General characteristics of Cells

The cell is an amazingly complex structure.  It has a control center, internal transportation system, power plants, factories for making needed materials, and packaging plants.

Cells, the building blocks of the body, have specialized organelles that carry out specific functions.

Most organelles are dispersed within the cytoplasm which is the jellylike material of the cell.

Most cells are very small and must be studied with microscope.  The microscope is one of the biologist's most important tools for studying the internal structure of cells.  Most cell structures were first identified with an ordinary light microscope, which uses visible light as the source of illumination.  The development of the electron microscope, which came into widespread use in the 1950s, enabled researches to study the fine detail (ultra structure) of cells and their parts.  The electron microscope uses an electron beam of energized electrons.  Photographs taken with the light microscope are referred to as light micrographs (LMs) and photographs taken with the electron microscope are referred to as electron micrographs (EMs) .

The size and shape of the cell are related to the specific functions it must perform.  For example, sperm cells are tiny cells with long, whiplike tails (called flagella) .  The tail is used to move towards the ovum or egg.  The ovum is one of the largest cells in the human body.  Epithelial cells which look like little building blocks cover body surfaces and line body cavities.  Muscle cells are elongated and specialized for contraction.  Nerve cells have long extensions that permit them to transmit messages over long distances within the body.  Lymphocytes, a type of white blood cells, change their shape as they move through the tissues of the body, destroying invading bacteria.

Levels of organization within the human body

The body is highly organized.The simplest level of organization is the chemical level,consisting of atoms and molecules.All matter is composed of chemical elements -pure chemical substances like iron,calcium or oxygen.About 98% of the human body is composed of 6 elements-oxygen,carbon,hydrogen,nitrogen,calcium and phosphorus.An atom is the smallest amount of a chemical element that has the characteristic properties of that element.Atoms can combine chemically to form molecules.For example,two atoms of hydrogen chemically combine to form one molecule of water.

Atoms and molecules associate to form the organelles and cells(building block of the body)of the cellular level.The human body is composed of about 100 trillion cells of many types,such as bone cells,muscle cells and blood cells.Cells vary in size and shape according to their function,most of them are so small that they can be seen only with a microscope.Each cell consists of specialized cell parts called organelles.One organelle,nucleus,serves as the information and control centre of the cell.Several other kinds of organelles scattered throughout the cell perform specific functions like breaking down fuel molecules to provide energy and manufacturing needed substances.

Cells associate to form tissues such as muscle or bone tissue.A tissue is a group of closely associated cells specialized to perform particular functions.The four main types of tissues are muscle tissue,nervous tissue,epithelial tissue and connective tissue.

Tissues may be organized to form organs such as the brain or heart.The heart consists mainly of muscle tissue but it is covered by epithelial tissue and also contains connective and nervous tissue.

Certain tissues and organs function together to make up an organ system or a body system.For example,the circulatory system consists of the heart,blood vessels,blood,lymph structures and several other organs.

The body systems work together to make up the human organism.