ribosomes attached to its surface and makes proteins. Choose: (a) A and R both true, R explains A; (b) A and R both true, R does not explain A; (c) A true, R false; (d) A false, R true.
Hint
Which type of ER carries ribosomes?
Answer
(c). A is true: smooth ER makes and stores fats and hormones. R is false: it is the rough ER that has ribosomes and makes proteins; smooth ER has none.
Q4 · 3 marks · Short answerHarder board level
A potato tuber cell and a leaf cell are compared. (a) Which type of plastid mainly occurs in each and what does it do? (b) Why is the inner membrane of a mitochondrion folded into cristae?
Hint
Colourless versus green plastids.
Answer
(a) Potato tuber: leucoplasts, colourless, store starch (1 mark). Leaf: chloroplasts, contain chlorophyll and carry out photosynthesis (1 mark). (b) The folds give more surface area for the reactions of cellular respiration and ATP production (1 mark).
Q5 · 4 marks · Case-basedHOTS / competency
Three different chemicals are tested on a cell: Chemical 1 stops lysosomes working, Chemical 2 damages the Golgi apparatus, and Chemical 3 damages the mitochondria. (a) Predict the effect of each. (b) Which chemical would affect the cell's energy supply first, and why?
Hint
Match each organelle to its job.
Answer
(a) Chemical 1: waste and worn-out parts are not broken down and accumulate (1 mark). Chemical 2: proteins and lipids made in the cell are not sorted, modified and packaged into vesicles, so secretion is disrupted (1 mark). Chemical 3: less respiration, so less ATP is made and energy-requiring activity falls (1 mark). (b) Chemical 3, because mitochondria release the energy as ATP (1 mark).
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.Where is DNA found in a bacterium?Show answer
In the nucleoid, a region with no nuclear membrane.
2.What is the energy currency of the cell?Show answer
ATP.
3.Which plastids store starch?Show answer
Leucoplasts.
4.Which organelle has cristae?Show answer
The mitochondrion.
5.Name the organelle that packages proteins into vesicles.Show answer
The Golgi apparatus.
6.How do you find the total magnification of a microscope?Show answer
Multiply the objective magnification by the eyepiece magnification.
Cell Organelles, Prokaryotic and Eukaryotic Cells: frequently asked questions
Do plant cells have mitochondria?
Yes. Plant cells carry out respiration, so they have mitochondria as well as chloroplasts.
Is a virus a prokaryotic cell?
No. Viruses are acellular, so they are not cells at all.
Why do eukaryotic cells need organelles?
Organelles divide the work: different reactions can be carried out in separate membrane-bound compartments, which makes a large cell efficient.
What is the difference between chromatin and chromosomes?
Chromatin is DNA with proteins in the nucleus; it coils into visible chromosomes when the cell is about to divide.
Why can a bacterium survive without a nucleus?
Its DNA lies in the cytoplasm in the nucleoid and does the same job of carrying genes; the cell is small and simple.