Living Organisms
October 5, 2011
CELLS
A cell is a self contained basic unit of all living organisms. Organisms can consist of a single cell or
trillions of cells. Each cell can take in nutrients and convert to nutrients to energy. The energy is used to
help the cell carry out a wide variety of specialized functions such as reproducing.
Cells can be classified into two categories prokaryotic and eukaryotic. Prokaryotic cells are structurally
simple only found in single celled organisms. Eukaryotic cells have their own membranes and are found
in single celled and multi-cellular organisms.
Eukaryotic Cells
Eukaryotic cells (from the Greek meaning
truly nuclear) comprise all of the life
kingdoms except monera. They can be easily
distinguished through a membrane-bound
nucleus.
Eukaryotic cells also contain many internal
membrane-bound structures called organelles.
These organelles such as the mitochondrion or
chloroplast serve to perform metabolic functions
and energy conversion. Other organelles like
intracellular filaments provide structural support
and cellular motility.
Plant Cells
Another important member of the eukaryote
family is the plant cell. They function essentially
in the same manner as other eukaryotic cells, but
there are three unique structures which set them
apart. Plastids, cell walls, and vacuoles are
present only in plant cells.
Diagram of a Plant Cell
Diagram of an Animal Cell
Centrioles are a pair of structures composed of microtubules. The primary function of centrioles is to generate the cell's
cytoskeleton.
Ribosomes perform precisely the same function in eukaryotic cells as they perform in prokaryotic cells. Eukaryotic
ribosomes are larger and more complex than prokaryotic ribosomes, but they are very similar.
The nucleus of the cell is a chamber specialized in DNA functions. It's enclosed by a double layer of membrane called the
nuclear envelope. The function of the nuclear envelope is to confine the materials necessary for DNA and RNA synthesis
inside the nucleus, and controlling movement into and out of the nucleus.
Mitochondria are very complex, double-membrane-bound organelles. Their function is to perform the aerobic portions of
aerobic cellular respiration, the essential energy-producing process of the cell. This is the same function performed by the
mesosomes in many prokaryotic cells. Mitochondria contain their own naked, circular DNA and their own ribosomes.