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Ch. 6 and 7 (Nucleus (Ribosomes (Endomembrane System (Endoplasmic…

- - - - Many of the membrane-bounded organelles of the eukaryotic cell are part of the endomembrane system.
      - The system carries out a variety of tasks including synthesis of proteins, transport of proteins, metabolism and movement of lipids and detoxification of poisons.
      - The membranes of this system are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles, or sacs made of membrane.
      - Endoplasmic Reticulum
        
        The endoplasmic reticulum (ER) is an extensive network of membranes that accounts for more than half of the total membrane in eukaryotic cells.
        
        The ER consists of a network of membranous tubules and sacs called cisternae.
        
        The ER membrane separates the internal compartment of the ER, called the ER lumen.
        
        There are two regions of the ER that differ in structure and function: smooth and rough ER.
        
        Smooth ER is named because its outer surface lacks ribosomes.
        
        Rough ER is studded with ribosomes on the outer surface of the membrane.
        
        Enzymes of the ER are important in the synthesis of lipids, including oils.
        
        Other enzymes of the smooth ER help detoxify drugs and poisons, especially in the liver.
        
        The smooth ER stores calcium ions. For example, in the muses it pumps calcium ions from the cytosol into the ER lumen.
        
        Many cells secrete proteins that are produced by ribosomes attached to rough ER.
        
        Most secretory proteins are glycoproteins, proteins with carbohydrates covalently bonded to them.
        
        After security proteins are formed, the ER membrane keeps them separate from proteins in the cytosol and produced by free ribosomes.
        
        Secretory proteins depart from the ER wrapped in the membrane of vesicles from transitional ER.
        
        Vesicles transit from one par of the cell to another called transport vesicles.
        
        Golgi Apparatus
        
        After leaving the ER, many transport vesicles travel to the Golgi apparatus.
        
        Products of the ER, such as proteins, are modified and stored and then sent to other destinations.
        
        It consists of flattened membranous sacs, cisternae, looking like a stack of pita bread.
        
        The membrane of each cisterna in a stack separates its internal space from the cytosol.
        
        Vesicle in the Golgi apparatus are engaged in the transfer of material between parts of the Golgi and other structures.
        
        A Golgi stack has a distinct structural directionality, with the membranes of cisternae on opposite sides of the stack differing in thickness and molecular composition.
        
        Cis face and trans face are the two side of a Golgi stack.
        
        Transport vesicles move material from the ER to the Golgi.
        
        A vesicle that buds from the ER can add its membrane and the contents of its lumen to the cis face by fusing with a Golgi membrane on that side.
        
        The trans face gives rise to vesicles that pinch off and travel to other sites.
        
        The Golgi apparatus also manufactures some macromolecules.
        
        Many polysaccharides secreted by cells are Golgi products.
        
        Lysosomes
        
        Lysosome is a membranous say of hydrolytic enzymes that many eukaryotic cells use to digest macromolecules.
        
        Lysosomal enzymes work best in the acidic environment found in lysosomes.
        
        If a lysosome breaks open or leaks, the released enzymes are not very active because the cytosol has a near-neutral pH.
        
        Some lysosomes arise by budding from the trans face of the Golgi apparatus.
        
        Lysosomes carry out intracellular digestion in a variety of circumstances.
        
        Amoebas and many other unicellular eukaryotes eat by engulfing smaller organisms or food particles by phagocytosis.
        
        The food vacuole formed fuses with a lysosome, whose enzymes digest the food.
        
        Digestion products, including simple sugars, amino acids and other monomers, pass into the cytosol and become nutrients.
        
        Lysosomes use their hydrolytic enzymes to recycle the cell's own organic material by autography.
        
        In autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a double membrane and a lysosome fuses with the outer membrane of this vesicle.
        
        Vacuoles
        
        Vacuoles are large vesicles derided from the endoplasmic reticulum and Golgi apparatus.
        
        The vacuolar membrane is selective in transporting solutes.
        
        Many unicellular eukaryotes living in fresh water have contractile vacuoles that pump water out of the cell, maintaining a concentration of ions and molecules inside the cell.
        
        Mature plants contain a large central vacuole which develops by the coalescence of small vacuoles.
        
        The central vacuole plays a role in he growth of plant cells, which enlarge as a vacuole absorbs water, enabling the cell to become larger with a minimal investment in new cytoplasm.
        
        Mitochondria and Chloroplasts
        
        Mitochondria are the sites of cellular respiration, the metabolic process that uses oxygen to drive the generation of ATP by extracting energy from sugars, fat and other fuels.
        
        Chloroplasts, found in plants and algae, are the sites of photosynthesis.
        
        The process in chloroplasts converts solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds.
        
        Mitochondria are found in nearly all eukaryotic cells.
        
        Each of the two membranes enclosing the mitochondria is a phospholipid bilayer with a unique collection of embedded proteins.
        
        The outer layer is smooth, but the inner membrane is convoluted with infoldings called cristae.
        
        The inner membrane is divides the mitochondria into two compartments, the first being the intermembrane space and the second the mitochondrial matrix.
        
        Chloroplasts contain green pigment chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar.
        
        The contents of chloroplasts are partitioned from the cytosol by an envelope consisting of two membranes separated by a very narrow inter membrane space.
        
        Inside the chloroplast is another membranous system in the form of flattened, interconnected sacs called thylakoids.
        
        Thylakoids are stacked and each stack is called a granum.
        
        The fluid outside the thylakoids is the stroma, which contains the chloroplasts DNA and ribosomes and enzymes.
        
        Peroxisomes
        
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- - - - All cells are bound by a selective barrier called the plasma membrane.
      - Inside all cells is the cytosol, which is a semifluid, jellylike substance in which subcellular components are suspended.
      - All cells have chromosomes that carry genes in the form of DNA and ribosomes, the tiny complexes that make proteins according to instructions from the genes.
      - In eukaryotic cells, most of the DNA is in an organelle called the nucleus, which is bound by a double membrane.
      - Prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed called the nucleoid.
      - The interior of either type of cell is the cytoplasm.
      - Eukaryotic cells are large than prokaryotic cells.
      - At the boundary of every cells is the plasma membrane which functions as a selective barrier that allows passage of enough oxygen, nutrients and wastes to service the entire cell.
- - - - Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.
      - The tonicity if a solution depends in part on its concentration of solutes that can't cross the membrane relative to that inside the cell.
      - If a cell without a wall, like an animal cell, is immersed in an environment that is isotonic, there is no net movement of water across the plasma membrane.
      - Water diffuses across the membrane, but at the same rate in both directions.
      - In hypertonic, the cell will lose water, shrivel and die.
      - Taking up too much water can be just as bad as losing water.
      - In hypotonic, water will enter the cell faster than it leaves, and the cell will swell and burst.
      - Organisms that lack rigid cell walls must have other adaptations for osmoregulation, the control of solute concentrations and water balance.
      - The cells of plants, prokaryotes, fungi and some unicellular eukaryotes are surrounded by cell walls.
      - If a cell is turgid, the healthy state for most plant cells.
      - Plants that are not woody, depend for mechanical support on cells kept turgid by a surrounding hypotonic solution.
      - If a plants cells and surroundings are isotonic, there is no net tendency for water to enter and the cells become flaccid.
      - As a plant shrivels in a hypertonic environment, its plasma membrane pulls away form the cell wall.
      - The phenomenon that causes the plant to wilt and lead to plant death is known as plasmolysis.
      - Facilitated Diffusion: Passive Transport
        
        Facilitated diffusion is the passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins.
        
        There are two types of transport proteins: channel and carrier proteins.
        
        Channel proteins provide corridors that allow specific molecules or ions to cross the membrane.
        
        The hydrophilic passageways provided by these proteins can allow water molecules or small ions to diffuse quickly from one side of the membrane to other.
        
        Aquaporins, the water channel proteins, facilitate the massive levels of diffusion of water that occurs in plant cells and in animal cells.
        
        Channel proteins that transport ions are called ion channels.
        
        Many ion channels function as gated channels which open or close in response to stimulus.
        
        Carrier proteins undergo a subtle change in shape that translocate the solute-binding site across the membrane.
        
        Such a change in shape may be triggered by the binding and release of the transported molecule.
        
        Carrier proteins involved in facilitated diffusion result in the net movement of a substance down its concentration gradient.
        
        Active Transport
        
        To pump a solute across a membrane against its gradient requires work; the cell must expend energy. This is known as active transport.
        
        The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins.
        
        Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment.
        
        ATP hydrolysis supplies the energy for most active transport.
        
        One transport system that works in this way is the sodium-potassium pump which exchanges Na+ for K+ across the plasma membrane of animal cells..
        
        Ion Pumps Membrane Potential
        
        All cells have voltage across their plasma membranes.
        
        The voltage across a membrane, called a membrane potential, ranges from about -50 to -200 millivolts (mV).
        
        The inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell.
        
        Electrochemical gradient is the diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane and the ion's tendency to move relative to the membrane potential.
        
        A transport protein that generates voltage across a membrane is called an electrogenic pump.
        
        The sodium-potassium pump appears to be the major electrogenic pump of animal cells.
        
        The main electrogenic pump of plants, fungo and bacteria is proton pump which transports protons out of the cell.
        
        The pumping of H+ transfers positive charge from the cytoplasm to the extracellular solution.
        
        Exocytosis
        
        The cell secretes certain molecules by the fusion of vesicles with plasma membrane. The process is known as exocytosis.
        
        A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane.
        
        When the vesicle membrane and palms membrane come into contact, specific proteins rearrange the lipid molecules of the two bilayers so that the two membranes fuse.
        
        The contents of the vesicles spill out of the cell, and the vesicle membrane becomes part of the plasma membrane.
        
        Many secretory cells use exocytosis to export products such as the cells in the pancreases to make insulin.
        
        Endocytosis
        
        In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from plasma membrane.
        
        The three types of endocytosis are phagocytosis, pinocytosis and receptor-mediated endocytosis.
        
        Phagocytosis occurs when a cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole.
        
        In pinocytosis, a cell "gulps" droplets of extracellular fluid into tiny vesicles, formed by inholdings of the plasma membrane. The cell obtains molecules dissolved in the droplets.
        
        Receptor-mediated endocytosis is a type of pinocytosis that enables the cell to acquire bulk quantities of specific substances. Specific solute bind to the receptors. The receptor proteins then cluster in coated pits, and each coxed pit forms a vesicle containing the bound molecules.
- - - - A membrane is a collage of different proteins, often clustered together in groups, embedded in the fluid matrix of the lipid bilayer.
      - Phospholipids form the main fabric of the membrane, but proteins determine most of the membranes functions.
      - Different types of cells contain different sets of proteins, and the various membranes within a cell each have unique collection of proteins.
      - Integral proteins penetrate the hydrophobic interior of the lipid bilayer.
      - The majority are transmembrane proteins which span the membrane.
      - The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids.
      - The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane.
      - Some proteins also have one or more hydrophilic channels that allow passage through the membrane of hydrophilic substances.
      - Peripheral proteins are not embedded in the lipid bilayer; they are loosely bound to the surface of the membrane.
      - On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the cytoskeleton.
      - On the extracellular side, certain membrane proteins attach to materials outside the cell.
      - A single cell may have cell-surface membrane proteins that carry out several different functions, such as transport through the cell membrane, enzymatic activity or attaching a cell to either a neighboring cell or the extracellular matrix.
      - There are six major functions performed by proteins of the plasma membrane.
      - The Role of Membrane Carbohydrates in Cell-Cell Recognition
        
        Cell-cell recognition, a cell's ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism.
        
        Its also the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrate animals.
        
        Cells recognize other cells by binding to molecules, often containing carbohydrates on the extracellular surface of the plasma membrane.
        
        Membrane carbohydrates are short, branched chains of less than 15 sugar units.
        
        Some are covalently bonded to lipids, forming molecules called glycolipids.
        
        Most are covalently bonded to proteins, which are glycoproteins.
        
        The carbohydrates on the extracellular side of the plasma membrane vary from species, among individuals and cell types.
        
        The diversity of the molecules and their location on the cell's surface enable membrane carbohydrates to function as markers that distinguish one cell from another.
        
        Permeability of the Lipid Bilayer
        
        Nonpolar molecules, such as Co2 and O2, are hydrophobic.
        
        They can dissolve in the lipid bilayer of the membrane and cross easily without the aid of membrane proteins.
        
        The hydrophobic interior of the membrane impedes direct passage through the membrane of ions and polar molecules, which are hydrophilic.
        
        Polar molecules and other sugars pass only slowly through a lipid bilayer, and even water, a very small polar molecule, does not cross rapidly relative to nonpolar molecules.
        
        The lipid bilayer is only one aspect of the gatekeepers system responsible for a cell's selective permeability.
        
        Transport Proteins
        
        Specific ions and a variety of polar molecules can't move through a cell membranes on their own.
        
        Hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane.
        
        Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane.
        
        For example, the passage of water molecules through the membrane in certain cells is facilitated by channel proteins known as aquaporins.
        
        Without aquaporins, only a small fraction of water molecules would pass through the same area of the cell membrane in a second, so the channel protein brings about a tremendous increase in rate.
        
        Other transport proteins, called carrier proteins, hold onto their passengers and change shape in a way that shuttles them across the membrane.
        
        A transport is specific for the substance it translocates, allowing only a certain substance to cross the membrane.