AS Experiments

Question 1

How are temporary slides made?

Answer 1

1. The material is placed on a clean glass slide and one or two drops of skin added
2. A cover slip is carefully lowered over the specimen to protect the microscope lens and to help prevent the specimen from dying out
3. A drop of glycerine mixed with the stain can also help prevent drying out
   - Animal: humans cheek cells
   - Plant: onion epidermal cells

Question 2

What are reducing sugars?

Answer 2

1. The reducing sugars include all monosacarhides such as glucose and some disaccharide such as maltose
2. The only common non-reducing sugar is sucrose
3. Reducing sugars can carry out reduction, and so are oxidised

Question 3

What is Benedict’s test?

Answer 3

1. Benedict’s reagent is copper (II) sulfate in an alkaline solution and has a blue colour
2. Reducing sugars reduced soluble blue copper sulfate containing copper (II) ions to insulate brick red copper oxide containing copper (I)
3. The copper oxide is seen as brick-red precipitate
   reducing sugar + Cu2+ (blue) -> oxidised sugar + Cu+ (red-brown)

Question 4

How do you carry out Benedict’s test?

Answer 4

1. Add Benedict’s reagent to solution you are testing and heat it in a water bath
2. If reducing sugar present turn through green, yellow, organ to red-brown as insoluble copper (I) oxide forms a precipitate
3. As long as an excess Benedict’s reagent use the intensity of the red colour is related to the concentration of the reducing sugar

Question 5

How can you estimate the concentration of sugar?

Answer 5

1. Using colour standards made by comparing the colour against colours obtained in tests done with reducing sugar solutions of known concentration
2. You should also measure the time taken for the colour to change
3. You can use colorimeter to measure subtle differences in colour precisely

Question 6

What are non-reducing sugars?

Answer 6

1. Some disaccharides such as sucrose are not reducing sugars so you would get. negative result from Benedict’s test
   - If just non-reducing then get brick red in this test
   - If both reducing and non-reducing the precipitate obtained in this test will be heavier than one in Benedict’s test
2. The disaccharide is first broken down into its two monosacchsrdie constituents, in hydrolysis by HCL
3. They will be reducing sugars and their presence can detested for using Benedict’s test after the acid has been neutralised

Question 7

How do you carry out the non-reducing test?

Answer 7

1. Heat sugar solution with HCL to release free monosaccharides
2. Neutralise with NaOH to make alkali conditions for Benedict’s reagent to work
3. Add Benedict’s greasy and heat as before and look for colour change
4. If solution goes red now but afferent before non-reducing sugar present
5. If still no colour change, then there is no sugar of any kind present

Question 8

What is starch test?

Answer 8

1. Starch molecules tend to curl up into long spirals
2. The hole that runs down the middle of this spiral is just the right size for iodine molecules to fit in
3. Use Iodine solution (iodine in potassium iodide solution) and the starch-iodine complex forms has a strong blue-black colour

Question 9

How do you test for starch?

Answer 9

1. Iodine solution is orange brown
2. Add a drop of iodine solution tot he solid or liquid substance to be tested
3. A blue-black colour is produced if starch is present

Question 10

How do you test for lipids?

Answer 10

• Lipids are insoluble in water, but soluble in ethanol, emulsion test
  1. The substance thought to contain lipid is shaken vigorously with some absolute ethanol
  2. Allows any lipid in the substance to dissolve in the ethanol
  3. Ethanol is then poured into a tube contains water
  4. If lipid present a cloudy white suspension is formed

Question 11

What happens if no lipid is present?

Answer 11

1. If there is no lipid present, the ethanol just mixes into the water
2. Light can pass straight through the mixture so it looks completely transparent
3. If there is lipid dissolved in the ethanol it cannot remain dissolved when mixed with water
4. The lipid molecules form tiny droplets throughout he mixture (an emulsion)
5. The droplets reflect and scatter light making the liquid look white and cloudy

Question 12

What is the protein test based on?

Answer 12

1. All proteins have peptide bond, containing nitrogen atoms
2. These form a purple complex with copper (II) ions
3. Biuret reagent, a dilute solution of KOH or NAIH and dilution soliton of copper (II sulfate), or use ready made buret Reagan that contains copper (II) slate solution and hydroxide ready mixed
4. To strop the copper ions reacting with the hydroxide ions and forming a precipitate, this ready mixed reagent contain sedum potassium tartrate or sodium citrate

Question 13

How do you test for proteins?

Answer 13

1. Biuret reagent added to solution to be tested
2. No heating is required and a purple colour indicates that protein is present
3. The colour develops slowly over several minutes

Question 14

How do you measure the reaction rate for enzymes?

Answer 14

1. Catalase-hydrogen peroxide reaction, one of product is gas and can be collected
2. If measure rate of starch production, measure rate at which starch disappears from the reaction mixture by taking samples from the mixture at known times and adding each sample to some potassium iodide solution and Starch forms a blue-balck colour with this solution
3. Using a colorimeter you cans measure the amount intensity of blue-black colour and use this as a measure of amount of starch still remaining
4. Do this over. period of time plot curve of amount of starch remains against time

Question 15

Why is it not ideal to obese course by mixing starch iodine in potassium iodide solution and amylase in a. tube and take regular reading of the colour of the mixture in this one tube in colorimeter?

Answer 15

The iodine interfere with the rate of the reaction and slows it down

Question 16

How do you immobilise enzymes in alginate?

Answer 16

1. Enzyme mixed with solution of sodium alginate
2. Little droplets of the mixture are then added to a solution of calcium chloride
3. The sodium alginate and calcium chloride instantly react to form jelly which turns each droplet into a little bead
4. The jelly bead contains the enzyme and the enzyme is held in the bead or immobilised
5. These beads can be packed gently into a column and a liquid containing the enzymes substrate can be allowed to trickle steadily over them
6. As the substrate runs over the surface of the beads, the enzymes in the beads catalyse a reaction that converts the substrate into product
7. The product continues to trickle down the column, emerging from the bottom where it can be collected and purified

Question 17

What is Visking tubing?

Answer 17

• Visking tubing (dialysis tubing) is a partially permeable, non-living membrane made from cellulose
• It posses molecular sized pores which are small enough to prevent the passage of large molecules chain as starch and sucrose, but will allow the passage of smaller molecules by diffusion such as glucose

Question 18

How can you demonstrate diffusion using Visking tubing?

Answer 18

1. Fill length of Viking tubing (15cm) with a mixture of glucose and starch solutions
2. Suspend tubing in boiling tube of water for period of time
3. Presence of starch and glucose outside the tubing can be tested for at intervals to monitor whether diffusion out of the tubing has occurred
4. The results should indicate that glucose, but not starch diffuses out of the tubing

Question 19

How can you make the diffusion using a Visking tubing more quantitative?

Answer 19

• Estimate concentration of glucose at each time interval by setting up separate tube one for each applied time intervals and using semi-quanititave Benedict’s test each time
• A colorimeter
• Set of colour standards prepared
• Graph drawn show how rate of diffusion changes with the concentration gradient between the inside and outside of the tubing
• Further experiments could be designed if sucrose and an enzyme that breaks down sucrose (sucrase) are added to Visking tubing
• Experiments involving amylase, which breaks down starch