AP Biology Unit 3: Cellular Energetics - ATP & Photosynthesis | StarSpark

Every organism needs energy. Where does it come from? How do cells capture it and use it? Unit 3 answers these questions. This unit covers enzymes, ATP, photosynthesis, and cellular respiration. These are the pathways that power life. They're also heavily tested, so understanding them matters.

Topic 3.1: Enzymes and Catalysis

Enzymes are catalysts. They speed up chemical reactions without being used up themselves. Cells would move at a glacial pace without enzymes. Most enzymes are proteins, and their structure determines how they work.

Key concepts to know:

• What enzymes do: Enzymes lower the activation energy of chemical reactions. The activation energy is the energy barrier that has to be overcome for a reaction to proceed. By lowering this barrier, enzymes allow reactions to happen at body temperature and at speeds compatible with life. Without enzymes, the same reactions would happen, but so slowly that organisms couldn't survive.
• Enzyme structure: Enzymes are proteins with a specific 3D shape. The shape of the active site, the part of the enzyme where the substrate binds, is crucial. The active site's shape and charge distribution are complementary to the shape and charge of a specific substrate (reactant).
• Enzyme-substrate complex: For a reaction to happen, the shape and charge of the substrate must be compatible with the active site of the enzyme. When the substrate binds to the active site, it forms an enzyme-substrate complex. The enzyme stabilizes a transition state, lowering the activation energy and allowing the reaction to proceed quickly.
• Specificity: Enzymes are highly specific. Each enzyme catalyzes only one reaction or a very limited set of reactions. This specificity comes from the exact shape and chemical properties of the active site. A slight change in the substrate's shape or charge can prevent it from binding at all.
• The enzyme isn't consumed: After the reaction, the enzyme releases the product and returns to its original state, ready to catalyze the reaction again. An enzyme can catalyze thousands of reactions per second.

Topic 3.2: Environmental Impacts on Enzyme Function

Enzymes are proteins, and their structure can be disrupted. When that happens, their function suffers.

Key concepts to know:

• Denaturation: When environmental conditions change enough to disrupt an enzyme's structure, it denatures. The 3D shape is lost, and the active site no longer fits the substrate. The enzyme loses its catalytic activity. Common denaturants include extreme temperature and extreme pH.
• Temperature effects: As temperature increases from cool to moderate, molecules move faster, leading to more frequent collisions between enzymes and substrates. The reaction rate increases. But past the optimal temperature, the enzyme begins to denature. Hydrogen bonds break, and the protein structure unravels. The active site loses its shape. At very high temperatures, the enzyme is permanently denatured and non-functional.
• pH effects: Each enzyme has an optimal pH. For digestive enzymes in your stomach, optimal pH is very acidic (around 2). For enzymes in your small intestine, it's slightly basic (around 8). Outside the optimal range, enzymes denature as hydrogen bonds are disrupted. The enzyme loses shape and function.
• Reversibility: Sometimes denaturation is reversible. If you cool a heated enzyme or return pH to normal, the enzyme can refold and regain activity. Sometimes it's permanent. Cooking an egg denatures the proteins permanently. You can't un-cook it.
• Substrate and product concentration: The more substrate available, the higher the reaction rate, up to a point. Once the enzyme is saturated (all active sites are occupied all the time), adding more substrate doesn't increase the rate. The enzyme is working at maximum velocity. Similarly, if products accumulate, they can inhibit the reaction by competing with substrates for binding.
• Competitive inhibition: Inhibitor molecules structurally resemble the substrate and can bind to the active site. When an inhibitor occupies the active site, the actual substrate can't bind. But this inhibition is reversible. If you add more substrate, the substrate can outcompete the inhibitor for binding. Many drugs and poisons work this way.
• Noncompetitive inhibition: Inhibitor molecules bind to an allosteric site on the enzyme, not the active site. This binding changes the enzyme's shape, distorting the active site. Now the substrate can't bind properly, even though the active site is "empty." This inhibition is not overcome by adding more substrate because the inhibitor isn't competing for the active site. It's just changing the enzyme's shape.

Topic 3.3: Cellular Energy and Thermodynamics

Energy is the currency of life. Cells need to capture it, store it, and use it. Understanding cellular energy requires understanding a bit about thermodynamics.

Key concepts to know:

• Energy input requirement: All living systems require energy input to maintain their highly organized structure and carry out their functions. Without continuous energy input, organisms move toward entropy, disorder, and death.
• The laws of thermodynamics: Life doesn't violate the first and second laws of thermodynamics. The first law says energy can't be created or destroyed, only transformed. The second law says that in any transformation, some energy is lost as heat, and the universe tends toward increased disorder (entropy).
• Energy balance: For an organism to stay alive and organized, energy input must exceed energy loss. Photosynthetic organisms capture energy from sunlight. All organisms must get energy from food. This energy is used to power cellular processes and is eventually lost as heat.
• ATP: The energy currency: Adenosine triphosphate (ATP) is the molecule that cells use to power almost every cellular process. ATP has a high-energy phosphate bond. When this bond is broken, energy is released. The cell couples the energy release from ATP hydrolysis to power reactions that would otherwise be unfavorable (like active transport).
• Coupled reactions: Cells link an energy-releasing reaction (like ATP hydrolysis) with an energy-requiring reaction. The energy released from one reaction drives the other. This allows cells to do useful work.
• Sequential pathways: Metabolic pathways are organized in a sequence for a reason. Each step produces a product that becomes the reactant for the next step. This controlled transfer of energy is much more efficient than releasing energy all at once. It allows the cell to capture energy in ATP rather than wasting it as heat.
• Conserved pathways: Core metabolic pathways like glycolysis and oxidative phosphorylation are found in all organisms. This conservation suggests common ancestry. All life uses the same basic energy-capture mechanisms.

Topic 3.4: Photosynthesis

Photosynthesis is the process by which organisms capture energy from sunlight and convert it to chemical energy in glucose. This is the foundation of most food webs on Earth.

Key concepts to know:

• The big picture: Photosynthesis uses carbon dioxide, water, and light energy to make carbohydrates (glucose) and oxygen. 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. Photosynthesis evolved in prokaryotes first, probably cyanobacteria. These organisms were responsible for releasing the oxygen in our atmosphere. Prokaryotic photosynthesis was the foundation for eukaryotic photosynthesis.
• Chloroplast structure: Photosynthesis happens in the chloroplast. Inside the chloroplast, you find two main compartments: thylakoids (stacked membrane sacs containing chlorophyll) and the stroma (the fluid surrounding the thylakoids). The stacks of thylakoids are called grana. This separation of compartments is important.
• Light reactions: These happen in the thylakoid membranes. Light energy is captured by chlorophyll molecules organized into two photosystems (Photosystem II and Photosystem I). The photosystems use the light energy to excite electrons to higher energy levels. Water is split, releasing electrons to replace those lost from Photosystem II, and releasing O2 as a byproduct. As electrons pass through an electron transport chain from Photosystem II to Photosystem I, protons are pumped across the thylakoid membrane, creating a proton gradient. Photosystem I reduces NADP+ to NADPH. The proton gradient drives ATP synthase, making ATP from ADP. So the light reactions produce ATP and NADPH, the energy carriers used in the Calvin cycle. Image: OpenStax Biology 2e (CC BY 4.0)
• The Calvin cycle (light-independent reactions): These happen in the stroma, not in the light. The Calvin cycle uses the ATP and NADPH produced by the light reactions to fix carbon dioxide into glucose. Specifically, CO2 is combined with a 5-carbon molecule to form a 6-carbon molecule that's immediately split into two 3-carbon molecules. These 3-carbon molecules are reduced using ATP and NADPH to form carbohydrates. The cycle is called a "cycle" because some of the 3-carbon molecules are rearranged to regenerate RuBP, allowing the cycle to continue. The ATP and NADPH are consumed during the Calvin cycle, which is why the light reactions need to keep producing them.
• The role of structure: The thylakoid membrane's organization is critical. Chlorophyll molecules are organized into photosystems, along with electron carriers and ATP synthase. The proximity of these components allows for efficient energy transfer and ATP production. The compartmentalization of the light reactions (in thylakoids) and Calvin cycle (in stroma) allows the cell to control each process independently.

Topic 3.5: Cellular Respiration

Cellular respiration is how organisms extract energy from glucose and other macromolecules and convert it to ATP. Most cells use aerobic respiration when oxygen is available.

Key concepts to know:

• The big picture: Aerobic respiration takes glucose (or other macromolecules) and, in the presence of oxygen, produces large amounts of ATP, carbon dioxide, and water. The overall equation is essentially the reverse of photosynthesis: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. Respiration is characteristic of all forms of life. It's a conserved pathway, which tells us about common ancestry.
• Glycolysis: This happens in the cytosol (not in mitochondria). Glucose is broken down into two 3-carbon molecules (pyruvate). During glycolysis, a small amount of ATP is made (2 ATP net per glucose), and electrons are captured in NADH molecules. Glycolysis happens in all cells, aerobic or anaerobic, which makes sense given how ancient and fundamental it is.
• Pyruvate oxidation and the Krebs cycle: Pyruvate is transported into the mitochondrion and oxidized, releasing electrons that reduce NAD+ to NADH and forming a 2-carbon molecule that enters the Krebs cycle (also called the citric acid cycle). The Krebs cycle is a circular pathway of reactions. During the cycle, carbon dioxide is released from the glucose skeleton, ATP is made, and more electrons are captured in NADH and FADH2. The Krebs cycle takes place in the mitochondrial matrix.
• The electron transport chain (ETC) and oxidative phosphorylation: The electrons carried by NADH and FADH2 are passed along a chain of protein carriers embedded in the inner mitochondrial membrane. As electrons move through the chain, energy is released and used to pump protons across the membrane. This creates a proton gradient, with high proton concentration in the intermembrane space and low concentration in the matrix (low pH outside, high pH inside). The folding of the inner membrane (cristae) increases surface area for the ETC, allowing more ATP to be synthesized. Electrons eventually are transferred to oxygen, the terminal electron acceptor, forming water. The proton gradient drives ATP synthase, which allows protons to flow back across the membrane. The energy from this flow is used to phosphorylate ADP to ATP, producing ATP. This process is called oxidative phosphorylation because phosphorylation (making ATP) is driven by oxidation (electron transfer).
• Heat generation: In aerobic respiration, some energy is released as heat, not captured as ATP. In some organisms (especially mammals), this heat production can be decoupled from ATP synthesis to generate warmth for body temperature regulation.
• Fermentation: When oxygen is unavailable, cells can still perform glycolysis, but they need a way to recycle NAD+ so that glycolysis can continue. Fermentation does this. In lactic acid fermentation, pyruvate is converted to lactic acid, regenerating NAD+. In alcoholic fermentation (like in yeast), pyruvate is converted to ethanol and CO2, regenerating NAD+. Fermentation only produces the small amount of ATP from glycolysis (2 ATP per glucose). Aerobic respiration produces significantly more ATP per glucose molecule. This is why aerobic organisms can generate much more energy than anaerobic ones.

Study Tips for Unit 3

The key to Unit 3 is understanding energy flow. Enzymes control the rates of these pathways. Don't memorize every step. Understand the big picture and how compartmentalization and electron transfer are the keys to everything.

Summary, Review Questions & Practice

You've covered all the topics in Unit 3. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're in great shape for this section of the exam.

Explore the Full AP Biology Study Guide

Unit 3 is the foundation for understanding how organisms capture and use energy. These concepts come back again and again in photosynthesis, cellular respiration, and even in cell signaling and regulation.

Check out the full AP Biology study plan to see how this unit connects to the rest of the course.

Other Unit Reviews:

• AP Biology Unit 1: Chemistry of Life
• AP Biology Unit 2: Cells
• AP Biology Unit 4: Cell Communication and Cell Cycle
• AP Biology Unit 5: Heredity
• AP Biology Unit 6: Gene Expression and Regulation
• AP Biology Unit 7: Natural Selection
• AP Biology Unit 8: Ecology

For official AP Biology resources, visit apcentral.collegeboard.org.

This review is aligned with the AP Biology Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.