Membrane


The membrane has a double lipid layer through which fat- soluble substances will
move, by passive transport or diffusion. The molecules will only move through if
they are small enough. There are globular protein regions, which are
hydrophilic. These regions allow water and water soluble substances through, but
only if they are small enough, for example starch grains are too big. These
substances move through by passive transport or diffusion. Diffusion is the
passive movement of substances from a high concentration to a low concentration
( N.B., you don't need a membrane present, e.g. diffusion occurs in the air). In

CONTRAST, diffusion of water across a semi- permeable membrane is called

OSMOSIS. Active Transport Active transport requires energy from ATP, and
involves carrier molecules studding the membranes. The carrier combines with the
molecule on the outside, which requires ATP. It then moves across, (which also
requires ATP), and returns. An example is glucose being transported by active
transport by insulin. Why is it called the Fluid Mosaic Model? Fluid- it is
called a fluid because proteins are said to wander through the membrane (hence
fluid) Mosaic- it is a mosaic because it is a collection (or mosaic) of
structures Model- because no matter how real the diagram looks, it is not real.