Cell Theory & Types of cells - Biology Class 11 - NEET PDF Download

Table of Contents
1. What is a Cell?
2. Cell Theory - Historical Development and Statements
3. An Overview of Cells - Examples and General Features
4. Size and Shape of Cells
5. Types of Cells
6. Ribosomes and Inclusion Bodies in Prokaryotes
7. Key Differences: Prokaryotic vs Eukaryotic Cells (summary)
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When observing the world around us, we encounter both living and non-living entities. A clear distinction between them is the presence of cells - the fundamental unit of life found in all living organisms. Some organisms are unicellular (consisting of a single cell) while others are multicellular (made up of many cells, for example humans). The structure and organisation of cells underlie all life processes.

What is a Cell?

Unicellular organisms are capable of independent existence and perform all vital functions within one cell. Without a functional cell, independent living is not possible.

Thus, the cell is the fundamental building block and the basic functional unit of all living organisms.

Early microscopists made key discoveries about cells. Anton van Leeuwenhoek was the first to observe and describe living cells using simple microscopes. Robert Brown discovered the nucleus in 1831. Advances in light and, later, electron microscopy revealed detailed internal structures and organelles.

Structure of Cell

Cell Theory - Historical Development and Statements

• In 1838, Matthias Schleiden (a German botanist) concluded that plant tissues are composed of cells.
• At about the same time, Theodor Schwann (a German zoologist) studied animal tissues and proposed that animals are also composed of cells; he identified the cell membrane in animal cells and noted the cell wall in plant cells.
• Schleiden and Schwann together formulated the early cell theory that living organisms are built of cells and their products.
• In 1855, Rudolf Virchow added the crucial principle that cells arise from pre-existing cells - summarised as Omnis cellula e cellula.

From these contributions the classical cell theory may be stated as follows:

All living organisms are composed of cells and their products.

All cells arise from pre-existing cells.

An Overview of Cells - Examples and General Features

• Onion epidermal cells (typical plant cells) show a rigid cell wall outside the cell membrane.
• Human cheek (buccal) cells are animal cells with an outer plasma membrane and a distinct membrane-bound nucleus containing chromosomes and DNA.
• Eukaryotic cells have a membrane-bound nucleus and many membrane-bound organelles (endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, microbodies, vacuoles).
• Prokaryotic cells lack a true nucleus and membrane-bound organelles; they have cytoplasm, ribosomes and genetic material in a nucleoid region.
• Ribosomes are non-membrane-bound particles present in all cells and are the sites of protein synthesis. In eukaryotes they are 80S (60S + 40S) and in prokaryotes 70S (50S + 30S).
• Animal cells contain a centrosome (with centrioles) that plays a role in cell division; most higher plant cells lack centrioles.

​NEET PYQ from this Topic:

From the statements given below choose the correct option:    (NEET 2025)

(a) A, B and E are true
(b) B, D and E are true
(c) A, B and C are true
(d)  A, B and D are true

Size and Shape of Cells

• Cells vary greatly in size and shape depending on type and function.
• Mycoplasma are among the smallest known cells (~0.3 µm). Typical bacteria range from ~1-5 µm in length. Mammalian red blood cells (erythrocytes) have diameter ≈ 7 µm. The ostrich egg is the largest single cell visible to the naked eye.
• Some nerve (neuronal) cells are the longest cells, with axons extending over a metre in large animals.
• Common cell shapes include disc-like, polygonal, columnar, cuboidal, thread-like (filamentous), spherical and irregular; shape relates closely to cell function.

Types of Cells

1. Prokaryotic Cells

A prokaryotic cell is generally unicellular and lacks a true nucleus and membrane-bound organelles. Organisms of domains Bacteria and Archaea are prokaryotic.

General Characteristics of Prokaryotic Cells

• Usually smaller in size and reproduce rapidly (binary fission) compared to eukaryotic cells.
• Shapes commonly include coccus (spherical), bacillus (rod-shaped), vibrio (comma-shaped) and spirillum (spiral).
• Most have a cell wall outside the plasma membrane (exception: Mycoplasma).
• Genetic material is not enclosed in a nuclear membrane; DNA is usually a single circular chromosome located in the nucleoid. Extra-chromosomal circular DNA molecules called plasmids may be present; plasmids often carry genes for traits such as antibiotic resistance.
• Lack membrane-bound organelles; have 70S ribosomes in the cytoplasm and sometimes attached to membrane regions.
• Contain inclusions (reserve materials) and specialised membrane infoldings such as mesosomes (historically described; many mesosome observations are now believed to be artefacts of preparation, though membrane infoldings for respiration and other functions do occur in some bacteria).

NEET PYQ from this Topic:

A specialised membranous structure in a prokaryotic cell which helps in cell wall formation, DNA replication is:     (NEET 2025) 
(a) Cristae 
(b) Endoplasmic Reticulum
(c) Mesosome
(d) Chromatophores

Cell Envelope and its Modifications (Bacteria)

• The bacterial cell envelope is often a complex multi-layered structure functioning as a protective barrier and interface with the environment.
• Typical layers from outside to inside: glycocalyx (capsule or slime layer) → cell wall → plasma membrane.
• Glycocalyx may be a loose slime layer (helps in adhesion, biofilm formation) or a well-defined capsule (protects against host defences and desiccation).
• Cell wall provides shape and structural strength, preventing lysis due to osmotic pressure.
• The constitution and arrangement of the cell envelope determine the behaviour in Gram staining:
  • Gram-positive bacteria: thick peptidoglycan layer (retains crystal violet stain); often contain teichoic acids.
  • Gram-negative bacteria: thin peptidoglycan layer plus an outer membrane containing lipopolysaccharides (LPS); they do not retain crystal violet after decolourisation and appear pink/red after counterstaining.
• The plasma membrane is selectively permeable and structurally similar to eukaryotic membranes (protein-lipid bilayer), carrying out transport and metabolic processes.

Surface Structures, Extensions and Motility

Bacterial cells may show various membranous and non-membranous surface structures: vesicles, tubules, lamellae, chromatophores in photosynthetic bacteria, flagella, pili and fimbriae.

Membranous Extensions (examples)

• Vesicles: small membrane-bound sacs used for transport and storage (in some bacteria).
• Tubules: slender projections or internal tubules involved in various processes.
• Lamellae: flattened membrane layers that may assist in respiration or photosynthesis.
• Chromatophores: membrane systems containing pigments for photosynthesis in cyanobacteria and purple bacteria.

Flagella

Flagella are filamentous structures that enable motility in many bacteria. A typical bacterial flagellum has three main parts:

1. Filament - long helical structure external to the cell.
2. Hook - curved segment connecting filament to basal body.
3. Basal body - motor and anchoring structure spanning cell envelope and rotating the filament.

Structure of Bacterium Flagellum

Pili and Fimbriae

• Pili (singular: pilus) are tubular protein structures; specialised pili (sex pili) mediate conjugation (transfer of plasmid DNA) between bacteria and can also assist in adherence.
• Fimbriae are short, bristle-like fibres that help bacteria attach to surfaces and host tissues (important in colonisation).

Ribosomes and Inclusion Bodies in Prokaryotes

• Ribosomes: Prokaryotic ribosomes are 70S particles composed of 50S and 30S subunits and are the sites of protein synthesis.
• Multiple ribosomes may associate with a single mRNA forming a polyribosome (polysome) for efficient translation.
• Inclusion bodies: reserve materials stored in the cytoplasm (not membrane-bound), e.g., glycogen granules, polyphosphate granules, cyanophycean granules.
• Gas vacuoles occur in some photosynthetic bacteria and provide buoyancy.

NEET PYQ from this Topic:

Which of the following statements about inclusion bodies is incorrect?    (NEET 2020)
(a) They lie free in the cytoplasm
(b) These represent reserve material in cytoplasm.
(c) They are not bound by any membrane.
(d) These are involved in ingestion of food particles.

2. Eukaryotic Cells

Eukaryotic cells have a well-defined nucleus enclosed by a nuclear membrane and contain membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus and lysosomes.

Characteristics of Eukaryotic Cells

• Include organisms from the groups: protists, fungi, plants and animals.
• Exhibit extensive compartmentalisation by membrane-bound organelles, each having specialised functions.
• Possess an organised nucleus with a nuclear envelope; the nucleus contains chromosomal DNA and nucleolus for ribosome assembly.
• Have an elaborate cytoskeleton (microfilaments, microtubules, intermediate filaments) providing structural support, intracellular transport pathways, and roles in cell division and motility.
• Genetic material is linear chromosomes with associated histone proteins (in most eukaryotes).
• Not all eukaryotic cells are identical: plant cells have cell walls (cellulose), chloroplasts (photosynthesis) and a large central vacuole, which are generally absent in animal cells.
• Animal cells typically have centrioles within the centrosome which are absent in higher plant cells.

Major Organelles (brief functions)

• Nucleus: stores genetic information; site of transcription and ribosome assembly (nucleolus).
• Mitochondria: site of aerobic respiration and ATP production.
• Chloroplasts: in plants and algae, sites of photosynthesis; contain chlorophyll and have their own DNA (supporting endosymbiotic origin).
• Endoplasmic reticulum (ER): RER (with ribosomes) synthesises proteins; SER synthesises lipids and detoxifies compounds.
• Golgi apparatus: modifies, sorts and packages proteins and lipids for secretion or delivery to other organelles.
• Lysosomes: contain hydrolytic enzymes for intracellular digestion (more prominent in animal cells).
• Peroxisomes (microbodies): contain enzymes for oxidative reactions (e.g., breakdown of H2O2).
• Vacuoles: store water, metabolites and maintain turgor in plant cells (large central vacuole).
• Centrosome and centrioles: organise microtubules during cell division in animal cells.
• Cytoskeleton: enables shape, movement and intracellular transport.

Key Differences: Prokaryotic vs Eukaryotic Cells (summary)

• Nucleus: Prokaryotes - no true nucleus; Eukaryotes - nucleus present.
• Organelles: Prokaryotes - lack membrane-bound organelles; Eukaryotes - numerous membrane-bound organelles present.
• Ribosomes: Prokaryotes - 70S; Eukaryotes - 80S (but mitochondria and chloroplasts have 70S ribosomes).
• Chromosomes: Prokaryotes - usually single circular DNA; Eukaryotes - multiple linear chromosomes with histones.
• Cell wall: Present in most prokaryotes (peptidoglycan) and in plant eukaryotes (cellulose), absent in animal cells.