www.soran.edu.iq m. saadatian cell structure 1. 2
TRANSCRIPT
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PeroxisomesPeroxisomes = Membrane-bound organelles that contain specialized teams of enzymes for specific metabolic pathways; all contain peroxide-producing oxidases.
• Bound by a single membrane
• Found in nearly all eukaryotic cells
• Often have a granular or crystalline core which is a dense collection of enzymes
• Contain peroxide-producing oxidases that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide
• Contain catalase, an enzyme that converts toxic hydrogen peroxide to water
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Peroxisomal reactions have many functions, some of which are:
• Breakdown of fatty acids into smaller molecules (acetyl CoA). The products are carried to the mitochondria as fuel for cellular respiration.
• Detoxification of alcohol and other harmful compounds. In the liver, peroxisomes enzymatically transfer H from poisons to O2.
• Specialized peroxisomes (glyoxysomes) are found in heterotrophic fat-storing tissue of germinating seeds.
• Contain enzymes that convert lipid to carbohydrate.
• These biochemical pathways make energy stored in seed oils available for the germinating seedling.
• Current thought is that peroxisome biogenesis occurs by pinching off from preexisting peroxisomes. Necessary lipids and enzymes are imported from the cytosol.
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Mitochondria and chloroplasts are the main energy transformers of cells
Mitochondria and chloroplasts are organelles that transducer energy acquired from the surroundings into forms useable for cellular work.
• Enclosed by double membranes.
• Membranes are not part of endomembrane system. Rather than being made in the ER, their membrane proteins are synthesized by free ribosomes in the cytosol and by ribosomes located within these organelles themselves.
• Contain ribosomes and some DNA that programs a small portion of their own protein synthesis, though most of their proteins are synthesized in the cytosol programmed by nuclear DNA.
• Are semiautonomous organelles that grow and reproduce within the cell.
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MitochondriaMitochondria = Organelles which are the sites of cellular respiration, a
catabolic oxygen-requiring process that uses energy extracted from organic macromolecules to produce ATP.
• Found in nearly all eukaryotic cells
• Number of mitochondria per cell varies and directly correlates with the cell's metabolic activity
• Are dynamic structures that move, change their shape and divide
Structure of the mitochondrion:
• Enclosed by two membranes that have their own unique combination of proteins embedded in phospholipid bilayers
• Smooth outer membrane is highly permeable to small solutes, but it blocks passage of proteins and other macromolecules
• Convoluted inner membrane contains embedded enzymes that are involved in cellular respiration. The membrane's many infoldings or cristae increase the surface area available for these reactions to occur.
• The inner and outer membranes divide the mitochondrion into two internal compartments:
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Intermembrane space
• Narrow region between the inner and outer mitochondrial membranes.
• Reflects the solute composition of the cytosol, because the outer membrane is permeable to small solute molecules.
Mitochondrial matrix
• Compartment enclosed by the inner mitochondrial membrane
• Contains enzymes that catalyze many metabolic steps of cellular respiration
• Some enzymes of respiration and ATP production are actually embedded in the inner membrane.
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ChloroplastsPlastids = A group of plant and algal membrane-bound organelles that
include amyloplasts, chromoplasts and chloroplasts.
Amyloplasts = (Amylo = starch); colorless plastids that store starch; found in roots and tubers.
Chromoplasts = (Chromo = color); plastids containing pigments other than chlorophyll; responsible for the color of fruits, flowers and autumn leaves.
Chloroplasts = (Chloro = green); chlorophyll-containing plastids which are the sites of photosynthesis.
• Found in eukaryotic algae, leaves and other green plant organs.
• Are lens-shaped and measure about 2 nm by 5 nm.
• Are dynamic structures that change shape, move and divide.
Structure of the chloroplast:
Chloroplasts are divided into three functional compartments by a system of membranes
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• Intermembrane space
The chloroplast is bound by a double membrane which partitions its contents from the cytosol. A narrow intermembrane space separates the two membranes.
Thylakoid space
Thylakoids form another membranous system within the chloroplast.
The thylakoid membrane segregates the interior of the chloroplast into two compartments: thylakoid space and stroma.
• Thylakoid space = Space inside the thylakoid
• Thylakoids = Flattened membranous sacs inside the chloroplast
• Chlorophyll is found in the thylakoid membranes.
• Thylakoids function in the steps of photosynthesis that initially convert light energy to chemical energy.
• Some thylakoids are stacked into grana.
Grana = (Singular, granum); stacks of thylakoids in a chloroplast.• Stroma• Photosynthetic reactions that use chemical energy to convert carbon
dioxide to sugar occur in the stroma.• Stroma = Viscous fluid outside the thylakoids
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The Cytoskeleton
Cytoskeleton = A network of fibers throughout the cytoplasm that forms a dynamic framework for support and movement and regulation
• Gives mechanical support to the cell and helps maintain its shape
• Enables a cell to change shape in an adaptive manner
• Associated with motility by interacting with specialized proteins called motor molecules (e.g., organelle movement, muscle contraction, and locomotor organelles)
• Play a regulatory role by mechanically transmitting signals from cell’s surface to its interior
• Constructed from at least three types of fibers: microtubules (thickest), microfilaments (thinnest), and intermediate filaments (intermediate in diameter)
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Microtubules
Found in cytoplasm of all eukaryotic cells, microtubules:
• Are straight hollow fibers about 25 nm in diameter and 200 nm – 25 nm in length
• Are constructed from globular proteins called tubulin that consists of one a- tubulin and one b-tubulin molecule
Functions of microtubules include:
• Cellular support; these microtubule function as compression-resistant girders to reinforce cell shape
• Tracks for organelle movement. Protein motor molecules (e.g., kinesin) interact with microtubules to translocate organelles (e.g., vesicles from the Golgi to the plasma membrane).
• Separation of chromosomes during cell division
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Centrosomes and centrioles
Centriole = Pair of cylindrical structures located in the centrosome of in animal cells, composed of nine sets of triplet microtubules arranged in a ring
• Are about 150 nm in diameter and are arranged at right angles to each
other.
• Pair of centrioles located within the centrosome, replicate during cell
division.
• May organize microtubule assembly during cell division, but must not be mandatory for this function since plants lack centrioles.
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Cilia and flagella
Cilia and flagella = Locomotor organelles found in eukaryotes that are formed from a specialized arrangement of microtubules.
• Many unicellular eukaryotic organisms are propelled through the water by cilia or flagella and motile sperm cells (animals, algae, some plants) are flagellated.
• May function to draw fluid across the surface of stationary cells (e.g., ciliated cells lining trachea).
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• Microfilaments (actin filaments)
Solid rods about 7 nm in diameter
• Built from globular protein monomers, Gactin, which are linked into long chains
• Two actin chains are wound into a helix
Function of microfilaments:
a. Provide cellular support
b. Participate in muscle contraction
c. Responsible for localized contraction of cells
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Cell Surfaces and JunctionsPlant Cell walls
Plant cells can be distinguished from animal cells by the presence of a cell wall:
• Thicker than the plasma membrane (0.1–2 nm)
• Chemical composition varies from cell to cell and species to species.
• Basic design includes strong cellulose fibers embedded in a matrix of other polysaccharides and proteins.
• Functions to protect plant cells, maintain their shape, and prevent excess water uptake
• Has membrane-lined channels, plasmodesmata, that connect the cytoplasm of neighboring cells
Plant cells develop as follows:
• Young plant cell secretes a thin, flexible primary cell wall. Between primary
cell walls of adjacent cells is a middle lamella made of pectins, a sticky polysaccharide that cements cells together.
• Cell stops growing and strengthens its wall. Some cells:
1. secrete hardening substances into primary wall.
2. add a secondary cell wall between plasma membrane and primary wall.
Secondary cell wall is often deposited in layers with a durable matrix that supports
and protects the cell
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The extracellular matrix (ECM) of animal cells functions in support, adhesion, movement, and development
Animal cells lack walls, but they do have an elaborate extracellular matrix (ECM).
Extracellular matrix (ECM) = Meshwork of macromolecules outside the plasma membrane of animal cells.
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Intercellular junctionsIntercellular junctions in plants:
Plasmodesmata (singular, plasmodesma) = Channels that perforate plant cell walls, through which cytoplasmic strands communicate between adjacent cells.
Allows free passage of water and small solutes. This transport is enhanced by cytoplasmic streaming.
Intercellular junctions in animals:
Tight junctions = Intercellular junctions that hold cells together tightly enough to block transport of substances through the intercellular space.
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Desmosomes = Intercellular junctions that rivet cells together into strong sheets, but still permit substances to pass freely through intracellular spaces (glycoprotein).
Gap junctions = Intercellular junctions specialized for material transport between the Formed by two connecting protein rings (connexon), e cytoplasm of adjacent cells. Have pores with diameters (1.5 nm) large enough to allow cells to share smaller molecules (e.g., inorganic ions, sugars, amino acids, vitamins), but not macromolecules such as proteins.