BIOMG 1350 Lecture 8 - Membrane Structure

Cell Membrane

Membranes act as selective barriers

Membranes are composed of lipids – that provide the barrier – and proteins that provide most of the selective

properties.

The plasma membrane is more than a barrier as it’s the interface the cell has with the environment. The plasma membrane has the capacity to import and export molecules, send signals, receive information/signals, permits the cell to divide without rupture, and the membrane can also move and expand when needed.

Membrane structure (simplified) is essentially composed of a phospholipid bilayer with two phospholipids and their tails consisting on the middle portion of the membrane and the heads consisting of the outer (facing water) side of the membrane.

Hydrophilic outside, and hydrophobic core. This is why large molecules or charged molecules cannot pass through the membrane layer.

The plasma membrane lipid bilayer is asymmetric as it carries a negative charge on the internal side of the cell membrane and they also don't contain the same composition of proteins on either side of the cell membrane.

Prokaryotic vs Eukaryotic Cell Membrane Differences

Prokaryotes have only one membrane and that is the plasma membrane and this membrane encloses the cell and makes a clear outside inside difference

Eukaryotes have a plasma membrane and also additional internal membranes which allows for compartmentalization which allows for a clear inside outside organelle and an inside outside cell difference. More compartmentalization means more complexity in functions the cell can perform.

What molecules are and are not able to pass through the cell membrane?

Membranes block the passage of most water soluble molecules.

• What can pass a cell membrane: small hydrophobic molecules (O2, CO2, N2, Steroid Hormones) as they can pass through the hydrophobic phospholipid bilayer, a small number of small uncharged polar molecules at a slow rate (H2O, glycerol, ethanol)

• What cannot pass through a cell membrane: larger uncharged polar molecules (amino acids, glucose, and nucleosides), and ions (H+, Na+, Ca2+, Mg2+, HCO3-, K+) as these cannot get past the hydrophobic and uncharged phospholipid bilayer

Synthesis and Redistribution of Lipids

Phospholipids are synthesized in the ER (and some in the mitochondria). Specifically on the cytosolic side of the ER membrane is where these phospholipids are synthesized.

Flippases selectively move specific lipids between membrane leaflets, while scramblases move lipids between leaflets more randomly.

How and why do cells generate asymmetric membranes?

Scramblases do not produce an asymmetric membranes only flippases do.

Flippases selectively send certain phospholipids to either side of the lipid bilayer.

The Golgi helps establish and maintain lipid and protein asymmetry in the plasma membrane by sorting and modifying lipids/proteins before they are delivered to the cell surface.

Flippases produce an assymetric membrane which allows the inside of a cell or an organelle to perform a different function with a different environment than outside of the cell or outside of the organelle.

Flippase

Creates an asymmetry in the golgi apparatus and plasma membrane and essentially serves as a pump to pump specific phospholipids from the inner leaflet of the bilayer to the outer leaflet of the bilayer selectively.

Flippases solve a functional identity problem by using ATP to actively pump specific phospholipids to designated sides of the membrane. This creates a strict chemical asymmetry, allowing the inside and outside surfaces of the cell to perform completely different jobs. For instance, they keep specific signaling phospholipids strictly on the inner leaflet so the cell isn't accidentally targeted for destruction by the immune system.

Scramblase

Provide a hydrophilic surface so the head group of one phospholipid can flip over to the other side of the plasma membrane.

Scramblases make the ER phospholipid bilayer homogenous making sure newly synthesized phospholipids are evenly distributed between both sides of the phospholipid bilayer. Equalizes the cell membrane so there is equal growth on both halves of the bilayer.

Scramblases solve a physical space problem in the ER, where newly synthesized phospolipids are only added to the cytosolic side (phospholipids are synthesized on the cytosolic side of the ER). By randomly flipping these new phospholipids to the opposite leaflet, they prevent the membrane from buckling or warping. This ensures both halves of the phospholipid bilayer grow evenly as a stable sheet.

Membrane Lipids or Phospholipids

Phospholipids or membrane lipids are amphipathic meaning a portion of the molecule is hydrophobic (tail) and another portion is hydrophilic (head)

Phospholipids arrange themselves into bilayers when placed in water to minimize the hydrophobic portion exposed to water and maximize the hydrophilic portion exposed to water.

A phospholipid bilayer is made of two monolayers of phospholipids and each monolayer of the bilayer is also called a leaflet.

Phospholipid bilayers spontaneously close to form sealed compartments as if they are in a flat sheet, that positioning is energetically unfavorable. Rather when they close into vesicles or enclosed membrane this position is more energetically favorable as this, again, orients the hydrophobic and hydrophilic portions of the phospholipid bilayer properly.

Phospholipid Structure and Charge

Phospholipids are made of a polar group (such as a choline group), then attached to a phosphate group, then attached to a glycerol group and then two hydrocarbon tails, also known as fatty acid tails, one is saturated and one is unsaturated (this is the top to bottom view as if the phospholipid is "standing up" with its tails touching the ground and head portion facing up)

Different phospholipids have different "head groups." Two examples of these are the phospholipids with the "PS" head and phospholipids with the "PC" heads. PS (phosphatidyl serine) carries a negative charge whereas "PC" phosphatidyl choline is neutral.

• This is important as most lipids have either a net neutral or a net negative charge. As a result, the surfaces of most cell membranes in cells have a net negative charge.

Saturated vs Unsaturated Fatty Acid Tail

Unsaturated fatty acids have a kink or double bond (one or more) in their hydrocarbon chain whereas saturated fatty acids are strictly made of single bonds.

Unsaturated fatty acids thus are wider than saturated fatty acids and often experience less van der waals forces with other phospholipids.

Phospholipid Leaflet

A phospholipid bilayer is made of two monolayers of phospholipids and each monolayer of the bilayer is also called a leaflet.

Phospholipid movement in the bilayer

Phospholipids have lots of degrees of freedom and can move to a large extent in membranes. They are dynamic structures and can undergo lateral diffusion (right and left movement), flexion, and rotation.

• Flip flopping of phospholipids in the bilayer (two phospholipids opposite one another swapping spots) basically never happens unless a protein catalyzes this movement.

• Fluidity of a membrane refers to the lateral movement of lipids (and membrane proteins) and the fluidity needs to be just right for the membrane to be able to perform its function.

Fluidity of cell membranes and factors influnecing membrane fluidity

Membranes with saturated lipids are less fluid than ones containing unsaturated lipids or fatty acids.

• Less fluid membranes are composed of densely packed saturated lipids or fatty acids. 

• More fluid membranes are composed of more unsaturated lipids with double bond kinks present.

• Unsaturated fatty acid chains have less hydrogen atoms than saturated fatty acid chains.

• Van der waals interactions play an important role in the rigidity of saturated fatty acid membranes as their dense packing increases the probability of these interactions.

Membrane fluidity depends on several factors

• Temperature as higher temperature can lead to more fluidity (temp is constant in human cells)

• Fatty acid tail length as shorter chains reduce the tendency of chains to interact with one another via van der waals, increasing fluidity

• Ratio of saturated to unsaturated phospholipids as higher unsaturation means higher fluidity

• Cholesterol content as the cholesterol steroid usually stiffens the membrane of animal cells (at a normal temperature)

How can lateral mobility of phospholipids and proteins be restricted?

Lateral mobility of both phospholipids and proteins in the plasma membrane can be restricted in several ways.

• Proteins can be bound to the cell cortex.

• Proteins can be bound to other proteins on the surface of another cell.

• Proteins can be bound to the extracellular matrix.

• Diffusion barriers on the cell membrane can prevent proteins in the membrane from moving past a certain point. Like giant water barriers preventing the movement of water onto land.

Cholesterol

Membranes contain the cholesterol steroid which contains a hydrophilic and hydrophobic portion allowing it to fit in between the gap of phospholipids and their hydrophilic and hydrophobic  

At lower temperatures cholesterol allows for fluidity of the membrane and at high temperatures cholesterol prevents too much fluidity of the membrane, this is the maintenance function of cholesterol. Cholesterol generally stiffens membranes though and makes the membrane less permeable.

Cholesterol's most important function is that it alters membrane fluidity and its importance is reflected in the fact that cholesterol makes up about 20% of the plasma membrane of animal cells.

Cholesterol makes membranes stiffer and less permeable and cholesterol's structure matches nicely to the phospholipids it slides itself between

What % of the plasma membrane is made up of cholesterol in animal cells?

Cholesterol's most important function is that it alters membrane fluidity and its importance is reflected in the fact that cholesterol makes up about 20% of the plasma membrane of animal cells.

What % of the plasma membrane consists of lipids verses proteins?

About 50% of the plasma membrane is lipids and 50% are proteins by mass.

However, there are more lipids in the plasma membrane if one is counting by number.

Cholesterol Structure

Structure: polar head group, rigid planar steroid ring structure, and nonpolar hydrocarbon tail

Cell membrane proteins

Membranes are composed of both lipids and proteins (about 50% lipids and 50% proteins, but in a given area there are usually a larger number of lipids due to their smaller count)

Lipids and membrane proteins can both diffuse relatively freely within the bilayer

Cell membrane proteins are proteins associated with the plasma membrane that perform functions such as transport, signaling, adhesion, and enzymatic activity. They can be integral (embedded in the membrane) or peripheral (associated with its surface).

Cell membrane protein functions

Plasma membrane protein functions

• Transporters (channel proteins, gated channel proteins, or carrier proteins that transport items in and out of the membrane)

• Anchors (helps anchor the cell membrane to the extracellular matrix or the cytoskeleton)

• Receptors (ligands can bind to these and send signals to the inside of cells)

• Enzymes (can catalyze reactions near the cell membrane)

Different ways cell membrane proteins associate with the lipid bilayer

Membrane proteins associate with the lipid bilayer in several different ways

• Integral proteins are permanently attached or embedded into the phospholipid bilayer (transmembrane proteins, lipid monolayer associated proteins, and covalently lipid linked proteins)

• Peripheral proteins are attached on only one side of the phospholipid bilayer and attached to the surface rather than embedded (associated with a membrane anchored protein)

• There are also peripheral membrane proteins that bind directly to the hydrophilic surface of the membrane unlike the example where the peripheral protein binds to the surface of deeply embedded transmembrane, integral proteins.

How do bilayer proteins cross the bilayer?

Proteins often cross membrane bilayers with an alpha helix. Hydrophobic amino acid side chains can interact with the hydrophobic portions of the phospholipid bilayer. (Remember alpha helixes have side chains that extend outwards). This basically means that when a protein needs to span across a cell membrane, it twists itself into a alpha helix to ensure nonpolar side chains face outwards whereas the polar polypeptide backbone is contained within the interior.

A hydrophilic pore can be formed by multiple transmembrane alpha helixes positioned around one another to form a tube with a hollow inner. Pore contains polar side chains of the alpha helix to interact with the aqueous environment whereas the nonpolar side chains interact with hydrophobic lipid tails.

Single verses multipass transmembrane proteins

Single pass and multipass transmembrane proteins:

Transmembrane proteins span the lipid bilayer using hydrophobic -helices, with single-pass proteins crossing exactly once to function primarily as signaling receptors. Meanwhile, multipass proteins weave through the membrane multiple times, clustering their helices together to form functional pores or transport channels.

Measuring protein diffusion within membranes

FRAP (fluorescence recovery after photobleaching) is one technique to do this. The process is as follows.

• The plasma membrane contains GFP tagged protein and this GFP membrane protein fusion is expressed in cells and fluorescence under a fluorescence microscope

• Part of the fluorescent area is bleached with a laser (no longer fluoresces)

• Then a graph is derived from the time it takes for the area that was photobleached with a laser to the time where the area recovers and proteins from other parts of the membrane that still fluoresce move to the photobleached area.

• These experiments assess mobility in membranes and a faster recovery looks like the fluorescence in the bleached area being restored to normal levels or near normal fluorescence levels in a shorter amount of time as opposed to a longer amount of time

Glycolipids

Lipids with a carbohydrate (sugar) chain attached to them. They are found mainly on the outer surface of cell membranes and help with cell recognition and communication.

Glycoproteins

Proteins with carbohydrate (sugar) chains attached to them. They are important for cell recognition, communication, and immune responses, and are often found on the cell membrane.

Liposomes

Small, spherical structures made of a phospholipid bilayer that forms a closed compartment around an aqueous interior. They can carry and deliver substances such as drugs or other molecules into cells.