Membrane Transport

Question 1

What is the cell membrane made from?

Answer 1

Phospholipid bilayer
Cholesterol
(Transmembrane proteins - go right across from one end to the other end) Proteins
Carbohydrates

Question 2

Structure and function of phospholipid bilayer

Answer 2

Phospholipids forms the basic structure of the cell membrane
Polar hydrophilic head (includes phosphate group) - water loving
Non-polar hydrophobic (fatty acid) tails prevent water-soluble substances from crossing, forming a boundary/fence - water hating
(Phospholipids can move side to side and rotate but they rarely flip to the other layer)

Question 3

Role of cholesterol in the cell membrane

Answer 3

Cholesterol molecules are present in this phospholipid bilayer
Stiffens membrane
Further decreases the water solubility of the membrane
(Cholesterol can flip easily to the other layer)

Question 4

Role of proteins in the cell membrane

Answer 4

Proteins can determine what functions (e.g facilitated diffusion, communication, transport etc) the membrane can perform
Proteins with different shapes have different functions
E.g of roles of proteins: transport, communication (acting as receptors for signal molecules) and joining cells to each other and to the extracellular matrix
Integral proteins are embedded in the lipid bilayer
Peripheral proteins are anchored to the membrane or to other proteins

Question 5

Role of carbohydrates in the cell membrane

Answer 5

Acts as identity molecules - for example during development so cells can sort themselves into tissues and organs. And allows immune cells to recognise our own cells and pathogens
Carbohydrates are found only on the outer surface of the membrane. Together, the carbohydrates on the outer surface of the cell membrane form a coating called the GLYCOCALYX
Glycoproteins - Carbohydrates attached to proteins
Glycolipids- Carbohydrates attached to lipids

Question 6

How do the hydrophilic polar heads and hydrophobic fatty acid tails of the phospholipid bilayer behave with the intracellular and extracellular fluid?

Answer 6

Polar heads face the water inside and outside of the cell
Non-polar hydrophobic fatty acid tails face away from the water

Question 7

What can/cannot enter the cell?

Answer 7

Can pass through:
Water - although it’s polar, it’s small so can pass through cell membrane via diffusion. But it passes very slowly
Small non-polar molecules, e.g gases, CO2, O2
Steroid hormones (steroids are already part of the membrane so it’s lipid soluble so can pass through)

Can’t pass (requires help to pass through):
Charged molecules, e.g ions
Large molecules, e.g glucose (because it’s large and polar)

Question 8

Osmosis definition

Answer 8

Net movement of water (a solvent) across a semi-permeable membrane by diffusion (from lower solute concentration to higher solute concentration)

Question 9

Definition and units of osmolarity. And does temperature or pressure affect it?

Answer 9

Definition: The number of solute particles present in 1 litre of a solution
Units: Osm/L or osmol/L
Independent of temperature and pressure (as the mass/moles don’t change with temperature or pressure)

Question 10

Definition and units of osmolality. And does temperature or pressure affect it?

Answer 10

Definition: The number of solute particles dissolved in 1 kg of water
Units: Osm/kg or osmol/kg
Independent of temperature and pressure

Question 11

Definition of osmole

Answer 11

Number of moles of solutes that contribute to the osmosis pressure of a solution