Cell Organelles and Their Functions
~9 min read
- Cell types: Prokaryotes (bacteria) lack a nucleus. Eukaryotes (plant, animal, fungi) have membrane-bound organelles.
- Key organelles: Mitochondria (ATP), chloroplast (photosynthesis), ribosome (protein synthesis), lysosome (digestion).
- Self-DNA: Mitochondria and chloroplasts have their own DNA and ribosomes (endosymbiotic origin).
The cell is the structural and functional unit of life. CDS/OTA has asked 35+ questions on organelle functions — especially mitochondria, ribosomes, lysosomes and endoplasmic reticulum.
The Living Content of a Cell
- Protoplasm = cytoplasm + nucleoplasm + all organelles. The 'living jelly' of the cell.
- Prokaryotic cells (bacteria, archaea): no true nucleus; DNA lies free in the nucleoid region.
- Eukaryotic cells (plant, animal, fungi, protists): membrane-bound nucleus and organelles.
- Cells without nucleus in humans: mature red blood cells (RBC) — extruded the nucleus during maturation to maximise haemoglobin.
Organelles and Functions
| Organelle | Function | Special note |
|---|---|---|
| Nucleus | Houses DNA; controls cell activities | Bound by double membrane with nuclear pores |
| Mitochondria | Aerobic respiration; ATP synthesis | "Powerhouse"; has own DNA & ribosomes |
| Chloroplast (plants) | Photosynthesis; traps light energy | Contains chlorophyll; double membrane; own DNA |
| Ribosome | Protein synthesis | Free or attached to RER |
| Rough ER | Protein transport and folding | Looks "rough" due to ribosomes on surface |
| Smooth ER | Lipid and steroid synthesis; detoxification | No ribosomes |
| Golgi apparatus | Packaging and modification of proteins | Forms lysosomes |
| Lysosome | Intracellular digestion of waste/foreign material | "Suicide bags" — release enzymes on damage |
| Vacuole | Storage; turgor pressure in plants | Large central vacuole in plants |
| Centrosome | Organises spindle in animal cell division | Absent in higher plants |
Organelles with Their Own DNA
- Mitochondria and chloroplasts have their own circular DNA, ribosomes and can self-replicate.
- This supports the endosymbiotic theory — both evolved from prokaryotes engulfed by an ancestral eukaryote.
- Both have double membranes; the inner membrane resembles a bacterial plasma membrane.
- Histone proteins: present in the nucleus in association with DNA. They package and regulate DNA.
Where RNA is Found
- RNA is present in nuclei, cytoplasm, mitochondria, chloroplasts and ribosomes.
- Three main types: mRNA (carries genetic message), tRNA (transfers amino acids), rRNA (forms ribosomes).
- Protein synthesis happens on ribosomes (free in cytoplasm or attached to RER).
CDS/OTA PYQ Examples
Q: Which one of the following cell organelles is known as 'suicide bags' of a cell?
(a) Lysosomes (b) Plastids (c) Endoplasmic reticulum (d) Mitochondria
Answer: (a) Lysosomes — contain digestive enzymes that destroy the cell on rupture. [CDS-II 2019]
Q: Which cell organelles have their own DNA and Ribosomes?
(a) Golgi body and ER (b) Mitochondria and Plastids (c) Lysosome and Golgi body (d) Vacuole and Plastids
Answer: (b) Mitochondria and Plastids — endosymbiotic origin. [CDS-I 2021]
Q: Which cells in human body have no nucleus?
(a) Muscle cells (b) Phagocytic cells (c) Red blood cells (d) Ciliated cells
Answer: (c) RBCs — extruded the nucleus to maximise haemoglobin. [CDS-I 2024]
Q: Which organelle in animal cells can digest foreign materials?
(a) Endoplasmic reticulum (b) Lysosome (c) Golgi body (d) Mitochondria
Answer: (b) Lysosome. [CDS-II 2024]
Q: Why does the Rough Endoplasmic Reticulum look 'rough' under a microscope?
(a) Ribosomes on its surface (b) Absence of ribosomes (c) Lysosomes on surface (d) Mitochondria on surface
Answer: (a) Ribosomes studded on the surface. [CDS-I 2025]
Q: Lysosome is formed from which of the following cell organelles?
(a) Nucleus (b) Endoplasmic reticulum (c) Golgi bodies (d) Ribosomes
Answer: (c) Golgi bodies. [CDS-I 2018]
Drill Cell Organelles and Their Functions for CDS/OTA
CDS/OTA-pattern items on Cell Organelles and Their Functions with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
What is the difference between cytoplasm and protoplasm?
Cytoplasm is the jelly between the nucleus and cell membrane (organelles + cytosol). Protoplasm is the entire living content — cytoplasm plus nucleoplasm.
Why is mitochondrion called the powerhouse?
It carries out aerobic respiration, oxidising glucose to produce ATP — the universal energy currency of cells.
Do plant cells have lysosomes?
Plant cells usually lack typical lysosomes; their large central vacuole performs similar digestive and storage functions.