Topic 4 - Transport Across Membranes

Question 1

Cell Membrane Function

Answer 1

1. Controls what enters + leaves the cell

Question 2

What are the three ways substances move across cell membranes

Answer 2

1. Diffusion (Simple and Facilitated)
2. Osmosis
3. Active Transport (including Co-transport)

Question 3

Cell Membranes are described as permeable. What does this mean?

Answer 3

1. Has tiny holes running through it

2. Some substances can pass through. Others cannot

Question 4

Cell Membrane Structure

Answer 4

1. Cell Membranes consist of lipids (mainly phospholipids), proteins and carbohydrates (usually joined to proteins +lipids)
2. “Fluid mosic” model is used to describe arrangement of molecules in cell membrane / phospholipid bilayer

Question 5

Fluid Mosaic Model

Answer 5

1. Used to describe arrangement of molecules in cell membrane
2. Phospholipid molecules form a continuous bilayer (double layer)
3. “Fluid” because phospholipids constantly moving
4. “Mosaic” because proteins scattereted throughout bilayer similar to mosaic tiles. This includes carrier proteins, channel proteins and receptor proteins
5. These proteins are either in fixed positions or can sometimes move sideways
6. Cholestrol also present in cell membranes

Question 6

Function of Phospholipids in cell membrane

Answer 6

1. Phospholipids consist of a large glycerol molecule bonded to 2 fatty acid tails and a phosphate head by ester bonds
2. The phosphate group / head is hydrophillic. It attracts water
3. The fatty acid tails are hydrophobic. Repel water
4. This causes phospolipid molecules to automatically arrange themselves into a bilayer (double layer) where the phosphate heads point outwards towards water on each side and the fatty acid tails point inwards away from water on each side
5. So, centre of bilayer is hydrophobic. Water soluble substances cannot enter

Question 7

Function of cholestrol in cell membrane

Answer 7

1. Type of lipid
2. Present in all cell membranes except bacteria
3. Fit between the phospholipids in the bilayer and join the fatty acid tails together, making the phospholipids more closely backed together. Therefore, restricts movement of phospholipids
4. This makes the bilayer less fluid, more rigid
5. Helps animal cells keep their shape (they do not have a cell wall to do this)

Question 8

Glycolipids

Answer 8

1. Lipids with a polysaccharide chain attached

Question 9

Glycoproteins

Answer 9

2. Proteins with a polysaccharide chain attacehd

Question 10

Receptor Proteins

Answer 10

1. Found on cell surface membranes
2. Detect chemicals released by other cells
3. These chemicals signal the cell to respond accordingly

Question 11

Factors affecting Cell Membrane Permebility

Answer 11

1. Solvent conc

2. Temp

Question 12

Investigating how temp affects cell membrane permeability

Answer 12

1. Use beetroot cells
2. Because they release a coloured pigement
3. The more permeable the cell membrane of beetroot cells, the more coloured pigement released
4. Cut up 5 beetroot pieces. Same size (length, width, height). Then rinse with water, to remove any coloured pigement which may have leaked when beetroot was being cut
5. Place each piece in a test tube with 5cm3 water
6. Place each test tube in a different waterbath with a different temp (10,20,30,40,50). For same amount of time
7. Then remove test tubes from water bath + remove the beetroot pieces from the test tubes, so the test tubes only contain the coloured solutions
8. Then, use a colorimeter. This is a machine which passes light through a solution and measures how much light is absorbed. The more light absorbed, the more pigement released and therefore, the more permeable the membrane
9. Beforer using the colorimeter, ensure it is calibrated to zero by passing light through pure water and give it 5 mins to stabilise

Question 13

How temp affects membrane permeability

Answer 13

below 0: Phospholipids in bilayer do not have much energy. Do not move much. Phospholipids more closely packed together Bilayer more rigid, less fluid. However channel proteins and carrier proteins deform, therefore, they cannot control what enters and leaves the cell through the membrane, increasing membrane permeability
0-45: Phospholipids in bilayer have more energy. Move more, phospholpids less closely packed together. Bilayer more fluid, less rigid. This increases permeability. Here, the membrane is partially permeable
above 45: Phospholipid bilayer melts and breaks down. Water inside the cell expands, putting pressure on the membraneThe channel proteins and receptor proteins deform. They cannot control what enters and leaves cell. Permeability increases

Question 14

How solvent conc affects membrane permeability

Answer 14

Increasing conc of solvent, increases permeability of membrane
Solvent dissolves the lipids in the bilayer, causing it to lose its structure

Question 15

Simple Diffusion

Answer 15

1. Net movement of particles from a region of high concentration to low concentration, down a concentration gradient. This is a passive process. Directly though cell membrane
2. Only small molecules and uncharged particles can move through cell membranes via simple diffusion. E.g. CO2, and O2. Small molecules can fit between the spaces between the phospholipids in the bilayer. Charged particles are water soluble. Not lipid soluble, cannot dissolve in hydrophobic centre of bilayer

Question 16

Facilitated Diffusion

Answer 16

1. Net movement of charged particles and large molecules, down a concentration gradient, from an area of high conc to low conc, facilitated by channel proteins or carrier proteins. Passive process.
2. Large molecules would take too long to move acroess the membrane by simple diffusion. Molecules would not fit between the gaps between phoshpolipids in the bilayer
3. Charged particles would also take too long. They are water soluble. Not lipid soluble and would not dissolvel in the hydrophobic centre of bilayer
4. Therefore, chargd particles and large molecules move across cell membrane by faciliated diffusion using carrier proteins and channel protiens

Question 17

Carrier Proteins

Answer 17

1. Facilitate the diffusion of large molecules
2. Different carrier proteins facilitate the diffusion of different large molecules
3. The large molecule attaches to the carrier protein on cell membrane
4. This causes carrier protein to change shape
5. Therefore, large molecule is released on opposite side of membrane

Question 18

Channel Protein

Answer 18

1. Facilitates diffusion of charged particles (polar molecules / ions)
2. Different channel proteins facilitatethe diffusion of different charged particles
3. Chanel proteins form pores in the membrane for the charged particle to diffuse through

Question 19

Factors affecting rate of simple diffusion

Answer 19

1. Conc gradient: The higher the conc gradient, the faster the rate of simple diffusion. (Difference in conc between each side of the membrane)
2. Thickness of exchange surface: Thinner = faster rate. Less distance for particles to diffuse across
3. Surface area of exchange surface: Larger SA = faster rate

Question 20

Factors affecting rate of facilitated diffusion

Answer 20

1. Conc gradient: The higher the conc gradient, the faster the rate of facilitated diffusion. (Difference in conc between each side of the membrane)
   ]2. No. of carrier proteins: The more carrier proteins = faster rate. If all carrier proteins are in use, even if conc gradient is increased, rate will not increase, carrier protein no. is limiting factor, must be increased to increse rate
2. No. of channel proteins: Higher no. = higher rate

Question 21

Osmosis

Answer 21

1. Diffusion of water molecuels from a region of high water potential to low water potential across a partially permeable membrane
2. Passive process
3. Water potential = conc of water molecules
4. Pure water has highest WP. Any other solution as WP lower than pure water
5. Isotonic = 2 solutions with same WP

Question 22

Factors affecting rate of osmosis

Answer 22

1. Water potential gradient. The higher the WP gradient, the higher rate of osmosis
2. Thickness of exhange surface: Higher = higher rate
3. SA of exchange surface: higher SA = higher rate

Question 23

Active Transport

Answer 23

Net movement of molecules / ions against a concentration gradient, from area of low conc to high conc, using energy released from hydrolysis of ATP

Carrier Proteins are involved

Question 24

Factors affecting rate of active transport

Answer 24

1. No. of carrier proteins. Higher no. = faster rate
2. Rate of respiration. Respiration produces ATP. Active transport requires energy from hydrolysis of ATP into ADP and Pi to occur
3. Speed of individual carrier proteins: The faster they work, faster rate

Conc does not affect rate of active transport. Active transport moves molecules / ions against conc gradient

Question 25

Differences between active transport and faciitated diffusion

Answer 25

1. Active transport moves molecules / ions against conc gradient. Facilitated diffusion moves ions / molecules along conc gradient
2. Active transport is an active process. It requires energy from ATP. Facilitated diffusion is a passive process. It does not require energy
3. Facilitated diffusion involves use of channel proteins and carrier proteins. Active transort only involves use of carrier proteins

Question 26

Co-Transporter Carrier Proteins

Answer 26

1. Type of carrier protein
2. Uses conc gradient of one molecule to move the other against its conc gradient
3. For example, sodium-glucose co-transporter proteins

Question 27

Co-Transport

Answer 27

1. Type of active transport which uses co-transporter carrier proteins
2. Glucose is absorbed by co-transport from the ileum (final part of small intestine) into the blood stream
3. This is because, in the ileum, the conc of glucose is low. And in the blood, conc is high
4. Sodium ions are actively transported out of ileum epithelial cells into the blood via sodium-potassium pump. This creates a conc gradient. There is a higher conc of Na+ in lumen of ileum than in epithelial cells
5. Na+ then are transported back into epithelial cells from lumen of ileum via sodium-glucose co-transporter proteins
6. Now, the conc of glucose in the ileum epithelial cells is high
7. Glucose then diffuses from epithelial cells into blood stream, down conc gradient, by facilitated diffusion, by through channel proteins