Textbook Notes Chapter 6.2

selective permeability

A property of biological membranes that allows some substances to cross more easily than others.

What happens when amphipathic lipids are dispersed in an aqueous solution?

Cages of water form around each of the nonpolar tails

What is the permeability of very small substances like ions?

Charged solutes, even small ones, do not effectively move across lipid bilayers without help from membrane proteins

What is responsible for the difference in permeabilities of solutes?

Charged substances and polar molecules above a certain size are better dissolved in a polar environment like water than in the nonpolar environment of membranes, where bilayers are found

Why is the difference in membrane permeability a critical issue?

Controlling what passes between the exterior and interior environments is a key characteristic of cells

What is cholesterol's effect on permeability?

Decreases as cholesterol content increases because cholesterol molecules interact with the hydrocarbon tails, making them more tightly packed

What is the saturation of hydrocarbon tails on permeability?

Decreases as degree of saturation increases because saturated fatty acids have straight hydrocarbon tails that pack together tightly, leaving few gaps

What is the length of hydrocarbon tails' effect on permeability?

Decreases as the length of hydrocarbon tails increases because longer hydrocarbon tails have more interactions (membrane is more dense)

What do micelles tend to form from?

Fatty acids or other simple amphipathic hydrocarbons

What do packed saturated hydrocarbon tails have?

Fewer spaces and more van der waals attractions

Suppose the investigators had created a set of liposomes using phospholipids with fully saturated tails and compared them to two other sets of liposomes where either 20 percent or 50 percent of the phospholipids contained polyunsaturated tails. Label the three lines on the graph above with your prediction for the three different sets of liposomes in this new experiment.

Increasing the number of phospholipids with polyunsaturated tails would increase permeability of the liposomes. Starting from the left, the first line (no cholesterol) would represent liposomes with 50 percent polyunsaturated phospholipids, the second line would be 20 percent polyunsaturated phospholipids, and the third line would contain only saturated phospholipids.

Why does cholesterol decrease membrane permeability?

The bulky cholesterol rings force phospholipid tails closer to each other, increasing their packing density. This closer packing of tails causes the membranes to become less permeable

What affects key aspects of phospholipid behavior in a membrane?

The degree of saturation and the length of the hydrocarbon tails

What influences the physical properties of a membrane and its permeability?

The length and saturation state of the hydrocarbon tails, in addition to the presence of cholesterol molecules

How do micelles and phospholipid bilayers form?

They form spontaneously in water—no input of energy is required

When are lipid bilayers created?

When lipid molecules align in paired sheets

When does the rat of transport for a molecule decrease?

When the small molecules are polar but uncharged

When are lipid bilayers more permeable?

When they contain many short, kinked, unsaturated hydrocarbon tails

What is temperature's effect on permeability?

decreases as temperature decreases because lower temperature shows movements of hydrocarbon tails, allowing more interactions

What happens when the length of saturated hydrocarbon tails increase?

the forces that hold them together also increase, making the membrane even denser

Micelles

tiny spherical aggregates created when the hydrophilic heads of a set of lipids face outward and interact with the water, while the hydrophobic tails interact with each other in the interior, away from the water

what do phospholipids tend to form?

Bilayers

If you understand this hypothesis, you should be able to predict where amino acids and nucleotides would be placed on the permeability scale and explain your reasoning.

Amino acids have amino and carboxyl groups that are ionized in water, and nucleotides have negatively charged phosphates. Due to their charge and larger size, both of these compounds would be placed below the small ions at the bottom of the permeability scale (<10−12 cm/sec).

liposomes

An artificial vesicle formed by mixing amphipathic lipids, such as phospholipids, together in an aqueous solution.

What bilayers are less permeable?

Bilayers containing mostly long, straight, saturated hydrocarbon tails

What does the fluid physical state of phospholipids in the plasma membrane allow them to do?

It allows individual lipid molecules to move laterally within each layer (like a person moving through a dense crowd)

What does the amphipathic nature of phospholipids cause them to do?

It causes them to spontaneously form into bilayers consisting of two lipid sheets held together by hydrophobic interactions

What effect does adding cholesterol have on a membrane?

It dramatically reduces their permeability

Why do largely unsaturated membranes allow more materials to pass?

It's interior is held together less tightly

What happens when unsaturated hydrocarbon tails are packed into a lipid bilayer?

Kinks created by double bonds produce spaces in the tails, which in turn reduce the number of van der waals interactions that help hold the hydrophobic tails together, weakening the barrier

What type of molecules move across the bilayers the slowest?

Large polar molecules

phospholipid bilayers

Lipid bilayers consisting of phospholipids

What happens at very low temperatures?

Lipids can begin to solidify

level of fluidity

Measure of molecular mobility

What happens to molecules as temperature drops?

Molecules in the bilayer move more slowly and become less fluid, causing the hydrophobic tails to pack more tightly together

Do amphipathic lipids dissolve in water?

No, their hydrophilic heads interact with water but their hydrophobic tails do not, instead of dissolving amphipathic lipids form one of two structures, micelles and lipid bilayers

Describe the characteristics of phospholipids that allow them to spontaneously form lipid bilayers in water.

Phospholipids are amphipathic, so their hydrophilic end will interact with water and their hydrophobic end will not. Water molecules are more ordered when surrounding the hydrophobic regions. When phospholipids are brought together to form a membrane, the hydrophobic regions are tucked away from water and thus the entropy in water increases. Events that increase entropy tend to be spontaneous.

What type of molecules move across bilayers quickly?

Small nonpolar molecules (like oxygen O2)

What is one characteristic of lipid bilayers?

They are highly selective

What do amphipathic lipids do instead of dissolving in water?

They form either a micelle or lipid bilayer

lipid bilayer

flexible double-layered sheet that makes up the cell membrane and forms a barrier between the cell and its surroundings

Planar bilayer

makes up membranes of cells, most stable, both outside and inside of membrane is aqueous

Vesicles

small membrane sacs that specialize in moving products into, out of, and within a cell

permeability

tendency to allow a given substance to pass through it