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    THE MOVEMENT OF SUBSTANCES

    ACROSS THE PLASMAMEMBRANE.

    ENTER

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    STRUCTURE OF

    PLASMA

    MEMBRANE.

    THE

    PERMEABILITY

    OF THE PLASMA

    MEMBRANE.

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    About the plasma membrane.

    Plasma membrane, are composed mainly ofphoslipids and proteins.

    Each phospholipid molecule consists of two

    parts:. A polar head that gives it a hydrophilic property

    [attrated to water], and

    A pair ofnon-polar fatty acid tails that give ithydrophobic property [repelled by water].

    Fluid mosaic model proposed by S.J Singer andG.L Nicholson.

    View the

    pictures and

    video..

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    G. L. Nicholson S. J. Singer

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    The permeability of the plasma

    membrane. A membrane is said to be impermeable.

    Plasma membrane partially permeable or semi-

    permeable.

    The selective permeability of the plasmamembrane depends on the.

    a) Selective barrier of the phospholipid bilayer, and

    b) Specific transport proteins built into the

    membrane. The size and polarity of molecules determine the

    movement of these molecules across the plasma

    membrane.

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    The phospholipid bilayer is permeable to.

    Non-polar molecules

    [ lipids soluble].

    Small, uncharged

    molecules.

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    The phospholipid bilayer is not permeable.

    Charged ions.Polar molecules

    [ lipid-insoluble].

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    There are two ways in which molecules can moves across the

    plasma membrane: passive transport and active transport.

    Movement of substances across the plasma membrane.

    Molecules transported

    down the concentration

    gradient .

    Molecules transported

    against the concentration

    gradient.

    Simple

    diffusion. Osmosis.Facilitated

    diffusion.

    Passive transport Active transport

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    Simple diffusion

    Simple diffusion is net movement of molecules orions from a region of high concentration to aregion of lower concentration, that is, down theconcentration of a substance between two

    regions. Molecules which move across the plasma

    membrane by simple diffusion include:.

    i. lipid-soluble molecules such as vitamins A, D, E,K.

    ii. Small molecules such as water and

    iii. Dissolved gases such as oxygen and carbondioxide.

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    Osmosis

    Osmosis is the diffusion of water

    molecules across a semi-permeable

    membrane.

    Water molecules move across the

    phospholipid bilayer and pore protein.

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    Facilitated diffusion

    Facilitated diffusion refers to the

    movement of hydrophilic molecules or

    ions across the plasma membrane with

    the help of transport proteins.

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    Active transport

    Active transport is the movement of

    molecules or ions from region of lower

    concentration to a region of higher

    concentration [against the concentration

    gradient] across the plasma membrane.

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