Cell Structure
The cell is the basic structural and functional unit of all living organisms — the smallest unit capable of carrying out life processes independently.
A. Types of Cells
Prokaryotic vs Eukaryotic Cells
The most fundamental division in cell biology
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | No membrane-bound nucleus — DNA floats freely in cytoplasm (nucleoid region) | True membrane-bound nucleus containing DNA |
| Size | Smaller: 1–10 µm | Larger: 10–100 µm |
| DNA | Circular, naked (no histones), in cytoplasm | Linear, wrapped around histones, inside nucleus |
| Membrane-bound organelles | Absent — no mitochondria, ER, Golgi | Present — mitochondria, ER, Golgi, lysosomes etc. |
| Ribosomes | Present — but smaller (70S) | Present — larger (80S) |
| Cell wall | Present — made of peptidoglycan | Present in plants (cellulose), fungi (chitin); absent in animals |
| Examples | Bacteria, Cyanobacteria (blue-green algae) | Plant, Animal, Fungal, Protist cells |
⚡ MCQ Tip "Pro" = before, "karyon" = nucleus → Prokaryotes came first evolutionarily. All bacteria are prokaryotes. Mitochondria and chloroplasts are found ONLY in eukaryotes.
B. Cell Organelles & Their Functions
Organelle Functions
| Organelle | Found In | Function | Nickname |
|---|---|---|---|
| Cell Membrane | All cells | Selectively permeable barrier — controls what enters and exits the cell via phospholipid bilayer | Gatekeeper |
| Cell Wall | Plants (cellulose), Fungi (chitin), Bacteria (peptidoglycan) | Provides rigid structural support and protection; prevents over-expansion | Outer armour |
| Nucleus | Eukaryotes only | Control centre — contains DNA (genetic instructions) and directs all cell activities | Brain of cell |
| Nucleolus | Inside nucleus | Produces and assembles ribosomal RNA (rRNA) — makes ribosomes | Ribosome factory |
| Mitochondria | All eukaryotes | Site of aerobic cellular respiration — produces ATP (energy) via Krebs cycle and ETC | Powerhouse ⚡ |
| Chloroplast | Plant cells only | Site of photosynthesis — traps sunlight energy to convert CO₂ + H₂O into glucose; contains chlorophyll | Solar panel 🌱 |
| Ribosome | All cells (70S prokaryotes, 80S eukaryotes) | Site of protein synthesis — reads mRNA and assembles amino acids into proteins | Protein factory |
| Rough ER | Eukaryotes | Has ribosomes on surface; synthesises and transports proteins (especially secretory proteins) | Protein highway |
| Smooth ER | Eukaryotes | No ribosomes; synthesises lipids, steroids, and phospholipids; detoxifies drugs in liver | Lipid factory |
| Golgi Apparatus | Eukaryotes | Packages, modifies, labels, and dispatches proteins and lipids to their destinations | Post office 📦 |
| Vacuole | All cells | Storage of water, nutrients, waste; large central vacuole in plant cells maintains turgor pressure | Storage tank |
| Lysosome | Animal cells mainly | Contains digestive enzymes; breaks down worn-out organelles, bacteria, and cellular debris | Suicide bags 💀 |
| Centrosome | Animal cells | Organises spindle fibres during cell division; contains two centrioles | Division organiser |
| Plastids | Plant cells | Chloroplasts (green/photosynthesis), chromoplasts (pigments/colour), leucoplasts (starch storage) | – |
Cell Membrane
Found InAll cells
FunctionSelectively permeable barrier — controls what enters and exits the cell via phospholipid bilayer
NicknameGatekeeper
Cell Wall
Found InPlants (cellulose), Fungi (chitin), Bacteria (peptidoglycan)
FunctionProvides rigid structural support and protection; prevents over-expansion
NicknameOuter armour
Nucleus
Found InEukaryotes only
FunctionControl centre — contains DNA (genetic instructions) and directs all cell activities
NicknameBrain of cell
Nucleolus
Found InInside nucleus
FunctionProduces and assembles ribosomal RNA (rRNA) — makes ribosomes
NicknameRibosome factory
Mitochondria
Found InAll eukaryotes
FunctionSite of aerobic cellular respiration — produces ATP (energy) via Krebs cycle and ETC
NicknamePowerhouse ⚡
Chloroplast
Found InPlant cells only
FunctionSite of photosynthesis — traps sunlight energy to convert CO₂ + H₂O into glucose; contains chlorophyll
NicknameSolar panel 🌱
Ribosome
Found InAll cells (70S prokaryotes, 80S eukaryotes)
FunctionSite of protein synthesis — reads mRNA and assembles amino acids into proteins
NicknameProtein factory
Rough ER
Found InEukaryotes
FunctionHas ribosomes on surface; synthesises and transports proteins (especially secretory proteins)
NicknameProtein highway
Smooth ER
Found InEukaryotes
FunctionNo ribosomes; synthesises lipids, steroids, and phospholipids; detoxifies drugs in liver
NicknameLipid factory
Golgi Apparatus
Found InEukaryotes
FunctionPackages, modifies, labels, and dispatches proteins and lipids to their destinations
NicknamePost office 📦
Vacuole
Found InAll cells
FunctionStorage of water, nutrients, waste; large central vacuole in plant cells maintains turgor pressure
NicknameStorage tank
Lysosome
Found InAnimal cells mainly
FunctionContains digestive enzymes; breaks down worn-out organelles, bacteria, and cellular debris
NicknameSuicide bags 💀
Centrosome
Found InAnimal cells
FunctionOrganises spindle fibres during cell division; contains two centrioles
NicknameDivision organiser
Plastids
Found InPlant cells
FunctionChloroplasts (green/photosynthesis), chromoplasts (pigments/colour), leucoplasts (starch storage)
Nickname–
C. Plant Cell vs Animal Cell
Key Differences
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | ✅ Present (cellulose) | ❌ Absent |
| Chloroplasts | ✅ Present (photosynthesis) | ❌ Absent |
| Vacuole | ✅ Large central vacuole | Small or absent |
| Centrioles | ❌ Absent (in most) | ✅ Present (cell division) |
| Lysosomes | Rare | ✅ Present |
| Shape | Regular, fixed (box-like) | Irregular, flexible |
| Plastids | ✅ Present | ❌ Absent |
⚡ MCQ Trick Plants have 3 extras animals don't: Cell Wall, Chloroplasts, Large Vacuole. Animals have 2 extras plants don't: Centrioles, Lysosomes.
Interactive Cell Diagram
Click an organelle
Select any part of the cell diagram or use the list below.
Quick MCQ Revision
| Organelle | Key Fact |
|---|---|
| Mitochondria | Powerhouse of cell — produces ATP by aerobic respiration |
| Ribosome | Site of protein synthesis — present in ALL cells (even prokaryotes) |
| Lysosome | "Suicide bags" — contains digestive enzymes; in animal cells mainly |
| Chloroplast | Only in plant cells — photosynthesis, contains chlorophyll |
| Cell wall: plants | Made of cellulose |
| Cell wall: fungi | Made of chitin |
| Cell wall: bacteria | Made of peptidoglycan |
| Nucleus | Contains DNA; controls all cell activities; only in eukaryotes |
| Golgi apparatus | Packages and sends proteins — "post office of the cell" |
| Prokaryote examples | Bacteria, Cyanobacteria (blue-green algae) |
| Eukaryote extras vs prokaryote | Membrane-bound nucleus, mitochondria, ER, Golgi, lysosomes |
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