Plant Cell Features PDF

# TEMA 13: PECULIARIDADES DE CÉLULA VEGETAL ## PECULIARIDADES DE LA CÉLULA VEGETAL The vegetal cell is eukaryotic, these are the cells that we find in more evolved organisms. We can find eukaryotic cells in animals, higher plants, fungi and mosses, and also cell walls in some algae. Prokaryotic...

# TEMA 13: PECULIARIDADES DE CÉLULA VEGETAL ## PECULIARIDADES DE LA CÉLULA VEGETAL The vegetal cell is eukaryotic, these are the cells that we find in more evolved organisms. We can find eukaryotic cells in animals, higher plants, fungi and mosses, and also cell walls in some algae. Prokaryotic cells can be found in organisms with a lower degree of evolution, such as bacteria and cyanobacteria. There are differences between eukaryotes and prokaryotes. The vegetal kingdom includes algae, fungi and bacteria, but we are going to refer to higher plants. The plant kingdom includes multicellular organisms with eukaryotic cells that have a cell wall and organisms that perform photosynthesis. Higher plants are those that produce flowers, fruits and reproduce through seeds. In this vegetal kingdom, we can also find ferns, mosses and algae, but these do not produce flowers nor do they reproduce by seeds. In a vegetal cell, we are going to find the same components as we find in an animal cell: - Nucleus - Endoplasmic Reticulum - Microtubules - Mitochondria - Golgi Apparatus - Ribosomes - Plasma membrane The difference between an animal and a vegetal eukaryotic cell is that: * **Large central vacuole**. The distinctive feature of this characteristic is not that the vegetal cell has a vacuole and the animal cell doesn't. What differentiates them, is that in animal cells, vacuoles are numerous and small. In contrast, in the vegetal cell, there is one vacuole and it is large, taking up almost the entire volume of the cell in some cases. When we are studying the entry of water into this cell, it is taken into account that water enters from the cellular exterior into the vacuole and exits from the vacuole to the exterior, why are there large vacuoles in vegetal cells?. This large vacuole could serve as a reservoir in vegetal cells, a place where substances can be stored, ranging from organic compounds of the cells, vegetal hormones, mineral elements to compounds that should not exist in the cell like herbicides or insecticides (compounds that are XENOBIOTICS, that is to say, foreign to the plant and are found in the cytosol but are sequestered by the vacuole to avoid rapid damage to the cell). Vegetal cells need a large vacuole because, unlike animals, they do not have the ability to move freely, and while they can move, they cannot be transported. This implies that if a plant has a lot of water one day, it has to take advantage of it and store it because it does not know if it will have it the next day. This vacuole is also important so that the cells can grow, this vacuole must occupy most of the cellular volume and induce pressure on the plasma membrane. Also, in vegetal cells, in some cases 80-90% of the fresh weight is water (volume of the vacuole). * **Presence of plastids.** There are many types of plastids such as chloroplasts (the green organelle that performs photosynthesis) which are the best known. However, vegetal cells have several types of plastids. * **Presence of a cell wall**. This structure can be found outside the cell. It is in close contact with the plasma membrane. The cell wall does not exist in animal eukaryotic cells. ## CELL WALL The cell wall acts as a mediator in the relationships of the cell with its surroundings (both with other cells and with the exterior). It is made up of a middle lamella that serves as a link between neighboring cells and contains pectin, a primary wall that allows the exchange of substances, it is thin, flexible and found in growing cells and, a secondary wall that does not allow the exchange of substances, it is more rigid and thicker, it is the last one to form and appears when cells have stopped growing, that is to say, they have specialized (although not all cells have it). It is noteworthy to mention the presence of plasmodesmata which are cytoplasmic bridges that cross both walls and connect the cytoplasms of two adjacent cells, promoting the exchange of signals and substances (they can never move cell organelles) and, inside plasmodesmata, desmotubules can be found which are membranous channels containing remnants of RER. Also, it is important to know that there is a great diversity in the cell wall (fimbriae, hairs, trichomes, thickenings) and that the cell wall can determine the structure of the cell. ## PRIMARY WALL **PHASES:** The crystalline microfibrilar phase is very well structured, it is formed of cellulose microfibrils (the only component of this phase) and does not allow growth. It is a poorly hydrolyzed and rigid phase. The microfibrils contain multiple chains of cellulose linked together by hydrogen bonds and in one of them we find between 40 and 70 chains of cellulose (250 microfibrils form a cellulose fibril and 1500 fibrils of cellulose form 1 fiber of cellulose). Its so well-structured organization is due to the hydrogen bonds (due to its large number of links, not because of the nature of those links). On the other hand, the amorphous or non-crystalline matrix is less organized and very hydrolyzed (containing up to 65% water) and, it is related to cell growth and the growth of the cell wall. This phase joins the microfibrils and prevents them from separating (that is, it prevents growth) at certain times and allows it at other times. There are three components found in this phase: hemicellulose which binds cellulose microfibrils, pectins and proteins. **COMPOSITION:** The percentage (add presentation) by dry weight shows us that carbohydrates predominate in the primary wall and, in addition, they form complex networks. ### Cellulose It is a polymer made up of a number of glucose molecules linked (two glucose molecules forming cellobiose, which is the one that is repeated in cellulose) by beta-1,4 bonds, forming a very rigid and linear chain without branching and insoluble in water. The synthesis of cellulose in the primary wall is produced from UDP-glucose (uridine diphosphate, a molecule rich in energy, very reactive and which allows the binding of other molecules by linkage) and takes place in a protein complex found in the plasma membrane called cellulose synthase. Sucrose (transport form of plant carbohydrates) is hydrolyzed and the glucose from this is used to form the primary wall cellulose in a way that the cellulose synthetase complex uses UDP to achieve it and is made up of the following groups of proteins together with their functions: * Proteins that make up the catalytic subunit that are responsible for producing cellulose. * Proteins that make up the pore or rosette that allow the exit of cellulose chains formed outside of the cell. * Proteins of assembly that are responsible for linking cellulose chains with each other by hydrogen bonds to form microfibrils. ### Hemicellulose Synthesized in the Golgi apparatus, they are complex branched polysaccharides that do not have an exact nature and that, in a primary wall, we can find hemicellulose formed of glucose (glucans), xylose (xylans), and xyloglucans (the most abundant and which are made up of the main glucose chain and ramifications of xylose). In other examples, we can see how the main chain is at the end of the name, while the ramification / ramifications are at the beginning of the composite name (eg: fucogalactoxyloglucan or arabinoxyloglucan). In addition, it may have between 200 and 500 units of monosaccharides, it does not have a defined structure and does not form bonds by hydrogen bonds. The function of hemicellulose in the primary wall is the linking of cellulose microfibrils, although hemicellulose in the amorphous matrix frequently undergoes enzymatic attack by enzymes that break the hemicellulose causing the microfibrils to separate and, therefore, causing the growth of the cell wall and the cell. ### Pectin Synthesized in the Golgi apparatus, they are the hegemonic components of the middle lamella of the cell wall, they are complex and they can be formed of acidic monosaccharides (galacturonic acid) or neutral monosaccharides (galactose and arabinose). They are components soluble in water where they form gels, they are used in the food industry as a thickener for food and they are attributed medicinal properties (depurative). The chains present many hydroxyl groups that give negative charges between the two pectin chains that enclose calcium ions (Ca2+) and form egg box structures. These can be linear or branched and determine the size of the pore in the cell wall matrix that allows the access of certain molecules or symbionts (bacteria that associate with radical cells). In addition, in dicotyledonous cells they represent 30%-50 % of the dry weight and in monocotyledonous cells between 2%-3%. Homogalacturonans are pectins with a linear chain formed of galacturonic acid that are linked by alpha-1,4 bonds. Ramnogalacturonans are branched and there are two types: * 1 (look for name) that has a central axis of galacturonic acid that is occasionally interrupted by rhamnose and whose bonds are: alfa-1,2 but it can also have galactose or araminosa with alfa-1,5, alfa-1,2 or alfa-1,3 links * 2 (look for name) are smaller, more complex, less frequent and have a main chain of galacturonic acid that is not interrupted and to which other sugars can be added such as arabinose, fucose or galacturonic acid among others. They may also contain rare sugars such as acetic acid, 3-deoxy-manooctuluronic acid and apiose. ### Proteins The protein part is synthesized in the ER but if it consists of other parts, they are synthesized in the Golgi apparatus. In the primary wall, we find protein rich in amino acids such as threonine, proline, hydroxyproline or glycine that intervene in cell growth (extensive, glycoproteins) and in recognition (lectins, glycoproteins) (important for symbiosis with bacteria). The extensive ones are proteins with enzymatic activity that make cuts, preventing cell growth and that allow hemicellulose synthesis, which strengthens the cell wall when there is a weakening in this area. Also, there are other groups in the primary wall with enzymatic activity (lesterases, peroxidases, hydrolases). ## SECONDARY WALL It is difficult establish the limit between the primary wall and the secondary wall. The secondary wall is smaller than the primary wall and, in addition, it is in contact with the membrane. It is formed in cells that are not growing and it is rigid and hard with a similar content to the primary wall but with some variations: cellulose predominates in wall 2 (tracing proteins, low abundance of pectins): up to 90 % and it generally does not have proteins. The rigidity of the wall is due to the proportion of the components that make it up: * Lignin, a phenolic polymer that provides hardness to the wall, it is located between microfibrils preventing their separation. * Others: suberin (or cork), cutins, waxes, oils ... In addition, when the secondary wall suffers attacks from pathogens, it releases oligosaccharids (which are unpleasant to taste) as a defense mechanism. ## PHYSIOLOGICAL ROLES OF THE CELL WALL * It serves as an exoskeleton (giving shape and rigidity) and, due to this, determines the form of the multicellular organism (plants). * It makes it possible for turgor pressure to form, which allows the wall to grow due to its swelling. * It keeps the cells joined (middle lamella). * It determines the resistance of the plant against external factors (such as wind) because in areas with a greater risk there are cells with a secondary wall that provides greater resistance. * It makes it possible for the recognition of pathogens because in the cell wall there are recognition proteins (lectins) that determine the type of molecule that will enter the cell. In addition, lectins determine what plants are associated with. # TEMA 14: CHLOROPLASTS ## TYPES OF PLASTIDS In general, the chloroplasts of vegetal cells are classified into two large groups: leucoplasts that lack pigments and chromoplasts that have pigments (the group to which chloroplasts belong). Leucoplasts have a storage function (such as amyloplasts (storing starch) or proteinoplasts (proteins) or they may not have a storage function such as proplastids which are the precursors (origin) of the rest of the plastids and are found in meristematic cells, and etioplasts. Chromoplasts can perform photosynthesis (chloroplasts) and have pigments such as chlorophyll or they may be inactive in photosynthesis such as chromoplasts themselves. A chromoplast gives color to the flower or the fruit (important for attracting insects, for fruit and seed dispersal), it contains carotenoids (pigments formed of xanthophylls and carotenes), anthocyanins, flavonoids (they are found in vacuoles and are important in the above processes but not in chromoplasts). The proplast (found in a cell that has just emerged from cell division and is the driving force of plastids) receives light and begins to take on a greenish hue because chlorophyll forms in it, in other words, it originates a chloroplast. Within it, there are a series of structures that the proplast does not have and that are formed from the inner membrane of the chloroplast (the chloroplast has two structures). However, in the absence of light, an etioplast is formed where there are no green structures of the chloroplast or the green of chlorophyll. An etioplast would be a leucoplast (it does not have storage substances or pigments) such as the proplast. One of the cases where the etioplast is formed is when the seed germinates and the etioplast is considered as a tubular structure or prolamellar body, because there are lamellae in the etioplast that allow the formation of a proplast with a little light. Once in the chloroplast (which is only formed exclusively in the presence of light) there are lamellae formed by an invagination process (which the inner membrane undergoes) and it results in the lamellar system. The underdeveloped chloroplast can divide because it has its own DNA, which allows it to self-divide, self-replicate. Through a developed chloroplast, more proplastids can be formed by invagination and also a chromoplast itself (pigment that is not chlorophyll) such as, for example, oranges that are not ripe (chloroplast) and when they ripen (chromoplast). That is to say, chloroplasts degrade and they originate chromoplasts. Finally, it is worth mentioning that from a proplast any type of plastid can be formed and from a chloroplast, a leucoplast can be formed (the latter under conditions of darkness). ## EVOLUTIONARY ORIGIN It is produced by endosymbiosis by a prokaryotic ancestor and a eukaryotic cell. In this case, it is not a normal endosymbiosis, the eukaryotic cell "swallows" the cyanobacteria, which causes the latter to lose its potential, remaining with the photosynthetic function. The first test shows that the chloroplast genome is similar to the prokaryotic one. Secondly, the ribosomes of chloroplasts are 70s (prokaryotic ribosomes). Finally, chloroplasts have two membranes: one from the ancestor and another from the eukaryotic cell. ## MOVEMENTS Chloroplasts, of which there are approximately 40 in one cell, move with the cytoplasmic current (cyclosis) and, in addition, they have two other types of movement taking into account the intensity of light: when they receive little light, they orient themselves perpendicularly to the projection of the intensity and, when they receive a lot of light, parallel to it. They also have movements of contraction and dilation thanks to the chloromyosin. ## MORPHOLOGY Chloroplasts, which are oval-shaped, they are formed in cells that make up the green tissue of the plant (stems, leaves, fruit peel of unripe fruits). In the transversal section of the leaf, the cells of the palisade parenchyma and the cells of the spongeous or lacunar parenchyma (close to the back of the leaf) are appreciated. In both cases, the cells are parenchyma tissue, tissue that fills spaces (lightly specialized) but the cell wall is specialized in leaves. Epidermal cells lack chloroplasts but there is an exception which are the guard cells of stomata or "wanda". In them, we find two membranes (inner and outer) and between them, the intermembranous matrix. The set of membranes is called an envelope. The internal and external membranes have differences in composition and function but being biological membranes, they must be made up of a phospholipid bilayer where the non-polar inside is predominant, the polar outside. However, in chloroplasts, phospholipids predominate over sulfolipids and galactolipids. In addition, they have proteins but not carbohydrates. On the one hand, the external membrane is porous, not selective (it lets metabolites that are in the cytoplasm through), it only discriminates in terms of molecular size (it is not specific and allows small molecules to pass) and it has the porin protein. On the other hand, the inner membrane is a selective barrier of the chloroplast, that is to say, it does not let ionic compounds through but it is permeable to CO2, to dicarboxylic acid and to monocarboxylic acid (acetic, glycolic ...), slightly permeable to amino acids and does not let sucrose or other sugars like pentoses or hexoses pass through. In the chloroplast and in the photosynthetic process, sucrose is not produced, it is produced outside the chloroplast. However, in the photosynthetic process, sugars are produced but much simpler ones such as the fibrosa (3-carbon sugars). The external membrane has proteins such as the phosphate transporter (it allows phosphate to pass through the internal membrane (from the cytosol to the chloroplast) and triose phosphate (it leaves the chloroplast to the membrane)), the glycerol transporter, the glycolate transporter (these last two are important for photorespiration (a process that depends on light and enters oxygen and releases carbon dioxide)), dicarboxylates of 4 and 5 carbon atoms. Inside the chloroplast, we find drops of lipids, starch grains (in fact, they are produced inside chloroplasts), DNA and a lipid rich in enzyme complexes called stroma, and thylakoids (lamellas or lamellae) which give the chloroplast its green color (chlorophyll is located in the thylakoid membranes). The thylakoids have their thylakoid membrane that separates them from the liquid of the stroma and these, can be grouped and form grana thylakoids or they can be loose and called stroma thylakoids. The space of a thylakoid is covered by the intercolloidal space or lumen. In thylakoid membranes, reactions of light capture, electron transport, reducing power formation and energy formation (light phase of photosynthesis) take place, among others. On the other hand, the Calvin cycle (CO2 fixation) takes place in the stroma (dark phase of photosynthesis). The number of grana that there are in a chloroplast is variable and depends on the type of photosynthetic cell, its physiological characteristics, where it develops, on average there are 50 grana per chloroplast. A granum takes up between 8 and 10 thylakoids and the size of a granum is 0.2 by 0.3 micrometers. The thylakoid membrane is separated from the envelope of the chloroplast in most cases although, on some occasions, it is attached to the inner membrane of the chloroplast. The chloroplast DNA allows it to self-divide, self-replicate and, it allows it to produce or synthesize chloroplast-specific proteins (such as, for example, the **rubisco** that is found in the stroma where CO2 is fixed in the Calvin cycle and is also a macromolecule, a large protein that is formed of 16 units (8 large and 8 small). To produce this macroprotein, the large units are produced thanks to chloroplast DNA, while the small ones are formed through the information of the nuclear DNA of the cells. Therefore, there are also protein transporters in the inner membrane. The structure is understood as what is found inside the thylakoid membranes: * Photosnthetic pigments (chlorophylls of photosynthesis) To study the structure of the chloroplast, the thylakoids are put in a liquid, they are stained with C and platinum and they are broken. When the study is carried out, 4 complexes of the thylakoid membrane are appreciated: (chlorophyll is found in the photosystems - in higher plants, chlorophyll a and chlorophyll b which are always linked to proteins - chlorophyll a has two absorption peaks : 420 nm (peaks in the blue) and 663 nm (peaks in the red), and b at 430 and 644 nm (peaks in the red) and that's why we see it green. * **Photosystem 2 (complex).** They are only found in stacked regions. * **Photosystem 1.** They are only found in non-stacked regions. * **Complex CF0/Cf1 ATP synthetase.** It produces ATP in chloroplasts and it is only found in non-stacked regions. * **Cytochrome b6f.** It is found in both regions because it serves as a coupling between the two photosystems, allowing the flow of electrons, for example. The function of these 4 complexes does not depend on the type of thylakoid and they can be called stromatic thylakoids. They depend on whether the membrane of that thylakoid is bathed or not by the stromatic liquid. The membranes that have no contact are called stacked regions and those that do are called non-stacked regions. * **plastoribosomes** are found in the chloroplast ## CHEMICAL COMPOSITION They depend on the envelope of the membrane (lipids such as galactolipids (digalactosyl diacyl glycerol in the external membrane and monogalactosyl diacyl glycerol in the internal membrane) and sulfolipids (in both external and internal membranes) although there are also phospholipids (phosphatidylcholine in the external membrane and phosphatidylglycerol in the internal membrane) and lipid derivatives such as carotenoids), in the thylakoid membranes, in the stroma and in the thylakoid. We can also find proteins such as transporters (phosphates, glycerol...) and glycosyltransferases that are enzymes that catalyze the formation of glycolipids of the internal membrane of the chloroplast. They depend on the thylakoids: complexes (in photosystems), pigments and lipids, electron transporters (cytochromes (cytochromes, quinones, plastocyanins), and protons, the ATP synthetase, lipids, carotenoids (xanthophylls, carotenes that are lipid derivatives) They depend on the stroma (which is a liquid rich in proteins such as those of the Calvin cycle that carry out the CO2 fixation): a set of proteins that catalyze the reduction of CO2 (the inorganic compounds of the presentation are oxidized and they have to be reduced (eg: NO3- is reduced to NH2- or NH4+); the plant although it absorbs nitrate, what it uses is the reduced one; there are enzymes that reduce nitrate and this process takes place in the stroma), nucleic acids, plastoribosomes (60 s), organic molecules (sugars from photosynthesis, fatty acids, proteins, starch, lipids), inorganic compounds (magnesium, phosphates, chloride). ## PHYSIOLOGICAL FUNCTIONS OF CHLOROPLAST * CO2 reduction center (a vital organ for plants and for life on Earth because the photosynthetic process closes the carbon cycle in the biosphere, that is to say, the CO2 that we, aerobic organisms, expel or that is produced in combustion processes returns to Earth through the photosynthetic process. In addition, it constitutes an energy storage process (solar radiation in the form of C-C links) on Earth, which influences the living conditions of organisms on Earth. The Rubisco enzyme is the most abundant in the world because it represents 25 percent of the proteins in the leaf.) * It is the center for the reduction and assimilation of nitrate and sulfate in the form of amino acids. * The chloroplasts allow for stomatic opening and closure that are mediated by changes in the water status of the guard cells (opening (gain) or loss (closure) of the water of the guard cells). # TEMA 15: CELL DIVISION ## CELL CYCLE Cells have a fundamental characteristic: their capacity for cell division or reproduction. Despite the vast diversity of cells, cell divisions have common points: * DNA replication must occur. * The copy of DNA must be separated. * The two daughter cells must be separated, completing the division process. In multicellular organisms, all cells are obtained through the division capacity of a cell that is newly formed. In a human being, there are 100^14cells through this division capacity of a cell. Erythrocytes have an average lifespan of 4 months, so that the organism can maintain the red blood count, a million divisions must take place. Every cell in its lifetime goes through the cell cycle. This cycle has two periods: the largest one is the interphase and the cell division period. Interphase is larger, it is variable and metabolic activities, DNA synthesis takes place and it is understood in 3 stages: * G1 * S * G2 Cells can remain in the interphase for a long time. Other cells remain in the cycle (meristematic cells) and they are constantly dividing. In animal cells, certain levels of order are maintained, whereas certain restrictions apply regarding regions of division in plants, for example: neurons do not divide and they are always in the interphase, even though this does not mean that new neurons are not formed (neurogenesis occurs in certain parts with the capacity to form new neurons). Lymphocytes (white cells) have a low division capacity. Apart from these, new cells are constantly being formed in the body. The division phase consists of karyokinesis (nuclear division) and cytokinesis (cytoplasmic division). In total there are 4 stages: G1, S, G2 and M. ## G1 Growth phase in the cell cycle just before DNA replication occurs, where there is a lot of protein and DNA synthesis. The cell doubles its volume and mass. Usually, when vegetal cells specialize, they exit the cell cycle at this stage. G stands for grow. ## S (stands for synthesis) DNA replication takes place ## G2 Continued synthesis of proteins and RNA (mainly the ones involved in mitosis). In vegetal cells, some cells exit the cycle after the chromosomes are duplicated, forming polyploid cells that contribute to the formation of new species. After this phase in eukaryotes, phase M is induced by the nucleoplasmatic ratio (size of the nucleus in relation to the size of the cytoplasm----nucleus/cytoplasm); the nucleus finds it difficult to control so much cytoplasm and division is induced. In the case of prokaryotes, cell division is by binary fission where the DNA is circular and is not separated from the rest of the membrane but it is found in the nucleoid (a similar process occurs in eukaryotes but in this case, they are unicellular organisms where cell division constitutes a method of asexual reproduction where copies identical to the progenitor are obtained). ## M PHASE OR MITOSIS To study it it is divided into several stages (it is actually a continuous process): prophase, metaphase (between these, the prometaphase or late prophase is found), anaphase and telophase (in these two last phases, cytokinesis begins to take place). ## PROPHASE The mitotic spindle begins to form (a set of microtubules that are responsible for driving chromosomes during cell division). Centrioles duplicate and they begin to disappear the nuclear membrane and chromosomes condense? . The nucleolus also disappears. ## LATE PROPHASE The nuclear membrane has dissolved completely and kinetochores are created (protein structures where microtubules are attached). Movement of the microsomes begins. ## METAPHASE Chromosomes are placed in the equator of the cell by the mitotic spindle fibers, ensuring that when they are separated, each nucleus receives a copy of each chromosome. In metaphase of mitosis, centromeres do not divide. Chromosomal alignment occurs, but no chromosome pairing or genetic recombination occurs. ## ANAPHASE Chromosomes are pulled to opposite sides of the cell by the acromatic spindle fibers. Cytokinesis may start at this stage, the cell has an elongated shape so that the new nuclei are very far apart and the acromatic spindle fibers cause this. ## TELOPHASE The nuclear membrane is restored, chromosomes decondense, the separation of daughter nuclei increases and cytokinesis continues (or starts). The process ends with karyokinesis (separation of organelles). The process ends with cytokinesis in which daughter cells that are identical to the parent cells are obtained. Cytokinesis in animal cells takes place by strangulation of the cytoplasm, where filaments and intermediate microfilaments of the cytoskeleton participate, forming a fibrous ring composed of the protein actin. In the case of plant cells, cytokinesis has the hindrance of the cell wall, so the process takes place by means of a phragmoplast, a septum that forms between the daughter nuclei, formed from Golgi apparatus vesicles that fuse and attach to side walls of the cell and, where there is no fusion, plasmodesmata are formed. When cells exit the cycle, they go into a resting phase and specialize (a process called cell differentiation, which makes the organism more efficient, increases sensitivity to stimuli) except for meristematic cells. The distribution of organelles is not always equal, which may result in two cells of different sizes. Some organelles, such as mitochondria divide during interphase, guaranteeing that both daughter cells have mitochondria ## CELL CYCLE CONTROL There are several factors such as changes in temperature, the availability of nutrients, the pH... that influence the speed of the cell cycle. However, it depends on a molecular clock that cells have that determines when the cell should divide.. A set of specialized cells constitute tissues and the grouping of tissues constitutes organs and these, constitute apparatuses or systems. That is to say, the differentiation process is a distribution of functions. Apoptosis is a natural process (cells are constantly being eliminated, for example, in the bone marrow) that causes programmed cell death, which helps to control growth and development of organisms, eliminating damaged cells or cells infected with viruses or abnormally growing or, in general, cells that are not useful.. This process is important for preventing diseases such as cancer since it eliminates cells that are multiplying abnormally due to DNA alteration. On the other hand, autophagy is a process whereby certain organelles are engulfed by a double membrane and they are literally digested and it occurs when there are damaged organelles or there is an excess of production or ROS. That is, it is a recycling process that the cell has that allows it to obtain energy and to live in stressful conditions. ==End of OCR for page 1==