1.3 membrane structure

Question 1

what does the ability of a compound to dissolve in water depend on?

Answer 1

whether the molecule is hydrophilic or hydrophobic

Question 2

when are molecules hydrophilic / hydrophobic?

Answer 2

• organic compounds which are hydrophilic tend to be polar molecules, and are soluble in water
• organic compounds which are hydrophobic tend to be non-polar molecules and are not soluble in water

Question 3

what is polarity?

Answer 3

• ability of a molecule to form a positive and/or a negative charge at parts of the molecule (dipole moment)

Question 4

difference between polar and non-polar?

Answer 4

• non-polar molecules do not form charges and hence cannot interact with water molecules (usually via hydrogen bonds) and hence are not soluble

Question 5

what is a phospholipid?

Answer 5

• a phospholipid molecule is considered to be a bipolar molecule, with one end being polar (charged) while the other end being non-polar (not able to form charges)
• phosphate (PO4-) end of a phospholipid molecule is polar and hence is hydrophilic and is able to attract and form weak bonds with other polar molecules like water
• other end is made up of two fatty acid tails (chains) and this region retains the non-polar nature and is hydrophobic
• can form monolayers, bilayers and micelles

Question 6

how do phospholipids form bilayers?

Answer 6

• most cells have water based environment on both sides of membrane
• water attracts the polar phosphate ends of the phospholipid molecules
• phospholipids align themselves to form a bilayer, with the polar ends in contact with water and the hydrophobic non- polar fatty acid tails inside the bilayer away from the water molecules

Question 7

how was it first discovered that the plasma membrane was a bilayer?

Answer 7

• with the advent of electron microscopy, scientists discovered that the plasma membrane is a bilayer, made up of two layers of material
• presence of 2 layers of dark material that sandwiches a lighter layer in between
• Davson-Danielli models explains this observation as arising from proteins on both surfaces surrounding a core made up of phospholipids

Question 8

what is the davson-danielli model?

Answer 8

• protein-lipid “sandwich”
• lipid bilayer made up of phospholipids with hydrophilic heads pointing outwards and hydrophobic tails pointing inwards
• proteins coat the surface and stabilises the bilayer and does not penetrate into the bilayer

Question 9

how was davson-danielli model falsified?

Answer 9

• use of freeze-fracturing to study cell membrane
• cells rapidly frozen and fractured and viewed under
  an electron microscope
• fracture occurs along lines of weakness, including the center of membranes
• fracture shows an irregular surface within the phospholipid bilayer, showing the presence of globular structures within the bilayer
• globular structures most likely proteins that were integral (and hence within) to the phospholipid bilayer, which falsified the Davson-Danielli model of having proteins only outside the phospholipid bilayer

Question 10

what replaced davson-danielli model?

Answer 10

singer-nicolson model (fluid mosaic theory)

• proposed by singer and nicolson in 1972
• states that many proteins found within plasma membrane, within phospholipid bilayer
• supported by evidence from freeze fracturing analysis of the plasma membrane
• “fluid” in the sense that the phospholipid molecules are not bound in a fixed crystal lattice structure, but can move independently like a fluid
• “mosaic” in the sense that the entire membrane is made up of smaller phospholipid components

Question 11

what are membrane proteins?

Answer 11

• membrane is a collage of different proteins embedded within the fluid matrix of the phospholipid layer
• proteins determine most of the specific functions of the plasma membrane
• weak hydrogen bonds between the polar regions of the proteins and the phospholipids keep the entire membrane stable

Question 12

difference between integral and peripheral membrane proteins

Answer 12

INTEGRAL
- permanently embedded within the membrane and may go all the way through and emerge from one surface (monotopic) or both surfaces (polytopic) of the plasma membrane

PERIPHERAL
- do not interact with the hydrophobic core of the phospholipid bilayer, and are loosely bound to the plasma membrane via interactions with the hydrophilic heads of the bilayer

Question 13

what are the major functions of membrane proteins?

Answer 13

J – joining / junctions (intercellular joining where it serves as a junction btwn 2 cells)
E – enzyme (fixing to membrane localises metabolic pathways)
T – transport (facilitated diffusion and active transport)
R – recognition (cell-cell recognition)
A – anchorage (attachment to cytoskeleton and extracellular matrix (ECM))
T – transduction (signal transduction; functions as receptors for peptide hormones)

Question 14

how do transport membrane proteins work?

Answer 14

• some act as a hydrophilic channel that spans across the membrane that allows for specific molecules to pass through the membrane past the hydrophobic region of the plasma membrane
• others shuttle substances from one side to the other side of the plasma membrane by binding to the molecule and changing shape, with some being able to spontaneously do so or with the expenditure of energy from ATP

Question 15

how do membrane proteins carry out enzymatic activity?

Answer 15

• protein embedded in a membrane may be an enzyme with their active site exposed to substances either in contact with the inner or outer surface of the membrane
• the enzymes may be organised as a group to carry out sequential steps in a metabolic pathway
• ATP synthase is an example of an embedded enzyme found on the inner membrane of mitochondria

Question 16

how do membrane proteins carry out signal transduction?

Answer 16

• membrane protein (receptor) may have a binding site on one end of the protein, usually on the external surface of the plasma membrane
• a messenger molecule, usually in the form of a hormone like insulin and adrenaline, will bind to the membrane receptor proteins when they are released into the bloodstream
• induces a change in shape of the membrane protein and this can relay the message into the cell (either by releasing another molecule within the cell, or binding to specific proteins in the cytoplasm)
• steps are as follows: reception, transduction, response

Question 17

how do membrane proteins carry out cell-cell recognition?

Answer 17

• cells recognise each other by binding to surface molecules which are often carbohydrates found on the plasma membrane
• membrane carbohydrates are covalently bonded to lipids (glycolipids) or proteins (glycoproteins) found in the plasma membrane
• carbohydrates found on the external side of the plasma membrane can vary from species to species, and can even vary within the different cell types in an organism
• example of one such glycoprotein are the antigens that determine the blood type of a person (A or B antigens)

Question 18

how do membrane proteins contribute to intercellular joining?

Answer 18

• membrane proteins of adjacent cells may bind together to form various types of junctions like gap junctions or tight junctions
• glycoproteins may be involved in cell adhesion

Question 19

how do membrane proteins carry out attachment to cytoskeleton and extracellular matrix?

Answer 19

• microfilaments or other elements of the cytoskeleton may be non-covalently bound to membrane proteins, to maintain cell shape or stabilise the location of certain membrane proteins

Question 20

do membrane proteins move? what experiment was conducted?

Answer 20

• frye and edidin conducted an experiment where the cells of a mouse and a human were used and their plasma membrane proteins were labelled differently (mouse labelled w rhodamine, human labelled w fluorescein), and the cells were fused
• experiment showed that the membrane proteins distributed themselves evenly, proving that membrane proteins can move

Question 21

how is plasma membrane fluid?

Answer 21

• phospholipids within the plasma membrane can move freely
• most of the lipids, and some membrane proteins, drift laterally (side to side)
• rare but possible for a phospholipid molecule to flip flop transversely across the membrane
• as membrane cools, membrane will gradually switch from a fluid to a more solid state
• temp at which a membrane solidifies depends on the types of lipids within the membrane
• membranes rich in unsaturated fatty acids are more fluid compared to those rich in saturated fatty acids
• membranes must remain fluid in order to work properly

Question 22

fatty acid tails

Answer 22

• fatty acids can be saturated or unsaturated
• degree of saturation can be seen to refer to the degree of “saturation of hydrogen”
• more carbon-carbon double bonds present, more unsaturated the molecule as they have the potential to be further saturated with hydrogen atoms
• carbon-carbon double bonds are not straight, and this can tilt the rest of the fatty acid tail to one side, increasing the overall volume taken up by the molecule
• spaces the phospholipids further apart and makes the membrane more fluid at a given temperature as compared to a membrane consisting of more saturated fatty acids

Question 23

what is cholesterol?

Answer 23

• steroid that makes up about 20% of the lipids found in animal cells, but are rarely found in plant cells
• fits between phospholipid molecules and gives the membrane greater stability
• affects membrane differently at different temperatures, but will always result in greater membrane stability
• higher temp, cholesterol restricts the movement of phospholipids and prevents membrane from being too fluid
• lower temp, cholesterol prevent phospholipids from packing too closely together to maintain fluidity