A Level Edexcel Biology B practicals

How do you investigate the effect of sucrose concentration on the growth of pollen tubes?

1. Make up solutions of sucrose between 0.2-2 moldm-3

2. Collect a petri dish for each sucrose concentration and place a moistened filter paper on each

3. Collect one microscope slide per concentration

4. Place a few drops of sucrose plus mineral salt medium in the dip of each slide

5. Gently rub the point of a mounted needle over the mature anthers of a flower so the pollen falls on the slide

6. Repeat for each sucrose solution and note the time at which the pollen was added

7. placate slides in the petri dish

8. Use a microscope at X100 magnification and a calibrated eye piece graticule to observe the slides

9. Make observations quickly and return the slide to the petri dish to prevent drying out

10. Once the pollen tubes have started to grow, measure the length of 4 or 5 tubes at 3 minute intervals for 30 minutes

What are the controlled variables when investigated pollen tube growth?

Volume of sucrose solution, volume of mineral solution, time left in solution, pollen source

What are the independent and dependent variables when investigating pollen tube growth?

IV: Concentration of sucrose

DV: Length of pollen tube

Why must the pollen tube slides be kept in a moistened petri dish?

To prevent drying out

How often should you measure the length of pollen tubes?

Every 3 minutes for 30 minutes

What must be added to a sucrose solution before pollen?

A mineral salt medium

How do you investigate the effect of temperature on membrane permeability?

1. Use a cork borer and knife to cut 1 cm lengths of beetroot

2. Place in distilled water overnight to remove dye, wash and blot dry

3. Place 8 boiling tubes in 8 water baths of different temperatures

4. Place a beetroot segment in each tube and leave for 30 minutes.

5. Remove the beetroot and shake the tubes to disperse the dye

6. Set colorimeter to % absorbance with a blue/green filter

7. Calibrate the colorimeter with distilled water

8. Calculate % absorbance for 2cm3 samples from each temperature

What are the controlled variables when investigating the effect of temperature on membrane permeability?

Volume of distilled water, time left in water, size of beetroot piece, source of beetroot

How is dye removed from beetroot pieces when investigating the effect of temperature on membrane permeability?

Placed in distilled water overnight to remove dye, washed and blotted dry

How is a colorimeter used when investigating the effect of temperature on membrane permeability?

Set to % absorbance with blue/green filter

What is used to calibrate the colorimeter when investigating the effect of temperature on membrane permeability?

Distilled water

How do you investigate the water potential of plant epidermal cells?

1. Take 5 thin sections of the red part of a red onion, as close to 1 cell thick as possible

2. Label 5 watch glasses and add 5 different concentrations of sucrose solution

3. Add a piece of onion to each glass and leave for 20 minutes

4. Put a drop of each solution on separate slides and float each tissue on the matching slides

5. Cover each section with a coverslip and observe under a microscope

6. Count 25 cells and record how many show plasmolysis

What are the independent and dependent variables when investigating the effect of temperature on membrane permeability?

IV: Temperature of water

DV: Transmission of light through solution

What are the controlled variables when investigating the water potential of epidermal cells?

Time left in solution, number of cells counted, part of onion used, same onion used

What are the independent and dependent variables when investigating the water potential of epidermal cells?

IV: Concentration of sucrose solution

DV: Plasmolysed cells

What is plasmolysis?

Shrinkage of a cell due to water being removed due to a hypertonic solution.

How do you investigate gas exchange in a locust?

1. Observe a live locust to see spiracles on the thoracic and abdominal segments of the locust

2. Look for rhythmic ventilation movements on the underside of the abdomen

3. Take a dead locust and play it on a dissecting board

4. Cut off the wings and remove the exoskeleton by cutting along each side then across the head and tip of abdomen

5. Flush with water to see the tracheae that will looks silvery

6. Remove the spiracles and mid gut tissue

7. Tissue can be stained with methylene blue and viewed under a microscope

Why do you need to flush the tissue with water when investigating gas exchange of locusts?

To see the tracheae that will look silvery

What can tissue be stained with when investigating gas exchange of locusts?

Methylene blue

How do you investigate the effect of environmental conditions on the uptake of water in a plant shoot?

1. Fill a capillary tube by placing it under water

2. Choose a leafy shoot that will fit into the rubber connector of a potometer

3. Cut the end of the shoot under water and insert into the rubber connector to prevent air bubbles forming in the xylem

4. Clamp the capillary tube and place the other end in water

5. Spread petroleum jelly around the rubber connector and tube to create an air tight seal

6. Dry all the leaves

7. Leave for 5 minutes to allow water to be drawn up the tube and for the plant to acclimatise

8. Time how long it takes for an air bubble to move along the capillary tube

9. Repeat the readings for different independent variables

10. Calculate water uptake by: water uptake per unit area of leaf = rate/ leaf area

What are some controlled variables when investigating the effect of environmental conditions on water uptake in a plant shoot?

Time left for bubble to move, type of plant, reset bubble to 0 each time, all other variables that aren't being tested

What are the independent and dependent variables when investigating the effect of environmental conditions on water uptake in a plant shoot?

IV: Light intensity/ temperature/ humidity

DV: Distance bubble moves in a set time

Why must a leafy shoot be cut and inserted into the bung underwater when investigating the effect of environmental conditions on water uptake in a plant shoot?

To prevent air bubbles forming in the xylem and interrupting the transpiration stream

Why must petroleum jelly be used around the rubber connector and tube when investigating the effect of environmental conditions on water uptake in a plant shoot?

To create an airtight seal

How do you calculate water uptake when investigating the effect of environmental conditions on water uptake in a plant shoot?

water uptake per unit area of leaf= rate/ leaf area

How would you investigate factors affecting the rate of aerobic respiration using a respirometer?

1. Assemble respirometer- test tube filled with CO2 absorbent covered with a wire gauze with an organism on it. Plug the tube with a bung with a hole for a glass tube with a 3 way tap, one to the test tube, one to a syringe and one to a capillary tube with a scale

2. Clamp the syringe and respirometer in position

3. Weigh one type of organism

4. Place a drop go coloured fluid in the open end of the glass tube with a dropping pipette

5. Open the connection between the syringe and respirometer and use the syringe to pull the fluid onto the scale

6. Close the tap to the syringe and start the clock

7. Note the position of the fluid at 1 minute intervals for 5 minutes

8. Calculate the distance travelled each minute and plot on a graph

What are the controlled variables when investigating the factors affecting the rate of aerobic respiration?

Time left for fluid to move, temperature, reset to 0

What are the independent and dependent variables when investigating the factors affecting the rate of aerobic respiration?

IV: type of organism

DV: Rate of O2 uptake

How do you investigate the effects of different wavelengths of light on the rate of photosynthesis?

1. Place 1cm piece of pondweed into a large beaker of water

2. Remove bubbles by running a finger over the surface of the pondweed underwater

3. Add half a spatula of NaHCO3 to the water to provide CO2 and leave for 5 minutes

4. Place a lamp close to the beaker with no filter and leave for 5 minutes to use as a control

5. Fill the capillary tube of the photosynthometer with water and place a funnel at the end of the tubing and add the pond weed under the funnel, a paperclip can be used to weigh the pond weed down

6. Start the stop clock as bubbles start to form

7. Use a syringe to draw up the oxygen produced to the scale to record the volume of gas produced

8. Repeat with other filters

What are the controlled variables when investigating the effects of different wavelengths of light on the rate of photosynthesis?

Light intensity, type of weed, amount of weed, temperature, CO2 concentration, volume of water

What are the independent and dependent variables when investigating the effects of different wavelengths of light on the rate of photosynthesis?

IV: wavelength of light

DV: rate of CO2 production

Why must NaHCO3 be added when investigating the effects of different wavelengths of light on the rate of photosynthesis?

To provide CO2

How do you investigate different chloroplast pigments using chromatography?

1. Take a piece of chromatography paper that fits the full length of a boiling tube without touching the sides

2. Draw a pencil line across the bottom of the paper

3. Grind a few leaves with a pestle and mortar, adding propanone as a solvent

4. Use a capillary tube to drop the extract on the pencil line

5. Allow each drop to dry before adding another drop on top of it

6. Pour chromatography solvent (propanone + petroleum spirit) so it comes up below the pencil line in the tube

7. Put the paper in and insert the bung