membrane structure

Question 1

Phospholipids

Answer 1

phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules

Question 2

structure of phospholipids

Answer 2

1. polar head - hydrophilic polar head that is composed of a glycerol and a phosphate molecule
2. non polar tails - hydrophobic two non polar tails composed of fatty acid (hydrocarbon) chains
3. amphipathic - contains both hydrophilic and lipophilic regions

Question 3

arrangement in membranes

Answer 3

phospholipids spontaneously arrange into a bilayer. the hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while the two hydrophilic head regions associate with the cytosolic and extracellular fluids respectively.

Question 4

properties of the phospholipid bilayer

Answer 4

1. bilayer held together by weak hydrophobic interactions
2. Hydrophobic/hydrophilic layers restrict the passage of many substances
3. individual phospholipids can move within the bilayer
4. fluidity allows spontaneous breaking and reforming of membranes

Question 5

membrane proteins

Answer 5

phospholipid bilayers are embedded with proteins which may either be permanently or temporarily attached to the membrane

Question 6

two proteins at membrane

Answer 6

1. integral proteins - permanently attached to the membrane and are typically transmembrane
2. Peripheral proteins - temporarily attached by non covalent interactions and associate with one surface of the membrane

Question 7

phospholipid bilayer

Answer 7

Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane

Question 8

Structure of Membrane proteins

Answer 8

amino acids of a membrane protein are localised according to polarity; Non polar (hydrophobic) and polar (hydrophilic)

Question 9

location of amino acids (polar and non polar)

Answer 9

1) Non-Polar amino acids associate directly with the lipid bilayer
2) Polar amino acids are located internally and face aqueous solutions

Question 10

tertiary structure if Transmembrane

Answer 10

1) Single helices (helical bundles)
2) Beta barrels (common in channel proteins)

Question 11

Integral Proteins

Answer 11

1) a-helix (Recognition, receptors)
2) Helical bundle (Enzymes, transporters, receptors)
3) b-barrel (Transporters, channel proteins)

Question 12

Peripheral proteins

Answer 12

1) Enzymes
2) Anchorage
3) Transporters (carriers)

Question 13

Functions of Membrane Proteins

Answer 13

1) Junctions - connect and join 2 cells together
2) Enzymes - Fixing membranes to localise metabolic pathways
3) Transport - facilitated diffusion and active transport
4) Recognition - markers for cellular identification
5) Anchorage - attachment points for cytoskeleton and extracellular matrix
6) Transduction - receptors for peptide hormones

Question 14

Cholesterol

Answer 14

Cholesterol is a component of animal cell membranes where it functions to maintain integrity and mechanical stability (absent in plant cells since they have a rigid cell wall made of cellulose)

Question 15

cholesterol molecule type

Answer 15

amphipathic molecule (both hydrophobic and hydrophilic regions)

• Cholesterol hydroxyl is hydrophilic and aligns towards the phosphate heads of phospholipids
• the remainder of the molecules is hydrophobic and associates with the phospholipid tails

Question 16

Cholesterol function

Answer 16

1) interacts with the fatty acid tails of phospholipids to moderate its properties
2) immobilises the outer surface of the membrane, reducing fluidity
3) makes membrane less permeable to very small water soluble molecules
4) separate phospholipid tails and prevent membrane to crystallise
5) helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

Question 17

cell membranes respresented through fluid mosaic model du to:

Answer 17

1) being Fluid - phospholipid bilayer is viscous and individual phospholipids can move position
2) being Mosaic - phospholipid bilayer is embedded with proteins, resulting in a mosaic of components

Question 18

components of the plasma membrane

Answer 18

1) Phospholipids - Form a bilayer with phosphate heads facing outwards and fatty acid tails facing inwards
2) Cholesterol - found in animal cell membranes and functions to improve stability and reduce fluidity
3) Proteins - May be either integral (transmembrane) or peripheral and serve a variety of roles

Question 19

Danielli and Davson model

Answer 19

proposed in 1935, model whereby two layers of protein flanked a central phospholipid bilayer. Lipid layer was sandwiched between two protein layers.

Question 20

problems of the Danielli and Davson model

Answer 20

1) constant lipid-protein ratio
2) all membranes had symmetrical surfaces
3) did not account permeability of certain substances
4) temperature was inaccurate

Question 21

Singer-Nicolson Model

Answer 21

promoted in 1972, model preferred by scientist (accurate). Known as the fluid mosaic model