Biological membranes

Question 1

What are lipids?

Answer 1

compounds that are primarily non-polar, hydrophobic, and insoluble in water. They include fatty acids, triacylglycerol, membrane lipids, and cholesterol

Question 2

What is the general formula for fatty acids?

Answer 2

CH3(CH2)NCOO-

Question 3

What does it mean for a fatty acid to be saturated? Unsaturated?

Answer 3

• Saturated fatty acids have no double bonds.
• Unsaturated fatty acids have one or more double bonds. Most naturally-occurring double bonds are in the cis conformation

Question 4

How does the cis conformation of a double bond affect the melting point of a fatty acid?

Answer 4

Cis double bonds introduce “kinks” into the structure of the fatty acid, which lowers the melting point. Trans fatty acids, on the other hand, do not have this kink and can pack more tightly, resulting in a higher melting point.

Question 5

What is the shorthand notation for fatty acids?

Answer 5

(# of carbons):(# of double bonds)Δ(locations of double bonds) For example, 18:1Δ9 represents an 18-carbon fatty acid with one double bond at carbon 9.

Question 6

What two factors affect the melting point of fatty acids?

Answer 6

○ Length: Longer fatty acids have higher melting points than shorter fatty acids.
○ Unsaturation: Saturated fatty acids have higher melting points than unsaturated fatty acids. Unsaturation has a greater effect on melting point than length.

Question 7

How are fatty acids stored?

Answer 7

Fatty acids are stored as triacylglycerol (TAG), which consists of three fatty acyl chains attached to a glycerol molecule. TAG is very hydrophobic

Question 8

What are the three main types of membrane lipids?

Answer 8

Glycerophospholipids, sphingolipids, and cholesterol.

Question 9

What is the structure of a glycerophospholipid?

Answer 9

Like triacylglycerol, a glycerophospholipid has fatty acyl groups covalently attached to glycerol. However, unlike triacylglycerol, the presence of a large polar group makes these molecules amphipathic. Variations exist in both polar head groups and acyl chains, affecting size and melting points.

Question 10

Describe the characteristics of cholesterol.

Answer 10

• Mostly hydrophobic.
  ○27 carbons, 1 OH (very weakly amphipathic).
  ○ Membrane lipid.
  ○ ~30% of mammalian plasma membranes.
  ○ Maintains fluidity and rigidity.
  ○ Does not form membranes alone.
  ○ OH associates with polar headgroups of other lipids.
  ○ Non-polar portion is found in the membrane.

Question 11

What structures do amphipathic molecules form in water?

Answer 11

Amphipathic molecules form micelles or bilayers in water to minimize unfavorable contact between water and hydrophobic tails while allowing solvation of polar head groups. Fatty acids form micelles, while membrane lipids form bilayers

amphipathic= both hydrophillic + hydrophobic

Question 12

What are some characteristics of lipid bilayers?

Answer 12

○ Vary depending on lipid composition (acyl chain and polar head group).
○ Non-covalent assembly
○ “Fluid yet stable”
○ 4-6 nm thick

Question 13

Define transition temperature in the context of lipid bilayers.

Answer 13

The melting temperature (transition temperature) of a lipid bilayer is the temperature at which it transitions from an ordered crystalline state to a more fluid state. This temperature depends on acyl-chain unsaturation and length.

Question 14

How does the transition temperature differ between artificial and biological membranes?

Answer 14

The transition temperature may be very sharp for an artificial membrane due to its homogenous preparation. However, biological membranes have a mixture of compounds (different lipids/proteins), resulting in a less defined transition temperature. Biological membranes must function above their gel temperature but not be completely disordered.

Question 15

How do cells maintain membrane fluidity in response to temperature changes?

Answer 15

Cells adapt to temperature differences by adjusting the lipid composition of their membranes. As temperatures decrease, more unsaturated fatty acids and shorter fatty acids are incorporated into membrane lipids. Conversely, with increasing temperatures, more saturated fatty acids and longer fatty acids are incorporated

Question 16

How does cholesterol affect membrane fluidity?

Answer 16

Cholesterol increases membrane rigidity by limiting the rotational movement of neighboring acyl tails, thereby increasing van der Waals interactions. This effect allows for increased fluidity at lower temperatures (by preventing tight packing) and increased rigidity at higher temperatures (by decreasing acyl chain motion).

Question 17

Describe lipid movement within the bilayer.

Answer 17

Lipids can move freely and rapidly laterally within the bilayer. However, transverse diffusion (flip-flop) is much slower and requires the assistance of enzymes like flippases. This restricted movement allows for differences in lipid composition between the leaflets of the bilayer.

Question 18

What are the types of membrane proteins?

Answer 18

○ Integral membrane proteins: These proteins are embedded within the lipid bilayer and have hydrophobic amino acid side chains on their surface that interact with the acyl tails.
○ Peripheral membrane proteins: These proteins are associated with the membrane but not embedded within it.

Question 19

What is the fluid mosaic model of membrane structure?

Answer 19

The fluid mosaic model describes the membrane as a dynamic, non-covalent, complex assembly of lipids and proteins (and carbohydrates). Both lipids and proteins move laterally within the membrane, but transverse movement is limited. The movement of proteins may also be restricted by the cytoskeleton. Carbohydrate chains are attached to the extracellular surface of some proteins and lipids.

Question 20

What types of molecules can cross the lipid bilayer by simple diffusion?

Answer 20

Small, non-polar molecules can cross the lipid bilayer by simple diffusion.

Question 21

What factors affect the rate of simple diffusion?

Answer 21

○ Size of molecule: Smaller molecules diffuse faster.
○ Concentration gradient: A larger gradient leads to a faster rate of diffusion.
○ Lipid solubility: More lipid-soluble molecules diffuse faster.

Question 22

What are the two major types of transport across biological membranes?

Answer 22

○ Passive transport: Movement of molecules down the concentration gradient; no energy required.
○ Active transport: Movement of molecules against the concentration gradient; requires energy.

Question 23

How do transport proteins facilitate transport across membranes?

Answer 23

Transport proteins reduce the activation energy barrier for transport, making it easier for molecules to cross the membrane.

Question 24

Describe the characteristics of porins and ion channels.

Answer 24

Porins and ion channels enable passive transport through membrane-spanning pores. Porins, typically trimers, have a relatively non-specific water-filled pore that allows free diffusion of molecules up to 1.5 kDa. Ion channels are highly selective and their selectivity depends on pore size and the properties of the side chains/functional groups within the pore

Question 25

How do transporter proteins function?

Answer 25

Transporter (carrier) proteins facilitate transport through conformational changes that alternate the opening of the protein from one side of the membrane to the other. They are selective for the substrate transported and may be involved in passive or active transport.

Question 26

What are the different types of transporter proteins?

Answer 26

○ Uniport: Transports one molecule in one direction.
○ Symport: Transports two molecules in the same direction.
○ Antiport: Transports two molecules in opposite directions.

Question 27

Contrast primary and secondary active transport.

Answer 27

Both types of active transport move molecules against their concentration gradient. Primary active transport directly utilizes energy from ATP hydrolysis or other reactions, while secondary active transport uses an ion gradient (often Na+) established by primary active transport as its energy source.

Question 28

What is an example of a primary active transporter?

Answer 28

The Na+/K+ ATPase is a primary active transporter that pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell for each ATP hydrolyzed. This process creates concentration gradients for both Na+ and K+ across the membrane.

Question 29

Describe how the Na+/glucose transporter functions.

Answer 29

The Na+/glucose transporter is an example of secondary active transport. It utilizes the Na+ gradient established by the Na+/K+ ATPase to transport glucose into the cell against its concentration gradient. The inward movement of Na+ (down its gradient) provides the energy for the inward movement of glucose (against its gradient).