Lecture 13: Membranes, Organelles, Cytoskeleton

Question 1

Lysosomes

Answer 1

Single layer vesicles with acidic lumen containing enzymes for breakdown of biochemical macromolecules, endocytosed material, and old organelles. Primary site of proteolysis.

Question 2

Lysosomal storage diseases

Answer 2

Caused by sphingolipid buildup in lysosomes due to inability to break down, e.g. Tay-Sachs disease

Question 3

Peroxisomes

Answer 3

Single layer vesicles carrying hydrogen peroxide, responsible for beta-oxidation of cellular lipids and synthesis of some glycolipids

Question 4

Spherocytes

Answer 4

RBCs shaped as spheres; due to hereditary spherocytosis leading to hemolytic anemia

Question 5

Major plasma membrane lipids

Answer 5

1. Glycerophospholipids
2. Sphingolipids
3. Cholesterol

Question 6

How does cholesterol affect the lipid bilayer?

Answer 6

Its steroid rings partially immobilize the FA tails of phospholipids, creating stiffer membranes and regulating membrane fluidity

Question 7

How does cholesterol regulate membrane fluidity?

Answer 7

“Anti-freeze” effect; at cold temperatures, cholesterol prevents phospholipids from condensing too much and freezing. At hot temperatures, cholesterol holds phospholipids together, preventing dissolution.

Question 8

Types of membrane protein

Answer 8

1. Integral
2. Peripheral

Question 9

Integral membrane proteins

Answer 9

Proteins that cross both layers of a phospholipid bilayer

Question 10

Peripheral membrane proteins

Answer 10

Proteins that only interact with one layer of a phospholipid bilayer

Question 11

What causes atypical “walking” pneumonia?

Answer 11

Mycoplasma consume membrane cholesterol, increasing membrane fluidity leading to limp cilia. Mucus accumulation provides a growth medium for more bacteria.

Question 12

Factors that affect membrane fluidity

Answer 12

1. Temperature (hotter = more fluid)
2. FA chain length (longer = less fluid)
3. Unsaturation (more = more fluid)
4. Cholesterol (more = less fluid)

Question 13

Liposomes

Answer 13

Small artificial vesicle with phospholipid bilayer; materials can be embedded in the membrane or within the lumen. Often used for targeted drug delivery.

Question 14

Main components of the cytoskeleton

Answer 14

1. Actin
2. Microtubules
3. Intermediate filaments

Question 15

Microtubule structure

Answer 15

αβ tubulin heterodimers form long cylinders of non-covalent αβ polymers (protofilaments), which assemble laterally into microtubules (~13 proto-f in mammals). Capping proteins stabilize ends and nucleated at centrosome from (-) end.

Question 16

Different types of tubulin

Answer 16

α-tubulin: towards (-) end (slower growing end)
β-tubulin: towards (+) end (faster growing end)
γ-tubulin: template for correct assembly

Question 17

How are microtubules regulated?

Answer 17

Various proteins for assembly, stabilizing, destabilizing, cutting, and motor movement (kinesin +, dynein -).
Also tubulin isoforms, post-trans. mods

Question 18

Microtubule therapeutics

Answer 18

Taxol (stabilizes microtubules), colchicine (inhib. polymer.), vinblastine (inhib. polymer.); these stop cell function by preventing microtubule growth.

Question 19

Intermediate filament structure

Answer 19

α helical monomer -> coiled coil dimer -> staggered tetramer -> 2 packed tetramers, staggered -> 8 tetramers twisted into a filament

Question 20

Intermediate filament roles

Answer 20

Mechanical integrity, sequester/position/scaffold signaling molecules

Question 21

Actin structure

Answer 21

Double stranded helical polymers of monomeric actin forming linear bundles, 2D networks, and 3D gels with faster growing (+) end and slower growing (-) end

Question 22

Types of actin monomer

Answer 22

1. α
2. β
3. γ

Each actin filament has only 1 type of actin