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Living matter is the one able to carry out the three so called vital functions:

• · Nutrition, which consists of taking in matter and energy in order to grow, survive and reproduce; waste matter and waste energy are produced as by-products.
• · Interaction is (a) the ability to perceive what is going on in both the environment and the inside of the organism itself, and (b) the ability to produce responses coherent with the information that was perceived.
• · Reproduction, the ability to produce living beings similar to the parental organisms.

Every living being, and every cell in the multicellular living beings, is able to carry out these three vital functions.

All living beings are made up of complex structures called cells. Cells are made up of billions of biomolecules working together. Viruses are not regarded as living beings because they are not made up of cells.

Living beings can be (a) unicellular: made up of just one cell (bacteria, protozoa, many algae, yeasts); and (b) multicellular: made up of more than one cell: (some algae, many fungi, plants and animals).

All the cells have (a) a cell membrane, which is the cellular envelope, (b) a cytoplasm with organelles, which are specialized cell compartments where specific functions are fulfilled, and (c) genetic material, that carries the instructions that allow both the cellular work and its self-construction.

There are two main kinds of cells: (a) the cells of the bacteria and archaea have no real nucleus: they are said to be prokaryotic; (b) the cells of all the other living beings have their genetic material separated from the cytoplasm by a nuclear membrane: they are said to be eukaryotic.

The cells of the plants can be easily distinguished from those of the animals because (a) they have a semi-rigid cell wall, made of cellulose, surrounding the cell membrane, that usually gives the cell a polyhedral shape; and (b) they have one kind of organelles, called chloroplasts, where sunlight energy is used to start building their own organic substances through a chemical process called photosynthesis.

Living beings are formed by clusters of matter increasingly bigger and complex. These clusters of matter are classified in levels: every new level is more complex than the previous one. The simplest organisms (bacteria, protozoans, many algae and some fungi) only reach the cellular level, but the more sofisticated ones also have tissues, organs and systems of organs.

	Examples
Bioelements	C, H, O, N, P, S
Biomolecules	Carbohydrates, proteins, lipids, vitamins, nucleic acids, water, mineral salts
Cells	Sperm cell, palisade cell, muscular cell, neurone...
Tissues	Muscular tissue, nervous tissue, blood...
Organs	Brachial biceps, heart, brain, leaf, root...
Systems of organs	Digestive system, nervous system, skeleton...
Apparatus	Motor apparatus
Multicellular organism	A person, a cat, a fungus, a black poplar...

This is not the most modern way of classification of living beings, but still proves to be quite useful to understand and organise the huge diversity of the living beings:

	Groups of organisms	Type of cells	Organisation
Monera	Bacteria and archaea	Prokaryotic	Unicellular
Protista	Algae, protozoa	Eukaryotic	Unicellular to multicellular
Fungi	Yeasts, mould, mushrooms, toadstools	Eukaryotic	Unicellular to multicellular
Plants	Mosses, ferns, flowering plants	Eukaryotic	Multicellular
Animals	Animals	Eukaryotic	Multicellular