Cell Structure and Functions Revision Notes

Cell as the Basic Unit of Life 

  • The cell is the basic structural and functional unit of all living organisms. 
  • It is the smallest part of the body of an organism, is capable of independent existence and is able to perform the essential functions of life. 

Discovery of the Cell

  • Robert Hooke observed cork cells under a simple microscope designed by him.
  • He observed compartments resembling honeycombs and each compartment was separated by a wall.
  • He termed each compartment as a ‘cell’.
  • It was the first indication that living organisms are made of cells.

Cell Theory 

  • In 1838, Matthias Schleiden and Theodor Schwann proposed the basic cell theory. In 1858, another scientist Virchow made an addition to the existing cell theory.
  • The postulates of the modern cell theory are
    • The cell is the smallest unit of structure of all living things.
    • The cell is the unit of function of all living things.
    • All cells arise from pre-existing cells.

Variety in Cells

On the basis of the number of cells 

Single-celledFew-celledMulti-celled
Organisms made of a single cell. They are called unicellular organisms. Examples: Bacteria, Yeast, Chlamydomonas, Amoeba, Paramoecium Organisms made of a few hundred to few thousand cells.Examples: Spirogyra, VolvoxOrganisms made of millions to billions of cells. They are called multicellular organisms.Examples: Man, cow, mango tree, crow

On the basis of size of cells 

On the basis of size of cells
  • Smallest cell: Examples: Bacteria (0.3–5.0 µm), red blood cells (7 µm)
  • Longest cell: Example: Nerve cell in the neck of a giraffe (˃3 m long) 
  • Largest cell: Example: Ostrich egg (170 mm × 130 mm)

On the basis of shape of cells

  • Columnar: Epithelial cells
  • Spherical: Human ovum
  • Oval: Fat cells
  • Spherical, biconcave: Red blood cells
  • Rectangular: Spirogyra
  • Spiral: Sperm cell
  • Rod-shaped: Bacteria
  • C-shaped: Cartilage cells
  • Cylindrical: Striated muscle fibre cells
  • Branched: Nerve cells
  • Spindle-shaped: Smooth muscle cells
  • Bean-shaped: Guard cell from a plant leaf

Structural Organisation of a Cell 

Structural Organisation of a Cell

CELL ORGANELLES

Plasma membrane/Cell membrane

  • Forms the outermost covering in animal cells
  • Lies next to the cell wall in plant cells
  • Separates cellular material from its surroundings.
  • Acts as an effective barrier and regulates the entry of substances in and out of the cell

Cell wall (in plant cells only)

  • Found in plant cells
  • Situated just outside the plasma membrane
  • Mainly composed of cellulose
  • Provides protection
  • Gives rigidity and shape to plant cells

Cytoplasm

  • Transparent jelly-like material
  • Centre of all metabolic activities
  • Different organelles contained in it perform different functions

Nucleus

  • Largest cell organelle
  • Mostly spherical and dense
  • Nuclear membrane with pores, which allow substances to enter and leave the nucleus
  • Regulates cell functions
  • Contains chromosomes, made of genes, which control hereditary characteristics

 Nucleolus

  • Embedded within the nucleus of the cell.
  • One or more in number 
  • Produces ribosomes
  • Participates in protein synthesis by forming and storing RNA

Chromatin fibers

  • Network of thread-like structures made of DNA
  • Chromosomes carry hereditary information or  Genes

Vacuoles

  • Fluid-filled membrane bound spaces
  • Larger and permanent in plant cells
  • Small and temporary in animal cells
  • Storage of water and other substances,  food, pigments and waste products
  • Provides turgidity to the cells

Study of Plant and Animal Cells

Differences between Plant and Animal Cells 

Plant and Animal Cells
PLANT CELLFEATUREANIMAL CELL
 Structural differences
Presence of a definite cell wall made of celluloseCell wallAbsence of cell wall
Present internal to the cell wallCell membraneForms the boundary of the cell
Presence of one or more prominent vacuolesVacuoles Presence of small and temporary vacuoles
Presence of plastidsPlastids Absence of plastids
 Functional differences
Usually larger with distinct outlinesSize Usually smaller with less distinct boundaries
Not so dense Cytoplasm Denser and more granular
Only a thin lining of cytoplasm, which is mostly pushed to the peripheryArrangement of cytoplasm Cytoplasm fills up almost the entire cell
 Other differences
Rectangular Shape Spherical 
Starch Storage materialGlycogen 

Prokaryotic and Eukaryotic Cells

PROKARYOTIC CELL    EUKARYOTIC CELL  
Absence of well-defined nucleus Presence of well-defined nucleus with a nuclear membrane
Absent Present 
Presence of a single length of only DNA  Presence of several lengths of DNA wound around certain proteins
Presence of smaller ribosomesPresence of larger ribosomes
Absence of other cell organellesPresence of several other cell organelles such as mitochondria, ER, chloroplasts etc.
Cell division occurs by fission or budding but not by mitosis Cell division occurs by mitosis or meiosis
Bacteria, blue green algaeEuglena, Amoeba, plants, animals