Cell Anatomy

Cell Anatomy.

Cell (from Flank Organ of Syrian Hamster)

A cell is the functional unit of living beings. All the living beings are made up of one or more cells (the only exception are viruses, not considered living beings by many scientists). Each cell in a multicellular organism is a living being capable of carrying out all the vital functions (although in complex organisms the cells are extremely specialised, so they have lost their individuality and can not live by themselves).

There are to groups of cells according to their characteristics: Prokaryotic Cells and Eukaryotic Cells.

Prokaryotes are primitive cells, without nucleus or complex inner organelles (they have no inner membranes, so the only complex organelles that can be found are ribosomes). The most important prokaryotic organisms are bacteria.

Eukaryotes, on the other hand, are more modern cells, with a nucleus and complex inner organelles. All the multicellular organisms are made up of eukaryotic cells. There are two types of eukaryotic cells: plant cells and animal cells. Plant cells have cell walls made up of cellulose. And chloroplasts, the organelles used to make photosynthesis. Animal cells, however, never have cell walls or chloroplasts (they never photosynthesise). 

In this unit we will analyse the anatomy, physiology and reproduction of eukaryotic animal cells.

Cell Anatomy: Parts of the cell.

Eukaryotic cells have three parts:

• Cell Membrane: Physical barrier that surrounds the cell and separates the interior of the cell from the environment.
• Cytoplasm: Inner part of the cell, where all the reactions and cellular processes take place. It is also the place where cell organelles can be found.
• Nucleus: Located in the interior of the cell, surrounded by a double membrane, it is the place where DNA is stored. 

Cell: Cytoplasm, membrane and nucleus.

Cell Membrane.

The cell membrane is the barrier that separates the interior from exterior of the cell. This barrier is also the system which controls the transport of substances between interior and exterior. Furthermore, the cell membrane protects the inner part of the cell.

The cell membrane is a type of biological membrane, and biological membranes are made up of two main components:

• Phospholipids: these are the most abundant component. These macromolecules form a structure called lipid bilayers. They are the main isolator component of the membrane, and many chemical substances are unable to penetrate this barrier. In fact, lipid bilayer can only be penetrated by small sized molecules without electrical charge. This bilayer has tow very different zones. The peripheral zone is hydrophilic, the inner or central zone, much thicker than peripheral, is hydrophobic. Due to this, to transport any molecule through the membrane, this molecule must pass through two thin hydrophilic zones and one thick hydrophobic zone. So it can not present remarkable hydrophobic or lipophobic properties,
• Membrane Proteins: These are proteins attached to the membrane. According to their position, there are two types of membrane proteins:
• Integral Membrane Proteins: They cross the membrane.
• Peripheral Membrane Proteins: They are anchored to the inner or outer part of the membrane, but never cross the it. 

Scheme of cell membrane.

According to their function, there are several types of proteins. These are the most important ones:

• Transport Proteins: Although the membrane separates interior and exterior of the cell, there must be a system to transport substances from the cytoplasm to the exterior and from the exterior to the cytoplasm.  This is the function of the transport proteins. These proteins are specific for one or a few molecules. According to the process used to transport the molecules, there are two groups of proteins:
• Passive Transport Proteins: Also called Channel Proteins, they form a channel that crosses the membrane. This transport system does not consume energy, so the substances must travel from the lower concentration to the higher concentration places.
• Active Transport Proteins: These proteins transport substances from higher to lower concentration places. In other words, the substances must be forced to cross the membrane. To promote this movement of substances, the substances can be exchanged: one substance crosses the membrane from a higher concentration place toa  lower concentration place and, at the same time, other molecule crosses the membrane from a lower concentration place to a higher concentration place. The other option is the transport of substances from lower to higher concentration places promoted by the consumption of energy, mainly the energy released when ATP is transformed into ADP. 

Transport Proteins.

Scheme of Receptor.

• Receptors: The cells must be related to the external environment, receiving stimuli and signals. Some stimuli are molecules that cross the membrane and are detected in the interior. But most frequently they are signals or molecules detected by receptors that  receive thee stimuli from the exterior and transmit information to the interior. This inner signal is called secondary messenger.
• Structural Proteins: Some membrane proteins support or fix other cell structures. Other membrane proteins join the cells to other adjacent cells or to extracellular structures, such as the basal membrane (which are called desmosomes and hemisesmosomes respectively).
• Other proteins: There are other membrane proteins with different functions. Some enzymes work attached to the membrane, for instance.

Cytoplasm.

The cytoplasm is the inner part of the cell. It is the place where all the characteristic cell actions take place: metabolic cell reactions (anabolic and catabolic), cell respiration, etc.

The cytoplasm has two main components. One of these is the liquid component, made up of proteins and many other substances dissolved or suspended in water. This liquid component is called cytosol. The other one is made up of complex specialised structures, where some concrete actions are carried out, and are called Cell Organelles. 

Cell: scheme of cell organelles.

Let’s analyse the most important cell organelles.

• Ribosomes: The ribosomes are spherical tiny organelles, made of two joined subunits. They are made up of special proteins (ribosomal proteins) and RNA, a concrete type of  RNA called ribosomal RNA (RNAr). The function of these organelles is the production of proteins, mainly the proteins that are going to carry out their actions in the cytoplasm. To produce these proteins they use messenger RNA (RNAm) as a guide. The ribosomes are a very abundant organelle, and the amount of ribosomes is related to the cell activity: the higher cell activity, the higher amount of ribosomes. Commonly, a regular cell has thousands ribosomes in its cytoplasm.

Ribosome.

• Rough Endoplasmic Reticulum: this is a cell organelle made up of a plasmatic membrane (similar to the cell membrane that surrounds the cell), that form internal sac shaped structures. These sacs are complex flat tubules. The surface of the sacs are covered by ribosomes attached to the membrane. In fact, these ribosomes are seen as little points when they are observed using a electron microscope, giving the structure a characteristic rough aspect and this is the reason of the name of the organelle. The main function of the organelle is producing three types of proteins: proteins that are going to be expelled to the exterior, proteins that are going to be part of the cell membrane (cell membrane proteins), or proteins that are going to be sent to the main digestive organelle of the cell, the lysosome. 

Production of proteins in the Rough Endoplasmic Reticulum.

• Smooth Endoplasmic Reticulum: just like the rough endoplasmic reticulum, this organelle is a tubular system, although they are not flat tubes, but with circular or elliptical section. These ducts, besides, do not have ribosomes attached to the surface, and this is the reason why it is called smooth. The tubular network is communicated with the rough endoplasmic reticulum: in fact, they form a complex structure with two different shapes. The smooth endoplasmic reticulum do not have ribosomes, so the function of the organelle is not producing proteins. The main functions of the organelle are related to the anabolism of lipids. This is the place where the phospholipids that made up the cell membrane or the inner organelles are produced. Other complex lipids are also metabolised in the organelle too. Finally, it is also related to the transformation or elimination of toxic products (this process is called detoxification).
• Golgi Body: This organelle is made up of a group of flat sacs, in parallel disposition and frequently slightly curved, so they have a convex and a concave face. There are usually between six and eight sacs, forming a group, but without direct connections between them. The Golgi Body receive proteins from the rough endoplasmic reticulum. These proteins arrive at the organelle enclosed into vesicles. And the Golgi Body is responsible for the transformation and distribution of that proteins. So, the proteins produced in the rough endoplasmic reticulum are sent to the Golgi body, where they are transformed and sent to the correct destination: the cell membrane, the exterior of the cell or the lysosomes. Due to this, we can say that the Golgi body is the router of the cell proteins. 

Proteins from the Rough Endoplasmic Reticulum.

• Lysosomes: Spherical organelles responsible for the destruction of other cell components. Sometimes, the cell components must be destroyed, because they are deteriorated or simply because they are not useful at the moment. Lysosomes are also related to the destruction or digestion of products that the cell has captured from the exterior, such as bacteria that have been phagocyted in defensive cells. The interior of the lysosomes is full of digestive enzymes, proteins that carry out degradative processes. These enzymes are produced by the rough endoplasmic reticulum, and are transformed and sent to the lysosome through the Golgi body.

Endoplasmic Reticulum and Golgi Body.

• Peroxisomes: This organelles have a similar morphology to the lysosomes, because they are also spherical structures and similar sized too. Both organelles are, however, very different. Peroxisomes are not related to the reticulum-Golgi route. And their functions are very different too, because the main function of the peroxisomes is the chemical transformation of oxidative products, mainly oxygen peroxide, that is produced in the cytoplasm as toxic secondary substances of the regular metabolism. The oxidant products are very dangerous to the cell, and must be eliminated by these organelles. Peroxisomes and Lysosomes can sometimes  be differentiated because peroxisomes are rich in a enzyme called peroxidase, that usually form visible crystals in the centre of the sphere.  
• Mitochondrion: This organelle has two membranes, one outer membrane and one inner membrane. It is rod shaped, and the inner membrane forms lots of protrusions and infoldings called mitochondrial called mitochondrial cristae. The mitochondrial is the main energetic organ of the cell. In fact, is the organelle where the cell respiration takes place: glucose (the most important sugar) reacts with oxygen and is catabolised and transformed into water and carbon dioxide, obtaining energy in the process. All the living cells have between a few dozen to several thousands mitochondrions. The amount of mitochondrions depends on the activity of the cell: the higher the cell activity, the larger amount of mitochondrions the cell has. The mitochondrions are very abundant in muscular cells, for instance. 

Mitochondrion.

• Cytoskeleton: We call the group of fibrillar proteins that form the inner skeleton to the group of fibrillar proteins that form the inner skeleton of the cell cytoskeleton. They are responsible for maintaining and supporting the shape and structure of the cell, the inner distribution of organelles, and the movement of the cell (for instance, the contraction of muscle cells). There are three different types of cytoskeletal fibres: microtubules, microfilaments and intermediate filaments. The main component of the cytoskeleton is called actin. 

Mitochondrion and cytoskeleton.

• Centrosome: This organelle have two subunits, and is made up of fibrillar proteins. Each one of these subunits is called a centriole. The centrioles are cylindrical and both centrioles are disposed perpendicular, in a structure similar to a letter T. The centrosome is responsible for controlling the cytoskeleton, its distribution and the movements of the cell or the chromosomes during the cell division.

Nucleus.

Nucleus and membrane

The nucleus is an important zone of the cell, usually spherical and surrounded by double membrane (that continues with the endoplasmic reticulum) called nuclear membrane. The  nucleus is the place where the DNA is stored. Although the DNA never exits from the nucleus (except during the cell division), it can not be isolated, so there must be connections between the nucleus and the cytoplasm. These connections are called Nuclear Pores, and are circular holes in the membrane that allow the exchanges of macromolecules between the nucleus and the cytoplasm (for instance, the RNAm must be produced in the nucleus, but is used in the cytoplasm, in the ribosomes, to carry out the protein synthesis in the ribosomes). The non condensed DNA of the nucleus is called chromatin. There are zones of the DNA with different grades of condensation.

The nucleolus is a circular zone of the nucleus where the chromatine is very dense. This is  the place where the RNAr (this RNA is the main component of ribosomes) is produced. This structure is related to the cell anabolism, because the ribosomes are the cell organelle responsible for the production of proteins. 

Just before the cell reproduction, the chromatine in the nucleus is condensed, forming dense enlarged structures called chromosomes. These structures ensure that, during cell division, the DNA of the cell is correctly distributed between the two descendant cells. The number of chromosomes formed during the cell division is always the same for each species of living beings. The chromosomes are organised as pairs of homologous chromosomes. Human beings, for instance, have 23 pairs of chromosomes (46 total chromosomes). The number of pairs is called n. As the total chromosomes are the double, this number is called 2n. In human beings n=23 and 2n=46. 

Nucleus and nuclear pore.

The last pair of chromosomes are responsible for deciding the sex of the living being. They are called sexual chromosomes. In human females, there are 22 pairs of somatic chromosomes plus two chromosomes called X. In males, there are 22 pairs of somatic chromosomes plus one X and one Y chromosomes.

Human chromosomes (male)

Just before the cell division, the DNA duplicates. Due to this, the chromosomes have two chromatids identical one another. In other words, in the cells we can find a variable number of pairs of chromosomes. Each chromosome and its partner are homologous. This means that they have information about the same things, although this information can be different in both chromosomes. For example, if one chromosome has information about the colour of the eyes, its partner has information about the colour of the eyes too. However one of the chromosomes could promote the colour blue for the eyes, whereas the other one could promote the colour black for the eyes, for instance. The chromatids of the chromosomes, on the other hand, are identical: they have not only information about the same things, but also exactly the same information. The two chromatids of the chromosome are attached by an structure called cinetocore.

Nucleus in a cell.

Cell Division.

The reproductive process of the cell is called cell division. The basic cell division carried out by regular cells is called Mitosis.

Mitosis.

During mitosis a simple mother cell divides into two identical daughter cell and identical to the mother cell. As we have studied, just before the mitosis the DNA of the cell duplicates, so all the chromosomes have two identical chromatids. This process ensure that, after the cell division, the daughter cells receive all the information and besides this information is identical in both cells. 

Mitosis.

Mitosis can be divided into two different processes:

• Chariokynesis: division and distribution of DNA, that is condensed in the chromosomes.
• Cytokinesis: physical division of the cell. This process takes place during the last phases of the mitotic process.

The mitotic process can be divided into four consecutive phases:

• Prophase: the chromatin in the nucleus condenses, forming the chromosomes. The nuclear membrane is disintegrated. Two centrosomes move to opposite poles of the cell and, between them, a special cytoskeletal structure called aster is generated. 

Prophase.

• Metaphase: the chromosomes align in the centre of the cell, attached to the aster. These chromosomes aligned in the centre of the cell, attached to the cytoskeleton, are called metaphasic plate. 

Metaphase.

• Anaphase: the two chromatids of each chromosome separate, and each one moves towards one of the poles (the other one moves towards the opposite pole). Due to this, the genetic information is distributed correctly. 

Anaphase.

• Telophase: the chromatids gather in the pole and start the decondensation process. The aster is disintegrated. The nuclear membrane is formed again, surrounding the chromatids. 

Telphase.

The cytokinesis usually begins during anaphase, and ends at the final part of telophase.

Meiosis.

The second type of cell division is called meiosis. This only takes place in reproductive cells and is the process carried out to produce gametes. These cells must have half of the chromosomes, or in other words, only one of the two homologous chromosomes.

So, during the sexual reproduction two gametes join, adding n chromosomes each one of them and producing a cell with the common 2n genetical information.

The meiotic process produces four daughter cells, different one another and different to the mother cell. It can be divided into two main parts, called Meiosis I and Meiosis II. Each part is also divided into four phases, called Prophase, Metaphase, Anaphase and Telophase (just like in mitosis).

During Prophase I the homologous chromosomes join forming an structure called synapsis.  In this process, the chromosomes exchange fragments. Due to this, after Prophase I the two chromatids of the chromosomes are not identical, because they have exchanged parts with their homologous chromatid. This factor is responsible for the production of different daughter cells. 

Prophase (synapsis).

During Anaphase I, the complete chromosomes travel towards the pole, instead of braking into two chromatids. The homologous chromosome travels towards the opposite pole. This is how the number of chromosomes is divided into two and, after Meiosis I, the cells obtained have half amount of chromosomes, in other words they have only one of the two homologous chromosomes. Each chromosome has two chromatids (that, after synapsis, are not identical). This issue is solved during Meiosis II.

Anaphase I.

Meiosis I produce two different cells. Each one starts the next process, called Meiosis II.

During Meiosis II the process that takes place is very similar to an ordinary mitosis, but with two important differences. The first one is that there are not homologous chromosomes, all the chromosomes are different. And the second one, during Anaphase II the chromatids that travel towards opposite poles are not identical, they are different (due to the synapsis that had taken place during Prophase I). This leads to the production of two different cells. 

Anaphase II.

Meiosis I produce two cells. Each one starts Meiosis II. So when the process ends, four different cells with only half chromosomes (n) have been produced.

Four cells after meiosis.

Insectos: Órdenes Hemypteroides.

Hemypteroide, por Mick Talbot

No existe nunca una boca masticadora típica. Cuando es masticadora, está muy modificada, nunca tiene cercos. El sistema nervioso va a estar muy concentrado, con pocos ganglios en la cadena. Presenta muy pocos tubos de Malpighi.

Orden Anoplura.

Poseen un aboca chupadora – picadora. Sus antenas son cortas y los ojos están ausentes o muy reducidos. El tórax no presenta signos externos de segmentación. Son ectoparásitos, como los piojos de los mamíferos. Se alimentan sólo de sangre.

Los ectoparásitos poseen una serie de características generales. Por un lado, antenas reducidas, los ojos reducidos o ausentes. Esto hace que los sistemas sensoriales estén muy reducidos. Los cuerpos aparecen aplastados (en los piojos, aplanado dorsoventralmente, en estoparásitos de otros órdenes, como las pulgas, planos dorsolateralmente). Todos poseen pinzas en las patas o filas de sedas rígidas en el cuerpo (para anclarse al animal que parasitan). Su cuerpo aplanado y las pinzas sirven para dificultar ser eliminado por el rascado. Los ectoparásitos son muy selectivos y específicos. No se alimentan de cualquier mamífero, solo de una especie y de una parte determinada del cuerpo, aunque

Pediculus humanus, por CDC/ Dr. Dennis D. Juranek

especies próximas de hospedadores poseen especies de anopluros parecidos. No pueden sobrevivir fuera del hospedador, si lo cambiamos de especies se mueren y si su hospedador muere deben buscar rápidamente a otro hospedador. A veces se busca un transporte, lo que se denomina loforesis. Es decir, se sube a otra especie animal diferente a la que parasita, de la cual en general no se alimentará, pero sobre el que vivirá hasta que encuentra a su hospedador original. Algunas especies pueden alimentarse del transportador, pero lo que nunca podrán es reproducirse en el. Muchas veces se acoplan a la reproducción del hospedador, llegando a percibir señales hormonales del hospedador para reproducirse cuando el hospedador se reproduce.

En los seres humanos encontramos, por un lado el Pediculus humanus, con dos subespecies, la capitis que hospeda el pelo y la corporis que vive sobre los tejidos (la ropa) y se alimenta del cuerpo (para eliminarlos hay que lavar la ropa al vapor). Y por otro lado, Pthyrius pubis, el piojo de la zona púbica, de donde no sale (en casos de sobreinfestación llega a pasar al vello torácico, axilas o cejas).

Orden Hemiptera (órdenes/subórdenes Heteroptera y Homoptera).

Heteróptero, por Thomas Schoch

Dentro del orden Hemiptera encontraremos dos subórdentes, el suborden Heteroptera y el suborden Homoptera (aunque en algunos libros aparecen como órdenes separados). Poseen un tipo de boca picadora-chupadora totalmente homóloga y de ahí que en muchos casos se agrupen en un solo orden. Esa boca tiene una especie de trompa formada por el labio inferior y el labro. Las mandíbulas se han transformado en una especie de estiletes. Encontraremos internamente las maxilas y externamente las mandíbulas, encerradas entre el labio inferior y el labro. Las piezas constituidas por la maxila y la mandíbula salen al exterior y su ápice es diferente, el de la mandíbula tiene forma de cincel y el de la maxila es más roma y con pinchos.

Esquema de la boca de Hemiptera.

 El rostro, es decir, la trompa, no es una pieza rígida. Tiene artejos diferenciados. El animal acerca el rostro, se arrolla  salen los estiletes. Primero clava los estiletes angulares, constituidos por la mandíbula y cuando ya ha realizado un agujero entran los dos maxilares. Cuando ya tienen metidos los estiletes, inyecta saliva por uno de los orificios interiores de la maxila. Esta saliva está cargada de enzimas, de forma que se predigiere el alimento. La inyección de saliva genera una presión que hace que el alimento tienda a subir por el otro orificio. A esta succión ayuda, además, el esófago, con el que el insecto es capaz de general vacío.

Heteróptero: Lygus rugulipennis, por Mick Talbot

La diferencia entre los Heterópteros y los Homópteros está, en primer lugar, en las alas delanteras. En Heterópteros están convertidas en hélitros. En Homópteros nunca están convertidas en hélitros, o bien son membranosas, o bien son helitroides, o bien algo que puede parecerse a un hélitro, pero siempre con la misma dureza a lo largo de todo el ala, desde el ápice al final. En Heterópteros el ala, en reposo, se coloca plana sobre el cuerpo. En Homópteros se colocan como tejas en un tejado.

Los Homópteros son siempre terrestres y fitófagos. Muchos constituyen plagas graves. Los Heterópteros no tienen tanta tendencia a ser fitófagos, resultando más habituales los predadores y pueden ser animales terrestres, acuáticos o anfibios.

Hay dos superfamilias Homópteras: Aphidoide y Coccoidea.

Ecdisis de una cigarra, por Macau500

Constituyen plagas muy importantes. Tienen partes partenogénicas y partes sexuales. Una parte de su ciclo de vida puede tener lugar sobre una parte de un árbol y otra sobre una parte del árbol diferente, por ejemplo. Una sola hembra (proveniente de partenogénesis) realiza una puesta en primavera que va produciendo en toda la temporada una generación por semana. Entre los individuos aparecerán hembras partenogénicas que completan el ciclo. Esto hace que puedan extenderse muy rápidamente.

Un ejemplo de insecto de la familia Paphodoidea son los pulgones, o la Philoxera de la vid, que en su momento ocasionó la desaparición de vides en toda Europa. Un ejemplo de insecto de la familia Coccoidea son las cochinillas.

En los Heterópteros encontramos ciclos normales y en general no son causantes de problemas de plagas. Hay tres tipos de Heterópteros, los terrestres los anfibios y los acuáticos. Los Heterópteros terrestres típicos son los chinches. Los anfibios, las zapateras, que poseen unos pelos bajo el cuerpo y las patas para no mojarse y de esta forma no romper la tensión superficial del agua y no hundirse. Además, en las patas poseen unas uñas extendidas hacia atrás que les ayudan a flotar. Son todos predadores. Las zapateras en general no tienen buena vista, pero tienen muy buen tacto, detectan las vibraciones de las moscas al caer al agua y e dirigen hacia ellas para capturarlas. En cuanto a los Heterópteros acuáticos, pueden ser predadores o saprófitos.

Structure of Living Beings.

Differentiation between living beings and inert matter is an easy question, at least in appearance. This is, however, because we usually think about living things that can be clearly seen growing, moving or, summing up, changing.  But if we take a look at our environment, we discover that we are surrounded by living beings, such as lichens, that live attached to rocks, that can be hardly differentiated of a simple stain. We need to do a deeper analysis to prove that it is really an organism, because it grows very slowly, it does not move and reproduces in a peculiar way, with no seeds, or fruits.

We usually forget that many living beings are not visible to the naked eye. And sometimes the boundaries of life are quite diffuse. Viruses, for instance, are a nucleic acid surrounded by a capsule made up of proteins, unable to reproduce by themselves, so are they real living beings? We can take the matter further, and talk about plasmids, those simple circular nucleic acids that live in bacteria and plant cells, are they living beings only because they are able to control their own replication? Or prions, made up only of proteins, but capable of reproduction and even to cause diseases.

Vital Functions and Complexity.

The main difference between living beings and inert matter is the ability of living beings to carry out the three vital functions.

• Nutrition: ability of living beings to take matter or elements from the environment (these elements are called nutrients) and use them to obtain energy or to be transformed into prime matters that will be used to produce their own inner medium.
• Relation (also called Interaction): Ability of living beings to perceive stimuli from the environment, analyse them and make a response. Relation is a complex process related not only to communication, but also to all the perceptions and responses. For instance, when an animal sees a predator and decide to run away or hide, that is a type of relation too. Or simply when a bacterium moves towards food.
• Reproduction: Ability of living beings to produce new living beings, similar or identical to the progenitor. This is the system used to transmit genetic information to the next generation, preserving this information and the species at the same time.

Is there any chemical difference between living beings and inert matter? Although the living beings and the inert matter that surround them is based on the same chemical elements, there are some important differences related to the amount of these elements. There are chemical elements that are very abundant in inert matter but not in living beings, and chemical elements very abundant in living beings but rare in matter. For instance, carbon or nitrogen are very abundant in living beings, but rare in inert matter. On the other hand, aluminium is very abundant in rocks, but is a trace element in living beings.

There are, besides, important differences related to:

• Complexity: Living beings have chemical components more complex than inert matter. Complex organic molecules are exclusive of living beings.
• Order: The inner structure of living beings is always extremely ordered. This order, besides, must be preserved. On the other hand, inert matter tends to become chaotic or disordered.

The chemical reactions in living beings must take place under severe control. These chemical reactions are usually very complex. We call metabolism to all the chemical reactions that take place in a living being.

Metabolism can be divided into two groups of chemical reactions:

• Catabolism: destructive chemical reactions. During these chemical reactions, complex structures are transformed into simpler molecules. The catabolic reactions have two transcendental  functions:
• Provision of energy.
• Renew structures (and turnover).
• Anabolism: constructive reactions. Complex structures are produced from simpler molecules. These chemical reactions consume energy. 

Living beings are, in some way, systems with rising complexity. We can draw a diagram showing this this rising complexity, beginning with the atoms and finishing in the complete organism, or further, with the populations, ecosystems or even the whole complete biosphere.

Atoms and Elements.

Around one hundred different atoms, or chemical elements can be found in nature. The rest of elements (until the 119 currently discovered) are not natural (must be produced in laboratories and last millesimal parts of second). Not all the natural elements can be found in living beings, though many of them are present, in high or low quantities. 

The chemical elements of living beings can be divided into two groups:

• Biogenic (or Essential) Elements:  They are present in important amounts. These are the eleven Biogenic Elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Calcium (Ca), Sodium (Na), Phosphorus (P), Potassium (K), Magnesium (Mg), Sulphur (S) and Chlorine (Cl). Carbon, Hydrogen, Oxygen and Nitrogen are the most abundant by far. Some books include Iron (Fe) as a Biogenic Element (in other books it is included into the Trace Elements).
• Trace Elements: These elements are present in living beings in low quantities, but they are essential (living beings cannot live without them). There are many different trace elements. Maybe the better known ones are Iodine (I), Copper (Cu), Silicon (Si),  Selenium (Se) or Manganese (Mn). The absence of one trace element cause a characteristic disease. For instance, lacking of Iodine is related to a disease called goitre.

Although some chemical elements can be found isolated, they usually join forming molecules.

Molecules.

Molecules can be defined as two or more atoms, identical or different, joined forming a chemical substance. There are two types of molecules: inorganic and organic.

Inorganic Molecules.

They are simpler, and can be formed in a natural way with no intervention of living beings. The most abundant and important inorganic molecule in living beings is the water (H2O). Life is related to water. The first living beings were evolved in the oceans. And is always the main component of living beings. In human beings, for instance, our mass is 70% water (some tissues, such as bone or fat tissue, have low percentage of water, others such us nervous tissues have high amounts). Another important inorganic molecule is O2, called molecular oxygen. This is the oxygen we breathe and the molecule used by our cells to respire. Another very important inorganic molecule is CO2, the carbon dioxide produced in the respiration. And mineral salts, a large group of salts solved in our body water, or crystallised such as the hydroxyapatite slats (made up of calcium carbonate crystals) in our bones.

Organic Molecules.

These molecules are more complex than inorganic ones, and a living being is required to their formation (although there are some exceptions, such us methane, that can be formed in volcanoes, for instance). There is a huge variety of Organic Molecules. They can be classified into the following groups: 

• Sugars: Also called carbohydrates. There are many types of sugars, but the most important are glucose and fructose. The main function of these model clues is the provision of energy to the living being: they are metabolised to obtain the energy that the living being needs to stay alive. Animals use carbohydrates to obtain energy, but they do not store high amounts of these molecules, because the movement of the body is very important to animals, and carbohydrates are too heavy. Sugars have other functions. For  instance, they are an important component of the biological membranes. In the cell membrane they are very important as an informative or signalling molecule: cells can recognise other cells using sugar chains attached to the cell membrane. They can be used by cells as inner signalling molecules (the sugar mannose is used in the cell to control the movement of vesicles, for instance). Other sugar chains are used as structural support in some tissues, such as glycosaminoglycans in the dermis.
• Lipids: Also known as fats. They are the main system to store energy in animals, because fats are lighter than sugars. They have also a important structural function, because lipids are the main components of phospholipids, and phospholipids are the most abundant macromolecule in biological membranes. Some lipids can be also signalling molecules. Some hormones are lipids or derived substances.  

• Amino Acids: They are the component of proteins. There are only 20 different amino acids, common to all the living beings. These 20 amino acids join to form proteins. In  fact, proteins can be defined as lineal chains of amino acids with no ramifications. So amino acids are clearly structural molecules. Sometimes living beings use amino acids to obtain energy, but only if there are not sugars available. They can be also signalling molecules, some hormones and neurotransmitters are transformed amino acids.

• Vitamins: Vitamins are organic molecules that the living being needs to live but can not produce, so they must be consumed from other living beings. They can be classified according to their chemical composition. We can find hydrophilic vitamins (all the B vitamins) and lipophilic (D orE vitamins, for instance).
• Other Organic Molecules: There are other important organic molecules that do not belong to any of these groups. Xantic bases, for instance, that are an important component of nucleic acids (DNA and RNA).

Macromolecules.

Macromolecules are large organic molecules, made up of several organic molecules joined. There are lots of different organic molecules. We are going to examine the most important ones:

• Polysaccharides: Macromolecules made up of many sugars joined. These sugars form chains that can be lineal or with ramifications. They can have different functions. They can be used to store energy. For instance, glycogen in animals. Animals build up glycogen in their liver an in their muscles. Plants build up polysaccharides to store energy. Some polysaccharides are an important structural molecule. We talked about glycosaminoglycans when we saw sugars. Polysaccharides are also a signalling molecule, as we saw. The sugar trees in the surface of erythrocytes are an example. These sugars are the responsible of the blood groups in humans.
• Proteins: Chains of amino acids with no ramifications. Proteins are usually very large, and they are made up of hundreds or even thousands amino acids. 

Proteins can have other molecules attached, such us sugars (they are called glycoproteins), lipids (they are called lipoproteins) or other proteins (they are called complex proteins). According to their function, there are several types of proteins. Enzymes are not only the most important type of proteins, but also the main functional molecule of the living beings. Enzymes are proteins that accelerate the chemical reactions that take place in living beings. Due to this, enzymes are the main functional molecule, because they control the chemical reactions: many chemical reactions would  not ever take place without the intervention of an enzyme. And all the chemical reactions of the living being are controlled by one enzyme, so enzymes control all the chemical processes of the cells. Other proteins have structural functions. Collagen, for instance, is a filament-shaped protein that give physical support to many tissues (such as the conjunctive, the cartilage or the bone tissues). And finally there are some non enzymatic proteins that carry out several functions, such us the proteins that transport substances through the plasmatic membrane of the cell (channel proteins).

• Phospholipid: Made up of two lipids joined, by a phosphate link, to a organic molecule, mainly a diglyceride. Phospholipids are the main structural component of the biological membranes. Phospholipids have two different parts, one of them made up of lipids, the other one made up of other chemical substances. The lipids form the tiles of the molecule and have lipophilic and hydrophobic properties. The diglicerides and the phosphate group form the head of the molecule and have hydrophilic and lipophobic properties. These properties is responsible for the ability of phospholipids to organise themselves as lipid bilayers. 

• Nucleic Acids: Nucleic acids are the main system to store and transmit information. This information must be stored, codified and translated. It must be, besides, transmitted to next generations. There are two types of nucleic acids: DNA and RNA.
• DNA (Deoxyribonucleic Acid): This macromolecule is the main system to store information in the whole known living beings. DNA is a lineal molecule made up of two strands forming a double helix. The information is codified according to the sequence of the xantic basis in the molecule (there are four different basis called Adenine, Thymine, Cytosine and Guanine (A, T, C, G). This sequence is translated into the sequence of amino acids of the proteins. What it means that DNA has the information  about how to produce all the proteins of the living being. As we have seen, proteins are the main functional proteins of the cell, and enzymes control all the chemical reactions. DNA control the enzymes that must be produced in a cell at a moment, so DNA control the functions of the cell in a indirect way. It is decided which structural proteins are necessary too. Summing up, controlling the production of proteins, DNA is also controlling all the processes that take place in the cell. DNA is a very large molecule that is always stored and protected in the nucleus of the cell (it is only outside of the nucleus during the cell reproduction). In multicellular organisms, all the cells have the same DNA, but different cells only use the part of the DNA information that they need. The cells of our skin, for instance, have DNA with the information about how to make digestive proteins, although this information will not ever be used by these cells.
• RNA (Ribonucleic Acid): This is other important nucleic acid. RNA has some chemical differences with the DNA. First, in RNA we find the sugar Ribose, whereas in DNA the sugar that can be found is Deoxyribose. RNA has the xantic base Uracil instead of Thymine. And RNA is a simple strand that never forms double helix. We have said that DNA is always located in t
  
  he nucleus of the cell. RNA, however, is mainly located out of the nucleus, in the cytoplasm of the cell. In fact, DNA is so important that it must be  always in the nucleus to be protected. Proteins are not produced in the nucleus, they are produced in the cytoplasm. To avoid possible damage to the DNA, when the cell needs to synthesise a protein it produce a copy of the part of the DNA that codifies for that protein. The copy of that part of DNA is called RNAm (messenger RNA). The RNAm can exit of the nucleus and be used in the cytoplasm to produce the required protein. To complete the synthesis of proteins other two RNA are necessary. The first one is called RNAt (transferential RNA), that have the decodification system to transform a sequence of xantic basis into a sequence of amino acids. The second one is called RNAr (ribosomal RNA), and it is a part of the ribosomes, the cytoplasmic organs where protein synthesis takes place.