🧬AP Biology Unit 3 – Cellular Energetics

Key Concepts

• Energy is the capacity to do work and is required for life processes
• Metabolism encompasses all chemical reactions involved in maintaining the living state of cells and organisms
• Autotrophs produce complex organic compounds from simple inorganic molecules using light energy (photosynthesis) or chemical energy (chemosynthesis)
• Heterotrophs utilize organic compounds produced by other organisms for energy and building materials
• Bioenergetics is the study of how organisms manage their energy resources
  • Includes the storage, transfer, and utilization of energy
• Thermodynamics governs energy transformations in biological systems
  • First law of thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another
  • Second law of thermodynamics states that every energy transfer increases the entropy of the universe

Energy and Metabolism Basics

• Energy exists in various forms, including chemical, electrical, light, mechanical, and thermal energy
• Potential energy is stored energy due to an object's position or chemical composition
  • Examples include chemical bonds and water behind a dam
• Kinetic energy is the energy of motion
  • Examples include light, heat, and a flowing river
• Chemical reactions involve the making and breaking of chemical bonds, which absorb or release energy
• Exergonic reactions release energy and are spontaneous
  • Examples include cellular respiration and the breakdown of glucose
• Endergonic reactions require an input of energy to proceed and are non-spontaneous
  • Examples include photosynthesis and the synthesis of proteins
• Coupled reactions link endergonic and exergonic reactions, allowing cells to perform necessary endergonic reactions by utilizing the energy released from exergonic reactions

ATP and Cellular Energy Currency

• Adenosine triphosphate (ATP) is the primary energy currency in cells
• ATP consists of an adenosine molecule bonded to three phosphate groups
• The high-energy bonds between the phosphate groups store a significant amount of potential energy
• ATP is produced through substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation
• Hydrolysis of ATP to ADP (adenosine diphosphate) and inorganic phosphate (Pi) releases energy for cellular work
• ATP is continuously recycled in cells, with ADP being phosphorylated back into ATP
• The ATP/ADP cycle allows for the efficient storage, transfer, and utilization of energy in biological systems
• ATP powers various cellular processes, including:
  • Synthesis of complex molecules (proteins, lipids, carbohydrates)
  • Active transport of molecules across membranes
  • Muscle contraction
  • Nerve impulse transmission

Enzymes and Biochemical Reactions

• Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process
• Enzymes lower the activation energy required for a reaction to occur, making it more likely to happen under cellular conditions
• Enzymes are typically proteins with specific three-dimensional structures that determine their function
• The active site of an enzyme is a specific region where the substrate binds and the reaction occurs
• Enzymes are highly specific, often recognizing only one substrate or a small group of related substrates
• Enzyme activity is influenced by factors such as temperature, pH, and substrate concentration
  • Optimal conditions allow for maximum enzyme activity
• Enzymes can be regulated through various mechanisms, including:
  • Allosteric regulation, where the binding of a molecule at a site other than the active site alters enzyme activity
  • Competitive inhibition, where a molecule similar to the substrate binds to the active site and prevents substrate binding
  • Feedback inhibition, where the end product of a metabolic pathway inhibits an earlier enzyme in the pathway

Cellular Respiration Overview

• Cellular respiration is the process by which cells break down organic molecules to release energy in the form of ATP
• Glucose is the most common substrate for cellular respiration, but other organic molecules (fatty acids, amino acids) can also be used
• Cellular respiration occurs in three main stages: glycolysis, the citric acid cycle, and the electron transport chain
• Glycolysis takes place in the cytosol and partially breaks down glucose into two pyruvate molecules, producing a small amount of ATP and NADH
• The citric acid cycle occurs in the mitochondrial matrix and completely oxidizes pyruvate, generating CO2, NADH, FADH2, and a small amount of ATP
• The electron transport chain is located in the inner mitochondrial membrane and generates the majority of ATP through oxidative phosphorylation
  • NADH and FADH2 from previous stages donate electrons to the electron transport chain
  • As electrons move down the chain, protons (H+) are pumped from the matrix into the intermembrane space, creating an electrochemical gradient
  • ATP synthase uses the proton gradient to drive the synthesis of ATP
• The overall equation for the complete oxidation of glucose through cellular respiration is:
  • $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

Glycolysis and Fermentation

• Glycolysis is the first stage of cellular respiration and occurs in the cytosol
• Glucose is converted into two molecules of pyruvate through a series of ten enzyme-catalyzed reactions
• Glycolysis has two phases: the preparatory phase and the payoff phase
  • The preparatory phase consumes 2 ATP to convert glucose into fructose-1,6-bisphosphate
  • The payoff phase splits fructose-1,6-bisphosphate into two three-carbon molecules, which are then oxidized to form pyruvate, generating 4 ATP and 2 NADH
• The net yield of glycolysis is 2 ATP and 2 NADH per glucose molecule
• Under anaerobic conditions, pyruvate is converted into lactate (in animals) or ethanol (in yeast and plants) through fermentation
  • Fermentation regenerates NAD+ from NADH, allowing glycolysis to continue in the absence of oxygen
• Fermentation produces no additional ATP but is essential for the continuation of glycolysis under anaerobic conditions
• Lactic acid fermentation occurs in exercising muscle cells when oxygen demand exceeds supply, leading to muscle fatigue and soreness

The Citric Acid Cycle

• The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is the second stage of cellular respiration
• It takes place in the mitochondrial matrix and completely oxidizes pyruvate from glycolysis
• Prior to entering the citric acid cycle, pyruvate is converted into acetyl-CoA by the pyruvate dehydrogenase complex
• Acetyl-CoA combines with oxaloacetate to form citrate, the first compound in the cycle
• Through a series of eight enzyme-catalyzed reactions, citrate is oxidized, releasing CO2 and generating NADH, FADH2, and a small amount of ATP
• The cycle regenerates oxaloacetate, allowing it to continue as long as acetyl-CoA is available
• The net yield of one turn of the citric acid cycle is 2 CO2, 3 NADH, 1 FADH2, and 1 ATP (or GTP) per acetyl-CoA molecule
• The NADH and FADH2 produced in the citric acid cycle are used in the electron transport chain to generate more ATP
• The citric acid cycle is a central metabolic hub, with intermediates being used for the synthesis of amino acids, nucleotides, and other important biomolecules

Electron Transport Chain and Oxidative Phosphorylation

• The electron transport chain (ETC) is the final stage of cellular respiration and is located in the inner mitochondrial membrane
• The ETC consists of a series of protein complexes (I, II, III, and IV) and mobile electron carriers (ubiquinone and cytochrome c)
• NADH and FADH2 from previous stages donate electrons to the ETC
  • NADH donates electrons to Complex I, while FADH2 donates electrons to Complex II
• As electrons move through the complexes, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient
• The proton gradient is used by ATP synthase (Complex V) to drive the synthesis of ATP through oxidative phosphorylation
  • Protons flow back into the matrix through ATP synthase, causing it to rotate and catalyze the formation of ATP from ADP and inorganic phosphate (Pi)
• Oxygen serves as the final electron acceptor in the ETC, combining with protons to form water
• The ETC and oxidative phosphorylation are highly efficient, generating approximately 34 ATP per glucose molecule (assuming 2.5 ATP per NADH and 1.5 ATP per FADH2)
• Chemiosmosis is the process by which the proton gradient is generated and utilized for ATP synthesis
• The ETC and oxidative phosphorylation are tightly regulated to maintain cellular energy balance and prevent the formation of harmful reactive oxygen species (ROS)

Photosynthesis Fundamentals

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose or other sugars
• The overall equation for photosynthesis is:
  • $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$
• Photosynthesis occurs in two stages: the light-dependent reactions and the light-independent reactions (Calvin cycle)
• The light-dependent reactions take place in the thylakoid membranes of chloroplasts and convert light energy into chemical energy (ATP and NADPH)
  • Photosystems I and II absorb light energy and use it to excite electrons
  • The excited electrons are passed through an electron transport chain, generating a proton gradient across the thylakoid membrane
  • ATP synthase uses the proton gradient to produce ATP (photophosphorylation)
  • NADP+ is reduced to NADPH using electrons from the electron transport chain
• The light-independent reactions (Calvin cycle) occur in the stroma of chloroplasts and use the ATP and NADPH from the light-dependent reactions to fix CO2 into organic compounds
  • Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the fixation of CO2 to ribulose bisphosphate (RuBP), forming 3-phosphoglycerate
  • 3-phosphoglycerate is reduced to glyceraldehyde 3-phosphate (G3P) using ATP and NADPH
  • Some G3P is used to regenerate RuBP, while the rest is used to synthesize glucose and other organic compounds
• Photosynthetic pigments, such as chlorophyll a, chlorophyll b, and carotenoids, absorb light energy and transfer it to the photosystems
• C3, C4, and CAM plants have different adaptations for carbon fixation, which help them optimize photosynthesis under various environmental conditions

Practical Applications and Real-World Connections

• Understanding cellular respiration and photosynthesis is crucial for developing strategies to address global challenges such as food security, renewable energy, and climate change
• Biofuels, such as ethanol and biodiesel, are produced from plant-derived sugars or oils and can be used as renewable alternatives to fossil fuels
  • Algae-based biofuels are a promising avenue for sustainable energy production
• Crop improvement through genetic engineering and selective breeding can increase photosynthetic efficiency and yield, helping to meet the growing global demand for food
  • Examples include the development of drought-resistant and pest-resistant crops
• Artificial photosynthesis aims to mimic the natural process to produce clean energy and valuable products, such as hydrogen fuel and biodegradable plastics
• Metabolic engineering of microorganisms can optimize the production of desired compounds, such as pharmaceuticals, enzymes, and biopolymers
  • Example: the production of artemisinin, an antimalarial drug, in engineered yeast
• Understanding the role of cellular respiration in exercise physiology helps athletes optimize their training and performance
  • Lactate threshold training and VO2 max testing are based on the principles of cellular respiration
• Mitochondrial disorders, such as MELAS syndrome and Leigh syndrome, are caused by defects in cellular respiration and can have severe health consequences
  • Research into these disorders can lead to the development of targeted therapies and treatments
• The study of photosynthesis and cellular respiration in extreme environments, such as deep-sea hydrothermal vents and Antarctic ice, provides insights into the adaptability and diversity of life on Earth