3 Cellular Energetics
- Syllabus
- 2025
- Section
- 3
- Level
- —
Exam analysis
3.1 Enzymes
- Syllabus
- 2025
- Topic
- 3.1
- Level
- —
How Enzymes Speed Biological Reactions
Enzymes are protein catalysts: they increase the rate of biological reactions by providing a reaction pathway with lower activation energy. With a smaller energy barrier, a greater proportion of reacting molecules can reach the transition state under the same cellular conditions.
Catalysis begins when a substrate enters the enzyme's active site. The substrate's shape and charge must be compatible with the active site, allowing an enzyme-substrate complex to form. Interactions in this complex position the substrate so the reaction can proceed more readily; products are then released and the enzyme can catalyze another reaction.
Compatible substrate and active site → enzyme-substrate complex → lower-activation-energy pathway → faster product formation. Because active sites differ in shape and charge, an enzyme catalyzes only a limited set of compatible reactions.
An enzyme does not supply energy to the reaction and is not used up by it. It changes the pathway and therefore the rate; it does not make an incompatible substrate fit its active site.
3.2 Environmental Impacts on Enzyme Function
- Syllabus
- 2025
- Topic
- 3.2
- Level
- —
When Enzyme Structure Changes, Function Changes
An enzyme functions only while its three-dimensional structure maintains a suitable active site. If that structure changes, the active site's shape or charge can become less compatible with the substrate, so fewer enzyme-substrate complexes form and catalytic efficiency decreases.
Temperature, pH, or the chemical environment can disrupt interactions that stabilize protein structure, including hydrogen bonds. A moderate change may reduce activity by altering the active site. A sufficiently large disruption can denature the enzyme, eliminating its ability to catalyze the reaction. Changes to the enzyme's amino-acid sequence can also alter folding and therefore function.
Environmental or molecular change → stabilizing interactions disrupted → enzyme shape changes → substrate binds less effectively → reaction rate decreases.
Denaturation does not mean that every bond in the protein is broken. It is loss of the functional protein structure. In some cases the change is reversible and activity returns when suitable conditions are restored; in other cases it is not.
How the Cellular Environment Controls Enzyme Activity
The cellular environment changes enzyme activity by affecting how often enzyme and substrate molecules meet, whether a substrate can occupy the active site, and whether binding elsewhere changes enzyme activity.
| Environmental factor | Mechanism | Effect on reaction efficiency |
|---|---|---|
| Relative substrate and product concentrations | Changes the availability of molecules participating in the reaction | Alters how efficiently the enzymatic reaction proceeds |
| Temperature below the optimum | Higher temperature increases average molecular speed and enzyme-substrate collision frequency | Reaction rate increases until the optimum is reached |
| Competitive inhibitor | Binds reversibly to the active site and competes with substrate | Reduces substrate binding |
| Noncompetitive inhibitor | Binds at an allosteric site and changes enzyme activity | Reduces catalysis without occupying the active site |
A prediction should name both the changed condition and its mechanism. For example, adding a competitive inhibitor lowers the rate because fewer active sites are available to substrate, whereas a noncompetitive inhibitor acts through binding at a different site.
Temperature does not increase enzyme activity indefinitely. The collision-based increase applies only up to the enzyme's optimal temperature; outside the optimal range, structural disruption can reduce activity.
3.3 Cellular Energy
- Syllabus
- 2025
- Topic
- 3.3
- Level
- —
Energy Flow Sustains Living Systems
Living systems require a continual input of energy to maintain their highly ordered organization and power cellular processes. As energy is transferred, some becomes unavailable for biological work, so maintaining life requires energy input to exceed energy loss.
Cells couple processes that release energy to processes that require it. For example, energy released during catabolic reactions can be transferred through ATP to drive cellular work. Coupling lets an energy-requiring process proceed without implying that the cell creates energy.
Energy-related pathways occur as controlled sequences: the product of one enzyme-controlled reaction becomes the reactant for the next. Releasing or transferring energy in multiple linked steps gives the cell more control than transferring it in one large step.
Biological order does not violate thermodynamic laws. Organisms are not closed systems: they take in energy and release energy to their surroundings. The Gibbs free-energy equation is not required for this AP objective.
Conserved Metabolism Reveals Common Ancestry
A biological process conserved across very different groups can support common ancestry because the shared process can be inherited from an ancestral population and retained through descendant lineages.
| Domain | Conserved core metabolic pathways |
|---|---|
| Archaea | Glycolysis and oxidative phosphorylation |
| Bacteria | Glycolysis and oxidative phosphorylation |
| Eukarya | Glycolysis and oxidative phosphorylation |
These pathways perform fundamental energy transformations and are present across all currently recognized domains. Their broad conservation is consistent with an early origin followed by inheritance as the domains diverged. The shared underlying pathway is therefore evidence of evolutionary continuity, not merely a list of similar names.
Common ancestry does not require every organism to perform metabolism identically. Lineages can modify regulation and pathway details while retaining a conserved core inherited from a common ancestor.
3.4 Photosynthesis
- Syllabus
- 2025
- Topic
- 3.4
- Level
- —
Chloroplast Structure Organizes Photosynthesis
Photosynthesis uses carbon dioxide, water, and light energy to produce carbohydrates and oxygen. The carbohydrates store captured energy for later biological processes, while chloroplast compartments keep the light-capturing reactions and carbon-fixation reactions organized.
| Chloroplast region | Key structures | Photosynthetic role |
|---|---|---|
| Thylakoids, stacked as grana | Membranes containing chlorophyll, photosystems, and electron-transport proteins | Light reactions capture light energy and produce ATP and NADPH |
| Stroma | Fluid inside the inner chloroplast membrane and outside thylakoids | Calvin-cycle reactions use ATP and NADPH to fix carbon dioxide into organic molecules |
The two regions form one coupled system: products of the thylakoid light reactions supply energy and reducing power for carbohydrate production in the stroma. This spatial organization links membrane-based energy capture to chemical energy storage.
Photosynthesis first evolved in prokaryotes; cyanobacterial photosynthesis contributed to an oxygenated atmosphere and provided the foundation for eukaryotic photosynthesis. AP Biology requires the overall Calvin-cycle role, not memorization of its individual steps, molecular structures, or enzyme names other than ATP synthase.
From Light Energy to ATP, NADPH, and Carbohydrates
The light reactions transfer energy from light into ATP and NADPH by coordinating photosystems, electron transport, and chemiosmosis across the thylakoid membrane. ATP and NADPH then power carbohydrate production in the stroma.
- Chlorophyll in photosystems II and I absorbs light, raising electrons to higher energy levels.
- Water splits and replaces electrons lost from photosystem II; oxygen is produced.
- Electrons move between the photosystems through an electron transport chain in the thylakoid membrane.
- Redox transfers through the chain establish high proton concentration inside the thylakoid and lower concentration outside.
- Protons flow through ATP synthase, driving ADP + inorganic phosphate → ATP by chemiosmosis (photophosphorylation).
- Electrons ultimately reduce NADP+ to NADPH at photosystem I.
ATP supplies usable energy and NADPH supplies high-energy electrons for the Calvin cycle. In the stroma, these products of the light reactions power the conversion of carbon dioxide into carbohydrates, transferring captured light energy into stored chemical energy.
The proton gradient stores potential energy; ATP synthase uses proton flow rather than electron flow directly to make ATP. Specific ETC carrier names, enzyme names, and pathway intermediates are outside the required AP scope.
3.5 Cellular Respiration
- Syllabus
- 2025
- Topic
- 3.5
- Level
- —
Mitochondrial Structure Enables ATP Synthesis
Cellular respiration transfers energy from biological macromolecules into ATP through coordinated enzyme-catalyzed reactions. In eukaryotes, the mitochondrion's compartmentalized inner membrane makes electron transport and chemiosmosis possible.
| Mitochondrial feature | Functional contribution |
|---|---|
| Inner membrane and its folds | Holds the ETC and ATP synthase; folds increase surface area for ATP synthesis |
| Intermembrane space | Accumulates protons, creating higher proton concentration than in the matrix |
| Matrix | Lower proton concentration provides the other side of the electrochemical gradient |
NADH and FADH2 deliver electrons to the ETC. Redox transfers move electrons toward a terminal electron acceptor and are coupled to proton movement across the inner membrane. Protons then flow back through ATP synthase, driving ADP + inorganic phosphate → ATP. In aerobic respiration oxygen is the terminal acceptor; aerobic prokaryotes run this membrane process across the plasma membrane, while anaerobic prokaryotes may use other acceptors.
Electron flow does not drive ATP formation by direct contact with ATP synthase; it establishes the proton gradient that powers chemiosmosis. If oxidative phosphorylation is decoupled from electron transport, gradient energy can be released as heat. Specific ETC carrier names and pathway intermediates are outside AP scope.
How Cells Extract Energy from Glucose
Cells obtain usable energy from glucose in stages. Early reactions make some ATP directly and transfer high-energy electrons to NADH and FADH2; these carriers then supply electrons to the membrane ETC that supports most ATP synthesis in aerobic respiration.
| Stage and location | Main outcome |
|---|---|
| Glycolysis, cytosol | Glucose energy yields ATP, NADH, and pyruvate |
| Pyruvate oxidation and Krebs cycle, mitochondrial matrix | Carbon dioxide is released; ATP, NADH, and FADH2 are produced |
| ETC and oxidative phosphorylation, inner mitochondrial membrane | NADH/FADH2 electrons establish a proton gradient; ATP synthase uses chemiosmosis to make ATP |
Electron transfer through the ETC creates higher proton concentration in the intermembrane space and lower concentration in the matrix, so matrix pH is higher. Proton flow down this electrochemical gradient through ATP synthase couples electron-energy transfer to ATP production.
When oxygen is absent, fermentation allows glycolysis to continue and produces organic products such as alcohol or lactic acid; it does not replace the ETC with another high-yield ATP pathway. AP Biology requires the purposes, locations, and major products—not memorization of every glycolysis or Krebs-cycle step, intermediate, or enzyme.