Required Practicals Flashcards
Give examples of variables that could affect the rate of an enzyme controlled reaction:
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Enzyme concentration / volume
- Substrate concentration / volume
- Temperature of solution
- pH of solution
- Inhibitor concentration
Describe how temperature can be controlled.
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Use a thermostatically controlled water bath
- Monitor using a thermometer at regular intervals
- add hot / cold water if temperature fluctuates
Describe how pH can be controlled.
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Use a buffer solution
- Monitor using a pH meter at regular intervals
Why were the enzyme & substrate solutions left in the water bath for 10 mins before mixing?
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
So solutions equilibrate / reach the temperature of the water bath
Describe a control experiment.
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Use denatured enzymes (eg. by boiling)
- Everything else same as experiment, eg. same conc. / volume of substrate (at start) and enzyme, same type / volume of buffer solution, same temperature
Describe how the rate of an enzyme-controlled reaction can be measured
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
Suggest a safety risk and explain how to reduce this risk.
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Handling enzymes may cause an allergic reaction
- Avoid contact with skin by wearing gloves and eye protection
Explain why using a colorimeter to measure colour change is better than comparison to colour standards.
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Not subjective
- More accurate
Explain a procedure that could be used to stop each reaction.
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Boil / add strong acid / alkali → denature enzyme
- Put in ice → lower kinetic energy so no E-S complexes form
- Add high concentration of inhibitor → no E-S complexes form
Describe how processed data can be presented as a graph
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Independent variable on x axis, rate of reaction on y axis, including units
- Linear number sequence on axis, appropriate scale (graph should cover at least half of grid)
- Plot coordinates accurately as crosses
- Join point to point with straight lines if cannot be certain of intermediate values OR draw a smooth curve but do not extrapolate
Explain why the rate of reaction decreases over time throughout each experiment
Required Practical (1) : Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.
- Initial rate is highest as substrate concentration not limiting / many E-S complexes form
- Reaction slows as substrate used up and often stops as there is no substrate left
Describe how to prepare squashes of cells from plant root tips (5)
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Cut a thin slice of root tip (5mm from end) using scalpel and mount onto a slide
- Soak root tip in hydrochloric acid then rinse
- Stain for DNA eg. with toluidine blue
- Lower coverslip using a mounted needle at 45 degrees without trapping air bubbles
- Squash by firmly pressing down on glass slip but do not push sideways
Common questions
Why are root tips used?
Required practical (2) : Preparation of stained squashes of cells from plant root tips
Where dividing cells are found / mitosis occurs
Common questions
Why is a stain used?
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- To distinguish chromosomes
- Chromosomes not visible without stain
Common questions
Why squash / press down on cover slip?
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- (Spreads out cells) to create a single layer of cells
- So light passes through to make chromosomes visible
Common questions
Why not push cover slip sideways?
Required practical (2) : Preparation of stained squashes of cells from plant root tips
Avoid rolling cells together / breaking chromosomes
Common questions
Why soak roots in acid?
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Separate cells / cell walls
- To allow stain to diffuse into cells
- To allow cells to be more easily squashed
- To stop mitosis
Describe how to set-up and use an optical microscope (5)
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Clip slide onto stage and turn on light
- Select lowest power objective lens (usually x 4)
- Use coarse focusing dial to move stage close to lens then turn coarse focusing dial to move stage away from lens until image comes into focus
- Adjust fine focusing dial to get clear image
- Swap to higher power objective lens, then refocus
What are the rules of scientific drawing?
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Look similar to specimen / image
- No sketching / shading - only clear, continuous lines
- Include a magnification scale (eg. x 400)
- Label with straight, uncrossed lines
Explain how the prophase stage of mitosis can be identified
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Chromosomes visible / distinct → because condensing
- But randomly arranged → because no spindle activity / not attached to spindle fibre
Explain how the anaphase stage of mitosis can be identified
Required practical (2) : Preparation of stained squashes of cells from plant root tips
Chromosomes lined up on equator → because attaching to spindle
Explain how the metaphase stage of mitosis can be identified
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Chromatids (in two groups) at poles of spindle
- Chromatids V shaped → because being pulled apart at their centromeres by spindle fibres
Explain how the telophase stage of mitosis can be identified
Required practical (2) : Preparation of stained squashes of cells from plant root tips
Chromosomes in two sets, one at each pole
What is a mitotic index?
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Proportion of cells undergoing mitosis (with visible chromosomes)
- Mitotic index = number of cells undergoing mitosis / total number of cells in sample
Explain how to determine a reliable MI from observed squashes
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Count cells in mitosis in field of view
- Count only whole cells / only cells on top and right edges → standardise counting
- Divide this by total number of cells in field of view
- Repeat with many / at least 5 fields of view selected randomly → representative sample
- Calculate a reliable mean
Suggest how to calculate the time cells are in a certain phase of mitosis
Required practical (2) : Preparation of stained squashes of cells from plant root tips
- Identify proportion of cells in named phase at any one time
- Number of cells in that phase / total number of cells observed
- Multiply by length of cell cycle
