Movement of substances in and out of the cell Passive Transport Definition: movement of molecules across cell membranes without the expenditure of cellular energy. Direction of movement is determined by the concentration gradient (difference between a region of high concentration and a region of lower concentration). Diffusion - The movement of molecules from an area of higher concentration to an area of lower concentration. It is the most common type of transportation in the cell. Facilitated Diffusion Definition: movement of materials from an area of high concentration to one of lower concentration by a protein carrier present in the cell membrane. This form of diffusion is more rapid than normal diffusion. Example of facilitated diffusion: Transport of glucose from your blood into the cells of your body. Osmosis Definition: a special type of diffusion. It is a movement of water molecules from an area of greater concentration to an area of lesser concentration through a semipermeable membrane. Osmosis will continue until the rate of water diffusion across the cell membrane in both directions is equal. This is called osmotic balance. The pressure generated from this movement of water is called osmotic pressure. Effects of Osmosis The effects of osmosis can be seen by placing animal cells into solutions containing different concentrations of water. (page 55; figure 2.26) Isotonic Solution Has the same concentration of dissolved substances as the cell placed in it. The concentration of water molecules in the cell and in an isotonic solution is the same. Since the concentration gradient is zero, there is no net gain or loss of water in the cell. Hypotonic Solution Contains a lower concentration of dissolved substances than the cell put in it. (i.e. the solution is mostly H2O) The concentration of water molecules is higher in the hypotonic solution than it is in the cell. Since the concentration of water molecules is higher outside the cell than inside, there is a net movement of water into the cell. Osmotic pressure produced by water entering the cell causes it to swell and burst. (e.g. blood cell in water will burst) Hypertonic Solution Contains a higher concentration of dissolved substances than the cell placed in it. The concentration of water molecules is lower in the hypertonic solution than it is in the cell. Since the concentration of water molecules is higher inside the cell than outside, there is a net movement of water out of the cell. Water leaving the cell causes it to shrink. Active Transport Definition: movement of molecules across a cell membrane requiring the expenditure of cellular energy. It is called an uphill process. Movement is from an area of low concentration to an area of high concentration. There are 2 mechanisms of active transport: Carrier Proteins Carrier proteins located in the cell membrane receive an energy boost. The energized proteins capture specific solute molecules and move them either into or out of the cell against the concentration gradient. Vesicles Energy is used to bring about a folding of the cell membrane to form pockets. 2 forms of this movement Endocytosis (movement in) and Exocytosis (movement out). Two Forms of Endocytosis: (i) Pinocytosis - liquid droplets are taken into the cell. (ii) Phagocytosis - solid particles are taken into the cell. Exocytosis – when a vesicle makes contact with the cell membrane, releasing the contents to the outside of the cell.
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