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Cell: Structure and expression, Energy Conversion and Cytoskeleton

S. Rajalakshmi
Written by S. Rajalakshmi. Posted in General Reference on 29 August 2009.
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Cell: Structure and expression

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The DNA and various levels of packaging.

Eukaryotic cells have their genetic material, usually a single nucleus, bounded by an envelope consisting of two lipid bilayers pierced by numerous nuclear pores and in continuity with the endoplasmic reticulum. Inside, there is the genetic material, DNA, observable in cells in interphase as chromatin heterogeneous distribution. This chromatin are associated with many proteins, among which the histones, and RNA, other nucleic acid. 

Such genetic material is immersed in an ongoing activity for regulation of gene expression, the RNA polymerases transcribe mRNA continuously, which exported to the cytosol, is translated into protein, according to physiological needs. Also, depending on the timing of cell cycle, the DNA can enter replication, as a prelude to mitosis. However, eukaryotic cells possess extranuclear genetic material: specifically in mitochondria and plastids, if any, these organelles retain a partial genetic independence of the nuclear genome. 

Synthesis and degradation of macromolecules

Within the cytosol, ie aqueous matrix that holds the organelles and other cellular structures are embedded many types of machinery metabolism cell: organelles, inclusions, elements of the cytoskeleton, enzymes ... In fact, the latter corresponding to 20% of the total enzyme in the cell. 

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Structure of ribosomes; 1: subunit, 2: small subunit.

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Image of a nucleus, endoplasmic reticulum and Golgi apparatus: 1, Core. 2, Poro nuclear.3, rough endoplasmic reticulum (RER) .4, smooth endoplasmic reticulum (SER). 5 Ribosome on the rough ER. 6, proteins being transportadas.7, Gall (transport). 8 Golgi apparatus. 9, cis side Golgi.10 gear, side gear trans Golgi.11, Cisterns of the Golgi apparatus.

Ribosome: The ribosomes, visible to the electron microscope as spherical particles are responsible for assembling supramolecular complex proteins from genetic information that reaches them from the DNA transcribed as messenger RNA. Elaborated in the nucleus, play its role in protein synthesis in the cytoplasm. They consist of ribosomal RNA and various types of proteins. Structurally, they have two subunits. In cells, these organelles appear in various states of dissociation. When complete, may be isolated or forming clusters (polysomes). They can also appear associated with rough endoplasmic reticulum or nuclear envelope.

Endoplasmic Reticulum: The endoplasmic reticulum is interconnected vesicular organelle that forms tanks, tubes and flattened saccules interconnected. Involved in functions related to protein synthesis, glycosylation of proteins, metabolism of lipids and some steroids, detoxification, and the trafficking of vesicles. In specialized cells such as myofibrils or muscle cells, differs in the sarcoplasmic reticulum, organelle crucial to produce the muscular contraction.

Golgi Apparatus: The Golgi apparatus is an organelle composed of stacks of saccules called dictyosomes, although as a dynamic entity, they can be interpreted as specific structures result from the coalescence of vesicles. Get the vesicles of endoplasmic endoplasmic reticulum remain to be processed. Among the functions held by the Golgi apparatus are the glycosylation of proteins, selection, destination, glycosylation of lipids and the synthesis of polysaccharides of the extracellular matrix. It has three compartments, one proximal to the endoplasmic reticulum, called the cis compartment ', which produces the phosphorylation of the Mafia of the enzymes should be directed to the lysosome, and the "intermediate compartment" with abundant Mannosidases and N-acetyl-glucosamine transferases and the "trans compartment or network ', the most distal, where waste is transferred galactose and sialic acid, and the emerging vesicles with different cell fates.

Lysosome: Lysosomes are organelles that contain many hydrolytic enzymes. Very variable morphology, has not been found to occur in plant cells. One feature that unites all the lysosome is the possession of acid hydrolases: proteases, nucleases, glycosidases, lysozyme, arylsulfatase, lipases, phospholipases and phosphatases. It comes from the fusion of vesicles from the Golgi apparatus, which, in turn, coalesce into a type of organelle called the endosome early, which, when acidified and hydrolytic enzymes win passes to become the lysosome function. Their functions range from the degradation of endogenous macromolecules or from the phagocytosis intervention in processes of apoptosis. 

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The state regulates vacuole of plant cell turgor.

Vacuole Plant: The plant vacuoles, numerous small cells meristematic and little and large differentiated cells are organelles unique to the representatives of the plant world. Immersed in the cytosol, are bounded by the tonoplast, a membrane lipid. Its functions are to facilitate the exchange with the external environment, maintain turgor cellular, cellular digestion and accumulation of reserve substances and products of metabolism. 

Cytoplasmic Inclusion: Inclusions are delimited by membrane accumulations never substances of various kinds, both plant and animal cells. Typically these are substances that are kept as reserve metabolic acquis: starch, glycogen, triglycerides, proteins ... although there are of pigments. 

Energy Conversion

The metabolism is based on cellular processing of some chemicals, called metabolites, in others, such chemical reactions evolve catalyzed by enzymes. While much of the metabolism occurs in the cytosol, such as glycolysis, There are specific organelles.

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Model of mitochondria: 1, inner membrane; 2, outer membrane; 3, mitochondrial crest, 4, mitochondrial matrix.

Mitochondria: Mitochondria are organelles in appearance, variable number and size involved in the Krebs cycle, oxidative phosphorylation and the electron transport chain of respiration. They have a double membrane, external and internal, including a left perimitocondrial space, inner membrane folded into cristae inside the mitochondrial matrix, has a large surface area. The interior generally has a single molecule of DNA, the mitochondrial genome, typically circular, and ribosomes more similar to bacteria than to eukaryotes. According to the endosymbiotic theory, it is assumed that the first protomitocondria was a type of proteobacteria . 


Chloroplasts: Chloroplasts are cell organelles in eukaryotes involved in the photosynthesis. They are limited by an envelope formed by two concentric membranes and contain vesicles, the thylakoids, which are organized pigments and other molecules involved in converting light energy into chemical energy. Besides this function, the plastids are involved in intermediary metabolism, producing energy and reducing power, synthesizing purine bases and pyrimidine, some amino acids and all fatty acids. Furthermore, it is common inside the reserve accumulation of substances such as starch. is considered to have analogy with the cyanobacteria. 

Peroxisome: Peroxisomes are organelles common in the form of vesicles containing abundant type enzymes oxidase and catalase; of so abundant, it is common crystallize inside. These enzymes perform functions of detoxification cell. Other functions of peroxisomes are general flavin oxidation, catabolism of purines, the beta-oxidation of fatty acids, the glyoxylate cycle, metabolism of glycolic acid and detoxification in general. vesicles are formed from the endoplasmic reticulum. 

Cytoskeleton


Cells possess a scaffold that allows the maintenance of its form and structure, but more importantly, this is a dynamic system that interacts with other cellular components to generate a high degree of internal order. This framework is composed of a series of proteins that are grouped resulting filamentary structures through other proteins interact with each other resulting in a kind of lattice. The aforementioned scaffolding called the cytoskeleton, and its major elements are microtubules, microfilaments and intermediate filaments.


Microfilaments: Microfilaments or filaments of actin are formed by a globular protein, actin, which can polymerize resulting filamentary structures. This actin is expressed in all cells of the body and especially in the muscle as it is involved in muscle contraction by interaction with myosin. Besides, it has binding sites for ATP, which provides its polarity filaments. can be found in free form or polymerized into microfilaments, which are essential for such important cellular functions such as mobility and cell contraction during the cell division. 

Eukaryotic cytoskeleton: microfilaments in red, microtubules in green and nucleus in blue.
Microtubules: Microtubules are tubular structures 25 nm in diameter and about 12 nm outer internal diameter, with lengths ranging from a few nanometers to micrometers, which originate from the microtubule organizing centers that are spread throughout the cytoplasm. They are found in eukaryotic cells and are formed by the polymerization of a dimer of two protein globular, the alpha and beta tubulin. Tubulins possess the ability to bind GTP. The microtubules are involved in various cellular processes that involve movement of vesicles of secretion, movement of organelles, intracellular transport of substances as well as in cell division (mitosis and meiosis) and which, together with microfilaments and intermediate filaments form the cytoskeleton. They constitute the internal structure of cilia and flagella. 
Intermediate Filaments: Intermediate filaments are cytoskeletal components. Groups formed by fibrous proteins, its name derives from its diameter of 10 nm, less than that of microtubules, 24 nm, but higher than that of the microfilaments, 7 nm. They are ubiquitous in animal cells, and do not exist in plants or fungi. Are a heterogeneous group, divided into five families: the keratins in epithelial cells, the neurofilaments in neurons, the gliofilaments in glial cells, the desmin in smooth muscle and striated, and vimentin, in cells derived from the mesenchyme. 

Micrograph the scanning electron microscope showing the surface of ciliated epithelial cells of the bronchioles.
Centrioles: Centrioles are a pair of structures that make up the cytoskeleton of animal cells. Such a hollow cylinders, are surrounded by a dense protein material called pericentriolar material, all of which form the centrosome or microtubule-organizing center that allow microtubule polymerization of tubulin dimers that make up the cytoskeleton. The centrioles are positioned perpendicular to each other. Its functions are involved in mitosis, during which generate the spindle and in cytokinesis, and is postulated to intervene in the nucleation of microtubules. 
Cilia and flagella: These are cell surface specializations with motility, with a structure based on clusters of microtubules, both differ in the greater length and lower number of flagella, and increased variability of the molecular structure of the latter .


S. Rajalakshmi

Author: S. Rajalakshmi

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