Plan and conduct an investigation, and analyze data, to explain how enzymes lower activation energy and how temperature and pH affect enzyme activity (Louisiana Student Standards for Science, High School Biology, HS-LS1).

What this topic is asking

Louisiana's LS1 standards ask you to explain and investigate the chemical processes inside organisms, and enzymes are what make those reactions fast enough to support life. For LEAP 2025 Biology you should know that enzymes are proteins that act as catalysts by lowering activation energy, that each enzyme is specific to its substrate, and how temperature and pH affect enzyme activity. Because this is an investigation-and-data standard, the test often gives a graph of reaction rate and asks you to explain its shape.

What an enzyme is and does

Cells run thousands of reactions, and at body temperature most would happen far too slowly without help. Enzymes provide that help, which is why life depends on them. Because enzymes are proteins, their function comes from their folded shape, linking this topic back to macromolecules.

Lowering activation energy

A useful image is a hill the reaction must climb: the enzyme lowers the height of the hill, so more reactant molecules have enough energy to get over it, and the reaction proceeds quickly.

Specificity and the active site

Each enzyme has a region called the active site, a pocket with a shape that fits only one kind of substrate, the way a key fits one lock. This is why enzymes are specific: an enzyme that breaks down starch will not break down protein. The substrate binds the active site, the reaction happens, the products are released, and the enzyme is free to act again.

Temperature, pH, and denaturation

Because an enzyme's function depends on its shape, conditions that change the shape change its activity:

• Temperature. As temperature rises toward the optimum (around 37 degrees Celsius for human enzymes), molecules move and collide faster, so the rate increases. Above the optimum, heat denatures the enzyme: its shape changes, the active site no longer fits the substrate, and the rate falls sharply.
• pH. Each enzyme has an optimum pH; away from it, the enzyme's shape is disrupted and activity drops. A pH far from the optimum also denatures the enzyme.

Try this

Q1. State what an enzyme does to the activation energy of a reaction, and what this does to the reaction rate. [2]

• Cue. The enzyme lowers the activation energy, which increases (speeds up) the reaction rate.

Q2. Explain why an enzyme stops working at a very high temperature. [2]

• Cue. The high temperature denatures the enzyme, changing the shape of the active site so the substrate no longer fits and the reaction cannot be catalyzed.