Plasma Membrane

Question 1

What are the general features of the plasma membrane?

Answer 1

• self-forming
• contains proteins, lipids, and sugars moieties
• contains molecules that anchor to ECM and cytoskeletal structures
• asymmetrical with respect to lipid distribution and protein structure
• includes the cell coat (glycocalyx)

Question 2

Describe the basic organization of the plasma membrane

Answer 2

a phospholipid bilayer with hydrophobic tails facing the interior and the hydrophilic head groups facing out

not a rigid structure, both proteins and phospholipids diffuse

Question 3

What molecules make the plasma membrane a selectively permeable structure?

Answer 3

Cholesterol and integral membrane proteins

Question 4

What are the major classes of lipids found in the membrane?

Answer 4

phospholipids, glycolipids, and cholesterol

Question 5

What is the diagnostic significance of the plasma membrane?

Answer 5

It contains specific proteins that may be targets for therapy or used for diagnosis

Question 6

What are integrins and what is their mechanism of function?

Answer 6

Integrins are integral membrane proteins that integrate extracellular matrices with the cytoskeleton.

Question 7

Describe the structure of the plasma membrane

Answer 7

Trilaminar structure:

electron dense inner region facing cytoplasm, electron dense outer region facing ECM, and translucent central portion

Question 8

Describe the mechanism by which phospholipids diffuse through membrane versus flip-flop.

Answer 8

Lateral diffusion of lipids occurs passively in the plasma membrane (does not require enzymes).

Flip-flop occurs but needs to be facilitated by specialized proteins (scrambles and flippase).

Question 9

Name the four main phosphate head groups and their typical orientations in the plasma membrane.

Answer 9

ethanolamine, choline, serine, inositol

PI and PS generally face cytosol
PC faces ECM

Question 10

Where is sphingomyelin oriented in the plasma membrane? Why?

Answer 10

Sphingomyelin is primarily found on the ECM side of the plasma membrane.

Question 11

Describe the differences between saturated and unsaturated fatty acids and their affect on the plasma membrane.

Answer 11

Saturated fatty acids are hydrocarbon chains formed completely with single bonds. As a result, these fatty acids pack together tightly which makes the plasma membrane

Unsaturated fatty acids are hydrocarbon chains formed with at least one double bond, causing a kink in the tail. Therefore these fatty acids create a more fluid plasma membrane but it can be more leaky, leading to the need for cholesterol.

Question 12

What are two functions of cholesterol in the plasma membrane?

Answer 12

1. Cholesterol helps with leakiness in the plasma membrane by intercalating between two phospholipids (between the kinks of an unsaturated tail adjacent to a saturated fatty acid).
2. Cholesterol helps maintain membrane fluidity at physiological temperature, preventing the crystallization of adjacent fatty acid tails.

Question 13

What is a main function of glycolipids with regarding to cell-cell signaling?

Answer 13

Glycolipids act as antigen determinants (A, B, O), depending on whether you have the enzymes to build the specific sugar groups.

It also functions as a toxin receptor.

Question 14

What are the glycolipid sugar groups for each blood type?

Answer 14

A = N-acetylgalactosamine
B = galactose
AB = both sugars
O = neither sugar

Question 15

Describe some of the main functions of lipids in the plasma membrane.

Answer 15

• cause charge differential across membranes
• participate in signaling reactions
• antigenic determinants
• determine fluidity of membranes
• influence clearance of cells from circulation

Question 16

peripheral proteins

Answer 16

• associate with other proteins that are embedded in the membrane via ionic interactions
• easy to remove from membrane

Question 17

integral proteins

Answer 17

• contain hydrophobic inner domain that interfaces with fatty acid tails and hydrophilic ends on either side of plasma membrane
• asymmetrically distributed
• must use harsher treatments to remove them since they span the membrane

Question 18

What three types of proteins always face the extracellular environment? Why?

Answer 18

• N-linked glycosylations
• Intrachain disulfide bonds
• GPI-linked proteins

Question 19

What type of protein faces the intracellular environment? Why?

Answer 19

• Pre-sulfhydryl groups

Question 20

What role does isoprenylation play in the normal and abnormal functioning of cells?

Answer 20

• cell-cell recognition
• cell-matrix interactions
• signal transduction processes
• cell migration
• transport

Question 21

glycocalyx

Answer 21

sugar moieties on the outer portion of the membrane (looks fuzzy in slides)

Question 22

What role does the glycocalyx play in the interaction of platelets and the lining of blood vessels (non-thrombogenic surfaces)? Biological significance?

Answer 22

causes repulsion between platelets and the blood vessel linings because sugar moieties on both cells is negative

Question 23

What size is the plasma membrane? Can it be viewed using a light microscope?

Answer 23

plasma membrane = 5-10nm

need special stains and should use electron microscope to see trilaminar structure (resolution of 2nm)

Question 24

What is the purpose of knowing whether the slide is looked at via LM or EM?

Answer 24

• helps viewer determine whether they are looking outside or inside cell
• use scale bars!!

Question 25

What are the net charges of the different phospholipids and why are these important?

Answer 25

- phosphatidyl-ethanolamine = 0 - phosphatidyl-serine = -1 - phosphatidyl-choline = 0 - phosphatidyl-inositol = -1 anionic phospholipids (PI and PS) generally facing cytosol --> important for signal transduction (activation of protein kinase C)

Question 26

What is the significance of flipped phospholipids?

Answer 26

When PS flips, it signals several biological processes:] - apoptosis--programmed cell death, important for development - hemostasis--important for blood clotting - RBC aging--recognition by macrophages

Question 27

annexin V

Answer 27

- high affinity for PS | - can be used to identify different pathologies

Question 28

annexin V

Answer 28

- high affinity for PS | - can be used to identify different pathologies

Question 29

N-linked glycosylations

Answer 29

formed in the ER

Question 30

CD (proteins)

Answer 30

- cluster differentiation - used in diagnostics to identify surface proteins? - CD markers on integral membrane proteins

Question 31

fatty acyl group

Answer 31

synthesized in cytosol | anchored to membrane on cytosol side

Question 32

GPI-linkage

Answer 32

occurs in lumen of ER (like N-linked sugars)

Question 33

How does paroxysmal nocturnal hemoglobinuria (PNH) demonstrate the importance of GPI-linkages?

Answer 33

In PNH, hemolysis of RBC's is the consequence of complement activity in normal serum (set of proteins put holes in membranes, killing cells). This complement system is generally inactive due to the presence of GPI-linkages. It is caused by a mutation of the PIG-A (phosphatidylinositol glycan class A) gene in marrow still cells. In other words, the GPI-linked moiety can't be put on proteins. Normally, GPI-linkages act as an inhibitor to the complement activation pathway. Complement activation leads to the leakage of hemoglobin and red urine. Flow cytometry can be used to diagnosis PNH using CD 55 or CD 59 (antibodies).

Question 34

lectin

Answer 34

"sugar-loving" | important for cell-cell interactions (binds with surface carbohydrates)

Question 35

lipid rafts & caveolae

Answer 35

- cholesterol- & sphingolipid-rich (microdomain) - lipids and proteins together exist in outer portion of PM - brings receptors and signaling molecules together

Question 36

different ways to bring material into cell?

Answer 36

1. phagocytosis: 2. pinocytosis: 3. endocytosis:

Question 37

Na+ ion concentration inside and outside cell (mM)

Answer 37

- intracellular: 5-15 mM (low inside) | - extracellular: 145 mM (high outside)

Question 38

K+ ion concentration inside and outside cell (mM)

Answer 38

- intracellular: 140 mM (high inside) | - extracellular: 5 mM (low outside)

Question 39

Ca++ ion concentration inside and outside cell (mM)

Answer 39

- intracellular: 10^-4 mM (extremely low inside) | - extracellular: 1-2 mM (very high outside)

Question 40

how are ion levels inside/outside cell controlled?

Answer 40

- channel-mediated--bulk flow of ions (passive) | - transport protein--requires ATP, one ion at a time (active)

Question 41

What role do microvilli play in Na+ absorption?

Answer 41

- presence of microvilli increases absorption of nutrients such as Na+ - passive transport of Na+ into epithelial cell (paired with glucose) - Na+/K+ pump on basal domain pumps Na+ out of cell into basement membrane vasculature (needs ATP, moving against gradient)

Question 42

functions of PM

Answer 42

- pivotal in: endocytosis/exocytosis, adhesion, movement (chemotaxis), signal transduction - selectively permeable barrier controlling movement of nutrients and waste