Cell – The Unit of Life is Chapter 8 of CBSE Class 11 Biology, and it is the chapter that everything else in biology quietly depends on. Every leaf, every muscle, every thought you have is the work of cells — the smallest units that can be called alive. Master this chapter and the rest of cell biology, plant physiology, and human physiology suddenly start to make sense.
By the end of these notes you will be able to state the cell theory, tell a prokaryotic cell from a eukaryotic one in a single glance, describe every major organelle and its job, and answer any board or NEET question on cell structure with confidence. This is a high-weightage chapter carrying roughly 6–8 marks in boards and forming the foundation for Biomolecules, Cell Cycle, and all of physiology.
Table of Contents
- Key Concepts — Cell theory, prokaryotes vs eukaryotes, membrane, cell wall, organelles, nucleus
- Weightage in Board & Entrance Exams
- Important Definitions
- Solved/Illustrative Explanations
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. The Cell and Cell Theory
A cell is the basic structural and functional unit of all living organisms — the smallest unit capable of independent existence and of carrying out all life functions. Robert Hooke first observed dead cork cells in 1665, and Anton von Leeuwenhoek first saw living cells.
The cell theory was proposed by Matthias Schleiden (botanist, 1838) and Theodor Schwann (zoologist, 1839). It was later refined by Rudolf Virchow (1855), who gave the famous line Omnis cellula-e cellula — every cell arises from a pre-existing cell.
The Cell Theory states:
- All living organisms are composed of cells and products of cells.
- All cells arise from pre-existing cells (Virchow’s contribution).
Exception: Viruses are not made of cells, so the cell theory does not strictly apply to them.
2. Overview: What All Cells Share
Despite enormous variety in shape and size, every cell shares three basics: a covering plasma membrane, a jelly-like cytoplasm where reactions happen, and genetic material (DNA) that controls the cell.
- The smallest cells are Mycoplasma (about 0.3 µm), the smallest living cells known.
- Bacteria are 3–5 µm; a typical eukaryotic cell is much larger.
- The largest isolated single cell is the egg of an ostrich.
- The longest cells are nerve cells.
On this basis cells are grouped into two great types: prokaryotic (no true nucleus) and eukaryotic (true, membrane-bound nucleus).
3. Prokaryotic Cells
Prokaryotic cells are found in bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO. They are generally smaller and multiply more rapidly than eukaryotic cells.
Their defining feature is the absence of a true, membrane-bound nucleus — the genetic material lies free in the cytoplasm in a region called the nucleoid. They also lack membrane-bound organelles like mitochondria, plastids, ER and Golgi.
Key features of a prokaryotic cell
- Genetic material: single, circular, naked DNA in the nucleoid; no nuclear membrane.
- Plasmids: small extra-chromosomal circular DNA that gives traits like antibiotic resistance.
- Ribosomes: 70S type (smaller than eukaryotic 80S).
- Mesosome: infolding of the plasma membrane that helps in respiration, secretion and DNA replication.
- Cell envelope: a three-layered covering — glycocalyx, cell wall, and plasma membrane.
Cell Envelope and Bacterial Types
The bacterial cell wall is made of peptidoglycan. Based on the Gram-staining response (developed by Gram), bacteria are Gram-positive (retain the stain) or Gram-negative (do not). Some bacteria also have flagella for motility and pili/fimbriae for attachment.
[DIAGRAM: A bacterial cell showing glycocalyx, cell wall, plasma membrane, nucleoid, ribosomes, mesosome, flagellum and pili.]
4. Eukaryotic Cells
Eukaryotic cells are found in protists, fungi, plants and animals. They have a true membrane-bound nucleus and a variety of membrane-bound organelles, allowing a clear division of labour inside the cell.
Their genetic material is organised into chromosomes, and the ribosomes are of the larger 80S type.
Plant cell vs Animal cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Plastids | Present (e.g. chloroplast) | Absent |
| Vacuole | Large, central | Small or absent |
| Centriole / Centrosome | Usually absent | Present |
| Lysosomes | Rare | Common |
5. Cell Membrane (Plasma Membrane)
The plasma membrane is the living, selectively permeable boundary of the cell. The most accepted model is the fluid mosaic model proposed by Singer and Nicolson (1972).
It is mainly a lipid bilayer (phospholipids arranged with polar heads outside and hydrophobic tails inside) with proteins floating in it. Because the lipids and proteins can move laterally, the membrane is described as fluid — this fluidity is vital for growth, division and secretion.
Transport across the membrane
- Passive transport: movement down a concentration gradient without energy — by simple diffusion or osmosis.
- Facilitated diffusion: down the gradient but with the help of carrier/channel proteins.
- Active transport: movement against the gradient using ATP (e.g. the Na⁺–K⁺ pump).
6. Cell Wall
The cell wall is a non-living, rigid outer covering found in plant cells, fungi, algae and prokaryotes (but not in animal cells). It gives shape, mechanical strength, and protection against mechanical and osmotic stress.
- In plants it is made of cellulose, hemicellulose, pectin and proteins; in fungi of chitin.
- The middle lamella (made of calcium pectate) cements adjacent cells together.
- The young cell has a thin, elastic primary wall; on maturity a thick secondary wall forms.
- Plasmodesmata are cytoplasmic bridges connecting neighbouring cells.
7. Endomembrane System
Several organelles work together as a coordinated unit called the endomembrane system: the endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles. (Mitochondria, chloroplasts and peroxisomes are not included, as they are not functionally coordinated with these.)
(a) Endoplasmic Reticulum (ER)
The ER is a network of membrane-bound tubules and sheets continuous with the nuclear membrane.
- Rough ER (RER): studded with ribosomes; active in protein synthesis and secretion.
- Smooth ER (SER): no ribosomes; site of lipid and steroid synthesis and detoxification.
(b) Golgi Apparatus
Discovered by Camillo Golgi, it is a stack of flattened, disc-shaped cisternae. It packages, modifies and dispatches materials — the cell’s “post office.” It has a cis (forming) face near the ER and a trans (maturing) face. It is the main site of glycoprotein and glycolipid formation.
(c) Lysosomes
Lysosomes are membrane-bound vesicles rich in hydrolytic (digestive) enzymes that work best in an acidic medium. They digest worn-out organelles, food and foreign material — earning the nickname “suicidal bags” of the cell.
(d) Vacuoles
A vacuole is a membrane-bound space (its membrane is the tonoplast) filled with cell sap. In plants it can occupy up to 90% of the cell volume and maintains turgidity and osmotic pressure.
8. Mitochondria
Mitochondria are double-membrane organelles known as the “powerhouse of the cell” because they are the site of aerobic respiration and produce ATP.
- The outer membrane is smooth; the inner membrane folds inward to form cristae, increasing surface area for ATP synthesis.
- The inner space is the matrix, which contains enzymes, ribosomes (70S) and its own circular DNA.
- Because they have their own DNA and ribosomes and divide on their own, mitochondria are called semi-autonomous organelles.
9. Plastids
Plastids are double-membrane organelles found only in plant cells and some protists. They are classified by the pigment they carry.
- Chloroplasts: contain green chlorophyll; site of photosynthesis. Inside are stacks of thylakoids (grana) embedded in a fluid stroma with 70S ribosomes and DNA.
- Chromoplasts: contain fat-soluble carotenoid pigments (yellow, orange, red) — give colour to flowers and fruits.
- Leucoplasts: colourless plastids that store food — amyloplasts (starch), elaioplasts (oils/fats), aleuroplasts (proteins).
Like mitochondria, chloroplasts are semi-autonomous (own DNA + 70S ribosomes).
10. Ribosomes
Ribosomes are tiny, non-membranous granules made of rRNA and proteins; they are the site of protein synthesis. They were discovered by George Palade.
- 80S ribosomes (60S + 40S subunits) in the cytoplasm of eukaryotes.
- 70S ribosomes (50S + 30S subunits) in prokaryotes and in mitochondria/chloroplasts.
- Several ribosomes on one mRNA form a polysome (polyribosome).
11. Microbodies, Cytoskeleton, Cilia, Flagella & Centrosome
(a) Microbodies
Membrane-bound minute vesicles containing enzymes — e.g. peroxisomes and glyoxysomes, found in both plant and animal cells.
(b) Cytoskeleton
A network of proteinaceous filaments — microtubules, microfilaments and intermediate filaments — that gives the cell mechanical support, shape, and helps in movement.
(c) Cilia and Flagella
Hair-like projections of the plasma membrane that help in movement. Both have a core called the axoneme with a 9 + 2 arrangement of microtubules (9 peripheral pairs + 2 central). Cilia are small and numerous; flagella are longer and fewer.
(d) Centrosome and Centrioles
The centrosome contains two cylindrical centrioles arranged at right angles, each with a 9 + 0 (cartwheel) arrangement of microtubule triplets. Centrioles form the spindle fibres during cell division and the basal body of cilia and flagella.
12. Nucleus and Chromosomes
The nucleus, described by Robert Brown (1831), is the control centre of the cell. It is bound by a double nuclear envelope with nuclear pores that allow exchange of material with the cytoplasm.
- The fluid inside is the nucleoplasm, containing the nucleolus and chromatin.
- The nucleolus is a non-membranous structure that is the site of ribosome (rRNA) synthesis.
- Chromatin is a network of DNA + histone proteins; it condenses into chromosomes during cell division.
Chromosome Structure
Each chromosome has a primary constriction called the centromere, bearing disc-shaped kinetochores. Based on the centromere’s position, chromosomes are classified as:
| Type | Position of Centromere | Shape |
|---|---|---|
| Metacentric | Middle | Forms two equal arms (V-shaped) |
| Sub-metacentric | Slightly away from middle | Unequal arms (L-shaped) |
| Acrocentric | Near the end | One very long, one very short arm (J-shaped) |
| Telocentric | At the tip | Rod-shaped |
A chromosome with a secondary constriction bearing a satellite is called a satellite chromosome (SAT chromosome).
Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 11) | 6–8 marks | Cell theory, prokaryote vs eukaryote, organelle functions, plant vs animal cell |
| NEET | 3–4 questions | Ribosomes (70S/80S), mitochondria, plastids, fluid mosaic model, cell wall |
| State CETs | 2–3 questions | Cell organelles, nucleus, semi-autonomous organelles |
[TABLE: Question-type split — VSA (1 mark): definitions, scientists, organelle functions; SA (2–3 marks): prokaryote vs eukaryote, plant vs animal cell, endomembrane system; LA (5 marks): structure of mitochondria/chloroplast/nucleus with labelled diagram.]
Important Definitions
| Term | Definition |
|---|---|
| Cell | Basic structural and functional unit of life; smallest unit capable of independent existence |
| Cell theory | All organisms are made of cells, and all cells arise from pre-existing cells |
| Nucleoid | Region of a prokaryotic cell where the naked DNA lies, with no nuclear membrane |
| Mesosome | Infolding of the plasma membrane in bacteria, aiding respiration and DNA replication |
| Fluid mosaic model | Membrane model (Singer & Nicolson) of proteins floating in a fluid lipid bilayer |
| Cristae | Infoldings of the inner mitochondrial membrane that increase surface area for ATP synthesis |
| Grana | Stacks of thylakoids in a chloroplast where the light reactions of photosynthesis occur |
| Semi-autonomous organelle | Organelle with its own DNA and 70S ribosomes (mitochondria, chloroplast) |
| Tonoplast | Single membrane surrounding a plant vacuole |
| Centromere | Primary constriction of a chromosome bearing the kinetochore, holding sister chromatids |
Solved/Illustrative Explanations
Example 1
Why are mitochondria and chloroplasts called semi-autonomous organelles?
Answer: Because each has its own circular DNA and 70S ribosomes and can synthesise some of its own proteins and divide by fission — yet they still depend on the nucleus for most proteins, so they are only semi-autonomous.
Example 2
A scientist observes a cell under the microscope and finds no nuclear membrane and 70S ribosomes. What type of cell is it?
Answer: The absence of a nuclear membrane (DNA lies in a nucleoid) and the presence of 70S ribosomes are signatures of a prokaryotic cell, e.g. a bacterium.
Example 3
Why are lysosomes called the “suicidal bags” of the cell?
Answer: Lysosomes contain powerful hydrolytic enzymes. If their membrane ruptures, these enzymes are released into the cytoplasm and digest (autolyse) the cell’s own contents — hence the name.
Example 4
Distinguish the 9 + 2 and 9 + 0 microtubule arrangements and where each is found.
Answer: The 9 + 2 arrangement (9 peripheral doublets + 2 central microtubules) is found in the axoneme of cilia and flagella. The 9 + 0 arrangement (9 peripheral triplets, no central tubule) is found in centrioles and basal bodies.
Example 5
State one function each of the rough ER and smooth ER.
Answer: Rough ER (with ribosomes) is the site of protein synthesis and secretion; smooth ER is the site of lipid and steroid synthesis and detoxification.
Example 6
Name the organelle responsible for packaging and dispatching cell products, and describe its two faces.
Answer: The Golgi apparatus. Its cis (forming) face lies near the ER and receives material; its trans (maturing) face packages and releases the modified products as vesicles.
Important Questions for Board Exams
1-Mark Questions (VSA)
- Who proposed the cell theory and who modified it later?
- Name the smallest living cell known.
- What is the chemical nature of the bacterial cell wall?
- Which organelle is called the “powerhouse of the cell”?
- What is the function of the nucleolus?
2–3-Mark Questions (SA)
- Differentiate between prokaryotic and eukaryotic cells (any three points).
- Explain the fluid mosaic model of the plasma membrane.
- Distinguish between rough and smooth endoplasmic reticulum with their functions.
- Why are mitochondria and plastids called semi-autonomous organelles?
5-Mark Questions (LA)
- Describe the structure of a mitochondrion with a labelled diagram and explain its role in respiration.
- Describe the structure of a chloroplast and explain the function of grana and stroma.
- Draw a well-labelled diagram of an animal cell and describe the structure and function of the nucleus.
Quick Revision Points
- Cell theory: Schleiden & Schwann; modified by Virchow (Omnis cellula-e cellula)
- Mycoplasma = smallest living cell; ostrich egg = largest single cell; nerve cell = longest
- Prokaryotes: no nuclear membrane, nucleoid, 70S ribosomes, mesosome, peptidoglycan wall
- Eukaryotes: true nucleus, membrane-bound organelles, 80S ribosomes, chromosomes
- Plasma membrane: fluid mosaic model (Singer & Nicolson); selectively permeable lipid bilayer
- Cell wall: cellulose in plants, chitin in fungi; middle lamella of calcium pectate
- Endomembrane system: ER, Golgi, lysosomes, vacuoles
- Mitochondria (cristae, matrix) and chloroplast (grana, stroma) = semi-autonomous, 70S
- Ribosomes: 80S (eukaryote cytoplasm), 70S (prokaryote/organelle); site of protein synthesis
- Cilia/flagella axoneme = 9 + 2; centriole = 9 + 0
- Nucleus: Robert Brown; nucleolus makes rRNA; chromosome types by centromere position
Next Chapter: Chapter 9 — Biomolecules
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