1. Independent Variable: The independent variable is the temperature at which the catalase and hydrogen peroxide reaction is carried out. The student deliberately varied the temperature.

2. Controlled Variables: The controlled variables are factors that were kept the same across all three test tubes to ensure that only the temperature affected the rate of reaction. These would include:

• The concentration of catalase
• The concentration of hydrogen peroxide
• The volume of hydrogen peroxide
• The volume of catalase
• The volume of the test tube
• The time period over which the oxygen production was measured

3. Increase in Reaction Rate with Temperature (up to a point): Increasing the temperature increases the kinetic energy of the molecules involved in the reaction. This leads to more frequent and more forceful collisions between the enzyme and substrate molecules. A higher proportion of collisions will have enough energy to overcome the activation energy barrier, resulting in a faster rate of reaction. More enzyme molecules are also in a more active conformation at higher temperatures.

4. Decrease in Reaction Rate at Higher Temperatures: At very high temperatures, the enzyme molecules begin to denature. Denaturation is the unfolding of the enzyme's 3D structure. This disrupts the shape of the active site, preventing the substrate from binding effectively. As more enzymes denature, the overall catalytic activity decreases, leading to a slower reaction rate. This is why there is an optimal temperature for enzyme activity; beyond this temperature, the rate of reaction declines sharply.

Diagram: (A diagram would be inserted here. Since this is text-based, I will describe the diagram. The diagram should show a key (enzyme) with a unique, specific shape. The lock (active site) is designed to perfectly fit the key. The substrate is represented as a molecule that fits into the lock. Arrows indicating the reaction occurring within the lock are also included.)

Explanation: The lock-and-key model proposes that the active site of an enzyme has a specific three-dimensional shape that is complementary to the shape of its substrate. This complementary shape allows the substrate to bind to the active site in a precise way, much like a key fits into a lock.

The specificity of an enzyme arises from this precise fit. Only substrates with a shape that matches the active site can bind effectively. Substrates with different shapes will not be able to bind, and therefore the enzyme will not be able to catalyze a reaction with them. This explains why enzymes are highly specific – each enzyme typically only works on a particular substrate or a small group of similar substrates. A change in the shape of the active site, due to mutations or denaturation, can alter the specificity of the enzyme, potentially preventing it from binding to its substrate.

Enzyme activity is highly sensitive to both temperature and pH. These factors influence the enzyme's structure and, consequently, its ability to bind to the substrate and catalyze the reaction.

Temperature:

• Increasing Temperature (up to a point): As temperature increases, the rate of the reaction generally increases. This is because molecules have more kinetic energy, leading to more frequent and more forceful collisions between enzyme and substrate.
• Optimum Temperature: Each enzyme has an optimum temperature at which it works most efficiently. This is the temperature at which the enzyme's 3D structure is optimal.
• High Temperatures (beyond the optimum): Above the optimum temperature, the enzyme's 3D structure begins to denature. Denaturation involves the unfolding of the protein, disrupting the active site and rendering the enzyme inactive. This denaturation is often irreversible.

pH:

• pH and Enzyme Structure: pH affects the ionization state of amino acid side chains in the enzyme. This can alter the enzyme's 3D structure and the charge of the active site.
• Optimum pH: Each enzyme has an optimum pH at which it is most active. This is the pH at which the enzyme's structure is optimal.
• Extreme pH Values: Outside the optimum pH range, the enzyme can denature. Extreme pH values can disrupt the ionic bonds and hydrogen bonds that maintain the enzyme's 3D structure. This can lead to irreversible denaturation and loss of activity.

Summary Table:

Therefore, maintaining the correct temperature and pH is crucial for enzymes to function effectively and for organisms to sustain life.