Cells: The Unit of Life

Introduction

The cell is the basic structural and functional unit of all living organisms. It is the smallest part of the body of an organism, capable of independent existence and able to perform all the essential functions of life. Cells vary in number (singlecelled: Amoeba; few-celled: Spirogyra, multi-celled: Man), size (smallest: Bacteria, longest: Nerve cell, largest: Ostrich egg) and shape (columnar: Epithelial cells).


The history of cell science began in 1665, with the observation of a thin section of a bottle cork by the English scientist Robert Hooke.


In 1838, Matthias Schleiden and Theodor Schwann proposed a basic 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.
  • Also Read Tissues: Plant Tissues and Animal Tissues

Structural Organization Of a cell

Cytoplasm

NATURE AND
OCCURRENCE
Content inside the
plasma membrane,
excluding the nucleus.
MAIN CHARACTERISTICSContains a mixture of water and soluble organic and inorganic compounds and various cell organelles.
MAIN FUNCTIONSa. Centre of all metabolic activities.
b. Seat of occurrence of glycolysis (production of pyruvic acid).

Endoplasmic reticulum (ER)

NATURE AND
OCCURRENCE
Irregular network of
tubular double
membrane.
MAIN CHARACTERISTICSMay be smooth (SER) or rough (RER).
MAIN FUNCTIONSa. Supportive framework of the cell.
b. RER synthesises proteins, while
SER secretes lipids.

Mitochondria

NATURE AND
OCCURRENCE
Have varied shapes, but usually are sausage-like.
MAIN CHARACTERISTICSa. Double-walled, inner wall thrown into folds, called
cristae.
b. Contain their own
ribosomes and DNA, with several genes.
MAIN FUNCTIONSa. Seat of aerobic respiration.
b. Synthesis of respiratory enzymes
and energy-rich compounds.

Golgi Apparatus (In animal cells)
Dictyosomes (In plant cells)

NATURE AND
OCCURRENCE
Stacks of flattened
membrane sacs.
MAIN CHARACTERISTICSConsists of a set of
membrane bounded, fluid-filled vesicles and vacuoles.
MAIN FUNCTIONSa. Synthesis and secretion of enzymes, hormones etc.
b. Involved in the synthesis of plasma membrane, cell wall etc.

Ribosomes

NATURE AND
OCCURRENCE
Dense, spherical,
small granules, either scattered in the cytoplasm or attached to the outside of ER.
MAIN CHARACTERISTICSa. Single-walled, dense, spherical bodies composed
mainly of RNA and proteins.
b. Not bounded by a membrane.
c. Two types: 70S and 80S
MAIN FUNCTIONSSynthesis of proteins.

Lysosomes

NATURE AND
OCCURRENCE
Simple, tiny, single
membrane bound
sacs.
MAIN CHARACTERISTICSContains 40 different types of enzymes.
MAIN FUNCTIONSa. Intracellular digestion.
b. Destroy foreign substances.

Centrosome (In animal cells only)

NATURE AND
OCCURRENCE
Region surrounding
the centrioles, located near nucleus.
MAIN CHARACTERISTICSContains one or two
centrioles that are
surrounded by radiating
microtubules to form an aster shape.
MAIN FUNCTIONSInitiates and regulates cell division.

Plastids (In plant cells only)

NATURE AND
OCCURRENCE
a. Have their own
genome.
b. Self-replicating
organelles.
MAIN CHARACTERISTICSa. Double membrane,
proteinaceous matrix containing DNA.
b. Possess disc-like
structures called thylakoids, containing chlorophyll.
MAIN FUNCTIONSa. Chromoplasts: Impart colour to
flowers and fruits.
b. Chloroplasts: Trap solar energy for photosynthesis.
c. Leucoplasts: Store starch.

Nucleus

NATURE AND
OCCURRENCE
Centrally located
spherical cellular
component.
MAIN CHARACTERISTICSa. Mostly spherical and dense.
b. Surrounded by nuclear membrane with pores.
MAIN FUNCTIONSa. Regulates cell functions.
b. Regulates cell cycle.

Nucleolus

NATURE AND
OCCURRENCE
Embedded within
the nucleus of the cell.
MAIN CHARACTERISTICSa. One or more in number.
b. Round-shaped.
MAIN FUNCTIONSa. Produces ribosomes.
b. Participates in proteins synthesis
by forming and storing RNA.

Chromatin fibres

NATURE AND
OCCURRENCE
Embedded within
the nucleus of the cell.
MAIN CHARACTERISTICSNetwork of thread-like structures, made up of DNA.
MAIN FUNCTIONSChromosomes carry hereditary information or genes.

Vacuoles

NATURE AND
OCCURRENCE
Fluid-filled or solid-filled and membrane-bounded spaces.
MAIN CHARACTERISTICSa. Non-living structures.
b. Larger in plant cells and fewer and smaller in animal
cells.
MAIN FUNCTIONSa. Storage of water and other substances, food, pigments and
waste products.
b. Gives turgidity to the cells.
c. Helps to maintain an osmotic pressure in a cell.

Granules

NATURE AND
OCCURRENCE
Non-living
structures.
MAIN CHARACTERISTICSSmall particles, crystals or droplets.
MAIN FUNCTIONSStarch (in plant cells), glycogen
(in animal cells) and fat-containing
granules serve as food for the cell.

Differences between plant and animal cells

Structural differences:

FEATUREPLANT CELLANIMAL CELL
Cell wallPresence of a definite cell wall, made up of celluloseAbsence of a cell wall
Cell membranePresent internal to the cell wallForms the boundary of the cell
CentrosomeAbsence of centrosomePresence of centrosome
CentrioleAbsence of centriolePresence of centriole
VacuolePresence of one or more
prominent vacuoles
Presence of small and
temporary vacuole
PlastidPresence of plastidsAbsence of plastids

Functional differences:

FEATUREPLANT CELLANIMAL CELL
SizeUsually larger with distinct
outlines
Usually smaller with less distinct boundaries
CytoplasmNot so denseDenser and more granular
Arrangement of
cytoplasm
Only a thin lining of
cytoplasm which is mostly
pushed to the periphery
Cytoplasm fills up almost
the entire cell

Differences between prokaryotic and eukaryotic cells

FEATUREPROKARYOTIC CELLEUKARYOTIC CELL
NucleusAbsence of a well-defined
nucleus
Presence of a well-defined
nucleus with a nuclear
membrane
NucleolusAbsentPresent
Genetic materialPresence of a single length of
only DNA
Presence of several lengths of DNA, wound around certain protein
RibosomesPresence of smaller
ribosomes
Presence of larger
ribosomes
Cell organellesAbsence of other cell
organelles
Presence of several other
cell organelles such as
mitochondria, ER, chloroplasts etc
Cell divisionCell division takes place by
fission or budding, but not by mitosis
Cell division takes place by
mitosis or meiosis
ExamplesBacteria, blue green algaeEuglena, Amoeba, plants, animals
  • All organisms grow due to the growth in size and increase in the number of cells. This growth is because of the production of more body substance and cell substance.
  • Repair of an injury or regeneration of a lost part is because of cell division.
  • Movement of the body is because of contractility of the cells or cellular parts.
  • Feeding and nutrition have many steps, and each step is a result of cellular activities.
  • Circulation of blood as well as the movement of other fluids in the body takes place through various forces that are set up due to the contraction of muscle cells of the heart and other parts of the body.
  • Respiratory gases are transported from the lungs to different parts of the body by blood cells such as RBCs.
  • Protection of the body from different germs and diseases occurs through cells called white blood cells (WBCs). These cells either devour the germs or give out antibodies or antitoxins, which destroy the germs and neutralize their effect.
  • The sensation of touch, pain, heat, cold etc. occurs due to sensory cells. The brain orders the muscles to contract or glands to secrete through their cells.
  • Thermoregulation or maintenance of our body heat also takes place through cellular activity.
  • All living organisms reproduce through cellular activity.
  • In plants, transportation takes place due to cellular activity

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