Properties of membranes

Question 1

Structure of phospholipids

Answer 1

-Phospholipids contain two fatty acid molecules bonded to a glycerol molecule, which is also bonded to a phosphate group

Question 2

Charges and hydrophillic/hydrophobic regions

Answer 2

-The fatty acid molecules are non-polar, so are hydrophobic
-The phosphate group is highly charged (polar) so is hydrophillic
-Therefore phospholipids contain both a hydrophobic region and a hydrophilic region

Question 3

Phospholipid bilayer

Answer 3

-Because phospholipids contain both hydrophobic and hydrophilic regions, when placed in water they form a phospholipid bilayer
-In this bilayer, the hydrophilic phosphate heads face outwards (towards the water), while the hydrophobic fatty acid tails are buried in the center, away from the water
-The phospholipid bilayer is the basis of all membranes
-This can be either the cell surface membrane, or the membrane of cell organelles

Question 4

Function of membranes (basic)

Answer 4

-BARRIER
-Membranes can act as a barrier between the internal contents of a cell and the external environment, or the contents of an organelle and the cytoplasm
-LOCATION FOR CHEMICAL REACTIONS
-The membrane can also act as a location for chemical reactions, for example some of the stages of respiration
-CELL SIGNALLING
-The cell surface membrane can also be involved in cell signalling

Question 5

-How does the cell surface membrane prevent molecules from entering

Answer 5

-The cell surface membrane acts as a barrier between the internal contents of the cell and the external environment
-Hydrophobic substances, such as steroid hormones, can easily pass through the hydrophobic core of the cell surface membrane, as they are both non-polar (uncharged)
-However, hydrophilic substances cannot easily pass through the hydrophobic core of the cell surface membrane, as they are polar or charged and are therefore repelled by the non-polar core
-However, water molecules can pass through the cell surface membrane easily, even though it is polar, because it is extremely small (even so this takes place at a slow rate)

Question 6

How does the structure of cholesterol help it to attract the phospholipid molecules

Answer 6

-Cholesterol molecules have a polar hydrophilic group on one side of the molecule, which helps to attract the polar hydrophilic head of the phospholipid molecule
-The rest of the molecule is non-polar and hydrophobic, which helps to attract the hydrophobic fatty acid tails of the phospholipid molecule

Question 7

Function of cholesterol in the membrane

Answer 7

-Cholesterol interacts with the phospholipids, which increases the strength of the membrane
-This makes the membrane more stable and less likely to get damaged
-Cholesterol also reduces the sideways movements of phospholipids and other molecules within the membrane
-This helps to control the fluidity of the membrane
-This prevents the membrane from becoming too fluid under warm conditions and too rigid under cold conditions
-Finally, by packing the spaces between the phospholipid molecules, cholesterol helps to reduce the movement of hydrophillic molecules across the cell membrane

Question 8

Why is it called the fluid mosaic model

Answer 8

-Fluid because the phospholipids and proteins can move around within the membrane, making it flexible
-Mosaic because the membrane is composed of multiple proteins with many different shapes and sizes

Question 9

Membrane proteins

Answer 9

Membrane proteins can be split into two broad categories; intrinsic/ integral and extrinsic/ peripheral proteins

Question 10

Intrinsic proteins

Answer 10

-Intrinsic proteins are proteins that span the length of the cell membrane from one side to the other
-As they pass through the phospholipid bilayer, intrinsic proteins have hydrophobic amino acids on the outside surface of the protein
-These hydrophobic amino acids can interact with the hydrophobic fatty acid tails on the inside of the bilayer
-Intrinsic proteins can be further split into channel and carrier proteins

Question 11

Channel proteins

Answer 11

-Channel proteins are intrinsic proteins that contain a channel that runs down the center of the protein
-This channel is lined with hydrophilic amino acids and is filled with water
-This channel allows polar or charged molecules to pass through the phospholipid bilayer

Question 12

Carrier proteins

Answer 12

-Carrier proteins are also intrinsic proteins
-Carrier proteins can change their shape or position to transfer a polar or charged molecule from one side of the membrane to the other

Question 13

Extrinsic proteins

Answer 13

-Extrinsic proteins do not span the membrane and are found on one side of the membrane or the other

Question 14

Functions of extrinsic proteins

Answer 14

-Some extrinsic proteins have a structural ole in the membrane
-Others act as enzymes
-Some extrinsic proteins act as receptors for other molecules such as hormones

Question 15

Glycoproteins

Answer 15

-Glycoproteins are carbohydrate molecules that attach to many membrane proteins

Question 16

Function of glycoproteins

Answer 16

-Some glycoproteins can allow cells to attach to each other to form tissues e.g nervous tissue
-Some glycoproteins are used to present antigens on the cell surface membrane for T cells
-Some glycoproteins are used as receptors for hormones

Question 17

Glycolipids definition and function

Answer 17

-Carbohydrate molecules can also attach to the phospholipid molecules, and these are known as glycolipids
-Glycolipids are often used when cells come into contact with each other, as glycolipids on the surface membrane of cells are recognised by other cells and can determine whether cells come into contact
-Glycolipids can also act as antigens e.g in determining blood group

Question 18

Core practical investigating effect of variables on membrane permeability (AIM temp)

Answer 18

Aim: To investigate the effect of temperature on membrane permeability using the pigment betalain released from beetroot as an indicator

Question 19

Method (temp on membrane permeability)

Answer 19

1. Cut up the beetroot into 1cm length cylinders using a cork borer and a ruler
2. Gently rinse off the beetroot pieces to remove excess surface pigment
3. Prepare water baths at different temperatures e.g 10C, 30C, 50*C etc
   4.Place a beetroot piece into a test tube with 5cm3 of distilled water
4. Place the test tubes into the water baths at different temperatures for 10-15 minutes
5. Remove the beetroot pieces from the test tube and carefully swirl the solution
6. Use a colorimeter to measure the absorbance of the solution
   -The higher the reading of the colorimeter, the higher the membrane permeability of the beetroot was
   -Compare the membrane permeability at each temperature and plto a graph of membrane permeability against temperature

Question 20

Control variables (temp on membrane permeability)

Answer 20

-Size and surface area of beetroot piece
-Time left in water bath
-Volume of distilled water in test tube (5cm3)
-same beetroot
-wavelength used on colorimeter

Question 21

Risk assessment

Answer 21

-Scalpel used to cut beetroot pieces
-Could cut themselves while cutting the beetroot
-To prevent this cut away from the self