Plasma Membrane Flashcards
What are the 6 membrane properties?
fluid (in 2 dimensions)
asymmetric (two sides of membrane are different)
Specific to the function of the cell/organelle
self-assembled - held together mostly by non-covalent interactions
semi-permeable to control contents
mosaic of lipids and proteins and some carbohydrate chains attached
Approximately how wide is the plasma membrane?
5-10 nm wide
What are the four major properties of the membrane? (FASS)
Fluid
Asymmetric
Self-assembled
Specific
What are 7 membrane functions
- compartmentalization
- basis for biochemical reactions
- selectively permeable membrane controls contents
- transport
- responds to external stimuli
- intracellular interactions
- energy transductions
Explain how the structure of the membrane allows it to function in compartmentalization
Membranes are continuous lipid sheets which encapsulate the cell or regions of the cell
How does the membrane function in compartmentalization?
- creates discrete compartments that can house different contents
- allows activities to be chemically isolated (ex. intracellular digestion, photosynthesis)
- cellular activities can be regulated independent of one another in their compartments
How does the membrane act as a basis for biochemical reactions?
Reactants of biochemical reactions can be embedded in membranes
What are the benefits of reactants being embedded in membranes?
- increases likelihood of interactions because they are not just free-floating in the cytosol
- effectively orders components to increase speed of interactions
How does the membrane act as a semi-permeable barrier between the inside and outside of a cell or organelle?
membranes control the exchange of molecules between the inside and the outside of a cell/organelle and prevent unregulated movement
membranes also allow for communication between the compartments they separate by being semi-permeable
How does a membrane facilitate transportation?
contains machinery to physically transport substances across the membrane (ex. ions, amino acids, sugars)
from concentration gradients of either high –> low or low –> high
What does functioning in transport allow a cell to do?
Accumulate substances it needs to power its metabolism and build the macromolecules it needs
What needs to be established across the membrane and how for the function of nerve and muscle cells?
ionic gradients established by transport
What do membranes have that allows them to respond to external stimuli?
receptors that interact with ligands or other stimuli (ex. light or mechanical pressure)
T or F: all types of cells have the membranes with the same receptors. why/why not?
FALSE because there is more than one kind of external stimuli, different cells will need to have different receptors to respond appropriately
Different types of cells have membranes with different receptors in order to respond to different stimuli
What do membrane receptors allow for?
signal transduction
What is signal transduction? Give some examples of responses
When an external stimulus triggers an internal reaction
ex. release specific ions, divide, move, synthesize specific compounds, die by apoptosis
How does the membrane function in intracellular interactions?
by allowing the communication between cells
ex. contact or adhesion between 2 cells, exchange of materials, interaction between the inner cytoskeleton and the ECM
How does the membrane function in energy transduction?
Membranes are crucial for converting energy into different forms
ex. the chlorophyll pigments required to absorb photons and the photosystems are embedded in membranes –> required for photosynthesis
ex. the electron transport chain and ATP synthase are embedded in membranes –> required for cellular respiration
T or F: lipids only have a few functions
FALSE. Lipids have SO many functions
List at least 5 functions of lipids
energy storage
components of cell membranes
can be enzyme cofactors
help in folding proteins
act as electron carriers
act as light absorbing pigments
hormones
act as membrane anchors for proteins
intracellular messages
Which key lipid is involved in most lipid functions?
fatty acids
Describe the structure of a fatty acid
a carboxyl group bonded to a long hydrocarbon tail
In what 3 ways can the hydrocarbon tails of fatty acids vary?
- can be from 4-36 carbons (most common is 12-24)
- can include branches
- can include double bonds
Define an amphipathic molecule
A molecule that has both a polar region and a non polar region
Are fatty acids hydrophobic, hydrophilic, or amphipathic? why?
amphipathic
they contain a carboxylic head which is polar AND
a hydrocarbon tail which is non polar
What does a carboxyl group look like?
An OH bonded to a C which is double bonded to an O
What two structures could a fatty acid have?
Either saturated or unsaturated
Define a saturated fatty acid
A fatty acid that does not contain any double bonds
This is because every single carbon is ‘saturated’ with a hydrogen - these have the max number of hydrogens
Define an unsaturated fatty acid
A fatty acid that has at least one double bond
The double bond means there are less hydrogens
Define a monounsaturated fatty acid
An unsaturated fatty acid that contains only one double bond
Define a polyunsaturated fatty acid
An unsaturated fatty acid that contains more than one double bond
Are most fatty acid double bonds cis or trans?
cis
Describe the structure of a saturated fatty acid
Straight
packed tightly
no double bonds
Describe the structure of an unsaturated fatty acid
Kinked
less tightly packed because of kinks
If a fat has more unsaturated fatty acids, will it be liquid or solid at room temperature? why?
liquid
it is less tightly packed because of the double bonds that create the kinks in the structure
If a fat has more saturated fatty acids, will it be liquid or solid at room temperature? why?
Solid
it is more tightly packed due to the lack of double bonds
Give examples of common food fats with unsaturated fatty acids
vegetable oils
Give examples of common food fats with saturated fatty acids
butter
dairy products
animal fat
What compound do almost all fats and oils have in their structure? What does this make them?
almost all have a fatty acid in their structure
makes them FATTY ACID DERIVATIVES
What will happen to a free carboxyl group of a fatty acid?
it will be ionized
What part of a fatty acid will link to other groups? and to form what?
a free carboxyl group will often link to other groups to form
ESTERS or AMIDES
Describe the structure of an ester
The C in the carbon skeleton is double bonded to an O and single bonded to an O which is bonded to another C
O = C - O - C
Describe the structure of an amide
The C in the carbon skeleton is double bonded to an O and single bonded to an N-H
| O = C - N - H
What kinds of linkages will a fatty acid form when it links to other groups?
Either an ester linkage to form esters
Or an amide linkage to form amides
What are fatty acids called when they are attached to another group via an ester or amide linkage?
Acyl chains or acyl groups
Fatty acyl group
What does TAG stand for?
Triacylglycerol
Describe the structure of a TAG
three fatty acids (tri) attached to a glycerol molecule by an ester linkage (acyl)
When most people talk about fats, what are they talking about?
TAGs
How are TAGs formed?
A dehydration reaction combines the glycerol and fatty acid
by removing an OH from the carboxyl group of the fatty acid and an H from the glycerol (to remove H2O) and form an ester linkage
T or F: TAGs are in membranes
FALSE FALSE FALSE FALSE
What is the function of TAGs? Why are they well suited for their function?
Energy storage
Well suited because they are so highly reduced, so oxidizing them at a later time will produce a LOT of ATP
Where are TAGs found?
Packed tightly into adipocytes (fat cells) in huge quantities
Are TAGs polar or non-polar?
essentially Non-polar even with the ester linkages
What 3 lipids are in membranes?
- glycerophospholipids
- sphingolipids
- cholesterol
What type of backbone do most phospholipids have? And what does this make them?
Glycerol backbones
Makes them glycerophospholipids or phosphoglycerides
Describe the structure of a glycerophospholipid
BACKBONE: glycerol
Attached to the glycerol:
- 1 fatty acid on C1
- 2nd fatty acid on C2
- A phosphate + head group on C3
What are common head group chemicals for glycerophospholipids?
serine
ethanolamine
choline
What type of backbones do sphingolipids have? which functional group does it include?
sphingosine
an amine
Why is it called sphingosine?
named after the Sphinx due to its mysteriousness
Describe the structure of sphingolipids
A sphingosine backbone which has 3 carbons (like glycerol) but attached to
C1 is a head group
C2 is an amine which will link to a fatty acid with an amine linkage
C3 is a long hydrocarbon chain of sphingosine
On a sphingosine backbone, which two places are available for other groups to bond to?
The OH and the amine (NH2)
because the third carbon is involved in a very long hydrocarbon chain
Where does the fatty acid attach to the sphingosine backbone? How?
The fatty acid attaches to the amine on the sphingosine backbone at C2 by an amine linkage
What kind of linkage connects the sphingosine backbone to a fatty acid?
amine linkage (C-N)
What are common head groups for sphingolipids? when attached, what does this make sphingolipids?
monosaccharides (simple sugars)
a sugar head group makes a GLYCOLIPID
T or F: when sugars are attached to a sphingolipid, the molecule becomes polar. Why/why not?
True. Sugars are polar
What are the other most common head groups aside from sugars for sphingolipids?
What is this specific lipid called?
Phosphate + Choline
Called Sphingomyelin
What is the only sphingolipid that is also a phospholipid? Why?
sphingomyelin
because it has an phosphate (and choline) head group
What are the major differences between glycerophospholipids and sphingolipids?
Glycerophospholipids have glycerol backbones
Sphingolipids have sphingosine backbones
All Gs have a phosphate group + a head group that can be a variety of things
Only one (Sphingomyelin) sphingolipid has a phosphate head group
G: have 2 fatty acids and they are attached by an ester linkage
S: have only one and it’s attached to an amine in the backbone by an amine linkage
Describe the structure of cholesterol
Four fused rings and a hydroxyl (OH) group
Is cholesterol hydrophobic, hydrophilic, or amphipathic? why?
Amphipathic because the hydroxyl (OH) group is polar and the hydrocarbons are non-polar
How does cholesterol align itself in the membrane bilayer?
It aligns its polar hydroxyl groups with the polar head groups of phospholipids in the bilayer
T or F: fluidity of a membrane is critical to membrane function
True
What 5 things dictate levels of fluidity in the membrane?
Van der Waals forces
Temperature
levels of unsaturated fatty acids
length of hydrocarbon tails
cholesterol
How do Van der Waals forces contribute to membrane fluidity?
Non-polar molecules or non polar portions of molecules (ex. hydrocarbon tails) interact with each other and attract one another
How does temperature contribute to membrane fluidity?
A decrease in temperature will cause lipids to pack closely together –> solidifying the membrane
Describe transition temperature
the temperature at which the lipid bilayer is converted from relatively fluid state (liquid crystalline) to FROZEN crystalline GEL
In what ways will membranes counteract solidification/freezing?
- Having double bonds - intro of unsaturated fatty acids to reduce packability of the membrane
- having lots of short hydrocarbon chains
How do many organisms (yeast, bacteria) keep their membranes fluid when temperatures change?
By adding or removing unsaturated fatty acids
How does cholesterol contribute to membrane fluidity?
Cholesterol acts as a FLUIDITY BUFFER by preventing any extremes in fluidity
How does cholesterol prevent membranes from gelling?
Prevents hydrocarbon chains from packing too closely together
How does cholesterol prevent membranes from melting?
Limits the movement/rotations of the phospholipid tails
What is the net result of cholesterol in the membrane?
the change between phases in the membrane is more GRADUAL
How does cholesterol impact membrane solubility?
Adding a rigid, planar cholesterol molecule REDUCES solubility of the membrane and reduces the ability of small molecules to move through it
Describe the asymmetry of a membrane
OUTER LAYERS:
glycolipids and anything with sugars
INNER LAYERS:
PS and PE on the inside to create a more negative charge
phospholipid head group will differ between the inner and outer layers
Why might some lipids be required on one side of the bilayer while others are required on the other side?
Certain lipids are involved in signalling and are needed on one side of the bilayer or the other to transfer a signal from the outside to the inside of a cell
How important is it that the correct phospholipids are on the correct side of the bilayer?
VERY
If they are on the wrong monolayer, it can trigger apoptosis
What is the glycocalyx? What is it composed of?
a layer of sugars on the outer surface of the membrane bilayer which all face the outside of the cell
Composed of glycolipids and glycoproteins
T or F: different membranes in the cell and in different cell types will have different lipid compositions?
TRUE
The lipid content of each membrane type is very specific
How do membranes self-assemble?
Phospholipids can spontaneously cluster to form bilayers because they are amphipathic
Water molecules move freely when hydrophobic portions of molecules cluster –> increasing entropy
What two structures can a phospholipid form depending on its shape?
Micelle
Bilayer
What is the shape of a phospholipid dictated by?
its lipid composition
Describe the micelle structure
A single layer of individual fatty acids that have aggregated with their fatty acyl chains on the inside to form a wheel-like shape
Individual fatty acids are wedge shaped, so when they aggregate with their tails in they form a pie/wheel
Inside is a hydrophobic centre
Describe the structure of a bilayer
Phospholipids are cylindrical (even head: tail ratio) and form a flat bilayer (TWO LAYERS)
Describe the structure of a liposome and how it forms?
When a phospholipid bilayer forms, it folds in on itself to protect its edges from water to form a liposome
Have two layers and an inside cavity that is hydrophilic
Why are liposomes so useful in scientific research?
They can be artificially created and the exact lipids and proteins of interest can be added to be studied
Why are liposomes so useful in delivering drugs?
They can act as vehicles to deliver drugs to the body
their hydrophilic polymer protects from immune cells
they can be antibodies and be targeted at specific body tissues
How do lipids move around within a layer? How do they move around BETWEEN layers?
WITHIN:
very quickly - their HC tails move around and they spin on their axis
BETWEEN:
Very rare for lipids to move from one monolayer to another because the polar head section would have to move through a hydrophobic core to get to the other layer –> unlikely
Describe flip-flopping
When a lipid moves from one monolayer to another
In what circumstance would it be required for a lipid to move from one monolayer to another?
Ex. when they are first synthesized in the ER
ex. maintains membrane symmetry
How is flip flopping facilitated?
A group of membrane proteins called PHOSPHOLIPID TRANSLOCATORS or FLIPPASES catalyze the rapid flip flop of the phospholipids from one monolayer to another
Describe lipid rafts
Occurs when lipids (with the same function) and proteins (with the same signalling pathway) cluster and float around the membrane in a group –> creates a thicker bit of membrane because of the tightly packed and long saturated HC tails
this contributes to asymmetry because it means the lipids and proteins are not evenly distributed throughout the membrane
What structures are common in lipid rafts?
- cholesterol
- sphingolipids
- GPI anchored proteins
- longer hydrocarbons
- saturated hydrocarbon tails
T or F: Rafts are less ordered than the rest of the membrane
FALSE
Rafts tend to be more ordered than the rest of the membrane
T or F: rafts are thicker than the rest of the membrane
True because they contain long hydrocarbon chains and saturated HC tails so they tightly pack together
