CDS Biology · Cell Biology
Cell Organelles
The working parts of a eukaryotic cell: the endoplasmic reticulum and Golgi body, lysosomes and microbodies, mitochondria and plastids with their own DNA, and how to match each organelle to its job.
Why this matters
Seventeen CDS questions from 2017 (II) to 2026 (I), the biggest group in Cell Biology. Four ask what lysosomes do or which cell has the most of them, three are match-the-organelle lists, and two ask which organelles carry their own DNA.
Concept 1 of 4: Endoplasmic reticulum, ribosomes and the Golgi body
Think of the cell as a factory. Ribosomes are the machines that join amino acids into proteins. Many of them sit on a network of membranes called the endoplasmic reticulum, which makes that part look rough; the part without ribosomes looks smooth and makes fats instead. The Golgi body is the packing and dispatch room: it wraps the products in little sacs and sends them where they are needed.
Definition
The membrane network:
- Ribosomes: the site of protein synthesis. Not bounded by a membrane; made of RNA and protein.
- Rough ER (RER): ER with ribosomes on its surface, which is why it looks rough. Makes proteins and carries them through the cell.
- Smooth ER (SER): no ribosomes. Makes lipids and steroid hormones, and detoxifies drugs and poisons in liver cells. It cannot make proteins.
- Golgi body (Golgi apparatus): stores, modifies, packs and dispatches products in vesicles. It also forms the lysosomes.
| Organelle | Main work |
|---|---|
| Rough ER | Carries ribosomes on its surface; makes and transports proteinsQ CDS 2025 (I): it looks rough because of the ribosomes on its surface. |
| Smooth ER | Makes lipids and steroid hormones; detoxifies drugs in the liverQ CDS 2019 (I): protein synthesis is the one job smooth ER does not do. |
| Ribosome | Joins amino acids into proteins |
| Golgi body | Packs and dispatches products; forms lysosomesQ CDS 2018 (I): lysosomes are formed from the Golgi bodies. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q73Easy
Example 1 · Cell Biology · Cell Organelles
Why does the Rough Endoplasmic Reticulum look 'rough' under a microscope ?
Smooth ER makes fats, not proteins
Proteins need ribosomes, and ribosomes sit on the rough ER. The smooth ER makes lipids and steroids, detoxifies and transports, but never synthesises protein.
Lysosomes come from the Golgi
The digestive enzymes are made on the rough ER, but they are packed and budded off as lysosomes by the Golgi body. 'Formed from the ER' or 'from the nucleus' is the trap.
Concept 2 of 4: Lysosomes and microbodies
A lysosome is a small sac full of digestive enzymes. It digests food the cell has swallowed, germs and worn-out parts of the cell. If the sac bursts, the enzymes digest the cell itself, so lysosomes are called the suicide bags of the cell. Cells whose work is to eat germs, such as white blood cells, are packed with them. Microbodies are other small sacs of enzymes, found in both plant and animal cells.
Definition
Lysosomes:
- Membrane sacs holding hydrolytic (digestive) enzymes that work best in acid.
- Work: intracellular digestion of engulfed food, foreign particles and germs, and worn-out organelles.
- Called suicide bags: if one bursts, its enzymes digest the cell.
- Most numerous in phagocytic white blood cells, which engulf and digest bacteria.
- Formed by the Golgi body.
Microbodies (such as peroxisomes and glyoxysomes): small membrane sacs found in plant and animal cells that contain enzymes (for example catalase).
| Question about lysosomes | Answer |
|---|---|
| What do they store? | Digestive (hydrolytic) enzymesQ CDS 2019 (I): the lysosome is the storehouse of digestive enzymes. |
| What can they digest? | Food, germs, foreign particles and worn-out cell partsQ CDS 2024 (II): the organelle that digests foreign material in animal cells is the lysosome. |
| Which cell has the most? | White blood cells that engulf germsQ CDS 2026 (I): a white blood cell, not a pancreatic, ovarian or muscle cell. |
| Which organelle makes them? | The Golgi body |
| What do microbodies contain? | EnzymesQ CDS 2024 (I): microbodies in plant and animal cells contain enzymes. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (II) 2019 — General Knowledge · Q85Easy
Example 2 · Cell Biology · Cell Organelles
Which one of the following cell organelles is known as 'suicide bags' of a cell ?
Suicide bags are lysosomes, not vacuoles
A vacuole stores sap and water. The organelle full of digestive enzymes, which can destroy the cell, is the lysosome. Ribosomes and desmosomes are distractors too.
Phagocytes, not busy secretory cells
A pancreatic cell makes and exports enzymes (lots of ER and Golgi) and a running muscle needs energy (lots of mitochondria). The cell richest in lysosomes is the one that eats: the white blood cell.
Concept 3 of 4: Mitochondria and plastids: energy and their own DNA
Mitochondria are the power stations of the cell: they finish breaking down food with oxygen and store the energy in ATP, the cell's energy currency. Chloroplasts, a kind of plastid, do the opposite job, trapping sunlight to make food. Both have two membranes and carry their own DNA and ribosomes, so they can make some of their own proteins. This is why they are thought to have been free-living bacteria long ago.
Definition
The energy organelles:
- Mitochondria: site of aerobic respiration; make ATP (the energy currency of the cell). Called the powerhouse of the cell.
- Plastids: in plant cells. Chloroplasts (green, photosynthesis, with stacks of membranes called grana), chromoplasts (coloured), leucoplasts (store starch, oil or protein).
- Both mitochondria and plastids have a double membrane and their own DNA and ribosomes; they are called semi-autonomous.
- Glycolysis, in the cytoplasm, splits a six-carbon glucose into two pyruvate molecules of three carbons each; pyruvate then enters the mitochondrion.
| Question | Answer |
|---|---|
| Which organelles have their own DNA and ribosomes? | Mitochondria and plastidsQ Asked twice: CDS 2020 (I) (mitochondrion) and 2021 (I) (mitochondria and plastids). |
| Which molecule is the cell's energy currency? | ATP |
| How many carbon atoms does pyruvate have? | ThreeQ CDS 2025 (I): pyruvate is a three-carbon molecule. |
| How many membranes surround a mitochondrion or chloroplast? | Two |
| Which plastids store starch? | Leucoplasts |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q18Easy
Example 3 · Cell Biology · Cell Organelles
Which one of the following is the 'energy currency' for cellular processes?
Golgi, ER and lysosomes have no DNA
Apart from the nucleus, only mitochondria and plastids (chloroplasts) carry DNA and ribosomes of their own. A vacuole paired with plastids is half wrong, so the option fails.
ATP is the currency, glucose is the fuel
Glucose is broken down to make energy available, but the molecule the cell actually spends is ATP. ADP is what is left after spending it, and pyruvic acid is a step in between.
Concept 4 of 4: Matching organelles to their jobs
Several CDS items give four organelles and four jobs and ask you to pair them. The fastest way is to fix the one pair you are surest of, usually mitochondria with respiration or ATP, and strike out every code that does not have it. Then one more sure pair settles the rest.
Definition
The pairs to know cold:
- Mitochondria: respiration, ATP.
- Chloroplast: photosynthesis.
- Ribosome: protein synthesis.
- Rough ER: transport of proteins. Smooth ER: lipids, detoxification.
- Lysosome: intracellular digestion.
- Cytoskeleton (microtubules and filaments): keeps the shape of the cell and helps it move.
- Vacuole: stores cell sap and chemicals; in a plant cell one large central vacuole makes the cell swell and grow (enlarge).
- Nucleus: control centre, holds the DNA.
| Organelle | Job |
|---|---|
| Mitochondria | Respiration and ATP formationQ In all three CDS match items (2017 (II), 2023 (II), 2026 (I)) mitochondria go with respiration or ATP. |
| Chloroplast | Photosynthesis |
| Ribosome | Protein synthesis |
| Lysosome | Intracellular digestion |
| Cytoskeleton | Keeps the shape of the cellQ CDS 2026 (I): cytoskeleton with cell shape, smooth ER with detoxification in liver cells, vacuoles with storage and cell enlargement. |
| Vacuole | Storage of sap and chemicals; enlarges the plant cell |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2017 · CDS (II) 2017 — General Knowledge · Q7Easy
Example 4 · Cell Biology · Cell Organelles
Match List-I with List-II and select the correct answer using the code given below the Lists :
Code :
| List-I (Cell Organelle) | List-II (Function) |
|---|---|
| A. Mitochondria | 1. Photosynthesis |
| B. Chloroplast | 2. Protein synthesis |
| C. Ribosomes | 3. Intracellular digestion |
| D. Lysosomes | 4. ATP formation |
Ribosome makes; rough ER carries
Options love to swap these two. Ribosomes synthesise protein; the rough ER they sit on transports it. Likewise mitochondria (respiration) and chloroplasts (photosynthesis) are swapped in the wrong codes.
Fix one sure pair first
Do not try to match all four at once. Mitochondria with respiration usually removes two of the four codes immediately; one more sure pair leaves only the answer.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
Endoplasmic reticulum, ribosomes and the Golgi body4 rows
| Organelle | Main work |
|---|---|
| Rough ER | Carries ribosomes on its surface; makes and transports proteinsQ CDS 2025 (I): it looks rough because of the ribosomes on its surface. |
| Smooth ER | Makes lipids and steroid hormones; detoxifies drugs in the liverQ CDS 2019 (I): protein synthesis is the one job smooth ER does not do. |
| Ribosome | Joins amino acids into proteins |
| Golgi body | Packs and dispatches products; forms lysosomesQ CDS 2018 (I): lysosomes are formed from the Golgi bodies. |
Lysosomes and microbodies5 rows
| Question about lysosomes | Answer |
|---|---|
| What do they store? | Digestive (hydrolytic) enzymesQ CDS 2019 (I): the lysosome is the storehouse of digestive enzymes. |
| What can they digest? | Food, germs, foreign particles and worn-out cell partsQ CDS 2024 (II): the organelle that digests foreign material in animal cells is the lysosome. |
| Which cell has the most? | White blood cells that engulf germsQ CDS 2026 (I): a white blood cell, not a pancreatic, ovarian or muscle cell. |
| Which organelle makes them? | The Golgi body |
| What do microbodies contain? | EnzymesQ CDS 2024 (I): microbodies in plant and animal cells contain enzymes. |
Mitochondria and plastids: energy and their own DNA5 rows
| Question | Answer |
|---|---|
| Which organelles have their own DNA and ribosomes? | Mitochondria and plastidsQ Asked twice: CDS 2020 (I) (mitochondrion) and 2021 (I) (mitochondria and plastids). |
| Which molecule is the cell's energy currency? | ATP |
| How many carbon atoms does pyruvate have? | ThreeQ CDS 2025 (I): pyruvate is a three-carbon molecule. |
| How many membranes surround a mitochondrion or chloroplast? | Two |
| Which plastids store starch? | Leucoplasts |
Matching organelles to their jobs6 rows
| Organelle | Job |
|---|---|
| Mitochondria | Respiration and ATP formationQ In all three CDS match items (2017 (II), 2023 (II), 2026 (I)) mitochondria go with respiration or ATP. |
| Chloroplast | Photosynthesis |
| Ribosome | Protein synthesis |
| Lysosome | Intracellular digestion |
| Cytoskeleton | Keeps the shape of the cellQ CDS 2026 (I): cytoskeleton with cell shape, smooth ER with detoxification in liver cells, vacuoles with storage and cell enlargement. |
| Vacuole | Storage of sap and chemicals; enlarges the plant cell |
Watch out for (8)
- Smooth ER makes fats, not proteins→ Endoplasmic reticulum, ribosomes and the Golgi body
- Lysosomes come from the Golgi→ Endoplasmic reticulum, ribosomes and the Golgi body
- Suicide bags are lysosomes, not vacuoles→ Lysosomes and microbodies
- Phagocytes, not busy secretory cells→ Lysosomes and microbodies
- Golgi, ER and lysosomes have no DNA→ Mitochondria and plastids: energy and their own DNA
- ATP is the currency, glucose is the fuel→ Mitochondria and plastids: energy and their own DNA
- Ribosome makes; rough ER carries→ Matching organelles to their jobs
- Fix one sure pair first→ Matching organelles to their jobs
Test yourself on Cell Biology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.