Lecture 6: The Cell Membrane

Question 1

Describe the basic characteristics of the phospholipid bilayer outer leaflet

Answer 1

Cholesterol, Phosphatidyl Choline, Sphingomyelin, Glycolipids, Glycosylphosphatidyl Inositol: (Important in cell signaling)

• Cholesterol is the second most common membrane lipid. It’s hydrophobic, but it does have a polar hydroxyl group that draws it to the polar heads of two adjacent phospholipids.

Question 2

What are the components of the phospholipid bilayer inner leaflet. (Just the half that’s inside the cell. In case that wasn’t clear)

Answer 2

Cholesterol, Phosphatidyl Ethanolamine, Phosphatidyl Serine (Negatively charged), Phosphatidyl Inositol

Question 3

Describe glycolipids

Answer 3

• Found only in the outer leaflet with carbohydrate portion facing the extracellular environment.
• Fatty acid tail is coupled via sphingosine to a carbohydrate head group.
• Create a cell coat involved in cell-to-cell interactions and conveys antigenicity.

Question 4

Describe Lipid Rafts

Answer 4

Small patches of:

• Cholesterol, Sphingolipids (sphingomyelin and glycolipids)
• Lipid rafts compartmentalize cellular processes by serving as organizing centers for: Assembly of signaling molecules, influencing membrane fluidity, Membrane protein trafficking, Regulating neurotransmission and receptor trafficking
• Essentially, they’re chunks of various stuff that change fluidity and protein trafficking. They regulate their own transmission and receptors.

Question 5

Describe the glycocalyx

Answer 5

• Not an integral part of the membrane
• Carbohydrate coat on the extracellular surface of the cell membrane composed of carbohydrate portions of glycolipids and glycoproteins.
• The glycocalyx is not technically part of the membrane.
• It protects cell from ionic and mechanical stress, serves as a barrier against microorganisms, and is involved in cell-to-cell interactions.

Question 6

How does cholesterol moderate cell fluidity in relation to temperature?

Answer 6

At high temperatures it interferes with FA chain movement, makes the outer part of the membrane less fluid, and reduces permeability to small molecules.
- At low temperatures it prevents membranes from freezing and maintains membrane fluidity.

Question 7

Describe the Fluid Mosaic Membrane Model

Answer 7

• Widely accepted theory invented in the 70s by two dudes named Singer and Nicolson.
• The phopholipid bilayer forms a thin layer of oily fluid.
• Two types of proteins fill this model. You should already know them but in case ya don’t,
• Membrane proteins: Associated with membranes through protein-protein interactions typically involving ionic bonds that can easily be dissociated from the membrane.
• Integral proteins: Proteins that are inserted into the membrane and that can only be dissociated by reagents that disrupt hydrophobic interactions. (Remember those?)
• Reminder, Integral proteins HAVE to have a hydrophobic section in order to fit into the membranes. Does that make them amphoteric? I honestly don’t know.

Question 8

1. What mass of the cell is differentiated between proteins and lipids?
2. Describe Peripheral Proteins
3. Describe Integral Proteins

Answer 8

• The mass is 50/50 between proteins and lipids. But proteins are a lot large so it’s like 1 protein per 50-100 lipids
• Found on both the outer and inner leaflets of the cell membrane facing either the extracellular or the intracellular fluid, these can be removed more easily than integral proteins by changing ionic concentrations or pH.
• Embedded within the phospholipid bilayer, extracellular portion is typically glycosylated. Also associated with the nonpolar core of the lipid bilayer either directly or indirectly. Can include proteins that span the lipid bilayer one or more times and proteins that are anchored to the lipid bilayer but do not pass through it.

Question 9

Describe Transmembrane Proteins

Answer 9

Integral proteins that pass completely through both phospholipid layers. These typically serve as channel and transporter proteins. They may be single-pass or multiple-pass

Question 10

Describe the 4 types of transportation that occurs between the membrane…in case you forgot.

Answer 10

• Diffusion: Movement of particles from high to low concentrations. Does not require energy or transport molecules
• Osmosis: Diffusion of solvent (water) across a semipermeable membrane. Does not require energy or transport molecules
• Facilitated diffusion: A type of diffusion (high to low concentrations) Requires transport molecules but not energy.
• Active transport: Movement against a concentration gradient. Requires energy and transport molecules

Question 11

Describe the 3 main types of transport proteins

Answer 11

• Unitransporters: Carrier single molecule or ion unidirectionally
• Symporters: Co-transporter. Carries 2 molecules or ions simultaneously or sequentially in same direction
• Antiporters: Co-transporter. Carries 2 molecules or ions simultaneously or sequentially in opposite directions

(This is a little more confusing than I remember. May wanna review this.)