Membranes Notes

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Tuesday, 25 September 2012
F211 Module 1 Membranes
Weekly Learning Outcomes
Students should be able to:
Outline the roles of membranes within cells and at the surface of cells.
State that plasma (cell surface) membranes are partially permeable barriers.
Describe, with the aid of diagrams, the fluid mosaic model of membrane structure.
Describe the roles of the components of the cell membrane: phospholipids, cholesterol, glycolipids,
proteins and glycoproteins.
Outline the effect of changing temperature on membrane structure and permeability.
Explain the term cell signalling.
Explain the role of membrane-bound receptors as sites where hormones and drugs can bind.
The Roles of Membranes
The major roles of membranes include:
Separating cell contents from the outside environment
Separating cell components for cytoplasm
Cell recognition and signalling
Holding the components of some metabolic pathways in place
Regulating the transport of materials into
and out of cells
Membranes have two layers of
phospholipid, forming a bilayer.
The molecules in the two layers have
opposite orientations, so that the non-polar
`tails' associate with each other and the polar
`heads' face the cytoplasm and the fluid out side
the cell. The polar heads form hydrogen bonds
with water in the surroundings.
Membrane proteins are embedded in the phospholipid bilayer. Transmembrane proteins extend right
through the bilayer with one end in the cytoplasm and the other end extending to the outside.
Transmembrane proteins are held in the membrane because they have hydrophobic regions that
span the hydrophobic interior of the membrane.
These are short chains of sugar molecules that branch to give `tree-like' attachments to proteins
and lipids.
Glycolipids are lipids with chains of sugar molecules attached.
Carbohydrate proteins are on the external surface of the membranes.

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Glycoproteins are proteins with chains of sugar molecules attached.
Carbohydrates are attached to lipids and proteins only on the external surfaces of cell membranes.
These molecules have polar and non-polar regions. Polar regions bind to
polar heads of phospholipids; non-polar regions bind to phospholipid tails.
It maintains the stability of membranes by preventing phospholipids
solidifying at low temperatures and becoming too fluid at high
Cholesterol reduces the permeability of membranes to water, ions and
polar molecules.…read more

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Viruses enter cells by binding with receptors on the cell's plasma membrane that would normally bind to
the host's signalling molecules. An example of this is HIV.
Common Misconceptions
Understanding hydrophilic and hydrophobic behaviour and distinguishing between them
Students often have trouble conceptualising very small things at a nm level. Simple thought
experiments could help.
Understanding that the fluid mosaic model, as a model, can be counter-intuitive, given the way in
which scientific information is often presented as fact.…read more

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Receptor site ­ Protein or glycoprotein molecules on cell surfaces, used for attachment of specific
substances such as hormones or viruses.
Solute ­ A solid that dissolves in liquid.
Xylem ­ A plant tissue containing xylem vessels (and other cells) that are used to transport water in a plant
and provide support.…read more


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