Calculations for enzymes

Qualitative results

Show presence or absence of a substance only

Quantitative results

Measure how much of a substance is present

Rate of reaction

Amount of substrate used or product formed per unit time

Example rate calculation

Rate = 0.1 g ÷ 60 s = 0.002 g s⁻¹ (2 mg s⁻¹)

Why rate decreases over time

Substrate concentration decreases so fewer collisions occur

How to measure starch concentration

Use iodine and a colorimeter to measure absorbance

Colorimeter

Measures optical density (absorbance or % transmission)

Calibration graph

Graph used to convert colorimeter readings into concentrations

Independent variable

Variable you change (plotted on x-axis)

Dependent variable

Variable you measure (plotted on y-axis)

Graph rules

Use pencil, clear scale (1,2,5,10), label axes with units, plot clear points

Line of best fit

Smooth line showing trend with roughly equal points on both sides

Trend in starch hydrolysis graph

Starch concentration decreases over time

Why reaction stops

All substrate is used up

Why initial rate is highest

Substrate concentration is highest so collisions are most frequent

Initial rate of reaction

Rate at the very start of a reaction

How to determine initial rate

Calculate from first data point or draw tangent at start of curve

Tangent method

Draw line touching curve at start and calculate its gradient

Catalase enzyme

Enzyme that breaks down hydrogen peroxide into water and oxygen

Catalase reaction

2H2O2 → 2H2O + O2

How to measure catalase activity

Measure volume of oxygen produced over time

Apparatus for catalase experiment

Water bath, reaction mixture, gas collection (gas syringe or water displacement)

Control variable in enzyme experiments

Temperature must be kept constant

Why use plant extract for catalase

Cheaper source of enzyme (e.g. potato, celery)

Volume vs time graph

Shows increase in product (oxygen) over time

Shape of enzyme graph

Curve that rises quickly then levels off

Reason graph levels off

Substrate is used up so reaction slows and stops

Turnover number

Number of substrate molecules converted per enzyme per unit time

Catalase turnover

Very high, making it one of the fastest enzymes

Why enzyme efficiency varies

Depends on fit between substrate and active site

Not all collisions successful

Only some enzyme-substrate collisions lead to reaction

Advantage of colorimeter

Provides accurate, quantitative measurements

Precautions with colorimeter

Use same volumes, same timing, clean cuvette, calibrate properly

Why consistent method is important

Ensures valid and reliable results

Semi-quantitative test

Estimates concentration using colour comparison

Quantitative starch test

Uses iodine + colorimeter + calibration curve for exact concentration