CBSE Class 11 Biology Revision Notes Chapter 8 Cell The Unit of Life
Cell The Unit of Life explains the cell as the basic structural and functional unit of all living organisms. In CBSE Class 11 Biology, this chapter covers cell theory, prokaryotic cells, eukaryotic cells and major cell organelles.
Cell The Unit of Life explains why the cell is considered the basic unit of every living organism. A single cell can perform essential life processes such as nutrition, respiration, growth and reproduction in unicellular organisms. In multicellular organisms, different cells work together through tissues, organs and organ systems.
Use CBSE Class 11 Biology Revision Notes Chapter 8 for 2026–27 to revise cell theory, prokaryotic cells, eukaryotic cells, plant cell, animal cell and organelles such as mitochondria, plastids, ribosomes, nucleus and endoplasmic reticulum.
Key Takeaways
- Cell: The fundamental structural and functional unit of all living organisms.
- Cell theory: All organisms are made of cells and all cells arise from pre-existing cells.
- Prokaryotic cells: Lack a membrane-bound nucleus and membrane-bound organelles.
- Eukaryotic cells: Have an organised nucleus and membrane-bound organelles.
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Cell The Unit of Life Class 11 Biology Notes: What is a Cell?
A cell is the basic unit of life. Unicellular organisms perform all essential functions inside one cell, while multicellular organisms have many specialised cells.
| Term | Meaning |
| Cell | Basic structural and functional unit of life |
| Unicellular organism | Organism made of one cell |
| Multicellular organism | Organism made of many cells |
| Cytoplasm | Semi-fluid matrix inside the cell |
| Nucleus | Membrane-bound structure containing genetic material |
Anything smaller than a complete cell cannot carry out independent living.
Discovery of Cell
| Scientist | Contribution |
| Antonie van Leeuwenhoek | First observed and described a living cell |
| Robert Hooke | Observed cells in cork |
| Robert Brown | Discovered the nucleus |
| Schleiden | Studied plant cells |
| Schwann | Studied animal cells |
| Rudolf Virchow | Proposed that cells arise from pre-existing cells |
The invention and improvement of microscopes helped scientists study cell structure in detail.
Cell Theory in CBSE Class 11 Biology Notes Chapter 8
Cell theory explains the cellular organisation of living organisms.
| Scientist | Contribution to Cell Theory |
| Matthias Schleiden | Plants are made of cells |
| Theodore Schwann | Animals are made of cells and cell products |
| Rudolf Virchow | New cells arise from pre-existing cells |
Main Points of Cell Theory
| Point | Explanation |
| All organisms are cellular | Living organisms are made of cells and cell products |
| Cell is the basic unit | Cell is the structural and functional unit of life |
| Cells arise from cells | New cells are formed from pre-existing cells |
Virchow gave the statement Omnis cellula-e cellula, meaning every cell arises from a cell.
Overview of Cell in Class 11 Biology Chapter 8 Notes
Cells differ in size, shape and function. Their structure depends on the activity they perform.
| Cell Feature | Description |
| Cell membrane | Outer boundary in animal cells |
| Cell wall | Outer boundary in plant cells |
| Nucleus | Contains chromosomes and DNA |
| Cytoplasm | Main site of cellular activities |
| Organelles | Specialised structures inside cells |
Plant cells have a cell wall, cell membrane, cytoplasm, nucleus and organelles. Animal cells have a plasma membrane as the outer boundary.
Cell Size and Shape
| Example | Feature |
| Mycoplasma | Smallest known cells |
| Bacteria | Usually smaller than eukaryotic cells |
| Ostrich egg | Largest isolated single cell |
| Human RBC | Round and biconcave |
| Nerve cell | Long and branched |
| Columnar epithelial cell | Long and narrow |
Cells may be disc-like, polygonal, columnar, cuboid, thread-like or irregular.
Prokaryotic Cells in Cell The Unit of Life Class 11 Notes
Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO. They are generally smaller and multiply faster than eukaryotic cells.
| Feature | Prokaryotic Cell |
| Nucleus | Membrane-bound nucleus absent |
| Genetic material | Naked DNA in cytoplasm |
| Organelles | Membrane-bound organelles absent |
| Ribosomes | Present |
| Cell wall | Present in most, absent in mycoplasma |
| Examples | Bacteria, cyanobacteria, mycoplasma, PPLO |
The genetic material is usually a single circular DNA molecule. Many bacteria also have small circular DNA called plasmids.
Shapes of Bacteria
| Shape | Name |
| Rod-shaped | Bacillus |
| Spherical | Coccus |
| Comma-shaped | Vibrio |
| Spiral | Spirillum |
These shapes help identify and classify bacteria.
Cell Envelope in Prokaryotic Cells
The bacterial cell envelope has three layers: glycocalyx, cell wall and plasma membrane.
| Layer | Position | Function |
| Glycocalyx | Outermost layer | Protection and attachment |
| Cell wall | Middle layer | Shape and structural support |
| Plasma membrane | Inner layer | Selective transport |
Glycocalyx may be a loose slime layer or a thick capsule.
Gram Positive and Gram Negative Bacteria
Bacteria are grouped based on their response to Gram staining.
| Type | Staining Response |
| Gram positive bacteria | Take up Gram stain |
| Gram negative bacteria | Do not take up Gram stain in the same way |
This difference is linked with cell envelope structure.
Mesosome, Flagella, Pili and Fimbriae
Some prokaryotes have specialised structures that support movement, attachment and cell processes.
| Structure | Function |
| Mesosome | Helps in cell wall formation, DNA replication, respiration and secretion |
| Flagella | Helps in movement |
| Pili | Helps in attachment and conjugation |
| Fimbriae | Helps bacteria attach to surfaces or host tissues |
| Chromatophores | Pigment-containing structures in some cyanobacteria |
A bacterial flagellum has three parts: filament, hook and basal body.
Ribosomes and Inclusion Bodies in Prokaryotic Cells
Ribosomes are the sites of protein synthesis.
| Structure | Description |
| Prokaryotic ribosome | 70S ribosome |
| Subunits | 50S and 30S |
| Polyribosome | Several ribosomes attached to one mRNA |
| Inclusion bodies | Reserve materials in cytoplasm |
Inclusion bodies store materials such as phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles occur in blue-green, purple and green photosynthetic bacteria.
Eukaryotic Cells in CBSE Class 11 Biology Notes Chapter 8
Eukaryotic cells have an organised nucleus and membrane-bound organelles. Protists, plants, animals and fungi are eukaryotes.
| Feature | Eukaryotic Cell |
| Nucleus | Membrane-bound nucleus present |
| Organelles | Membrane-bound organelles present |
| Genetic material | Organised into chromosomes |
| Cytoplasm | Compartmentalised |
| Ribosomes | 80S in cytoplasm |
| Examples | Protists, plants, animals, fungi |
Plant Cell and Animal Cell Difference
| Feature | Plant Cell | Animal Cell |
| Cell wall | Present | Absent |
| Plastids | Present | Absent |
| Central vacuole | Large | Small or temporary |
| Centrioles | Absent in most higher plant cells | Present |
| Shape | Usually fixed | Usually flexible |
Plant cells have cell walls, plastids and a large central vacuole. Animal cells usually have centrioles.
Plasma Membrane in Cell The Unit of Life Class 11 Biology Notes
The plasma membrane is the outer boundary of the cell. It controls movement of substances into and out of the cell.
| Component | Description |
| Phospholipids | Arranged in a bilayer |
| Proteins | Integral and peripheral proteins |
| Cholesterol | Present in many membranes |
| Carbohydrates | Present on membrane surface |
Phospholipid heads face the outer aqueous side, while hydrophobic tails face inward.
Fluid Mosaic Model
Singer and Nicolson proposed the fluid mosaic model in 1972.
| Term | Meaning |
| Fluid | Lipids allow lateral movement within the bilayer |
| Mosaic | Proteins are embedded in the lipid bilayer |
| Fluidity | Ability of membrane components to move |
Fluidity helps in cell growth, secretion, endocytosis, cell division and formation of cell junctions.
Transport Across Plasma Membrane
| Type of Transport | Energy Requirement | Direction |
| Simple diffusion | No ATP | Higher to lower concentration |
| Osmosis | No ATP | Water moves by diffusion |
| Facilitated transport | Carrier proteins involved | Along concentration gradient |
| Active transport | ATP required | Lower to higher concentration |
Na+/K+ pump is an example of active transport.
Cell Wall in Class 11 Biology Chapter 8 Notes
The cell wall is a rigid, non-living outer covering present in plants and fungi.
| Feature | Description |
| Nature | Non-living and rigid |
| Position | Outside plasma membrane |
| Function | Gives shape and protection |
| Plant wall material | Cellulose, hemicellulose, pectin and proteins |
| Algal wall material | Cellulose, galactans, mannans and minerals |
The cell wall protects the cell from mechanical damage and infection. It also helps in cell-to-cell interaction.
Parts of Plant Cell Wall
| Part | Description |
| Primary wall | Formed in young plant cells and allows growth |
| Secondary wall | Formed inside primary wall after maturity |
| Middle lamella | Calcium pectate layer joining neighbouring cells |
| Plasmodesmata | Cytoplasmic connections between neighbouring cells |
Plasmodesmata help connect the cytoplasm of adjacent plant cells.
Endomembrane System in Cell The Unit of Life
The endomembrane system includes organelles whose functions are coordinated.
| Included in Endomembrane System | Excluded from Endomembrane System |
| Endoplasmic reticulum | Mitochondria |
| Golgi apparatus | Chloroplasts |
| Lysosomes | Peroxisomes |
| Vacuoles |
These organelles help in synthesis, modification, packaging and transport of materials.
Endoplasmic Reticulum in CBSE Class 11 Biology Notes Chapter 8
Endoplasmic reticulum is a network of tiny tubular structures present in the cytoplasm.
| Type | Structure | Function |
| Rough ER | Ribosomes present on surface | Protein synthesis and secretion |
| Smooth ER | Ribosomes absent | Lipid synthesis and steroid hormone synthesis |
ER divides the intracellular space into luminal and extra-luminal compartments.
RER is common in cells involved in protein synthesis. SER is the major site of lipid synthesis.
Golgi Apparatus in Class 11 Biology Chapter 8 Notes
Camillo Golgi first observed the Golgi apparatus in 1898. It consists of flat, disc-shaped sacs called cisternae.
| Feature | Description |
| Structure | Stacked cisternae |
| Location | Near nucleus |
| Cis face | Forming face |
| Trans face | Maturing face |
| Main function | Packaging and secretion |
Golgi apparatus modifies proteins from ER and sends them to their targets. It also helps form glycoproteins and glycolipids.
Lysosomes and Vacuoles
Lysosomes
Lysosomes are membrane-bound vesicles formed by packaging in the Golgi apparatus.
| Feature | Description |
| Enzymes | Hydrolytic enzymes |
| Active pH | Acidic |
| Function | Digestion of carbohydrates, proteins, lipids and nucleic acids |
| Formation | From Golgi apparatus |
Lysosomes contain enzymes such as lipases, proteases and carbohydrases.
Vacuoles
Vacuoles are membrane-bound spaces in the cytoplasm. They contain water, sap, excretory products and other materials.
| Feature | Description |
| Membrane | Tonoplast |
| Plant cell vacuole | May occupy up to 90% cell volume |
| Amoeba vacuole | Contractile vacuole helps osmoregulation and excretion |
| Food vacuole | Formed by engulfing food particles |
In plant cells, vacuoles store substances and help maintain cell pressure.
Mitochondria in Cell The Unit of Life Class 11 Notes
Mitochondria are double membrane-bound organelles. They are sites of aerobic respiration.
| Feature | Description |
| Shape | Usually sausage-shaped or cylindrical |
| Membranes | Outer membrane and inner membrane |
| Inner folds | Cristae |
| Inner compartment | Matrix |
| Genetic material | Circular DNA |
| Ribosomes | 70S ribosomes |
| Function | ATP production |
Mitochondria are called powerhouses of the cell because they produce ATP.
Structure of Mitochondria
| Part | Function |
| Outer membrane | Forms outer boundary |
| Inner membrane | Forms cristae |
| Cristae | Increase surface area |
| Matrix | Contains DNA, RNA, ribosomes and enzymes |
Mitochondria divide by fission.
Plastids in CBSE Class 11 Biology Revision Notes Chapter 8
Plastids are found in plant cells and euglenoids. They may contain pigments and help in photosynthesis or storage.
| Plastid | Pigment/Colour | Function |
| Chloroplast | Chlorophyll and carotenoids | Photosynthesis |
| Chromoplast | Carotenoid pigments | Gives yellow, orange or red colour |
| Leucoplast | Colourless | Stores food |
Types of Leucoplasts
| Leucoplast | Stored Material |
| Amyloplast | Starch |
| Elaioplast | Oils and fats |
| Aleuroplast or Proteinoplast | Proteins |
Chloroplast Structure
| Part | Description |
| Outer membrane | Outer covering |
| Inner membrane | Inner covering |
| Stroma | Fluid matrix |
| Thylakoids | Flattened membrane sacs |
| Grana | Stacks of thylakoids |
| Stroma lamellae | Connect grana |
Chloroplasts also contain circular DNA and ribosomes.
Ribosomes in Cell The Unit of Life Class 11 Biology Notes
Ribosomes are non-membrane-bound organelles made of RNA and proteins.
| Type | Location | Size |
| Prokaryotic ribosome | Cytoplasm or plasma membrane association | 70S |
| Eukaryotic cytoplasmic ribosome | Cytoplasm | 80S |
| Organelle ribosome | Mitochondria and chloroplasts | 70S |
Ribosomes help in protein synthesis. Several ribosomes attached to one mRNA form a polysome.
Nucleus in Class 11 Biology Chapter 8 Notes
The nucleus is a membrane-bound structure that contains genetic material.
| Feature | Description |
| Nuclear envelope | Double membrane covering |
| Nuclear pores | Allow exchange of materials |
| Nucleoplasm | Fluid inside nucleus |
| Chromatin | DNA and protein material |
| Nucleolus | Site of ribosome assembly |
The nucleus controls cell activities and stores hereditary information.
Chromatin and Chromosomes
| Term | Meaning |
| Chromatin | Thread-like genetic material in non-dividing nucleus |
| Chromosome | Condensed chromatin during cell division |
| Euchromatin | Less condensed and transcriptionally active region |
| Heterochromatin | Densely packed region |
DNA carries instructions for proteins and hereditary traits.
Cytoskeleton in Cell The Unit of Life Class 11 Notes
The cytoskeleton is a network of protein filaments in the cytoplasm.
| Function | Explanation |
| Mechanical support | Helps maintain cell shape |
| Cell movement | Supports movement of cell parts |
| Organelle organisation | Holds organelles in position |
| Intracellular transport | Helps movement of materials inside cell |
The cytoskeleton is important in eukaryotic cells.
Cilia and Flagella
Cilia and flagella are hair-like or whip-like structures that help in movement.
| Feature | Cilia | Flagella |
| Size | Shorter | Longer |
| Number | Usually many | Usually fewer |
| Function | Movement of substances or cell | Cell movement |
| Core structure | Axoneme | Axoneme |
The axoneme has a 9+2 arrangement of microtubules: nine peripheral doublets and two central microtubules.
Centrosome and Centrioles
Centrosome is a non-membrane-bound organelle present in animal cells. It contains two centrioles.
| Feature | Description |
| Centriole shape | Cylindrical |
| Arrangement | Nine peripheral triplet fibrils |
| Function | Helps form spindle fibres during cell division |
| Role in flagella and cilia | Forms basal bodies |
Centrioles help organise microtubules.
Prokaryotic and Eukaryotic Cells Difference
| Feature | Prokaryotic Cell | Eukaryotic Cell |
| Nucleus | Membrane-bound nucleus absent | Membrane-bound nucleus present |
| Genetic material | Naked circular DNA | DNA organised into chromosomes |
| Organelles | Membrane-bound organelles absent | Membrane-bound organelles present |
| Ribosomes | 70S | 80S in cytoplasm |
| Cell size | Usually smaller | Usually larger |
| Cell division | Usually binary fission | Mitosis or meiosis |
| Examples | Bacteria, cyanobacteria | Plants, animals, fungi, protists |
Plant Cell and Animal Cell Quick Comparison
| Feature | Plant Cell | Animal Cell |
| Cell wall | Present | Absent |
| Plastids | Present | Absent |
| Vacuole | Large central vacuole | Small or temporary vacuoles |
| Centrioles | Absent in most higher plants | Present |
| Shape | More fixed | More flexible |
| Reserve food | Starch | Glycogen |
Important Cell Organelles and Their Functions
| Organelle | Function |
| Plasma membrane | Selective transport |
| Cell wall | Shape and protection |
| Endoplasmic reticulum | Protein and lipid synthesis |
| Golgi apparatus | Packaging and secretion |
| Lysosomes | Intracellular digestion |
| Vacuoles | Storage and osmoregulation |
| Mitochondria | ATP production |
| Plastids | Photosynthesis and storage |
| Ribosomes | Protein synthesis |
| Nucleus | Control and heredity |
| Cytoskeleton | Shape, support and movement |
| Centrioles | Spindle formation |
NCERT-Based Exam Points
- Cell is the fundamental structural and functional unit of life.
- Unicellular organisms can exist independently.
- Antonie van Leeuwenhoek first observed and described a living cell.
- Robert Brown discovered the nucleus.
- Schleiden and Schwann formulated cell theory.
- Rudolf Virchow explained that cells arise from pre-existing cells.
- Cytoplasm is the main arena of cellular activities.
- Ribosomes occur in both prokaryotic and eukaryotic cells.
- Prokaryotic cells lack membrane-bound organelles.
- Plasmids are small circular DNA molecules in bacteria.
- Mesosome is an infolding of the plasma membrane.
- Prokaryotic ribosomes are 70S.
- Eukaryotic cytoplasmic ribosomes are 80S.
- Plant cells have cell walls, plastids and a large central vacuole.
- Animal cells have centrioles.
- Singer and Nicolson proposed the fluid mosaic model.
- Osmosis is movement of water by diffusion.
- Na+/K+ pump is an example of active transport.
- Cell wall gives shape and protection.
- Middle lamella contains calcium pectate.
- ER, Golgi apparatus, lysosomes and vacuoles form the endomembrane system.
- RER helps in protein synthesis.
- SER helps in lipid synthesis.
- Golgi apparatus helps in packaging and secretion.
- Lysosomes contain hydrolytic enzymes.
- Mitochondria produce ATP.
- Chloroplasts contain chlorophyll.
- Nucleus contains genetic material.
- Cilia and flagella have 9+2 microtubule arrangement.
- Centrosome contains centrioles.
Useful Links for Class 11 Biology Revision Notes
| Section | Useful Links |
| Revision Notes | CBSE Class 11 Biology Revision Notes |
| Biology Notes | CBSE Class 11 Biology Revision Notes Chapter 1 |
| Biology Notes | CBSE Class 11 Biology Revision Notes Chapter 2 |
| Syllabus | CBSE Class 11 Biology Syllabus |
| NCERT Solutions | NCERT Solutions Class 11 Biology |
| Sample Papers | CBSE Sample Papers for Class 11 Biology |
| Important Questions | Important Questions Class 11 Biology |
| NCERT Books | NCERT Books for Class 11 Biology |
FAQs (Frequently Asked Questions)
Revise the chapter in groups: cell theory, prokaryotic cells, eukaryotic cells and organelles. Use tables for prokaryotic vs eukaryotic cells, plant vs animal cells and organelle functions.
Focus on four points: plant cells have cell wall, plastids and a large central vacuole. Animal cells have centrioles and usually lack cell wall and plastids. This comparison is useful for short-answer questions.
The nucleus stores genetic material, controls cell activities and helps in transmission of hereditary information. The nucleolus helps in ribosome assembly, while chromosomes carry DNA.
Mitochondria are called the powerhouse of the cell because they produce ATP during aerobic respiration. ATP acts as the energy currency used for cellular activities.
The fluid mosaic model explains that lipids form a fluid bilayer and proteins are embedded in it. This structure allows selective transport, secretion, endocytosis, cell growth and membrane flexibility.
For some students, Class 11 Biology Chapter 8 may be difficult. However, with a proper strategy in place and the revision notes provided by Extramarks, students will be able to tackle this chapter with ease. Here are some ideas to make the process of comprehension go more smoothly:
- Make sure you understand the material being taught at school.
- Talk about the chapter with your friends and teachers.
- Note relevant points and diagrams in a separate notebook. That would help with memorising all the important names and terms covered in the chapter.
- Use the revision notes provided by Extramarks to your benefit. Revise the chapter at least twice before the exam and you will be good to go.