[3.2.3] - Transport Across Cell Membranes (+RP3)

Question 1

Describe the roles of membranes

Answer 1

• controls what substances enters and leaves the cell
• partially permeable (only letting certain molecules in the membrane)
• substances can mobe via diffusion, osmosis or active transport

Question 2

Describe the “Fluid Mosaic Structure”

Answer 2

• a model which describes the arrangements of molecules in the membrane
• presents phospholipid molecules to form a continuous bilayer
• the bilayer is “fluid” because the phospholipid is constantly moving
• channel proteins and receptor proteins are scattered around the bilayer (like a mosaic) enabling the cell to detect chemicals and transport molecules in the cell
• some proteins mive sideways while others are fixed
• some proteins have a polysaccharide attached - these are called “glycoproteins”
• some lipids have a polysaccharide attached - these are called “glycolipids”

Question 3

Describe the role of phospholipids for transport

Answer 3

• forms a bilayer
• “hydrophilic head” attracts water
• “hydrophobic tail” repels water
• head faces outwards (towards water) and tails face inwards (away from water)

Question 4

Describe the role of cholesterol in the membrane

Answer 4

• fits between the phospholipids
• binds to the hydrophilic tail of phospholipids causing them to pack more closely
• causes the membrane to become more rigid and help maintain the shape of the animal cell

Question 5

What is diffusion

Answer 5

The net movement of particles from an area of high concentration to an area of low concentration until there is an even distribution of particles

• occurs down a concentration gradient
• passive process (no energy required)
• across a cell membrane

Question 6

Describe Facilitated diffusion and describe the process of which it happens

Answer 6

the process of passive transport of large molecules across a membrane, with the help of carrier proteins and channel proteins.

How does it work:

1) Large molecule attaches to a carrier protein in the membrane
2) The protein changes shape
3) The molecule is released on the opposite side of the membrane

Question 7

Name and explain the factors that affect the rate of diffusion

Answer 7

1) “The Concentration Gradient”: higher conc. grad = faster diffusion (rate of diffusion slows down as it gets closer to reaching equilibrium)
2) “Thickness of Exchange Surface”: thinner exchange surface = faster diffusion (shorter distance travelled)
3) “Surface Area”: larger surface area = faster diffusion
4) “The Number of Channel/Carrier Proteins”: the grater number of channel/carrier proteins the faster rate of diffusion

Question 8

Define osmosis

Answer 8

The net movement of water across a partially permeable membrane from a area of “high water potential” (low solute conc.) to an area of “low water potential” (high solute conc.)

Question 9

Name and explain 3 factors affecting the rate of osmosis

Answer 9

1) “The Water Potential Gradient”: higher water potential gradient = faster osmosis
2) “Thickness of Exchange Surface”: thinner surface = faster osmosis
3) “Surface Area”: larger surface area = faster surface area

Question 10

[Required Practical 3] How would you investigate the water potential of potatoes using serial dilutions

Answer 10

1) Make a series of dilutions of 1m sucrose solutions (0.0, 0.2, 0.4, 0.6, 0.8, 1.0) and dilute using distilled water
2) Measure 5cm^3 of each solution into separate test tubes
3) Use a cork borer to cut out 6 potato chips and cut them down into identically sized chips.
4) Dry and weigh each chip before starting the experiment
5) Place the chip in each test tube and leave for 20 minutes
6) Remove each chip, dry gently using a paper towel and weigh them out again
7) Calculate the percentage change in mass for each sucrose solution

++ Use results to make a calibration curve, showing the change in mass against sucrose concentration

Question 11

What is active transport and how does it work

Answer 11

the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

How does it work:

1) Molecule binds to carrier protein, on one side of the membrane.
2) ATP is hydrolysed into ADP and P. The phosphate binds to carrier protein on the other side of the membrane, causing the carrier protein to change shape.
3) Molecule travels through the newly-opened channel in the cell.
4) Phosphate molecule is released and recombines with ATP.
5) Carrier protein returns to original shape.

Question 12

What are co-transporter and how do they work?

Answer 12

A type of carrier protein.

1) Co-transporter binds two molecules at a time
2) The concentration gradient of one of the molecules is used to move the other molecule against its own concentration

Question 13

Explain how glucose is co-transported into the ileum via sodium ions

Answer 13

1. Sodium ions are actively transported out of the epithelial cell into the blood
2. This reduces the sodium ion concentration in the epithelial cell.
3. Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell
4. The protein the sodium ions diffuse through is a co-transporter protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient