• Anton Leeuwenhoek invented
the microscope in the late
1600’s, which first showed that
all living things are composed
of cells. Also, he was the first
to see microorganisms.
• Light microscopes have a
magnification of more than
about 2000-fold does not
improve what you can see.
• Electron microscopes use
electrons instead of light. The
short wavelength of electrons
allows magnifications much
better than visible light.
The Cell Theory
• Use of the microscope for 150 years or so led to these
basic beliefs about cells:
• 1. All living things are made of cells.
• 2. The cell is the smallest unit of life.
• 3. All cells arise from pre-existing cells.
Basic Cell Organization
• All cells contain:
– 1. cell membrane that keeps the
inside and outside separate.
– 2. DNA-containing region that
holds the instructions to run the
processes of life.
– 3. Cytoplasm: a semi-fluid region
containing the rest of the cell’s
• Prokaryotes: (bacteria): simple
cells with no internal membrane-
bound structures. DNA is in a
special region of the cytoplasm.
• Eukaryotes: complex cells with
internal membranes. DNA is in a
nucleus separated from the
cytoplasm by a membrane.
The basic problem is surface-to-volume ratio. All food and oxygen has to come in through the
cell’s surface. As size increases, you get less surface area to support a given volume of cell
For example, if the cell’s diameter increases: 1-2-3-4-5, its surface area increases 1-4-9-16-25,
and its volume increases 1-8-27-64-125. A 5-fold increase in diameter cuts the amount of surface
area per volume to 1/5 of the original: the cell starves.
Also, consider how a cell responds to a change in the environment: the signal must travel from the
surface of the cell to the nucleus, then the nucleus issues new instructions to deal with the
situation. The instructions must reach all parts of the cell. The bigger the cell, the longer it takes
to respond to the envi