3- Lipids and transport across membranes

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15 Terms

1

Triglyceride

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

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2

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 ).

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3

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).

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4

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.

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5

Facilitated diffusion

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

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6

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

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7

Active transport

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

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8

Fick’s Law

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

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9

To increase diffusion rate we need to...

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

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10

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.

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11

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.

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12

Triglycerides have many biological roles such as:

  • storage of energy

  • insulation- electrical and heat

  • protection

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13

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.

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14

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.

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15

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.

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