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.

• Small, non polar molecules through the bilayer

Facilitated diffusion

· The movement of substances from a region of higher concentration to a region of lower concentration

· Uses a channel or carrier protein

• Large, charged, polar molecules

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

• Large, charged, polar molecules 

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.

Lipid-soluble, small, non-polar e.g oxygen and carbon dioxide

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.

Charged, polar, large, water-soluble molecules e.g. ions, glucose, amino acids

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.

Why are triglycerides not polymers?

They are not made of monomers- they are made of two different types of molecule

Describe the formation of a triglyceride

A condensation reaction- producing 3 molecules of water. Ester bond formed between the hydroxyl group of the glycerol and the carboxyl group of the fatty acids

What is surrounded by a plasma membrane?

All cells and some organelles (some organelles have double e.g. chloroplast and nucleus)

What are cell membranes made from?

A phospholipid bilayer 

Label and describe a plasma membrane

Why does the rate of facilitated diffusion become limited?

Limited by the number of carriers/channels above a certain concentration as all proteins are occupied/saturated

What limits the rate of active transport? 

• Number of carrier proteins

• Rate of respiration

  • Lack of oxygen, glucose, low temp

• example of active transport is roots taking up ions

Endocytosis

Exocytosis

• The process of importing material in vesicles

• The process of exporting material in vesicles

• Both require ATP

What is the process of endocytosis/exocytosis?

• Cells need to transport large amounts of material

• Cells make containers from the plasma membrane called vesicles to transport solids or liquids across the plasmas membrane, in or out a cell

• Requires ATP

Functions of plasma membranes

• To control the transport of substances into and out of the cell or organelles 

• To act as a receptor site

• To compartmentalise, separate off the cell from the environment and forming organelles 

Fatty acids

• Saturated- contain no double bonds between carbon atoms

• Unsaturated- one or more double bonds between carbon atoms

Function of glycoproteins and glycolipids

• Glycoproteins- extrinsic proteins with carbohydrate chains attached

• Glycolipids- phospholipids with carbohydrate chains attached

• Act as recognition sites and receptors and antigens

Function of cholesterol

A lipid that sits with phospholipids in the core of the membrane and helps to make it less fluid/ more rigid

Examples of intrinsic proteins

Proteins which span the whole length of the phospholipid bilayer e.g. carrier (active transport) and channel

Hypertonic definition

The solute concentration in the solution is higher than in the cell- so water moves by osmosis into the solution

Isotonic definition

The solute concentration in the solution is the same as the solute concentration of the cell- therefore there is no net movement of water

Hypotonic definition

The solute concentration in the solution is lower than in the cell- so water moves by osmosis into the cell

How does surface area affect the rate of movement across a membrane

A larger surface area increases the rate of movement across a membrane, because there is more space for molecules to pass through simultaneously and more channel and carrier proteins available at once

How does the number of carrier or channel proteins affect the rate of movement across a membrane

increases the rate of facilitated diffusion and active transport, as more proteins means more active sites for molecules to bind and cross