Human Body Structure: Cells, Tissues, and Systems

Human Body Structure: Classification and Composition

Firstly, human beings are classified within the animal kingdom, in the group of vertebrate animals because we have an internal skeleton and a spine. We are classified within the group of mammals because we are viviparous; we feed on our mother’s milk during the first stages of our lives.

Secondly, human beings are formed by cells; this is the most important characteristic of all living beings. Living things are classified into two different groups according to the number of cells that they have. These groups are unicellular beings and pluricellular beings.

Unicellular Organisms

They are formed by one single cell which performs all their vital functions. They are small, simple organisms such as bacteria and protozoa.

Pluricellular Organisms

They are composed of many cells, developed from one initial cell. The cells in a multicellular being are not independent of each other. However, most of them have three main parts:

• The membrane
• The cytoplasm
• The nucleus

Cell Components Detail

Membrane

This is a thin and elastic external cover.

Cytoplasm

This is a substance that fills the cell’s interior. Inside, there are some structures, called organelles, which perform many specific functions.

Nucleus

This is a substance that controls all cellular functions and activities.

Human Cells

The cellular organelles are very small. The human cells are eukaryotic cells. They have a nucleus separated from the cytoplasm by the nuclear membrane.

The Cellular Organelles Are:

Mitochondria, endoplasmic reticulum (rough and smooth), Golgi apparatus, ribosomes, centrioles, cytoskeleton, and lysosomes.

Levels of Organization in Multicellular Organisms

The Tissues

A tissue is a group of cells made up of one or more types that fulfil the same function.

The Organs

The tissues come together to make organs. An organ is made up of various different types of tissues which work in co-ordination.

The System

The system is a set of organs that act in co-ordination in order to perform a complex function.

The Apparatus

In some cases, different systems with related functions come together to form an apparatus. For example, the muscular and skeletal systems form the motor apparatus.