Biological Membranes Flashcards
What is the main component of cell membranes?
Lipids
What is often embedded in or associated with the cell membrane?
Proteins
What are carbohydrates called when they attach to lipids and proteins in the cell membrane?
Glycolipids and glycoproteins
Why have red blood cells been a particularly useful model for studying membrane structures?
They don’t contain nuclei or internal membranes
What is the major type of lipid found in cell membranes?
Phospholipids
What does it mean for phospholipids to be amphipathic?
They have both hydrophilic and hydrophobic regions
What three things make up a phospholipid?
A glycerol backbone attached to a phosphate group and two fatty acids
Which region of a phospholipid is hydrophobic?
The fatty acid tails
Is the hydrophobic region of a phospholipid polar or non-polar?
Non-polar
Which region of a phospholipid is hydrophilic?
The phosphate head group
Is the hydrophilic region of a phospholipid polar or non-polar?
Polar
When phospholipids spontaneously arrange themselves, which region faces out and which region faces inwards?
The polar head groups face outwards and the non-polar tail groups come together inside away from the water
What determines the shape of the phospholipid structure?
The bulkiness of the head group relative to the tails
What is a micelle?
A spherical structure in which lipids can arrange themselves
What is the structure of phospholipids that form micelles?
Those with bulky heads and a single hydrophobic tail
How do phospholipids that are roughly rectangular with less bulky head groups and two hydrophobic tails arrange themselves?
A bilayer
Where does phospholipid synthesis occur?
The interface of the cytosol and outer endoplasmic reticulum membrane
Two of which molecule always starts phospholipid synthesis?
Two fatty acids
Where are the two fatty acids in a phospholipid initially derived from?
Carbohydrates via the glycolytic pathway
Where are fatty acids activated during phospholipid synthesis?
In the cytosol
During phospholipid synthesis, how are fatty acids activated?
By the attachment of a CoA molecule
What causes activated fatty acids to bind to glycerol-phosphate and inserted themselves into the cytosolic leaflet of the ER membrane?
Acyl transferase
During phospholipid synthesis, what does the phosphatase enzyme do?
Removes the phosphate from a protein
During phospholipid synthesis, what is attached via choline phosphotransferase?
A choline already linked to a phosphate
What is the difference between flippases and floppases?
- Flippases: transfer phospholipids from the outer leaflet to the inside
- Floppases: transfer phospholipids from the inner leaflet to the outside
After being synthesized, phospholipids destined for the cytoplasmic cellular membrane will be on which leaflet of a vesicle?
The exterior leaflet
After being synthesized, phospholipids destined for the exoplasmic cellular membrane will be on which leaflet of a vesicle?
The interior leaflet
T or F: The lipid bilayer is self-healing.
True
How are small tears in the bilayer sealed?
Spontaneous rearrangement of the lipids surrounding the damaged region
T or F: The bilayer forms spontaneously.
True
What is the structure of the bilayer dependent on?
Properties of the phospholipid
T or F: The formation of the bilayer relies on the action of enzymes.
False
Why is the pH of a solution important for phospholipids to arrange in a bilayer?
The pH ensures the head groups are in their ionized (charged) form and are suitably hydrophilic
What structures do phospholipids form when they are in environments with a neutral pH like water?
Liposomes
Can liposomes form, break, and re-form?
Yes
How can liposomes grow?
By incorporating lipids from the environment and capturing molecules in their interiors
What is proposed about the creation and evolution of membranes?
Membranes formed originally by straightforward physical processes, then their composition and function evolved overtime
T or F: Early membranes may have been either leaky or impermeable to molecules.
True
What does it mean to say that a cell membrane is dynamic?
They are continually moving, forming, and re-forming during the lifetime of a cell
Why are lipids able to freely associate with each other?
Because of the extensive van der waals forces between their fatty acid tails
Why is the cell membrane said to be fluid?
Because membrane lipids are able to move in the plane of the membrane
What two factors influence membrane fluidity?
- Lipid composition
- Temperature
In a single layer of the lipid bilayer, what does the strength of van der waals interactions depend on?
The length of the fatty acid tails and the presence of double bonds between neighbouring carbon atoms
How do longer fatty acid tails affect membrane fluidity?
They result in tighter packing of lipids and reduced mobility
In terms of temperature, what changes the membrane into a liquid crystal state?
An increase in temperature
In terms of temperature, what changes the membrane into a crystalline gel state?
A decrease in temperature
In response to changes in temperature, how is the lipid composition of membranes affected?
By desaturation of lipids and change of lipid chains
What is the difference between saturated and unsaturated fatty acids?
- Saturated: Fatty acid tails don’t have double bonds
- Unsaturated: Fatty acid tails have double bonds, causing kinks
Do saturated or unsaturated fatty acid tails reduce lipid mobility in the membrane?
Saturated
Do saturated or unsaturated fatty acid tails enhance lipid mobility in the membrane?
Unsaturated
Is cholesterol amphipathic?
Yes
Which region on a cholesterol molecule is hydrophilic?
The hydroxyl group (-OH)
What is the hydrophobic region of cholesterol composed of?
Four carbon rings with an attached hydrocarbon chain
Which region of cholesterol interacts with the head groups of phospholipids in the bilayer?
The hydroxyl head group
Which region of cholesterol participates in van der waals interactions with the fatty acid tails of the phospholipids in the bilayer?
The carbon ring structure
What component of animal cell membranes acts as a bidirectional regulator of membrane fluidity?
Cholesterol
How does cholesterol affect membrane fluidity at high temperatures?
It decreases fluidity
How does cholesterol affect membrane fluidity at low temperatures?
It increases fluidity
If cholesterol is added to a liquid crystal membrane, what will happen to the membrane’s fluidity?
Membrane fluidity will decrease
If cholesterol is added to a crystalline gel membrane, what will happen to the membrane’s fluidity?
Membrane fluidity will increase
What are lipid rafts?
Lipids that have assembles in a defined patch in the cell membrane
What tends to accumulate around lipid rafts?
Cholesterol and other membrane components like proteins
What is lipid flip-flop?
The spontaneous transfer of a lipid between layers of the bilayer
Why is it not surprising that lipid flip-flop is rare?
Because flip-flop requires the hydrophilic head group to pass through the hydrophobic interior of the membrane
Why does a lack of lipid flip-flop allow the two membrane layers to differ in composition?
Because there is little exchange of components between layers
What are the three classes of membrane proteins?
- Integral membrane proteins
- Peripheral membrane proteins
- Lipid-anchored proteins
What are peripheral membrane proteins?
A protein that is temporarily associated with the lipid bilayer or with integral membrane proteins through weak non-covalent bonds
T or F: Peripheral membrane proteins can only be associated with the external side of the membrane.
False. They can be associated with either the internal or external side of the membrane.
Which type of membrane protein is transiently associated with the membrane and can play a role in transmitting information received from external signals?
Peripheral membrane proteins
What is an integral membrane protein?
A protein that is permanently associated with the cell membrane
What are three different examples of functions played by different integral membrane proteins?
- Transport of nutrients and ions
- Cell-to-cell communication
- Attachment
What are transmembrane proteins?
Proteins that span the entire lipid bilayer
T or F: Most integral proteins are transmembrane proteins.
True
What are the three regions composing transmembrane proteins?
- Two hydrophilic regions (one protruding from each face of the membrane)
- A connecting hydrophobic region that spans the membrane
What is the most common protein structure element crossing biological membranes?
The alpha-helix
What is a transmembrane protein domain?
A peptide sequence that is largely hydrophobic (uncharged) spans across the plasma membrane
What does the transmembrane protein domain do?
This sequence permanently attaches the protein to the plasma membrane
What are tetraspanins (TM4SFs)?
A family of membrane proteins found in all multicellular eukaryotes
What are tetraspanins composed of?
Four transmembrane alpha-helices and two extracellular domains
Which extracellular domain of a tetraspanin is shorter and which one is longer?
- EC1 is shorter
- EC2 is longer
On which extracellular domain/loop can tetraspanins be glycosylated?
The long extracellular domain (EC2)
What does it mean for a tetraspanin extracellular domain to be glycosylated?
There is a carbohydrate molecule attached to it
What do tertraspanins play a role in?
Cell adhesion, motility, proliferation, and more
What are transporters?
A transmembrane protein that moves ions or other molecules across the cell membrane
What are transmembrane receptors?
A transmembrane protein that allows the cell to receive signals from the environment
What are transmembrane enzymes?
A transmembrane protein that functions as a catalyst to accelerate the rate of a chemical reaction
What are transmembrane anchors?
A transmembrane protein that attaches to other proteins and helps maintain cell structure and shape
What are lipid-anchored proteins?
Proteins that attach to a lipid in the bilayer
Are biological membranes symmetrical or asymmetrical?
Asymmetrical
Which leaflet of biological membranes often contains glycolipids and glycoproteins?
The outer leaflet
What did the Frye-Edidin experiment prove?
That the membrane is dynamic
What was the procedure of the Frye-Edidin experiment and what observations were made?
- The surface proteins of mouse and human cells were dyed different colours
- The cells were fused together
- The surface proteins of the mouse and human cell diffused around the unified membrane
What is fluorescence recovery after photobleaching?
A technique used to measure the mobility of molecules in the plane of the membrane
During fluorescence recovery after photobleaching, what is labelling?
A process in which a fluorescent dye is attached to proteins embedded in the cell membrane
What does the fluorescence recovery after photobleaching procedure involve?
- Membrane proteins are labelled
- A laser is used to bleach a small area of the membrane, leaving a non-fluorescent spot on the cell surface
What was the hypothesis of the fluorescence recovery after photobleaching procedure?
If membrane components move in the plane of the membrane, the bleached spot should become fluorescent overtime as unbleached fluorescent molecules move into the bleached area
What observations and conclusions were made using fluorescence recovery after photobleaching?
- Observations: Overtime, fluorescence appeared in the bleached area
- Conclusions: Proteins move in the plane of the membrane
What is the fluid mosaic model?
The idea that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of lipids and proteins
What is the plasma membrane?
The membrane that surrounds the cytoplasm of the cell
The plasma membrane is what kind of structure?
A trilaminar structure made up of a phospholipid bilayer
What is the most studied cell membrane?
The plasma membrane
What is homeostasis?
The active regulation and maintenance of a constant environment within cells
Why is the plasma membrane able to maintain homeostasis?
Because it is selectively permeable
What does it mean for the plasma membrane to be selectively permeable?
It lets some molecules in and out freely, some in and out only under certain conditions, and prevents others from passing through at all
What allows the plasma membrane to be selectively permeable?
The lipids and embedded proteins of which it is composed
What types of molecules are able to move unassisted across the plasma membrane?
Gases (ex. oxygen and CO2) and non-polar molecules (ex. lipids)
What types of molecules require assistance to move across the plasma membrane?
Ions and charged polar molecules
What is the simplest form of movement into and out of cells?
Passive transport
Which type of transport works by diffusion?
Passive transport
What is diffusion?
The random motion of molecules, with net movement occurring from areas of higher to lower concentration
In terms of diffusion, what happens when there is no longer a concentration gradient?
Net movement stops, but movement of molecules in both directions continues
What is facilitated diffusion?
Diffusion of a molecule through a membrane protein, bypassing the lipid bilayer
What are the two membrane transporters that can be used for facilitated diffusion?
- Channels
- Carriers
What is the difference between channel and carrier proteins?
- Channels: Only transport ions and molecules down the concentration gradient
- Carriers: Transport substances both down and against the concentration gradient
What is a protein channel?
A transporter with a passage that allows the movement of molecules through it, depending on their shape and charge
What does it mean for a channel to be gated?
They open in response to some sort of chemical or electrical signal
What are voltage-gated channels?
Channels that respond to changes in charge across the membrane
What are ligand-gated channels?
Channels that respond to binding of a specific molecule on its surface
What the molecule called that binds to and activates ligand-gated channels?
A ligand
T or F: Any ligand can bind to a ligand-gated channel and cause the proper conformational change.
False. Only a ligand adapted to the binding site can produce an effect (lock-and-key model)
Under what conditions can molecules flow through an open channel?
Molecules can only flow through channels when a concentration gradient is present
What is tetrodotoxin?
A very potent neurotoxin
What type of ion channels do tetrodotoxin target?
Binds to voltage-gated sodium channels
What is curare?
A mixture of organic compounds found in plants
What does it mean for curare to be a competitive antagonist?
It binds to the same site on the receptor with an equal or greater affinity to the actual ligand, but elicits no response
What are the three types of protein carriers?
- Uniporter
- Symporter
- Antiporter
What is a uniporter?
A type of carrier that passively transports molecules across the cell membrane
What is a symporter?
A carrier protein that uses the chemical gradient of one molecule to transport a second molecule in the same direction
What is an antiporter?
A carrier protein that uses the chemical gradient of one molecule to transport a second molecule in the opposite direction
What is osmosis?
The net movement of a solvent (ex. water) across a selectively permeable membrane from regions of high solute concentration to low solute concentration
What are aquaporins?
A protein channel that allows water to cross the plasma membrane
What is osmotic pressure?
The pressure needed to prevent water from moving into a solution by osmosis
What is active transport?
The movement of substances against the concentration gradient that requires the input of energy
What is the role of proteins in active transport?
They act as pumps, using energy directly to move a substance into or out of a cell
Within cells, how do sodium and potassium concentrations differ?
- Sodium concentration is much lower that the external environment
- Potassium concentration is much higher than the external environment
What is the sodium-potassium pump?
A transmembrane protein that actively moves sodium out of the cell and potassium into the cell
How many sodium and potassium ions can be transported via the sodium-potassium pump with one ATP molecule?
1 ATP allows 3 sodium ions out and 2 potassium ions in
What is a hypertonic solution?
A solution with a higher solute concentration than the cell surrounded by it
What would happen to a human red blood cell if placed in a hypertonic solution?
Water leaves the cell by osmosis and it shrinks
What is a hypotonic solution?
A solution with a lower solute concentration than the cell surrounded by it
What would happen to a human red blood cell if placed in a hypotonic solution?
Water moves into it the cell by osmosis and it lyses/bursts
What are contractile vacuoles?
A type of cellular compartment that takes up excess water and waste products from inside the cell and expels them into the external environment
What is an example of a single-celled organism that exists in hypotonic environments
Paramecium
How do single-celled organisms existing in hypotonic solutions avoid bursting from water intake by osmosis?
Contractile vesicles take up and expel excess water from inside the cell
What is the cell wall?
The structural layer external to the plasma membrane
What is turgor pressure?
The force exerted by water pressing against the cell wall
How does turgor pressure build?
Turgor pressure builds as a result of water moving into the cell by osmosis and the tendency of the cell wall to resist deformation
What is the membrane bound organelle that can absorb water and contribute to turgor pressure?
Vacuoles
Why do plants wilt when dehydrated?
The loss of water in vacuoles reduces turgor pressure and the cell can no longer maintain its shape
What type of cell wall is composed of polysaccharides, including cellulose?
Plant cell walls
What is the most abundant biological material in nature?
Cellulose
What type of cell wall is composed of cellulose, silicon, or calcium carbonate?
Algae cell walls
What type of cell wall is composed of chitin?
Fungi cell walls
What type of cell wall is composed primarily of peptidoglycan?
Bacteria cell walls
How do membrane proteins play a major role in signal transduction?
Convert extracellular signals into intracellular signals
What are the three stages of signal transduction?
- Binding of ligand to receptor
- Signal transduction via secondary messengers
- Cellular response
Does the ligand fully enter the cell during signal transduction?
No
What causes signal transduction via secondary messengers?
Caused by cytosolic side of receptor protein being affected by ligand induced conformational change
What is gycogenolysis?
The process in which epinephrine/adrenaline activates the conversion of glycogen stored in the liver to glucose
What is glycogen?
A molecule with a glycogenin protein at its core surrounded by branches of glucose
What is glycogenin?
An enzyme that acts as a primer to polymerize the first glucose molecules before other enzymes take over
What is the signal that starts glygogenolysis?
Epinephrine
Where does epinephrine bind to during gygogenolysis?
The adrenergic receptor on the liver cell
What activates the adrenergic receptor on the liver cell during gygogenolysis?
The binding of epinephrine
During the process of gygogenolysis, what does the active receptor do?
Recruits G-protein and allows the binding of GTP to turn that G-protein on
During gygogenolysis, what occurs after the G-protein is turned on?
One of the G-protein subunits will dissociate and turn on adenyl cyclase
What is the role of adenyl cyclase during gygogenolysis?
Turns ATP into cyclic AMP (cAMP)
What is the secondary messenger during gygogenolysis?
Cyclic AMP (cAMP)
During gygogenolysis, what causes the cellular response?
The accumulation of cAMP
What does the accumulation of cAMP lead to as the final step of gygogenolysis?
Causes a molecular cascade that eventually results in the release of a glucose unit from the glycogen
