Cells are the basic unit of life in all living organisms.
Unicellular
organisms (e.g.,
Amoeba, Bacteria): A single cell performs all life functions.
Multicellular
organisms (e.g.,
Plants, Animals): Composed of specialized cells working together.
Seven
Basic Properties of Life
Movement
Sperm
cells move with flagella.
White
blood cells move through the bloodstream.
Organelles
also move within cells.
Nutrition
Cells
obtain nutrients to produce energy, build structures, and run biochemical
reactions.
Respiration
Cells
break down glucose to release ATP, the energy currency of the cell.
Excretion
Waste
products are removed by diffusion and active transport to prevent
toxicity.
Homeostasis
Cells
maintain a stable internal environment by controlling the movement of
substances across membranes.
Growth
Cells
grow by absorbing nutrients and converting them into cellular components.
Reproduction
Mitosis: Produces identical daughter cells
for growth and repair.
Meiosis: Produces gametes (sperm and eggs)
for sexual reproduction.
3.2
– Cell Theory
Early
Discoveries
1665
– Robert Hooke
Observed
cork under microscope.
Discovered
“little boxes” and named them cellulae (cells).
1673
– Anton van Leeuwenhoek
Improved
microscope and observed living cells in pond water ("animalcules").
1809
– Jean Baptiste de-Lamarck
Observed
cells in animals and plants.
1831
– Robert Brown
Discovered
the nucleus.
1838
– Matthias Schleiden
Concluded
all plants are made of cells.
1839
– Theodor Schwann
Concluded
all animals are made of cells.
1855
– Rudolf Virchow
Proposed
“All cells come from pre-existing cells.”
1862
– Louis Pasteur
Experimentally
proved Virchow’s idea.
Three
Points of Cell Theory
All
living organisms are composed of one or more cells.
The
cell is the basic unit of structure and function in organisms.
All
cells come from the division of pre-existing cells.
Validation
of Cell Theory
Microscopy: Light and electron microscopes
show cells are structural units of life.
Cell
Imaging & Genetics:
Track cell replication → confirm new cells arise from existing cells.
DNA
Studies: Show
cells share genetic material and metabolic pathways.
Cell
Culture & Tissue Engineering:
Show cell growth, division, and differentiation.
Exceptions
to Cell Theory
Viruses,
Prions, Viroids
Not
made of cells.
Cannot
reproduce independently → need host cell.
Mitochondria
& Chloroplasts
Have
their own DNA and can replicate independently.
Suggest
endosymbiotic origin.
Multinucleated
Structures
Certain
fungi and algae have many nuclei in shared cytoplasm.
Muscle
Cells (Myocytes)
Can
fuse to form multinucleated fibers, challenging the concept of a
single-cell unit.
Comparison: Cell Theory vs Exceptions
|
Aspect |
Cell Theory (General Rule) |
Exceptions / Special Cases |
|
Living organisms |
All living things are made up of
one or more cells. |
Viruses, prions, and viroids are
not made of cells but show some life-like properties. |
|
Basic unit of life |
Cell is the basic structural and
functional unit of life. |
Multinucleated organisms (some
fungi, algae) and muscle fibers challenge this concept. |
|
Origin of cells |
All cells come from pre-existing
cells (Virchow’s principle). |
Viruses do not arise from cells
but need a host cell for reproduction. |
|
Organelles |
Organelles function under the
control of the nucleus. |
Mitochondria and chloroplasts
have their own DNA and replicate independently (endosymbiotic theory). |
|
Cell boundaries |
Each cell is usually a separate
compartment with its own membrane. |
Syncytial structures (shared cytoplasm with multiple nuclei) blur the concept of individual cells. |
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