3.4: Determining the initial rate of reaction

What is meant by the initial rate of reaction?

The rate of reaction at the very beginning (time zero), when substrate concentration is highest.

Why is the initial rate the fastest?

Because substrate concentration is highest → maximum collisions between enzyme and substrate.

Why does the rate decrease over time in enzyme reactions?

Substrate is used up → fewer collisions → fewer enzyme-substrate complexes.

Why is it important to measure the initial rate?

It gives the most accurate measure of enzyme activity before substrate becomes limiting.

What are the two ways to follow an enzyme reaction?

• Disappearance of substrate

• Appearance of product

Which enzyme is commonly used to study product formation?

Catalase

What reaction does catalase catalyse?

Hydrogen peroxide → water + oxygen

Write the balanced equation for catalase activity

2H₂O₂ → 2H₂O + O₂

How can catalase be obtained for experiments?

From plant extracts (e.g. potato, celery, lettuce) or yeast.

How is catalase extracted from plant material?

• Cut plant tissue

• Blend with water

• Filter → obtain enzyme extract

How is the rate of catalase activity measured?

By measuring the volume of oxygen produced over time.

How can oxygen be collected in this experiment?

• Gas syringe

• Downward displacement of water

What is a key control variable in this experiment?

Temperature (kept constant using a water bath)

What type of graph is plotted in this experiment?

Volume of oxygen vs time

What is the shape of the graph for enzyme reactions?

A curve that rises quickly then levels off.

Why does the graph level off?

All substrate is used up → no more product formed.

How can the initial rate be determined from a graph?

By drawing a tangent at time zero and calculating its gradient.

What is a tangent?

A straight line that touches a curve at one point and shows the slope at that point.

Alternative way to estimate initial rate?

Use the first data point(s) to calculate rate.

What is meant by gradient in this context?

Change in volume ÷ change in time

What are the units for rate in this experiment?

cm³ s⁻¹

What is a line of best fit?

A line that best represents the overall trend of the data.

How should a line of best fit be drawn?

• Pass through most points

• Equal points above and below

• Smooth curve if needed

What if data points don’t show a clear pattern?

Join points with straight lines to show uncertainty.

What is turnover number?

The maximum number of substrate molecules converted to product per enzyme per unit time.

What does turnover number indicate?

Enzyme efficiency

What factors affect enzyme efficiency?

• Fit between substrate and active site

• Ability to catalyse reaction

• Frequency of successful collisions

Do all enzyme-substrate collisions result in a reaction?

No — only successful collisions form products.

What happens to initial rate when substrate concentration increases?

It increases (more collisions → more enzyme-substrate complexes)

What happens to initial rate when temperature increases?

It increases (up to optimum) due to more kinetic energy and collisions.