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How do enzymes speed up reactions, and what changes their activity?

Explain how enzymes act as catalysts for biochemical reactions and how factors such as temperature and pH affect enzyme activity (North Carolina Standard Course of Study, Biology, LS.Bio.1).

A standard-level answer on enzymes for the North Carolina Biology EOC: how enzymes lower activation energy, the lock-and-key model, and how temperature, pH, and concentration affect enzyme-controlled reactions.

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  1. What this topic is asking
  2. What an enzyme does
  3. The lock-and-key model: why enzymes are specific
  4. Temperature, pH, and concentration
  5. Try this

What this topic is asking

North Carolina LS.Bio.1 asks you to explain how enzymes act as catalysts and how temperature and pH affect their activity. For the Biology EOC you need to know that an enzyme lowers activation energy, that it is a protein with a specific active site (the lock-and-key idea), and that each enzyme has an optimum temperature and pH, beyond which it denatures. Items very often give you a graph and ask you to interpret the rise and fall.

What an enzyme does

Reactions need a minimum push to get started, called the activation energy. Without help, many reactions in a cell would be far too slow to keep you alive. An enzyme works by lowering the activation energy, so the reaction proceeds quickly at the cell's normal temperature. Crucially, the enzyme is not consumed: after it releases its product, it is free to bind another substrate molecule, so a small amount of enzyme can process a lot of substrate. The enzyme does not change how much energy the reaction releases overall; it only lowers the barrier to getting there.

The lock-and-key model: why enzymes are specific

The substrate binds in the active site to form an enzyme-substrate complex; the reaction happens, the product is released, and the enzyme is reused. Because everything depends on the shape of the active site, anything that changes that shape changes how well (or whether) the enzyme works.

Temperature, pH, and concentration

Three factors control the rate of an enzyme-controlled reaction, and all three are common EOC graph questions.

  • Temperature. As temperature rises toward the optimum (about 37 degrees Celsius for human enzymes), molecules move faster and collide more often, so the rate increases. Above the optimum, the heat denatures the enzyme: its shape changes, the active site no longer fits the substrate, and the rate falls sharply. This is why the graph rises to a peak and then drops, rather than rising forever.
  • pH. Each enzyme has an optimum pH. Most human enzymes work best near neutral (pH 7), but the stomach enzyme pepsin works best at a strongly acidic pH (around 2). Move too far from the optimum and the enzyme denatures.
  • Substrate concentration. Adding more substrate increases the rate until all the active sites are occupied; after that the rate levels off, because the enzymes are working as fast as they can.

Try this

Q1. Explain what is meant by saying an enzyme is "specific." [2]

  • Cue. Its active site has a shape that fits only one substrate (lock and key), so it catalyzes only that one reaction.

Q2. A digestive enzyme has an optimum pH of 2. Predict its activity at pH 7 and explain. [2]

  • Cue. Low activity, because pH 7 is far from its optimum; the wrong pH changes the active site's shape (denatures it), so the substrate no longer fits.

Exam-style practice questions

Practice questions written in the style of NCDPI exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

NC Biology EOC (style)1 marksHow does an enzyme speed up a chemical reaction? (A) It raises the temperature. (B) It lowers the activation energy needed. (C) It adds energy to the products. (D) It is used up in the reaction.
Show worked answer →

A 1-point item on enzyme action.

The correct answer is B. An enzyme is a catalyst that lowers the activation energy, the energy barrier a reaction must overcome, so the reaction happens faster at body temperature. A is wrong (enzymes do not heat the cell), C misstates energy, and D is wrong because a catalyst is not consumed and can be reused.

Catalyst means it lowers activation energy and is not used up.

NC Biology EOC (style)2 marksA graph shows an enzyme's reaction rate rising as temperature increases to about 37 degrees Celsius, then falling sharply above 45 degrees Celsius. (a) Explain the rise. (b) Explain the sharp fall.
Show worked answer →

A 2-point item interpreting an enzyme graph.

(a) 1 point: as temperature rises toward the optimum, molecules move faster and collide more often, so the reaction rate increases.
(b) 1 point: above the optimum the heat denatures the enzyme, changing the shape of its active site so the substrate no longer fits, and the rate falls.

Markers reward the collision explanation for the rise and denaturation for the fall.

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