enzyme prac prep

Define an enzyme.

An enzyme is a protein that acts as a biological catalyst, speeding up chemical reactions without being chemically changed or used up in the process.

Explain how enzymes work.

Enzymes have an active site with a specific shape. Only substrates with a complementary shape can fit into the active site to form an enzyme-substrate complex. The enzyme catalyses the reaction, converting the substrate into products, which are released. The enzyme remains unchanged and can be reused.

Explain why enzymes are specific. (4)

• Active site has a unique shape.

• Only one substrate fits.

• Complementary shape.

• Other substrates cannot bind.

Model Answer

Enzymes are specific because each enzyme has an active site with a unique shape. Only substrates with a complementary shape can bind to the active site and be catalysed. Substrates with a different shape cannot fit, so no reaction occurs.

Explain the effect of temperature on enzyme activity. (5)

• Low temperature → molecules move slowly.

• Fewer successful collisions.

• Activity increases as temperature increases.

• Optimum temperature reached.

• Above optimum, enzyme denatures.

• Active site changes shape.

• Substrate no longer fits.

Model Answer

At low temperatures, enzyme and substrate molecules have little kinetic energy, so they collide less frequently and the reaction is slow. As the temperature increases, the molecules move faster and more successful collisions occur, increasing the rate of reaction until the optimum temperature is reached. Above the optimum temperature, the enzyme becomes denatured because the active site changes shape, preventing the substrate from binding and causing the reaction rate to decrease rapidly.

Explain the effect of pH on enzyme activity (5)

• Each enzyme has an optimum pH.

• Maximum activity at optimum pH.

• pH changes affect bonds.

• Active site changes shape.

• Enzyme denatures.

• Substrate cannot bind.

Model Answer

Each enzyme has an optimum pH at which it works most efficiently. If the pH changes significantly from the optimum, bonds within the enzyme are disrupted, causing the active site to change shape. The enzyme becomes denatured and the substrate can no longer bind, so the reaction rate decreases.

Explain why increasing substrate concentration increases enzyme activity. (4)

• More substrate particles.

• More frequent collisions.

• More enzyme-substrate complexes.

• Rate increases until all active sites are occupied.

• Plateaus when enzymes become the limiting factor.

Model Answer

Increasing the substrate concentration increases the number of substrate particles available to collide with enzyme molecules. This results in more enzyme-substrate complexes being formed and a faster reaction rate. Eventually all the active sites become occupied, so adding more substrate has no further effect because the enzyme becomes the limiting factor.

Explain why increasing enzyme concentration increases the rate of reaction. (4)

• More enzyme molecules.

• More active sites.

• More enzyme-substrate complexes.

• Faster reaction while substrate is available.

Model Answer

Increasing the enzyme concentration provides more enzyme molecules and therefore more active sites for substrate molecules to bind. This increases the number of enzyme-substrate complexes formed each second, increasing the reaction rate as long as there is enough substrate available.

Compare the effects of temperature and pH on enzyme activity. (6)

• Both have an optimum.

• Rate increases to optimum.

• Rate decreases after optimum.

• High temperature denatures enzymes.

• Extreme pH denatures enzymes.

• Active site changes shape.

• Substrate no longer fits.

Model Answer

Both temperature and pH affect the rate of enzyme activity and each enzyme has an optimum value at which it works best. As temperature or pH approaches the optimum, the reaction rate increases. Beyond the optimum, the reaction rate decreases because the enzyme becomes denatured. Denaturation changes the shape of the active site so the substrate can no longer bind, preventing the reaction from taking place.

Explain why enzymes are important in living organisms. (4)

• Catalyse metabolic reactions.

• Increase reaction rate.

• Allow reactions at body temperature.

• Essential for life.

Model Answer

Enzymes catalyse metabolic reactions by increasing their rate, allowing them to occur quickly enough at normal body temperatures. Without enzymes, many essential reactions would occur too slowly to support life.

A student investigated the effect of temperature on amylase activity. Explain why the reaction was fastest at 37°C but slower at 20°C and 70°C. (6)

• 20°C → less kinetic energy.

• Fewer successful collisions.

• 37°C → optimum temperature.

• Maximum enzyme activity.

• 70°C → enzyme denatured.

• Active site changed.

• Starch no longer binds.

Model Answer

At 20°C, enzyme and substrate molecules have less kinetic energy, so they move more slowly and collide less frequently, resulting in a slower reaction. At 37°C, the enzyme is at its optimum temperature, so the greatest number of successful collisions occur and the reaction is fastest. At 70°C, the enzyme becomes denatured because its active site changes shape. The starch substrate can no longer fit into the active site, so the reaction rate decreases sharply.

Describe how you would investigate the effect of temperature on amylase activity.(6)

• Independent variable: Temperature (20°C, 30°C, 40°C, 50°C, 60°C).

• Use water baths to maintain temperatures.

• Mix equal volumes of amylase and starch.

• Test samples with iodine at regular intervals.

• Record the time when iodine remains brown (no starch present).

• Repeat each temperature twice and calculate the mean.

Model Answer

Prepare water baths at 20°C, 30°C, 40°C, 50°C and 60°C. Place equal volumes of amylase and starch into each water bath until they reach the required temperature. Mix the solutions and remove a drop every 30 seconds to place onto iodine on a spotting tile. Record the time when the iodine remains brown, showing that all the starch has been digested. Repeat the experiment twice for each temperature and calculate the mean.

Why is iodine used in the amylase experiment? (5)

• Tests for starch.

• Blue-black = starch present.

• Brown/orange = no starch.

• Indicates when digestion is complete.

Model Answer

Iodine is used because it tests for starch. If starch is present, iodine turns blue-black. When all the starch has been broken down by amylase, the iodine remains brown/orange, showing that the reaction is complete.

State four control variables in an amylase experiment. (5)

Any four:

• Volume of amylase.

• Volume of starch.

• Concentration of amylase.

• Concentration of starch.

• pH (buffer solution).

• Time intervals.

• Same apparatus.

Model Answer

The volume of amylase, volume of starch, concentration of both solutions and the pH should all be kept constant. A buffer solution should be used to maintain the same pH throughout the investigation.

Describe how to investigate the effect of pH on enzyme activity. (6)

• Independent variable: pH.

• Use buffer solutions.

• Keep temperature constant.

• Mix enzyme and substrate.

• Measure time taken or product formed.

• Repeat and calculate the mean.

Model Answer

Prepare buffer solutions of different pH values, for example pH 3, 5, 7, 9 and 11. Add equal volumes of enzyme and substrate to each buffer solution while keeping the temperature constant. Measure the time taken for the reaction to finish or the amount of product produced. Repeat each pH twice and calculate the mean.

Why is a buffer solution used? (4)

• Maintains constant pH.

• Resists changes in pH.

• Makes the investigation fair.

Model Answer

A buffer solution is used to keep the pH constant throughout the investigation. It resists changes in pH, ensuring that only the independent variable is affecting the enzyme activity.

Catalase breaks down hydrogen peroxide. Describe this investigation (5)

• Add catalase to hydrogen peroxide.

• Measure oxygen produced.

• Gas syringe or counting bubbles.

• Change one variable.

• Keep others constant.

Model Answer

Add catalase to hydrogen peroxide and measure the oxygen produced using a gas syringe or by counting the number of bubbles released in a fixed time. Change only one independent variable, such as temperature or pH, while keeping all other variables constant.

Suggest a suitable control for an enzyme investigation. (5)

• Boiled enzyme.

• Water instead of enzyme.

• No reaction expected.

• Shows changes are due to the enzyme.

Model Answer

A suitable control is to use boiled enzyme or replace the enzyme with distilled water. No reaction should occur. This confirms that any reaction observed in the investigation is caused by the enzyme.

6 marks)

Question

A graph shows that the reaction rate increases and then levels off as substrate concentration increases. Explain this.

• More substrate molecules.

• More collisions.

• More enzyme-substrate complexes.

• Active sites become occupied.

• Enzyme is limiting.

• Rate becomes constant.

Model Answer

As substrate concentration increases, there are more substrate molecules available to collide with enzyme molecules, so more enzyme-substrate complexes form and the reaction rate increases. Eventually, all the enzyme active sites become occupied. The enzyme becomes the limiting factor, so increasing substrate concentration further has no effect and the reaction rate levels off.

Why should each experiment be repeated? (5)

• Improves reliability.

• Identifies anomalous results.

• Calculate the mean.

• Reduces random error.

Model Answer

Repeating the experiment improves the reliability of the results. It helps identify anomalous results and allows a mean value to be calculated, reducing the effect of random errors.

7 marks)

Question

Design an investigation to determine the effect of temperature on amylase activity

Aim

• Investigate the effect of temperature on amylase activity.

Independent Variable

• Temperature (°C).

• Use 20°C, 30°C, 40°C, 50°C and 60°C.

• Maintain using water baths.

Dependent Variable

• Time taken for starch to be digested.

• Measure using the iodine test (record the time when iodine remains brown/orange).

Standardised Variables

• 3 cm³ of amylase.

• 3 cm³ of starch.

• Same concentration of amylase.

• Same concentration of starch.

• Same pH (use a buffer solution).

Method

• Place amylase and starch in the water bath.

• Mix them together.

• Test with iodine every 30 seconds.

• Record the time when the iodine remains brown/orange.

• Repeat the procedure for each temperature.

Control

• Use boiled amylase (or distilled water instead of amylase).

Safety

• Wear safety goggles.

• Handle hot water baths carefully.

Repeat

• Repeat each temperature twice.

• Calculate the mean.

Model Answer

Investigate the effect of temperature on amylase activity by preparing water baths at 20°C, 30°C, 40°C, 50°C and 60°C. Place 3 cm³ of amylase and 3 cm³ of starch into each water bath until they reach the correct temperature, then mix them together. Remove a drop of the mixture every 30 seconds and add it to iodine on a spotting tile. Record the time taken for the iodine to remain brown/orange, showing that all the starch has been digested. Keep the pH constant using a buffer solution, and keep the volume and concentration of the enzyme and starch the same throughout the investigation. Repeat the procedure for every temperature, carrying out each experiment twice and calculating the mean. Use boiled amylase as the control. Wear safety goggles and take care when handling the hot water baths.

7 marks)

Question

Design an investigation to determine the effect of enzyme concentration on the rate of amylase activity.

Mark Scheme)

Aim

• Investigate the effect of enzyme concentration on the rate of amylase activity.

Independent Variable

• Enzyme concentration (%).

• Use 20%, 40%, 60%, 80% and 100%.

Dependent Variable

• Time taken for starch to be digested.

• Measure using the iodine test.

Standardised Variables

• 3 cm³ of starch.

• Temperature (keep constant using a water bath).

• pH (keep constant using a buffer solution).

• Concentration of starch.

• Volume of enzyme.

Method

• Prepare the different enzyme concentrations.

• Add the same volume of starch to each.

• Test with iodine every 30 seconds.

• Record the time when iodine remains brown/orange.

• Repeat for each enzyme concentration.

Control

• Use distilled water instead of enzyme.

Safety

• Wear safety goggles.

• Clean up spills immediately.

Repeat

• Repeat each concentration twice.

• Calculate the mean.

Model Answer

Investigate the effect of enzyme concentration on amylase activity by preparing enzyme solutions of 20%, 40%, 60%, 80% and 100%. Add 3 cm³ of each enzyme solution to 3 cm³ of starch solution. Test the mixture with iodine every 30 seconds and record the time taken for the iodine to remain brown/orange, showing that all the starch has been digested. Keep the temperature constant using a water bath, maintain the pH using a buffer solution, and keep the volume and concentration of the starch the same throughout the investigation. Repeat each concentration twice and calculate the mean. Use distilled water instead of enzyme as the control. Wear safety goggles throughout the experiment.

Design an investigation to determine the effect of substrate concentration on the rate of amylase activity.(7)

Investigate the effect of substrate concentration on amylase activity by preparing starch solutions of 2%, 4%, 6%, 8% and 10%. Add 3 cm³ of each starch solution to 3 cm³ of amylase solution. Test the mixture with iodine every 30 seconds and record the time taken for the iodine to remain brown/orange, indicating that all the starch has been digested. Keep the temperature constant using a water bath, maintain the pH using a buffer solution, and keep the volume and concentration of the enzyme the same throughout the investigation. Repeat each concentration twice and calculate the mean. Use distilled water instead of starch as the control. Wear safety goggles and handle glassware carefully.