Videos: Diseases

Estos vídeos han sideo elaborados por alumnado de tercero de ESO, materia Biología y Geología (programa bilingüe).

Tratan de enfermedades y podemos encontrar vídeos de enfermedades infeccionsas como la varicela y el SIDA, enfermedades genéticas como la piel de mariposa, alteraciones fisiológicas como el infarto de miocardio y enfermedades de etiología diversa como el ELA.

ALS

CHICKENPOX

MYOCARDIAL INFARCTION

AIDS

EPIDERMOLYSIS BULLOSA

Circulatory System: Videos

I have updated two videos of the Circulatory System.

The first one shows the most relevant veins and arteries of our body.

The second one explains the most important events that take place during the cardiac cycle.

I have also updated the diagrams of the circulatory system and the cardiac cycle in the page: "Materiales Didácticos", in this blog.

The Manna for the Acari.

Biologists define commensalism as an inter-specific relationship, that takes place between two different species where one of them obtains some profit without affecting the other one.

Dust Mites, by Jacopo Werther

One typical example of commensalism is the relationship between human beings and those myriads of acari, also called dust mite, that live in our pillows, sheets or mattresses. Some of these small arthropoda are adapted to live close to humans and feed on our dead epithelial cells that are permanently being removed from our skin. The animal obtains its food without affecting us, unless we are allergic to them. But, even in that case, we must talk about commensalism because the dust mites are not responsible for the damage, it is our defensive system which is working incorrectly. Summing up, the bug causes indirect damage and after all, it is not to blame.

Dust mites are one of the smallest known animals, they measure around 250 microns so they are hardly visible to the naked eye. They belong to the phylum arthropoda, and the class arachnida, like spiders or scorpions and just like them, they have eight legs and their body has two main parts, the cephalothorax and the abdomen. As they feed on dead organic matter, we can say that they are scavengers.

Evolution has transformed some of these animals into our ordinary neighbours. They dwell in places where dust and organic matter are especially abundant. And we lose lots of dead epithelial cells when we are sleeping in our beds. Due to this, they are glad to combine our rest and their meal.

The fact of being surrounded by animals capable to live eating dead parts of our body is, in my opinion, slightly overwhelming. Superficial cells of our skin are shed constantly. Indeed, a fraction of the dust that it is built up in our house is made up of these cells we have released. This is the reason why we talk about dust allergy; we should talk about allergy to dust mites, because the animals that live in the dust are the real allergen.

Skin Anatomy

Our skin is a complex organ with three parallel layers. The lower one, in contact with other inner tissues, above all bones and muscles, is called hypodermis. This is a connective tissue, rich in adipocytes (also called fat cells) and fibrillar proteins. Just above this layer we can find the dermis, a thick layer made also of connective tissue, although this one is richer in water, elastic fibers and fibroblasts, the cells that produce and support the fibrilar system.

Human skin

Finally, the upper layer is called epidermis and it is made of epithelial tissue. The lowest level of this layer is called basal layer and it is made of epithelial cells that are dividing constantly. They produce new epithelial cells that move to upper levels. New cells push older cells, so that the longer the time a cell has been produced, the higher the level it can be found.

The cells move through the epidermis towards the surface and at the same time, they carry out a maturative process, they build up a fibrilar protein called keratin that make them harder. They also lose all their inner organelles and, step by step, they are transformed into a block of keratin. After finishing the process, after arriving in the surface, they are dead. To sum up, all our exterior is a sort of sheet, a tissue of keratin and dead cells (called squamous cells). The older cells are removed and other cells take their place. This is the way our body keeps our exterior in optimum condition.

From the dust mites point of view, these lost cells are a sort of manna released by those enormous animals that God created in order to provide them their food.

Respiratory System.

Respiratory System.

The cells of our body consume oxygen in order to obtain energy. In this process, called oxidation, the cells burn glucose using oxygen, releasing carbon dioxide, and obtaining the necessary energy to carry out several metabolic processes. The respiratory system is responsible for transporting and providing oxygen from the air to the blood and carbon dioxide from the blood to the air. This air will be exhaled after the exchange process. Anatomy of the Digestive System.

Respiration includes the whole exchange process. It has three different phases:

• Pulmonary ventilation: Inhalation and exhalation of air: the air flows from the exterior of our body to our lungs and after that is expelled from the lungs to the exterior.
• External respiration: Exchange of gases between the lungs and the blood.
• Internal or tissue respiration: Exchange of gases between the blood and the cells or tissues.

Anatomy of the Respiratory System.

Respiratory Organs.

The respiratory system can be divided into the following parts:

• Upper respiratory tract: made up of the nose, nasal cavity, pharynx and other structures.
• Lower respiratory tract: made up of the larynx, trachea, bronchi, bronchioles and lungs.

The respiratory system can also be divided into two big divisions:

• Respiratory tract.
• Lungs.

Structure of the Respiratory System.

We are going to study, one by one, the most important anatomical structures of the respiratory system:

• Nose: The nose has an external and a internal part. The external part is divided into two nasal channels called nostrils. The inner part is a large cavity located between the facial bones, just above the mouth. The floor of the nasal cavity is the hard palate, and the roof is a part of the ethmoid bone called cribriform plate. It is divided into two parts, the right and the left ones. It is connected to the pharynx through two openings called choanes. The nasal cavity is responsible for filtrating, heating and wetting the inhaled air. It also receives olfactory stimulus and modifies our voice.
• Pharynx: This is a thirteen centimetre long duct similar to a funnel, that connects the nasal cavity with the cricoid cartilage, that it is the upper part of the larynx. It has three parts. The upper part is located just beneath the nasal cavity and is called nasopharynx. The nasopharynx is connected with the nasal cavity through the choanes, and with the mid ear through the Eustachian tube. The next part of the pharynx is the oropharynx, and it is located behind the oral cavity. The oropharynx connects the mouth with the respiratory and the digestive systems. The lower part of the pharynx is the hypopharynx, that connects the oropharynx with the oesophagus (digestive system) and the larynx (respiratory system).
• Larynx: It is a short duct, made up of nine cartilaginous pieces, that connects the pharynx with the trachea (windpipe). The larger piece of the larynx is the thyroid cartilage, that it is also known as the Adam’s apple. This structure covers the thyroid gland. Another important piece is the epiglottis, a sort of flap that opens and closes the duct in order to prevent food from going into the trachea. The vocal folds are also located in the larynx.
• Trachea: This is a twelve centimetre long and two and a half centimetre diameter duct that connects the larynx with the bronchi. It lies just in front of the oesophagus. The trachea is surrounded by fifteen incomplete cartilaginous rings, that protect and maintain the airway. This rings are C shaped, to allow the dilation of the oesophagus when the food is passing through it. At the level of the fifth dorsal vertebra the trachea bifurcates into two principal bronchi.
• Bronchi: The trachea bifurcates into two principal bronchi, called right bronchus, that enters in the right lung and left bronchus, that enters in the left lung. After penetrating in the lung, each bronchus divides into two secondary bronchi. The secondary bronchi divides into tertiary bronchi. Each tertiary bronchi divides into two bronchioles. The bronchioles continue dividing successively, completing sixteen total divisions. The air that fills the bronchi and bronchioles (around 150ml) is not used to breathe, because there are not special structures to allow the exchange of gases, so it does not takes place. The structure where this exchange of gases takes place is called alveoli and these are located after the last bronchiolar division.
• Lungs: the lungs are two large conic shaped organs, located in the thoracic cavity and separated by the heart and the mediastinum. Each lung is covered by two membranes. The outer one is attached to the thoracic wall and it is called parietal pleura. The inner one is attached to the lung surface and it is called visceral pleura. Between both layers there is an internal fluid that keeps both membranes together and that lubricates them the to avoid friction when they move. This liquid is called pleural effusion. The lower and broader part of the lung is called base of the lung. The upper part of the lung is called anterior border. The cavity from where blood vessels and bronchi enter is called hilum. The right lung is slightly larger than left one, because the left one must leave a space for the heart. The left one is, however, longer than right one, because it must leave a space for the liver. Both lungs have fissures that divide them into lobes. The left lung has one fissure that divides it into an upper and a lower lobe. The right lung has two fissures that  divide it into a lower, a middle and a upper lobe. The lungs are the organs where the bronchioles divide. After the last division, the alveolar sacs can be found. Each one of these sacs has two or three alveoli. The alveoli are covered by blood vessels, because these are the anatomical structures where the exchange of gases between the air and the blood takes place.

Respiratory System: Anatomy.

Physiology of Respiration.

Introduction.

First, we will study the process called pulmonary ventilation, that explains how the air flows from the exterior of our body to the lungs and from the lungs to the exterior of our body. Then, we will study the exchange process between the air and the blood, that takes place in the pulmonary alveoli and it is called external respiration. Finally we will study the exchange of gases between the blood and the internal tissues, that is called internal or tissue respiration.

Pulmonary Ventilation.

Pulmonary ventilation, also called breathing, is the movement of the air between the exterior of the body and the lungs. The air enters the lungs from the environment to in a process called inhalation. The air exits the lungs to the environment in a process called exhalation. These movements of air are a consequence of the changes of pressure in the lung and of the special properties of this organ, that is capable of increasing its volume by distension and to return to its original size by elasticity.

The process of entrance of air into the lungs is called inhalation. It takes place when the lungs expands, increasing their volume. The expansion results from the contraction of the respiratory muscles: the diaphragm and the internal intercostal muscles. The most important muscle is, by far, the diaphragm. When it contracts, its convex morphology changes, becoming flatter. This movement pulls the lung down, enlarging its lower part. The internal intercostal muscles raise the thoracic cage, causing the expansion of the lungs because they are closely attached to the ribs. These two processes increase the volume of the lungs. The higher volume leads to a drop in the internal pressure, so that the air moves from the environment to the lung.

The release of air is called exhalation. It is a passive process, no muscular contraction is required. The elastic fibres of the lungs and the weight of the thoracic cage decrease the volume of the lungs when the respiratory muscles relax. The reduction of volume leads to an increment of the internal pressure, so that the air moves from the interior to the exterior.

Although this is a passive process, the contraction external intercostal and the abdominal muscles can accelerate the release of air. This is called forced exhalation, and is carried out when the body needs to improve the exchange of air.

Ventilation Volumes.

During normal breathing around 500ml of air is exchanged between the lungs and the environment. This amount of gas that enters and afterwards exits from the lungs is called Tidal Volume (VT).

Not all this gas is available to exchange oxygen and carbon dioxide. Around 150ml of air never reaches the alveoli and stand in the outer respiratory ducts: nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles. This volume that is not directly used in the pulmonary respiration is called Dead Space (DS).

The Respiratory Minute Volume (MV) is the amount of air exchanged between the lungs and the environment per minute. An adult human being breathes approximately  twelve times per minute, exchanging 500ml per breathing (Tidal Volume), so it is easy to calculate that the MV is 6000ml/min. 

We can breathe more deeply, inhaling more than 500ml. We can reach between 3000ml and 3500ml more in a forced inhalation. This is called Inspiratory Reserve Volume (IRV). We can even take more air if, just before the forced inhalation, we exhale as much air as we can. This air that we can release through forced exhalation, around 1200ml, is called Expiratory Reserve Volume (ERV).

After exhaling all the air that forced expiration allows, there is a volume of air that remains in the respiratory system. This amount of gas that we cannot release is very important, because it prevents the duct and alveolar sacs from collapsing. It is 1200ml more or less, and it is called Residual Volume (RV).

If we add the Tidal Volume to the Inspiratory Reserve Volume we obtain the Inspiratory Capacity (IC). It is around 3600ml. If we add the Residual Volume to the Expiratory  Reserve Volume we obtain the Functional Residual Capacity (FRC). It is around 2400ml.

The Inspiratory Reserve Volume added to the Tidal Volume and to the Inspiratory Reserve Volume is called Vital Capacity. It is around 4800ml. If we add all the volumes (IRV+VT+ERV+RV) we obtain the Total Lung Capacity (TLC). It is around 6000ml. 

Ventilation Volumes

Pulmonary Respiration Physiology.

The physiology of pulmonary respiration is based on the concentration gradients or differences in partial pressure. The internal membrane of the lungs is extremely thin (around 10.5μm), so that the gases are easily exchanged. And the internal surface of the lung is really broad, around 70m2 if we count all the alveolar surface.

The air that reaches the alveoli is very rich in oxygen, between 100-105mmHg. The concentration of oxygen in the blood in the capillaries of the lung, however, is quite low, around 40mmHg. Due to this, the oxygen tends to flow from the air to the blood, until both concentrations become equivalent. When the blood exits the capillaries of the lung, its concentration of oxygen is approximately 110mmHg.

To improve the movement of oxygen in the blood, it is transported linked to a special protein called hemoglobin.

Carbon dioxide concentration in the air is around 40mmHg. When the blood arrives in the alveoli, its concentration of carbon dioxide is around 45mmHg. Due to this, the carbon dioxide tends to flow from the blood to the air until both concentrations become equivalent. When the blood exits the capillaries of the alveoli the concentration of carbon dioxide is 40mmHg.

The carbon dioxide is not transported by any protein, but is transformed into a different substance called bicarbonate.

Tissue Respiration Physiology.

The situation in the tissues is just the opposite to in the lungs. The extracellular fluid that surrounds the cells is very poor in oxygen, because it has been consumed by the cells. Its concentration is 40mmHg. As we studied, the concentration of oxygen in the blood that comes from the lungs is 100mmHg. Due to this, the oxygen tends to flow from the blood to the tissues until both concentrations become equivalent.

Carbon dioxide, however, is more concentrated in the extracellular matrix, because the cells produce and release this substance during their metabolic activity. Its concentration is 45mmHg, whereas in the blood the concentration is around 40mmHg. So the carbon dioxide flows from the extracellular matrix to the blood until both concentrations become equivalent.

A part of this carbon dioxide is transported by the blood linked to hemoglobin, but only a low quantity, around the 23%. A small amount, the 7%, is transported dissolved in the plasma. The rest of the carbon dioxide, around the 70%, is transformed into bicarbonate by an enzyme called carbonic anhydrase. It is so how it is transported, because this substance can be easily dissolved in the plasma.

Respiratory System: Anatomy.

Control of Breathing. 

Breathing is an extremely controlled process, because it must be finely adjusted to the requirements of the body. An ordinary human being consumes around 200ml of oxygen per minute. While intense exercising, however 30 times this amount can be consumed. To increase the amount of taken oxygen the body increases the respiratory rate and depth.

The respiratory rate at rest is controlled by some areas of the nervous system located in the bulb and the pons. The Bulb Rhythmic Area controls the basic system of respiration and the respiratory rate at rest. The Pneomotaxis Centre controls the coordination between inhalation and exhalation. The Apneustic Centre controls the inhalation.

Other zones of the brain have connections to these respiratory centres and they can raise or decrease the respiratory rate when it is necessary. When the pH of the blood decreases, for instance, it is related to an increment of the bicarbonate dissolved in the plasma, so the respiratory rate must be increased to release the excess of carbon dioxide. When some receptors detect that the amount of oxygen drops, they promote an increment of the respiratory rate. There are many different chemical receptors in out body, such us the carotid and aortic receptors.

Some hormones can also have different effects on the respiratory system. Adrenalin, for instance, affects not only to the respiratory rate, but also to the amount of air inhaled by changing the diameter of the bronchioles and increasing the air flow to the alveoli. Other hormones have just the opposite effects.

The Deep Essence

Changing their appearance to look better or at least different to the rest has been an obsession to for human beings since ancient times. Even our most primitive ancestors used chemical products to modify the colour of their skin and hair or their bodily smell.

 

Perfume container from Rome, by Dennis Jarvis

In fact, perfumes are not only one of the first cosmetics produced by primitive humans, but also one of the most common, produced and sold nowadays. Classically perfumes are made up of three different groups of fragrant substances according to the duration of their smell, called low, medium and high notes.

High notes are fleeting essences, usually derived from flowers or other odorous parts of plants. Medium notes are slightly more lasting than high notes and also come from plants. Finally, low notes are the most persistent ones, are the responsible for the central smell of the perfume, that one that is present for hours. Low notes are mainly obtained from animal fluids, such us some sexual glands.

One special product used to obtain low notes is distilled from an strange material that sometimes appears in on the coast of Nordic countries. It is a spherical grey greasy structure called ambergris. The origin of this substance was a mystery for centuries.

 

Ambergris, by Peter Kaminski

To discover its precedence we must travel to the deep cold parts of the Atlantic Ocean. It is not easy to assume that three quarters parts of our planet surface is covered by several kilometres of water, and we know much better the characteristics further places such as the Moon or Mars’ surface than this profound zone of our own planet.

Hidden in that obscure world lives one of the most mythological animals, the giant squid, that is the largest known invertebrate and one of the five largest animals in general. It is so strange that nobody achieved recording a live giant squid until 21st century and we were only able to meet them when they died in superficial zones and their corpses were carried by oceanic current to the coast. Something quite infrequent.

 

Giant Squid (1920 Source: Bright, M. 1989)

The main predator of this legendary animal is the sperm whale. This cetaceous is capable of diving to the deepest part of the ocean to hunt giant squid. It is not, however, an easy task. We must realise that in abyssal zones there is not light, so the preys must be located by echolocation. Some parts of the squid, besides, are not digested easily. Although the animal is covered by a soft muscular mantle, in its interior it has a cartilaginous skeleton (that lots of us have tasted when we eat roasted squids). And its powerful beak is also extremely hard.

When a sperm whale eats a giant squid, all these hard remains can not be digested properly, so they build up in the intestine of the animal. Afterwards they are compacted, forming spherical structures. These spheres grow slowly in the guts of the animal and, when they are large enough, the whale vomits them.

 

Sperm whales, by Gabriel Barathieu

And, yes, sometimes they arrive floating at the coast and they are that substance that we call ambergris.

What a wonderful world we live in, a place where an strange substance used to produce perfumes by humans is the remains of a giant animal after being digested by the intestine of the largest carnivore on the planet, and has traveled from the deepest part of the sea to a little glass bottle in a wardrobe of our bathroom.

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.