Lipids & Bio Membranes Flashcards
(29 cards)
1
Q
What are lipids?
A
- provide energy reserves, predominantly in the form of triacylglycerols
- serve as structural components of biological membranes
- all lipids are insoluble or poorly soluble in aqueous solutions
- both lipids and lipid derivatives serve as vitamins and hormones
- lipophilic bile acids aid in lipid solubilisation
2
Q
4 main lipid families
A
- Triacylglycerols (fat/oil) = glycerol backbone and 3 fatty acid (acyl) groups
- Glycerophospholipids (membrane component) = glycerol backbone, contain phosphate moieties and polar head group
- Sphingolipids (membrane component) = built on Sphingosine backbone unit and often has glyco-conjugates
- Isoprenoids = include steroids, lipid vitamins & hormones and cholesterol
3
Q
Properties of the lipid families
A
- Triacylglycerols are hydrophobic (can’t mix with water at all)
- Glycerophospholipids and Sphingolipids are amphipathic and form biological membranes
- These 3 have fatty acid chains
- Isoprenoids are largely hydrophobic and have variable polar group content
4
Q
How do fatty acids differ from one another?
A
- Length of the hydrocarbon tails (generally 12-22 carbon atoms)
- Degree of unsaturation (up to 6 double bonds at most)
- Position of the double bonds in the chain
5
Q
Structure and nomenclature of fatty acids
A
- most fatty acids have 12 to 22 carbons
- carboxyl carbon is carbon 1
- carbon furthest from the carboxyl is the omega carbon
- categorises by unasaturatedness —> monounsaturated (one double bond) or polyunsaturated (many double bonds)
- saturated has no double bonds
6
Q
Importance of cis double bonds in fatty acids
A
- Cis double bonds are healthy
- They introduce kinks
- Less intermolecular Van der Waals interaction (packed less tightly together)
- more fluid (lower melting point)
7
Q
What are triacylglycerols?
A
- Fatty acids are an important fuel source = stored as neutral lipids called triacylglycerols (TAG)
- TAGs are composed of 3 fatty acyl residues esterfied to a glycerol (a single fatty acid)
- Are very hydrophobic and are stored in cells in an anhydrous form
8
Q
Describe glycerophospholipids
A
- the most abundant lipid in membranes
- possess a glycerol backbone
- 2 fatty acids esterfied to glycerol = very hydrophobic
- one phosphate esterfied to 3rd carbon of glycerol
- the phosphate can be further esterfied to a polar group (X)
9
Q
What can the polar group be in glycerophospholipids?
A
Hydrogen = phosphatidate/phosphatidic acid
Ethanolamine
Choline
Serine
Inositol
10
Q
Describe Sphingolipids
A
- large family of membrane lipids
- hydrophobic chain with no ester linkage
- instead of a glycerol backbone, they have a sphingosine unit = 3 carbons that can be considered similar to those in glycerol
- have 2 nonpolar (hydrophobic) tails and a polar head group = amphipathic
11
Q
Types of Sphingolipids
A
- Ceramide (has OH)
- Sphingomyelin (has phosphocholine or ethanolamide)
- Cerebroside (has glucose or galactose)
- Ganglioside (complex oligosaccharide)
12
Q
Describe steroids
A
- From a large class of Isoprenoids that originate from 5 carbon isoprene units
- 30 carbons long
- Form a 4 ring system
13
Q
Describe cholesterol
A
- a structural component of membranes (about 20%)
- a precursor to other important steroids e.g. steroid hormones and bile acids
- has an OH group at the end of chain
- has 4x ring structures
- amphipathic
- fused ring system = less flexible than a fatty acid chain
- fluidity buffer for membranes = increases fluidity at low temperatures and decreases at high temperatures
14
Q
Biological membrane composition
A
- define the external boundaries of cells and separate cellular compartments
- provide a regulated barrier to transport selected solutes
- consists of proteins embedded in or associated with a lipid bilayer
- 25-50% lipid and 50-75% proteins by mass
- variable depending on type of membrane and species
- lipids include phospholipids, Sphingolipids and cholesterol
15
Q
Important functions of membranes
A
- contain proton pumps for ions or small molecules
- generate proton gradients for ATP production
- membrane receptors respond to extracellular signals and communicate them to the cell interior
- approximately 30% of genes in the human genome encode for membrane proteins
16
Q
Key feature of membranes
A
Lipids provide the permeability barrier
Proteins mediate nearly all the other membrane processes
17
Q
Describe the lipid bilayer
A
- noncovalent interactions among amphipathic lipid molecules make them flexible and self-sealing
- polar head groups contact aqueous medium
- nonpolar tails point towards the interior
- the 2 leaflets of a bilayer membrane tend to differ in their lipid composition —> more Sphingolipids in the outer leaflet and more glycerophospholipids in the cytosolic leaflet (inside)
18
Q
How is the membrane a dynamic structure?
A
Fluid mosaic model
Lipid fluidity
- lateral diffusion (within a monolayer) is very rapid = can move forward or backward
- transverse diffusion from one half of the bilayer is very slow = flips from one side of the leaflet to the other
19
Q
3 classes of membrane proteins
A
- Integral
- Peripheral
- Lipid anchored
20
Q
What are integral membrane proteins?
A
- contain hydrophobic regions = insert into the hydrophobic lipid bilayer
- usually span the bilayer completely
- can provide channels through a membrane
- in bacteria = porins and allow passage of certain ions/polar molecules
21
Q
What is Bacteriorhodopsin?
A
- harnesses light for energy production in certain bacteria
- high proportion of nonpolar residues interact with lipids or each other
- some polar/charged residues on interior that interact with pigment
22
Q
What are peripheral membrane proteins?
A
- associated with membrane face through charge-charge or hydrogen bonding interactions to integral proteins and polar head groups of membrane lipids
- more readily dissociate from membranes
- change in pH or ionic strength often releases these proteins
23
Q
What are lipid anchored membrane proteins?
A
Tethered to membrane through a protein-lipid covalent bond
- Ester or thioester bond linking amino acids (Ser, Cys) to a fatty acyl group
- Amide bond linking an N-terminal Gly to a fatty acyl group
- Thioether bond linking amino acid (Cys) to an Isoprenoid chain
- Protein anchored by its C-terminus to glycosylphosphatidylinositol (GPI)
1-3 are in the cytosolic leaflet and 4 is on the outer leaflet of the plasma membrane
24
Q
3 types of membrane transport mediated by integral proteins
A
- Channels and pores
- Passive transporters
- Active transporters
25
Channels and pores
- central passage allows molecules and ions to transverse the membrane
- solutes of appropriate size, charge and molecular structure can diffuse down a concentration gradient
- process requires no energy and occurs at rates approaching the diffusion limit
26
Passive transporters
- carriers and permeases
- protein binds specific solutes and transports them down a concentration gradient
- this process does not require an energy source
- much slower than channels
- involves conformational change
27
Active transport
- requires energy to move a solute against its concentration gradient
- transport of charged molecules or ions may thus result in a charge gradient across the membrane
28
Types of passive and active transport systems
Uniport = carries a single type of solute
Symport = cotransporter of 2 solutes in the same direction
Antiport = cotransporter of 2 solutes in opposite directions
29
Primary vs secondary active transport
Primary transport is powered by a direct source of energy such as ATP, light or electron transport
Secondary transport is driven by an ion concentration gradient
