OCR A Level Biology A: Core Practicals

Question 1

Preparations and examination of slides

Answer 1

Preparation
Staining samples with methylene blue (DNA)/ eosin (Cytoplasm)
- staining adds contrast ∴ easier to identify different parts of the cell

Dry mount:
1. thinly sliced
2. use tweezers to pick up your specimen and put it on clean slide
3. place a coverslip over the top of the specimen

Wet mount:
1. pipette a small drop of water onto the slide
2. use tweezers to place the specimen on top of the water
3. stand the coverslip upright on the slide next to the water
4. carefully tilt and lower the cover slide so it covers the specimen
5. stain the specimen by putting a drop of the stain next to the cover slip.
6. put a piece of paper towel next to the opposite edge ∴ stain will get drawn under the slip across specimen.

Microscope
1. clip the slide containing the specimen onto the stage
2. select lowest powered objective lens
3. use coarse adjustment knob to bring the stage up to below objective lens
4. look down the eyepiece and use the coarse adjustment knob to move the stage so the image is roughly in focus.
5. adjust the focus with the fine adjustment knob until a clear image is obtained
6. swap to greater magnification with higher-powered objective lens

Eyepiece graticule + stage micrometer
1. place stage micrometer on stage
2. place eyepiece graticule on eyepiece
3. calibrate and line up the scales
4. measure specimen using eyepiece units
5. take REPEAT measurements and calculate mean diameter

REMEMBER

• MAGNIFICATION = IMAGE SIZE / OBJECT SIZE
• stage micrometer is the bigger ‘ruler’
• eye piece graticule is the smaller ‘ruler’

Question 2

Chemical test for:

1. proteins
2. starch
3. lipids
4. reducing sugars (colorimetry)
5. non-reducing sugars

Answer 2

Proteins
1. mix equal volumes of biuret test with protein solution
2. add NaOH then, CuSO4
3. colour change blue to purple

Starch
1. add iodine dissolved in KI (aq)
2. colour change brown-orange to blue-black

Lipids
1. shake lipid solution with ethanol then pour into cold water
2. white/milky emulsion forms at surface of water

Reducing sugars
1. using known conc. of reducing sugar, heat with equal volumes of Benedict’s (aq)
2. colour change from blue, green, yellow, orange, brick red
3. remove ppt
4. calibrate colorimeter using water => calibrates colorimeter to 0
5. using a red filter, measure how much light is transmitted => more transmission ∴ more r sugar present
6. obtain calibration curve by plotting conc. against transmission

Non-reducing sugars (sucrose)
1. breakdown sucrose into its monosaccharides by adding dil. HCl (aq)
2. add NaHCO3 to neutralise it
3. carry out Benedict’t test
4. coloured ppt should form but if it remains blue ∴ no reducing/non-reducing sugars are present

Question 3

How are biosensors used?

biosensors determines conc. of certain molecules in solution

Answer 3

1. biological molecules (enzymes) detects chemical
2. enzyme produces a signal => converted to electrical signal by a transducer
3. electrical signal is processed ∴ can work out (eg - glucose conc.)

Question 4

Investigating cell membrane permeability

Answer 4

Beetroot permeability
1. cut 5 equal sized pieces then rinse and dry
2. place each piece into different test tubes each with 5cm3 water
3. place each test tube in a water bath at different temperatures (10, 20 30…) for same length of time using stopwatch
4. remove pieces of beetroot from tubes, leaving the coloured liquid
5. carry out colorimetry to measure % absorbance/transmission
6. at higher temps ↑ permeability of membrane => more pigment released => ↑ % absorbance and ↓ % transmission of liquid

Control variables
- same age/size/type of beetroot/potato => diff potatoes have diff water potential
- same vol of water
- same length of time
- same temp => affects rate of osmosis

Errors:
- inadequate drying => more mass gained, less mass lost

Question 5

Investigating diffusion in model cells

Answer 5

1. make agar jelly with phenolphthalein and dil. NaOH

Concentration
2. prepare test tubes containing diff conc. of HCl with same vol
3. put an equal sized cube of agar jelly into each test tube and time how long it takes each one to go colourless
4. highest conc. HCl will turn colourless the fastest => steepest conc. gradient

SA
2. cut agar jelly into diff sized cubes and work out SA:V
3. put the diff sized cubes into same conc. + vol of HCl
4. time how long it takes each one to go colourless
5. largest SA:V will turn colourless fastest

Temperature
2. prepare several boiling tubes with same conc. HCl and put them in water baths of varying temps
3. put an equal sized cube into each boiling tube and time how long it takes each one to go colourless
4. highest temp will turn colourless fastest

Question 6

Serial dilution SF 10

Answer 6

1. start with 100% catalase solution and add 1cm3 to first boiling tube along with 9cm3 distilled water
2. using a syringe add 1cm3 of the first solution into the second boiling tube along with 9 cm3 distilled water => makes 10% catalase solution
3. repeat process to make 1%, 0.1% …

Question 7

Investigating the effect of temperature on catalase activity

Answer 7

1. Set up boiling tubes with same vol/conc. of H2O2 => keep pH constant by adding equal vol of buffer solution to each tube
2. set up apparatus to measure vol of O2 produced from each boiling tube => use delivery tube and upside down measuring cylinder
3. put each boiling tube in a water bath set to a diff temp along with another tube containing catalase => equilibriate
4. using a pipette, add same vol/conc. of catalase to each boiling tube => -ve control = boiling tube w/o catalase added to each temp
5. record how much O2 is produced in 60 seconds using a stopwatch
6. repeat at each temp 3x ∴ find mean vol of O2 produced at each temp
7. calculate mean rate of reaction at each temp

To investigate diff variables on catalase activity:
- substrate conc. => keep temp same, but prepare boiling tubes with diff conc. of H2O2 (serial dilution)
- pH => add a buffer solution with diff pH to each tube

Question 8

Dissection

Answer 8

Fish
1. place fish in a dissection tray
2. push back on the operculum and use scissors to carefully remove the gills
3. cut each gill arch through the bone at the top and bottom
4. observe gill filaments

Heart
1. place heart in dissection tray
2. look/feel inside of heart and try to identify 4 main blood vessels
3. identify atria and ventricles
4. use a scalpel to cut along lines to look inside ventricle and use scissors to cut through wall of atrium, follow down to the apex of ventricle
5. open up the atrium and ventricle to examine them
6. measure diameter/thickness of walls of left and right ventricles and atria

Kidney
1. look around the outside of kidney => covered in a renal capsule and beneath is renal cortex
2. part of kidney is indented => renal hilum
3. identify the tubes: renal vein, renal artery and ureter => wall of artery is thicker than wall of the vein => ureter has most adipose (fatty) tissue
4. cut kidney in half lengthways and look at internal structures
5. cortex is denser and a lighter shade than medulla
6. find renal pyramids in the medulla => in between pyramids are the renal columns => base of pyramids are renal calyces
7. renal calyces lead to renal pelvis => connects to ureter

Question 9

Investigating transpiration rates using a potometer

Answer 9

1. cut shoot underwater/slanted => prevents air entering xylem/↑ SA for water uptake
2. assemble potometer underwater and insert shoot underwater => prevents air entering
3. remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water
4. check the apparatus is watertight/airtight using screws/petroleum jelly
5. dry the leaves
6. allow time for the plant to acclimatise then shut the tap
7. remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into water
8. record the starting position of the air bubble
9. start a stopwatch and record the distance moved by the bubble at regular time intervals (every 30 mins)
10. Calculate the rate of air bubble movement by dividing the distance travelled by time => is an estimate of the transpiration rate

Limitations:
- not all the water taken up by the plant is used for transpiration => some is used in cells to maintain turgidity
- some water is used in photosynthesis
- the plant is dying once you cut off its roots ∴ takes up less water

To ↑ validity:
- same light source => light int = constant
- use water baths => temp = constant

Question 10

Investigating phototropism

Answer 10

1. take nine wheat shoots that are equal in height and plant them in individual pots in the same type of soil
2. prepare the shoots as follows:
   - cover the tips of three shoots​ ​with a foil cap​ ​and label A
   - leave three shoots without foil​ ​and label B.
   - wrap the bases of the final three shoots with foil => leaving only the tip exposed and label Shoot C
3. set up the shoots in a light source => ensuring roots are the same distance from the light source and experience the same light intensity => control other variables: moisture, temperature and nutrient concentration (put in same propagator)
4. leave the shoots to grow for 2 days
5. then ​record the amount of growth and the direction​ => give qualitative and quantitative data

Question 11

Investigating geotropism

Answer 11

1. line three petri dishes with moist cotton wool => same vol of water and the same amount of cotton wool in each dish
2. space out 10 cress seeds on the surface of the wool and press them down slightly
3. put a lid on each dish and wrap the dishes in foil => prevents light reaching the seeds
4. leave the dishes where the temperature is constant and warm (airing cupboard)
5. set up the dishes so ​they’re placed at​ ​different angles​:
   - place one dish at 90° ​and label
   - place another dish at ​45°
   - place the third dish on a ​flat/horizontal surface​
6. leave the seeds for 4 days
7. then unwrap each dish and ​note the direction of the shoot and root growth of cress seedlings​ => record in a table

The roots should all have grown ​towards gravity

Question 12

Investigating role of auxins in apical dominance

Answer 12

1. plant 30 similar plants​ ​(same height/age/weight) in pots containing the ​same soil​
2. count and record no. of side shoots​ growing from the main stem of each
   plant
3. for ​10 plants​, remove the tip of the shoot and ​apply a paste containing auxins​ ​to the top of the stem
4. for another ​10 plants​, remove the tip of the shoot and ​apply a paste without auxins to the top of the stem
5. leave the final ​10 plants​ ​as they are =>​controls​ ​for comparison to see the effect of hormone
6. leave the plants to grow for ​6 days​ => keep control variables constant (same light intensity/water/temp)
7. after 6 days, count the ​number of side shoots​ growing from the main stem of each of plant => record in a table

The plants with the auxin paste should prevent extra side shoots from growing

Question 13

Investigating role of gibberellins in stem elongation

Answer 13

1. plant ​40 plants​ => similar age/height/mass in pots containing the same type of soil
2. leave 20 plants to grow => water them in the same way and keep all other conditions the same => negative controls
3. leave the other 20 plants to grow in the same conditions, except water them with a dil. solution of ​gibberellin​ ​(100 mg dm​-3)​
4. let all the plants grow for 28 days
5. every 7 days, measure the ​length of stem of each plant​
6. calculate the ​mean stem length​ ​for the plants watered normally and the plants watered with gibberellin
7. plot a graph of stem length against time for both

Question 14

Investigating the rate of transpiration using a respirometer

Answer 14

1. each tube has NaOH/soda lime => absorbs CO2
2. vol/mass of glass bead = vol/mass of organism
3. control tube is set up in the same way as the test tube but w/o woodlice and with glass beads ∴ results are only due to organism respiring
4. coloured fluid is added to the manometer by dipping end of capillary tube into a beaker of fluid => capillary action makes fluid move into tube => syringe sets fluid to known level
5. leave apparatus for set amt of time (20 mins)
6. there will be ↓ in vol of air in test tube => O2 consumption by organism and CO2 is absorbed
7. ↓ vol => ↓ pressure in test tube ∴ coloured liquid in manometer moves towards test tube (away from control tube w glass bead)
8. measure distance moved by liquid in a given time ∴ calculate vol of O2 taken in by organism => you need to know diameter of capillary tube to do this

Controls
- same temp/vol of NaOH/soda lime
- use same mass of soda lime
- allow apparatus to stabilise in a water bath

To produce precise results => repeat experiment ∴ calc mean vol of O2

Errors
- oxygen uptake may not be a good representation of rate of respiration

• invertebrates move around ∴ higher rate of respiration than seeds
• use larger gas syringe for animals
• use same mass of animals as seeds

Question 15

Investigating factors affecting rate of photosynthesis

Answer 15

1. connect test tube containing pondweed and water with a capillary tube full of water => connected to a syringe
2. place source of light @ specific distance from pondweed
3. leave pondweed to photosynthesise for set amt of time ∴ O2 released collects in capillary tube
4. at the end => use syringe to draw gas bubble in tube alongside a ruler ∴ measure length of gas bubble ∝ to vol of O2 produced
5. repeat whole experiment at diff distances of light source from pondweed

Controls:
- same temp/time pondweed is left to photosynthesise
- same pH

To produce precise results => repeat experiment ∴ calc mean length of gas bubble

Question 16

Measuring effect of temperature on growth of bacteria

Answer 16

1. use a sterile pipette to add set vol of bacteria sample to agar plate and discard pipette
2. spread bacteria across agar plate using sterile spreader and discard after use
3. tape lid on agar plate shut
4. repeat until there’s 6 plates
5. place 3 plate at 4°C and 3 in an incubator at 25°C => plates should be incubated upside down ∴ prevents condensation forming on lid and dropping onto agar
6. put another lidded agar plate => uncultured in the 2 diff temps => controls
7. leave plate for same amt of time (2 days) and observe results
8. count no. of colonies formed on each plate => record results in a table
9. calc mean no. of colonies formed at each temp

Question 17

culturing microorganisms

Answer 17

1. transfer microorganism onto agar plate using sterile inoculation loop/pipette/spreader
2. incubate plates => allow them to grow
3. add nutrients to agar => improve growing conditions

Control:
- agar plate w/o microorganism
- ensure growth is only due to that microorganism
- ensures conditions are aseptic

Question 18

Investigating rates of respiration on yeast - aerobic conditions

Answer 18

1. put a known vol/conc. of substrate solution (glucose) in a test tube
2. add a known vol of buffer solution => keeps the pH constant (choose the optimum pH for the yeast you’re testing => usually 4-6)
3. place the test tube in a water bath at 25˚C => ensures temp stays constant
4. leave for 10 mins to allow temp of the substrate to stabilise
5. add known mass of dried yeast to the test tube and stir for 2 mins
6. after the yeast has dissolved into the solution, put a bung with a tube attached to a gas syringe in the top of
   the test tube => set gas syringe to 0
7. start a stop watch as soon as the
   bung has been put in the test tube
8. as the yeast respire => CO2 formed travels up tube and into gas syringe => measures the vol of CO2 released
9. at regular time intervals (every min) record the volume of CO2 present in the gas syringe => do this for
   a set amount of time (10 mins)
10. a control experiment should also be set up => where no yeast is present ∴ no CO2 should form
11. Repeat the experiment three times ∴ calc the mean rate of CO2 production

Question 19

Investigating rates of respiration on yeast - anaerobic conditions

Answer 19

1. put a known vol/conc. of substrate solution (glucose) in a test tube
2. add a known vol of buffer solution => keeps the pH constant (choose the optimum pH for the yeast you’re testing => usually 4-6)
3. place the test tube in a water bath at 25˚C => ensures temp stays constant
4. leave for 10 mins to allow temp of the substrate to stabilise
5. add known mass of dried yeast to the test tube and stir for 2 mins
6. after the yeast has dissolved into the solution => cover surface of solution with liquid paraffin => forces yeast to respire anaerobically
7. put a bung with a tube attached to a gas syringe in the top of the test tube => set gas syringe to 0
8. start a stop watch as soon as the
   bung has been put in the test tube
9. as the yeast respire => CO2 formed travels up tube and into gas syringe => measures the vol of CO2 released
10. at regular time intervals (every min) record the volume of CO2 present in the gas syringe => do this for
    a set amount of time (10 mins)
11. a control experiment should also be set up => where no yeast is present ∴ no CO2 should form
12. Repeat the experiment three times ∴ calc the mean rate of CO2 production