|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.|
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.
|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.
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.
- 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.
- 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 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.|
The reproductive process of the cell is called cell division. The basic cell division carried out by regular cells is called 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 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.
- 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.
- 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.
- 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.
The cytokinesis usually begins during anaphase, and ends at the final part of telophase.
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.
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.
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.
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.|