Robert Hooke 1665

coined the word `cell’ for the box-like structures of cork

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

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.

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)  

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

Different Shapes & Forms

Scanning electron micrograph of human

red blood cellsSickle cells

Why are cells so small?

a. Efficiency:  smaller cells have larger surface area

b. Specialization: Having numerous small cells permits specialization

•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?

Getting around this problem

Divide to become multicelled

- Embryo

long and thin or skinny and

flat

many protists, nerve cells, muscle cells

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

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”

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

Prokaryotic cell-Bacterium

A general prokaryotic cell

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.

Fine structure of an animal cell- based on electron microscope studies

Eukayotic cell as seen with an electron microscope

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.

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

Some protists

E

n

d

o

S

Y

m

b

I

o

si

s

Why double membrane of chloroplast?

Outer membrane of organelle

originally belonged to the host

cell.

Invagination

Why double membrane?

Images with electron microscope

Mitochondrium

Cutaway view of mitochondrion

Inner membrane more active

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

Centrifuge