Module 4 - Lipids and Membranes

Question 1

Define 3 broad areas for lipid functions and provide examples.

Answer 1

• Energy storage (Fat deposit)
• Structural components of biological membranes
• Signals, pigments, and cofactors

Question 2

What is lipid?

Answer 2

A amphipathic, chemically diverse organic compounds with different function.

Question 3

Explain the trends in fatty acid melting temperature and solubility.

Answer 3

As the hydrocarbon chain gets longer, the melting temp increases while the solubility decreases. Hydrophobic effect and van der Waals interactions favor hydrophobic compounds to aggregate to each other (highest entropy) leading to higher intermolecular forces, requiring more energy to break the bond. The more longer the chain, the more hydrophobic it is, hence less soluble.

Question 4

______ are important component of most lipid groups. Describe it.

Answer 4

Fatty acid:

• hydrocarbon derivative
• contain 1 carboxylic acid group
• and a hydrocarbon chain (4-36)

Question 5

Describe the characteristics of unsaturated fatty acid.

Answer 5

• One carboxylic acid chain
• hydrocarbon chain containing more than 1 or more double bond
  
  • double bond -> cis- configuration
  • when there are 2 or more double bonds, these are not conjugated -
  • interfere with tight packing of saturated fatty acid - lower melting point

Question 6

Omega-3 fatty acids are essential nutrients, which means _______. It has a double bond in the ________ carbon, located three carbons away from the omega carbon.

Answer 6

we need them, but cant produce them

omega-3

Question 7

Describe the characteristics of triacylglycerol (or fats)

Answer 7

• fatty acid esters of glycerol
• 3 OH- of glycerol provide the 3 sites for fatty acid linkage
• can be simple (identical fatty acids) or mixed fatty acids
• provide stored energy and insulation

Question 8

Explain how sterols are described as amphipathic molecules.

Answer 8

Sterols have a polar OH- group while the 4 fused carbon rings is not polar.

Question 9

Explain why triacylglycerol is an effective means to provide energy and insulation.

Answer 9

• Higher energy yield compared to the oxidation of other fuel sources
• Not hydrated (more compact)

Question 10

Define the differences between lateral diffusion and trans-bilayer translocations and describe how these might be measured experimentally.

Answer 10

Lateral diffusions:

• membrane lipids (and proteins) diffuses diffuse laterally in each leaflet of the membrane
• contained in each leaflet
• measured Fluorscence Recovery After Photobleaching (FRAP)

Trans-bilayer:

• called flip-flops, movement of lipids between the leaflet
• occurs very slowly
• catalysis is essential (flippase, floppase, scramblase)
• measured by Radioactive labelling

Question 11

Describe how temperature and composition impact on membrane fluidity.

Answer 11

Low temperature –> semi solid state, all motions of the individual lipids are strongly constrained

High temperature –> liquid-disordered stated, fatty acids are in constant motion, lateral diffusion

Question 12

Compare the properties of peripheral and integral membrane proteins,

Answer 12

Peripheral membrane proteins: released from membranes by disruption of ionic interactions and hydrogen bonding

Integral membrane proteins: released from membranes by disruption of hydrophobic chains

Question 13

List 3 functions of membrane proteins.

Answer 13

• recognition signals
• structural support for the cell
• transport: selective entry and exit of molecules

Question 14

Explain using examples the differences between active and passive transport.

Answer 14

Passive transport:

• movement of the molecule across a membrane from a region of higher to lower concentration
  
  • Eg. GLUT 1 - GLUT 1 exist in 2 conformation, T1 and T2
  • Thermal energy drives conformational changes (T1–> T2)

Active transport:

• protein-mediated transport against a gradient of concentration or charge.
  
  • Eg. use of P-type ATP-ase
  • ATP hydrolysis (release of energy)
  • energy used to induce conformational change (E2 conformation favoured over E1)
  • changes in affinity for binding with Ca2+ , Ca2+ released
  • removal of phosphate restart the chain

Question 15

Describe the structure of sphingolipids.

Answer 15

Sphingolipids:

• perform distinctive structural and signalling role
• derivatives of the amino alcohol sphingosine and contain no glycerol
• part of the sphingosine contribute one of the three tails.
• amide bond between fatty acid and the sphingosine
• head group may contain either a phosphate (phospholipid) group or not (glycolipid).

Question 16

Describe the structure of glycolipids.

Answer 16

• contain mono or oligosaccharide units in their head groups
• components of outer membrane leaflet
• contribute to sites of biological recognition (exp. determinant of the blood groups A, B, O)

Question 17

Describe the structure of sterols.

Answer 17

Sterols:

• have 4 fused carbon rings (three 6-carbon rings, one 5-carbon ring)
• almost planar and relatively rigid
• amphipathic
• plays an important role in signalling acting as a precursor for steroid hormones.

Question 18

Membrane proteins have a _________ in the membrane. Explain it.

Answer 18

unique orientation, the amino and carboxyl terminus position (external or internal) is fixed based on the type of membrane proteins

Question 19

Type II membrane protein has the carboxylate terminus in the ______ of the membrane, while the amino terminus in the _______. Type 1 is vice versa.

Answer 19

outside, inside

Question 20

What is membrane rafts?

Answer 20

Membrane rafts are microdomaind of the membrane with the foollowing features:

• rich in sphingolipids and cholestorl
• slightly thicker, less fluid
• lateral diffusion of components into and out of the raft is possible
• good for signalling

Question 21

Describe the structure of phospholipids.

Answer 21

They are analogous to triaceylglycerols but with a phosphate group as their polar head groups. Each has one polar phosphate group and 2 non polar tails.