Enzymes are biological catalysts—proteins that speed up chemical reactions without being consumed. They are essential for virtually every metabolic process in living organisms.
Key Enzyme Vocabulary
Term
Definition
Substrate
The molecule(s) that the enzyme acts upon
Active Site
The specific region on the enzyme where the substrate binds
Product
The molecule(s) produced after the reaction
Enzyme-Substrate Complex
The temporary structure formed when substrate binds to active site
Enzyme Substrate Products
Lock and Key Model (Old)
Suggested that the substrate fits perfectly into the active site like a key in a lock. The enzyme's shape doesn't change.
❌ Now considered too simplistic
Induced Fit Model (Current)
The active site changes shape slightly when the substrate binds, creating a tighter fit. This conformational change helps catalyze the reaction.
✓ Accepted model today
How Enzymes Lower Activation Energy
Enzymes work by lowering the activation energy (Ea) needed for a reaction to proceed. They don't change the overall energy of reactants or products—just make it easier to get there.
Key insight: Enzymes don't change the ΔG (overall energy change) of a reaction. They only lower the activation energy, making the reaction happen faster.
AP Exam Tip: Enzymes are specific — each enzyme typically catalyzes only one type of reaction due to the shape of its active site. This is why cells need thousands of different enzymes!
3.2
Environmental Impacts on Enzyme Function
Enzyme activity is affected by environmental factors. Understanding these factors helps explain how organisms regulate metabolism and respond to their environment.
Factors Affecting Enzyme Activity
Temperature
↑ Temp = ↑ Activity (to a point)
Too hot → denaturation
pH
Each enzyme has optimal pH
Extreme pH → denaturation
Substrate Concentration
↑ [S] = ↑ Activity (until saturated)
Saturation = Vmax reached
Enzyme Concentration
↑ [E] = ↑ Activity
More enzymes = more reactions
Each enzyme has an optimal temperature and pH where it works best. Beyond these optima, the enzyme's shape changes and activity decreases.
Denaturation: When an enzyme loses its 3D shape due to extreme temperature or pH, it can no longer function. The active site is distorted, so the substrate can't bind. This is usually irreversible.
Enzyme Inhibition
Competitive Inhibition
Inhibitor competes with substrate for the active site. Has similar shape to substrate.
Many enzymes have allosteric sites — binding sites separate from the active site. When molecules bind here, they change the enzyme's shape and activity.
Allosteric Activators
Stabilize the active form → ↑ activity
Allosteric Inhibitors
Stabilize the inactive form → ↓ activity
Feedback Inhibition: The end product of a metabolic pathway inhibits an enzyme early in the pathway. This is a form of negative feedback that prevents overproduction. Classic example: ATP inhibits phosphofructokinase in glycolysis.
3.3
Cellular Energy
Cells need a constant supply of energy to perform work. ATP (adenosine triphosphate) is the universal energy currency of all living cells.
Cells use energy coupling to power unfavorable reactions. The energy released from ATP hydrolysis (exergonic) is used to drive endergonic reactions that wouldn't happen spontaneously.
The ATP Cycle: ATP is constantly being recycled. We use about our body weight in ATP every day, but we only have about 250g at any moment. Cellular respiration regenerates ATP from ADP + Pᵢ.
3.4
Photosynthesis
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose). It occurs in the chloroplast.
Overall Equation for Photosynthesis
6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂
Carbon dioxide + Water + Light → Glucose + Oxygen
Chloroplast Structure
Photosynthesis occurs in two distinct regions:
Thylakoid (Grana)
Stacked membrane discs
Site of Light Reactions
Stroma
Fluid-filled space
Site of Calvin Cycle
Light energy Water Oxygen (released) ATP NADPH/NADP⁺ G3P/Glucose
For every 3 CO₂ fixed → 1 G3P exits → 2 G3P make 1 glucose
AP Exam Tip: Know that O₂ comes from water, not CO₂! The light reactions split H₂O and release O₂ as a byproduct. Also remember: light reactions need light; Calvin Cycle doesn't directly need light but requires ATP/NADPH from light reactions.
3.5
Cellular Respiration
Cellular respiration is the process that breaks down glucose to release energy and produce ATP. It occurs in the cytoplasm and mitochondria.
Overall Equation for Cellular Respiration
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~30-32 ATP
Glucose + Oxygen → Carbon dioxide + Water + Energy
Notice: This is the REVERSE of photosynthesis! Plants do BOTH processes — photosynthesis to make glucose, respiration to use it.
The Four Stages of Cellular Respiration
📍 Cytoplasm
1. Glycolysis
Glucose → 2 Pyruvate
Yield: 2 ATP, 2 NADH
📍 Mitochondrial matrix
2. Pyruvate Oxidation
Pyruvate → Acetyl-CoA
Yield: 2 NADH, 2 CO₂
📍 Mitochondrial matrix
3. Krebs Cycle
Acetyl-CoA oxidized
Yield: 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂
📍 Inner membrane
4. ETC + Oxidative Phos.
NADH/FADH₂ → ATP
Yield: ~26-28 ATP
ATP Accounting (Per Glucose)
Stage
ATP (direct)
NADH
FADH₂
ATP from ETC*
Glycolysis
2 ATP
2 NADH
—
~3-5 ATP
Pyruvate Oxidation
—
2 NADH
—
~5 ATP
Krebs Cycle
2 ATP
6 NADH
2 FADH₂
~18 ATP
TOTAL
4 ATP
10 NADH
2 FADH₂
~26-28 ATP
*Each NADH ≈ 2.5 ATP; Each FADH₂ ≈ 1.5 ATP via oxidative phosphorylation
Chemiosmosis (How ETC Makes ATP)
The electron transport chain pumps H⁺ ions from the matrix into the intermembrane space, creating a proton gradient. H⁺ flows back through ATP synthase, which uses this energy to phosphorylate ADP → ATP.
O₂ is the final electron acceptor — it combines with electrons and H⁺ to form H₂O.
Anaerobic Respiration (Fermentation)
When O₂ is absent, cells use fermentation to regenerate NAD⁺ so glycolysis can continue.
Fermentation doesn't use the Krebs cycle or ETC, so most of the energy in glucose remains trapped in lactate or ethanol.
Aerobic: ~30-32 ATP per glucose
Anaerobic: 2 ATP per glucose
Aerobic respiration is ~15x more efficient!
Connection to Photosynthesis: The products of cellular respiration (CO₂ and H₂O) are the reactants of photosynthesis, and vice versa. These two processes form a cycle that recycles carbon and oxygen in ecosystems!