Cell Membranes : Structure And Function

Question 1

Describe the chemical compositions of cell membrane

Answer 1

Membranes are dynamic structures in which proteins float in a sea of lipids
Eukaryotic cells contain internal membranes that form boundaries of the organelles
Membranes are sheet-like 60-100 angstroms

Question 2

How are bacteria classified and how do the membranes differ

Answer 2

Bacteria are classified into gram negative which has 2 membranes
And gram positive which has a single membrane

Question 3

What properties do fatty acids present in lipids?

Answer 3

Hydrophobic properties
It may be saturated or unsaturated

Question 4

What are the 3 common types of membrane lipids?

Answer 4

Phospholipids
Glycolipids
Cholesterol

Question 5

What are the properties of membrane lipids

Answer 5

Amphipathic - form bimolecular sheets in solution.
Polar heads - hydrophilic
Non polar hydrocarbon tails- hydrophobic

Question 6

What are peripheral and integral membrane proteins?

Answer 6

Peripheral membrane proteins bind to the surface of integral proteins
The integral proteins interact with the hydrocarbon region

Question 7

where is alpha helices found in membrane

Answer 7

Found in bacteriohodopsin
Certain non-polar amino acid and very few are charged

Question 8

How are beta strands found in membrane

Answer 8

Found I porin
Each strand is diagonally hydrogen bonded to its neighbour- antiparallel
Forms a single beta sheet which curls to form a hollow cylinder
Porin has hydrophilic and hydrophobic AA in adjacent positions
Hydrophilic residues tend to lie outside the structure

Question 9

How come sometimes detergents are needed to release proteins and in what protein

Answer 9

Eg prostaglandin and synthase-1
Lies along the outset studrgace of the membrane bound by alpha helices
The alpha helices extend into the membrane
The linkage is strong enough that only detergents can release the proteins
Technically integral, although not membrane spanning

Question 10

What is the fluid mosaic model

Answer 10

All is lateral no event but not rotation though the membrane
Lateral diffusion of proteins is rapid
Spontaneous rotation/ flip flop is slow

Question 11

How is membrane fluidity controlled

Answer 11

Fatty acid composition and cholesterol content
Cholesterol is the key regulator and disrupts interactions between fatty acid chains
- also forms specific complexes with soem phospholipid
Makes membrane more rigid and less subject to phase transition s

Fatty acids composition - number of double bond or chain length
Packing of fatty acid chains is disrupted by cis double bonds
Increased chain length decrease membrane fluidity

Question 12

What is the role of cell membrane

Answer 12

• prevent molecules generated from cell to leak out
• barriers that define inside and outside of cells
  -prevent unwanted molecules from diffusing into a cell
• contain transport systems that allow spruced molecules to be taken up and removed

Question 13

How does receptor-mediated endocytosis (LDL)?

Answer 13

• LDL. binds to a specific receptor
• The complex invaginates to form an internal vesicle
  -The vesicle separates from its receptors and fuses with a lysosome
  -LDL is degraded and it’s cholesterol is released

Question 14

Name the different types of secondary transporter

Answer 14

Anti-porter - eg. Sodium-calcium exchanger - molecules flow in opposite directions

Symporters- eg. Sodium-glucose symporter - molecules flow in the same direction

Question 15

What does eversions means?

Answer 15

Turning inside out

Question 16

what are phospholipids made up of

Answer 16

Made up of two fatty acids, glycerol, phosphate and an alcohol

Question 17

what is cholesterol formed from

Answer 17

four linked hydrocarbon rings
▪ Oriented parallel to fatty acid chains in membranes
▪ Hydroxyl group interacts with nearby phospholipid heads

Question 18

what can be predicted from the AA sequence

Answer 18

Transmembrane proteins can be predicted from amino acid sequences
Amino acids with more negative transfer free energy values are more likely
to be located in an aqueous environment

Question 19

what do hydropathy plots predict

Answer 19

Hydropathy plots can predict the window of 20 residues from which the
transmembrane domain originates

Question 20

what a lipid bilayers held by and what are its properties

Answer 20

Lipid bilayers are held by covalent bonds
o No exposed hydrocarbon chains – form compartments
o Self-sealing – holes are energetically unfavourable

Question 21

what transports require a mixture of ATPase and secondary transporters:

Answer 21

Na+/K+ ATPase converts free energy into a sodium ion gradient
o This gradient is used to pump other materials into cells using secondary transporters
▪ E.g. sodium-glucose symporter
The mechanism of P-type ATPase action

Question 22

describe the the mechanism of P-type ATPase

Answer 22

• The Na+
  /K+ ATPase binds with ATP and 2 calcium ions
• Phosphorylation then occurs and the ATP converts to ADP and is lost leaving the Phosphate attached onto the complex.
• eversion occurs
• the calcium molecules are released
• hydrolysis occurs removing the phosphate

Question 23

In a phospholipid what bond is present

Answer 23

Ester bond bewteren phosphate and alcohol

Question 24

What is cholesterols

Answer 24

Membrane lipid based on steroid nucleus