Transport across the cell membrane

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  • Movement Across The Cell Membrane
    • Diffusion
      • movement of particles/ molecules from an area of high concentration to low
      • Passive so no ATP required
      • small molecules like oxygen and carbon dioxide and non polar molecules like fatty acids, glycerol and vitamins diffuse directly through
      • what affects diffusion rate?
        • temp - increases/ decreases particles KE
        • concentration gradient - higher gradient the faster
        • size- smaller the molecule the faster it diffuses
        • thickness - the thicker the membrane the longer it takes
        • surface area-  large SA = faster diffusion
      • graph to show diffusion is a straight increasing line
    • Facilitated Diffusion
      • The diffusion of substances from a high concentration to low using channel proteins and carrier proteins.
      • Passive no ATP required
      • Relies on protein channels and carrier proteins
      • used by specific molecules e.g. glucose, water (aquaporins), ions ect..
      • graph  increases then levels off
        • levels off because not enough proteins and become saturated
    • Methods of facilitated diffusion
      • Channel Proteins
        • Forms a water filled pore or channel in the membrane
        • allows charged substances to diffuse across the membrane e.g. ions
      • Carrier Proteins
        • have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite ends of the membrane
        • e.g. large sugars, amino acids
      • co transport (example in epithelial cells)
        • molecules move across the membrane with ions
        • in epithelial cells glucose moves across the membrane with the Na+ ions through a carrier protein. a protein transports the ions out the cell so theres a concentration gradient so this can happen. the third protein moves the glucose through into the capillary
    • Movement of bulk materials
      • Exocytosis
        • releases large molecules out of the cell. occurs by fusion of the vesicle with the plasma membrane releasing them to the outside
      • Endocytosis
        • takes large molecules into the cell by creating a vesicle to ingest the molecules and transport them into the cell


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