LIPIDS AND MEMBRANES

Question 1

Which body compartments are lipids found in?

Answer 1

Plasma, adipose tissue and biological membranes

Question 2

What is a biological membrane?

Answer 2

Aa selectively permeable membrane that separates cell from the external environment or creates intracellular compartments

• the bulk of lipid in a cell membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning
• dynamic structures which proteins float in a sea of lipids
• in addition to phospholipids, other membrane lipids include glycolipids and cholesterol

Question 3

What are membrane lipids?

Answer 3

Small molecules that have both hydrophilic and hydrophobic elements
-there are amphiphilic (has both hydrophilic and phobic properties)

Question 4

What are fatty acids?

Answer 4

Long hydrocarbon chains of various lengths and a terminal carboxyl group

• may be saturated or unsaturated
• single double bond=monounsaturated
• two or more double bonds=polyunsaturated
• composed of a carboxyl (HO-C=O) and methyl (CH3) end

Question 5

What are essential fatty acids?

Answer 5

EFA’s are essential to the body, homeostatic balance and normal bodily functions

• indispensable to the body because of the body’s inability to synthesise these endogenously (within the body)
• lack of EFA can result in skin conditions

Question 6

What are non-essential fatty acids?

Answer 6

NEFA’s can be derived from within the body and via food stuffs
-eg.Oleic acid, occurs naturally in animal and vegetable fat, the most abundant fatty acid in human adipose tissue, desaturase can add a double bond to stearic acid to make oleic acid

Question 7

What is the important of essential and non-essential fatty acids?

Answer 7

Maintains the integrity of the cell membranes, normal function of our nervous system, vision, cognition and memory. nutrient metabolism and cell growth

Question 8

What is fatty acid synthesis?

Answer 8

The creation of fatty acids from acetyl-coA and NADPH through the action of enzymes called fatty acid syntheses

Question 9

Where does FA synthesis take place?

Answer 9

In the cytoplasm

Question 10

What are the cis-double bonds in the FA chain?

Answer 10

Places a kink in the linear structure of the fatty acid chain, interfering with the structure and lowering the overall melting point of the FA

Question 11

What are FA stored as?

Answer 11

Triglycerides

• they are hydrolysable, can be used in aerobic respiration
• they are an efficient energy source
• 1g triglyceride=6 times the energy source of glycogen

Question 12

What are the two fatty acids known to be essential?

Answer 12

Alpha linolenic acid (omega 3 fatty acid) and linoleic acid (omega 6 fatty acid)

• cannot be synthesised in humans because humans lack the desaturase enzymes required for their production and to remove hydrogens from omega 3 and 6
• needed from diet

Question 13

What is the COX-enzyme?

Answer 13

An enzyme that is responsible for formation of prostanoids, including thromboxane and prostaglandins such as prostacyclin, from arachidonic acid

• target for Non-steroidal anti-inflammatory drugs
• eg.aspirin, ibuprofen, indomethacin

Question 14

What is membrane formation?

Answer 14

The consequence of the amphipathic nature of membrane molecules

• polar head groups favour contact with water while the hydrocarbon tails interact with one another
• hydrophobic and hydrophilic properties of membrane lipids can be satisfied by forming a lipid bilayer, composed of two lipid sheets, the two opposing sheets are called leaflets

Question 15

What are micelles?

Answer 15

An aggregate (or supramolecular assembly) of surfactant molecules dispersed in a liquid, forming a colloidal suspension

• globular structure
• formed by lipid molecules in membranes

Question 16

What property of lipids enables it form membranes?

Answer 16

Hydrophobic properties
-lipids spontaneously form closed bimolecular sheets in aqueous media (spheres) that are barriers to the flow of most solutes (polar molecules)

Question 17

What are lipid bilayers?

Answer 17

The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules

• the layers tend to close in on themselves
• self-sealing because a hole in the bilayer is energetically unfavourable

Question 18

What are liposomes?

Answer 18

A liposome is a spherical vesicle having at least one lipid bilayer

• the propensity to form membrane sheets favours the formation of Liposomes
• an be loaded with hydrophobic and/or hydrophilic molecules such as DNA or drugs
• can be used to study membrane permeability or to deliver chemicals to cells (drug targets)
• long circulating liposomes concentrate in regions of increased blood circulation such as tumors and sites of inflammation

Question 19

What is the favoured structure for most phospholipids and glycolipids in aqueous media?

Answer 19

A bimolecular sheet rather than a micelle

• two fatty acid chains are too bulky to fit into the interior of the micelle
• lipid bilayers form spontaneously by a self-assembly process largely driven by hydrophobic interactions
• van der waals attractive forces between the hydrocarbon tails favour close packing of the tails
• there are also electrostatic and hydrogen bonding attractions between the polar head groups and water molecules

Question 20

What are exosomes?

Answer 20

Exosomes are a type of extracellular vesicle that contain constituents (protein, DNA, and RNA) of the cells that secrete them. They are taken up by distant cells, where they can affect cell function and behaviour.

Question 21

What is the fluid mosaic model?

Answer 21

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character

• the membrane is a fluid-like phospholipid bilayer into which other lipids and proteins are embedded
• membrane lipids and proteins easily move on the membrane surface by lateral diffusion
• lipids and many membrane proteins are constantly in lateral motion and rotation
• acyl chains can flex within the bilayer

Question 22

What is membrane fluidity?

Answer 22

The viscosity of the lipid bilayer of a cell membrane
-many membrane processes such as transport or signal transduction, depend on the fluidity of the membrane lipids, which in turn depends on the properties of fatty acid chains

Question 23

Why is the asymmetry of the lipid bilylayer functionally important?

Answer 23

Provides the two sides of the plasma membrane with different biophysical properties and influences numerous cellular functions

Question 24

What are phospholipids?

Answer 24

A class of lipids whose molecule has a hydrophilic “head” containing a phosphate group, and two hydrophobic “tails” derived from fatty acids, joined by a glycerol molecule

• a key component of all cell membranes
• the platform on which the phospholipids are built may be glycerol, a 3 carbon alcohol or sphingosine
• the common alcohols or glycerophospholipids are serine, ethanol amine, choline and inositol

Question 25

What is sphingosine?

Answer 25

18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phospholipid

Question 26

What are glycolipids?

Answer 26

Sugar containing lipids

• derived from sphingosine
• orientated in a completely asymmetric fashion with the sugar residues always on the extracellular side
• the simplest glycolipid called cerebroside, contains a single sugar residue, either glucose or galactose
• more complex glycolipids such as gangliosides contain a branched chain of as many as 7 sugar residues

Question 27

What are the functions of phospholipids?

Answer 27

-provide barriers in cellular membranes to protect the cell, and they make barriers for the organelles within those cells —-work to provide pathways for various substances across membranes

Question 28

What are the functions of glycolipids?

Answer 28

• maintains the stability of the cell membrane
• facilitates cellular recognition, which is crucial to the immune response and in the connections that allow cells to connect to one another to form tissues

Question 29

What is cholesterol?

Answer 29

A steroid built from four linked hydrocarbon rings

• a hydrocarbon tail is linked to the steroid at one end and a hydroxyl group at the other
• contains a bulky steroid nucleus
• inserts into phospholipid bilayers with its long axis perpendicular to the plane of the membrane
• the different shape of cholesterol compared with that of phospholipids disrupts the regular interactions between fatty acid chains
• a drop of temp increases membrane fluidity, important for endocytosis

Question 30

What is the function of cholesterol?

Answer 30

Its main function is to maintain the integrity and fluidity of cell membranes and to serve as a precursor for the synthesis of substances that are vital for the organism including steroid hormones, bile acids, and vitamin D

Question 31

How does glycolipids serve as recognition sites for cell to cell interaction?

Answer 31

Saccharide of the glycolipids will bind to a specific complementary carbohydrate or to a lectin (protein that binds to certain carbohydrates) of a neighbouring cell
-the interaction of these cell surface markers is the basis of cell recognition and initiates cellular responses that contribute to activities such as regulation, growth and apoptosis

Question 32

How do glycolipids participate in immune responses?

Answer 32

Selectins, a class of lectins found on the surface of leukocytes and endothelial cells bind to the carbohydrates attached to glycolipids to initiate the immune response

• this binding causes leukocytes to leave the circulation and congregate near the site of the inflammation (initial binding mechanism)
• following the initial binding is the expression of integrins which form stronger bonds and allow leukocytes to migrate towards the inflammation site

*also recognises host cells by viruses

Question 33

What is an example of how glycolipids on cell membranes mediate cell interactions with the surrounding environment?

Answer 33

Blood types

• the four main human blood types (A,B, AB and O) are determined by the oligosaccharide attached to a specific glycolipid on the surface of red blood cells which act as an antigen
• blood type A has an N-acetylgalactosamine added as the main determining structure
• blood type B has a galactose
• blood type AB has both of those antigens

Question 34

What are glycerophospholipids?

Answer 34

Glycerol-based phospholipids

-glycerophospholipid bilayer with cholesterol is more densely packed, less fluid and more permeable

Question 35

What is the effect of cholesterol on sphingolipids?

Answer 35

Reduction in the packing density, keeps membrane more fluid
-cholesterol is enriched along with sphingolipids in membrane invaginations (clarithin-coated pits) that are important for endocytosis

Question 36

What percentage of cholesterol is absorbed during passage through the gut?

Answer 36

30-60%

-transported to the liver and peripheral tissues as a component of lipoprotein particles, the chylomicrons

Question 37

What membrane transporter does the intestines used to absorb dietary cholesterol?

Answer 37

NPC1L1

Question 38

Which transporters transports cholesterol into the lumen from the enterocyte?

Answer 38

Two half transporters ABCG5 and ABCG8

Question 39

What do exogenous cholesterol bind to be taken up by the cell?

Answer 39

ApoB receptors (LDLR)

Question 40

What are the membranes surrounding a fat droplet different?

Answer 40

Because it’s contents are hydrophobic

Question 41

What are lipid rafts?

Answer 41

Highly dynamic complexes formed between cholesterol and specific lipids

• different lipid and protein content, specialised function
• brings together proteins that work together (eg. for endocytosis or signalling)

Question 42

Examples of how the different lipid compositions and different chemical nature of lipids is related to the function of the cell

Answer 42

1. Hepatocytes are high in phosphatidylcholine which are released into bile
2. ER is low in cholesterol, increasing membrane fluidity
3. E.coli lacks cholesterol, peptidoglycan provides its stability
4. Myelin is enriched in glycolipids to insulate nerve axons

Question 43

What are the limitations of the fluid mosaic model?

Answer 43

• while some proteins can diffuse laterally within the membrane, others are anchored in protein complexes or tied to the ECM or intracellular cytoskeleton
• the lipid contents of rafts is important for raft integrity (the lipids influence the protein-protein contact)
• lipids can also be signalling molecules and a source of secondary messengers
• lipids can also modulate membrane protein activity directly

Question 44

How does alcohol affect the functioning of membrane proteins?

Answer 44

The toxicity of ethanol alters membrane permeability and diffusion

• different alcohols have different effects on the cell membrane
• short chain alcohol have an amphiphilic character and do less damage to the cell membrane compared to long chain alcohol
• the addition of a CH2 group increases the hydrophobicity of the alcohol
• ethanol passes more readily into the lipid bilayer than methanol as it is more hydrophobic

Question 45

What is a consequence of a buildup of cholesterol?

Answer 45

Increased cholesterol in the diet, increases LDL synthesis, higher levels circulating
-Atherosclerosis is the buildup of fats, cholesterol and other substances in and on your artery walls. This buildup is called plaque. The plaque can cause your arteries to narrow, blocking blood flow. The plaque can also burst, leading to a blood clot.