structure

Question 1

how does the phospholipid bilayer form

Answer 1

The phospholipids align as a bilayer due to the hydrophillic/polar heads being attracted to water and the hydrophobic/non-polar tails being repelled by water.

Question 2

role of cholesterol

Answer 2

• regulation of fluidity
• The purpose of cholesterol is to restrict the lateral (sideways) movement of other molecules in the membrane.
• It is useful because it makes the membrane less fluid at high temperatures, which prevents water and dissolved ions leaking out of the cell.
  (- if they did leak this could cause dehydration, or cells bursting.)
• also prevents the membrane becoming too rigid in cold conditions.
• prevents the movement of water soluble molecules across the cell surface membrane.

Question 3

role of extrinsic proteins

Answer 3

• provide mechanical support, or connect to proteins or lipids to make glycoproteins and glycolipids. (if they are attached to a protein or a phospholipid)
• their function is cell recognition as receptors that trigger certain functions. Or they can function as antigens.
• examples incude glycoproteins

Question 4

role of intrinsic proteins

Answer 4

protein carriers or channel proteins
- involved in the transport of molecules across the membrane.
protien carriers
- will bind with other carrier proteins or larger molecules (e.g glucose, amino acids)
- This causes the carrier protein/ molecule to change shape (using energy) in order for them to be transported to the other side of the membrane
- mainly involved in active transport.
channell proteins
- hollow tubes that can fill with water
- This enables water soluble ions (e.g sodium) to dissolve and diffuse through the channel.
- mainly involved in facilitated diffusion
- can be selective (specific ones for specific molecules)

Question 5

what molecules can pass through the plasma membrane

Answer 5

Lipid soluble molecules/substances (e.g some hormones)
-Very small molecules (e.g CO2, O2, H2O)
-hydrophobic molecules
-molecules with a particular channel or carrier protein

Question 6

what molecules cannot pass through the membrane

Answer 6

• Water soluble (polar) substances
• Large molecules

Question 7

how temperature effects membrane permeability

Answer 7

• at a certain temperature increase the membrane starts to lose its integrity. The permeability increases and temperature increases.
• This is because with more kinetic energy the phospholipids move around more and there are bigger gaps between them. (intermolecular forces break)
• more molecules can pass through these gaps, increasing permeability.
• As temperature increases, membrane proteins also start to denature (bonds break in tertiary and quaternary structure) and this may create wider, non specific channels for molecules to travel through.

Question 8

how solvents effect membrane permeability

Answer 8

• phospholipds hold their form because of hydrophillic associations between the polar heads and the surrounding water, and the hydrophobic tails being repelled by water.
• As surrounding ethanol concentration increases, there is less water present for the polar heads to be attracted to, and the hydrophobic tails also dissolve in the ethanol.
• because of this the phospholipids don’t hold their shape as a bilayer and start to disperse. Fatty acid tails can kink and bend.
• This leaves gaps molecules can travel through, and permeability increases.
• proteins could also disperse and denature
• denatured proteins could create non-specific channels for molecules to travel through

Question 9

why is ethanol bad when drinking alcohol and good in wipes.

Answer 9

BAD:
(when drinking alcohol)- disrupts neurone membranes, allowing the diffusion of unwanted ions.
- this can trigger unwanted or unregulated nerve impulses to be transmitted.
GOOD:
(in wipes)- wipes contain a high ethanol concentration which disrupts bacterial membranes.
- if ethanol destroy the membrane enough it kills the bacteria.

Question 10

evidence for the fluid moasic model
(just name as many as possible, atleast 3)

Answer 10

1) TEM showing bilayer
2) Frye and Edidin (1970) – fusion of human and mouse cells with different fluorescent tags in the membranes of the two cell types
3) Study of ion channels by neuroscientists
4) SEM freeze fracture showing membranes
5) It explains how the properties of membranes change with temperature
6) It explains the association between proteins and membranes (e.g. hydrophilic and hydrophobic areas of intrinsic proteins)
7) It explains the dynamic nature of membranes (endo- and exocytosis etc.)