Basic Information

Levels of Organisation in Living Beings

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.

	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	Muscle tissue, nerve tissue, blood…
Organs	Brachial biceps, heart, brain, leaf, root…
Systems of organs	Digestive system, nervous system, circulatory system, skeleton…
Apparatus	Motor apparatus
Multicellular organism	A person, a cat, a fungus, a black poplar…

Bioelements and Biomolecules

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.

Cells

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.

Common Structures in Eukaryotic Cells

	Description	Function	Where
Cell wall	Outermost layer of a plant cell composed of cellulose and other complex carbohydrates.	Helps to support and protect the cell.	P
Flagella (flagellum)	Long and scarce threadlike structures that extend from the surface of the cell.	Used for movement of the cell or to move fluids over the cell's surface for absorption.	A
Cilia (cilium)	Short and abundant threadlike structures that extend from the surface of the cell.	Used for movement of the cell or to move fluids over the cell's surface for absorption.	A
Cell membrane	Outer layer composed of lipids and proteins.	Controls the permeability of the cell to water and dissolved substances.	A, P
Cytoplasm	Viscous fluid mixture that occupies most of the cell's interior. Wraps the nucleus and contains biomolecules, organelles and a protein fiber network (the cytoskeleton).	Medium in which organelles and other internal structures exist in. The fiber network makes the cytoskeleton, which supports the shape of the cell and anchor organelles to fixed positions.	A, P
Mitochondria (mitochondrion)	Elongated organelles enclosed in a double membrane, the inner one with folds called cristae.	Sites of cellular respiration, which converts sugars and fats into energy through oxidation.	A, P
Chloroplasts	Elongated organelles enclosed in a double membrane and with vesicles containing chlorophyll.	Sites of photosynthesis.	P
Ribosomes	Tiny organelles composed of proteins and RNA, not enclosed in a membrane. Some are free in the cytoplasm, some are attached to endoplasmic reticulum. They are the only organelles present in all cells, including prokaryotics.	Sites of protein synthesis.	A, P
Endoplasmic reticulum	Extensive system of internal membranes. May be smooth or rough: the latter has ribosomes attached to its membrane.	Site of synthesis, modification and transport of various organic biomolecules.	A, P
Golgi apparatus	Flattened stacks of membranes.	Used in the collection, packaging, and distribution of synthesized molecules.	A, P
Secretory vesicles	Membrane enclosed sacks created at the Golgi apparatus.	These structures contain cell secretions, like hormones and neurotransmitters. The secretory vesicles are transported to the cell surface where they release those substances outside the cell (exocytosis).	A, P
Vacuoles	Elongated organelles enclosed in a membrane. Few and large in plant cells.	Used to store sap (water and sugars) or waste products.	A, P
Lysosomes	Spherical organelles enclosed in a membrane.	Contain digestive enzymes for breaking down old cellular components or ingested food (smaller cells, big macromolecules).	A, P
Centrosome	A pair of hollow tubes (the centrioles) surrounded by protein fibers in a star-like arrangement. Plant cells have an equivalent structure.	Move and organise chromosomes during mitosis and meiosis.	A
Nucleus	Double membrane structure that encases chromatine.	Controls the cellular activity.	A, P
Chromatine	Long strands of DNA and protein. During cell division it is packaged into chromosomes.	The DNA stores hereditary information in small units of information called genes, and expresses it.	A, P
Nucleolus	Highly condensed chromatine loops.	Area were ribosomes are manufactured.	A, P

Tissues

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:

Epithelial tissue	Composed of layers of cells that line organ surfaces such as the surface of the skin or the inner lining of the digestive tract. Serves for protection of organs (as in the skin), secretion of substances (when it forms glands - in the skin, in the digestive tract, etc.), and absorption of substances (as in the intestine).
Muscle tissue	Composed of very long cells (up to several cm) called muscle fibres. They have more than one nucleus, are able to expand and contract (thanks to a dense protein network that takes up most of the cellular space), and so, are specialized in movements. There are three kinds: cardiac muscle (found in the heart), skeletal muscle (attached to bones and under voluntary control) and smooth muscle (not in the heart or attached to bones and under involuntary control, as in the wall of the stomach).
Nerve tissue	Composed of cells with many projections that are specialized in contacting other cells and transmitting messages via electrical signals.
Connective tissues	Usually specialized in holding together different organs or tissues. It is composed of cells usually very separated by an abundant extracellular matrix. The main types are the bone tissue (in bones, with matrix rich in apatite, a mineral rich in P and Ca), the cartilage tissue (in cartilages), the adipose tissue (as in the fatty layer under the skin - the hypodermis), the fibrous connective tissue (in ligaments and tendons), the loose connective tissue (as in the skin's dermis) and the blood.

Organs, Systems and Apparatuses

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.