practicals

Question 1

method for light microscopes

Answer 1

Method:
Stain the specimen after slicing thin e.g iodine
Place on stage and adjust
Start with lowest magnification and objective lens
Adjust the coarse focusing knob then fine knob.

Question 2

Eye piece graticule calculations ALL IN MICROMETERS:

Answer 2

Measure actual length of stage micrometer and count number of total divisions. Find the value of one division on the micrometer
Place on stage then line up with eye piece graticule
For each magnification, equate the number of divisions on eyepiece to stage micrometer by lining them up.
Rearrange to find 1 division on eye piece= x divisions on stage micrometer
Sub in the value of 1 SM division to find value of 1 division on EP.
Then repeat for other magnifications.

Question 3

food tests

Answer 3

Starch: add set value of food to a test tube with iodine solution should turn blue-black
Proteins: biuret test, add sample then add biuret A (NaOH) with biuret B (copper sulphate) . colour will go from blue to purple
Lipids- emulsion test. Add ethanol and mix thoroughly so lipids dissolve. Add water if there are lipids there will be cloudy white substance at the top as lipids are not soluble in water as they are hydrophobic.

Question 4

method for quantitative glucose

Answer 4

Prepare set of different glucose concentration by adding glucose solution and distilled water.
Starting with most dilute solution, to minimise cross contamination, add 3cm to test tube with benedict’solution and heat the tubes in water bath for 5 minutes
Allow test tubes to cool and the precipitate to settle at the top
Set up colorimeter with an orange filter. Test by using cuvette filled with distilled water to be 0.
Add each solution to the cuvette and record the OD (OPTICAL DENSITY).
Plot data and use it to estimate concentration of unknown glucose.

Question 5

explanation of quantitative glucose results

Answer 5

The solutions with the highest concentrations were clearer in colour and the lower concentrations were darker blue. The higher concentrations had lower optical density value for absorption. (downwards curve)
As concentration increases, absorbance decreases because glucose is a reducing sugar so Cu2+ ions (blue) in benedicts-> Cu+ (clear). so if there is more glucose it will be more clear as more is reduced. If it is clear there is less absorption (light absorbed but more transmission).

Question 6

potential issues in quantitative glucose

Answer 6

Mixing in the precipitate and not allowing it to settle
Touching the cuvette so its not clear- not accurate
Cross contamination from not clearing syringes.

Question 7

method for effect of temperature on membrane permeability

Answer 7

Add 10cm^3 distilled water into 6 test tubes and place in different temperature water baths eg. 30-80. For 5 minutes till temp is correct
Cut beetroot cylinders with cork borer, us kine and ruler on a white tile to ensure they are all the same size and thickness. Rinse with distilled water and pat dry to get rid of surface dye.
Add one cylinder to each test tube and leave in water bath for 15 mins
After, shake the tubes and remove the cylinders
Take liquid form each tube and use a colorimeter on green filter to find absorption.

Question 8

sources of error for beetroot practical

Answer 8

Some cylinders of beetroot came from different beetroots or sections
Temperature of water bath and room fluctuated

Question 9

results for beetroot membrane permeability

Answer 9

As temperature increases, absorption increases/ transmission decreases.
Because when temp increases, phospholipids om ,e,nrane gain kinetic energy to become more and more fluid. Pigment diffuses across the membrane easier so it is more coloured. Transmission decreases as less light can pass through. Above 45 degrees membrane protein tertiary structures denature so bigger gaps, more osmosis.

Question 10

mitosis in root tip method

Answer 10

Fill small bottle with 1m HCL and using foam floater put in water bath at 55 degrees for 15 mins
Using scalpel cut 1cm of the root tip and place in the acid for 5 mins
take it out and rinse roots with water and dry.
Place the root in the acetic orcein (purple red)
After the root tips have been in the stain for 5 minutes, use forceps to take them out of the vial, and place them on a microscope slide.
Squashing the root spreads the cells, so that there is just a single layer.This makes the individual cells easier to see
Add a drop of water to the root tip on the slide and Tease the root tip with mounted needle , to spread out the cells and get rid of air bubbles
Place a cover slip over the root tip
Wrap the slide in several layers of a thick paper towel
Use your thumbs to gently press on the slide
Examine under a microscope.

Question 11

why to use hot acid for seeing mitosis in root tip

Answer 11

Hot acid softens tissue to break down the pectin in the middle lamella, and separate the cells
walls, making it easier to squash the root tip into a single layer of cells, which is crucial for viewing chromosomes under a microscope as stain can enter

Question 12

Why should tissues be thin for microscopy
2 marks

Answer 12

So light can pass through
Single layer of cells can be viewed

Question 13

method for determining water potential of potato cell

Answer 13

Prepare six sucrose solutions with the same volume
Cut twelve cylinders from the same potato using cork borer. Remove skin and trim to same length
Dry the potato cylinder with paper towel
Record initial mass
Tie 2 cylinders together and put in each solution, with bung on top
Allow to settle for 1 hour
Dry and reweigh

Question 14

osmosis practical Why do we find percentage mass:

Answer 14

because potatoes will have different staring masses. % allow comparison

Question 15

osmosis practical Why do we remove skins from potato and make them the same size

Answer 15

skins make it waterproof and we need the same surface area

Question 16

osmosis practical Why are bungs put on boiling tubes:

Answer 16

prevent water evaporation from solution which would change their concentration and rate of osmosis

Question 17

results and analysis of osmosis potato practical

Answer 17

As concentration increases, % change in mass decreases and becomes negative .At lower concentrations (higher water potential) water moves into the cell- +mass. At higher concentrations (lower water potential) water moves out of the cell hence -mass.

Where the line crosses the x axis (0 change in mass) is isotonic solution, no net movement of water so that is where incipient plasmolysis happens. concentrations/water potential = in potato and solution.
Find the sucrose concentration as osmotic potential which = water potential. Due to the equation because there is no turgor pressure (pressure potential=0).

Question 18

method for osmosis practical on onion cell

Answer 18

Cut five one-cell thin slices of onion cell
Make 5 different sucrose concentrations in watch glass and leave each slice there for 20 minutes
Remove each tissue with forceps and put on a slide. Put a drop if the solution and cover with cover slip
Count 25 cells per slice and count how many in plasmolysis.

Question 19

results for onion cell osmosis

Answer 19

Higher concentration, more plasmolysed cells
Playmolsys- when cell loses water leading to the cell wall pulling away. At higher concentration water leaves the cell to go to an area with lower water potential.
Convert numbers into percentages of plasmolysed cells. Draw a graph and estimate which concentration had 50% cells plasmolysed= incipient plasmolysis.