Paper 1 required practicals

Required practical 1:

• Use a light microscope to look at cells

Microscope:

• Stage: where slides go

• Clips: hold slides in place

• Lamp/ mirror (to reflect light back up through the slide): light passes through the microscope slide to illuminate specimen

• Three different objective lenses with different magnifications (x4, x10, x40)

• Eyepiece: look through

  • Contains an objective lens (magnification of 10x)

• Coarse focusing dial: changes height of stage significantly

• Fine focusing dial: changes height of stage very slowly and minutely

Method:

• Peel off a layer of onion using forceps

• Place a drop of water onto slide with pipette

• Place onion layer

• Add 2 drops of iodine to stain cells

• Lower cover slip using forceps, make sure no air bubbles are trapped

• Place slide onto stage with clips

• Turn coarse focusing dial to move the stage up so that the objective lens almost touches the slide

• Look into eyepiece and adjust the coarse focusing dial again but to move the stage away, until it comes into focus

• Turn the nosepiece to select an objective lens

• Look into eyepiece and turn the fine adjustment dial until the image comes into focus

• Make a labelled diagram and mark down the magnification

Required practical 2:

• Investigate the effects of different antiseptics/antibiotics on bacterial growth using agar jelly

• Wash hands with antibacterial hand wash

• Spray work bench with disinfectant then wipe dry with paper towels

• Light a bunsen burner by the work station to create an updraught of air, preventing unwanted bacteria from settling on the agar

• Sterilise petri dish and agar jelly by heating to a high temperature

• Sterilise inoculating loop and spreader by passing them through bunsen burner flame

• Use the inoculating loop to transfer and streak bacteria

• Use the spreader to spread it evenly

• Take four sterile paper disks that have each been soaked in different antibiotics and lower it into the agar using sterile tweezers

• Lightly tape the lid on the petri dish and store upside down; don’t put tape all around the sides so that air can still get in for the bacteria to respire

• Incubate the dish at 25 degrees for 2 days

  • This reduces the chance that harmful bacteria will grow

• Calculate and compare the zones of inhibition

Required practical 3:

• Investigate the effect of a range of concentrations of solution on the mass of potato tissue

• Peel the potato skin off of the potato (as skin can affect osmosis)

• Use a cork borer to produce 5 cylinders of potato of equal diameter

• Use a scalpel to trim the lengths of the cylinders so that they are equal

• Measure the length using a ruler and the mass using a balance

• Place each cylinder in a test tube of different concentration solution (one should be distilled water)

• Leave the potatoes in overnight to allow osmosis to take place

• Remove the potatoes and gently roll them in a paper towel to remove any surface moisture

• Measure the mass again using a balance

• Calculate the percentage change of the mass

• Plot a graph where the y axis is % change in mass and the x axis is concentration of sugar solution

• Where the graph crosses the x axis and there is no change of mass, that concentration is the approximate concentration of inside the cell

Required practical 4

• Food tests for

  • Starch

  • Reducing sugars (eg. glucose)

  • Proteins

  • Lipids

Include sources of error and safety precautions

Food sample for: starch, reducing sugars, proteins

• Take the food sample and grind this with distilled water using a pestle and mortar to make a paste

• Transfer paste to beaker and add more distilled water; stir so chemicals in the food dissolve into the water

• Filter solution to remove suspended food particles

Food sample for lipids:

• Same process, but do not filter the solution as fat molecules will stick to the filter paper

Starch

• Iodine

• Orange → blue black

Reducing sugars

• Add Benedict’s solution

• Heat using a water bath

• If present in low quality: green

• If present in high quality: brick red

• If none, solution remains blue

Proteins

• Biuret’s reagent

• Blue to purple

• Keep away from skin (as corrosive) and do not ingest (as poisonous)

Lipids:

• Equal parts ethanol and distilled water

• If lipids are present, a cloudy/ white emulsion forms

• Keep solution away from flames as ethanol is highly flammable!

Sources of error:

• Colour change may be subtle and difficult to judge if concentration is low

Required practical 5:

• Investigate the effect of pH on the rate of activity on amylase enzyme

Include sources of error

Amylase is a carbohydrase (produced in small intestine, pancreas and salivary glands) and breaks down starch molecules into simpler, soluble sugars

• Place on drop of iodine solution into each well of a spotting tile

• Label each well with the time (from 0 onwards)

• Have three different test tubes: one with starch solution, one with amylase solution, and one with pH 5 buffer solution

  • A pH buffer is a solution that remains the same pH

• Place them all in a water bath at 30 degrees; leave them for 10 minutes to allow all the solutions to reach the same temperature

• Combine all three solutions into one test tube and mix with stirring rod

• Return to the water bath and start a stopwatch

• After thirty seconds, use the stirring rod to transfer one drop of solution into a spotting tile

  • The iodine should turn blue black, showing that starch is present

• Continue to take a sample every thirty seconds until the iodine remains orange

• When the iodine remains orange, it means that the reaction is finished and the starch is no longer present

• Repeat the whole experiment several times, using different pH buffers (eg. 6, 7, 8)

Sources of error:

• We are only taking samples every thirty seconds; intervals may be too long to accurately find the time taken for the starch to be completely broken down

  • Take samples every 10 seconds

• Colour change may be gradual (eg. some wells may have a mixture of orange and blue-black), so it can be difficult to see when the reaction has finished

  • Have multiple people looking to jointly decide

Required practical 6:

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

• Take a boiling tube and place it 10cm away from an LED light source

  • LED light is used as they don’t release much heat

  • Too much heat would impact the results

• Fill the boiling tube with a fixed volume of sodium hydrogencarbonate solution

  • Releases carbon dioxide, which is needed for photosynthesis

• Cut the end of the pondweed

• Gently push it down using a glass rod

• Leave the boiling tube to rest for 5 minutes

• Start the stopwatch and count the number of bubbles produced in one minute

• For each distance/ light intensity, repeat the count three times and calculate a mean

• Repeat steps for more distances

Calculate light intensity using inverse square law

Light intensity = 1/d²

Plot graph for rate of photosynthesis (bubbles per minute) to light intensity

Sources of error:

• Bubbles are different sizes, so different amounts of oxygen

• Bubbles may form too quickly to be counted

  • Place pondweed under a funnel to catch bubbles in a measuring cylinder