Cell Membranes

Question 1

What is cellularity?

Answer 1

Universal property of life

Question 2

What is the basic structure of a membrane?

Answer 2

Thin (5nm) film of lipids and proteins held together by non-covalent interaction
The lipid bilayer forms an impermeable barrier to water soluble/ polar molecules

Question 3

What are the 3 types of membrane lipid?

Answer 3

Glycerophospholipids - Glycerol molecules is bonded to 2 fatty acids and a phosphate group which is bonded to an R group.
Sphingolipids - 2nd carbon is bonded to an amine group - only contain saturated fatty acids so are straight
Sterols- cholesterol - Rigid part made of rings which are connected to a polar head group and a non- polar hydrocarbon tail which decreases membrane fluidity

Question 4

Outline the link between thermodynamics and the lipid bilayer

Answer 4

Polar molecules dissolve- increased entropy
Non polar molecules dont dissolve - water molecules form an ice like cage structure around the molecule which decreases entropy

Question 5

How is a lipid bilayer formed?

Answer 5

Hydrophobic tails cluster together
The edges of the bilayer then meet to form a continuous spheroid and a sealed compartment

Question 6

How do cells respond to damage to their membranes?

Answer 6

Phospholipids are created in the cell

Question 7

What is the fluid mosaic model?

Answer 7

Fluid - lipids can move in the layer
Mosaic - Different shapes and sizes of molecules

Question 8

What are the 3 main methods of fluidity?

Answer 8

Lateral diffusion - phospholipids move around within the layer
Flexion - movement of the tail
Rotation- aids membrane function

Question 9

What are the regulators of membrane fluidity?

Answer 9

Unsaturated double bonds - as they dont pack together tightly and make the membrane thinner
Cholesterol - Decreases membrane fluidity as phospholipids cant move laterally but they prevent them from compacting and becoming too rigid

Question 10

What are lipid rafts?

Answer 10

Randomly distributed with mainly sphingolipids as they have long fatty acid chains and strong attractive forces to hold adjacent molecules together, and cholesterol - proteins are recruited in this area which increases the efficiency of processes

Question 11

What are glycoproteins and glycolipids?

Answer 11

Proteins and lipids that are bonded to a carbohydrate
These created the CHO coating of the membrane called the glycocalyx which is protective

Question 12

What are the membrane proteins?

Answer 12

Integral/ transmembrane proteins- single pass and multi pass
Peripheral membrane proteins -
embedded within the layer or covalently bonded to a lipid or non covalently attached to integral membrane proteins

Question 13

What is the difference between integral and peripheral membrane proteins?

Answer 13

Integral cross the entire membrane from outer to inner layer (multi pass cross multiple times)
whereas
Peripheral do not cross the entire membrane

Question 14

What 2 factors impact transport across the membrane?

Answer 14

Size and charge/ polarity of the molecule

Question 15

State the molecules and how easily they can move across the membrane

Answer 15

No problem - non polar molecules
Ok - Small uncharged polar molecules
Probably - Large uncharged polar molecules
Not at all - charged, large and polar / ions

Question 16

What are all transport proteins?

Answer 16

Trans multi pass proteins

Question 17

What are carrier proteins?

Answer 17

Bind a solute and undergo a conformational change to transfer across the membrane

Question 18

What are channel proteins ?

Answer 18

Interact only weakly with solute, form aqueous pore that solutes can pass through quickly - selective - size and charge of the ion thus only allowing the correct ions through

Question 19

What is facilitated diffusion?

Answer 19

Movement of a solute down a concn gradient through a partially permeable membrane

Question 20

What is passive transport?

Answer 20

No energy required
Simple diffusion
Channel mediated
Carrier mediated

Question 21

What is the electrochemical gradient?

Answer 21

If an ion has a charge then concn and charge determine the direction of flow
e.g. +ve ions outside of the membrane will be attracted to the negative charge on the inside whereas -ve ions will be attracted to the outside

Question 22

What is active transport?

Answer 22

Pumping solutes across a membrane against their concn gradient which requires energy e.g. energy from electrons transfer

Question 23

What are coupled carriers?

Answer 23

Secondary active transport
Symporters - move 2 solutes in the same direction- one down its concn gradient which releases energy for the other solute to move against its gradient
Antiporters- do the same but move them in different directions

Question 24

What are ATP driven pumps?

Answer 24

Primary active transport
e.g. Na+k+ ATPase pump - free energy released by ATP hydrolysis to actively pump 3 Na+ out of the cell and 2K+ into the cell

Question 25

What are the different types of channel proteins?

Answer 25

Voltage gated - change in membrane potential to open the gates
Ligand gated channels - Only opened by the binding of a ligand
Mechanically gated - disruption in the membrane causing the gate to open