Levels of organization

Living things are made in an organized way, from small parts to bigger ones.

The atom is the smallest part of matter. It has a center (nucleus) with electrons around it.

An atom has three main parts: protons, neutrons, and electrons.

Protons have a positive charge (+).

Electrons have a negative charge (–).

Neutrons have no charge; they are neutral.

Atoms join together to make molecules. Molecules are made of two or more atoms connected to each other.

H2O is the chemical formula for water, which is a molecule composed of two hydrogen atoms and one oxygen atom.

Some molecules are very big. These are called macromolecules. They are made when many small parts, called monomers, join together.

One example of a macromolecule is DNA. DNA has the information that tells every living thing how to grow and work.

All living things are made of cells. Cells are the smallest part of a living thing that can work by itself. Inside the cell, there are small parts called organelles. Organelles have special jobs that help the cell live and grow.

For example: Mitochondria give the cell energy.

Chloroplasts are found in green plants and help them make food from sunlight.

Some living things have only one cell, like bacteria. Others, like humans and trees, have many cells.

Cells can be of two types:

Prokaryotic cells: they don’t have a nucleus.

                                          Streptococcus bacteria

Eukaryotic cells: they do have a nucleus and other organelles with membranes.

                  

Level of Organization	Explanation	Example
Cells	The smallest part of living things.	Muscle cell
Tissues	Groups of cells that work together.	Muscle tissue
Organs	Tissues that form body parts.	Heart, roots, leaves
Organ Systems	Organs that work together.	Circulatory system
Organism	One living thing.	One tree
Population	Many organisms of the same kind.	All the trees in a forest
Community	Different living things in one place.	Trees, flowers, insects
Ecosystem	Living things and non-living things together.	Forest with soil, water, and sunlight
Biosphere	All ecosystems on Earth.	Land, water, and air where life exists