Levels of Complexity forming the Human Body:

Molecules, Cells, Tissues, Organs

The component parts of the human body can be described as follows, from the smallest to the largest parts:


Definition / Description:

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.
  • Mixtures and compounds include more than one type of atom, the difference between these being that mixtures consist of discrete (not attached together) combinations of atoms and molecules, which can therefore often be separated easily. Conversely, compounds consist of at least two different types (elements) of atoms that have joined together via chemical bonds to form molecules. Although molecules can also be formed from two or more atoms of the same type (same element), e.g. the gaseous molecules of oxygen (O2) and nitrogen (N2), unless atoms of different elements have joined together, the molecule is that of an element, not a compound.

Some scientific disciplines, such as particle physics, include consideration of atoms and sub-atomic particles (that is the even smaller parts that, together, form atoms). In most cases biologists and health scientists concentrate on studying 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 food.


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

Bulk Elements

Oxygen (O)
Carbon (C)
Hydrogen (H)
Nitrogen (N)
Calcium (Ca)
Phosphorus (P)
Potassium (K)
Sulphur (S)
Sodium (Na)

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

Trace Elements

Magnesium (Mg)
Chlorine (Cl)
Iron (Fe)
Iodine (I)
Cobalt (Co)
Manganese (Mn)
Copper (Cu)
Zinc (Zn)
Flourine (F)
Molybdenum (Mo)
Selenuim (Se)


Definition / Description:

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 concerned with 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.


Definition / Description:

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

The relative quantities of the types of organelles within cells depends the type of cell. There are many different types of cells in the body that are specialised for performing specific tasks.


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


Definition / Description:

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. Such comparison might be omitted from courses in specifically Human Biology because the human body includes only "animal cells" (unless the digestion of plant-matter is discussed at a cellular level).

Animal cells are surrounded by a cell membrane and are generally specialised for particular tasks within the body. When viewing animal cells in photographs or on microscope slides, they are therefore usually labelled according to the particular type of cell, which often relates to its location in the body.


For example, there are several types of nerve cells, also known as neurons or neurones:


Definition / Description:

4 Basic Types of
Animal Tissue

Epithelial Tissue

Connective Tissue

Muscular Tissue

Nervous Tissue

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:


Definition / Description:

(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'.

Organ Systems

Definition / Description:

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.

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

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

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.

Organism (the whole body)

Definition / Description:

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

It is obvious that every example is different, although some may be very similar such as identical twins and triplets, etc..


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

Note about this information:
Many courses in Biology, Human Biology, and Human Anatomy & Physiology begin by describing the Characteristics of Life and then the Structure of an (Animal) Cell.
It is useful to understand cells and the way in which they form tissues because this knowledge aids understanding of the more complicated structures described and explained later. This page summarises the stages of complexity at which human biology and related subjects can be studied. It establishes a scale of 'level of detail' along which descriptive pages, such as those in the 'Tissues Section', can be placed.

See also characteristics of life, a simple animal cell, structure of plant cells & classification of tissue types.

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