Biology core practicals

Use a light microscope to observe, draw and label a section of plant and animal cells

Sample method

1. Place a tissue sample on a microscope slide 

2. Add a few drops of a sutible stain

3. Lower a coverslip onto the tissue

4. Place the slide on the microscope stage and focus on the cells using low power

5. Change to high power and refocus

6. Draw any types of cells that can be seen

7. Draw a scale line on te diagram

Considerations, mistakes and errors

• The scale line can be added by focusing on the millimetre divisions of a ruler

Investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue

Sample Method

Potatoes can be used to measure the effect of sugar solutions on plant tissue:

1. Cut some cylinders of potato tissue and measure their mass

2. Place the cylinders in different concentrations of sugar solution

3. After about 30 minutes remove the cylinders and measure their mass again

Considerations, Mistakes and Errors

• The cylinders need to be left in the solution long enough for a significant change in mass to occur 
• Before the mass of the cylinders is measured again, they should be rolled on tissue paper to remove any excess solution
• If the cylinders change in mass, they have lost or gained water by osmosis

Investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue

Variables

• The independant variable is the one deliberately changed - in this case, the concentration of sugar solution
• The dependant variable is the one that is measured - in this case, the change in mass of the potato
• The control variables are kept the same - in this case, the tempurature, the length of time the cylinders are left in the solution and the volume of the solution

Hazards and Risks

• Care must be taken when cutting the cylinders of potato

Use qualitative reagents to test for a range of carbohydrates, lipids and proteins

Sample Method

1. To test for sugars, e.g. glucose, add Benedict's reagents and heat in water bath for two minutes. If sugar is present, it will       turn red

2. To test for starch add iodine solution. If satrch is present, it will turn blue-black

3. To test for protein add biuret reagent. If protein is present, it will turn purple

Considerations, Mistakes and Errors

• Do not boil the mixture for a long time, because any starch present could break down into sugar and test positive 
• Refer to 'iodine solution' not 'iodine'
• Sometime sthe purple colour is difficult to see. Try holding the test tube in front of a sheet of white paper

Investigate the effect of pH on the rate of reaction of amylase enzyme

Sample Method

1. Put a test tube containing starch solution and a test tube containing amylase into a water bath at 37°C

2. After 5 minutes add the amylase solution to the starch

3. Every 30 seconds take a drop from the mixure and test it for starch using iodine solution

4. Record how long it takes for the starch to be completely digested

5. Repeat the experiment at different pH values using different buffer solutions 

Considerations, Mistakes and Errors

• The solutions need to be left in the water bath for a while to reach the correct tempurature before they are mixed
• After mixing, the tube must be kept in the water bath 
• A buffer solution must be used to keep the reaction mixture at a certain fixed pH

Investigate the effect of pH on the rate of reaction of amylase enzyme

Variables

• The independant variable is the pH
• The dependant variable is the time taken for the starch to be digested
• The control variables are the temperature, concentration and volume of starch and amylase

Hazards and Risks

• Care must be taken if a bunsen burner is used to heat the water bath 
• Take care not to spill iodine solution on the skin

Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed

Sample method

1. Place a piece of pondweed in a beaker and shine a light at it using a lamp a specific distance away 

2. Record the number of bubbles of gas coming out of the pondweed in one minute

3. Repeat this with the lamp at different distances from the pondweed

Considerations, Mistakes and Errors

• It is best to take at least two readings at each distance and calculate the mean of the number of bubbles
• Carbon dioxide is provided by adding a small amount of sodium hydrogen carbonate to the water

Hazards and Risks

• Care must be taken to avoid any water being dropped onto the hot light bulb

Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed

Variables

• The independant variable is the light intensity (distance from the light)
• The dependant varible is the number of bubble in one minute
• The control variables are the piece of pondweed, the temperature, and the concentration of carbon dioxide

Investigating the effect of a factor on human reaction time

Sample Method

Reaction time can be investigated by seeing how quickly a dropped ruler can be caught between finger and thumb:

1. The experimenter holds a metre ruler vertically from the end 

2. The subject has their finger and thumb a small distance apart, either side of the ruler, on the 50cm line 

3. The experimenter lets go of the ruler and the subject has to trap it

4. The distance the ruler travels from the 50cm line is noted 

5. The experiment is repeated on subjects that have drunk coffee or cola and subjects that haven't

Considerations, Mistakes and Errors

• It is difficult to control the variables in this experiment
  
• To obtain reliable results, large numbers of subjects need to be tested and averages taken

Investigating the effect of a factor on human reaction time

Variables

• The independant variable is whether the subject has taken in caffine or not
• The dependant variable is the distance the ruler travels
• The control variables are the age, sex and mass of the subjects

Hazards and Risks

• There are limited risks in this experiment

Measure the population size of a common species in a habitat

Sample Method

1. Place a quadrat on the ground at random

2. Count the number of individual plants of one species in the quadrat

3. Repeat this process a number of times and work out the mean number of plants

4. Work out the mean number of plants in 1m²

5.  Measure the area of the whole habitat and multiply the number of plants in 1m² by the whole area

Considerations, Mistakes and Errors

• The main consideration in the experiment is making sure that the quadrats are places at random. Using random numbers to act as coordinates can help with this 
• The more samples that are taken, then the more accurate the estimate should be 

Measure the population size of a common species in a habitat

Variables

• The dependant variable is the number of plants in the quadrat

Hazards and Risk

• Care should be taken to wash hands after ecology work in a habitat
• Care should be taken when throwing quadrats - throw low to the ground, not up in the air