Membranes- structure, function, diffusion, osmosis


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A diagram to show the structure of membranes.

File:Cell membrane detailed diagram 4.svg

 Hydrophobic and hydrophilic properties of phospholipids 

List the functions of membrane proteins.

Some classic examples seen in markschemes:

They act as channels for passive transport

Protein pumps are there for active transport

Hormone binding sites

Communication between cells and cell adhesion to each other

Electron carriers



Diffusion and osmosis

 Diffusion is a passive movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient’ alternatively) as a result of the random motion of particles.

Osmosis is a passivemovement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

Passive transport across membranes by simple diffusion and facilitated diffusion.

Facilitated diffusion is different to simple diffusion in the sense that it is facilitated, or ‘aided’, by integral proteins in the bilayer and thus requires protein channels. However it still does not require energy and goes down the concentration gradient just like simple diffusion.

The molecules always move down a concentration gradient.

Simple diffusion deals with small, uncharged molecules while facilitated diffusion deals with charged and usually larger molecules.

The role of protein pumps and ATP in active transport across membranes.

Active transport is the movement of the molecules across the bilayer involving the use of energy from ATP (which is derived from hydrolysing ATP to make ADP which releases energy). An important thing you need to note is that active transport nearly always involve proteins, being either protein pumps or carrier proteins.

Compared to passive transport, active transport has the ability to move against the concentration gradient (from low to high concentration).

How vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus, and plasma membrane

The bilayer is very fluid. Vesicles are formed by parts of the cell membrane ‘caving in’ and then forming a new pocket. They are used to transport substances around the cell, importing nutrients for example using endocytosis and exporting substances such as waste by exocytosis.

Proteins, such as enzymes for example, are transported in vesicles. First from the RER to the Golgi, where it is packaged and processed, and then from the Golgi to the cell membrane. Vesicles always ‘fuse’ with the cell membrane to either expel or import material.

 How the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis

Endocytosis: membrane pulled inwards (caving in) and vesicle is ‘pinched’ off of the membrane

Exocytosis: vesicle fuses with plasma membrane and then the materials inside are expelled, then the plasma membrane flattens out

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2579 Administrator 03 Jun 2013 18:45:20
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