Week 3 - Organelles, Cell Growth and Cytoskeleton

Question 1

Functions of plasma membrane

Answer 1

Compartmentalisation
Barrier
Transport
Response (receptors)
Interaction (e.g. intracellular signalling)

Question 2

Components of the plasma membrane

Answer 2

• Lipid bilayer (including cholesterol)
• Glycoproteins
• Transmembrane structural proteins
• Transmembrane channels

Question 3

Why is fluidity important in plasma membranes?

Answer 3

• Provides compromise between rigid structure and fluid liquid
• Allows movement of membrane proteins for specific activities
• Allows growth of membrane with new components
• Allows coordinated movement of cells

Question 4

Application of lipid bilayer

Answer 4

Liposome and drug delivery

Question 5

Integral membrane proteins

Answer 5

• Transmembrane by penetrate into
  lipid bilayer
• Hydrophobic portion (usually in
  helical structure) in contact within
  bilayer
• Hydrophilic portion outside bilayer
• May form channel (hydrophilic
  side wall) for transport

Question 6

Peripheral membrane proteins

Answer 6

• Non-covalently attached to
  hydrophilic lipid ends or integral
  proteins ends
• Those found on cytosolic membrane
  surface function as membrane
  skeleton or signaling molecules
• Those found on external surface are
  part of extracellular matrix

Question 7

Facilitated diffusion

Answer 7

• Diffusion whereby
  substance binds specifically to
  membrane spanning protein and
  diffuses through it
• There is no release of energy in
  facilitated diffusion (vs active
  transport)
• Binding is possible on both sides of
  transporter

Question 8

Ion pumps/active transport

Answer 8

Ions gradients are needed and
maintained by pumps in active
transport

3 Na+ cytoplasmic side -> phosphorylation
Change of transporter conformation
2 K+ bind to outside -> dephosphorylation
Release of 2 K+ inside the cell

Question 9

Mitochondria

Answer 9

Centre of oxidative metabolism

Question 10

Endomembrane pathways

Answer 10

1. Constitutive secretory pathway: continual vesicular transport from the trans Golgi network to the plasma membrane (e.g. collagen release + cell membrane growth)
2. Regulated secretory pathway: release of hormones, triggering a response
3. Endocytic pathway

Question 11

Function of rough ER in the endomembrane system

Answer 11

Membrane biosynthesis, synthesis of membrane lipids

Question 12

Golgi complex

Answer 12

Flattened, disc-like
Cis: ER, trans: plasma membrane
Vesicles binding at trans side

Question 13

Protein coats of transport vesicles

Answer 13

COP II: ER to Golgi
COP I: Golgi to ER
Clathrin: Trans golgi to endosome/lysosome (endosome is to uptake molecules)

Question 14

Targetting of lysosomal enzymes

Answer 14

1. Enzyme synthesised in RER, glycosylated and phosphorylated -> mannose-6-phosphate side chain
2. M6P detected by receptors (MPR) with a coat protein and adaptor protein (Δ), at TGN and form vesicles
3. Δ dissociation from vesicle, enzyme transported to endosome
4. MPR returned to trans golgi network to restart cycle or present in plasma membrane

Question 15

Cytoskeleton

Answer 15

Made of microtubules, microfilaments and intermediate filaments

Question 16

Functions of cytoskeleton

Answer 16

Scaffold, hold organelles, intracellular transport, movement and aid in cell cycle

Question 17

Microtubules are made of…

Answer 17

• α-tubulin has a bound GTP which is not
  hydrolyzed
• β-tubulin has a bound GDP which can be
  exchanged for GTP

Question 18

Microtubule motor proteins are found in the forms of…

Answer 18

Kinesins (2 heavy 2 light; towards + direction)
Dyenins (2 heavy + variety of intermediate/light chains; towards - direction)

Question 19

Microtubules: motor proteins

Answer 19

Head - binds to the microtubules
Tail - binds to vesicles

Observe conformational change in the head after ATP binds to another head
“hand-over-hand”

Question 20

Microtubules mechanism

Answer 20

• rearrangement of existing microtubules
• disassembly of existing microtubules and reassembly of new ones
• dynamic changes associated with GTP hydrolysis

found in cell periphary

Question 21

Microfilaments: motor protein

Answer 21

Myosin (motor protein for actin)
* Conventional myosin (type II)
found in most muscle and non-
muscle cells
* Head bind to actin and ATP
(distinct domains)
* 2 light chains are associated and
form filaments

Question 22

Function of microfilaments

Answer 22

Vesicle trafficking, lateral mevement of membrane proteins, cytokinesis, phagocytosis

Question 23

Actin polymerisation and cell signalling

Answer 23

• Actin polymerization required for
  cell movement and change in
  shape
• Cell shape change mediated by
  signaling molecules
• Signaling molecules displays binary
  switch by state of GTP