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Cell Theory
The CELL THEORY, or cell
doctrine, states that all
organisms are composed of
similar units of organization,
called cells.
The concept was formally
articulated in 1839 by Schleiden
& Schwann and has remained as
the foundation of modern biology.
The modern tenets of the
Cell Theory include:
1. all known living things are made up
of cells.
2. the cell is structural & functional
unit of all living things.
3. all cells come from pre-existing
cells by division.
(Spontaneous Generation
does not occur).
4. cells contains hereditary
information which is passed
from cell to cell during cell
division.
5. All cells are basically the
same in chemical composition.
6. all energy flow (metabolism
& biochemistry) of life occurs
within cells.
Robert Hooke
In 1663 an English scientist, Robert
Hooke, discovered cells in a piece of
cork, which he examined under his
primitive microscope.
Hooke only observed cell walls
because cork cells are dead and
without cytoplasmic contents.
Hooke drew the cells he saw and also
coined the word CELL.
The word cell is derived from the
Latin word 'cellula' which means
small compartment.
Anton van Leeuwenhoek
Ten years later Anton van
Leeuwenhoek (1632-1723), a
Dutch businessman and a
contemporary of Hooke used his
own (single lens) monocular
microscopes and was the first
person to observe bacteria and
protozoa
Leeuwenhoek's skill at grinding
lenses, together with his naturally
acute eyesight and great care in
adjusting the lighting where he
worked, enabled him to build
microscopes that magnified over
200 times, with clearer and
brighter images than any of his
colleagues at that time.
Two Major Classes of Cells
One kind—a prokaryotic cell (pro
KAR ee oh tik)— lacks a nucleus
and most other organelles.
Bacteria and another group of
organisms called the archaea are
prokaryotic cells. Prokaryotic
organisms appear earliest in Earth's
fossil record.
A bacterium is an example of a
prokaryotic cell (pro means
"earlier than").
Without a true nucleus and the
organelles of eukaryotic cells,
prokaryotic cells are much
simpler in structure
The DNA in a prokaryotic cell is
concentrated in an area called
the nucleoid region:
which is not separated from
the rest of the cell by a
membrane, as is the case in a
eukaryotic cell
Eukaryotic cell (yoo KAR ee oh
tik) has a nucleus surrounded by
its own membrane, and has other
internal organelles bounded by
membranes.
Protists, fungi, plants, and animals
consist of eukaryotic cells.
Organisms with eukaryotic cells
appeared later in Earth's history.
Membrane Structure
• Membranes help keep the functions
of a eukaryotic cell organized. As
partitions, the membranes isolate
teams of enzymes within a cell's
compartments.
• Membranes, unlike walls, regulate
the transport of substances across
the boundary, allowing only certain
substances to pass.
plasma membrane
• The plasma membrane and other
membranes of a cell are
composed mostly of proteins and
a type of lipid called
phospholipids
A phospholipid molecule is
structured much like the fat
molecules but has only two fatty
acids instead of three.
The two fatty acids at one end
(the tail) of the phospholipid are
hydrophobic.
The other end (the head) of the
molecule includes a phosphate
group, which is negatively
charged and hydrophilic.
So the tail end of a phospholipid
is pushed away by water, while
the head is attracted to water
The structure of phospholipids
allows them to form boundaries,
or membranes, between two
watery environments.
The phospholipids form a twolayer "sandwich" of molecules,
called a phospholipid bilayer, that
surrounds the organelle or cell.
Many types of proteins are
embedded in the membrane's
phospholipid bilayer.
Other molecules, such as
carbohydrates, may be attached to
the membrane as well, but the
proteins perform most of the
membrane's specific functions.
Another function of membrane
proteins is to help cells—especially
cells that are part of a multicellular
organism—communicate and
recognize each other.
Example, chemical signals released
by one cell may be "picked up" by
the proteins embedded in the
membrane of another cell.
Still other membrane proteins,
called transport proteins, help
move certain substances such as
water and sugars across the
membrane
Diffusion
Molecules in a fluid are constantly in
motion, colliding and bouncing as
they spread out into the available
space.
One result of this motion is diffusion,
the net movement of the particles of
a substance from area of high
(more) concentrated to where they
are low (less) concentrated
Passive Transport
Cellular membranes are barriers to
the diffusion of some substances. A
selectively permeable membrane
allows some substances to cross the
membrane more easily than others
and blocks the passage of some
substances altogether.
Diffusion across a membrane is
called passive transport because
no energy is expended by the cell in
the process.
Only the random motion of the
molecules is required to move them
across the membrane.
Osmosis
The passive transport of water
across a selectively permeable
membrane is called osmosis (ahs
MOH sis).