THE CELL: THE UNIT OF LIFE
Class 11 Biology • Detailed NCERT-aligned study notes • NEET revision
How to use these notes: Read the concept first, learn the comparison tables, then practise labelled diagrams and
NCERT-based MCQs. These notes are a study aid; use your current NCERT textbook for exact wording and any
syllabus updates.
1. Cell: an introduction
The cell is the basic structural, functional and biological unit of living organisms. All organisms are made of one or more
cells. A single cell can perform essential life processes, while multicellular organisms show division of labour among
specialised cells.
• Unicellular organisms: one cell performs all life functions; examples include Amoeba, Paramecium and many
bacteria.
• Multicellular organisms: many cells are organised into tissues, organs and organ systems; examples include plants
and animals.
• Cytology / cell biology: the study of cells.
• Cell size: varies widely. Most cells are microscopic, but some are visible to the naked eye, such as the yolk-rich egg of
a bird.
2. Discovery of the cell and cell theory
Scientist Contribution
Robert Hooke (1665) Observed tiny box-like compartments in thin cork and called them “cells”. The cork cells he saw
Antonie van LeeuwenhoekObserved
(1670s) living cells and microorganisms using improved simple microscopes.
Robert Brown (1831) Described the nucleus in plant cells.
M. J. Schleiden (1838) Concluded that plants are made of cells.
Theodor Schwann (1839) Concluded that animals are made of cells; with Schleiden, developed the cell theory.
Rudolf Virchow (1855) Stated that new cells arise from pre-existing cells (Omnis cellula e cellula).
Cell theory
• All living organisms are composed of cells and cell products.
• The cell is the basic structural and functional unit of life.
• New cells arise from pre-existing cells through cell division.
Important: Classical cell theory explains the cellular organisation of life. Viruses are acellular and do not fit the usual
definition of a cell.
3. Overview: types of cells
Prokaryotic cells
• Usually small and structurally simple; examples include bacteria and archaea.
• No membrane-bound, true nucleus. DNA lies in a nucleoid region.
• Membrane-bound organelles such as mitochondria, ER and Golgi apparatus are absent.
Class 11 Biology | The Cell: The Unit of Life Page 1
, • Typically have 70S ribosomes; may have a cell wall, capsule, pili and flagella depending on the organism.
• Bacterial genetic material is generally a single circular DNA molecule; plasmids may also occur.
Eukaryotic cells
• Have a membrane-bound nucleus containing chromosomes.
• Contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus.
• Cytoplasmic ribosomes are generally 80S; mitochondria and chloroplasts contain 70S-type ribosomes.
• Found in protists, fungi, plants and animals.
Feature Prokaryotic Eukaryotic
Nucleus No true nucleus; nucleoid True nucleus with nuclear envelope
Membrane-bound organelles
Absent Present
Ribosomes 70S 80S in cytoplasm; 70S in mitochondria/chloroplasts
DNA Usually circular; not organised as typical eukaryotic
Linear chromosomes
chromosomes
associated with histone proteins in the
Cell division Usually binary fission Mitosis or meiosis in the appropriate context
Examples Bacteria, archaea Plants, animals, fungi, protists
4. Shape and size of cells
Cells vary in shape according to their function. Examples: red blood cells are biconcave; neurons are elongated and
branched; muscle cells are elongated; some white blood cells can change shape; plant cells often have a more fixed
outline because of the cell wall.
Cell dimensions are commonly measured in micrometres (µm; 1 µm = 10■■ m) and nanometres (nm; 1 nm = 10■■ m).
Bacteria are generally a few micrometres long, while many eukaryotic cells are roughly 10–100 µm, with exceptions.
5. Plasma membrane
The plasma membrane forms the boundary of the cell. It is selectively permeable, controlling movement of substances
into and out of the cell, and also helps in cell signalling and recognition.
Fluid mosaic model
• Proposed by S. J. Singer and G. L. Nicolson (1972).
• The membrane is mainly a phospholipid bilayer with proteins embedded in or associated with it.
• Phospholipids are amphipathic: hydrophilic heads face the watery environments and hydrophobic tails face inward.
• Membrane components can move laterally, giving the membrane a dynamic, fluid nature.
• In animal cells, cholesterol helps regulate membrane fluidity. Carbohydrates attached to lipids or proteins help in
recognition.
Transport across the membrane
Process Energy needed? Direction / mechanism
Simple diffusion No Net movement down a concentration gradient; e.g., small non-polar molecule
Facilitated diffusion No Down a gradient through channel or carrier proteins.
Osmosis No direct metabolic energy
Net movement of water across a selectively permeable membrane from highe
Class 11 Biology | The Cell: The Unit of Life Page 2
Class 11 Biology • Detailed NCERT-aligned study notes • NEET revision
How to use these notes: Read the concept first, learn the comparison tables, then practise labelled diagrams and
NCERT-based MCQs. These notes are a study aid; use your current NCERT textbook for exact wording and any
syllabus updates.
1. Cell: an introduction
The cell is the basic structural, functional and biological unit of living organisms. All organisms are made of one or more
cells. A single cell can perform essential life processes, while multicellular organisms show division of labour among
specialised cells.
• Unicellular organisms: one cell performs all life functions; examples include Amoeba, Paramecium and many
bacteria.
• Multicellular organisms: many cells are organised into tissues, organs and organ systems; examples include plants
and animals.
• Cytology / cell biology: the study of cells.
• Cell size: varies widely. Most cells are microscopic, but some are visible to the naked eye, such as the yolk-rich egg of
a bird.
2. Discovery of the cell and cell theory
Scientist Contribution
Robert Hooke (1665) Observed tiny box-like compartments in thin cork and called them “cells”. The cork cells he saw
Antonie van LeeuwenhoekObserved
(1670s) living cells and microorganisms using improved simple microscopes.
Robert Brown (1831) Described the nucleus in plant cells.
M. J. Schleiden (1838) Concluded that plants are made of cells.
Theodor Schwann (1839) Concluded that animals are made of cells; with Schleiden, developed the cell theory.
Rudolf Virchow (1855) Stated that new cells arise from pre-existing cells (Omnis cellula e cellula).
Cell theory
• All living organisms are composed of cells and cell products.
• The cell is the basic structural and functional unit of life.
• New cells arise from pre-existing cells through cell division.
Important: Classical cell theory explains the cellular organisation of life. Viruses are acellular and do not fit the usual
definition of a cell.
3. Overview: types of cells
Prokaryotic cells
• Usually small and structurally simple; examples include bacteria and archaea.
• No membrane-bound, true nucleus. DNA lies in a nucleoid region.
• Membrane-bound organelles such as mitochondria, ER and Golgi apparatus are absent.
Class 11 Biology | The Cell: The Unit of Life Page 1
, • Typically have 70S ribosomes; may have a cell wall, capsule, pili and flagella depending on the organism.
• Bacterial genetic material is generally a single circular DNA molecule; plasmids may also occur.
Eukaryotic cells
• Have a membrane-bound nucleus containing chromosomes.
• Contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus.
• Cytoplasmic ribosomes are generally 80S; mitochondria and chloroplasts contain 70S-type ribosomes.
• Found in protists, fungi, plants and animals.
Feature Prokaryotic Eukaryotic
Nucleus No true nucleus; nucleoid True nucleus with nuclear envelope
Membrane-bound organelles
Absent Present
Ribosomes 70S 80S in cytoplasm; 70S in mitochondria/chloroplasts
DNA Usually circular; not organised as typical eukaryotic
Linear chromosomes
chromosomes
associated with histone proteins in the
Cell division Usually binary fission Mitosis or meiosis in the appropriate context
Examples Bacteria, archaea Plants, animals, fungi, protists
4. Shape and size of cells
Cells vary in shape according to their function. Examples: red blood cells are biconcave; neurons are elongated and
branched; muscle cells are elongated; some white blood cells can change shape; plant cells often have a more fixed
outline because of the cell wall.
Cell dimensions are commonly measured in micrometres (µm; 1 µm = 10■■ m) and nanometres (nm; 1 nm = 10■■ m).
Bacteria are generally a few micrometres long, while many eukaryotic cells are roughly 10–100 µm, with exceptions.
5. Plasma membrane
The plasma membrane forms the boundary of the cell. It is selectively permeable, controlling movement of substances
into and out of the cell, and also helps in cell signalling and recognition.
Fluid mosaic model
• Proposed by S. J. Singer and G. L. Nicolson (1972).
• The membrane is mainly a phospholipid bilayer with proteins embedded in or associated with it.
• Phospholipids are amphipathic: hydrophilic heads face the watery environments and hydrophobic tails face inward.
• Membrane components can move laterally, giving the membrane a dynamic, fluid nature.
• In animal cells, cholesterol helps regulate membrane fluidity. Carbohydrates attached to lipids or proteins help in
recognition.
Transport across the membrane
Process Energy needed? Direction / mechanism
Simple diffusion No Net movement down a concentration gradient; e.g., small non-polar molecule
Facilitated diffusion No Down a gradient through channel or carrier proteins.
Osmosis No direct metabolic energy
Net movement of water across a selectively permeable membrane from highe
Class 11 Biology | The Cell: The Unit of Life Page 2
