3- Lipids and transport across membranes

Triglyceride

· 3 fatty acids combined with 1 molecule of glycerol.
· Joined with ester bonds

Phospholipid

• Contains two fatty acids joined to a glycerol and a phosphate group instead of one fatty acid

• Polar /hydrophilic (attracts water) head (glycerol and phosphate)

• Non-polar /hydrophobic (repels water) tails (fatty acids ).

Test for lipids

Add ethanol to the sample.
Shake thoroughly to dissolve any lipid
Then add water to the sample and shake gently.
A milky white emulsion indicates the presence of lipid (NB: not a precipitate).

Simple diffusion

· The movement of substances from a region of higher concentration to a region of lower concentration.
· The difference is called a concentration gradient.
· Diffusion is movement of molecules down a concentration gradient.

Facilitated diffusion

· The movement of substances from a region of higher concentration to a region of lower concentration
· Uses a channel or carrier protein

Osmosis

· The movement of water across a partially permeable membrane down a water potential gradient i.e. from higher (less negative) to lower (more negative) water potential

Active transport

· The movement of substances against their concentration gradient
· Uses a carrier protein and ATP

Fick’s Law

Rate of Diffusion is proportional to surface area x difference in concentration/
thickness of exchange surface

To increase diffusion rate we need to...

· maximise the surface area
· maximise the concentration difference
· minimise the thickness of the exchange surface.

Which molecules will be able to move through the phospholipid bilayer by simple diffusion and why?

Molecules that are non-polar can diffuse rapidly across the phospholipid bilayer, as they are lipid-soluble and are not repelled by the fatty acids in the hydrophobic tails.

Smaller molecules can also diffuse through more easily. Oxygen molecules are non-polar and small and so diffuse rapidly through the bilayer.

Which type of molecule moves via facilitated diffusion and why?

It is more difficult for polar molecules to pass through the bilayer as the fatty acid tails are a barrier to these molecules.

Triglycerides have many biological roles such as:

• storage of energy

• insulation- electrical and heat

• protection

The structure and properties of triglycerides related to their function

· They have a high ratio of energy storing carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy.
· They have a low mass to energy ratio, making them good storage molecules because so much energy can be stored in a small volume.
· They are large, , non-polar molecules, insoluble in water so they have no osmotic effect and do not affect the water potential in cells.

Why is the model described as fluid mosaic?

• Fluid refers to the fact that all the different molecules can move around.

• Mosaic because there are different types of molecule - the proteins and carbohydrates 'float around' within the phospholipid bilayer.

How can cells move large quantities in/out of the cell?

To do this, cells make containers from the plasma membrane itself, called vesicles.
· The process of importing material in vesicles is called endocytosis
· The process of exporting material in vesicles is called exocytosis. Both processes require ATP.