• Quick retrieval from earlier learning (Digestive enzymes and the molecules they make). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Which enzyme is described here: breaks fats down into fatty acids and glycerol? (amylase (a carbohydrase) / protease / lipase) → lipasePrompt: The enzyme's name often sounds like the food it breaks down.
2. Amylase was mixed with starch at different temperatures. Iodine was used to time how long the starch took to disappear: 20 °C: 10 min 30 °C: 6 min 40 °C: 3 min 50 °C: 7 min 60 °C: starch still present after 20 min At which temperature did amylase work fastest, in °C? → 40 °CPrompt: Fastest means the shortest time.
3. Which organ makes bile? (pancreas / gall bladder / stomach / liver) → liverPrompt: Bile is stored in one organ but made in another.

• Show a microscope image of cells and ask what students think they are looking at.
• Share the objective: How enzymes work: active sites, temperature and pH. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Why does a reaction speed up as temperature rises towards the optimum?
• Step 1: More kinetic energy means more successful collisions between enzyme and substrate.
• Step 2: So the answer is: Molecules move faster, so enzyme and substrate collide more often
• Answer: Molecules move faster, so enzyme and substrate collide more often
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Why can an enzyme only act on one type of substrate? (Enzymes are too big / Only that substrate fits the shape of its active site / Enzymes are used up after one reaction / Other substrates are dead) → Only that substrate fits the shape of its active sitePrompt: Think of a lock and its key.
2. Enzymes: which statement is NOT true? (Enzymes are proteins. / Cold temperatures denature enzymes permanently. / Enzymes are not used up in the reactions they speed up. / Most human enzymes work best at about 37 °C.) → Cold temperatures denature enzymes permanently.Prompt: Is the effect of cold permanent?

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. A human enzyme has an optimum temperature of about 37 °C. What is the enzyme most likely to be doing at 65 °C? (working slowly, but not denatured / working at or near its fastest rate / denatured and no longer working) → denatured and no longer workingPrompt: Cold slows molecules down; it does not change the active site's shape.
2. Enzymes: which statement is NOT true? (Enzymes are living things. / Enzymes are biological catalysts. / Enzymes are not used up in the reactions they speed up. / Enzymes are proteins.) → Enzymes are living things.Prompt: Is the effect of cold permanent?

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.