AQA A-Level New specification 3.2.3 Osmosis

3.2.3 Osmosis
The basic structure of all cell membranes, including cell-surface
membranes and the membranes around the cell organelles of
eukaryotes, is the same.
The arrangement and any movement of phospholipids, proteins,
glycoproteins and glycolipids in the fluid-mosaic model of
membrane structure. Cholesterol may also be present in cell
membranes where it restricts the movement of other molecules
making up the membrane.
Movement across membranes occurs by:
• simple diffusion (involving limitations imposed by the nature
of the phospholipid bilayer)
• facilitated diffusion (involving the roles of carrier proteins and
channel proteins)
• osmosis (explained in terms of water potential)
• active transport (involving the role of carrier proteins and the
importance of the hydrolysis of ATP)
• co-transport (illustrated by the absorption of sodium ions and
glucose by cells lining the mammalian ileum).
Cells may be adapted for rapid transport across their internal or
external membranes by an increase in surface area of, or by an
increase in the number of protein channels and carrier molecules
in, their membranes.
Students should be able to:
• explain the adaptations of specialised cells in relation to the
rate of transport across their internal and external
membranes
• explain how surface area, number of channel or carrier
proteins and differences in gradients of concentration or
water potential