Levels of Complexity forming the Human Body: Molecules, Cells, Tissues, Organs

Level of detail:

Definition / Description:

Example(s):

Elements

Knowledge of basic science includes the distinction between atoms and molecules, and elements, mixtures and compounds.

"Elements" are chemical substances that include only one type of atom (which used to be considered the most basic "building block" of matter - until physicists "split the atom" !).
"Mixtures" and "Compounds" include more than one type of atom, the difference being that "mixtures" can include discrete (not attached together) combinations of atoms and molecules, which can therefore often be easily separated. "Compounds" consist of at least two different type of atoms that have joined together via chemical bonds to form molecules. (Some molecules are formed from atoms of the same type, e.g. the gaseous molecules of oxygen (O₂) and nitrogen (N₂) but unless different atoms are involved, the molecule is that of an "element", not a "compound".

Other scientific disciplines, such as particle physics, are interested in atoms and sub-atomic particles (that is the even smaller parts that, together, form atoms). Biologists and health scientists usually study structures from the size of molecules (i.e. "molecular scale") upwards in size.

The human body needs a wide range of specific elements, which are sometimes called "Minerals". The different elements have different functions, work in different ways, are required in different quantities, and received in different ways, e.g. we breathe in oxygen but receive sodium from the salt in our foods.

The following are required by the human body in sufficient quantities that they are known as:

The following are also important but required in lower quantities, hence known as:

Molecules

The size of molecules varies enormously depending on the type of molecule. The smallest consist of only two tiny atoms. Others, such as certain modern plastics, consist of extremely long chains of carbon atoms together with groups of atoms of other elements.

Other scientific disciplines, such as particle physics, are interested in atoms and sub-atomic particles (that is the even smaller parts that, together, form atoms).

Biologists and health scientists usually study structures from the size of molecules (i.e. "molecular scale") upwards in size.

Molecules may be considered as the "building blocks" of animals such as humans. In most cases they must be obtained in the diet or manufactured (by the body) from dietary components.

For example, the amino acid called alanine (whose molecule is indicated below) is a sub-unit that forms part of proteins such as actin and myosin, and also the enzyme ATPase.

As indicated by its structure, alanine includes the elements Hydrogen, Carbon, Nitrogen and Oxygen.

Organelles

Organelles are sub-cellular structures specialised to perform specific functions within the cell.

The balance (relative quantities) of the types of organelles within cells varies with the type of cell - because cells also exist in many different forms, specialised for performing specific tasks.

The following organelles are found in animal cells - click on the clicks for further information about each.

Centrosomes;
Cytoplasm;
Goli Apparatus;
Lysosomes;
Microvilli;
Mitochondria;
Nucleolus;
Nucleus;
Ribosomes;
Rough Endoplasmic Reticulum (RER);
Smooth Endoplasmic Reticulum (SER).

Cells

Cells are units of living matter.

One of the first units taught in many biology courses includes the structures of, and differences between, animal cells and plant cells (though comparison may be omitted from "Human Biology" courses because the human body obviously includes only "animal cells", unless the digestion of plant-matter is discussed at a cellular level).

For example, there are several types of nerve cells (also known as neurons or neurones). Click on the diagram below for more information about nerve cells.

Tissues

A tissue is group of similar and closely associated cells that are specialised to perform a particular function (or group of functions).

Each of the 4 basic types of animal tissue listed on the left can be sub-divided into even more specialised tissue-types.

For example, there are three (3) distinctly different types of muscle tissue.
They are:

Skeletal Muscle (Tissue),
Cardiac Muscle (Tissue), and
Smooth Muscle (Tissue).

Organs

(Biological) organs are groups of physically associated tissues that operate together (as one unit) to perform a specific function with great efficiency.

There are many organs within the body. Examples include:

• The Heart
• The Lungs
• The Stomach
• The Liver
• The Kidneys
• The Intestines

Groups of organs and associated tissues that operate together are called "Organ Systems", see below.

Organ Systems

An organ system is defined as a group of organs and tissues that operate together in a co-ordinated way to perform a gross function.

Introductory courses in Human Biology, Anatomy & Physiology, or similar often begin with a general introduction to sub-structures such as cells and tissues then progress to cover each of the major organ systems as separate units. More advanced study may then include consideration of interactions between the systems of the body.

The example of the Urinary System shown on the right illustrates:

• The inferior vena cava (vein, in blue),
• The aorta (artery, in red)
• The right- and left- Kidneys
• The right- and left- Adrenal Glands (part of the Endocrine System but included in the diagram due to their physical proximity),
• Renal arteries and renal veins,
• The Urinary Bladder,
• The sphincter muscle (beneath the bladder), and
• The Urethra.

Other examples of "Organ Systems", also known as "Body Systems" include:

• The Endocrine System,
• The Respiratory System, and
• The Urinary System.

Organism

(i.e. the whole body !)

The "human organism" refers to a whole (complete) human.

It is obvious that every example is different, but this statement can lead to interesting discussions about identical twins and triplets, etc.. As such considerations belong in the section about genetics they so are not exploered further here.

To see a perfect example, just look in a mirror, or if you are reading this in a group, at the person next to you.