Module 1: Chapter 2: Required Practical

Question 1

Using a light microscope

Answer 1

1. Start with lowest power objective lens.
2. Put slide on stage.
3. Use course focus to move stage close to lens.
4. Adjust fine focus.

Question 2

Dissection - safety

Answer 2

• Use scissors instead of scalpel where possible.
• Replace blade covers when not in use.
• Make cuts with blades facing away.
• Sterilise equipment.

Question 3

Why is staining used?

Answer 3

• Identify cellular components.
• Identify different cell types in a tissue.
• We only see parts that absorb light.
• Stains make parts of the cell absorb more light - become visible.

Question 4

Test for carbs: Benedict’s test - reducing sugars

Answer 4

1. Add Benedict’s solution (blue).
2. Heat sample in water bath to 100 degrees.
3. If sugar is present turns red.

Question 5

Test for carbs: Benedict’s test - non-reducing sugars

Answer 5

1. Add HCl.
2. Boil in a water bath.
3. Test again for a reducing sugar - if non-reducing sugar is present it turns red.
4. If not present - stays blue.

Question 6

Biuret Test for Proteins

Answer 6

1. Add NaOH - alkali.
2. Add copper sulphate solution - blue.
3. If protein is present - turns purple.
4. If no protein - stays blue.

Question 7

The Emulsion Test for Lipids

Answer 7

1. Mix unknown substance with ethanol (alcohol).
2. Add water.
3. Observe - a milky layer if lipid is present.

Question 8

Colorimeter

Answer 8

A instrument to quantitatively measure the intensity of the colour of a solution.

Question 9

Colorimetry method

Answer 9

1. Use a blank corvette filled with distilled water to zero each reading.
2. Make a calibration curve.

Question 10

Calibration (standard) curve

Answer 10

A method for determining the concentration of an unknown solution by comparing it to a range of known concentrations.

Question 11

Purification of DNA by precipitation

Answer 11

1. Open cells with blender.
2. Mix washing up liquid to dissolve membrane, table salt to clump DNA and water.
3. Add cell mixture to other ingredients - ice cold to stop enzymes from breaking down DNA.

Question 12

Investigating permeability of cell membranes

Answer 12

1. Use colourful tissue.
2. Wash thoroughly.
3. Range of at least 5 water baths at different temp. and solvent concentrations.
4. Control all other variables.
5. Measure volumes.
6. Colour is determined by using a colorimeter set to absorbance.

Question 13

Investigating populations: Sampling

Answer 13

1. Used to measure biodiversity of habitat or the abundance of a species.
2. Impossible to count all of the organisms of a species.
3. Sample needs to be representative of whole habitat - repeats.
4. Use sample data to extrapolate to estimate.

Question 14

Investigating populations: Sampling - General rules

Answer 14

1. Large sample size/lots of repeats.
2. Sample at different times of day and weather conditions.
3. Standardise sampling methods.
4. Avoid capturing individuals twice.
5. Use a statistical test.

Question 15

Investigating populations: Sampling - Random sampling

Answer 15

• Avoid bias.
• Divide sampling area into a grid.
• Use a random number generator to pick coordinates.

Question 16

Investigating populations: Sampling - Non-Random Sampling: Systematic sampling

Answer 16

• Samples are taken at fixed intervals e.g. transect.
• Measures a change in the environment.

Question 17

Investigating populations: Sampling - Non-Random Sampling: Opportunistic sampling

Answer 17

• Sample sites are chosen by investigator - samples are biased.
• Easy and fast to do.

Question 18

Investigating populations: Sampling - Non-Random Sampling: Stratified sampling

Answer 18

• Survey area is divided into groups.
• Each area is surveyed separately - in proportion to its total coverage of the area.
• More representative than random sampling.

Question 19

Investigating diversity - comparing genetic diversity within a species (intraspecific)

Answer 19

Comparing DNA, mRNA, amino acid sequence.

Question 20

Investigating diversity - comparing genetic diversity between a species (interspecific)

Answer 20

Observe characteristics - put into groups.

Question 21

Investigating diversity - what is diversity/variation caused by?

Answer 21

• Genetic differences e.g. blood type.
• Environmental differences e.g. nutrition.

Question 22

Investigating diversity - sampling a population

Answer 22

• It’s impossible to sample all the individuals in a population.
• The date should be a representative of the whole population - can use a statistical test.
• Selected at random - avoid bias e.g. random number generator.

Question 23

Enzyme rate of reaction - changing substrate concentration

Answer 23

• Use a range of at least 5 different substrate concentrations.
• Use an enzyme volume and concentration that doesn’t limit that rate of reaction.
• Identify an observable dependant variable e.g. colour change.

Question 24

Enzyme rate of reaction - changing enzyme concentration

Answer 24

1. Use a range of at least 5 different enzyme concentrations.
2. Use a substrate and concentration that doesn’t limit that rate of reaction.
3. Identify an observable dependant variable e.g. colour change.

Question 25

Enzyme rate of reaction - volume of gas produced

Answer 25

1. Measure time using a stop watch. 2. Calculate the initial rate of reaction - plot data on a graph, draw a tangent at t=0, calculate initial rate. 3. Use a suitable control variable. 4. Repeat at least 5 times for each concentration - take mean values. 5. Control all other variables.

Question 26

Factors affecting membrane permeability - temperature: freezing

Answer 26

* Carrier proteins and protein channels lose their shape - open. * Cell membrane is damaged by ice crystals. * Membrane very permeable.

Question 27

Factors affecting membrane permeability - temperature: cold

Answer 27

* Low kinetic energy - phospholipids are close together - membrane stiff. * Less permeable.

Question 28

Factors affecting membrane permeability - temperature: warm

Answer 28

Phospholipids moving - more space between them - more permeable.

Question 29

Factors affecting membrane permeability - temperature: hot

Answer 29

* Carrier proteins and protein channels denature: open. * Phospholipid bilayer melts. * Membrane is very permeable.

Question 30

Factors affecting membrane permeability - solvents

Answer 30

* Some solvents can dissolve phospholipids in the cell membrane. * Loses its structure. * Increases its permeability.

Question 31

Serial dilutions

Answer 31

Progressively diluting a concentrated solution by known dilution factors.

Question 32

Serial dilution uses

Answer 32

Viable cell count. Calibration curve.

Question 33

Serial dilutions - 10 fold serial dilution

Answer 33

1. 1cm³ of original concentration. 2. Add to 9cm³ of distilled water. 3. Mix well - homogenous. 4. 1cm³ transferred to the next test tube.

Question 34

Potometer - variables to control

Answer 34

Light intensity Wavelength of light Temperature Humidity

Question 35

Potometer equipment

Answer 35

Bung Stem cut at an angle Capillary tube Reservoir: repeats, resets air bubble to start.

Question 36

Potometer - explanation of results

Answer 36

1. As leaves are covered or removed: 2. Surface are of leaves decreases. 3. Number of stomata decreases. 4. Transpiration decreases. 5. Cohesion-tension decreases. 6. When all leaves covered/removed: 7. Some evaporation (not transpiration) from upper surface of leaves.

Question 37

Potometer - sources of error

Answer 37

1. Water used to make cells turgid. 2. Water used by photosynthesis. 3. Water made by respiration. 4. Leaks in apparatus. 5. Stem not cut under water - can allow air into xylem which breaks cohesion tension.

Question 38

Water potential of plant tissue - method

Answer 38

1. Prepare a serial dilution of solute from a known conc. 2. Prepare cylinders of plant tissue - similar surface area. 3. Weigh mass of each cylinder. 4. Submerge cylinders in the range of solute conc. for 1 hour. 5. Weigh cylinders again. 6. Calculate % change in mass. 7. Calibration curve.