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What are Cells?
 Cells are the basic unit of life.
History of Cells & Cell Theory
Who Was Involved?

The cell theory grew from the work and
research of many scientists.
First to View Cells
• In 1665, Robert
Hooke was the first
to use a light
microscope to
examine a thin slice
of cork (dead plant
tissue)
• What he saw looked
like small boxes
Hooke's Microscope
Hooke's drawing of Cork Cells
First to View Cells
• Hooke is responsible
for naming cells.
• Hooke called them
“CELLS” because
they looked like the
small rooms that
monks lived in
(called cells).
Anton van Leeuwenhoek
(LAY-vuhn-hook)
• Dutch microscope maker.
• One of the first people to
use a microscope to study
living things in nature &
was the first to view
microorganisms .
• Leeuwenhoek used a
simple, handheld
microscope to view pond
water & scrapings from
his teeth.
• Observed living cells, which
he called “animalcules.”
Beginning of the Cell Theory
• In 1838, a German
botanist named
Matthias Schleiden
concluded that all
plants were made
of cells
• Schleiden is a
cofounder of the
cell theory
Beginning of the Cell Theory
• In 1839, a German
zoologist named
Theodore Schwann
concluded that all
animals were made
of cells.
• Schwann also
cofounded the cell
theory.
Beginning of the Cell Theory
• In 1855, a German
medical doctor named
Rudolph Virchow
observed, under the
microscope, cells dividing.
• He reasoned that all
cells come from other
preexisting cells by cell
division.
Three Principles of Cell Theory
Cell Theory
1. All living things are made
of one or more cells.
2. Cells are the basic unit of
structure and function in
an organism (they are the
smallest unit that can
perform life functions).
3. Cells come from the
reproduction of existing
cells (cell division).
Why is the Cell Theory called
a Theory and not a Fact?
Principle #1

All organisms are made of one or more cells.
• Some organisms are made up of one cell or
unicellular
• Others are multicellular or made up of many cells
Principle #2
 The cell is the most basic unit of life.
Principle #3

All existing cells are produced by
other living cells.
Characteristics of Cells
1. They are flexible in function
•
•
Become different types of tissues and material
Ex: fetal development
2. They can let things in and out
•
•
Communication
Getting nutrients, energy
3. They have their own command center
•
DNA tells cells what to do
4. They have the ability to divide and make more
•
Ex: growth & reproduction
Cell Size
Cells are small so they
can exchange materials
with their surroundings.
Surface area relative to the volume decreases as
size of cell increases.
- limits the size of cells
Cells need a large surface area to absorb enough
nutrients.
Factors Affecting Cell Size
Surface area = 24
• Volume of a cell is
Volume = 8
determined by
multiplying length
times width times
2
height (L x W x H).
• Surface area (cell
2
membrane) is the total
2
area (L x W) of all sides
(the entire surface of
the object).
4
• Therefore, Volume
increases FASTER
than the surface area.
Surface area = 96
(only 4 times greater)
Volume = 64
(8 times greater)
4
4
Cell Size
• When the surface area is no longer
great enough to get rid of all the
wastes and to get in enough food
and water, then the cell must divide.
• Therefore, all of the cells in an
organism are close in size.
Cell Size
Question:
Are the cells in
an elephant
bigger,
smaller, or
about the same
size as those in
a mouse?
Cell Size
Answer:
The elephant’s
cells are about
the same size
as those of the
mouse, but the
elephant has
MANY MORE
cells than the
mouse.
Sizes of living things