Cells

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  • Cells
    • Eukaryotic cell structure
      • Cell membrane
      • Nucleus
        • Containing chromosomes protein bound - linear - DNA  and one or more nucleoli
      • Mitochondria
      • Chloroplasts
      • Golgi apparatus and vesicles
      • Lysosomes
      • 80s Ribosomes
      • RER and SER
      • Cell wall (in plants, algae and Fungi) made of CELLULOSE and/or CHITIN
      • Cell Vacuole (in plants)
    • Prokaryotic Cell structure
      • SMALLER
      • No membrane bound organnelles
      • 70s Ribosomes
      • No nucleus
        • Single circular DNA molecules free in the cytoplasm
      • Cell wall made from MUREIN (peptidoglycan)
      • A capsule
      • Falgella
    • Method of Studying CELLS
      • TEM (transmission electron microscope)
        • passes a beam of electrons through a VERY thin specimen (stained with heavy metals to show fine internal structures)
        • More complex and time consuming
        • HIGH resolution
        • HIGH magnification- more detailed
        • Specimens have to be thinand dead
        • Large machine
        • Does not show in colour
      • SEM (scanning electron microscope)
        • Beams of electrons shot down at sample and bounce off the surface of an object
          • Secondary electrons emitted from sample and detected by secondary electron detectors
        • Develops a 3D image
        • VERY HIGH resolution and magnification
        • Specimen has to be coated in gold and dead
        • More expensive and bigger
          • No colour
      • Optical Microscope
        • Specimen mounted on microscope slide and light is passed through the specimen, up through the objective and eyepiece lenses, and into the eye.
        • Different levels of magnification
          • X4
          • X10
          • X100
        • The sie of the wavelength limits the resolution of the light microscope
        • Live specimens can be viewed
        • shows colour
        • Has a low resolution
      • Centrifugation
        • 1. PLaced in cold, isotonic, buffered solution
          • To stop the pH from fluctuating and altering organnelles shape
          • prevent organnelles from bursting or shriking because of osmosis
          • To slow down enyme activitythat may break down organnelles
        • 2. HOMOGENATION - Cell membrane broken to release organelles
        • 3. Homogenate filtered to remove any complete cells
        • 4. Ultracentrifugation is where the filtered homogenate is spun at v. high speeds to create centrifugal force - The heaviest organnelle (nucleus) forced to the bottom of the tube
        • 5. The supernatant is removed and spun again and the pellet contains the organnelle
        • 6. The tube is spun again at a faster speed and the next heaviest organnelle (mitochondria form a pellet at the bottom.
      • Magnification = image / actual sie
    • All cells arise from other cells
      • 1) Interphase - DNA replicates
      • 2) Mitosis
        • Prophase is where the chromosomesbecome visible as sister chromatids and spindle fibres from and attach to the centromeres
        • Metaphase is wherethe chromosmes line up on the equator
        • Anaphase is where the centromeres divide and the sister chromatids move to opposite poles because they are pulledby the spindle fibres which contract and shorten
        • Telophase is where the cell starts to constrict in the middle and a nuclear membrane froms
      • 3) Cytokinesis is where organnelles and cell content split into two new cells
      • Binary Fission:
        • Replication of the circular DNA and of plasmids. Division of the cytoplasm to produce two new daughter cells with a single copy of circular DNA and variable copies of plasmids
      • Uncontrolled cell division can lead to formation of tumours and of cancers.
    • Transport across cell membranes
      • Simple (passive) diffusion
        • A substance moves from an area of high concentration to an area of low concentration (net movement ALONG a concentration gradient)
        • Only SMALL, NON-POLAR, UNHCARGED molecules can diffuse
        • Molecules will diffuse at different rates depending on conditions
      • Fluid Mosaic Model
        • Selective permeability
      • Facilitated Diffusion
        • A type of passive diffusion that allows large, charged particles to move across via a specialised transport protein
          • Channel Proteins: allow water-soluble ions passage by 1) ions binding to the protein 2) It then changes shape so it closes to one side and opens to another
          • Carrier Proteins: attach to particular molecules and then change shape to fit the particular protein and then deposit them on the inside. When this is done the proteins go back to their original position
      • Osmosis
        • Water molecules diffuse DOWN a water potential gradient from a HYPOTONIC to a HYPERTONIC solution
          • Hypotonic solution = less solute and higher water potential SO water moves into the cell
          • Hypertonic solution = more solute and lowe water potential SO water moves out of the cell
        • Water is polar so has to use a transport protein or diffusion will be TOO slow
      • Active Transport
        • When molecules are moving against a concentration gradient
        • This movement requires energy - ATP - which is released during respiration
      • Co-Transport
        • Moves substances ALONG a concentration gradient BUT two molecules couple together in order to fit the required shape of the transport protein
    • Cell Recognition and the Immune System
      • T-Cell Response
        • 1) The phagocyte engulfs the pathogen and presents its antigen on the surface membrane using an MHC complex
        • 2) CLONAL SELECTION: There are lots of T-Cells within the blood and the complementary T-Cell is selected
        • 3) CLONAL EXPANSION: The binding of the complementary T-cell and the antigen activates the T-Cell to divide rapidly by mitosis, creating clones.
        • 4) CLONAL DIFFERENTIATION
          • Helper T-Cells: Release cytokines that stimulate the B-Cell resposne
          • Killer T-Cells: Release perforin proteins which make holes in the pathogens membrane, meaning it becomes freely permeable to all substance and SO dies
          • Memory T-Cells: Remain in the blood in case of a secondary response
      • Antibody: Protein produced by a plasma B-Cell in response to a specific antigen
      • Antigen: molecule on the surface of a cell that triggers an immune response
      • Phagocytosis
        • 1) Detection: The phagocyte detect pathogens by the chemicals they give off ( exotoxins) and are attracted to them by  chemotaxis
        • 2) Ingestion: Phagocytes engulf the pathogen by wrapping psuedopodia around to form vesicles called phagosome
        • 3) Fusion: The phagosome fuses with the lysosome in the cell (which contain hydrolytic enzymes called  lysosymes)
        • 4) Digestion: the lysozyme breaks down the pathogen into pus and debris
        • 5) Discharge: The indigestible material is released, also releasing endotoxins
      • B-Cell Response
        • 1) CLONAL SELECTION: The B-Cell that is complementary to the antigen is selected
        • 2) CLONAL EXPANSION: The cytokines that the T-Helper cells produce activate the B-Cells to divide rapidly by mitosis, creating clones
        • 3) CLONAL DIFFERENTIATION
          • B-Plasma Cells: Produce antibodies that bind to and destroy the pathogen
          • B-Memory Cells: Remain in the body in case of a secondary infection

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