Lipids & Membranes intro

Question 1

What are lipids?

Answer 1

• Major class of biomolecules

Question 2

What makes lipids different from other macromolecules?

Answer 2

• soluble in organic solvents, but barely in water if at all
• non-polymeric, don’t forms polymers
• instead they associate together by non covalent forces

Question 3

What are some examples of lipids ?

Answer 3

(Not generally found in membranes:)
• Free fatty acids (intermediates in biosynthesis)
• Triglycerides (high energy storage)

(membrane lipids)
• Phospholipids (phosphoglycerides, sphingolipids)
• Steroids (ie cholesterol)

Question 4

What are free fatty acids?

Answer 4

They are monocarboxylic acids ionised at neutral pH when free

(most are weak acids)

They are building blocks of lipids and a starting point for biosynthetic pathways

Question 5

What is the chain length of fatty acids and what are they used for?

Answer 5

They have variable chain lengths of 2-24 C atoms:
• Most commonly 16-18C
• Short chain 2-6C (fermentation of fibre in the colon)
• Medium chain 6-12C (many plant oils)

in mammals chains are always linear.
saturated = single bonds
unsaturated = one or more double bonds

Question 6

Give some examples of fatty acids

Answer 6

• Palmitic acid
• Stearic acid
• Oleic acid
• Linoleic acid

Question 7

What is the importance of double bonds in fatty acids?

Answer 7

Saturated fatty acids (single bonds) are flexible and can ‘flap around all over the place’ gives narrow width, however unsaturated becomes bulky due to double bonds

Question 8

In unsaturated fatty acids, are trans or cis forms more energetically stable?

Answer 8

Trans

Question 9

In unsaturated fatty acids, is cis or trans forms found in naturally occurring fatty acids?

Answer 9

Cis

Question 10

In unsaturated fatty acids, is cis or trans forms made by ruminant digestion?

Answer 10

Trans fatty acids

Question 11

In unsaturated fatty acids, is cis or trans forms mostly made in industrial hydrogenation of vege fats?

Answer 11

most dietary Trans fats made there

Question 12

What are the health implications of dietary trans fats?

Answer 12

• increase in cardiovascular diseases

Question 13

What determines the mp of fatty acids?

Answer 13

• MP increases w chain length due to more overall attraction/bonds/vdws

• Decreases w number of double bonds due to vdw further so weaker/fewer bonds to line up

• cis lower than trans

Question 14

What are examples of Arachidonic acid derived signalling molecules and what are they used for?

Answer 14

• Prostaglandin (uterine contractions
• Thromboxane (blood clotting)
• Leukotriene (blood clotting)
• Anandamide (Endocannabinoid)
• Tetrahydrocannabinol (Phytocannabinoid)

Question 15

What is triglyceride composed of?

Answer 15

one glycerol and three fatty acid chains

Question 16

What are triglycerides used for?

Answer 16

Bodys major energy store- fat more efficient at storing calories than glycogen.

Question 17

Where does the body store triglycerides?

Answer 17

Adipose cells

Question 18

Fats an oils are triglycerides. Differentiate the two:

Answer 18

• Fat are solid at room temp, oils liquid
• fat has high proportion of saturated fatty acids, oils unsaturated
• fat is linear (interactions w neighbouring chains), oils are kinked (fewer interactions w neighbouring chains)
• fats are often animal origin, oils are often plant origin

Question 19

What is the general structure of phosphoglyceride?

Answer 19

Glycerol, 2 fatty acids, phosphate group + alcohol
(gives hydrophilic head and hydrophobic tail)

Question 20

What are the alcohol substituents in phospholipids and their resulting name?

Answer 20

• Choline (phosphatidylcholine)
• Serine (phosphatidylserine)
• Ethanolamine (phosphatidylethanolamine)
• Inositol (phosphatidylinositol)

Question 21

What are sphingolipids?

Answer 21

• Similar shape and properties but built around sphingosine (rather than glycerol like phosphoglycerides)

• simplest sphingolipids are ceramides (have polarity)

• often glycosylated to form complex glycolipids (ie gangliosides and cerebrosides)
- always found on outer leaflet of plasma membrane
- carb exposures sed externally and role in cell recognition

Question 22

Membrane lipids self assemble to form bilayers. How does this help

Answer 22

The lipids in membranes determines its fluidity and symmetry to make the membrane intrinsically impermeable to hydrophilic solutes and ions

membrane lipids are very insoluble in water- instead it emulsifies

Question 23

What happens to entropy during interaction of hydrophobic molecules and water?

Answer 23

Entropy decreases

Question 24

What is the hydrophobic effect?

Answer 24

Entropy increases in water from:
• Dispersion of lipids (lowest entropy)
• clusters of lipids
• micelles (highest entropy)

Question 25

Why do lipid bilayers spontaneously close to form sealed compartments?

Answer 25

More energetically favourable as sealed form

Question 26

What makes membranes flexible?

Answer 26

Phospholipids can move within the plane

Question 27

What affects membrane fluidity?

Answer 27

Fatty acid compositions: the length of chains and no. double bonds

(ie fats have less membrane fluidity whereas oils have more)

Question 28

What decreases membrane fluidity?

Answer 28

Cholesterol

Question 29

Why is membrane fluidity important?

Answer 29

• membrane proteins can move rapidly within the bilayer
• membrane lipids and proteins can diffuse from sites of insertion/synthesis
• membranes can fuse with one another
• membrane molecules can be shared during cell division

Question 30

What allows transport between cellular organelles?

Answer 30

membrane vesicles

Question 31

Phospholipid bilayers have low permeability to solutes and ions, give examples for different molecules and how they interact w membranes

Answer 31

The smaller the molecule and fewer favourable interactions with water the more rapidly it diffused across the lipid bilayers

• small non polar molecules diffuse easily (O2, CO2, N2, steroids & hormones)
• small uncharge molecules diffuse not quite as easily at slow rate(H2O, ethanol, glycerol)
• Larger uncharged polar molecules have very low permeability and require protein transport channels (amino acids, glucose, nucleosides)
• Ions have no permeability so require channels (H+, Na+, K+ etc)