PAGS

Question 1

benedict’s test for reducing sugars

Answer 1

1) reducing sugars = all monosaccharides(glucose) and some dissacharides(maltose+lactose)
2) add excess blue benedict’s reagent to a sample and heat in water bath that’s been brought to boil
blue–>green–>yellow–>orange–>brick red

the higher the conc of reducing sugar, the further the colour change + filter solution and weigh precipitate

Question 2

benedict’s test for non-reducing sugars

Answer 2

non reducing sugars must be broken down into monosaccharides

1) add dilute hcl and careful heating in water bath that been brought to a boil
2) neutralise with sodium hydrogencarbonate

Question 3

glucose test

Answer 3

1) test strips coated in reagent
2) colour change comparison to a chart confirms glucose presence

useful for diabetes urine tests

Question 4

starch test

Answer 4

1) add iodine dissolved in potassium iodide solution to sample
+ browny orange to blue black
- browny orange

Question 5

protein test

Answer 5

1) add drops of sodium hydroxide solution(so it is alkaline)
2) add copper(II) sulfate solution
+ purple
- remains blue

Question 6

test for lipids

Answer 6

1) shake with ethanol for 1 min
2) then pour solution in water
+ milky(er)
- remains clear

Question 7

colorimetry

Answer 7

colorimeter measures strength of coloured solution via the amount of light that passes through it
aids a quantitative estimate of glucose conc via benedicts test
1) benedict’s test(same amount) + negative control(pure water)
2) remove any precipitate - leave for 2hr/centrifuge
3) use colorimeter with red filter to measure absorbance of remaining benedicts solution in each tube
4) plot calibration curve using results of absorbance against glucose conc

Question 8

serial dilution

Answer 8

1) line up 5 test tubes in a rack
2) add 10cm3 of initial 40mM glucose conc to first 5 test tubes an 5 cm3 to ither 4
3) using a pipette draw 5cm3 from first test tube and transfer to next tube
4) repeat process

Question 9

chromatography(amino acids)

Answer 9

solvent = mixture of butan-1-ol / glacial ethanoic acid /water
use of ninhydrin solution to visual amino acid dots

Question 10

DNA purification

Answer 10

1) use of blender to break up cells
2) make up solution of detergent(breaks down cell membranes) + salt(binds to the DNA causing clumps) + distilled water
3) add broken up cells into beaker containing prepared solution and incubate in a water bath for 15mins at 60’(temperature denatures enzymes from working properly and breaking down DNA)
4) once incubated put beaker in ice bath to cool
5) once cooled filter mixture
6) transfer sample to clean boiling tube
7) add protease enzymes(break down some proteins in mixture)
8) add RNase enzymes to break down RNA
9) slowly dribble cold ethanol down side of tube to form layer on top of DNA detergent mixture
10) DNA should form whie precipitate which can be collected with glass rod

Question 11

enzymes

Answer 11

catalase
collect oxygen produced + how fast its given off
measuring disappearance of substrate
use of iodine to have more clear end point

Question 12

the effect of temperature on catalase

Answer 12

1) set up boiling tubes containing same volume and concentration of hydrogen peroxide + buffer(pH)
2) set up apparatus to measure vol of oxygen produced (dlievery tube + upside down measuring cylinder)
3) place each test tube in different temperature along with catalase separate(5 mins to set temp)
4) use pipette to add same volume/conc of catalase to each boiing tube
5) record amount of oxygen produced in the first 60 seconds using stopwatch
6) repeat expriment at each temperature 3 times + mean of results via diving vol of oxygen/time taken = cm3s-1

Question 13

permeability of cell membranes

Answer 13

1) cut 5 equal sized pieces of beetroot + rinse to removes pigment released during cutting
2) place 5 test tubes in 5 diffent test tubes each with 5cm3 of water
3) different temps for each test tube for same time
4) remove beetroot pieces
5) use of colorimeter = higher the permeability = more pigment released = higher absorbance of liquid

Question 14

diffusion in model cells

Answer 14

1) agar jelly with phenolphthalein + sodium hydroxide (pinks)
2) fill a beaker with dilute hydrochloric acid + using a scapel cut out a few cubes from the jelly and place into acid
3) leave the cubes - they go colourless as the acid diffuses into agar jelly and neutralises the sodium hydroxide

SA - (largest sa:v cube goes colourless first)
conc gradient - (highest conc of hcl goes colourless first)
temp - (highest temp goes colourless first)

Question 15

investigating water potential

Answer 15

1) prepare sucrose concs
2) cut potatoes into same sizes peices(1cm in D)
3) divide chips into 3 groups + mass balnce to measure mass of each group
4) place one group in each conc
5) leave in solution for as long as possible
6) removes + pat dry gently
7) weigh each group + record
8) calc % change
9) plot results on graph

Question 16

potometer for transpiration rates

Answer 16

measures water uptake, but it is assumed this is directly related to water loss from leaves

1) cut shoot underwater(prevents air entering xylem) + slant to increase SA for water uptake
2) insert shoot into potometer so no air can enter + water/air tight apparatus submerged capillary tube end
3) dry leaves + allow time for shoot to acclimitise, then shut the tap
4) remove capillary tube end from beaker until one air bubble forms then place back
5) record starting position of air bubble(air-water meniscus) + stop watch + distance of bubble/per unit time
6) change one variable, others constant

Question 17

Another student suggested that he should repeat the investigation at least twice.
How would this have improved the investigation?

Answer 17

improve reliability ;
assess, variability / spread of results ;
allows calculation of mean ;

Question 18

Describe how the student could carry out a chemical test for reducing sugar and suggest how he could estimate the amount of reducing sugar in the sample from tube A.

Answer 18

add, Benedict’s (reagent) / CuSO4 + NaOH /
alkaline copper sulphate ;
heat ;
(forms) precipitate ;
(colour changes from blue to), green / yellow / orange
/ brown / (brick) red ;
concentration estimated from
EITHER
degree of colour change / use different colours ;
comparison (of final colour) with , standard / known,
solution ;
OR
filter / centrifuge , and weigh precipitate ;
greater mass = more sugar present / use of a
standard curve ;
OR
centrifuge ;
size , of pellet / colour of supernatant (liquid),
indicates concentration ;

Question 19

Another student suggested that the agarose may have been broken down to a non-reducing sugar.Describe how the test for reducing sugar could be modified to investigate this hypothesis.

Answer 19

Another student suggested that the agarose may have been broken down to a non-reducing sugar.Describe how the test for reducing sugar could be modified to investigate this hypothesis.

Question 20

Suggest how the student could check the reliability of the data.

Answer 20

repeat / replicate ;
compare replicate values / identify anomalous results ;
mean / range / standard deviation / error bars / % error ;
compare results with , others / book / internet ,
values / results ;

Question 21

State the colour of iodine solution in the presence of starch.

Answer 21

blue-black / black / dark blue ;

Question 22

State three variables that need to be controlled in this practical procedure in order to produce valid results.

Answer 22

temperature ;
pH ;
enzyme / amylase / chloride, concentration ;
substrate / starch / amylose, concentration ;
constant / regular, stirring ;
(fixed) volume of solution
(removed each time for sampling) ;