Unit 1: Membranes Flashcards
Lipids are (hydrophilic/phobic) and are composed mostly of…
What are the three types of biological lipids?
Lipids are hydrophobic and are composed mostly of hydrocarbons (C-C and C-H bonds). Due to these bonds, they are non-polar and hence have no partial charges, meaning that they do not interact with water and water’s hydrogen bonding potential.
The 3 biological lipids are:
1) Triglycerides - combination of fatty acids
2) Phospholipids
3) Sterols
What are fatty acids made of? How can you determine them from a picture? How many carbon atoms are usually in the chain?
Can they have double bonds? How does this affect structure (generally)
Fatty acids are hydrocarbons (C-C and C-H bonds) with a carboxyl group at one end (COOH or OH-C=O). If you see a long hydrocarbon chain with a carboxyl group at the end of it— and possibly double bonds within the chain — it is a fatty acid. The carboxyl group is what makes it an acid - it can gain or lose H’s. Usually there are around 14-22 carbon atoms in the chain.
They CAN have double bonds in their chain. In this case, they do not have the maximum number of C-H bonds they possibly can, and therefore the chain is UNSATURATED — not completely full. This causes a kink in the structure most of the time, although sometimes it is even more stable and linear.
What are triglycerides? What are they used for in our bodies?
Triglycerides: 3 fatty acid “tails” bound to a glycerol (C3H5) anchor. This is a highly compact fatty acid which allows us to store excess energy in our bodies, and which can be accessed when needed. The O which is normally attached to a hydrogen on the carboxyl group is instead attached to one of three carbons in the glyceride anchor — each of these three carbons has 2 hydrogens attached (or one for the middle one since it has an extra C-C bond).
What are phospholipids? What is their structure and what is this structure called? What does each group contribute to the phospholipid?
Phospholipids are an organic molecule, made of:
1) A phosphate group. This group is PO4, and will have a charged oxygen (since it only has one bond). This leaves it polar and hence hydrophilic, so it can react with water. There is also an R group attached to the top oxygen, which results in various properties on the outside of the phospholipid (and hence proteins that can attach to the outside of the membrane).
2) Glycerol. This group is what attaches to the two fatty acid tails and one phosphate group, acting like the barrier between hydrophilic and hydrophobic. However it will be hydrophobic itself as it is only made of non-polar hydrocarbons. (C3H5)
3) 2 fatty acid tails. These groups are long chains of hydrocarbons attached to the glycerol molecule through the carboxyl groups on their ends. This bonding however takes away any charges leaving the tails hydrophobic and not able to react with or interact with water. These tails are what prevent the majority of things from entering the cell, and act as the main barrier.
Phospholipids are AMPHIPATHIC — meaning both hydrophobic and hydrophilic.
What are the three types of layers that phospholipids form?
What property allowed these to spontaneously form?
1) Micelle: This layer is a monolayer of phospholipids wrapped up in a sphere. So the tails all interact to produce this sphere, with the head on the outside to interact with water.
2) Liposome: This is a hollow sphere made of a bilayer lipid body. So the heads form an outer shell and inner circle, with the tails interacting between the two. This is NOT a cell though because you need to add proteins and other molecules on the inside.
3) Phospholipid bilayer: This is a flat line of 2 layers of phospholipids that interact with tails together and heads on the outside. This will only form if it has something to attach to on either side.
The amphipathic nature of these phospholipids is what will allow the layers to form, because non-polar tails are attached and repelled from outside/ water, which the heads are attached to. Therefore it puts them at the lowest potential energy/ most natural state possible.
What is the main function of biological membranes?
Membranes are ________________ _______________ barriers.
Membranes are where __________ and ___________ __________ take place. Proteins are used on the outside to allow this to occur.
Biological membranes compartmentalize the cell by separating outer and inner reactions. This ensures that reactions do not harm other parts of the body. As well, in eukaryotic cells there are many more specialized organelles within the cell that are separated from the cytoplasm by these membranes as well. Again, this allows specialized reactions that need certain conditions to occur without effecting other different specialized reactions.
Membranes are selectively permeable barriers — based on the properties of those membranes and the various channels and proteins present. Membranes are scaffolds for communication and chemical reactions, because the various R-groups attached to the heads of phospholipids, or proteins embedded within the phospholipids allows for messages to be received and pathways to be regulated.
The fluid mosaic model was developed because…
Mosaic:
Membrane proteins can be ____________ or _____________. Sugars are also attached to these proteins and lipids are called __________ and ____________ respectfully. What is the sugar used for?
Fluid: (what experiment proved this?)
Mosaic: The membrane is a mosaic of lipids and proteins which coexist in the membrane. Essentially, these proteins are floating in a sea of phospholipids.
The proteins on the membrane can be PERIPHERAL — float around on one side of the other, but are attached to the outside, usually for communication and upholding the cytoskeleton of the cell — or INTEGRAL — are embedded within the membrane and can be seen from either side.
Sugar attached to the proteins are called glycoproteins and attached to the lipids is called glycolipds. These sugars are used for communication and receiving signals between cells.
Fluid: The lipid bilayer is fluid, or in other words it is just held together by attractions between tails, attraction between heads and water, and repulsion of tails and water. However, those phospholipids and proteins are free to move within the membrane — they are not bonded or fixed to one specific spot. So each lipid is free to move along the plane of the membrane.
This was proved when a human and mouse cell hybrid was made, so that membrane proteins were segregated (half and half). However after some time those membrane proteins completely intermixed, making a random distribution of the proteins, due to the membrane’s fluid nature!
What factors affect the fluidity of the membrane? (4)
1) Temperature increases fluidity because it increases molecular motion. So, the phospholipids will have enough energy to spread out more at higher temperatures, increasing fluidity and the leakage of the membrane. The more fluid it is the more gaps there are, and so on hot summer days or when one has a fever, electrolytes may be lost, effecting many electrochemical gradients, and hence cellular processes. At lower temps, that molecular motion will decrease and the lipids will be forced to pack closer together, creating a more viscous membrane — which can also be quite dangerous.
2) The structure of the fatty acid tails will also affect this fluidity. If there are any cis-unsaturated fatty acid tails (carbons are on the same side) then there will be a bend in the membrane, forcing the lipids to spread out more and hence be more fluid. This is why saturated fats are bad, because they are so tightly packed and humans do not have the mechanisms to break those tight bonds down.
If it is hot, the number of unsaturated tails will go down. But if it is cold, the cell will increase the number of tails, in order to maintain a fluid enough membrane state.
3) The length of the hydrocarbon tails will also have an effect. For these non-polar bonds, or any bonds in general, there will always be London-dispersion or Vander Waals forces. These forces are due to the random movement of electrons within their orbitals, and due to this at any given moment there could be more electrons in one place than another. When this occurs, it will cause an instantaneous polar pole, which will interact with the next molecule and force electrons away from that end, creating a polar pole on the neighbouring molecule. As well, the larger the atom, the further away those electrons are from the nucleus and so the more easily they can be distorted by these forces. This goes for non-polar bonds in the hydrocarbons as well, they are very unstable in their intermediate place between the two atoms and so these vander waals forces will have a big effect on them.
So overall, the larger the atom, more non-polar the bonds, and the longer the chains (more electrons present) the more vander waals forces that will be present. This allows the neighbouring hydrocarbon tails to interact, and hence the phospholipids to be more highly attracted to each other.
Therefore, smaller hydrocarbon chains will lead to less interactions and a more fluid membrane, and longer chains = more interactions and a less fluid membrane. Therefore, the length of these chains can also be edited by the cell to adapt to environmental conditions.
4) Sterols are another factor which can be used to regulate membrane fluidity. Specifically in animal cells, cholesterol is inserted into the bilayer and can both prevent excess viscosity and fluidity.
Viscosity: Stops phospholipids from getting to close together by getting between them to break them up. This allows for more space for those lipids to move around. By getting between the lipids, it breaks up those vander waals interactions that are so much stronger at lower temperatures and closer distance, and so the lipids can flow away from each other more easily.
Fluidity: Attracts the tails of the phospholipids together and fills in the gaps in order to bring them closer and decrease the fluidity of the membrane. By bringing them together this can increases forces of attraction between tails as well to make them more viscous.
Why is it bad when membranes are too fluid or too viscous?
When do we want equilibrium and when do we not?
Too fluid: The membranes are “leaky” and hence allow things to cross the membrane that shouldn’t be able to.
Too viscous: The membranes are strong “barriers” and prevent solutes that can normally cross from passing through the membrane.
In this case, we DO want equilibrium. We never want equilibrium of a chemical reaction or process, but we do want to be at a state of equilibrium.
How do the properties of lipid bilayers lead to selective permeability?
Rank the four different classes of molecules based on their abilities to get through the membrane. Give examples of each type of molecule…
Lipid bilayers have hydrophobic heads and hydrophilic tails. the heads are like the doors, and so hydrophobic molecules can get through them.
But the tails are what really create the barrier. These tails prevent any hydrophilic molecules from getting through the barrier, so that only essential molecules that are needed extremely quickly (to breathe) can get through.
Move through most —> least easily
1) Non-polar molecules (no matter the size) are able to get through the membrane EASILY as they interact with and dissolve in the non-polar tails.
Ex) O2, CO2, N2, etc. These are needed for breathing and so it makes sense that they are easily transferred through.
2) Small, uncharged polar molecules move through the membrane SLOWLY, but most of the time are able to make it through.
Ex) H2O, glycerol, and indole (a therapeutic agent in treatment of cancer, infections, migraines, and hypertension).
3) Large, uncharged polar molecules have a VERY HARD time getting through the membrane as they would move EXTREMELY SLOWLY. This is due to the large number of London dispersion forces present and the fact that such a large number of atoms will have a hard time getting through the repulsion of the tails.
Ex) Glucose, sucrose (more than 3 carbons)
4) Ions — any molecule with a full charge — will NEVER be able to cross the membrane. Even if the ions are insanely small, the lipid tails will repel them 100% of the time. Plus, they are too strongly attracted to water that they will never move away from those attractions to the tails which are repelling them.
Ex) Cl-, H+, K+, Na+
Since water moves slowly across the membrane, how does it cross the membrane in the large amounts that is needed for homeostasis? What are these channels called and what is their structure?
Water needs channel proteins in order to diffuse across the membrane, because it is needed in far too large amounts to rely on the slow diffusion across the hydrophobic tails.
So, channels called AQUAPORINS are used. These channels are made of a hydrophillic inner cylinder (made of polar proteins) and a hydrophobic outer cylinder (to interact with the hydrophobic membrane).
What is diffusion? Does it always work across membranes?
A conc. Gradient = a _______________ _______________ state.
Diffusion is the tendency of molecules to move from an area of high concentration to an area of low concentration (in other words: down their concentration gradient). Eventually, equilibrium will be reached, and the cell will be at a lower potential energy state — more stable.
A conc. Gradient = a high energy state, because molecules are separated from where they want to go naturally. This is why diffusion will naturally occur (it’s like an exergonic reaction).
But diffusion will only work across membranes if the solute is free to pass across that bilayer. If not, it will diffuse amongst the area it is free to move in.
Now when there is a concentration gradient in a cell, but the solute cannot cross the membrane, then how does it even out? (Not facilitated yet)
In this case osmosis — or the diffusion of water — will occur. The water will move from the less concentrated to more concentrated side, to dilute that side and balance out the concentrations. So there should be a larger volume water and larger amount solute on one side, but concentrations on either side should be equal!
What is tonicity? What does it affect?
Hypertonic…
Hypotonic…
Isotonic…
When a cell is bursting, this means….
When a cell is shrinking, this means…
What condition is the cell normally relative to the surroundings?
Tonicity is the relative concentration difference across a lipid bilayer, and this affects the diffusion/osmosis across that membrane.
Hypertonic is when the concentration in one place is LARGER than the thing it’s being compared to.
Hypotonic is when the concentration in one place is SMALLER than the thing it’s being compared to.
Isotonic is when the concentrations of the two things are EQUAL!
When a cell is bursting, this means that the cell is hypertonic relative to the solution. So the concentration in the cell is LARGER than outside the cell, and therefore water wants to move INTO the cell to try to even this out. However, the solution is hypotonic relative to the cell.
When a cell is shrinking, this means that the cell is hypotonic relative to the solution, so the concentration inside is less than on the outside. Therefore, water wants to move out of the cell to dilute the surroundings. However, the solution is hypertonic relative to the cell.
SET WHATEVER IT IS RELATIVE TO AS ZERO, AND THEN COMPARED TO THAT SEE IF THE CONCENTRATION IS POSITIVE OR NEGATIVE. ALSO THINK ABOUT WHERE THE WATER HAS TO MOVE TO MAKE THE NON-ZERO CONCENTRATION EQUAL TO ZERO. (Away from it or towards it?)
Relative to the surroundings, the cell is hypertonic and therefore it has to work very hard to keep water out of the cell. So drinking salt water can kill you because it will cause those cells to burst.
What are the two types of passive transport?
Simple diffusion and Facilitated diffusion.
