08 September 2014

#23.2 Cell membranes and transport - Syllabus 2016 - 2018

4.1 Fluid mosaic membranes 
4.2 Movement of substances into and out of cells

The fluid mosaic model introduced in 1972 describes the way in which biological molecules are arranged to form cell membranes. The model has stood the test of time as a way to visualise membrane structure and 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 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 the section on Biological molecules.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

Learning Outcomes

Candidates should  be able to:

4.1 Fluid mosaic membranes 

The structure of cell surface membranes allows movement of substances between cells and their surroundings and allows cells to communicate with each other by cell signalling.

a) describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins

b) outline the roles of cell surface membranes including references to carrier proteins, channel proteins, cell surface receptors and cell surface antigens

c) outline the process of cell signalling involving the release of chemicals that combine with cell surface receptors on target cells, leading to specific responses

4.2 Movement of substances into and out of cells

The fluid mosaic model allows an understanding of how substances enter and exit cells by a variety of different mechanisms.

Investigating the effect of increasing the size of model cells allows an understanding of the constraints of obtaining resources across the cell surface and moving substances out of cells.

a) describe and explain the processes of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis (no calculations involving water potential will be set)

b) investigate simple diffusion using plant tissue and non-living materials, such as glucose solutions, Visking tubing and agar
c) calculate surface areas and volumes of simple shapes (e.g. cubes) to illustrate the principle that surface area to volume ratios decrease with increasing size

d) investigate the effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes

e) investigate the effects of immersing plant tissues in solutions of different water potential, using the results to estimate the water potential of the tissues

f) explain the movement of water between cells and solutions with different water potentials and explain the different effects on plant and animal cells

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