Transport across membranes

Question 1

Describe the structure of a phospholipid molecule and explain how phospholipids are arranged in a plasma membrane (3 marks).

Answer 1

1- Glycerol joined to two fatty acid tails Phosphate group joined to glycerol on opposite side. (joined by condensation reaction with ester bond).;
2- Phospholipid has hydrophilic head (phosphate and glycerol) and hydrophobic tails (fatty acid chains)
3- Arrange to form a phospholipid bilayer; (Hydrophilic head facing out. Hydrophobic fatty acid chains facing in)

Question 2

Function of cholesterol in fluid-mosaic model

Answer 2

1- decreases permeability and increases the stability of the membrane
2- more cholesterol = less fluidity of the membrane
*different types of cell have different proportions of cholesterol

Question 3

Function of channel proteins in fluid-mosaic model

Answer 3

1- allow specific charged ions or small molecules to move across the membrane by facilitated diffusion
extra:
- Na+ ions can only pass through sodium ion channel proteins
- proteins have specific tertiary structure, so they are specific and can only transport molecules that are complementary to the shape of the channel protein

Question 4

function of carrier proteins in fluid-mosaic fluid

Answer 4

1- carrier proteins aid the transport of ions/polar molecules and large molecules by facilitated diffusion and active transport

Question 5

function of receptor proteins in fluid-mosaic fluid

Answer 5

1- other protein molecules act as specific receptors for complementary molecules

*only specific molecules can bind to specific receptor proteins

Question 6

function of enzymes in fluid-mosauc fluid

Answer 6

enzymes active site is specific and complementary to its substrate
1- allowing them to form enzyme-substrate complexes

Question 7

function of glycoproteins in fluid-mosaic fluid

Answer 7

are produced in golgi body and are composed of carbohydrate and protein
1- are on the outer surface of the membrane and are important in cell recognition, often acting as antigens

Question 8

function of aquaporins in fluid-mosaic fluid

Answer 8

1- specific types of ‘channel proteins’ specific to water
- a lot of aquaporins= very permeable to water and will carry out osmosis easily

Question 9

Define diffusion and list the 4 factors affecting rate of diffusion

Answer 9

diffusion is the net movement of molecules from an area of higher concentration to a lower concentration across a partially permeable membrane
1- temperature
2- surface area
3- concentration gradient
4- diffusion distance

Question 10

define Fick’s law

Answer 10

surface area x concentration gradient
divided by–
diffusion distance

Question 11

how does temperature affect rate of diffusion

Answer 11

increased kinetic energy therefore faster rate of diffusion of molecules

Question 12

how does surafce area affect rate of diffusion

Answer 12

-largER SA provides more space for molecules to pass through therefore fastER
-this allows for more proteins to be present

Question 13

how does diffusion distance affect rate of diffusion

Answer 13

1- the shorter the diffusion distance, the faster molecules will travel from one area to the next

Question 14

define osmosis

Answer 14

osmosis is the net movement of water molecules from highER WATER POTENTIAL to a solution with lowER WATER POTENTIALthrough a SELECTIVELY permeable membrane

Question 15

What is pressure/water potential (kPa)

Answer 15

pressure exerted when free water molecules are able to collide with the membrane