Cell Structure & Function – Class 11 (CBSE)
Aim of the chapter
Understand the structure of living cells, the organelles that give them their functions, and how these structures relate to the cell theory and the nature of life.
Key topics
- Cell theory & its implications
- Microscopy – types & use in biology
- Plant vs. animal cells – similarities & differences
- Cell components – nucleus, cytoplasm, membrane, wall, organelles
- Functions of organelles & transport across membranes
1. Simple Explanation
| Concept | What it means |
|---|---|
| Cell | The smallest structural and functional unit of life. |
| Cell theory | 1) All living organisms are made of cells. 2) All cells arise from pre‑existing cells. 3) All cells contain genetic material (DNA). |
| Cell membrane | Semi‑permeable lipid bilayer that controls entry/exit of substances. |
| Cell wall | Rigid layer (cellulose in plants) that gives shape and protects against osmotic lysis. |
| Cytoplasm | Gel‑like fluid where organelles reside. |
| Nucleus | Control centre containing DNA. |
| Organelles | Specialized structures that perform specific tasks (e.g., mitochondria = ATP factories). |
2. Key Points
2.1 Microscopy
| Type | Principle | Typical Use |
|---|---|---|
| Compound Light Microscope (CLM) | Uses visible light & lenses | General cell observation (size, shape, organelles) |
| Phase Contrast Microscope | Enhances contrast of transparent specimens | Live cell imaging |
| Fluorescence Microscope | Uses fluorophores & UV light | Detects specific proteins or structures |
| Electron Microscope (TEM, SEM) | Uses electron beam | Ultra‑fine details of organelles, membrane structure |
2.2 Cell Theory Implications
| Principle | Biological significance |
|---|---|
| All living organisms are made of cells | Basis for studying life at the microscopic level |
| All cells arise from pre‑existing cells | Explains growth, reproduction, and the continuity of life |
| All cells contain genetic material | DNA as the hereditary material, basis for heredity & evolution |
2.3 Plant vs. Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Shape | Usually rectangular | Variable (often round) |
| Chloroplasts | Present | Absent |
| Large central vacuole | Present | Rare (small vacuoles) |
| Centrioles | Absent | Present |
| Plasmodesmata | Present (intercellular channels) | Absent |
2.4 Organelles & Functions
| Organelle | Main Function | Key Features |
|---|---|---|
| Nucleus | Stores DNA, regulates gene expression | Nuclear envelope, nucleolus |
| Nucleolus | Ribosomal RNA synthesis | Dense, no membrane |
| Ribosomes | Protein synthesis | Free in cytoplasm or bound to ER |
| Mitochondria | ATP production (oxidative phosphorylation) | Double‑membrane, cristae |
| Chloroplast | Photosynthesis | Thylakoid membranes, grana |
| Endoplasmic Reticulum | Protein (RER) & lipid (SER) synthesis | Rough vs smooth |
| Golgi Apparatus | Modifies, sorts, packages proteins | Membrane‑bound cisternae |
| Lysosomes | Intracellular digestion | Enzyme‑rich vesicles |
| Peroxisomes | Fatty‑acid β‑oxidation & detoxification | Enzyme‑rich vesicles |
| Vacuole | Storage, turgor pressure | Large in plant cells |
| Cytoskeleton | Shape, movement, transport | Microfilaments, microtubules, intermediate filaments |
| Cell membrane | Selective permeability, signaling | Phospholipid bilayer + proteins |
| Cell wall | Structural support, protection | Cellulose (plants) / peptidoglycan (bacteria) |
2.5 Transport Across the Cell Membrane
| Mechanism | Energy requirement | Example |
|---|---|---|
| Passive diffusion | No energy | Oxygen, CO₂ |
| Facilitated diffusion | No energy | Glucose via GLUT, ions via channels |
| Osmosis | No energy (water) | Water movement in plant cells |
| Active transport | Requires ATP | Na⁺/K⁺‑ATPase, proton pump in chloroplasts |
| Endocytosis / Exocytosis | Requires ATP & vesicle machinery | Receptor‑mediated endocytosis |
3. Worked Example
Problem
A plant cell is placed in a 0.5 M sucrose solution (outside). The cell contains 0.1 M sucrose internally.
Question:
- Will water move into or out of the cell?
- What will happen to the cell’s turgor pressure?
Solution
| Step | Explanation |
|---|---|
| 1. Calculate osmotic potential (π) | π = –CRT ( |