Cell: The Unit of Life NEET Notes 2027 PDF and Key Points

Cell: The Unit of Life is Chapter 8 of NCERT Class 11 Biology and part of Unit 3, Cell Structure and Function, of the NEET UG syllabus. These notes cover cell theory, prokaryotic and eukaryotic cells, the cell membrane and wall, the endomembrane system, mitochondria, plastids, ribosomes, cytoskeleton, centrioles and nucleus, with a free notes PDF.

Cell: the Unit of Life NEET Notes

Section-wise key points from NCERT Chapter 8, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Cell: the Unit of Life at a Glance

Item	Details
NEET UG syllabus unit	Unit 3: Cell Structure and Function
NCERT textbook	Biology, Class 11, Chapter 8 (rationalised book)
Other chapters in the unit	Biomolecules (Chapter 9), Cell Cycle and Cell Division (Chapter 10)
Syllabus topics	Cell theory; prokaryotic and eukaryotic cells; plant and animal cells; cell envelope, membrane and wall; endomembrane system; mitochondria, ribosomes, plastids, microbodies; cytoskeleton, cilia, flagella, centrioles; nucleus
Official chapter	NCERT Chapter 8
Notes PDF	View PDF

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

What is a cell and what does cell theory say?

The cell is the fundamental structural and functional unit of all living organisms, because anything less than a complete cell cannot live independently.

Antonie von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus.
Matthias Schleiden (1838), a German botanist, found that all plants are made of different kinds of cells that form tissues.
Theodore Schwann (1839), a German zoologist, reported the thin outer layer of animal cells (today’s plasma membrane) and concluded that the cell wall is unique to plant cells.
Schleiden and Schwann together formulated the cell theory, but it did not explain how new cells form.
Rudolf Virchow (1855) explained that new cells arise from pre-existing cells (Omnis cellula-e cellula) and gave the theory its final shape.
Cell theory today: (i) all living organisms are composed of cells and products of cells; (ii) all cells arise from pre-existing cells.

How do cells differ in size and shape?

Cells vary greatly in size, shape and activity, and the shape of a cell often depends on the function it performs.

Cell or particle	Size given in NCERT
Mycoplasma (smallest cells)	0.3 µm in length
Bacteria	3 to 5 µm
Viruses (for comparison)	0.02 to 0.2 µm
Human red blood cell	About 7.0 µm in diameter
Typical eukaryotic cell	10 to 20 µm
Largest isolated single cell	Egg of an ostrich
Longest cells	Nerve cells are some of the longest

Shapes: disc-like, polygonal, columnar, cuboid, thread-like or irregular.
Examples from NCERT: RBCs are round and biconcave, WBCs amoeboid, nerve cells branched and long, columnar epithelial cells long and narrow, tracheids elongated, mesophyll cells round and oval.

What is the difference between prokaryotic and eukaryotic cells?

The key difference is the nucleus: eukaryotic cells have a membrane-bound nucleus and membrane-bound organelles, while prokaryotic cells have neither.

Feature	Prokaryotic cell	Eukaryotic cell
Examples	Bacteria, blue-green algae, mycoplasma, PPLO	Protists, plants, animals, fungi
Nucleus	No well-defined nucleus; genetic material is naked, not enclosed by a nuclear membrane	Organised nucleus with a nuclear envelope
Genetic material	Single circular genomic DNA; many bacteria also have plasmids	Organised into chromosomes
Membrane-bound organelles	Absent	Present (ER, Golgi, lysosomes, mitochondria, microbodies, vacuoles)
Ribosomes	70S (50S + 30S), associated with the plasma membrane	80S (60S + 40S) in cytoplasm and on rough ER; 70S inside mitochondria and chloroplasts
Mesosome	Present (characteristic of prokaryotes)	Absent
Cell wall	Present in all except mycoplasma	Present in plants and fungi, absent in animals
Size and growth	Generally smaller; multiply more rapidly	Generally larger

Prokaryotic Cells: Key Points

Prokaryotic cells share a similar basic organisation even though they vary widely in shape and function.

Four basic bacterial shapes: bacillus (rod), coccus (spherical), vibrio (comma) and spirillum (spiral).
Plasmids are small circular DNA outside the genomic DNA. They give unique phenotypic characters such as antibiotic resistance and are used to monitor bacterial transformation with foreign DNA.
Ribosomes are the only organelles found in prokaryotes; inclusions are unique to them.

Cell Envelope and Its Modifications

The cell envelope of most bacteria has three tightly bound layers: outer glycocalyx, then cell wall, then plasma membrane. Together they act as one protective unit.
Glycocalyx can be a loose sheath (slime layer) or thick and tough (capsule).
Gram staining divides bacteria by envelope differences: those that take up the stain are Gram positive; those that do not are Gram negative.
The cell wall determines shape and gives strong support so the bacterium does not burst or collapse. The plasma membrane is selectively permeable and similar in structure to that of eukaryotes.
Mesosomes are infoldings of the plasma membrane (vesicles, tubules, lamellae). They help in cell wall formation, DNA replication and distribution to daughter cells, respiration and secretion, and increase surface area and enzymatic content.
Chromatophores are pigment-containing membranous extensions in some prokaryotes such as cyanobacteria.

Flagella, Pili and Fimbriae

Structure	What it is	Role
Flagellum	Thin filamentous extension of the cell wall with three parts: filament (longest), hook and basal body	Motility
Pili	Elongated tubular structures made of a special protein	Not involved in motility
Fimbriae	Small bristle-like fibres	Not involved in motility; in some bacteria help attach to rocks in streams and to host tissues

Ribosomes and Inclusion Bodies

Prokaryotic ribosomes are about 15 nm by 20 nm, made of 50S and 30S subunits that together form the 70S ribosome.
Several ribosomes on one mRNA form a polyribosome (polysome), which translates the mRNA into proteins.
Inclusion bodies store reserve material, are not membrane-bound and lie free in the cytoplasm: phosphate granules, cyanophycean granules, glycogen granules.
Gas vacuoles occur in blue-green algae and in purple and green photosynthetic bacteria.

How are plant and animal cells different?

Both are eukaryotic, but plant cells have a cell wall, plastids and a large central vacuole, while animal cells have centrioles.

Feature	Plant cell	Animal cell
Cell wall	Present	Absent
Plastids	Present	Absent
Vacuole	Large central vacuole (up to 90% of cell volume)	Large central vacuole absent
Centrioles	Absent in almost all plant cells	Present

Cell Membrane: Fluid Mosaic Model and Transport

The cell membrane is a lipid bilayer with proteins, described by the fluid mosaic model of Singer and Nicolson (1972).

Its detailed structure was studied only after the electron microscope arrived in the 1950s; early chemical studies used human red blood cells.
Major lipids are phospholipids in a bilayer: polar heads face outwards and hydrophobic tails face inwards. The membrane also has cholesterol, proteins and carbohydrates.
The human erythrocyte membrane has about 52% protein and 40% lipid; the protein to lipid ratio varies between cell types.
Peripheral proteins lie on the surface; integral proteins are partly or totally buried in the membrane.
The quasi-fluid lipid allows lateral movement of proteins. This fluidity matters for cell growth, intercellular junctions, secretion, endocytosis and cell division.

Type of transport	How it works	Energy
Simple diffusion	Neutral solutes move along the concentration gradient (higher to lower)	No energy needed (passive)
Osmosis	Movement of water by diffusion	No energy needed (passive)
Carrier-mediated	Polar molecules cannot cross the nonpolar bilayer and need a carrier protein	Needs a membrane carrier protein
Active transport	Ions or molecules move against the gradient (lower to higher), e.g. Na+/K+ pump	Uses ATP

Cell Wall: Composition and Layers

The cell wall is a non-living rigid outer covering of the plasma membrane in fungi and plants.

Functions: gives shape, protects from mechanical damage and infection, helps cell-to-cell interaction and acts as a barrier to unwanted macromolecules.
Algae: cellulose, galactans, mannans and minerals such as calcium carbonate. Other plants: cellulose, hemicellulose, pectins and proteins.
Primary wall (young cell) can grow; the secondary wall forms later on the inner side, towards the membrane.
Middle lamella is mainly calcium pectate and glues neighbouring cells together.
Plasmodesmata cross the cell wall and middle lamella and connect the cytoplasm of neighbouring cells.

What is the endomembrane system?

The endomembrane system is the group of membranous organelles whose functions are coordinated: the endoplasmic reticulum, Golgi complex, lysosomes and vacuoles. Mitochondria, chloroplasts and peroxisomes are not part of it.

Organelle	Structure	Main function
Rough ER (RER)	ER with ribosomes on the outer surface; continuous with the outer nuclear membrane	Protein synthesis and secretion
Smooth ER (SER)	ER without ribosomes	Major site of lipid synthesis; steroidal hormones in animal cells
Golgi apparatus	Stacked flat, disc-shaped cisternae (0.5 to 1.0 µm); convex cis (forming) face and concave trans (maturing) face	Packaging and modification of materials; formation of glycoproteins and glycolipids
Lysosomes	Membrane-bound vesicles formed by packaging in the Golgi	Hydrolytic enzymes (lipases, proteases, carbohydrases), active at acidic pH, digest carbohydrates, proteins, lipids and nucleic acids
Vacuoles	Membrane-bound space bound by a single membrane, the tonoplast	Store water, sap, excretory products; tonoplast moves ions into the vacuole against the gradient

ER divides the intracellular space into luminal (inside ER) and extra-luminal (cytoplasm) compartments.
Camillo Golgi (1898) first saw the densely stained reticular structures near the nucleus.
Vesicles from the ER fuse with the cis face of the Golgi and move towards the trans face, which is why the Golgi stays close to the ER.
In Amoeba, the contractile vacuole helps osmoregulation and excretion; in protists, food vacuoles form by engulfing food particles.

Mitochondria and Plastids: Double-membrane Organelles

Mitochondria and chloroplasts are both double membrane-bound and both carry their own DNA and 70S ribosomes.

Feature	Mitochondrion	Chloroplast
Shape and size	Sausage-shaped or cylindrical; diameter 0.2 to 1.0 µm (average 0.5 µm), length 1.0 to 4.1 µm	Lens-shaped, oval, spherical, discoid or ribbon-like; length 5 to 10 µm, width 2 to 4 µm
Inner membrane	Folds into cristae that increase surface area	Relatively less permeable; encloses the stroma
Inner space	Matrix	Stroma with thylakoids stacked as grana, linked by stroma lamellae
DNA and ribosomes	Single circular DNA, a few RNA molecules, 70S ribosomes	Small double-stranded circular DNA, 70S ribosomes
Function	Aerobic respiration; ATP production (‘power houses’ of the cell)	Trapping light energy for photosynthesis
Other points	Number per cell depends on physiological activity; divide by fission	1 per cell in Chlamydomonas to 20 to 40 per mesophyll cell

Mitochondria are not easily visible under the microscope unless specifically stained.
Chlorophyll pigments are in the thylakoids. Grana are the site of light reactions; the stroma is the site of dark reactions and has enzymes for carbohydrate and protein synthesis.

Types of Plastids

Plastids are found in all plant cells and in euglenoids, and are classified by their pigments.

Plastid	Pigment or content	Example or note
Chloroplast	Chlorophyll and carotenoids	Most are in leaf mesophyll cells
Chromoplast	Fat-soluble carotenoids such as carotene and xanthophylls	Give yellow, orange or red colour
Amyloplast (leucoplast)	Stores carbohydrate (starch)	Potato
Elaioplast (leucoplast)	Stores oils and fats	Colourless
Aleuroplast (leucoplast)	Stores proteins	Colourless

Ribosomes, Cytoskeleton, Cilia, Flagella and Centrioles

These are the non-membrane-bound structures of the cell; ribosomes are found in all cells, while the cytoskeleton and centrioles belong to eukaryotes.

Ribosomes

First observed under the electron microscope as dense particles by George Palade (1953).
Made of RNA and proteins and not surrounded by any membrane.
Eukaryotic 80S ribosomes have 60S and 40S subunits; prokaryotic 70S ribosomes have 50S and 30S subunits.
S is the Svedberg unit, the sedimentation coefficient, an indirect measure of density and size.

Cytoskeleton

A network of proteinaceous filaments in the cytoplasm: microtubules, microfilaments and intermediate filaments.
Functions: mechanical support, motility and maintaining cell shape.

Cilia and Flagella

Hair-like outgrowths of the cell membrane. Cilia are small and work like oars to move the cell or the surrounding fluid; flagella are longer and move the cell.
Bacterial flagella are structurally different from eukaryotic flagella.
The core, called the axoneme, has nine doublets of peripheral microtubules and a central pair: the 9+2 array.
The central tubules are joined by bridges and enclosed by a central sheath, linked to one tubule of each peripheral doublet by a radial spoke, so there are nine radial spokes. Peripheral doublets are joined by linkers.
Both emerge from centriole-like basal bodies.

Centrosome and Centrioles

A centrosome usually has two cylindrical centrioles lying perpendicular to each other, surrounded by amorphous pericentriolar material.
Each centriole has a cartwheel organisation of nine evenly spaced peripheral fibrils of tubulin; each fibril is a triplet, and adjacent triplets are linked.
The central proteinaceous part of the proximal region is the hub, joined to the peripheral triplets by radial spokes.
Centrioles form the basal body of cilia and flagella, and the spindle fibres of the spindle apparatus during cell division in animal cells.

Feature	Cilium or flagellum axoneme	Centriole
Peripheral microtubules	9 doublets	9 triplets
Central microtubules	2 (central pair)	None; central proteinaceous hub
Arrangement	9+2	Cartwheel

Nucleus: Structure, Chromatin and Chromosome Types

The nucleus controls the activities of the cell and plays a major role in heredity; Robert Brown first described it in 1831.

Flemming named the material of the nucleus stained by basic dyes chromatin.
The interphase nucleus has chromatin, nuclear matrix (nucleoplasm) and one or more nucleoli.
The nuclear envelope has two parallel membranes with a perinuclear space of 10 to 50 nm. The outer membrane is continuous with the ER and bears ribosomes.
Nuclear pores form by fusion of the two membranes and let RNA and protein molecules pass in both directions.
Some mature cells lack a nucleus: erythrocytes of many mammals and sieve tube cells of vascular plants.
The nucleolus is not membrane-bound and is the site of active ribosomal RNA synthesis. Cells active in protein synthesis have larger and more numerous nucleoli.
Chromatin contains DNA, basic proteins called histones, non-histone proteins and RNA. A human cell has about 2 m of DNA spread over 46 chromosomes (23 pairs).
Every chromosome has a primary constriction, the centromere, with disc-shaped kinetochores on its sides. The centromere holds the two chromatids.
Some chromosomes have a non-staining secondary constriction that gives a small fragment called the satellite.

Chromosome type	Centromere position	Arms
Metacentric	Middle	Two equal arms
Sub-metacentric	Slightly away from the middle	One shorter, one longer arm
Acrocentric	Close to one end	One extremely short, one very long arm
Telocentric	Terminal	Centromere at the tip

Microbodies

Microbodies are many minute, membrane-bound vesicles that contain various enzymes and occur in both plant and animal cells.

Most-asked Facts for NEET: Cell: the Unit of Life

Revise these one-line facts from the NCERT chapter just before the exam.

Leeuwenhoek first saw a live cell; Robert Brown discovered the nucleus (1831); Virchow (1855) gave Omnis cellula-e cellula.
Mycoplasma are the smallest cells and have no cell wall.
Mesosomes are a characteristic of prokaryotes.
Slime layer is a loose glycocalyx; capsule is thick and tough.
Bacterial flagellum: filament, hook and basal body.
Fluid mosaic model: Singer and Nicolson, 1972.
Na+/K+ pump is active transport and uses ATP.
Middle lamella is mainly calcium pectate.
Endomembrane system: ER, Golgi, lysosomes, vacuoles only.
Golgi apparatus is the main site of glycoprotein and glycolipid formation.
Lysosomal hydrolases work best at acidic pH.
Tonoplast is the vacuole membrane; vacuoles can fill up to 90% of a plant cell.
Mitochondria and chloroplasts have their own circular DNA and 70S ribosomes.
Amyloplast stores starch, elaioplast stores oils and fats, aleuroplast stores proteins.
Ribosomes were first observed by George Palade (1953); 80S = 60S + 40S, 70S = 50S + 30S.
Cilia and flagella: 9+2 axoneme; centriole: nine triplets in a cartwheel.
Nucleolus is the site of rRNA synthesis and has no membrane.
Acrocentric chromosome: centromere near one end; telocentric: terminal centromere.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

Saying Robert Brown discovered the cell. He discovered the nucleus; Leeuwenhoek first saw and described a live cell.
Counting mitochondria, chloroplasts or peroxisomes as part of the endomembrane system.
Mixing up Golgi faces: the cis (forming) face is convex and the trans (maturing) face is concave.
Thinking pili and fimbriae help in motility. Only flagella do.
Calling inclusion bodies or the nucleolus membrane-bound. Neither has a membrane.
Giving centrioles a 9+2 arrangement. The 9+2 array belongs to the axoneme of cilia and flagella; centrioles have nine triplets and no central pair.
Adding S values: a 50S and a 30S subunit make a 70S ribosome, not 80S, because S measures sedimentation, not mass.
Saying the secondary wall forms outside the primary wall. It forms on the inner side, towards the membrane.
Writing that plasmids are part of the genomic DNA. They are separate small circular DNA molecules.
Assuming all plastids are green. Chromoplasts are yellow, orange or red, and leucoplasts are colourless.

Download the AglaSem notes for Cell: The Unit of Life (key notes and important questions with answers) free: View PDF

All chapters: NEET Biology notes
Full topic list: NEET syllabus
Practice: NEET question papers
Source text: NCERT Chapter 8 (NCERT, Class 11 Biology)

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Cell: the Unit of Life NEET Notes FAQs

Which NCERT chapter is Cell: The Unit of Life?

It is Chapter 8 of the NCERT Class 11 Biology textbook, the first chapter of Unit 3, Cell: Structure and Functions. Biomolecules (Chapter 9) and Cell Cycle and Cell Division (Chapter 10) complete the unit.

Is Cell: The Unit of Life in the NEET 2027 syllabus?

It is in the latest NEET UG syllabus (2026, released by NMC on 22 December 2025) under Unit 3, Cell Structure and Function. NMC is expected to release the 2027 syllabus later; check this page and the syllabus page once it is out.

Which organelles have their own DNA and ribosomes?

Mitochondria and chloroplasts. Both are double membrane-bound and contain circular DNA and 70S ribosomes, while the ribosomes in the rest of the eukaryotic cytoplasm are 80S.

Why are mitochondria not part of the endomembrane system?

The endomembrane system groups organelles whose functions are coordinated: ER, Golgi complex, lysosomes and vacuoles. The functions of mitochondria, chloroplasts and peroxisomes are not coordinated with these, so NCERT keeps them out.

Are these notes enough for NEET questions on the cell?

No. Read NCERT Chapter 8 line by line first, because NEET Biology questions closely follow the NCERT textbooks. Use these notes and the PDF for quick revision, then practise previous year questions.

Cell: The Unit of Life NEET Notes 2027: Key Points and PDF