❤️AP Bio Unit 4 – Cellular Energetics

Key Concepts

• Energy is the capacity to do work and is required for life processes
• Adenosine triphosphate (ATP) is the primary energy currency in cells
• Enzymes lower activation energy and speed up metabolic reactions
• Cellular respiration breaks down glucose to produce ATP in the presence of oxygen
• Photosynthesis captures light energy to synthesize glucose from carbon dioxide and water
• Fermentation allows for ATP production in the absence of oxygen
• Chemiosmosis couples the electron transport chain to ATP synthesis

Energy and Life

• Living organisms require a constant input of energy to maintain homeostasis and perform vital functions
• Energy is needed for chemical reactions, transport of molecules, and mechanical work (muscle contraction)
• Autotrophs (plants) capture energy from sunlight or chemical reactions to produce organic molecules
• Heterotrophs (animals) obtain energy by consuming organic molecules produced by other organisms
• Energy flows through ecosystems from producers to consumers and is eventually lost as heat
• The laws of thermodynamics govern energy transformations in biological systems

ATP: The Energy Currency

• ATP consists of an adenosine molecule bonded to three phosphate groups
• The hydrolysis of ATP to ADP + Pi releases energy that can be used to power cellular processes
  • This reaction is reversible, allowing ADP to be converted back to ATP when energy is available
• ATP is continuously recycled in cells, with each molecule being used and regenerated many times
• The high-energy phosphate bonds in ATP store energy in a form that can be easily accessed by enzymes
• Other nucleotides (GTP, UTP, CTP) can also serve as energy carriers in specific reactions

Enzymes and Metabolic Pathways

• Enzymes are biological catalysts that speed up chemical reactions without being consumed
• They work by lowering the activation energy required for a reaction to occur
  • This allows reactions to proceed at physiological temperatures and pH levels
• Enzymes are highly specific, binding only to particular substrates and catalyzing specific reactions
• Cofactors (metal ions) and coenzymes (organic molecules) assist enzymes in catalyzing reactions
• Metabolic pathways are series of enzyme-catalyzed reactions that transform starting materials into end products
  • Examples include glycolysis, the citric acid cycle, and the electron transport chain
• Regulation of enzyme activity allows cells to control metabolic pathways in response to changing conditions

Cellular Respiration Overview

• Cellular respiration is the process of breaking down glucose to produce ATP in the presence of oxygen
• It occurs in three main stages: glycolysis, the citric acid cycle, and the electron transport chain
• Glycolysis takes place in the cytoplasm and produces a small amount of ATP and NADH
• The citric acid cycle occurs in the mitochondrial matrix and generates NADH and FADH2
• The electron transport chain is located in the inner mitochondrial membrane and couples electron transfer to ATP synthesis
• The overall reaction for cellular respiration is: C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP

Glycolysis

• Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm
• It involves the breakdown of one glucose molecule into two pyruvate molecules
• The process can be divided into 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 produces 4 ATP, 2 NADH, and 2 pyruvate molecules
• The net yield of glycolysis is 2 ATP and 2 NADH per glucose molecule
• Glycolysis is an ancient metabolic pathway and is found in nearly all living organisms

The Citric Acid Cycle

• The citric acid cycle (also known as the Krebs cycle) is the second stage of cellular respiration
• It takes place in the mitochondrial matrix and involves the oxidation of acetyl-CoA derived from pyruvate
• The cycle begins with the condensation of acetyl-CoA and oxaloacetate to form citrate
• Through a series of redox reactions, citrate is converted back into oxaloacetate, releasing CO2 and generating NADH and FADH2
• One turn of the cycle yields 3 NADH, 1 FADH2, and 1 ATP (or GTP) per acetyl-CoA molecule
• The NADH and FADH2 produced are used to power the electron transport chain and generate additional ATP

Electron Transport Chain and Chemiosmosis

• The electron transport chain (ETC) is the final stage of cellular respiration and is located in the inner mitochondrial membrane
• It consists of a series of protein complexes that transfer electrons from NADH and FADH2 to oxygen
• As electrons move down the ETC, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space
  • This creates an electrochemical gradient known as the proton motive force
• ATP synthase, an enzyme embedded in the inner mitochondrial membrane, uses the proton motive force to generate ATP
  • Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi
• This process, in which the ETC is coupled to ATP synthesis, is called chemiosmosis
• The ETC and chemiosmosis together produce the majority of ATP generated during cellular respiration

Fermentation

• Fermentation is an anaerobic process that allows for ATP production in the absence of oxygen
• It occurs in the cytoplasm and involves the reduction of pyruvate to regenerate NAD+
• There are two main types of fermentation: lactic acid fermentation and alcohol fermentation
  • Lactic acid fermentation converts pyruvate to lactate and is used by muscle cells during intense exercise
  • Alcohol fermentation converts pyruvate to ethanol and CO2 and is used by yeast in the production of beer and wine
• Fermentation yields only 2 ATP per glucose molecule, compared to the 38 ATP produced by cellular respiration
• While less efficient, fermentation allows organisms to generate ATP in low-oxygen environments (anaerobic conditions)

Photosynthesis Basics

• Photosynthesis is the process by which plants and other autotrophs capture light energy to synthesize organic molecules
• The overall reaction for photosynthesis is: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
• Photosynthesis occurs in two stages: the light-dependent reactions and the Calvin cycle
• The light-dependent reactions take place in the thylakoid membranes of chloroplasts and convert light energy into chemical energy (ATP and NADPH)
• The Calvin cycle takes place in the stroma of chloroplasts and uses the ATP and NADPH to fix CO2 into glucose
• Photosynthetic pigments (chlorophylls and carotenoids) absorb light energy and transfer it to reaction centers

Light-Dependent Reactions

• The light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts
• They involve the absorption of light energy by photosynthetic pigments and the transfer of electrons through photosystems
• There are two types of photosystems: photosystem II (PSII) and photosystem I (PSI)
  • PSII absorbs light at a wavelength of 680 nm and splits water to release electrons and oxygen
  • PSI absorbs light at a wavelength of 700 nm and reduces NADP+ to NADPH
• Electron transport chains transfer electrons from PSII to PSI, pumping protons into the thylakoid lumen
• The proton gradient is used by ATP synthase to generate ATP in a process similar to chemiosmosis in cellular respiration
• The light-dependent reactions produce ATP and NADPH, which are used in the Calvin cycle to fix CO2 into glucose

Calvin Cycle

• The Calvin cycle (also known as the light-independent reactions) is the second stage of photosynthesis
• It takes place in the stroma of chloroplasts and uses the ATP and NADPH produced by the light-dependent reactions to fix CO2 into glucose
• The cycle can be divided into three phases: carbon fixation, reduction, and regeneration
  • In the carbon fixation phase, the enzyme RuBisCO combines CO2 with a 5-carbon sugar (ribulose bisphosphate) to form two 3-carbon compounds
  • In the reduction phase, ATP and NADPH are used to convert the 3-carbon compounds into 3-carbon sugars (glyceraldehyde 3-phosphate)
  • In the regeneration phase, some of the glyceraldehyde 3-phosphate is used to regenerate ribulose bisphosphate, allowing the cycle to continue
• The net output of the Calvin cycle is one 3-carbon sugar (glyceraldehyde 3-phosphate) per three CO2 molecules fixed
• These sugars can be used to synthesize glucose and other organic molecules needed by the plant

Connections to Other Topics

• Cellular respiration and photosynthesis are complementary processes that form the basis of energy flow in ecosystems
• The ATP and NADPH produced by photosynthesis are used in various metabolic pathways, including the synthesis of amino acids, lipids, and nucleotides
• Fermentation is used in the production of many food products (yogurt, cheese, bread) and biofuels (ethanol)
• Photosynthesis plays a crucial role in the global carbon cycle by removing CO2 from the atmosphere and storing it in organic molecules
• Understanding the mechanisms of cellular respiration and photosynthesis is essential for developing treatments for metabolic disorders (diabetes) and optimizing crop yields
• The evolution of photosynthesis and the oxygenation of Earth's atmosphere allowed for the development of complex life forms