6 Biological Membranes Flashcards
Lipids are compounds that are primarily:
- ______
- ______
- _______ (beh in water)
Compounds that are primarily:
- Non-polar (or amphipathic)
- Hydrophobic
- Insoluble in water
What are four types of lipids?
- Fatty acids
- Triacylglycerol
- Membrane lipids
- Cholesterol
What are Fatty acids?
- Long chain hydrocarbon carboxylic acid
- up to 24 carbons long (16 & 18) most common
- General formula (for a saturated Fatty Acid) CH3(CH2)NCOO-
- Amphipathic (Amphiphilic)
- Polar and non-polar portions
- May be saturated or unsaturated
- Saturated = no double bonds
- Unsaturated = double bonds
- Mono- or polyunsaturated (1+ DB’s)
- usually cis (z) double bonds
FATTY ACIDS are:
- Long-chain _____________\_
- up to __\_carbons long (__\_ & __\_ most common)
- General formula (for a saturated Fatty Acid) ________\_
-
_____\_(_____\_)
- Polar and non-polar portions
- May be _____\_or _____\_
- _____\_ = no double bonds
-
_____\_ = double bonds
- Mono- or polyunsaturated (1+ DB’s)
- usually _____\_ conformation double bonds
FATTY ACIDS are:
- Long-chain hydrocarbon carboxylic acid
- up to 24 carbons long (16 & 18 most common)
- General formula (for a saturated Fatty Acid) CH3(CH2)NCOO-
-
Amphipathic (Amphiphilic)
- Polar and non-polar portions
- May be saturated or unsaturated
- Saturated = no double bonds
-
Unsaturated = double bonds
- Mono- or polyunsaturated (1+ DB’s)
- usually cis (z) conformation double bonds
What type of molecules are shown in the image?
Fatty acids (Lipids)
(a) is saturated
(b) is monounsaturated (1 double bond between C9 and C10)
(c) is polyunsaturated (>1 double bond)
* ALL in Z or CIS configuration
Would CIS or TRANS conformation of Fatty acids be more favourable? Why?
- TRANS appears more energetically favourable
- HOWEVER impact of of CIS = Creates a kink in the structure
- affects melting point (lowers mp) = liquid at room temperature
- Most naturally occurring db’s are in the cis conformation
- Most naturally occurring double bonds are in the ___\_ conformation
- impact of __\_ = Creates a kink in the structure
- affects _______\_ = liquid at room temperature
- Most naturally occurring double bonds are in the cis conformation
- impact of CIS = Creates a kink in the structure
- affects melting point (lowers mp) = liquid at room temperature
What is the shorthand notation of fatty acids and what does it identify about the structure?
- Shorthand notation identifies: (#C):(#double bonds)Δ(locations of the double bonds)
- eg:
- CH3CH2CH2CH2CH2COOH is 6:0 (5 carbons, no double bonds)
- carbon 1 is associated with the -OOH (carboxyl group)
In a fatty acid, what Carbons would be designated alpha α, beta β, and omega ω?
Alpha: Carbon 2 (the carbon attached to the carboxyl group (COOH))
Beta: Carbon 3 - follows alpha
Omega: The last carbon in the chain
Give the shorthand notation for the fatty acid in the image
- Is it a cis or trans double bond?
16:1 Δ7
Cis double bond
Provide the shorthand notation for the fatty acid in the image:
16:1 trans Δ7
If a shorthand notation for a fatty acid doesn’t indicate cis or trans explicitly, what can you assume it’s configuration is?
CIS
How do double bonds affect fatty acids?
Double bonds alter the shape of fatty acids
As length of the fatty acid chain increases, what happens to the Melting Point?
Melting point increases with length
How does melting point change with increased degrees of unsaturation?
Melting point decreases with increased degrees of unsaturation (more double bonds)
What two properties affect the fatty acid melting points and how?
- Length
- longer fatty acids melt at higher temperatures
- shorter fatty acids melt at lower temperatures
- Unsaturation
- Saturated fatty acids melt at higher temperatures
- Unsaturated fatty acids melt at lower temperatures
- Has a greater effect on melting point than length
Does length or unsaturation have a greater effect on melting point (fatty acids)
Unsaturation has a greater effect
Why does unsaturation have such a dramatic effect on melting point of fatty acids?
- Saturated fatty acids can align closely to maximize van der waal’s interactions
- Unsaturated fatty acids are bent = interferes with close packing
- Trans fatty acids are able to pack better than Cis ones
What is triacylglycerol (TAG)?
Molecule that contains fatty acids as part of its structure; also contains glycerol
- Very hydrophobic (NOT amphipathic)
- way of storing fatty acids
- Three (ester-linked) acyl chains attached to glycerol
Three acyl chains attached to glycerol make up what molecule?
Triacylglycerol
- Acyl chains from fatty acids (ester linked)
What is the molecule in the image?
Triacylglycerol
- 3 acyl chains attached to glycerol
- acyl chains from fatty acids (ester-linked)
TAG (triacylglycerol) is extremely ________ (not amphipathic). Why?
TAG (triacylglycerol) is extremely Hydrophobic (not amphipathic). Why?
- This is due to the Ester-linkage between the polar hydroxyls of glycerol and the polar carboxylates of the fatty acids.
- TAG (triglycerides) are hydrophobic, non-polar, and insoluble in water
Forms droplets in aqueous environment.
Melting points for triacylglycerols will be lower for those containing ________ or ________
Melting points for triacylglycerols will be lower for those containing unsaturated fatty acids or Shorter chains
What are the three lipids found in the membrane (ie membrane lipids)?
- Glycerophospholipids
- Sphingolipids
- Cholesterol
Glycerophospholipids and sphingolipids contain fatty acids as part of their structure and are structurally similar
Compare glycerophospholipids and triacylglycerol
- Like triacylglycerol, glycerolphospholipids have glycerol with fatty acyl groups covalently attached
- Unlike triacylglycerol, the presence of a large polar group (phosphate) makes these molecules amphipathic
*variations exist in both polar head groups and acyl chains, affecting size and melting points
What is the molecule shown?
Cholesterol
Describe cholesterol
- Weakly amphipathic (because has single OH group - 3 h-bond interactions 2 accept 1 donor)
- Rigid, non-polar structure
- hydrocarbon/ring structure
- Accounts for ~35% of mammalian membranes
Cholesterol is mostly ______ (weakly amphipathic because of ____)
- ____ carbons and 1 ___
- Maintains _____ and _____ of membrane
Cholesterol is mostly hydrophobic (weakly amphipathic because of OH)
- 27 carbons and 1 OH
- Maintains fluidity and rigidity of membrane
How does cholesterol associate with the membrane?
Does not form membranes alone:
OH associates with polar headgroups of other lipids
- non-polar portion found in the membrane
Amphipathic molecules form _____ or ______ in water
Amphipathic molecules form micelles or bilayers in water
Membrane lipids form a ______ when mixed in water;
Fatty acids form _____ when mixed in water
Membrane lipids for a bilayer when mixed in water;
Fatty acids form micelles when mixed in water
What is the goal for Bilayer sheets, micelles and liposomes?
to eliminate unfavourable contact between H2O and hydrophobic tails while still permitting solvation of polar head groups
Why do fatty acids form micelle’s instead of bilayers or liposomes?
The geometry is different:
- Fatty acids are more conical in shape - favours micelle formation
- Membrane lipids are more cylindrical - favours bilayers/liposomes
What are liposomes?
Simplified lipid membrane bilayer structures
- spherical vesicles
- allow for separation of different environments
- Isolates core from exterior environment
The structure and physical characteristics of bilayers depends on ________ which includes
- ______
- ______
The structure and physical characteristics of bilayers depends on lipid composition which includes
- acyl chain
- polar head group
How are bilayers capable of being both fluid and stable?
They are non-covalently assembled
- no bonding between the individual lipid molecules and the surrounding aqueous environment
- relatively weak interaction
- individual components within bilayer are relatively mobile
Examine the image:
In what ways are the dimensions of a lipid bilayers variable?
- The lipid head groups have significantly different dimensions
- The acyl tails vary between 16-20 carbon atoms in length and degree of unsaturation
- Cholesterol is almost entirely buried in the bilayer
______ is about the same size as a 16:0 fatty acid. What does this imply?
Cholesterol is about the same size as a 16:0 fatty acid. What does this imply?
- If we have a 16:0 fatty acid and cholesterol nearby they are on the same scale which is important when looking at interactions within the membrane
What is the “ordered gel phase” of lipid bilayers?
Below the transition temperature (ie below melting point), acyl chains are packed together in van der Waals contact in a gel-like solid state
- Freeze membranes
- become fragile
- membranes can fracture
What is the disordered liquid crystalline phase of lipid bilayers?
Above the transition temperature (melting point), the lipid molecules and their acyl chains move freely and rapidly
What is the melting temperature (transition temperature) of a lipid bilayer?
The temperature of its transition from an ordered crystalline to a more fluid state
- Depends on acyl-chain unsaturation and length
A sharp transition of a short range is associated with a relatively _____ sample.
Biological membranes typically do not have a sharp transition temperature, why?
A sharp transition of a short range is associated with a relatively pure sample.
- Biological membranes are a mixture of compounds (various lipids/proteins)
- must operate above gel temperature but not be completed disordered
How can a membrane maintain its fluidity with decreasing temperature?
- Add more unsaturated fatty acids
- Shorter fatty acids
How can a membrane maintain (in this case reduce) its fluidity with increasing temperatures?
- More saturated fatty acids (fewer double bonds)
- Longer fatty acids
How does cholesterol reduce the dynamic ability of fatty acids that are adjacent to it?
Cholesterol is rigid and planar = limits the rotational movement of neighbouring acyl tails thereby increasing van der Waals interactions
What is the effect of cholesterol on membrane rigidity at each temperature extreme (Low temperature and high temperature)
- Low temperature
- Prevents close packing between acyl chains = increases fluidity (decreases rigidity)
- High temperature
- decreases motion/disorder of acyl chains = increases rigidity
What is the overall effect of cholesterol?
To increase the membrane fluidity for a larger effective temperature range
What type of individual lipid movement happens freely and rapidly within the bilayer?
- Lateral movement happens freely and rapidly (within leaflet)
- Transverse movement (flip-flop) does not happen freely (from one leaflet to another)
- requires flipases and other enzymes
What types of proteins allow for differences in lipid composition in the leaflets?
Flipases and other enzymes that increase the rate of transverse diffusion
- requires significant amount of energy to move the polar head group through the hydrophobic core
______ carry out most membrane processes
Proteins carry out most membrane processes
Which of the three types of membrane proteins has a hydrophobic surface and a hydrophilic surface (is amphipathic)?
Integral membrane protein
Peripheral membrane proteins are joined to the _______ of the membrane lipids via _______,________,________ interactions and can be easily disrupted (eg by changing ________)
Peripheral membrane proteins are joined to the polar heads of the membrane lipids via polar/ionic/electrostatic interactions and can be easily disrupted (eg by changing [Salt])
The portion of an integral membrane protein that is in contact with the acyl tails of the bilayer must have __________ on its surface
The portion of an integral membrane protein that is in contact with the acyl tails of the bilayer must have hydrophobic amino acid side chains on its surface
What can we ascertain about the sidechain type and distribution of integral membrane proteins from looking at the image of a K+ channel?
*What four side chains could be indicated by the red? (very polar/charged)
- Nonpolar side chains (green) make up most of the helix surfaces that face the lipid tails
- Polar side chains (yellow) are more prominent in the loops, interacting with the lipid head groups and solvent
- Very polar (charged) side chains (Asp, Glu, Lys, and Arg) (red) often flank these regions interacting with solvent
What issue arises when we have a transmembrane structure found within a very hydrophobic portion of the bilayer? (similar to what happens in the core of soluble globular proteins)
If we are in a very hydrophobic environment, and we have a polypeptide chaine, present, then the preferred structure that would be adopted by those pp chains would be a regular secondary structure (alpha helices and beta sheets/barrels)
- Regular structures satisfy the hydrogen bond potential of the polypeptide backbone
_________ and ______ predominate as the portions of these protein structures that are assoc with these transmembrane regions
Regular alpha-helices and beta-sheets/barrels predominate as the portions of these protein structures that are assoc with these transmembrane regions
What type of residues would we expect to predominate on the surface of the alpha-helices around the region associated with the transmembrane structure?
Hydrophobic residues - need to associate with the non-polar tails of the lipid membranes
In regular beta sheets/barrels, the portion exposed to the hydrophobic portion of the bilayer will be ______
In regular beta sheets/barrels, the portion exposed to the hydrophobic portion of the bilayer will be hydrophobic
- Forms hydrophobic interactions with the lipid core of the bilayer
Carbohydrate chains are attached to the _______ surface of some proteins and lipids
Carbohydrate chains are attached to the extracellular surface of some proteins and lipids
What type of molecules can freely cross the lipid bilayer via simple diffusion?
Small, non-polar molecules
What are the two major types of transport across biological membranes?
- Passive
- Active
_______ and _____ enable passive transport via membrane-spanning pores
Porins and Ion channels enable passive transport via membrane-spanning pores
Porins contain a relatively ______, _______ pore in the center of a ______
Porins contain a relatively non-specific, water-filled pore in the center of a Beta-barrel
Most porins exist as ________ (3 subunits)
- each individual subunit forms a ________ structure and contains a ______
Most porins exist as Trimers (3 subunits)
- each individual subunit forms a beta-barrel structure and contains a pore
- regular secondary structure which characterizes most integral membrane proteins is a Regula beta sheet
In ion channels, the channel is formed between ______.
______ subunits with channel at the core = highly ______
In ion channels, the channel is formed between subunits
Four subunits (tetramer) with channel at the core = highly selective
Selectivity of ion channels depends on the ________ and the _________
Selectivity of ion channels depends on the size of the pore and the properties of the side chains/functional groups found there
Using potassium channels as an example, explain how the properties of the side-chains/functional groups has more control over channel specificity than the size of the pore:
Potassium channel has four carbonyls forming the pore = K+ is capable of interacting with all four of these groups simultaneously.
Na+ (albeit smaller than K+) can only interact with one carbonyl at a time and therefore moves very slowly through the channel
What is the major structural difference between transport proteins (eg ion channels and porins) and Transporter proteins?
Transporter proteins (Carrier proteins) change shape (conformation)
- do not contain membrane-spanning pores
- May be passive or active
What type of relationship is shown by the kinetics of passive transport by carrier proteins?
Hyperbolic
Label the red boxes
What are the three classifications of carrier proteins?
- Uniport
- Symport
- Antiport
Two solutes transported in the same direction would be a _______ carrier protein;
Two solutes transported in opposite directions would be a ______ carrier protein;
One solute being transported is a ________ carrier protein
Two solutes transported in the same direction would be a symport carrier protein;
Two solutes transported in opposite directions would be an antiport carrier protein;
One solute being transported is a uniport carrier protein
What is the difference between primary (1°) and secondary (2°) active transporters?
- Primary:
- typically uses ATP as the source of free energy
- Secondary
- Uses an ion gradient (eg Na+) as the source of free energy
In _________ active transport, the free energy of the reaction is less than zero
ΔGrxn<0
In Primary active transport, the free energy of the reaction is less than zero
ΔGrxn<0
- ATP hydrolysis
- Redox rxn
In _______ active transport, the free energy of an ion which is less than zero plus the free energy of the reaction will give us a net free energy that is MORE NEGATIVE than the free energy associated with the transport of our solute in order for the process to occur
ΔGT(ion) <0 + ΔGrxn =
ΔGnet < 0 (spontaneous)
In secondary active transport, the free energy of an ion which is less than zero plus the free energy of the reaction will give us a net free energy that is MORE NEGATIVE than the free energy associated with the transport of our solute in order for the process to occur
ΔGT(ion) <0 + ΔGrxn =
ΔGnet < 0 (spontaneous)
Ie things have to add up to be less than zero in order for secondary active transport to occur
Fill in the table
The sodium potassium pump (Na+K+ ATPase) is an example of a __________ transporter
The sodium-potassium pump (Na+K+ ATPase) is an example of a primary active transporter
The Na+K+ ATPase requires ____ to move ions ______ their concentration gradients
The Na+K+ ATPase requires ATP to move ions against (up) their concentration gradients
The Na+ Glucose transporter is a ____________ transporter
The Na+ Glucose transporter is a secondary active transporter
How are the primary and secondary active transports for Na+K+ATPase and Na+ Glucose linked?
The primary active antiporter (Na+K+ATPase) creates a concentration gradient;
The secondary active symporter (Na+ Glucose) uses that concentration gradient (of sodium) to drive the import of a third molecule (glucose)
A transporter protein moves substances X and Y as shown in the following diagram. It moves X down its concentration gradient and Y up its concentration gradient. What type of transport is this?
A. Primary active antiport
B. Primary active symport
C. Secondary active antiport
D. Secondary active symport
A transporter protein moves substances X and Y as shown in the following diagram. It moves X down its concentration gradient and Y up its concentration gradient. What type of transport is this?
A. Primary active antiport
B. Primary active symport
C. Secondary active antiport
D. Secondary active symport
In the figure shown below, which of the labelled carbons is the β‐carbon?
Z
The diagram below shows the structure of an integral membrane protein. The solid lines indicate the approximate location of the polar head groups of the lipid bilayer.
Which arrow most correctly identifies a region in the protein where you would expect to find amino acid residues such as Leu, Ile, Val and Ala?
1
Leu, Ile, Val and Ala are HYDROPHOBIC and would therefore be found in the hydrophobic, non-polar acyl tail groups
Which of the following conclusions can be drawn regarding the protein shown below, given ONLY the information in the diagram?
A. This protein is a transporter protein.
B. This protein is an ion channel.
C. This protein is not a porin.
D. This protein is not an active transporter.
A. This protein is a transporter protein.
B. This protein is an ion channel.
C. This protein is not a porin.
D. This protein is not an active transporter.
Which of the following does NOT correctly describe the picture below?
A. Polyunsaturated
B. Spontaneously forms a bilayer in aqueous solution.
C. 18:2Δ9,12
D. Fatty acid
Which of the following does NOT correctly describe the picture below?
A. Polyunsaturated
B. Spontaneously forms a bilayer in an aqueous solution. (fatty acids form micelles when mixed with water)
C. 18:2Δ9,12 (18C:2db’s at C9 and C12)
D. Fatty acid
