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الأربعاء، 11 يناير 2012

Histology : human cell struture - تركيب الخلية

 
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human cell struture

Structure of a typical eukaryotic cell

The interior of the cell is divided into the nucleus and the cytoplasm. The nucleus is a spherical or oval shaped structure at the center of the cell. The cytoplasm is the region outside the nucleus that contains cell organelles and cytosol, or cytoplasmic solution.  Intracellular fluid is collectively the cytosol and the fluid inside the organelles and nucleus.
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Membranes are the gateways to the cell. The plasma membrane, is the selective barrier surrounding the cell. It provides a barrier to the movement of molecules between the intra and extracellular fluids. Recall that extracellular means outside the cell. The plasma membrane also serves to anchor adjacent cells together and to the extracellular matrix. Various signals and inputs can alter the sensitivity and permeability of membranes.
Cell, or Plasma, membrane - encloses every human cell
 Structure - 2 primary building blocks include protein (about 60% of the membrane) and lipid, or fat (about 40% of the membrane). The primary lipid is called phospholipid, and molecules of phospholipid form a 'phospholipid bilayer' (two layers of phospholipid molecules). This bilayer forms because the two 'ends' of phospholipid molecules have very different characteristics: one end is polar (or hydrophilic) and one (the hydrocarbon is non-polar (or hydrophobi

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The cytoskeleton is a filamentous network of proteins that are associated with the processes that maintain and change cell shape and produce cell movements. The cytoskeleton also forms tracks along which cell organelles move propelled by contractile proteins attached to their various surfaces. Like a little highway infrastructure inside the cell. Three types of filaments make up the cytoskeleton.



  • Cytoplasm consists of a gelatinous solution and contains microtubules (which serve as a cell's cytoskeleton) and organelles (literally 'little organs')


  • The three fibers of the cytoskeleton
    microtubules in blue, intermediate filaments in red, and actin in green–play countless roles in the cell.

    In these cells, actin filaments appear light purple, microtubules yellow, and nuclei greenish blue.
    The cyotoskeleton represents the cell's skeleton. Like the bony skeletons that give us stability, the cytoskeleton gives our cells shape, strength, and the ability to move, but it does much more than that. The cytoskeleton is made up of three types of fibers that constantly shrink and grow to meet the needs of the cell: microtubules, microfilaments, and actin filaments. Each type of fiber looks, feels, and functions differently. Microtubules consists of a strong protein called tubulin and they are the 'heavy lifters' of the cytoskeleton. They do the tough physical labor of separating duplicate chromosomes when cells copy themselves and serve as sturdy railway tracks on which countless molecules and materials shuttle to and fro. They also hold the ER and Golgi neatly in stacks and form the main component of flagella and cilia.
    Microfilaments are unusual because they vary greatly according to their location and function in the body. For example, some microfilaments form tough coverings, such as in nails, hair, and the outer layer of skin (not to mention animal claws and scales). Others are found in nerve cells, muscle cells, the heart, and internal organs. In each of these tissues, the filaments are made of different proteins.
    Actin filament are made up of two chains of the protein actin twisted together. Although actin filaments are the most brittle of the cytoskeletal fibers, they are also the most versatile in terms of the shapes they can take. They can gather together into bundles, weblike networks, or even three-dimensional gels. They shorten or lengthen to allow cells to move and change shape. Together with a protein partner called myosin, actin filaments make possible the muscle contractions necessary for everything from your action on a sports field to the automatic beating of your heart
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    Organelles


    Cell Organelles

    Cell organelles are the little workhouses within the cell. All the functions of life take place in each individual cell. Organelles can be released by breaking the plasma membrane, through homogenization and ultracentrifuging the mixture. The organelles are of different size and density and will settle out at specific rates.
    Overview of organelles
    1. The nucleus is in the center of most cells. Some cells contain multiple nuclei, such as skeletal muscle, while some do not have any, such as red blood cells. The nucleus is the largest membrane-bound organelle. Specifically, it is responsible for storing and transmitting genetic information. The nucleus is surrounded by a selective nuclear envelope. The nuclear envelope is composed of two membranes joined at regular intervals to form circular openings called nuclear pores. The pores allow RNA molecules and proteins modulating DNA expression to move through the pores and into the cytosol. The selection process is controlled by an energy-dependent process that alters the diameter of the pores in response to signals. Inside the nucleus, DNA and proteins associate to form a network of threads called chromatin. The chromatin becomes vital at the time of cell division as it becomes tightly condensed thus forming the rodlike chromosomes with the enmeshed DNA. Inside the nucleus is a filamentous region called the nucleolus. This serves as a site where the RNA and protein components of ribosomes are assembled. The nucleolus is not membrane bound, but rather just a region.
    1. Ribosomes are the sites where protein molecules are synthesized from amino acids. They are composed of proteins and RNA. Some ribosomes are found bound to granular endoplasmic reticulum, while others are free in the cytoplasm. The proteins synthesized on ribosomes bound to granular endoplasmic reticulum are transferred from the lumen (open space inside endoplasmic reticulum) to the golgi apparatus for secretion outside the cell or distribution to other organelles. The proteins that are synthesized of free ribosomes are released into the cytosol.
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    1. The golgi apparatus is a


    gilgi appratus
    1. membranous sac that serves to modify and sort proteins into secretory/transport vesicles. The vesicles are then delivered to other cell organelles and the plasma membrane. Most cells have at least one golgi apparatus, although some may have multiple. The apparatus is usually located near the nucleus.
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    1. Endosomes are membrane-bound tubular and vesicular structures located between the plasma membrane and the golgi apparatus. They serve to sort and direct vesicular traffic by pinching off vesicles or fusing with them.
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    1. Lysosomes are bound by a single membrane and contain highly acidic fluid. The fluid acts as digesting enzymes for breaking down bacteria and cell debris. They play an important from in the cells of the immune system.
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    1. Peroxisomes are also bound by a single membrane. They consume oxygen and work to drive reactions that remove hydrogen from various molecules in the form of hydrogen peroxide. They are important in maintaining the chemical balances within the cell.
      1. Microfilaments are the thinnest and most abundant of the cytoskeleton proteins. They are composed of actin, a contractile protein, and can be assembled and disassembled quickly according to the needs of the cell or organelle structure.
      1. Intermediate filaments are slightly larger in diameter and are found most extensively in regions of cells that are going to be subjected to stress. Desmosomes in the skin will contain filaments. Once these filaments are assembled they are not capable of rapid disassembly.
      1. Microtubules are hollow tubes composed of a protein called tubulin. They are the thickest and most rigid of the filaments. Microtubules are present in the axons and long dendrite projections of nerve cells. They are capable of rapid assembly and disassembly according to need. Microtubules are structured around a cell region called the centrosome, which surrounds two centrioles composed of 9 sets of fused microtubules. These are important in cell division when the centrosome generates the microtubluar spindle fibers necessary for chromosome separation.
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    1. The endoplasmic reticulum (ER) is collectively a network of membranes enclosing a singular continuous space. As mentioned earlier, granular endoplasmic reticulum is associated with ribosomes (giving the exterior surface a rough, or granular appearance). Sometimes granular endoplasmic reticulum is referred to as rough ER. The granular ER is involved in packaging proteins for the golgi apparatus. The agranular, or smooth, ER lacks ribosomes and is the site of lipid synthesis. In addition, the agranular ER stores and releases calcium ions Ca 2+

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    Mitochondria


    1. Mitochondria are some of the most important structures in the human body. They are they site of various chemical processes involved in the synthesis of energy packets called ATP (adenosine triphosphate). Each mitochondrion is surrounded by two membranes. The outer membrane is smooth, while the inner one is folded into tubule structures called cristae. Mitochondria are unique in that they contain small amounts of DNA containing the genes for the synthesis of some mitochondrial proteins. The DNA is inherited solely from the mother. Cells with greater activity have more mitochondria, while those that are less active have less need for energy producing mitochondria.


    Mitochondria are found exclusively in eukaryotic cells. These organelles are often called the "power plants" of the cell because their main job is to make energy (ATP). Mitochondria are highly unusual--they contain their own genetic material and protein-making machinery enwrapped in a double membrane
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    , cilia are hair-like motile extensions on the surface of some epithelial cells. They have a central core of 9 sets of fused microtubules. In association with a contractile protein, these microtubules produce movement in cilia. Ciliar movements propel the luminal contents of hollow organs lined with ciliated epithelium
    • Flagella & cilia - hair-like projections from some human cells
      • cilia are relatively short & numerous (e.g., those lining trachea)
      • a flagellum is relatively long and there's typically just one
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    The DNA stored in the nucleus of a single human cell spans over six feet in length if stretched from end to end. Made up of four chemical building blocks called A, C, T and G, for short, DNA contains the instructions for making all living things. The building blocks link to form the molecule's famous "double helix" structure, which allows genetic information to be copied and passed down from one generation to the next. Occasionally exposure to toxins or malfunction of cellular processes, among other things, does cause copying mistakes. Such changes over long time periods provide opportunities for organisms to adapt to new surroundings--or, cause them to die out. Discrete segments of DNA, called genes, encode the instructions for making proteins. Work horses of the cell, proteins serve as structural material, hormones, enzymes and neurotransmitters as well as play many other roles
    Tucked away inside the DNA of all of your genes are the instructions for how to construct a unique individual. Our genetic identity is "coded" in the sense that four building blocks, called nucleotides, string together to spell out a biochemical message—the manufacturing instructions for a protein. DNA's four nucleotides, abbreviated A, T, G, and C, can only match up in specific pairs: A links to T and G links to C. An intermediate in this process, called mRNA (messenger ribonucleic acid), is made from the DNA template and serves as a link to molecular machines called ribosomes. Inside every cell, ribosomes read mRNA sequences and hook together protein building blocks called amino acids in the order specified by the code: Groups of three nucleotides in mRNA code for each of 20 amino acids. Connector molecules called tRNA (transfer RNA) aid in this process. Ultimately, the string of amino acids folds upon itself, adopting the unique shape that is the signature of that particular protein.
    Transcription - DNA is used to produce mRNA




  • sequence of amino acids in a protein is determined by sequence of codons (mRNA). Codons are 'read' by anticodons of tRNAs & tRNAs then 'deliver' their amino acid.
  • Amino acids are linked together by peptide bonds (see diagram to the right)
  • As mRNA slides through ribosome, codons are exposed in sequence & appropriate amino acids are delivered by tRNAs. The protein (or polypeptide) thus grows in length as more amino acids are delivered.
  • The polypeptide chain then 'folds' in various ways to form a complex three-dimensional protein molecule that will serve either as a structural protein or an enzyme


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    Active Transport: The Sodium-Potassium Pump





  • Endo- & exocytosis - moving material into (endo-) or out of (exo-) cell in bulk form


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    Mitochondrial electron transport chain. Notice that the 'chain' of reactions that occurs with the conversion of NADH to NAD+ result in the transport of three pairs of hydrogens (2H+) (that will then result in the production of 3ATP), whereas the reactions occuring after the conversion of FADH2 to FAD result in the transport of two 2H+

    The cellular metabolism of substrates such as glucose and fatty acids (green arrows in the figure) generates hydrogens and, specifically, hydrogen carriers — NADH and FADH2. NADH and FADH2 donate electrons to the electron-transport chain that consists of proteins located in the mitochondrial inner membrane. Electrons are ultimately transported to molecular oxygen that is reduced to water in the last step of the electron-transport chain. As electrons are transferred along the electron-transport chain, the energy released is used to pump protons (H+) from the mitochondrial matrix into the mitochondrial intermembrane space. The energy produced drives the synthesis of ATP from ADP and inorganic phosphate (Pi) by ATP synthase. ATP is then made available to the cell for various processes (e.g., active transport)

    ATP synthase. The proton channel and rotating stalk are shown in blue
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