Levels of Cellular
Organisms
3A-2
Learning Goals:
-differentiate between unicellular and
multicellular organisms
-distinguish between tissues, organs, and
organ systems
-differentiate between colonial organisms
and organisms containing organ systems
-discuss the three types of cell junctions
Whether the organisms is a one-celled
amoeba or a gigantic sequoia tree, it is
composed of cells.
Those organisms that are composed of just
one cell are called unicellular organisms
Bacteria, as well as many protozoans,
algae, and fungi, are unicellular.
Example:
A paramecium is a unicellular organism
belonging to the kingdom Protista, so it
isn't exactly a plant or an animal. The
organism can digest food, move through
water by propelling itself with cilia, and
reproduce. As one of the oldest organisms
on earth, it has evolved and developed very
simple methods of defense, genetic
exchange, and mobility.
Multicellular organisms are made of
many cells.
Some algae and fungi are multicellular,
as are animals, humans, and plants.
A colonial organism is a collection of similar cells
living together, and except for certain specialized
reproductive cells, any of the cells could carry on
the processes of the organism if they were
separated from colony.
Colonial organisms can vary in complexity from
what appears to be a clump of cells to a highly
organized mushroom.
Sometimes unicellular or colonial organisms
are referred to as “first” or “early” organisms.
Use of these terms should alert the reader
that the writer may be writing from an
evolutionary perspective, implying that the
more complex organisms developed later as
the “early” organisms evolved.
However, these terms are not correct since
all life was created during the week of
Creation.
Most unicellular organisms are
made of cells that are organized
into tissues.
A tissue is a group of similar cells that
work together to carry out a specific
function.
The muscle cells of the body compose
tissues whose primary function is
movement.
The skin that covers and protects the body
is another example of a tissue.
Many multicellular organisms have
organs
Organs are composed of several types of tissues working together
to perform a specific function.
The stomach is an example of an organ.
It is made of muscle tissue that churns the food, a special
tissue that lines the inside of the stomach to protect it from
the acid, nerve cells that direct the movement of the stomach,
connective tissue cells that hold the stomach together, and
cells embedded in the stomach lining that produce acid and
other enzymes.
All these tissues work together to help digest the food that is
eaten.
Organ System
An organ system is made up of a group of
organs that work together to accomplish
life functions.
For example the digestive system:
The digestive system includes the
mouth, stomach, intestines, and other
organs that work together to digest
food and eliminate waste so that the
body can function properly.
However, none of the specialized systems
or tissues can survive alone.
They depend on other tissues and organ
systems to supply their needs.
So they are very interdependent
Muscles provide movement, but they depend
on the digestive system to provide the
energy needed for movement, the circulatory
system to bring them food and oxygen, and
the nervous system to tell them when and
how to move.