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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, protozoa, unicellular algae and yeasts) only reach the cellular level, but the more sofisticated ones also have tissues, organs and systems of organs.

The bioelements are most abundant chemical elements in a livig being, which are not much the same ones that you can find in a rock or in the air. The top six are C, H, O, N, P, S, and they're called primary bioelements.

The molecules that can be found in all living beings, from the simplest bacterium to the most complex animal, are called biomolecules, the molecules of Life. There are two types:

• Organic: Carbohydrates, proteins, lipids, vitamins and nucleic acids. They all have an inner skeleton built mainly with carbon atoms, which allows for a really large size. The organic biomolecules can only be produced by living beings, if we discard artificial synthesis. This is, they can not be produced by geological or atmospheric processes, for instance.
• Inorganic: Water and mineral salts. They don't have an inner skeleton of carbon atoms and can be produced in non-biological processes.

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, most fungi, plants and animals).

All cells are able to perform the three vital functions: (a) they reproduce, quite usually by mitosis, a process that yields two daughter cells with almost identical genetic material; (b) they interact with their environment, giving responses to specific stimuli, as when a white blood cell is able to detect and destroy a bacterium; and (c) they feed, meaning that they are able to exchange matter and energy with their environment, as when a human cell takes oxygen from the blood and releases carbon dioxide.

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 (algae, protozoa, fungi, plants and animals) 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; (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; (c) they use to have one or a few big vacuoles that contain sap (instead of lots of smaller ones without sap) which normally push the nucleus out to the periphery of the cell; and (d) although they have equivalent structures, they lack centrosomes, the organelles that control the arrangement of the chromosomes during mitosis in an animal cell.

In multicellular beings there may be different types of cells, each type being specialized in an specific function, and having the specific shape that allows them to fulfill that function the best. Each of those types is called a cellular tissue; examples are the vascular tissue (plants) or the blood tissue (animals). One tissue may have several subtypes of cells (e.g. white blood cells and red blood cells). The human body contains over 200 different types of cells.

The four main types of human tissues are the following:

There are some tasks in a multicellular being that must be achieved by cells of different kinds working together (such as pumping blood throughout the human body). In this case, cells of different tissues gather and make up an organ (epithelial, connective, muscle and adipose cells make up the heart).

Several organs working together in a common general task make up an organ system, e.g., the heart and the blood vessels make up the circulatory system. And when two organ systems work cosely together in a common function are said to constitute an apparatus: the muscular system and the skeleton form the motor apparatus, because both contribute to the function of locomotion in an animal.

Put simple, the human organ systems contribute to the three vital functions as follows:

• Nutrition is fulfilled through:
• The Digestive System, which (a) takes in the food, (b) breaks it down into nutrients and other substances, (c) absorbs the nutrients into the blood, and (d) gets rid of the non assimilable substances in the form of faeces.
• The Circulatory System (a) transports those absorbed nutrients to all the cells of the body and (b) transports waste substances to the kidneys, the sweat-glands and the lungs.
• The Excretory System expells of the waste substances arriving to the kidneys, by producing and releasing urine.
• The Respiratory System (a) takes in oxygen (a nutrient) which is absorbed into the blood and (b) gets rid of the carbon dioxide (a waste substance).
• Reproduction is carried out through the male and female reproductive systems which (a) produce the specialized reproductive cells (sperm and egg cells), (b) allow those reproductive cells to join in pairs, and (c) grow the embryo coming out of a fertilised egg-cell.
• Interaction is fulfilled through:
• The Sensory Organs, which continuously detect bits of information coming from the inside of the body or from the environment.
• The Nervous System, which collects that sensitive information, interprets it, and generates response orders.
• The Endocrine System, which cooperates in conveying those response orders by means of substances, called hormones, that are released by glands and travel through the blood.
• The Skeleton and the Muscle System, which carry out most of those response orders produced in the nervous system.