Cell Bio Chapter 11: Membrane Structure

How do lipids move from one side of plasma membrane to another?

There are certain enzymes that help them move from one side to another. -Scramblases -Flippases -Floppases

Not all transmembrane segments are entirely hydrophobic. How can this happen?

There are some hydrophilic molecules that the cell needs to get across the bilayer. The cell has these channels that have the inner parts all hydrophilic. This allows for hydrophilic molecules to go through the hydrophobic region of the cell and get to the other side of the bilayer completely surrounded by a hydrophilic enviornment. In order to do this, there are about 4 or 5 alpha helices that are arranged in a circle. Each alpha helix has hydrophobic molecules on the outer sides and hydrophilic molecules in the middle of the circle called the polar core. ex: channels *in picture, green are hydrophobic regions and red is the hydrophobic polar core

Composition of membranes

-all membranes have lipids and proteins -lipid and protein assemblies are held together in a thin sheet by noncovalent bonds -proteins in the lipid bilayer are also held by noncovalent bonds -lipids are going to be in charge of the barrier/ selective permeability and the the structural framework that the proteins are embedded in -proteins carry out most of the specific functions -carbohydrates (sugars) are attached to the membrane as well

Detergents

-amphipathic (polar & nonpolar ends) -can substitute for phospholipids and make hydrophobic proteins soluble in water ex: SDS (sodium dodecyl sulfate), triton X-100

Sphingosine

-an amino alcohol with long hydrocarbon chain -has hydrophobic hydrocarbon tails, an OH group (hydrophilic), and amino group (hydrophilic)

Glycolipid

-built on Sphingosines but do not have phosphate groups -when X group of sphingolipid is a carbohydrate (sugar) -lipid part with a sugar on it -amphipathic -can be bound to multiple sugars

Sphingolipids

-built upon sphingosines NOT glycerol -derivatives of sphingosines -smaller class of lipids -phospholipid because of the phosphate group

Phosphatidylinositol: head molecules

-can act as signaling molecules or binding sites for cell signaling

Phospholipid structure: fatty acid tails

-can be saturated or unsaturated -usually one saturated tail and the other is unsaturated -unsaturated tail has a double bond in cis configuration which causes it to kink

Cholesterol impact on bilayer

-cholesterol gets embedded in the lipid bilayer because it is mainly hydrophobic -cholesterol has rings in its structure so it isn't as flexible as the fatty acid tails -cholesterol disrupts close packing of fatty acids & interferes with their mobility - tends to increase durability & decrease permeability of membrane -can create conditions of intermediate fluidity

Lipid Rafts

-elevated sphingolipids (such as sphingomyelin) on the outer plasma membrane -move above membrane lipids -can help localize signaling molecules

Scramblases

-enzyme in the ER that RANDOMLY transfers some phospholipids to inner leaflet when there is asymmetry in the membrane of the ER -this enzyme just wants to even out the number of phospholipids on each side of the bilayer

Flippases

-enzyme in the golgi and plasma membrane that move SPECIFIC phospholipids to the inner leaflet

Floppases

-enzyme in the golgi, plasma membrane, and ER that move SPECIFIC phospholipids to the outer leaflet

plasma membrane of a red blood cell

-has a cell cortex (proteins on the inside of the plasma membrane); the cell cortex strengthens the plasma membrane -located just below plasma membrane -every cell has them, but the ones in red blood cells are really tough and strong because they have to move and squeeze through places -inner cortex has a hierarchy arrangement

Cholesterol structure

-hydrophobic hydrocarbons and small hydrophilic OH group

Movement of phospholipids in a membrane

-lipid fatty acid tails jiggle -head groups diffuse around rapidly: in about a millionith of a second, a head group can move from place to another on the same side of the membrane -about once every 28 hours, one hydrophillic head group can move across to the other side of the membrane -Flippases/scramblases can help lipids switch leaflets

Cholesterol

-looks structurally different from Phosphoglycerides and Sphingolipids -may constitute 50% of animal cell membranes -absent from most plants/bacteria -smaller and less amphipathic -won't form lipid bilayer on its own -not as flexible as phospholipid (rigid structure) -interferes with phospholipid packing -makes membrane more fluid at lower temps and less fluid at higher temps

What are the most common Phosphoglycerides

-most common are PC (phosphatidylcoline), PE (phosphatidylethanolamine), and PS (phosphatidylserine)

Laser tweezers and plasma membrane fluidity

-plasma membrane is very thin but very strong -laser tweezers can be used to physically distort the plasma membrane, but the plasma membrane will not rupture. It will move quickly to adapt to the distortion -pulling of the membrane with laser tweezers helps demonstrate that the plasma membrane is strong and does not rupture because of hydrophobic interactions

Lipid-anchored membrane proteins (Lipid-linked membrane proteins)

-protein itself is entirely out of the membrane but is associated covalently with a lipid tail that is inserted in the membrane -would be removed from the membrane with detergent (high salt would not be enough) -Differ in anchor used and membrane sidedness -ex: GPI and fatty acid anchor

Protein structure and crossing of the lipid bilayer

-proteins can cross lipid bilayer because of their alpha helices (secondary structure of protein) -transmembrane alpha helices (present in proteins that can cross the lipid bilayer) have a bunch of hydrophobic R groups (hydrophobic amino acids) that associate with the hydrophobic tails of the lipids in the bilayer

FRAP method: results of artificial membrane experiment

-proteins move more slowly in Plasma Membrane than in artificial lipids -large fraction (30-70%) not free to diffuse which tells us that in real cells, proteins are doing something else besides recovering.

Integral proteins

-proteins that are embedded in the lipid bilayer; go through the hydrophobic middle -amphipathic molecules

Peripheral membrane proteins

-proteins that can come on and off of the lipid bilayer membrane -proteins that are entirely out of the lipid bilayer -they are associated with the lipid bilayer by binding to something that is in the lipid bilayer -can bind to integral proteins or lipids that are directly in the lipid bilayer -weak electrostatic bonds (matching charges) with lipids or integral membrane proteins (noncovalently bonded) -they can exist on either side of the membrane

FRAP method: rate of the recovery and extent of recovery

-rate of recovery is a direct measure of diffusion rate (How fast the area is recovered is based on how fast the proteins diffuse.) -extent of recovery is a direct measure of percentage of labeled molecules free to move (Does the color go back to the original or does it stay a lighter color?): In some cases, proteins recover but never get back to the original color. This means that some of those proteins are not free to diffuse.

Lipid bilayer self assembly into a:_________________. Why doesn't the bilayer form a flat sheet?

-sealed compartment (shape of a sphere) to protect the hydrophobic fatty acids -It doesn't form a flat sheet because the fatty acids would still be exposed to water

Sphingolipid Structure

-sphingosine + fatty acid + X group -can contain a phosphate and be a phospholipid

Phosphatidylinositol: structure and function

-structure: 2 fatty acid tails attached to a glycerol, which is attached to a phosphate group that is attached to an inositol (head group is inositol) -acts as a signaling molecule in cell

Laser tweezers

-techniques used to move small particles- cells and organelles -basically shine laser and a particle with a high refractive index will be attracted to the laser

Fluid-Mosaic Model

-the current model of the Plasma membrane -lipids, which continuously move around the lipid bilayer, are the "fluid" part -the "mosaic" parts are the proteins that are embedded in the lipid bilayer (they move as well but a lot less than the lipids) -integral proteins -peripheral proteins -phospholipids, glycolipids, glycoproteins, oligosaccharides, hydrophobic alpha helix

Phosphoglyceride Structure

-there are various kinds of Phosphoglycerides, but all have 2 of the 3 carbons in the glycerol (3 carbons with -OH group but H comes off and just leaves O to bind with rest of molecule) bonded to two fatty acid tails (hydrophobic) and the O in the glycerol bonded to a phosphate group which is bonded to some hydrophilic end group. -Phosphoglycerides differ in the hydrophilic end group that's attached to the phosphate

What removes peripheral proteins from the membrane?

-these proteins can be solubilized (removed from membrane) with aqueous salt solutions/ high salt solutions

Phosphoglycerides

-very amphipathic: very hydrophobic hydrocarbon chains and very hydrophilic head group (amino acid and phosphate) -major lipids in membrane -as name implies, they have a phosphate group and a glycerol group -phospholipid because of the phosphate group

Sphingomyelin

-when X group of Sphingolipid is phosphorylcholine -found in our bodies

If a cell is enlarged 3 million times to the size of 100 yards, how thick will the Plasma membrane be?

0.6 inches

10,000 plasma membranes stacked together make the thickness of:

1 sheet of paper

What are some of the functions of membrane proteins?

1. transporters and channels: transport nutrients and ions across lipid bilayer (ex: sodium and potassium protein that generates Na and K gradients) 2. anchors: anchor membrane to macromolecules; help cells move along 3. receptors: detect chemical signals from environment (ex: insulin in the environment binds to receptors and causes events inside cell to occur) 4. enzymes: catalyze reactions at the membrane

Carbohydrate membrane percentages

2 - 10% of plasma membrane by weight <10% in glycolipid >90% in glycoprotein

Apical plasma membrane, lateral plasma membrane, and basal membrane functions

Apical plasma membrane: regulation of nutrients and water, regulated secretion, protection Lateral plasma membrane: cell contact and adhesion; cell communication Basal membrane: cell-substrate contact; generation of ion gradients

Integral protein association with the membrane

Because integral proteins are embedded in the membrane, they can't come off and will always be associated with the membrane

Movement of lipids and proteins in the lipid bilayer

Both lipids and proteins in the lipid bilayer move around. Lipids move more. Some proteins are anchored and do not move as much as other proteins.

The soft margarine - butter analogy

Butter is animal fat that's largely saturated. Butter coming out of the refrigerator is hard/solid. Oils are plant fats that are unsaturated. For margarine, which comes from oils and plants, the double bonds are switched to the trans configuration so that they are solid at room temperature

The internal tempereature of some animals (cold blooded animals) changes with outside temperature. How do these organisms respond to the change in temperature?

These organisms respond by altering the membrane lipid composition. They can do this through enzymes that work when the organisms are exposed to low temperatures. These enzymes are Desaturase, Phospholipases and acyltransferases

Where are proteins found in the bilayer?

They are found embedded in the lipid bilayer

Cholesterol and temperature

Cholesterol acts as a buffer. -at low temperatures, cholesterol will make membrane more fluid. This happens because cholesterol will stop the usual packaging of the fatty acid tails at low temperatures (it stops it because it is in the way) -at high temperatures, cholesterol will make membrane less fluid. At high temperatures, the fatty acid tails are normally jiggling around. Cholesterol cannot jiggle around and it makes the membrane less fluid.

How does cholesterol interfere with phospholipid packing?

Cholesterol can fill gaps in the bilayer and stabilize it. It gets embedded in the phospholipid bilayer. The hydrophobic carbons associate with the hydrophobic fatty acids of the phospholipid bilayer. The hydrophilic head group (OH) is associated with the hydrophilic parts of the phospholipid.

An integral protein that does not come off the lipid bilayer from changing the salt concentration, comes of when exposed to detergent, why? Why does detergent disrupt a membrane?

Detergent can extract a protein out of the lipid bilayer because hydrophobic tails of the detergent will bind to the hydrophobic regions of the proteins and are protected. The hydrophilic parts of the detergent will interact with water. The protein, which has hydrophobic parts, is now in water and taken out of membrane and the membrane disrupts.

Which has more membrane: plasma membrane or ER membrane?

In most cells, there is more membrane in the ER than there is in the plasma membrane

Phosphatidylinositol: Calcium example

In one case, a receptor binding to protein will cause the head of phosphatidylinositol to be cut off. The head group (inositol and phosphate) is hydrophilic (so it's on outsides of bilayer) and releases into the cell. This is a signal for the cell to release calcium. The tail (glycerol and fatty acid chains) will stay in membrane and signal other things

What is the symmetry of the lipids like in the membrane of the ER?

In the ER, there is not a lot of asymmetry

How do we study protein dynamics in the plasma membrane (how do we study their movement and how rapidly they move)? FRAP method

In the FRAP (fluorescence recovery after photobleaching) method, membrane proteins are fluorescently labeled. So, at time 0, the surface of the membrane is uniformly colored by fluorescent label. A small section of the membrane is then bleached using a laser beam. At some time, some unbleached fluorescently labeled proteins migrate into the bleached area and some bleached proteins move out, giving the bleached area some color. After some time, recovery will be complete and the area will completely return to the fluorescent color, making the entire surface one color again. This happens because of the diffusion of fluorescent molecules and bleached molecules. How fast the area is recovered is based on how fast the proteins diffuse. Timing how quickly it takes the surface to return to a uniform color can tell us how quickly the proteins on the surface of the membrane move. If no recovery happens, the beached area would stay white.

Red blood cells and detergent experiment

In this experiment, detergent was added to red blood cells. The detergent got rid of the lipids, and what stayed behind were plasma membrane "ghosts." What was left over were proteins that kept the shape of the red blood cells. After getting rid of the membrane, the proteins of the membrane still stay there.

Why is the fluidity of the membrane important?

It is important because the proteins are embedded in the lipids and they move around as part of their function. If the membrane is not fluid enough, the proteins can't move around enough and complete their functions

Red blood cells and peripheral and integral proteins

It was found that red blood cells have peripheral and integral proteins. After the high salt solutions, some proteins would come out of the membrane. If detergent was added to the red blood cells, other proteins would come out. Using the salt/detergent methods, the proteins in red blood cells were able to be identified

Why won't cholesterol form lipid bilayer on its own?

It won't form lipid bilayer on its own because there is not a big enough difference between the ends in this amphipathic molecule

Assembly of membrane bilayer

Lipid bilayer will spontaneously self assemble when lipids are put in water. In a lipid bilayer, lipids arrange themselves so that their polar head groups are exposed to water and the hydrophobic tails are sandwiched in the middle. The hydrophilic ends will hydrogen bond to water and hydrophobic forces will result in the hydrophobic ends coming together.

Distribution of lipids on the outer membrane

Lipids are not evenly distributed here

Why are lipids harder to study than proteins?

Lipids move around rapidly and they are difficult to preserve (couldn't really study them so the existence of lipid rafts were debated). Proteins you can fix them in place.

Distribution of lipids on the plasma membrane

Phospholipids and glycolipids are distributed asymmetrically, but certain phospholipids stay on the extracellular space/outer leaflet (ex: phosphatidylcoline and glycolipids) and others stay in the cytoplasm side of the bilayer/inner leaflet (ex: phosphatidylserine). Cholesterol can be found on either leaflet. Not only do different membranes have different lipid compositions, they also have different distribution of some of the lipids in the membrane. There are concentrated areas within the outer leaflet of only specific lipids.

Plasma Membrane: import/export of molecules

Sometimes things that are made inside cell need to be exported out of the cell or things outside the cell need to be imported into the cell

Different proteins exist on different membranes. Why?

This is because different cells have different functions and need proteins specific for their function

Why is the Plasma membrane only making up 2% of the cell membrane surprising?

This is surprising because the Plasma membrane extends and covers the entire cell so one might expect it to make up more than 2% of the cell membrane

How do unsaturated and saturated fatty acid tails impact the bilayer?

When forming a bilayer, unsaturated fatty acid tails pack loosely which allows the bilayer to remind fluid. If the fatty acid tails were saturated (no double bonds), bonds would solidify

What happens when new membrane from the ER is sent to the golgi to be added to the golgi membrane?

When membrane leaves the ER and are incorporated in the golgi, they encounter an enzyme called flippase. Flippase SELECTIVELY removes phosphatidylserine and phosphatidylethanolamine from the noncytosolic side (outer leaflet) and flips them to the cytosolic side (inner leaflet). This transfer leaves phosphatidylcholine and sphingomyelin concentrated on the noncytosolic side (outer leaflet)

What happens when phospholipids are added (phospholipid synthesis) to the cytosolic side of the lipid bilayer of an ER membrane?

When the new lipids are added to the cytosolic side of the membrane, that side gets longer and creates an asymmetry (cytosolic side has more phospholipids than ER lumen side). In order to move lipids from cytosolic side to the ER lumen side for symmetry, an enzyme called Scramblases is used. This enzyme randomly transfers some phospholipids to the ER lumen side. After this random transfer, there is symmetrical growth on both halves of the bilayer

Bacteriorhodopsin

a transmembrane protein that uses energy from light to pump protons (highly charged and cannot cross membrane alone) across the membrane. It has hydrophobic outside that associates with the bilayer and a hydrophilic inside that transports hydrophilic molecules

How thick is the plasma membrane?

about 5 nm thick (about 1/5 the size of a ribosome) 10,000 Plasma membranes stacked together make the thickness of a paper

Different types of membrane lipids are all:

amphipathic

Spectrin mutations

causes anemia- red blood cells break in capillaries so the individual has less red blood cells. The cells break because spectrin is what gives them strength; mutations in this can cause the red blood cells to be weaker and break more easily.

Fluidity depends on temperature, ___________________, and __________________________.

chain length and degree of saturation. Membranes need to reach a melting point to be fluid. Chain length and degree of saturation (double bond vs single bond) influence melting point temperatures. -carbons with longer tails need higher melting points -the more double bonds (cis configuration) present, the the lower the melting point has to be (can have a really low melting point like -20). This is because double bonds make kinks in the hydrocarbon tails. The more kinks there are, the less the hydrocarbon tails can be packed in together. Lipids with saturated fatty acids (single bonds) can pack together in the membrane

What is required for the integral proteins to separate from the membrane?

detergents are required to solubilize (separate from membrane)

Plasma Membrane

enclosing of a cell or of an intracellular compartment (in eukaryotes) that act as selective barriers (membranes are selectively permeable)

Charge of end group and phosphate in Phosphoglycerides

end group: + phosphate group: -

Phospholipases and acyltransferases

enzyme that reshuffles fatty acid chains to create lipids with two unsaturated chains at lower temperatures. Having two fatty acid tails that are next to each other creates more fluidity.

Desaturase

enzyme that takes single bonds in the fatty acid tails and turns them into double bonds so that the membrane can stay fluid at low temperatures

Why are red blood cells a good cell to use to understand Plasma Membrane?

human red blood cells are just plasma membrane; they have no internal membrane (no organelles) -blood is a renewable supply -blood only has red blood cells and white blood cells so it is easy to get red blood cells supply

Which bonds are present in the backbones of the alpha helix?

hydrogen bonds

In the lipid bilayer, the hydrophobic ends are embedded in the:

hydrophobic region

Which integral membrane proteins have their proteins out of the lipid bilayer and are only connected to the bilayer through covalent bonding of something in the bilayer?

lipid linked proteins

Number of R groups can help tell if the protein is a __________. What is this number of R groups?

membrane protein. about 20-25 hydrophobic R groups

Amphipathic

molecule with hydrophobic and hydrophilic regions

Which kind of bonds are found in the lipid bilayer?

noncovalent interactions are found here

Which molecules are always on the external side of the plasma membrane (extracellular leaflet)?

oligosaccharide and glycoproteins which have sugar

Which membrane lipids are found on the outer leaflet? Which are found on the inner leaflet?

outer leaflet: phosphatidylcoline, sphingomyelin, and glycolipids inner leaflet: phosphatidylserine

Proteins in bilayer are split in which groups?

peripheral and integral

Peripheral membrane protein: types

protein attached proteins: proteins that are attached to the membrane only by relatively weak, noncovalent interactions with membrane proteins

Restriction of protein movement

protein movement can be restricted in many ways: 1. restricted by the tails- the cytoplasmic tails of the proteins are bound to something so the proteins are restricted to one spot. (ex: cytoskeleton) 2. restricted by the head- proteins are bound to something outside of the membrane/cell 3. restriction by bonding- proteins might be bonding to proteins (or something else) on another cell and stop diffusing 4. restriction of movement by diffusion barriers- proteins are restricted from moving past "fences" or diffusion barriers. (ex: gut proteins are restricted from diffusing with blood proteins by diffusion barriers). Certain proteins have to stay on the apical side of the plasma membrane and others have to stay on the basal membrane; diffusion barriers help those proteins stay on those sides of the membrane

All membranes are made of:

proteins and lipids

Peripheral proteins

proteins that are not embedded in the hydrophobic middle; they are only attached to one of the outer layers of the lipid bilayer (inner or outer leaflet) associated with the surface

Endoplasmic Reticulum

site where proteins that will be released outside of the cell will be synthesized. It extends around the entire cytosol Ex: cells that work in producing lots of insulin for the body will have lots of ER

Peripheral protein association with the membrane

sometimes peripheral proteins are temporarily associated with the membrane. They can come off and on of the membrane.

Inner leaflet

the inner side of the Plasma membrane; the one facing the cyoplasm and inside of cell

The lipid:protein ratio varies greatly and depends on:

the membrane type (ex: Plasma membrane or ER) and membrane function

Outer leaflet of Plasma membrane

the outer side of the Plasma membrane; the one facing the outside environment (also called the extracellular)

Where do Phosphoglycerides get their names from?

their names come from their end group (the one bonded to phosphate)

Functions of peripheral proteins

these proteins may be enzymes, specialized coats, transmit transmembrane signals or provide mechanical support

melting point is also called the

transition temperature

How do we understand the arrangement of proteins the plasma membrane?

-A lot of what we know about PM proteins came from studies with human red blood cells

Why is it important for the membrane to be fluid or have fluidity?

-Changes in membrane fluidity can alter the ability of a membrane to function properly -if membrane is not fluid then proteins cannot move and carry out their functions -required for cell movement, growth, division, intercellular junction formation, secretion, endocytosis and anything that the membrane does

Cell surface: carbohydrates

-In addition to lipids and proteins, carbohydrates (sugars) are also present on plasma membrane -these sugars are on the outside -there are proteins and lipids with sugars on them -Carbohydrate content covalently linked to both lipids & proteins -can be used for protection of the cell surface or act as receptors/binding sites for certain things. Two molecules can bind to each other because of the sugars on their surface

GPI (glycophosphatidylinositol)

-Lipid-anchored membrane protein -found on external plasma membrane face -covalently linked to glycolipid -glycolipid connects GPI to membrane -function as various receptors, enzymes, cell adhesion proteins -helps certain proteins associate with certain regions of the plasma membrane

Fatty acid anchor

-Lipid-anchored membrane protein -protein is hydrophilic but is associated with a fatty acid anchor. When fatty acid anchor is present, protein inserts itself in the membrane. -some proteins have fatty acid anchor hidden inside and it comes out for signaling (temporary association) -found on internal Plasma Membrane face -covalently linked to Fatty acid -kinases- associated with the membrane under certain conditions because of fatty acid tail

Phospholipid structure

-Phospholipid has two fatty acid tails (one saturated and one unsaturated) attached to the glycerol which is attached to the phosphate which is attached to a head group. -the phosphate group and the head group are polar and hydrophilic -the fatty acid tails are hydrophobic

Lipid bilayer and temperature

-Structure of lipid bilayer depends on temperature -lipids jiggle around with thermal energy -in cold conditions there is not a lot of thermal energy. The long hydrocarbon tails of the fatty acids get packed together. The membrane resembles gel (frozen-like) texture -if you keep warming up the membrane, eventually you will reach some sort of melting point. In warmer conditions, the membrane is liquid like and the hydrocarbon tails separate and jiggle more

Identifying transmembrane domains using the computer

-You can put a sequence of a protein in the computer to find out where the protein is found in the cell. The computer will look to see if there are 20-25 amino acids in a row. If the computer sees that, it will say that the protein is associated with the membrane -Hydropathy plot will tell us which amino acids are hydrophobic and hydrophillic. 20-25 hydrophobic amino acids in a row will show up on plot -Hydropathy plot can show that there are mulitple proteins because mutliple spots will show up with hydrophobic regions

spectrin

-a protein that cross links "spokes" and "wheels" -spectrin meshwork forms cell cortex

Liposomes

-a spherical sac of phospholipid molecules that encloses water -sometimes inner leaflet has water so the hydrophilic parts bond to that and the outer leaflet hydrophilic parts bond to water too and form liposomes

How do we study protein dynamics in the plasma membrane (how do we study their movement and how rapidly they move)? Fusion of cell method

1. Some scientists took a mouse cell and labelled the proteins on the surface red. They took another cell, a human cell, and labelled all of their surface proteins green. The plasma membranes of the mouse and human cell were fused together. Now there is one cell. At time 0, one of the plasma membrane sides came from the mouse cell and the other came from the human cell (so half of the plasma membrane was red and the other half was green). At time 0, the red and green proteins are separated. With time, at 37°C, the proteins begin to mix. The reason for this mixing is because some of the red proteins on one half of the surface began to diffuse down to the green half and vice versa. If there was no diffusion of the proteins (if the proteins on the surface of a cell membrane did not move), one side would always be red and the other would be green (there would be no mixing). They tested this at different temperatures and found that the transition temperature (the temperature where the proteins began to mix) was about 59°F (about 15-17°C). Below that, the lipids were frozen and did not move. Once they became fluid, they could diffuse around. Once the proteins mixed, they stayed mixed regardless of temperature. This proved that the proteins moved around laterally, but did not tell how fast they moved.

Proteins show different patterns of diffusion (movement). What are the different patterns and how do we track this?

1. different patterns: -freely diffusing in lipid bilayer (line/movement is all over the place and covers a big distance): these proteins seem like they were working in an area for a while and then broke away to work in another area and then broke away again. This tells us that maybe something is going on and the proteins are interacting with something. A in photo -corralled in membrane domain by other proteins (movement is confined to a space but medium sized distance; seems to have a boundary). B in photo -tethered (tied) to cytoskeleton (movement is confined to a small space over a small distance). C in photo 2. track using Single particle tracking

Methods used to study and understand integral membrane proteins

1. solubilization with detergent- detergent pulls out protein and protein can now be studied 2. identifying transmembrane domains using hydrophobic alpha helices

Integral membrane protein: types

1. transmembrane proteins: extend across bilayer as signle alpha helix, mulitple alpha helices, and rolled up beta sheet (beta barrels) 2. monolayer-associated alpha helix proteins: are anchored to the cytosolic half of the lipid bilayer 3. lipid linked proteins: proteins that are linked to either side of the bilayer by a covalently attached lipid molecule

Plasma Membrane: cell growth/motility

Cells are moving throughout the body and environment and need to squeeze through small places. The plasma membrane needs to be changed to deal with the environment. Plasma membrane needs to be strong enough to keep cell intact but it can't be rigid because it has to adjust to the environment.

Nature and importance of lipid bilayer

Each type of cellular membrane has a characteristic lipid composition. The lipid composition can function in determining physical state of membrane, influencing activities of some membrane proteins, and providing precursors or chemical messengers

True or false: the lipid bilayer only has unsaturated carbons in the hydrocarbons of the fatty acid tails.

False. The bilayer has both saturated and unsaturated carbons.

GPI and lipid rafts

GPI are associated with the elevated regions (lipid rafts) on the outer membrane

Some molecules can't get through the membrane by itself. What are some of these molecules

Generally hydrophobic things can get across the membrane, but molecules like amino acids, ATP, sodium, glucose can't get across lipid bilayer by itself.

Plasma Membrane: receiving information

Hormones use receptors on the surface of the plasma membrane to transmit messages inside. Signaling molecules can be transmitted across the lipid bilayer so that the cell can receive information

Phosphatidylinositol: docking sites

In another case, Phosphatidylinositol acts as a docking site for proteins. Certain phosphates get added on to the Phosphatidylinositol. Now that the phosphates are present, a specific protein that was floating around in cytoplasm can recognize that and binds to the membrane to become activated

Where are most of the lipids in the cell synthesized?

Most are synthesized in the ER

Examples of lipid:protein ratio variation

Myelin Sheath of nerve axon: functions in insulation - 5 times more lipid than proteins (18% protein, 79% lipid) Mitochondrial inner membrane: ATP production- (78% protein, 22% lipid)

Energy for turning the flat sheet of the bilayer into a sealed compartment (sphere)

No ATP is needed for this change because the change is energetically favorable

What are the 3 major classes of lipids in membranes? What property do they all have in common?

Phosphoglycerides, sphingolipids (include glycolipids), and cholesterol They are all amphipathic and are the structural parts of the membrane

Sphingomyelin vs phosphotidylcholine

Phosphotidylcholine -has the two fatty acid tails Sphingomyelin -has longer sphingosine tails which make areas that have Sphingomyelin thicker (In photo, Sphingomyelin is redish objects that are elevated) -associated with GPI (glycophosphatidylinositol) anchor proteins -lipid rafts

Function of the Plasma Membrane

Plasma membrane is involved in cell communication, import/export of molecules and cell growth/motility

Percentage of Plasma membrane, ER, and mitochondria in the composition of the cell membrane

Plasma membrane: 2% of cell membrane ER: up to 50% of cell membrane mitochondria: inner membrane of the mitochondria can be about 30% of all the membrane in the cell

Primary function of the phospholipids are _________________________ but they also function in ___________________________

Primary function: membrane; to make up membrane; to create permeability of membrane Also function in: cell signaling

What would happen if the lipid bilayer was broken?

The lipid bilayer would spontaneously seal back up again because of hydrophobic forces

What would you see if you FRAP-ed spectrin?

The spectrin would stay bleached because it is associated with the cortex