Levels of Organization
Cellular Level
The simplest is the cellular level. Organisms on this level are either unicellular, like the protozoa, or colonial. Colonial organisms are composed of single-celled individuals that stay together to sustain the life of the whole colony. Examples might include the colonial algae which form seaweeds.
Tissue Level
Next on the scale of complexity is the level in which cells are arranged into tissues. A tissue is a group of similar cells which perform a unified function. Examples of tissue level of organization are muscle, fat, bone, cartilage, and blood.
Organ Level
The third level is the organ level, in which tissue is arranged into organs. Organs are groups of tissues arranged together to perform a specific function as a unit. Examples of animal organs are the kidneys, heart, stomach, and liver. Plant organs include roots, stamens, and leaves.
System Level
The fourth level of organization is the system level: Organs are arranged into groups called systems that perform a specific bodily process. Examples are the respiratory system, circulatory system, nervous system, and digestive system.
Organism Level
The fifth and highest level is the organism level, in which body systems work together in a structure capable of independent life. However, correctly speaking, all plants and animals exist at the organism level, even if they are only at the cellular level. From this circular observation, anyone can reason that even unicellular organisms are very complex. They depend upon the interrelated functions of multiple systems.
A Paradox?
Even The Smallest Building Blocks Of Life Operate At The Highest Level Of Organization
or think of it this way:
Even The Lowest Level Of Cellular Organization Operates At The Level Of An Organism.
Cell Specialization
A unicellular organism could be compared to Robinson Crusoe on his island. The cell, like Crusoe, has to do everything necessary to maintain itself. As you learned in Science 1003, the amoeba had to perform respiration, ingestion, digestion, excretion, and reproduction for itself. Robinson Crusoe had to hunt, build, doctor himself when hurt, defend himself against animal intruders, and many other things.
Division of labor. In human society, however, no one person performs all the tasks necessary to his life. We have doctors, plumbers, teachers, carpenters, mechanics, laborers, and so on to do specialized jobs. This specialization, called division of labor, leads to an increase in efficiency because people doing the jobs are experienced at one particular task. This concept is true also of multicellular organisms. No one cell does all the things necessary to sustain the life of the organism. The cells perform one or a few tasks and depend on other cells to provide them with their needs. This division of labor among cells is called cell specialization. Unicellular organisms like the Amoeba are not specialized but perform every task needed for life.
Examples of specialized cells. Nerve cells, blood cells, and muscle cells are examples of cells with specialized structures and functions.
Nerve cells are called neurons, and are responsible for the sensory function of the organism. Neurons make up not only the nerves but also the brain and spinal column, transmitting the impulses within the nervous system. A neuron from the brain looks something like a tall, slender tree with many branches. It has a cell body as well as dendrites, which transmit impulses from other neurons to the cell body. Extending from the cell body is an axon, a long thread-like structure that transmits impulses away from the cell body. Neurons in different parts of the nervous system have different forms of this basic structure.
Blood cells are of two types: red blood cells and white blood cells. Red blood cells, or erythrocytes, contain a red pigment called hemoglobin. This pigment enables the erythrocytes to carry oxygen to all the tissues of the body where it is used to release energy. Unlike other types of cells, the erythrocyte does not contain a nucleus. Red blood cells are concave on both sides because of the absence of a nucleus.
Erythrocytes
White blood cells, or leukocytes, do not contain hemoglobin but do contain nuclei. The function of leukocytes is to protect our bodies from disease. Some leukocytes eat bacteria by phagocytosis, just as the amoeba does. Others produce chemical substances called antibodies which attack material foreign to our bodies. Some leukocytes have a many lobed nucleus.
Leukocytes
Sometimes leukocytes eat so many bacteria that they die from overeating and form either the pus in a pimple or the white substance from an infected wound.
The skeletal muscle cell is often called a muscle fiber because it is quite long. It has cross-bands transversing the cell, which enable it to contract. A muscle cell contains many nuclei.
Skeletal Muscle Cell
The body contains two types of muscle cells besides the skeletal muscle type. One type makes up the heart, the other type lines internal cavities like the intestine. Heart and intestinal muscle cells differ in structure from skeletal muscle cells and work automatically without our conscious direction.
God's design. You have seen in this unit that the living things that God created are made of units called cells. You have considered the several transport mechanisms for feeding and flushing the cell, and the small structures within the cell called organelles. You differentiated between animal and plant cells. These scientific observations, along with many others, support the theory of creationism.
This view differs radically from the evolutionist view, which says matter organized itself, according to the physical laws of the universe and probability. By this view, the cell evolved from chemical compounds on a primitive earth. As you study the chemistry of life further, you will learn that biological organic chemicals cannot form spontaneously whether or not the "primitive" atmosphere of the earth contained oxygen.
The biological chemicals must be synthesized by pre-existing complex chemical
laboratories under the instruction of DNA - otherwise called "organisms".
Once these organic chemicals are produced they can remain stable only within an active "living" cell. Once the cell dies, organic chemicals self-destruct or are decomposed by natural reactions in the earth's environment.
