Biology PRACTICAL

If the first and last percentage concentration is given, what must be made sure for the concentrations in between

Percentage concentration is to have equal intervals

Explain why you chose this volume of sodium chloride solution (50.0cm³ of sodium chloride solution is chosen in previous part to be added to each Petri dish containing and piece of potato tissue)

Volume is sufficient to completely immerse the potato tissue

Ensures that all the surfaces of the potato are exposed to the same sodium chloride concentration, allowing osmosis to occur uniformly and improving the reliability of the results.

Explain the trend shown by your results in terms of water potential

As the percentage concentration of sodium chloride increases, the water potential of the solution becomes lower, water moves out of the plant tissue by osmosis from a region of higher water potential to a region of lower water potential. The cells lose water and become less turgid, causing greater shrinkage/bend more, so the distance between the mark and the horizontal line increases

Suggest how you could make improvements to the procedure so that a more accurate estimate of the concentration of sodium chloride in U can be obtained

Test more sodium chloride concentrations, especially in smaller intervals around the estimated concentration of U

Identify sources of error in this investigation on bending of potato strips in different concentrations of sodium chloride. Explain how this error could affect your results.

• Judging when there is “some resistance” when bending the potato strip → different amounts of force may be applied each time, so the measured distance may be higher or lower, reducing reliability

• Biological variation between potato strips e.g. not from the same potato → potato strips may differ in water potential and tissue properties, so they may bend by different amounts even in the same solution

• Potato strips in different solutions were removed and processed sequentially, meaning strips measured later had a longer immersion time than the specified 20 minutes → different immersion times may result in different amounts of osmosis and different bending distances

Suggest why the starch grains get smaller as plants germinate

As plants germinate, they require more glucose from the breakdown of starch, amount of starch in the plant decreases. Starch grains which is a storage of starch becomes smaller in size.

In land plants, one of the main functions of the epidermis is to reduce water loss.

From your observations of slide M1, suggest one way in which the upper epidermis of land plants is adapted to reduce water loss

• Absence of stomata

• Upper epidermis is thicker

How to describe a plasmolysed cell?

Cell membrane pulled away from the cell wall, cell is plasmolysed

In another investigation to find the water potential of onion tissue, the first step is to find the concentration of sucrose solution at which 50% of the cells in the epidermal tissue are plasmolysed and 505 are not plasmolysed

Describe how you could use the results to find the water potential of the onion tissue

1. Plot a graph of percentage plasmolysed against concentration of sucrose solution

2. Draw a line of 50% plasmolysis and read off the concentration OR

1. Find concentration above and below closest to those that give percentage of 50% plasmolysis

2. Make solutions in this range and repeat experiment to find the concentration giving 50% plasmolysis

Explain why it is necessary to collect a result for the 0% sodium chloride solution and yeast mixture

To show that decrease in catalase activity/temperature is due to the presence of sodium chloride solution

Show how variables could be controlled so that they do not affect the results of the investigation on the effect of the concentration of hydrogen peroxide on the activity of catalase

• Temperature → Use a thermostatically controlled water bath set at 35-37 degree celsius

• Concentration of catalase → Use yeast from the same species and culture of yeast and add it to a specific volume

• Equilibrium time → Place test tubes containing yeast cells and hydrogen peroxide separately in the water bath for 5 minutes before starting the reaction

• pH → Use a 1cm³ pH 7 buffer

Suggest one improvement in this experiment of observing colour corresponding to light intensity, other than including controls or repeating the procedure, that would increase confidence in the results

Use a colorimeter to record absorbance values of the water sample

Describe sources of error in the procedure in the investigation of the effects of an inhibitor, copper sulfate, on the enzyme catalase, that reduce confidence in your results recorded

• Stacking the potato discs reduces the surface area exposed to the copper sulfate solution → reduce the catalase concentration exposed to hydrogen peroxide → lower number of drops released per minute

• Drops produced may vary in size/volume → number of drops per minute is not directly proportional to the rate of decomposition of hydrogen peroxide

• Imprecise cutting of potato pieces results in inconsistent potato disc size or thickness → different surface areas and amounts of catalase affecting the measured rate

• Tapping the syringe to dislodge blocked discs may artificially increase rate of drop release

How to describe colour change for a test

Transparency + colour

e.g. Content remained a slightly cloudy, colourless solution. No white suspension

e.g. Content changed from a clear, blue solution to a clear, purple solution

What to note about independent variable when drawing table

Independent variable must be the most left column

Identify what type of error this is and the effect on the results.

The line at 1.0cm³ syringe used in step 21 actually measures a volume of 0.95cm³ and not 1.0cm³

Systematic error

No effect → Consistently wrong, trend is the same

What to note when describing distribution of something in the cells

In what manner

e.g. randomly scattered

Choose the lens that is most suitable for counting the number of stomata in the field of view.

State which objective lens you have decided to use and give a reason for your choice

Choice: Higher power objective → shows accurate stomata count, smaller number of stomata so less likely to miss or count twice

OR

Choice: low power objective → gives large sample size → more representative samples

What to note about sodium hydroxide, sodium carbonate and sodium hydrogen carbonate

Sodium carbonate and sodium hydroxide absorbs carbon dioxide

Sodium hydrogen carbonate supply carbon dioxide

What to note about small numbers when recording results in table

Don’t leave answer in d.p. → use standard form or 1000s^-1

For each concentration of sodium hydrogen carbonate solution, the time taken for the leaf discs to float varies.

Give a reason for this variation

• Variation in leaf disc mass or internal structure → due to differences in cellular composition → different volume of oxygen are required for the discs to become buoyant

• Shading effect between leaf discs → discs overlap or stack on one another during the experiment → reducing light exposure for some discs → this leads to differences in photosynthesis rate and oxygen production

Describe precautions that must be taken when preparing the leaf discs in order to increase the reliability of the results

• Avoid cutting through midrib → select discs only from the leaf blade to ensure uniform thickness and tissue compositin

• Use leaf discs taken from the same leaf to reduce variation in photosynthesis activity

Predict and explain what would happen if one were to place the syringe containing floating leaf discs in the dark for an hour. What additional information will one get from this experiment?

The leaf discs will gradually sink to the bottom of the syringe → photosynthesis stops in the absence of light because light is required for the light-dependent reaction but respiration continues → oxygen is required as the final electron acceptor → net decrease in gas in the leaf tissues → leaf discs reduced buoyancy and sinks

This shows that measured photosynthesis is a net/apparent rate and provides the indication of respiration rate

For roots, when asked to compare, what are the possible points of comparison

• Presence of pith

• Position of xylem (scattered vs arranged in a ring)

• Presence of endodermis

• Presence of root hairs

• Number of large xylem vessles

• Presence of air spaces

For stems, when asked to compare, what are the possible points for comparison

• Size of vascular bundle (but need to measure out or explain in proportion)

• Arrangement of vascular bundle (scattered vs found in a ring)

• Thickness of epidermis

• Bundle size (varying vs uniform)

• Location of vascular bundles (periphery only vs centre and periphery, note that periphery is nearer to the edge of stem)

Validity vs reliability vs accuracy

Validity → either suggest improve accuracy or reliability

Reliability → keep other variables constant

Note: repeatability is a subset of reliability, repeating checks for repeatability but does not improve it

Accuracy → changing measuring methods or collecting more data points for graph-plotting

Replicates vs repeats

Replicates → done within a single experimental setup or batch

Repeats → the entire experiment is run a new

Explain why isolated chloroplasts will need to be suspended in a cold buffer solution

• Slow down enzyme reactions that would damage chloroplasts

• Buffer solution to minimise fluctuations in pH to keep enzyme activity constant/prevent enzyme denaturation

Describe and explain the relationship between light intensity and the rate of reduction of DCPIP by isolated chloroplasts

• As light intensity increases from [quote], rate of reduction of DCPIP increases from [quote]

• Light provides energy for photosynthesis. As light intensity increases, more photons of light strike the photosynthesis pigments and more energy is absorbed and relayed to pigment molecules

• More electrons in special chlorophyll a in the reaction centre of PSI and PSII are excited and emitted

• Electrons combine with protons from water to form hydrogen atom which reduces NADP to NADPH

• Since DCPIP serves as a substitute for NADP, at higher light intensity, more electrons and more protons are available to reduce DCPIP, leading to higher rate of reduction

Suggest how the results of your investigation (increasing light intensity increases rate of reduction of DCPIP) might explain the differences in the growth of the plants shown (plants further from large tree which shades path, experience increasing light intensity and hence faster growth)

• As light intensity increases, rate of reduction of NADP to NADPH increases

• NADPH is used to provide reducing power in the light independent reaction

• At higher light intensity, plants further from the large tree produce more triose phosphate which can be combined to produce more hexose sugars such as glucose

• Polymerisation of glucose to form cellulose/faster production of cellulose for cell wall results in faster growth of plants.

Generally, how to record measurements based on apparatus

What are the exceptions

Record to half of the smallest division

Exceptions:

Ruler: 0.1cm if the smallest division but record to 0.1cm

Stopwatch: 0.01s smallest division but record to 1s

Electronic balance: read off

Suggest an advantage of having air spaces in plant stems that grow submerged in water

• Provides buoyancy for the plant

• Allows for rapid diffusion of carbon dioxide for photosynthesis

What is the use of staggering

e.g. transfer boiling tube with yeast culture to the water bath. Repeat steps for activated yeast cultures 2 to 4 at 2 min intervals

To ensure they are all incubated for the same amount of time

For the experiment of different substrates on the rate of gas production by yeast, other than a lack of replicate or repeat, suggest one source of error in this procedure and suggest an improvement that can be made to reduce the error stated

Yeast culture may leave the syringes during the experiment. Loss of yeast culture may result in inaccurate volume of gas collected in the syringe

• Place the yeast culture in a boiling tube with water and stopper immediately with a bung which can be connected to a gas syringe

• Place fine mesh or gauze securely over the nozzle of the syringe to prevent loss of yeast cells

Explain why the students calculated the percentage change in mass of the onions

Allows valid comparison between onions in different sodium chloride concentrations as, initial mass of onions is not constant

One of the students concluded that: “The water potential of the onion cells is the same as the water potential of a 4.2% sodium chloride solution”

With reference to the information provided, including the results shown in Fig 1.2. (graph), suggest reasons why this conclusion should not be accepted

• Line of best fit may give different more accurate value

• Intermediate concentrations of sodium chloride not tested → less accurate results

• Qualified experimental error

• Not all onions have the same water potential

Using Fig.1 and Fig. 2 to guide you, locate the pollen sac in your slides. Describe the difference in appearance of the genetic material observed in early prophase and late prophase

• In early prophase, chromatin is observed while in late prophase condensed chromosomes can be seen clearly

• In early prophase, nucleolus can be observed while in late prophase, nucleolus cannot be seen

If never ask for calibration step earlier but ask for actual size of cell, what to do

Include calibration step and objective lens used

Why were beetroot discs washed several times before they were used for the various treatments

To remove the pigments leaked from the cut cells which will affect the accuracy of the data collected

The colorimeter was blanked using distilled water. What is the purpose of this

Used to eliminate the background absorbance from the distilled water

OR

Distilled water is used as the standard value to be compared to

It was found that the betacyanin turned darker red when acid is added and become yellow when alkali is added. Use the universal indicator to determine the pH of lead acetate solution. In view of this information, explain the result with lead acetate at 40 degrees celsius (absorbance value is low for solution of lead acetate 40 degrees celsius with beetroot which contains betacyanin pigment)

The acidic lead acetate (pH=6) denatures the proteins by disrupting the ionic and hydrogen bonds, causing the leakage of the red pigment from the cell

Using the data obtained above, explain the results obtained

[Data: Increasing concentration of ethanol causes the exponential increase in absorbance values.]

Increasing concentration of ethanol causes greater loss of integrity of cell membrane due to ethanol dissolving phospholipids/denaturing membrane proteins

Increased leakage of red pigment out of the cell caused higher intensity of red colour in the liquid and was detected as higher readings by the colorimeter

What is the formula for cell potential

Cell potential = solute potential + water potential

In the experiment of determining the solute potential of onion cells using the incipient plasmolysis method, excluding reference to controls or repeats, state one limitation in the experimental procedure and suggest a change that can increase confidence in the results

The sharp increase in the percentage of plasmolysis with a small change in sucrose concentration at around the 50% plasmolysis makes the estimation of the actual sucrose concentration less accurate

Increase the number of different sucrose concentrations being used around the region of the graph where 50% of the cells are plasmolysed, this will lead to a more accurate determination of the solute potential of the epidermal cells from the graph

For starch grains, when asked to compare, what are the possible points for comparison

• Size of starch grain

• Shape of starch grain (e.g. oval vs angular)

• Pattern lines in starch grain (e.g. concentric rings vs star-like)

Typically, for the different objective lens, 1 division of the eyepiece graticule is equal to how many micrometers

4x objective lens (40x magnification): 2.5 divisions of the stage micrometer = 25 micrometers

10x objective lens (100x magnification): 1 divisions of the stage micrometer = 10 micrometers

40x objective lens (400x magnification): 0.25 divisions of the stage micrometer = 2.5 micrometers

What is sclerenchyma

• A supportive tissue that is often found near or surrounding vascular bundles as part of the pericycle

• Cell walls are thick with an empty lume

• Cells tend to be angular

Stem cells: dicot plants vs monocot plants

• In dicot plants, vascular bundles are arranged in a ring whereas in monocot plants, the vascular bundles are scattered throughout the stem

• In dicot stem, xylem and phloem is separated by a layer of of cambium whereas in monocot plants, cambium is absent

Root cells: dicot plants vs monocot plants

• In dicot root, cross section has an X-shaped structure at its center whereas in monocot root, center of the root is filled with pith

• In dicot root, X is made up of many xylem cells, phloem cell fill the space between the X, whereas in monocot root, phloem cells and the larger xylem cells form a ring around the central pith

Identify one source of error in the procedure that affects the temperature at which sucrose is broken down in this investigation

Suggest an improvement in the method to overcome this error

Error: The invertase solutions are not equilibrated to 40 degrees celsius, so when added at room temperature they lower the temperature of the reaction mixture, meaning the reaction does not start at 40 degrees celsius

Improvement: Place the invertase solutions in the water bath for 5 minutes before mixing, so both enzyme and substrate are at the same temperature before the reaction begins

Instead of counting the number of colonies that grew on each agar plate, the scientists could have counted the number of cells in each 1cm³ sample using a method that gives accurate results

Suggest why knowing the number of cells in each 1cm³ sample would not give a useful indication of the effectiveness of salicyclic acid in treating C.albicans infections

The method would have count both living and dead cells, so it would not show whether the salicyclic acid has actually killed or inhibited the yeast.