Ap Biology Unit 3 Review

AP Biology Unit 3 Review: Cellular Energetics – Mastering Cellular Respiration and Photosynthesis

This comprehensive review covers AP Biology Unit 3, focusing on cellular energetics. We'll delve into the intricate processes of cellular respiration and photosynthesis, examining their interconnectedness and importance in sustaining life. Understanding these processes is crucial for success on the AP Biology exam. This guide will provide a thorough overview, equipping you with the knowledge and understanding necessary to excel.

I. Introduction: The Flow of Energy in Living Systems

Life, as we know it, depends on energy. The sun provides the ultimate energy source for most ecosystems, with plants and other photosynthetic organisms converting light energy into chemical energy through photosynthesis. This chemical energy, stored in the bonds of organic molecules like glucose, is then transferred to other organisms through the process of cellular respiration. Understanding this energy flow – from sunlight to glucose to ATP – is fundamental to understanding cellular energetics. This unit examines the key biochemical pathways involved in capturing and utilizing energy at the cellular level, focusing on the intricate details of both photosynthesis and cellular respiration. We will explore the key enzymes, reactants, products, and regulatory mechanisms that govern these essential processes.

II. Cellular Respiration: Harvesting Energy from Glucose

Cellular respiration is the process by which cells break down glucose to generate ATP (adenosine triphosphate), the cell's primary energy currency. This process occurs in four main stages: glycolysis, pyruvate oxidation, the Krebs cycle (citric acid cycle), and oxidative phosphorylation (electron transport chain and chemiosmosis).

A. Glycolysis: This anaerobic process (occurs without oxygen) takes place in the cytoplasm and breaks down one molecule of glucose into two molecules of pyruvate. This stage generates a net gain of 2 ATP molecules and 2 NADH molecules (electron carriers). Glycolysis is a relatively simple pathway, but its regulation is crucial for maintaining cellular energy balance.

• Key Enzymes: Hexokinase, phosphofructokinase, pyruvate kinase.
• Products: 2 ATP, 2 NADH, 2 pyruvate.

B. Pyruvate Oxidation: Pyruvate, the product of glycolysis, is transported into the mitochondria. Here, each pyruvate molecule is converted into acetyl-CoA, releasing one CO2 molecule and generating one NADH molecule per pyruvate. This step is a crucial link between glycolysis and the Krebs cycle.

• Key Enzyme: Pyruvate dehydrogenase.
• Products: 2 acetyl-CoA, 2 NADH, 2 CO2.

C. Krebs Cycle (Citric Acid Cycle): This cycle takes place in the mitochondrial matrix and involves a series of oxidation-reduction reactions. Each acetyl-CoA molecule enters the cycle, eventually yielding 2 CO2 molecules, 1 ATP (or GTP), 3 NADH molecules, and 1 FADH2 molecule (another electron carrier). Because two acetyl-CoA molecules are produced from one glucose molecule, the total yield from the Krebs cycle for one glucose molecule is doubled.

• Key Enzymes: Citrate synthase, aconitase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase, malate dehydrogenase.
• Products: 2 ATP (or GTP), 6 NADH, 2 FADH2, 4 CO2.

D. Oxidative Phosphorylation: This process, occurring in the inner mitochondrial membrane, involves the electron transport chain (ETC) and chemiosmosis. Electrons from NADH and FADH2 are passed along a series of protein complexes embedded in the inner mitochondrial membrane. This electron flow drives the pumping of protons (H+) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.

• Electron Transport Chain (ETC): A series of redox reactions that release energy as electrons are passed down the chain. Oxygen acts as the final electron acceptor, forming water.

• Chemiosmosis: The flow of protons back into the mitochondrial matrix through ATP synthase, an enzyme that uses the proton gradient to generate ATP. This is where the majority of ATP is produced in cellular respiration – approximately 32-34 ATP molecules per glucose molecule.

• Products: Approximately 32-34 ATP, H₂O.

III. Photosynthesis: Capturing Light Energy

Photosynthesis is the process by which plants and other photosynthetic organisms convert light energy into chemical energy in the form of glucose. This process occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).

A. Light-Dependent Reactions: These reactions take place in the thylakoid membranes of chloroplasts. Light energy is absorbed by chlorophyll and other pigments, exciting electrons to a higher energy level. These excited electrons are passed along an electron transport chain, similar to that in cellular respiration. This electron flow drives the pumping of protons into the thylakoid lumen, creating a proton gradient. The flow of protons back into the stroma through ATP synthase generates ATP. Water is split (photolysis) to replace the electrons lost from chlorophyll, releasing oxygen as a byproduct. NADP+ is also reduced to NADPH, another electron carrier.

• Key Components: Photosystems I and II, electron transport chain, ATP synthase, water-splitting enzyme.
• Products: ATP, NADPH, O2.

B. Light-Independent Reactions (Calvin Cycle): These reactions take place in the stroma of chloroplasts. ATP and NADPH generated in the light-dependent reactions are used to fix carbon dioxide (CO2) from the atmosphere into organic molecules, specifically glucose. The Calvin cycle involves a series of enzyme-catalyzed reactions that ultimately convert CO2 into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that can be used to synthesize glucose and other organic molecules. RuBisCO, a key enzyme in the Calvin cycle, is responsible for fixing CO2.

• Key Enzyme: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).
• Products: Glucose (and other organic molecules).

IV. Connecting Cellular Respiration and Photosynthesis:

Cellular respiration and photosynthesis are fundamentally interconnected. The products of one process are the reactants of the other. Photosynthesis produces glucose and oxygen, which are used by cellular respiration to generate ATP. Cellular respiration produces carbon dioxide and water, which are used by photosynthesis to produce glucose and oxygen. This cyclical relationship is essential for maintaining the balance of life on Earth. The oxygen released during photosynthesis is crucial for aerobic respiration in many organisms, while the carbon dioxide released during respiration is essential for the process of photosynthesis.

V. Regulation of Cellular Respiration and Photosynthesis:

Both cellular respiration and photosynthesis are highly regulated processes. The rate of these processes is influenced by various factors, including:

• Substrate availability: The availability of glucose for cellular respiration and CO2 for photosynthesis directly impacts the rate of these processes.
• Enzyme activity: The activity of key enzymes is regulated by various factors, including allosteric regulation and feedback inhibition.
• Environmental conditions: Factors like temperature, light intensity (for photosynthesis), and oxygen availability (for cellular respiration) can significantly influence the rate of these processes.
• Hormonal regulation: Plant hormones can influence the rate of photosynthesis, while hormones in animals can affect cellular respiration.

VI. Scientific Inquiry and Experimental Design:

Understanding how scientists investigate these processes is critical. Experiments involving respirometers (to measure oxygen consumption and CO2 production), chromatography (to separate and identify pigments), and spectrophotometry (to measure light absorption) are commonly used to study cellular respiration and photosynthesis. Analyzing data from these experiments and drawing appropriate conclusions is key to mastering this unit. Designing controlled experiments to test specific hypotheses about these processes will enhance your understanding and ability to analyze data.

VII. Frequently Asked Questions (FAQ)

• What is the difference between aerobic and anaerobic respiration? Aerobic respiration requires oxygen, while anaerobic respiration does not. Aerobic respiration produces significantly more ATP than anaerobic respiration.

• What is the role of NADH and FADH2? These are electron carriers that transfer electrons from glycolysis and the Krebs cycle to the electron transport chain in cellular respiration.

• What is photolysis? Photolysis is the splitting of water molecules during the light-dependent reactions of photosynthesis, releasing oxygen as a byproduct.

• What is RuBisCO? RuBisCO is the enzyme that catalyzes the fixation of CO2 in the Calvin cycle.

• How is ATP produced in cellular respiration and photosynthesis? In both processes, ATP is produced by chemiosmosis, where the flow of protons across a membrane drives ATP synthase.

• What is the relationship between cellular respiration and fermentation? Fermentation is an anaerobic process that occurs when oxygen is unavailable, allowing glycolysis to continue. It produces less ATP than aerobic respiration.

VIII. Conclusion: Mastering Cellular Energetics for AP Biology Success

Mastering cellular energetics requires a thorough understanding of the intricate biochemical pathways involved in cellular respiration and photosynthesis. This review has provided a comprehensive overview of these processes, emphasizing the interconnectedness and importance of energy flow in living systems. By understanding the key enzymes, reactants, products, and regulatory mechanisms, you will be well-prepared to tackle the challenges of the AP Biology exam. Remember to practice applying your knowledge through problem-solving and experimental design to solidify your understanding. Good luck!