PAG 5: Colorimeter + Potometer Flashcards
Describe the method for using a colorimeter. (8)
- Switch the colorimeter on and allow it to stabilise for 5 minutes
- Select the red filter (for Benedict’s test - use a complementary colour to starting solution) on the colorimeter. If using Benedict’s test, centrifuge the solution or allow it to sit to precipitate out the copper solid
- Set the colorimeter to zero using a cuvette ¾ filled with distilled water
- Ensure the cuvette is placed in the colorimeter so the light passes through the clear sides
- Make sure the slides are clean and there are no bubbles in the solution
- Using a pipette, fill the cuvette ¾ with the sample
- Place in the colorimeter and read the absorbance of light
- Less light is absorbed by a paler solution, so there is greater transmission for a paler solution
How can a calibration curve be used to measure the concentration of reducing sugars using a colorimeter? (4)
- Use standard solutions of reducing sugars with known concentrations
- Carry out a Benedict’s test on each sample, ensuring the Benedict’s solution is in excess
- Use the colorimeter to measure the percentage transmission for each sample
- Plot a graph of transmission against reducing sugar concentration
What is the relationship between transmission and reducing sugar concentration when using a colorimeter?
Higher transmission and lower absorbance indicate more sugar present (greater concentration)
Why does absorbance not have units? (1)
Absorbance is a ratio comparing light passing through a solution to light passing into the solution
What key experiments in OCR A A level Biology is colorimeter used for? (4)
Measuring reducing sugar concentration
Using Benedict’s test followed by a colorimeter to measure absorbance/transmission
Investigating membrane permeability
Measuring the absorbance of pigment released from beetroot cells at different temperatures or ethanol concentrations
What is a potometer used for? (1)
A potometer is used to estimate the rate of transpiration in a plant by measuring water uptake
What assumption does a potometer make when measuring transpiration? (1)
It assumes that water uptake by the plant is directly related to water loss by the leaves through transpiration.
What are the limitations of using a potometer? (3)
Not all the water taken up by the plant is used for transpiration (some maintains cell turgidity)
Some water is used in photosynthesis
The plant may take up less water as it begins to die after being cut
Describe the method for using a potometer to estimate transpiration rate. (6)
- Cut a shoot underwater at a slant to prevent air entering the xylem and increase the surface area for water uptake
- Assemble the potometer in water and insert the shoot under water
- Remove the apparatus from the water while keeping the capillary tube submerged
- Check for watertight and airtight seals using screws or petroleum jelly
- Allow the plant to acclimatise, then shut the tap
- Introduce an air bubble into the capillary tube, record its starting position, and measure its movement at regular intervals
Describe and explain the precautions that need to be taken when setting up and using a potometer. (4)
Cut the stem underwater and insert it into the potometer while submerged to prevent air from entering the xylem
Dry leaves before starting to avoid reducing the transpiration rate due to a lower concentration gradient
Remove the end of the capillary tube from water only after a single air bubble has formed to prevent multiple bubbles
Ensure all joints are airtight and watertight to prevent leakage or inaccurate readings
Suggest a reason for an anomalous reading in an investigation using a potometer. (2)
The measurement was timed for longer than intended
Environmental factors such as temperature, light intensity, or air movement were not controlled
Describe how to calculate the rate of water uptake by the shoot. (2)
Use πr² to calculate the cross-sectional area of the capillary tube
Multiply by the distance moved by the air bubble and divide by time
How can you make a valid comparison between the rate of transpiration in two leafy shoots? (2)
Find the total leaf area in mm² by measuring around each leaf and multiplying by 2 for both surfaces
Calculate the transpiration rate per unit leaf area
Why would the rate of water movement through a shoot in a potometer not be the same as in a whole plant? (1)
The whole plant has roots, and the xylem in a shoot is narrower than that in the main stem
