Cell Membrane, Part 2

Question 1

membrane dynamics

Answer 1

• flexibility: ability to change shape without losing integrity or becoming leaky
• basis for this are non covalent interactions among lipids in the bilayer (thus, mobility)
• structure and flexibility of bilayer depend on kinds of lipids present
• changes with temperature

Question 2

vesicle

Answer 2

• intracellular structures consisting of liquid enclosed in lipid bilayer
• form naturally during:
  
  1. secretion (exocytosis)
  2. uptake (endocytosis)
  3. membrane transport

Question 3

liposomes

Answer 3

• synthetic spherical bilayers vesicles

- model membranes in experimental studies

Question 4

black membranes

Answer 4

• planar bilayers
• formed across a hole in a partition btwn 2 aqueous compartments
• used to measure permeability properties of synthetic membranes

Question 5

cell membrane fluidity

Answer 5

• movement of phospholipids within lipid bilayer

Question 6

transversal diffusion

Answer 6

• flip-flop

- happens rarely

Question 7

lateral diffusion

Answer 7

• happens readily and rapidly

Question 8

flippases and floppases

Answer 8

• need ATP

Question 9

scramblases

Answer 9

• does not need ATP

- special transport proteins move phospholipid and other lipids in the membrane

Question 10

lipid bilayer

Answer 10

• fluidity must be precisely regulated
  
  • depends on its composition and temperature
• stabilized by hydrophobic interactions btwn lipids’ fatty acid chains

Question 11

phase transition

Answer 11

• change of a lipid bilayer from a liquid to a 2-D rigid crystalline state (gel) at a characteristic temperature
• phase transition and temp at which this occurs is lower if phospholipid chains are short or have double bonds

Question 12

fluidity depends on:

Answer 12

• lipid composition
  
  • more cis-double bonds -> more fluid
    
    • double bonds make it difficult to pack chains together
    • more spread apart -> form a thinner membrane
  • shorter chain: reduces tendency of hydrocarbon tails to interact with one another -> membrane remains fluid at lower temps
• cholesterol content
• temperature
  
  • at low temps, lipids show less movement
  • lipid is in paracrystalline state (more rigid state)
  • at higher temps (20-40C), plasma membrane becomes more fluid

Question 13

saturated fatty acids

Answer 13

• tend to form paracrystalline structures (less space btwn phospholipid tails
• the more saturated fatty acids a lipid bilayer contains, the higher the paracrystalline-to-fluid transition temp of a membrane

Question 14

unsaturated fatty acids

Answer 14

• yields to formation of kinks -> inhibits paracrystalline conformation (more space btwn tails)

Question 15

lipid bilayer components

Answer 15

• also contains cholesterol and glycolipids
• eukaryotic plasma membranes contain large amounts of cholesterol
• cholesterol molecules improve permeability-barrier properties of lipid bilayer
  
  • orient themselves with hydroxyl groups close to polar heads of phospholipid molecules
  • rigid steroid ring can support hydrocarbon chains and stabilize them
  • inhibits possible phase transitions

Question 16

cholesterol

Answer 16

• orients itself in bilayer with its small hydroxyl groups close to the polar head groups of adjacent phospholipids and the hydrophobic chain btwn phospholipid tails (hydrophobic interaction)
• helps keep membrane more stable, stiffening bilayer and making it less fluid and less permeable
• stops tails from clumping and interacting
• helps membrane remain fluid

Question 17

cell membrane transport

Answer 17

• passive transport

- transporters or permeases

Question 18

passive transport

Answer 18

• simple diffusion
• facilitated diffusion
• electrochemical gradient
• channels

Question 19

transporters or permeases

Answer 19

• carriers
• active transporters
• pumps
• symport systems

Question 20

extracellular matrix (ECM)

Answer 20

• complex and intricate network of macromolecules
• provides structural and biochemical support to surrounding cells
• common functions:
  
  • cell adhesion, cell-cell communication and differentiation
• animal ECM includes: interstitial matrix and basement membrane/basal lamina

Question 21

interstitial matrix

Answer 21

• present btwn diff animal cells (in intercellular spaces)

- gels of polysaccharides and fibrous proteins fill intercellular spaces -> act as compression buffer to ECM

Question 22

basement membrane

Answer 22

• sheet-like depositions of ECM with epithelial cells

Question 23

connective tissue with respective type of ECM

Answer 23

• bone tissue: collagen fibers and bone mineral
• loose connective tissue: reticular fibers and ground substance
• blood: ECM is blood plasma

Question 24

extracellular matrix

Answer 24

• macromolecules in ECM produced locally by cells in the matrix and secreted via exocytosis

Question 25

fibroblasts

Answer 25

• in connective tissue (collagen fibers)
• secrete ECM
• chondroblasts - cartilage
• osteoblasts - bone tissue

Question 26

major classes of macromolecules in ECM

Answer 26

• mammals have >300 matrix proteins
  1. glycosaminoglycans (GAGs, ex: cartilage): large and highly charged polysaccharides
  
  • when linked to proteins -> proteoglycans
    
    2. fibrous proteins: like collagen (ex: skin and bone)
    3. non-collagen fibrous proteins: such as glycoproteins (elastin, fibronectin, laminin)

Question 27

collagen

Answer 27

• fibrous, long, stiff, triple-stranded helical proteins

- ‘fibrils’ rich in proline and glycine and glycosylated

Question 28

types of collagen and their properties

Answer 28

Type I: fibril-forming (fibrillar)
- bone, skin, tendons, ligaments, cornea, internal organs (accounts for 90% of body collagen)

Type III: fibril-forming (fibrillar)
- skin, blood vessels, internal organs

Type IV: network-forming
- basal lamina

Question 29

Elastin

Answer 29

• gives tissues their elasticity (skin, blood vessels, lungs)
• hydrophobic protein rich in proline and glycine (like collagen) but is NOT glycosylated
• molecules are joined together by strong covalent bonds to generate a cross-linked network
• each elastin molecule can extend and contract resembling a random coil, so entire assembly can stretch and recoil like a rubber band

Question 30

In ECM there are also glycoproteins

Answer 30

• with multiple domains each with specific binding sites for other matrix macromolecules and for receptors of the cell surface
• ex: Fibronectin (2500 AAs long) helps organize ECM and cells attached to it

Question 31

basal lamina

Answer 31

• aka basement membrane
• specialized form of ECM
• thin, flexible, tough -> essential component of all epithelia

Question 32

laminin

Answer 32

• primary organizer of sheet structure of basal lamina
• composed of 3 long polypeptide chains held together by DISULFIDE BONDS
• multi domain protein
• asymmetric molecule

Question 33

integrins

Answer 33

• transmembrane cell adhesion proteins
• act as ‘matrix receptors’
• link cytoskeleton with ECM