Lecture 7: Membranes Flashcards
Why are proteins amphiphilic?
They have stretches of hydrohpobic and hydrophilic amino acids
What part of alpha-helix proteins that integrate into the membrane protein interact with the membrane?
The side chains of the alpha helix protein stick out of the sides and interact with the plasma membrane
What side chains interact with the fatty acid chains of the phospholipid bilayer?
Non-polar(hydrophobic) side chains
Where are the polar(hydrophilic) amino acids found in the plasma membrane?
Near the polar phosphate head groups of the membrane
How do proteins move within the lipid bilayer?
Laterally
Why can’t proteins flip flop within the membrane?
There are specific proteins that interact with things on the outside of the bilayer and other proteins that only interact with things within the bilayer
Integral membrane proteins
Proteins that cross the lipid bilayer(found on both the outside and inside of the bilayer)
Peripheral membrane proteins
Bound to integral membrane proteins or found on the outsides of the membrane alone
Oligosaccharides in the membrane
Chains attached to proteins and help give them additional characteristics
-Retain water and help single celled organism stay in the right place
Why is the overall membrane structure called the fluid mosaic?
Fluid: All components within the membrane can move around laterally
Mosaic: A variety of different components(proteins, lipids) are inserted into the membrane, to carry out a variety of functions
What did the freeze fracture microscopy experiment prove?
Proved that the lipid bilayer contains many proteins
How did the freeze fracture microscopy experiment work?
- Cell were flash freezed
- Then the cell in the block of ice is fractured, this causes the lipid bilayer to fracture into two leaflets
- Electron microscopy is then done on the leaflet samples, this displays many black dots that indicate a membrane protein
Two main functions of the membrane and what they do?
- Acts as a barrier: Prevents toxins, chemicals and unwanted things from entering the cell
- Transportation: Allows good molecules(sugars) to enter the cell
What type of permeability do membranes have and what does it mean?
Membranes have selective permeability, meaning that they only allow what they want in and out
What is a highly permeable molecule and what are some examples?
Highly permeable molecules can cross the lipid membrane very easily
ex.
-Small non polar molecules(gases)
- Small uncharged polar molecules
What is a low permeable molecule and what are some examples?
A low permeable molecule is one that cannot cross through the lipid bilayer by itself
ex.
- Large uncharges polar molecules
-ions
Why is it so hard for ions to cross the lipid bilayer?
They are large and have a hydration shell surrounding them
What molecules are transported via passive transportation?
Highly permeable molecules
What is passive transport?
Type of membrane transport that does not require energy to transport molecules across the membrane
What are the three types of passive transport?
- Simple diffusion
- Facilitated diffusion
- Osmosis
What is simple diffusion?
The passive mixing of substances resulting in net transport along a concentration gradient.
What factors determines the rate of simple diffusion?
- Temperature
- Steepness of the concentration gradient
- Size of the molecule
What is steepness of concentration gradient?
The greater the difference in concentration of the molecules, the steeper the concentration gradient and faster molecules diffuse.
-Steep concentration gradient has a very negative delta G value that helps drive the diffusion
What is net transport?
How many molecules have diffused across the membrane
How does temperature affect the rate of diffusion?
The higher the temperature the higher the energy of the molecules and therefore the faster the rate of diffusion
How does size affect the rate of diffusion?
Smaller the molecule the faster it diffuses
What is random walk?
Random Walk: Distance is proportional to the square root of time (in the 1 hour you would only cover 60m)
How is diffusion based on random walk?
All individual molecules are constantly in motion as long as the temperature is above 0K and that is due to their thermal motion
- Molecules are based on random walks in diffusion and thus cannot go very far which makes diffusion very slow/ineffective
What is “directed walk”?
Distance is proportional to time (if you walk 1m per second, after 1hr you will have walked 3600m(very far))
Why is osmosis not the same as diffusion?
Osmosis is the transport of a solvent across a membrane, whereas diffusion is the transport of solutes across a membrane.
What is osmosis?
The diffusion of water across a membrane
Concentration gradient of water?
Water diffuses to the side that has a higher concentration of solute
What is a hypertonic solution?
Higher solute concentration outside of the cell membrane
- Water flows out of the cell
-Cell shrinks
What is facilitated diffusion?
Diffusion of bigger molecules across the cell membrane with the help of channel proteins or carrier proteins and with their concentration gradient
What is a hypotonic solution?
Higher solute concentration inside of the cell membrane
-Water flows into the cell
-Cell swells
How do electrochemical gradients use channel proteins?
Electrochemical gradient cause more charge on one side of the membrane, leading to the ions wanting to cross the membrane. Channel protein ns will open up for a short amount of time and allow ions to cross the membrane
What are channel proteins, how do they work and what diffuses through them?
- Proteins that aid in facilitated diffusion
- They are gated and open when they receive a signal and allow molecules to cross
- They allow ions to cross the membrane
What are carrier proteins and what diffuses through them?
- Molecule that wants to cross the membrane binds to the carrier protein, the protein then undergoes a conformational change that allows the molecule to cross the membrane and be released on the other side.
-They allow sugars and amino acids to cross
When are transport proteins considered as saturated?
When all of the biding sites of the carrier proteins are full or when the channel of the channel proteins is full
What limits the rate of diffusion of facilitated diffusion?
When the proteins are saturated
Why is diffusion linear?
It can never be saturated
What is active transport?
-Transportation that requires energy
-It is the transportation of molecules against the concentration gradient
What energy is typically used in active transport?
ATP hydrolysis
What is primary active transport?
-Directly relies on ATP hydrolysis to transport molecules across the membrane and against their concentration gradient
What is an example of primary active transport?
Sodium potassium ATPase is an enzyme that helps pump 3Na+ ions out and 2K+ ions in and it hydrolyzes 1 ATP as it does so.
Why is the sodium potassium pump so important? (3 reasons)
-Helps maintain osmotic equilibrium( helps counteract the drinking of a lot of water(enzyme would become more active if you drank more water)
-Generates a membrane potential in cells, there will be an overall positive charge on the outside which is important for nerve cells and other cells
-Sets up a concentration gradient that allows other transport processes to occur (these concentration gradients can be used for secondary active transport)
What is secondary active transport?
Uses the energy from the concentration gradient set up by primary active transport
- Uses gradients established by the sodium potassium pump
Why does freeze fracture split the bilayer into two leaflets?
Freezing tightly binds phospholipids to surrounding water molecules by hydrogen bonds, and the two lipid leaflets are only held together with weakvan Der Waals forces
Random walk is always found across membranes but diffusion is only when there is a concentration gradient
Random walk occurs constantly due to molecules wandering around but diffusion only occurs when there is a difference in concentration
When does delta G = 0 for diffusion?
Once the concentration gradient is gone
What is an isotonic solution?
Same concentration of solute inside and outside of the cell
True or False: integral membrane proteins are channel proteins and carrier proteins?
True
Does saturation occur earlier in channel proteins or carrier proteins?
Carrier Proteins
What is an example of secondary active transport?
The sugar sodium cotransporter, uses energy from the sodium inflow to transport sugar into cells with a higher interior concetration of sugar
