Membranes 2.1.5

Question 1

What is compartmentalisation?

Answer 1

The formation of seperate membrane bound areas in a cell

Question 2

What are the roles of membranes?

Answer 2

• Partially permeable barriers between the cell and its environment, between organelles and the cyptoplasm and within organelles
• Sites of celll communication

Question 3

Membrane structure

Answer 3

• The cell surface membrane which seperates the cell from its external environment is known as the plasma membrane.
• Phospholipid bilayer = Hydrophillic (phosphate) head facing outward and hydrophobic tails facing inwards

Question 4

Fluid mosaic model

Answer 4

• Proposed by Singer and Nicolson
• Known as fluid mosaic molecule because phospholipids are free to move within the layer relative to eachother, giving the membrane flexibility. And because the proteins embedded in the bilayer vary in shape, size and position

Question 5

Intrinsic proteins

Answer 5

• Transmembrane proteins which are embedded through both layers of membrane
• They have amino acids with hydrophobic R groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.
• Channel and carrier proteins are both intrinsic
• Channel proteins = provide a hydrophillic channel which allows for passive movement of polar molecules and ions down a concentration gradient through membranes. They are held in position by interactions between the hydrophobic core and hydrophobic R groups on outside
• Carrier proteins have an important role in both passive transport (down concentration gradient) and active transport (against gradient) into cells which often involves shape of the protein changing.

Question 6

Glycolipids

Answer 6

• Lipids with carbohydrate chain attatched
• These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self or non self

Question 7

Extrinsic proteins

Answer 7

• AKA peripheral proteins
• Normally have hydrophillic R groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins.
• Can be presdent in either layer and some move between them

Question 8

Glycoproteins

Answer 8

• Are intrinsic proteins
• Embedded in cell surface membrane with carbohydrate chain attatched of varying lengths and sizes.
• They play a role in cell adhesion and as receptors for chemical signals
• When chemicals bind to receptors it elicits a response from the cell, this is cell signaling, examples include: neurotransmitters, peptide hormones
• Some drugs act by binding to cell receptors e.g. beta blockers are used are used to reduce the response of the heart to stress.

Question 9

Cholesterol

Answer 9

• A lipid with a hydrophillic end and a hydrophobic end
• Positioned between phospholipids in the membrane bilayer
• Regualtes membrane fluidity and stability

Question 10

Cell signalling in membranes

Answer 10

• The protein receptors are glycoproteins that have a complementary binding site to a specific hormone on the extra celular side of the membrane
• The hormone does not enter the cell but the message is relayed to the inside of the cell by the second messenger.

Question 11

Receptors as drug binding sites (e.g. antihistamine)

Answer 11

• Agonist + receptor = drug action/effect
• antagonist + receptor = no drug action/effect
• Antihistamine has a similiar shape to the signalling molecule, can bind to the complementary receptor and block or enhance its action. The antihistamine blocks the receptor so less histamine binds to receptor and inflammation is reduced.

Question 12

Beta blockers

Answer 12

• Beta blocker block access of adrenaline to target tissue

Question 13

Site attachment for the cytoskeleton in the cell membrane

Answer 13

• Anchoring point for cytoskeleton
  -Microfilaments attach to move the plasma membrane

Question 14

cell to cell recognition and cell to cell adhesion

Answer 14

• Glycoproteins act as antigens on the surface of the cell

Question 15

Diffusion

Answer 15

The net movement of a substance down its concentration gradient until equilibrium is reached. The molecules have their own kinetic energy, there is no need for an input of energy.

Question 16

Simple diffusion

Answer 16

• Diffusion of molecules through the phospholipid bilayer
• This includes small nonpolar molecules (CO2, O2), H2O, lipid soluble hormones

Question 17

Facilitated diffusion

Answer 17

• Diffusion of molecules through transmembrane proteins
• There are 2 types of facilitated diffusion requiring 2 types of proteins:
• Channel proteins/ion channels = contain hydrophillic channels, allow diffusion of charged particles/ions across a membrane
• Carrier proteins = Diffusion of small organic molecules, e.g. glucose or amino acids, where the conformation (shape) of protein changes

Question 18

Active transport

Answer 18

Movement of molecules against their concentration gradient, using carriers as proteins (often called pumps) and input of energy.

Question 19

Factors affecting membrane structure - temperature

Answer 19

• When temperature increases the phospholipids will have more energy and will move more. This makes the membrane more fluid and it begins to lose its structure.
• The loss of structure increases the permeability of the membrane, making it easier for particles to cross it.
• Carrier and channel proteins in the membrane will be denatured at higher temperatures. These proteins are involved in transport across the membrane so as they denature membrane permeability will be affected.

Question 20

Factors affecting membrane structure - solvents

Answer 20

• Water (a polar solvent) is essential in the formation of the phospholipid bilayer.
• Organic solvents which are less polar than water, e.g. alcohols, can dissolve into membranes and disrupt cells.
• Pure or very strong alcohols are toxic as they destroy cells in the body. Less concentrated solutions of alcohols will not destroy but will damage cells.
• The non polar alcohol molecules can enter the cell membrane and the presence of these molecules between phospholipids disrupts the membrane.
• When the membrane is disrupted it becomes more fluid and more permeable.

Question 21

What is water potential?

Answer 21

The ability of a solution to donate water to another solution (measured in kPa)

Question 22

Define osmosis

Answer 22

The net movement of water from a higher water potential to a lower water potential across a partially permeable membrane (until equilibrium is reached)

Question 23

Dilute / less concentrated solutions

Answer 23

Hypotonic solution, higher water potential

Question 24

Less dilute / more concentrated solution

Answer 24

Hypertonic solution, lower water potential

Question 25

Pure water potential

Answer 25

• Pure water has a potential of 0kPa
• The more concentrated the solution the more negative the water potential

Question 26

How can the rate of osmosis be increased?

Answer 26

• The use of protein channels called aquaporins. These are inserted into the membrane by the exocytosis of vesicles.
• The active transport of ions, to lower the water potential and create a gradient across the membrane