1.1 - Cell Membrane Lipids Flashcards
Major functions of biological membranes
- continuous and highly selective permeable barrier
- control of the enclosed electrochemical environment
- Communication
- Signal generation in response to stimuli (electrical and chemical)
- Recognition, signalling, adhesion proteins and immune surveillance
Dynamic fluid mosaic model basics
⁃ phospholipid belayer with proteins and add-on structures
⁃ H20
⁃ Dynamic 3D macro molecular structure
⁃ Structural stability based on highly organised regions with defined charge density and distribution
⁃ Selectively permeable to biological molecules
Membranes are heterogenous within + between cells
⁃ ie don’t look the same, vary completely
⁃ Specialisations vary depending on tissue types and developmental status
⁃ Ie for interaction with basement membranes, interaction with adjacent cells, absorption of body fluids, secretion and transport
⁃ Membranes change over time (based on developmental status or disease)
⁃ Some (eg nerves) may specialise in electrochemical signalling - so need chemical signal generation and transmission.
⁃ Membranes are highly organised structure
What is the composition of cell membranes
⁃ measure using dry weight
⁃ Approx 40% lipid
⁃ 60% protein
⁃ 1-10% carbohydrate
⁃ However, membranes are hydrated structures, and about 20% of total membrane weight is water
⁃ Water dynamically interacts with membrane, and is vital to membrane structure + function
Why is water important for cell membranes
⁃ membranes are hydrated structures, and about 20% of total membrane weight is water
⁃ Water dynamically interacts with membrane, and is vital to membrane structure + function
⁃ Water interacts with charged/polar regions, stabilising the lipid head regions
⁃ Small proportion of water molecules also enter hydrophobic region of lipid bilayer
What are the different types of lipids found in membranes
3 main groups
⁃ phospholipids - can be unsaturated or saturated
⁃ Glycolipids
⁃ Cholesterol
Saturated vs unsaturated fatty acids
⁃ unsaturated have at least one double bond and are therefore kinked
⁃ saturated have no double bonds
What is amphipacity (lipids in cell membranes)
⁃ fundamental property of membrane lipids
⁃ Means one part of molecule is hydrophilic (polar or ionic charge). This is the head
⁃ The tails (fatty acids) are hydrophobic
⁃ Property is key to the formation of the lipid bilayer
Formation of the lipid bilayer in the aqueous environment
⁃ amiphathic phospholipids (PLs) heads strongly prefer contact with water, whereas tails avoid water
⁃ Forms 2D bilayer sheets or 3D micelles
⁃ This is the structure that is the most optimally energetically stable, thermodynamically
Features of glycerophospholipids
Consists of:
⁃ phosphate (+ charged/polar head) - aka hydrophilic head
⁃ Glycerol backbone
⁃ 2 Fatty acid chains aka hydrophobic tails
Also:
⁃ because glycerol residual has single bonds, the heads and tails can swivel
⁃ Hydrophilic head phosphate is charged, meaning it can interact with water molecules
⁃ In aqueous environments, phospholipids can arrange to form two bilayer leaflets
Features of the fatty acid chains
- Length of C12-C24
- Can be saturated (no C=C)
- Can be unsaturated (at least one C=C in one of the chains)
- unsaturated cis is C=C with ‘kink’ + sticky out leg
- saturated trans is C=C with legs straight
What is the reaction that forms phospholipids (ie glycerol bonding to fatty acid chains)
Esterification (between the acid group of the fatty acid + the alcohol group of the glycerol)
Polar head groups
- Amines
- Amino acids
- Choline (most common)
- Inositol (has key signalling role)
☞ the thing that makes it charged = phosphate C=O group
What is sphingomyelin
- Also a phospholipid (but not a glycerophospholipid)
- No glycerol backbone → instead use sphingosine molecule
- Can have both unsaturated or saturated fatty acids (but mainly saturated)
- Often the fatty acid tail chains are unequal in length
- Has a key role in lipid raft formation
- Strong interaction with cholesterol
- High concentration in myelin (for myelin sheaths)
What are the two types of glycolipids (no details)
cerebrosides and gangliosides
Glycolipids are also sphingolipids
Features of cerobrosides
- Polar head group is a single saccharide
- Mainly glucose or galactose (polar head group) → therefore hydrophilic
- Major glycoside found in CNS/PNS myelin sheath
- Only found in external outward facing membrane leaflet (for identity or signalling)
- Also found in lipid rafts
Main roles of cerebrosides
- Stabilising membrane
- Cell-cell recognition and identity markers
- Immune system: form the basis of blood groups (antigens on RBC surface)
- found in myelin sheath
Features of gangliosides
- polar head group with an oligosaccharide chain
- up to 9-10 saccharides
- includes sialic acids eg NANA
- only in external membrane leaflet
- extend out from cell surface (wide signalling capability)
- very important in CNS
Main roles of gangliosides
- stabilising membranes
- cell-cell recognition/identity markers
- immune system (form basis of blood groups, antigens on cell surface)
- have very important role in CNS
- form part of lipid rafts
What are some common structural features of membrane lipids
- amphipathic ie hydrophilic head and hydrophobic tail
- FA tails are saturated or unsaturated
- Bilayered 3D structure
☞ sphingomyelin and glycolipids present on outer face of membrane (concerned with cell identity)
Features of cholesterol in cell membranes
- Essential biomolecule
- Major membrane component
- Primary molecule in steroid hormone synthesis
- Structurally distinct membrane lipid
- Properties mean that it has significant biophysical interactions with other membrane lipids + proteins
Structure of cholesterol and how is cholesterol structurally distinct
- Amphipathic so hydrophobic + hydrophilic
- hydrophilic - small OH group ‘head’
- hydrophobic - planar steroid rings (rigid steroid rings) and smaller ‘tail’chain
- Not very ‘wobbly’ (and short single chain)
Main roles of cholesterol in cell membranes
- Typically 30-40% of the membrane lipids
- Enhancing/extending temperature range – for membrane fluidity + stability
- Lipid raft formation: cholesterol is large component in this
