Cell-the Unit Of Life Class 11 Notes Biology Chapter 8 - CBSE

Chapter : 8

What Are Cell-the Unit Of Life ?


(Fundamental, structural and functional unit of life)

Robert Hook

(Discovered cell)

Anton Von Leeuwenhoek

(Discovered living cell)



Observed that all plants are composed of different kinds of cells.

  • Schleidenn

Observed that all plants are composed of different kinds of cells.

  • Schwann

Found that cells have a thin outer layer (plasma membrane). He also found that plant cells have cell wall.

  • Schleiden and Schwann

Formulated the cell theory

  • All living organisms are composed of cells and products of cells.
  • All cells arise from pre-existing cells
  • Communicability

First explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the cell theory.


Prokaryotic Cell
  • They have no membrane bound nucleus and organelles.
  • They include bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
  • They are generally smaller and multiply more rapidly than the eukaryotic cells.
  • They vary in shape and size. E.g. Bacteria have 4 basic shapes: Bacillus, Coccus, Vibrio and Spirillum.
Eukaryotic Cell
  • They have well organized membrane bound nucleus and organelles.
  • Presence of membranes gives clear compartmentalization of cytoplasm.
  • Their genetic material is organized in to chromosomes.
  • They have complex locomotory and cytoskeletal structures.

The Endomembrane System

It is a membranous component of the eukaryotic cell. The cytoplasm of the cell contains a system of membranous organelles that are suspended in it. The organelles are termed as a system even though they have different structures and functions as they are essential to the working of the cell.

Cell Organelle Occurrence/ Characteristic & Structure Function
Cell Membrane/Plasma Membrane
  • Present in both plant cell and animal cell.
  • Selectively Permeable: Allows the materials in and out of the cell according to the requirement of the cell.
  • Made up of bilipid layer and protein (Fluid Mosaic Model)
  • Encloses the contents of the cell.
  • Provides shape: animal cell.
  • Allows transport: by Diffusion and Osmosis.
Cell Wall
  • Present only in a plant cell.
  • Hard and rigid.
  • Fully permeable.
  • Made up of Cellulose in plant and peptidoglycan in bacteria.
  • Protection
  • Gives shape and turgidity.
  • Contains 80-90% water and many organic and inorganic compounds.
  • Colloidal, Viscous, Jelly like fluid inside the cell.
  • Contains enzymes responsible for all the metabolic activity taking place inside the cell.
(Brain of the Cell)
  • Covered by a double membranous nuclear membrane in a Eukaryotic Cell.
  • Contains DNA, RNA, Protein, nucleolus, and Chromatin network.
  • Controls the activity of the cell.
  • Starts cell division.
  • It has the chromosomes and DNA which controls the hereditary characters.
(The Power House of the Cell / Storage Batteries)
  • Rod-shaped, Double membranous structure.
  • Autonomous body as contains its own DNA.
  • Self-duplicates.
  • The outer membrane is smooth while the inner membrane folds over many times to form cristae.
  • Cristae increase the surface area of the inner membrane by several times.
  • Matrix is the fluid inside the mitochondria.
  • Mitochondria act as energy production sites and are hence, known as the Powerhouse of the Cell.
    $$\text{Glucose + Oxygen}\xrightarrow{}\text{Carbon}$$
    Dioxide + Water + Energy (ATP)
Golgi Bodies
(Shipping Department of Cell)
  • Discovered by Camillo Golgi in 1898.
  • Smooth, flattened sac-like structures called Cisternae stack together in parallel rows to form Golgi bodies.
  • Golgi Cis face receives protein from Endoplasmic reticulum and modifies, packages and stores it.
  • Modification, Packaging, and transport of materials
  • Synthesis of lysosomes, plasma membrane.
  • Secretion of enzymes, hormones and proteins.
Endoplasmic Reticulum
(Framework of Cell)
  • A network of membranes.
  • RER bears ribosomes and appears rough.
  • SER does not have ribosomes.
  • Forms the skeletal framework of the cell.
  • Transport of materials from one cell to other.
  • Provides a surface for the synthesis of material – Proteins in RER and Lipids in SER.
  • Formation of lysosomes, Golgi bodies and vacuoles
  • Membrane Biogenesis
  • Detoxification of harmful substances in the liver.
  • Arise from ER and GB
  • Surrounded by tonoplast and filled with cell sap.
  • Plant cells usually have a large vacuole.
  • Store cell sap which may be liquid or solid food, toxic byproduct.
  • Provide rigidity and turgidity to plant cell.
Cell Organelle Occurrence/ Characteristic & Structure Function
Lysosomes (Suicidal bags of Cell, natural scavenger, cellular housekeeper)
  • Membrane-bound organelles
  • Present in all animal cells and few plant cells
  • Tiny circular single membrane-bound structures filled with digestive enzymes.
  • Intracellular digestion of food in unicellular organisms.
Ribosomes (Protein Factories)
  • Without a membrane
  • Consist of two subunits – 60S and 40S in eukaryote both made up of RNA.
  • Synthesis of Proteins
Plastids Double membrane-bound. Types:
  • Leucoplast –Colourless plastid;
  • Chromoplast –Coloured Plastid – blue, red, yellow
  • Chloroplast – Green plastid
  • Autonomous self-duplicating body
  • Chloroplast – Perform Photosynthesis – Helps in the release of oxygen
  • Chromoplast – impart colour to flowers which help in pollination
  • Leucoplast – Storage
  • Amyloplast –Store starch
  • Aleuroplast – Store Protein
  • Elaioplast – Store fat


A microbody is a cell organelle present in both plant and animal cells. Glyoxysomes, peroxisomes are included in the microbodies family.


The cytoskeleton is a structure that helps cells maintain their shape and internal organization, and it also provides mechanical support that enables cells to carry out essential functions like division and movement.


Cilia are small, slender, hair-like structures present on the surface of all mammalian cells. They are primitive in nature and could be single or many. Cilia play a major role in locomotion.


Flagella are microscopic hair-like structures involved in the locomotion of a cell. The word “flagellum” means “whip”. The flagella have a whip-like appearance that helps to propel a cell through the liquid.


Centrioles are paired barrel-shaped organelles located in the cytoplasm of animal cells near the nuclear envelope. Centrioles play a role in organizing microtubules that serve as the cell's skeletal system.

Plant And Animal Cells

  • Plants have larger vacuoles while animals have smaller vacuoles. Vacuoles store water or food for plants and even store waste products until they can be discarded. Plants need bigger vacuoles as they cannot move to satisfy their hunger or thirst.
  • Only plant cells have plastids. They help in manufacturing or storing food.
  • Only animal cells have centrioles. Centrioles help in the division of cells.