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CELL THEORY & CELL
DIVERSITY
An intro to the CELL!
What is a cell?
• (Def) the smallest unit
of life that can carry out
all the functions of a
living organism
The Cell Theory:
1. Cells are the basic units of all life.
2. All organisms are made of one or more cells.
3. All cells arise from pre-existing cells.
#1 Cells are the basic unit of life.
• All organisms are composed of one or more
cells.
– Reminder:
• One cell = unicellular
• Many cells = multicellular
– each cell is capable of carrying out all of the
functions of living organisms
– EXCEPTION to this theory - viruses*
#2 All organisms are made of one or more
cells.
• UNICELLULAR organisms are singlecelled, independently living cells
• I.e… bacteria, algae, yeasts & protozoa
•
http://videos.howstuffworks.com/hsw/12340-the-basic-units-of-life-unicellular-organisms-video.htm
MULTICELLULAR organisms are composed of
many cells, each type differentiated to perform a
specific function essential to the organism
 I.e…plants and animals
#3 All cells arise from existing cells.
• BIOGENESIS – “life comes from existing life”
• By means of either asexual or sexual reproduction
How did we discover the cell?
• Hint: remember cells
are MICROSCOPIC…
• the discovery & study
of cells progressed with
the invention &
improvement of the
MICROSCOPE!
• Let’s learn the basics of
the microscope .
CELL DIVERSITY
• Although all organisms are composed of cells,
there is great variety amongst the different
types of cells in terms of…
• SIZE
• SHAPE
• INTERNAL ORGANIZATION
•Although most cells are microscopic,
they vary greatly in SIZE!
• A human egg cell (oocyte),
seen here with red blood cells,
is the smallest cell in the human
body.
• An ostrich egg is the largest
animal cell.
The size, or volume, of all cells is determined by
the cell membrane.
The SHAPE of any particular cell is determined
by that cell’s specific function.
Muscle cells are elongated so
that they can contract.
Nerve cells have any long projections
which allow them to carry impulses to
and from the brain.
Red blood cells are circular
discs, allowing them flow easily
through narrow blood vessels.
Cells differ in what organelles they have inside
of them (aka: INTERNAL ORGANIZATION.)
• Organelles – structures that perform specific
functions within a cell
• PROKARYOTIC cells vs
EUKARYOTIC cells
• Plant cells vs Animal Cells
• Cells vs. Viruses
PROKARYOTIC
CELLS …
• are believed to be the first cells
– primitive
• are smaller than eukaryotic cells
• are simpler (less complex) than
eukaryotic cells
– lack most membrane-bound
organelles
• Contain genetic material (DNA)
but lack a true nucleus
• Examples: bacteria
EUKAYROTIC
CELLS…
• Evolved from prokaryotic cells
• Are generally larger than
prokaryotic cells
• Are more complex than
prokaryotic cells
– they have differentiated
organelles
• Have genetic material contained
in a true nucleus
• Examples: amoeba, skin cell
Plant and animal cells have
different organelles because
they have different methods of
gaining energy….
AUTOTROPHS – organisms that produce their own energy
I.e…plants, some bacteria and algae
HETEROTROPHS – organisms that consume other
organisms / chemicals as sources of energy
I.e…animals, fungi and some bacteria
So what is a virus then?
• Virus - a core of nucleic
acid (either DNA or RNA)
surrounded by a protein
coat
– Living?
• They can only reproduce
within a host cell.
– Using all of the cell’s energy
and resources!
• Outside of a host, virus are
inactive and cannot grow or
reproduce.