The document summarizes the history and key concepts of cell theory. It describes how early microscopists like Hooke, van Leeuwenhoek, and Brown discovered key cell structures and components. In 1838, Schleiden concluded that plants are made of cells, and in 1839, Schwann concluded the same for animals, laying the foundations for cell theory. Virchow later established that cells only arise from pre-existing cells. The modern cell theory states that all living things are made of cells, cells are the basic unit of life, and new cells arise from existing cells. The document also discusses cell size, shape, types (prokaryotic vs. eukaryotic), and some of the

1. cell theory, shape and size
Archana Soni
archi22ss@gmail.com

2. 1665 – Robert Hooke
Observed cells in cork.
Coined the term "cells”.
Cork Cells

3. 1673- Anton van Leeuwenhoek
Created a powerful
microscope

4. 1827-33 - Robert Brown
-noticed that pollen grains in water jiggled around
called “Brownian motion”
-discovered the nucleus
Nucleus
Human Cheek Cell

5. 1838 - Matthias Schleiden
A botanist who concluded that all plants are made of cells.
Typical Plant Cell

6. 1839 - Theodor Schwann
A zoologist who concluded that all animals
are made of cells.
Nerve Cells

7. 1855 - Rudolph Virchow
A physician who did
research on cancer cells
and concluded
“Omnis cellula e cellula”.
“All cells are from other
pre-existing cells.”

8. Cell theory
• Formally established by
Theodor Schleider and
Matthias Schwann in 1839, cell
theory (or cell doctrine) is the
most basic tenet underlying
the field of biology.
• Cell Theory is one of the basic
principles of biology. Credit for
the formulation of this theory is
given to German scientists
Theodor Schwann, Matthias
Schleiden, and Rudolph
Virchow.

9. The Cell Theory
The Cell Theory states:
•All living organisms are composed of cells. They may be
unicellular or multicellular.
•The cell is the basic unit of life.
•Cells arise from pre-existing cells. (They are not derived
from spontaneous generation.)
The modern version of the Cell Theory includes the ideas that:
•Energy flow occurs within cells.
•Heredity information (DNA) is passed on from cell to cell.
•All cells have the same basic chemical composition.

11. Cells are Diverse…
Cell Size and Shape
•The shapes of cells are quite
varied with some, such as
neurons, being longer than
they are wide and others such
as erythrocytes (red blood
cells) being equidimensional.
Cell shape varies depending
both on function and surface
area requirements.

12. The cell size is limited by the certain factors and that include surface area
to volume ratio, the nucleo-cytoplasmic ratio, the fragility of the cell
membrane and the structures that hold the cell together. Each of these
factors works in conjunction to limit the ability of the cell to support a larger
size
•Surface area to volume ratio
Cells diffuse nutrients and oxygen across their cell membrane to function
properly and obtain proper nutrition. Cells need a high surface area to
volume ratio, but the volume increases faster than surface area as cells
grow larger. Lets see the examples of formulae of simple shapes.
(where r is the length of each side of the cube)
(where r is the radius of the sphere, The diameter (d) of the sphere is twice
the radius so the above could be re-written in terms of diameter using the
relationship d=2r)
Using the above formulae, it is easy to express the ratios of surface area to
volume for these very simple shapes:
Surface area / Volume ratio for a cube = 6/r
Surface area / Volume ratio for a sphere = 3/r = 6/d

13. Why Are Cells So Small?
• Transport- Cell volume to surface area
ratios favor small size.
• Control- Nucleus to cytoplasm
consideration.
• Metabolic requirements-
……..we’ll come back to this later.

14. How small can a cell be?
Mycoplasmas - bacteria that are 0.1 to 1.0 µm.
(1/10 the size of regular
bacteria).
Note: 1.0 µm = one millionth of a meter

15. Cell Types
Prokaryotes- simple cells that do not have internal membranes
example = bacteria
Eukaryotes- more complex cells that do have internal, membrane-
bound structures
examples = plants and animals

16. Timeline
Prokaryotic
Organisms:
First appeared
3.5 BYA
include bacteria
and cyanobacteria
Eukaryotic
Organisms:
First appeared
2.0 BYA
include protists,
fungi, plants and
animals

17. Key Differences:
Prokaryotes
• Lack a nucleus and
other membrane
bounded structures.
• Have small ribosomes
• DNA is not organized
into chromosomes
• Flagella are not made of
microtubules and does
not have a 9+2 structure
• Cell walls are made of
peptidoglycan, not
cellulose
Eukaryotes
• Have a nucleus and other
membrane bounded
structures.
• Have large ribosomes
• DNA is organized into
chromosomes
• Flagella are made of
microtubules and have a
9+2 structure
• Cell walls are made of
cellulose

18. Prokaryotic Eukaryotic
Nucleus
Examples

19. Prokaryotic
Eukaryotic