U3 IAL edexcel biology (core practicals)

Describe the difference between semi-quantative and quantitative test


(2marks)

- semi-quantitative test:
gives an estimate/range of the concentration of a substance (by comparing against standard solutions)


- quantitative test: gives the exact value (of concentration)

( gives an exact quantity, done by using colorimeter and plotting calibration curve)

How to make a reliable semi-quantitive estimate? (1mark)

- a valid comparison is needed (to make a reliable semi-quantitative estimate)

State what is meant by the term reducing sugar

Give 5 examples

- reducing sugar is a sugar that can donate electrons to another molecule/chemical

- glucose, fructose, galactose, lactose & maltose

( sucrose is the disaccharide that is non-reducing sugar)

Explain what the value for standard deviation means (/definition of S.D)

(1mark)

- a measure of the spread of data around the mean

Describe a method to allow a quantitative comparison of the vitamin C content of two/three fruits.

(5 marks)

[CP2]

- Use equal masses of the fruits & same volume of DCPIP solution


- Use the same extraction method (e.g. same volume of distilled water, same time, same filtering)

- Add fruit extract dropwise until DCPIP changes from blue to colourless
[titration]

- number of drops needed (to decolorise) to determine vitamin C content
(or record the volume of extract used)

- repeat experiment and calculate an average and compare V-C content

State the reagent and color change when testing for protein content in food sample

(2 marks)

- biuret reagent

- blue to purple

[CP7 microscopy/observe and draw plant structures]

- cut thin section of a stem using sharp knife/scalpel

- add stain/ dye to it

- place the thin section on slide and under a coverslip

- draw the structure under low power plan

Describe a procedure for the accurate determination of the diameter of a cell
(5marks)

[CP5]

- use light microscope under low power & then view under high power

- calibrate eyepiece graticule

- count number of E.P.G units over the cell

- convert E.P.G to microns (μm)

- measure diameter of cell from different positions

Describe a SAFE method to prepare and examine the structure of human cheek cells
(5marks)

[CP5]

- wear gloves for safety

- use sterile cotton bud to collect the cheek cells (from inside your cheek/mouth)

- rub the cotton bud on glass slide

- add stain such as methylene blue to the cheek cell

- place cells under coverslip

- observe the slide under microscope using low power first and then high power

Describe how you would prepare cells in order to observe mitosis

(4 marks)

[CP 6 ]

-take sample of root tip and add HCL acid (to breakdown middle lamella/celloluse)

- add stain such as toluidine blue/methy blue and warm to make stain more visible

- Use a mounted needle to break open the root tip

- squash root tip
(squash so that cells are seperated, thus chromosomes are visible and mitosis can be seen)

- place cells under coverslip and view under high power microscope

- count no. of cells dividing(or in mitosis) and divide it with total no. of cells to determine mitotic index

- color of DCPIP solution changes from blue to colorless

- because DCPIP is reduced
(reduction = gains electrons, as Vitamin C is antioxidant containing free radicals)

Low power vs high power image

Low power image:

- do not draw individual cells

- only a few cells

High power image:

- include magnification
- cell wall

Devise a procedure to investigate the effect of different concentrations of sucrose on
the rate of growth of pollen tubes, using a sucrose solution.

(5 marks)

[Additional practical: Investigate factors affecting pollen tube growth ]

- use a range of concentrations of sucrose solution

- prepare range by diluting stock solution( i.e the 500 g dm⁻³) with distilled water

- use 100ppm boric acid solution

(Boric acid provides boron, needed for pollen tube growth)

- measure pollen tube length using microscope & eyepiece graticule

- record P.T length at regular time intervals to calculate rate

Design a valid investigation to determine the tensile strength of plant fibres

(6marks)

- take fibres of same mass, length and age

- make sure to control environmental factors such as temperature

- make sure procedure is controlled by using same sized masses

- hang mass to the fibre and keep adding mass until fibre breaks

- repeat the procedure to find average mass that breaks fibre, remove any anamolies

- (for safety:) wear goggles to protect eyes

[ Improve accuracy = use smaller mass intervals ]