robert hooke 1665 coined the word `cell’ for the box-like structures of cork
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Robert Hooke 1665
coined the word `cell’ for the box-like structures of cork
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M J Schleiden a lawyer turned botanist &
Theodore Scwann
an animal embryologist
met at dinner party in 1837
•1839 formally espoused cell theory
•the cells are of universal occurrence and are
• the basic units of an organism
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1849 Cell division described
1858 Rudolph Virchow forcefully states the paradigm of cell theory
``All cells come from previous cells’’
Omnis cellula e cellula
Thus:
all individuals derived from a single celled organism are related &
all cells in a multi-celled organism are descended from one fertilized egg.
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Basics of the Cell Theory
• all known living things (systems) are composed of cells
• cells arise only from other cells (not spontaneously - experiments of Redi and Pasteur)
• all energy flow (metabolism & biochemistry) of life occurs within cells
• cells contain DNA as genetic info - (for subsequent generations)
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Some important facts about cells:
- their diversity : many sizes, shapes, and types (165+ in humans)
- their similarity: have same basic structural plan
- surrounded by cell membrane
- contain nuclei (bacteria = genophore)
- similar sub-cell parts (ORGANELLES)
- all contain same macromolecules
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Different Shapes & Forms
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Scanning electron micrograph of human
red blood cellsSickle cells
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Why are cells so small?
a. Efficiency: smaller cells have larger surface area
b. Specialization: Having numerous small cells permits specialization
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•As an object increases in size its volume increases as the cube of its linear dimensions while surface area increases as the square
Surface area = L X B Square of two linear dimensions
Volume = L X B X H Cube of three linear dimensions
•As these cubes illustrate the surface area to volume ratio of a small object is larger than that of a large object of similar shape
•This ratio limits how large cells can be
How large cells can be?
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Getting around this problem
Divide to become multicelled
- Embryo
long and thin or skinny and
flat
many protists, nerve cells, muscle cells
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Smaller Particles React Faster Than Larger Particles
• Granular sugar dissolves easier than sugar cubes• Everything that the cell needs or has to get rid
of has to go through the cell membrane, the amount of which relates to the surface area
• Surface must allow sufficient exchange to support the contents of the cell
• stops growing when its surface area is insufficient to meet the demands of the cell's volume
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Cells Have Evolved Two Basic Architectural Plans
Cells without a nucleus Cells with a nucleus
“karyon” = kernel or nut; “pro” = before ; “eu” = good or true; • Eukaryotes - “cells with a true nucleus”
• Prokaryotes - “cells with no defined nucleus”
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Prokaryotes
• Cells whose nucleus is not enclosed in a membrane
• Lack other organelles too• First to evolve, The oldest accepted prokaryote fossils
date to 3.5 billion years
• Containa circular chromosome
Plasmids with satellite DNARibosomesfood storage granules
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Prokaryotic cell-Bacterium
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A general prokaryotic cell
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Eukaryotes• Cells whose nucleus is enclosed in a membrane
• Evolved later. Eukaryotic fossils date to between 750 million years and
possibly as old as 1.2-1.5 billion years.
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Fine structure of an animal cell- based on electron microscope studies
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Eukayotic cell as seen with an electron microscope
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Organelles
• Mitochondria and chloroplasts are energy-transducing organelles
• Both seem to share many properties with bacteria: – contain 70S ribosomes (whereas rest of
eukaryote cells contain 80S ribosomes)– divide by binary fission– contain circular DNA without nucleus, etc.
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Endosymbiont Theory Symbiosis = "living together“
• Chloroplasts and Mitochondria derived from ancient colonization of large bacteria (became the eukaryotic cell) by smaller bacteria
• Host cells acquired respiration from the precursor of the mitochondrion, and oxygenic photosynthesis from the precursor of the chloroplast
• Also acquired much of their genetic information• Eventually, organelles lost ability to exist as separate
organisms, cannot have independent existence from cell
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Some protists
E
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d
o
S
Y
m
b
I
o
si
s
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Why double membrane of chloroplast?
Outer membrane of organelle
originally belonged to the host
cell.
Invagination
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Why double membrane?
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Images with electron microscope
Mitochondrium
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Cutaway view of mitochondrion
Inner membrane more active
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Centrifugation
• A centrifuge is a device for separating particles from a solution according to their size, shape, density, viscosity of the medium and rotor speed
• In biology, the particles are usually cells, sub cellular organelles, viruses, large molecules such as proteins and nucleic acids
• Instrumental in working out the functions of organelles
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Centrifuge