4 Cell membranes and transport
The fluid mosaic model, introduced in 1972, describes the way in which biological molecules are arranged to
form cell
... [Show More] membranes. The model continues to be modified as understanding improves of the ways in which
substances cross membranes, how cells interact and how cells respond to signals. The model also provides the
basis for our understanding of passive and active movement of molecules and ions between cells and their
surroundings, cell-to-cell interactions and long-distance cell signalling.
Investigating the effects of different factors on diffusion, osmosis and membrane permeability involves an
understanding of the properties of phospholipids and proteins covered in Biological molecules (Topic 2).
4.1 Fluid mosaic membranes Learning outcomes
Candidates should be able to:
1 describe the fluid mosaic model of membrane structure with
reference to the hydrophobic and hydrophilic interactions that
account for the formation of the phospholipid bilayer and the
arrangement of proteins
2 describe the arrangement of cholesterol, glycolipids and
glycoproteins in cell surface membranes
3 describe the roles of phospholipids, cholesterol, glycolipids,
proteins and glycoproteins in cell surface membranes, with
reference to stability, fluidity, permeability, transport (carrier
proteins and channel proteins), cell signalling (cell surface
receptors) and cell recognition (cell surface antigens – see
11.1.2)
4 outline the main stages in the process of cell signalling leading
to specific responses:
• secretion of specific chemicals (ligands) from cells
• transport of ligands to target cells
• binding of ligands to cell surface receptors on target cells
4.2 Movement into and out of cells Learning outcomes
Candidates should be able to:
1 describe and explain the processes of simple diffusion,
facilitated diffusion, osmosis, active transport, endocytosis and
exocytosis
2 investigate simple diffusion and osmosis using plant tissue and
non-living materials, including dialysis (Visking) tubing and agar
3 illustrate the principle that surface area to volume ratios
decrease with increasing size by calculating surface areas and
volumes of simple 3-D shapes (as shown in the Mathematical
requirements)
4 investigate the effect of changing surface area to volume ratio
on diffusion using agar blocks of different sizes
continued
Cambridge International AS & A Level Biology 9700 syllabus for 2022, 2023 and 2024. Subject content
Back to contents page www.cambridgeinternational.org/alevel 19
4.2 Movement into and out of cells
continued
Learning outcomes
Candidates should be able to:
5 investigate the effects of immersing plant tissues in solutions
of different water potentials, using the results to estimate the
water potential of the tissues
6 explain the movement of water between cells and solutions in
terms of water potential and explain the different effects of the
movement of water on plant cells and animal cells (knowledg [Show Less]