Notes
Enzymes
Enzymes are biological catalysts (usually proteins) that lower activation energy. The substrate binds the active site, which has a specific shape and chemistry.
- Temperature and pH affect enzyme shape. Too far from the optimum and the enzyme denatures.
- More substrate increases rate until the enzymes are saturated.
- Competitive inhibitors bind the active site (overcome by adding more substrate). Noncompetitive inhibitors bind elsewhere (an allosteric site) and change the enzyme's shape.
- Cells use ATP to couple energy-releasing (exergonic) reactions to energy-requiring (endergonic) ones.
Photosynthesis
$6CO_2 + 6H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6O_2$
- Light reactions (thylakoid membranes): light excites chlorophyll in photosystems II and I. Splitting water releases $\mathrm{O_2}$ and electrons. The electron transport chain pumps $\mathrm{H^+}$ into the thylakoid space, and ATP synthase makes ATP as $\mathrm{H^+}$ flows back out. NADPH is also made.
- Calvin cycle (stroma): uses ATP and NADPH to fix $\mathrm{CO_2}$ (with the enzyme rubisco) into sugar.
Cellular respiration
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$
| Stage | Location | Produces |
|---|---|---|
| Glycolysis | Cytosol | 2 pyruvate, net 2 ATP, NADH |
| Pyruvate oxidation and Krebs cycle | Mitochondrial matrix | $\mathrm{CO_2}$, NADH, $\mathrm{FADH_2}$, a little ATP |
| Electron transport and chemiosmosis | Inner mitochondrial membrane | Most of the ATP; $\mathrm{O_2}$ is the final electron acceptor, forming water |
- NADH and $\mathrm{FADH_2}$ carry electrons to the ETC, which pumps $\mathrm{H^+}$ into the intermembrane space. The $\mathrm{H^+}$ gradient drives ATP synthase.
- Without oxygen, fermentation regenerates $\mathrm{NAD^+}$ so glycolysis can continue (making lactate or ethanol).
Topics in this unit
Each topic has its own short notes and practice.
Practice
Enzymes stabilize the transition state; they aren't consumed.
Competitive inhibitors compete for the active site, so more substrate outcompetes them.
Carbon fixation happens in the fluid surrounding the thylakoids.
Photosystem II splits water, releasing oxygen.
Oxygen accepts electrons at the end of the chain and forms water.
ATP synthase depends on the $\mathrm{H^+}$ gradient across the inner membrane.
Glycolysis needs $\mathrm{NAD^+}$; fermentation recycles it from NADH.
Below the optimum, slower molecular motion reduces collisions; denaturing happens above the optimum.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A student measures oxygen production by an aquatic plant at different light intensities.
(a) Identify the independent and dependent variables.
Show the answer and scoring guide
Independent: light intensity. Dependent: rate of oxygen production.
Check off the points you earned:
(b) Predict what happens to the oxygen production rate as light intensity increases far past the plant's needs. Justify.
Show the answer and scoring guide
It rises, then levels off. At high light another factor (like $\mathrm{CO_2}$ supply or the amount of enzyme in the Calvin cycle) limits the rate.
Check off the points you earned:
(c) Explain why oxygen production would stop if the plant were given a chemical that blocks photosystem II.
Show the answer and scoring guide
Photosystem II splits water to replace its excited electrons, which releases $\mathrm{O_2}$. If it is blocked, water isn't split and no oxygen is produced.
Check off the points you earned: