La célula | Los secretos del cuerpo humano

DISCOVERING THE CELL

Welcome to a new post! Today we will discover the wonders of the cell, from its functions and characteristics to each of its organelles. Accompany me!

According to the Cell Theory enunciated by Schleiden and Schwann, the cell is the morphological, physiological and genetic unit of living beings. In the first place, it is morphological because all living beings are made up of one or more cells. On the other hand, it is the physiological unit because it is capable of carrying out all the necessary processes to stay alive. Finally, it is genetic because it contains the information of each living being.

There are two types of cells: prokaryotic cells and eukaryotic cells that divide into animals and plants.

Prokaryotic cells are the simplest and smallest, and are unique to bacteria (Monera Kingdom), eubacteria and archaebacteria. Regarding cell organization, DNA is dispersed throughout the cell cytoplasm without being surrounded by a membrane in a specific area called the nucleoid. In addition to the plasma membrane, it also has the cell wall and some may have a third envelope, the capsule, which provides more resistance to the cell.

In the following image you can see the different parts of the prokaryotic cell, as well as some of its functions.

Source: own image.

As for the eukaryotic cell, we can distinguish between animal and plant cells. The plant cell has a cell wall that surrounds the plasma membrane, unlike the animal cell that does not have it. Similarly, the former contains large chloroplasts and vacuoles, while the animal cell does not have chloroplasts but does have vacuoles (they are called vesicles in this type of cell), although they are small. Regarding the composition of their plasma membranes, that of animal cells contains glycogen and that of plant cells contains starch. Generally, the shape of the first is irregular and its size varies from 10 to 30 microns. However, vegetables often have a regular shape and their size is usually between 10 and 100 microns.

Next, you can observe the composition of both cells in more detail.

Source: own image.

Source: own image.

The plasma membrane is the boundary between the external extracellular and the intracellular environment that is common to all types of cells. Furthermore, it is approximately 75 Å thick and can be observed with electron microscopes. Regarding its composition, we can conclude that it is made up of 40% lipids, 60% proteins and carbohydrates.

First, the plasma membranes of eukaryotic cells have three types of lipids. These are phospholipids, glycolipids, and sterols (like cholesterol).

Furthermore, as lipids are amphipathic in nature, in an aqueous medium they are spatially oriented, forming spherical micelles or lipid bilayers. In turn, this biomolecule is responsible for the fluidity of the membrane, which depends on the nature of the lipids, the temperature and the presence of cholesterol. So when the temperature increases, so does the fluidity, and when there are many unsaturated and short-chain lipids, the membrane is more fluid.

Lipids have several forms of movement: they can move laterally, in the plane of the membrane, they can rotate on themselves, they can have bending movements in which the chains move, and finally they can undergo flip-flop processes.

On the other hand, proteins give the membrane its specific functions. Most of them have globular structure and can be classified according to the place they occupy in the membrane in transmembrane or intrinsic proteins if they are embedded in the lipid bilayers, and in peripheral or extrinsic proteins if they do not cross the bilayer and are located both in the outside as inside the membrane.

Finally, the membrane also has carbohydrates, although to a lesser extent. The most abundant are the oligosaccharides that are linked by covalent bonds to the extracellular domains of proteins and lipids, forming glycoproteins and glycolipids. These carbohydrates form the cell cover or glycocalyx, which is the envelope made up of glycoproteins, glycolipids and hyaluronic acid, which protrude from the cell membrane. In addition, it serves as the mechanical protection of cells, allows cell adhesion and intervenes in processes of cell identification and hormonal reception.

I advise you to take a look at the following drawing to differentiate all the concepts seen above.

Source: own image.

The membrane has mechanisms to physically transport molecules, allowing cells to pass metabolites necessary for the synthesis of macromolecules and to release the products derived from catabolism and secretion substances. Therefore, it could be said that it behaves as a semi-permeable barrier, allowing the passage, through various mechanisms, of substances against or in favor of an osmotic or electrical concentration gradient. In the following diagram you can see the different types of transport with their main characteristics, as well as the substances involved in each one.

Source: own image.

Next, we find the extracellular matrix, typical of the cells of animal tissues and very abundant in connective tissues such as conjunctiva and cartilage. First, it is composed of an amorphous ground substance which is a gelatinous structure of hydrated glycoproteins containing a fine network of collagen, elastin and fibronectin protein fibers. These protein fibers provide consistency, elasticity and resistance to the cell and condition their shape, development and proliferation. In turn, the amorphous ground substance is made up of proteoglycans. As for the functions that this matrix performs, the most important are to fill intercellular spaces, give consistency and resistance and serve as a union between the cells that form tissues and the tissues that form organs.

Source: own image.

Regarding the cell wall, it should be noted that it is a thick covering located on the outer surface of the membrane and that it is present in plant cells, fungi and bacteria. The function of this structure is to give shape and rigidity to the cell so that the cell does not break when the osmotic pressure changes. As you can see in the following diagram, there are three types of cell wall corresponding to each cell in which it is present. The plant cell wall is made up of cellulose fibers linked together by a matrix of polysaccharides and proteins. It can also contain lignin, suberin, cutin, and mineral salts such as calcium carbonate and silica. The cellulose of its composition is secreted by the cell itself and is arranged in different layers: middle sheet, primary wall and secondary wall. Furthermore, as with the plasma membrane, this envelope also allows the passage of substances through pits and plasmodesmata. The former occur when the secondary wall is abruptly interrupted and perforations called pits appear in the middle sheet and primary wall. While the latter are cytoplasmic connections that cross the cell wall between contiguous cells.

Source: own image.

The time has finally come to get inside the cell. Although before discovering what is inside it, it is convenient to differentiate some concepts that are subject to confusion.

Nucleoplasm: cellular part that occupies the interior of the nucleus.
Cytoplasm: cell space between the plasma membrane and the nuclear envelope. It is made up of the cytosol, the cytoskeleton, and the cellular organelles.
Hyaloplasm or cytosol: intracellular fluid.

In the first place, the cytoskeleton is a network of protein filaments whose function is skeletal since they constitute the "internal scaffold" of eukaryotic cells. In addition, they intervene in the movement of the cell. We therefore find three types of protein filaments: Microfilaments, intermediate filaments and microtubules, arranged from smallest to largest size. Microfilaments are made up of actin protein subunits and are present in muscle cells where they intervene in contraction together with myosin. Intermediate filaments are made up of filamentous proteins and they mainly exert structural functions.Finally, microtubules are only present in eukaryotic cells. Their walls are made up of subunits of the protein tubulin, and they form structures such as cilia, flagella and centrioles.

Source: own image.

Organelles are found in the cytosol and are responsible for the proper functioning of cells. They can be organelles without a membrane, organelles with a single membrane, and organelles with a double membrane. They can also be classified into non-membrane organelles, membrane organelles and energy transducing organelles.

The centrosome is the area of the cytoplasm in which the microtubule organizing center is located. It is made up of a pair of centrioles that are called a diplosome. In a centrosome with centrioles, the pericentriolar material, the aster and the diplosome are also found.

Ribosomes are tiny membrane-less organelles made up of RNA and proteins. As we saw in the previous post, they are different in eukaryotes and prokaryotes. They are in charge of carrying out protein synthesis. Each ribosome is made up of 80% water, 10% rRNA, and 10% protein. These are formed in the nucleus and through the pores pass to the cytoplasm. To learn more about this organelle, click here .

Cytoplasmic inclusions are accumulations of hydrophobic substances that are found in the cytoplasm and are not surrounded by a membrane. They are present in eukaryotic (animal and plant) and prokaryotic cells. In addition, they can accumulate energy reserve substances, pigments with or without a protective function because they are waste products, finally, they can accumulate precipitated proteins.

The Golgi apparatus is an organelle present in all eukaryotic cells. Each complex is made up of flattened sacs, bounded by membranes, loosely stacked on top of each other, and surrounded by tubules and vesicles. In addition, it has functional units called dichthyosomes that are a set of a half dozen stacked saccules or cisterns, related to each other and surrounded by small membranous vesicles. Generally, it is located next to the nucleus although in animal cells, it is also located around the centrioles. Regarding the functions it performs, it is worth highlighting the transport of substances, maturation, accumulation and secretion of proteins, glycosylation of lipids and proteins, synthesis of polysaccharides, and it serves as compaction and distribution centers.

Lysosomes are small vesicles that form in the Golgi complex and contain a wide variety of hydrolytic enzymes involved in cellular digestion processes. This organelle is present in all cells except red blood cells. The lysosomal enzymes named above are hydrolases and their optimal activity occurs at acidic pH. On the other hand, there are two types of lysosomes, the primary ones that only contain digestive enzymes, and the secondary ones that contain substrates in the process of digestion. In turn, the latter differ into digestive or heterophagic vacuoles if the substrate comes from the outside by phagocytosis or pinocytosis, or autophagic vacuoles if the substrate comes from the inside. Finally, in terms of their function, lysosomes participate in cellular digestion processes, both intracellular and extracellular.

Peroxisomes and glyoxysomes are a type of vesicles present in most eukaryotic cells, similar to lysosomes but containing oxidative enzymes such as oxidase and catalase. They are organelles responsible for the elimination of toxic substances and for synthesizing carbohydrates from lipids respectively.

The endoplasmic reticulum is a membranous organelle composed of a network of flattened sacs and tubules joined together and that limit a space called the lumen. It communicates with the Golgi complex and with the outer nuclear membrane. It is divided into rough and smooth endoplasmic reticulum. The rough ER is made up of a system of cisterns, tubes and flattened sacs, interconnected with each other and with ribosomes adhered to the cytoplasmic face of its membrane. In addition, it performs functions such as the synthesis, modification and storage of proteins and the synthesis of phospholipids and secretion proteins. On the other hand, the smooth ER does not have ribosomes and forms a system composed of interconnected membranous tubules. Regarding its functions, it performs the synthesis, storage and transport of lipids, detoxification or detoxification and intervenes in some specific responses of the cell such as muscle contraction.

Vacuoles and vesicles are part of the endomembrane system and are constituted from the endoplasmic reticulum, the Golgi apparatus, or membrane invaginations. In animal cells they are usually small and are called vesicles, while in plant cells they are usually large and are called vacuoles. In the same way, the function of the former is the temporary storage of substances and the transport of materials both within the cell and towards the interior and exterior, and the function of vacuoles is to maintain cell turgor, although they can also temporarily store nutrients and waste products.

Source: own image.

The most important are mitochondria and chloroplasts, so we took notes from an explanatory video and added information on the topic. Below, you can see how a Cornell Notes template is populated with this information.

Source: own image.

The nucleus is a large and often spherical body. This can vary depending on the cell throughout the cell cycle. At the interface an interphase nucleus is observed with the envelope intact and the chromatin unwound, and at the end of this period DNA replication occurs. In the division phase the chromatin fibers condense on themselves and lead to the chromosomes.

The nucleus is surrounded by the nuclear envelope which is made up of two concentric membranes, each of which is a lipid bilayer. These membranes separated by 20-40nm, forming the perinuclear or intermembrane space, fuse creating small nuclear pores through which materials circulate between the nucleus and the cytoplasm.

The nucleoplasm has a composition similar to that of the cytosol and it is in it that the remaining components of the nucleus are suspended: the nucleolus and chromatin, which are kept fixed thanks to a network of fibrillar proteins. In addition, the synthesis and packaging of nucleic acids and RNA and DNA nucleotides occur in the nucleoplasm.

On the other hand, chromatin is the fibrous-looking substance found in the nucleoplasm that is made up of DNA and histones. It is responsible for providing the genetic information necessary for, through transcription, to carry out the synthesis of RNA and to preserve and transmit the genetic information contained in DNA.

Next, we find the nucleolus, which is a spherical, dense corpuscle, lacking a membrane and with a granular appearance, with a high content of RNA and proteins. In addition, ribosomal RNA is synthesized in it. In addition, electron microscopy has allowed the differentiation of two components. A strictly nucleolar component in which two zones are distinguished, the granular zone (corresponds to ribosomal subunits in the maturing process) and the fibrillar zone (found inside). And a nuclear component or associated chromatin that can be found surrounding the nucleolus (perinucleolar chromatin) or within it (intranucleolar chromatin).

Source: own image.

Source: own image.

When the cell goes into division, a series of changes take place in the structure of the nucleus that lead to its disorganization. The nucleolus disappears, the nuclear envelope disintegrates, the nuclear content is released to the cytoplasm and the chromatin condenses and forms the chromosomes.

Therefore chromosomes are rod-shaped structures that appear during the division of the nucleus, when the nuclear envelope is broken. Like chromatin, they are made up of DNA and histones. Its function is to facilitate the sharing of the genetic information contained in the DNA of the stem cell between its two daughter cells.

In condensed chromosomes it is possible to distinguish several elements:

CHROMATIDE: is each of the two identical halves of a duplicated chromosome.
CENTROMER: is the primary constriction region in chromosomes and is the site where sister chromatids unite during mitosis and meiosis.
On both sides of the centromere there are structures of a protein nature called CINETOCORES that constitute the anchoring points of the fibers of the mitotic spindle.
TELOMERE: end of the chromosome and area that does not have genetic information.
SECONDARY CONSTRICTION: they are the regions of the chromosomes that are located at the ends of the arms and that give rise to spherical portions located at the end of the chromosome called SATELLITES.
BANDS: more or less wide segments of the chromosome, which appear as light and dark bands, as they stain with different intensity.

Chromosomes can be classified according to their shape, which depends on the location of the centromere, into:

a) Metacentric: the arms are equal, centromere in the center.

b) Submetacentric: centromere in the middle of two unequal arms.

c) Submetacentric with satellite zone

d) Acrocentric: centromere in the subminal position. Very uneven arms.

e) Telocentric: a single arm, centromere at one end.

Source: own image.

ACTIVITIES.

1. Why is the plasma membrane said to have a fluid mosaic structure?

The plasma membrane membrane is said to have a fluid mosaic structure because the lipids are arranged forming a phospholipid bilayer, located with their hydrophilic heads towards the external medium or towards the cytosol, and their hydrophobic tails arranged towards the interior. Proteins are intercalated in this lipid bilayer depending on the interactions with the regions of the lipid zone.

The membranes are asymmetric structures in terms of the distribution of all their chemical components: lipids, proteins and carbohydrates. Furthermore, it is not a rigid structure, but a fluid one, and it allows the movement of proteins within the lipid bilayer.

Like lipids, integral proteins are also amphipathic, since they present hydrophilic and hydrophobic areas, so they can be partially embedded in the bilayer.

Whether the membrane has more or less fluidity depends on several factors:

Degree of saturation of fatty acids in membrane lipids. The higher the degree of saturation of the fatty acids, the fluidity is lower.
Length of fatty acid chains in membrane lipids. Fluidity decreases with increasing chain length.
Temperature. The fluidity decreases as the temperature decreases.
Cholesterol proportion. Cholesterol is a molecule located between phospholipids that stabilizes membranes, that is, making them less flexible and fluid. Cholesterol is present in almost all plasma membranes of eukaryotic cells and in some prokaryotes devoid of cell walls.

2. Which type of cells will contain the greatest number of ribosomes: one that stores fat or another that stores new cells, such as epidermal cells?

Ribosomes synthesize proteins based on the genetic information of the cell. The number of these organelles varies according to the activity of making membrane material or proteins, so there will be more ribosomes in those that store new cells because more material is generated in them than in those that store fats.

3. Is it possible that a smooth endoplasmic reticulum and a Golgi apparatus coexist in a cell, both highly developed? Why?

It is not possible, since if the Golgi apparatus is highly developed, it is a fundamentally protein-secreting cell, so the rough endoplasmic reticulum would be highly developed, which is responsible for storing and transporting proteins, and not the smooth one. that its function is related to lipids.

4. The hyaloplasm and cytoplasm, do they constitute the same structure?

They do constitute the same structure since the cytosol or hyaloplasm is the inner aqueous gel of the cell that is outside the inner membranes when they exist, and the cytoplasm is the set of the hyaloplasm and the cytoplasmic organelles.

5. The eukaryotic cell: indicate the main cellular structures and organelles, what characteristics each one has and what function they perform.

The eukaryotic cell is formed by the plasma membrane , which separates the external extracellular and intracellular environment, and whose approximate composition is 40% lipids, 60% proteins and to a lesser extent carbohydrates.

The cytoplasm , composed of the cytosol (hyaloplasm) and organelles . And the nucleus where the genetic material is located.

In plant cells we also find a cell wall , a thick layer located on the outer surface of the membrane, which gives it shape and rigidity, and prevents its rupture.

Among the cellular organelles, depending on the type of membrane we find:

Organelles with double membrane:

Mitochondria : they are energy transducing organelles that carry out cellular respiration.
Plastos: among them we find chloroplasts, which present chlorophyll and are responsible for photosynthesis.

Organelles without membrane:

80S ribosomes: whose main function is protein synthesis. Composed of water, rRNA and proteins, they consist of two subunits, one of 40S and the other of 60S.
Centrioles : they are part of the centrosome but are only present in animal cells and are responsible for the distribution of chromosomes in cell division and the regulation of the movement of cilia and flagella.
Cytoplasmic inclusions : such as glycogen or starch that act as an energy reserve.

Organelles with simple membrane:

Endoplasmic reticulum : a complex system of membranes that form flattened saccules and tubules connected to each other and that define an internal space called the lumen. They communicate with the Golgi apparatus and with the outer nuclear membrane. Two types are distinguished, the smooth endoplasmic reticulum (REL) and the rough endoplasmic reticulum (RER).
Vacuoles and vesicles : endomembrane system, which are formed from the endoplasmic reticulum, the Golgi apparatus or membrane invaginations. They contribute to the maintenance of cellular turgor and serve as a reserve store.
Golgi apparatus : formed by flattened sacs, limited by membranes, stacked loosely on top of each other and surrounded by tubules and vesicles. It is also made up of functional units called dichthyosomes. It is a dynamic structure that presents polarity, that is to say, in the dictyosomes, two faces with different structure and function differ.
Lysosomes : small vesicles, formed in the Golgi complex that contain a great variety of hydrolytic enzymes involved in the cellular digestion processes, in all cells except red blood cells.
Peroxisomes : type of vesicles similar to lysosomes but that contain oxidative enzymes such as oxidase and catalase that oxidize organic substances.

6. Explain the differences and similarities between the prokaryotic cell and the eukaryotic cell.

The main difference has to do with the core. The eukaryotic cell has a nucleus in which DNA is found, while prokaryotic cells have DNA dispersed throughout the cytoplasm in a specific area called the nucleoid.

On the other hand, eukaryotic cells are present in animals, fungi, plants, algae, and protozoa. However, prokaryotic cells are only found in bacteria.

Furthermore, the ribosomes of prokaryotic cells are 70S and those of eukaryotes are 80S. All prokaryotic cells have a wall that protects and shapes the cell, but not all eukaryotic cells have it.

The membrane of prokaryotic cells does not have cholesterol. Lastly, prokaryotic cells are simpler and smaller than eukaryotic cells.

7. Explain the similarities and differences between animal and plant cells.

-Both have:

Membrane-free structures, such as the 80s ribosomes that synthesize proteins, the centrosomes involved in cell division, and the cytoskeleton, made up of microtubules, intermediate filaments, and microfilaments.
Endomembrane system, such as the endoplasmic reticulum (smooth or rough), the Golgi apparatus (stores and distributes proteins and lipids), vacuoles (stores substances) and lysosomes (participate in cellular digestion).
Organelle energy transducer with are the mitochondria (involved in cellular respiration).
Nucleus, consisting of nucleoplasm, nuclear envelope, chromatin, and nucleoli.

-Differences between both cells:

The plant cell, unlike the animal, has a cell wall that covers the plasma membrane.
The plant cell contains another energy transducer organelle, such as chloroplasts, which are responsible for photosynthesis. So another difference could be that plant cells carry out photosynthesis, and animals do not.
Plant cells usually have a large vacuole, whereas in animal cells the vacuoles are smaller but numerous and are called vesicles.
Animal cells tend to be more irregular and spherical in shape than plant cells.
The animal cell has a centrosome with centrioles, unlike the plant cell that does not have centrioles.

8. What is the difference between the ribosomes of a prokaryotic cell and another eukaryotic?

The ribosomes of eukaryotic cells are 80S, with subunits of 60S and 40s. On the other hand, those of the prokaryotic cell are 70S, with subunits of 50S and 30S.

See you in the next post! ;-)