Biology ✿ required practical

Some students want to look at onion cells

Suggest a way to prepare the microscope slide [4]

• peel off epidermis layer with tweezers

• place drop of water onto slide and place sample on top

• add drops of iodine

• place cover clip using tweezers carefully at an angle to prevent air bubbles

Some students have prepared a slide of onion cells

Suggest a way to observe the cells using a microscope [5]

• place the slide on the stage

• choose the lowest power objective lens

• look through the eyepiece lens and slowly turn the course adjustment knob so the cells are in focus

• slowly turn the fine adjustment knob to bring cells into a clearer focus

• when cells have been found, switch to a higher power lens

• turn the fine adjustment knob to bring cells back into focus

give 3 ways which the student can improve their drawing

• write down magnification

• do not shade nuclei

• label cell parts

why is a thin layer of epidermis used to look through the microscope for the microscopy experiment?

to allow light to pass through

why is a drop of iodine used before looking through the thin layer of epidermis during the microscopy experiment?

allows subcellular structures to be seen

equation for change in mass as a percentage

(change in mass / initial mass) x 100

describe how a student could carry out an investigation on the effect osmosis using potato tissue [6]

• cut equal sizes of potato and remove skin, as it is non permeable

• weigh using a balance and record the initial mass

• place potatoes in test tubes with different concentrations 0 - 0.8 mol/dm³ of sugar solution and leave for a set time

• take out potatoes, dab off excess water and weigh

• calculate the percentage increase of mass

Explain why the potato pieces in the 0.4 mol/dm³ salt solution decreased in mass [3]

• water moves outside of cells

• by osmosis

• because the solution in the cells is less concentrated than outside

describe how a student could carry out an investigation on the effect of light intensity on the rate of photosynthesis [5]

• fill a tube with sodium hydroxide carbonate solution

• cut 10cm of pondweed at an angle while it is submerged in the solution

• place a LED lamp 10cm away from the tube and switch it on

• wait a few minutes for the rate of photosynthesis to stabilise

• count the number of bubbles in one minute

• repeat for 5 different distances

counting the number of bubbles per minute isn’t an accurate way of measuring the rate of photosynthesis.

suggest two other ways to accurately measure the rate of photosynthesis

• measure the volume of gas using an upside down measuring cylinder or gas syringe

• repeat each distance 3 times and calculate a mean

describe how a student could carry out an investigation to determine the optimum pH of an enzyme [5]

• prepare a spotting tile and place drops of iodine

• measure out 2cm³ of starch solution, pH 1-5 buffer solution and amylase in separate tubes and place in a water bath at 30°C and leave for 5 minutes

• mix all reactants, place back into water bath and start stopwatch

• at 30 seconds, place a drop of reactant into the spotting tiles using a pipette and continue for every 30 seconds until iodine remains orange (reaction stops)

• repeat this with different pH buffers solutions

suggest 2 ways to improve the investigation to determine the optimum pH of an enzyme?

• shorter time intervals

• use more pH values such as 6 and 7

what 2 things can be done to improve this experiment?

• repeat and calculate a mean

• control the water temperature

how would you test for starch?

• add iodine solution

• positive result → blue/black

what do you do to the food before using it to do a food test?

grind food using pestle and motor and add distilled water

how would you test for sugars?

• add bendicts’s solution

• positive result → green, yellow or brick red

how would you test for protien?

• add briuet solution

• positive result → lilac

independent variable

what is changed

dependant variable

what you measure

control variable

what stays the same in order to keep the test fair

precision

how large the spread of data is

Why did the student leave the starch solution and amylase solution for 5 minutes in the water bath before mixing them?

to allow both solutions to reach 30 °C

The student investigated the effect of temperature on amylase activity.

Describe how the student could extend the investigation to determine the effect of a different factor on amylase activity [2]

• keep the temperature the same

• but use different pH levels

Describe three aseptic techniques the student should have used. [3]

• sterilise equipment before using

• secure lid with tape

• only lift lid a little when setting up plate

explain why there is no loss or gain of mass of the potato at this point.

there is no net movement of the water as the concentration outside and inside the potato is equal

what causes a gain of mass inside the potato? [2]

• solution inside the potato is more concentrated

• so water moves inside by osmosis

what causes a loss of mass inside the potato? [2]

• solution outside the potato is more concentrated

• so water moves outside by osmosis