Try making jelly with fresh pineapple and it won't set. Use canned pineapple and it sets perfectly. The difference is one enzyme, and it's a good way into Unit 3.
Enzymes
Fresh pineapple contains bromelain, an enzyme that breaks down proteins, including the gelatin that makes jelly set. Canned pineapple has been heated, which denatures the enzyme: its shape changes, its active site no longer fits, and it stops working.
Enzymes speed reactions up by lowering the activation energy. Each one works best at an optimum temperature and pH. Inhibitors can slow them down:
- Competitive inhibitors block the active site; more substrate can outcompete them.
- Noncompetitive inhibitors bind elsewhere and change the enzyme's shape; more substrate doesn't help.
Photosynthesis: capturing energy
In the chloroplast, plants turn light energy into chemical energy:
6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
- Light-dependent reactions (thylakoid membranes): light splits water, releasing O₂, and the energy is stored as ATP and NADPH.
- Calvin cycle (stroma): ATP and NADPH are used to fix CO₂ into sugar, starting with the enzyme rubisco.
Cellular respiration: spending it
Every living cell, plant cells included, breaks down glucose to make ATP:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
- Glycolysis (cytoplasm): glucose → 2 pyruvate, a little ATP, no oxygen needed.
- Krebs cycle (mitochondrial matrix): carbons leave as CO₂; energy is captured as NADH and FADH₂.
- Oxidative phosphorylation (inner membrane): the electron transport chain pumps H⁺ ions, which flow back through ATP synthase to make most of the ATP. Oxygen is the final electron acceptor and becomes water.
Without oxygen, cells fall back on fermentation, which keeps glycolysis going but makes far less ATP.
Notice the mirror image: photosynthesis and respiration both use an electron transport chain and chemiosmosis to make ATP.
Common mistakes students make
- "Plants photosynthesise instead of respiring." Plants respire all the time; in the light, photosynthesis just outpaces it.
- "The oxygen from photosynthesis comes from CO₂." It comes from water.
- "Oxygen is used to break down glucose." Its job is at the very end, accepting electrons.
- "Enzymes are used up." They're reused; only denaturation stops them.
Try it
Say each answer before you flip. Then ask yourself: what would you expect to see if you added boiled pineapple juice to a bowl of setting jelly?
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