6 - Cell membranes

Question 1

Structure of the Phospolipid bilayer?

Answer 1

Hydrophilic head
Hydrophobic tail
Forms bilayer, heads out tails in, when in the water of the cytoplasm
Dosent allow in water soluble substances

Question 2

What does the membrane contain?

Answer 2

Phospholipids
Proteins
Glycoproteins
Glycolipids
Cholesterol

Question 3

Why is it called a fluid-mosaic model?

Answer 3

The phospholipid and protein molecules fit together to form a continuous pattern - mosaic
Fluid, the components are in constant motion and move laterally - fluid

Question 4

Role of phospholipids?

Answer 4

Head is hydrophilic - attracts water
Tail is hydrophobic - repels water
Forms bilayer
Centre of bilayer is hydrophobic - doesn’t allow water-soluble substances (ions) through it

Question 5

Role of cholesterol?

Answer 5

Provides stability by preventing too much movement in the cell

Question 6

Diffusion?

Answer 6

Net movement of particles from an area of high concentration to an area of low concentration
It is a passive process

Question 7

Factors affecting the rate of diffusion?

Answer 7

Concentration gradient
Length of diffusion pathway
Size of molecules or ions
Temperature - affects kinetic energy
Surface area

Question 8

What can diffuse through the phospholipid bilayer?

Answer 8

Small molecules
Non polar molecules - lipid soluble
O2, CO2 , Ethanol

Question 9

Why can’t ions diffuse through the phospholipid bilayer?

Answer 9

Water soluble (polar) and thus can’t go through the hydrophobic phospholipid bilayer

Question 10

How do ions get diffuse through the phospholipid bilayer?

Answer 10

Sodium ions are the main example
Channel proteins
cell specific sized
Passive
Facilitated diffusion
Cell can change the number of channels present, the more channel proteins the more ions diffuse through
Can use gated channels to control the diffusion of ions
‘Voltage-gated channels’ use an electrical charge to open their channels

Question 11

How does glucose pass through the phospholipid bilayer?

Answer 11

Large and have small charges on their OH groups means they can’t pass through the phospholipid tails
glucose binds temporarily to carrier proteins which changes shape and the glucose moves through
Passive
Facilitated transport

Question 12

Structure of channel and carrier proteins

Answer 12

Ion specific due to their specific territory structure

Question 13

How does water pass through the phospholipid bilayer?

Answer 13

Do have dipoles but are very small
Can diffuse through by osmosis using aquaporins - water channel proteins
Cell membranes are selectively permeable so water needs the aquaporins

Question 14

What is osmosis?

Answer 14

The net movement of water molecules from an area of high water potential to an area of lower water potential through a partially permeable membrane - moves down a water potential gradient

Question 15

The unit for water potential?

Answer 15

psi Ψ, Pure water has a Ψ of 0, water potential units are therefore negative

Question 16

What is active transport?

Answer 16

The movement of particles from an area of low concentration to an area of high concentration using energy produced from the hydrolysis of ATP
In the phospholipid bilayer, the active transport of ions uses carrier proteins

Question 17

Why do sodium ions help runners?

Answer 17

Stimulates the rate of glucose uptake in the blood as specialised carrier proteins in the cell surface membranes of the epithelial cells lining of the small intestine which co-transport glucose and sodium into the cell at the same time.

Question 18

Co-transport of Na+ and glucose

Answer 18

• Sodium ions actively transported out of the ileum epithelial cells into the blood, by sodium potassium pump
• Sodium ions diffuse from lumen of the ileum into the epithelial cell, down the concentration gradient
• Co-transporter carries glucose in the cell with sodium
• Concentration of glucose increases
• Glucose diffuses out of the cell into the blood through a protein channel by facilitated diffusion
• Sped up by large surface area of microvilli
• Sped up by having large number of carrier proteins
• Sped up by having a lot of mitochondria

Question 19

Test for permeability of cell membrane?

Answer 19

Use scape and cut five equally sized pieces of beetroot. Rinse to remove and pigment
Add five pieces to the test tube, each containing 5cm3 of water. Use a measuring cylinder or pipette to measure water.
Place each test tube in a water bath at a different temperature for the same time
Remove the pieces of beetroot from the tube leaving just the coloured liquid
Now use a colorimeter - passes light through and see how much is absorbed, higher the absorbance the more pigment released so the higher the permeability
Connect colorimeter to a computer and use software to collect data and create a graph

Question 20

Effect of temperature on the permeability of the membrane?

Answer 20

1. Below 0C - membrane is rigid as the molecules don’t have much energy - channel proteins and carrier proteins deform, increasing the permeability - ice crystals may form and pierce the membrane making it highly permeable
2. Temperatures between 0 and 45C - Partially permeable - molecules not packed together and not rigid - as it increases becomes more permeable as the molecules move more
3. Above 45C - Bilayer starts to melt (break down) - more permeable - Water expands and pressures the membrane - Channel proteins and carrier proteins deform - increases the permeability

Question 21

What is facilitated diffusion?

Answer 21

Down a concentration gradient - higher to lower concentration
Passive
Carrier proteins move large molecules
Channel proteins for ions - ion specific

Question 22

Making concentrated solutions?

Answer 22

Divide the moles by 1
Divide the volume by the answer
Thats how many cm3 of the solvent there is
Minus that from the total volume to find the volume of water needed