Enzymes.

Step 1 of 7

Shape and site

Nearly every chemical reaction in a cell, from splitting sugar to copying DNA, is sped up by an enzyme: a protein that acts as a catalyst. Without enzymes, most of those reactions would take years at body temperature.

In the beaker in the Scene three enzymes swim in water among molecules of a double sugar. Each enzyme has a pocket, the active site, whose shape matches the sugar molecule, the substrate, exactly. A substrate that enters the active site binds, splits into two simple sugars, and they leave.

Step 2 of 7

A lower barrier

For a reaction to happen, molecules must collide with enough energy to break bonds: that is the activation energy. The second chart is the reaction’s energy profile, with an enzyme and without.

Step 3 of 7

Again and again

Follow one enzyme for a few seconds. How many molecules does it split?

Step 4 of 7

Too hot

Two effects of temperature pull opposite ways. On one side, the hotter it is, the faster the molecules move, and the more collisions carry enough energy. On the other, the enzyme is a protein, and in heat its fold unravels, as you saw in the last Lesson.

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Too cold

In the Fridge collisions carry very little energy. But what happens to the fold?

Step 6 of 7

Too acidic

pH, how acidic the water is, changes how the enzyme works too. The substrate holds on in the active site not only by its shape but also by the electric charges of the side chains there.

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Lock and key

Now Temperature and pH are both free. Move each one and watch the activity curve. Every enzyme has an optimum temperature and an optimum pH, and every enzyme has its own substrate.