Required Practicals

Question 1

RP 1 - Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction (temperature)

Answer 1

Make control samples:
- Milk suspension + distilled water (no enzyme activity)
- Milk suspension + HCl (completely hydrolysed sample)

Main experiment:
1. Add milk suspension to 3 test tubes
2. Place in water bath at 10c for 5 mins
3. Add trypsin to each test tube simultaneously and start timer
4. Time how long it takes milk to completely hydrolyse and become colourless

Repeat and analysis:
- Repeat main experiment at 20c, 30c, 40c and 50c
- Find the mean time for each temp
- Rate of reaction = 1/mean time

Question 2

RP1 - Why does increasing temp increase rate of enzyme action?

Answer 2

As the temperature increases from 10°C, kinetic energy increases so more
enzyme-substrate complexes form. This means that the rate of reaction
increases up to the optimum temperature.

At temperatures beyond the optimum, bonds in the enzyme tertiary structure
break, which changes the shape of the active site. This means that the
substrate and enzyme are no longer complementary.

Question 3

RP 2 - Calculating mitotic index using plant cells. Describe how to prepare root tip.

Answer 3

Prepare root tip:
- Heat 1 mol/dm3 HCl to 60c in a water bath
- Cut a small sample of root tip using scalpel
- Transfer root tip to HCl and incubate for 5 mins
- Remove from HCl and wash in cold distilled water
- Remove very tip of root using scalpel

Question 4

RP 2 - Calculating mitotic index using plant cells. Describe to examine root tip.

Answer 4

Examine:
- Place tip on microscope slide and add a few drops toluidine blue stain (to make chromosomes visible)
- Lower the cover slip down carefully onto slide. Do not allow air bubbles which could distort image or the cover slip to slide sideways which could damage chromosomes.
- Place under microscope and set objective lens to lowest level
- Coarse and then fine focus

Question 5

RP 2 - Calculating mitotic index using plant cells. Describe how to calculate mitotic index.

Answer 5

Count cells undergoing mitosis (cells with visible chromosomes) and total number of cells

Mitotic index = no. of cells with visible chromosomes/ total no. of cells

Question 6

RP 3 - Investigating water potential. Describe method.

Answer 6

• Make a series of dilutions of 1M sucrose solution. These should be at 0.0, 0.2,
  0.4, 0.6, 0.8 and 1.0M sucrose. Dilute using distilled water.
• Measure 5cm3 of each dilution into separate test tubes
• Use a cork borer to cut out six potato chips and cut down the sections into identically sized chips
• Dry each chip using a paper towel to remove excess water but do not squeeze
• Weigh each before the start of the experiment
• Place a potato chip in each test tube (one per sucrose concentration) and leave for 20 minutes
• Remove each potato chip, dry gently using paper towel, and weigh them in turn
• Calculate the percentage change in mass for each sucrose solution

Question 7

RP 3 - Investigating water potential. Describe how to use results.

Answer 7

• Plot a graph of change in mass against concentration of sucrose solution.
• The point at which the line of best fit crosses the x axis (zero change in mass) indicates the point at which the solution is isotonic

Question 8

RP 3 - Investigating water potential. Describe conclusion.

Answer 8

• Potato chips in lower concentrations of glucose solution will increase in mass, whilst those in the higher concentrations of glucose solution will decrease in mass
• In the dilute glucose solutions, the solution has a higher water potential than the potato, so water passively moves via osmosis to the area of lower water potential (the potato). This causes the potato to increase in mass.
• In concentrated glucose solutions, water will move out of the potato, thus the
  potato will decrease in mass.

Question 9

RP 4 - Investigating cell membrane permiability. Describe method.

Answer 9

1. Cut beetroot cubes
2. Rinse off any pigment released

For temp:
1. Place each cube in equal volume distilled water
2. Put each test tube in water bath (30c to 80c)

For solvents:
1. Create dillution series of ethanol
2. Add each cube

Finish:
1. Leave samples for 20 minutes
2. Set colourimeter to blue filter and zero using curvette of distilled water
3. Filter each sample into a curvette using filter paper
4. Measure absorbance of each solution

Question 10

RP 4 - Investigating cell membrane permiability. Describe temperature conclusion.

Answer 10

● As the temperature increases, the permeability of the cell-surface membrane also increases. This is because the proteins in the membrane denature as the heat damages the bonds in their tertiary structure. This creates gaps in the membrane, so it is easier for molecules to pass through it.

● At low temperatures, phospholipids have little energy and are packed closely together to make the membrane rigid. This causes a decrease in permeability and restricts molecules from crossing the membrane

Question 11

RP 4 - Investigating cell membrane permiability. Describe conc of ethanol conclusion.

Answer 11

Ethanol causes the cell-surface membrane to rupture, releasing the betalain pigment from the cell. Higher concentrations of ethanol will cause more disruption to the membrane and more gaps will form. Thus, as concentration of ethanol increases, the permeability of the cell-surface membrane also increases.