Honors Biology Unit 5 ReviewCellular Energy: Respiration & Photosynthesis

Key Concepts

• Cellular respiration breaks down glucose and other organic molecules to release energy in the form of ATP
• Photosynthesis captures light energy to synthesize glucose and other organic compounds using carbon dioxide and water
• Glycolysis, the Krebs cycle, and the electron transport chain are the three main stages of cellular respiration
• Light-dependent reactions and the Calvin cycle are the two main stages of photosynthesis
• Chemiosmosis involves the flow of protons across a membrane down their electrochemical gradient through ATP synthase to generate ATP
• Photophosphorylation is the production of ATP using the energy of sunlight during photosynthesis
• Oxidative phosphorylation is the production of ATP using energy derived from the redox reactions of an electron transport chain during cellular respiration

Cellular Respiration Basics

• Cellular respiration is a metabolic process that breaks down glucose and other organic molecules to release energy in the form of ATP
• Occurs in the mitochondria of eukaryotic cells and the cytoplasm of prokaryotic cells
• Requires oxygen (aerobic respiration) or occurs in the absence of oxygen (anaerobic respiration or fermentation)
• Overall equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$
  • Glucose is oxidized to carbon dioxide and water, releasing energy that is used to generate ATP
• Consists of three main stages: glycolysis, the Krebs cycle, and the electron transport chain
• Produces approximately 30-32 ATP molecules per glucose molecule during aerobic respiration

Glycolysis and Fermentation

• Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm
• Breaks down one glucose molecule into two pyruvate molecules
• Produces a net gain of 2 ATP and 2 NADH molecules
• Steps of glycolysis:
  1. Glucose is phosphorylated by ATP to form fructose-1,6-bisphosphate
  2. Fructose-1,6-bisphosphate is split into two three-carbon compounds (DHAP and G3P)
  3. G3P is oxidized and phosphorylated to form 1,3-bisphosphoglycerate
  4. 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate, generating ATP
  5. 3-phosphoglycerate is converted to pyruvate, generating another ATP
• Fermentation occurs in the absence of oxygen and regenerates NAD+ from NADH
  • Allows glycolysis to continue by providing NAD+ for the oxidation of G3P
• Lactic acid fermentation (in animal cells and some bacteria) reduces pyruvate to lactate
• Alcoholic fermentation (in yeast and some bacteria) decarboxylates pyruvate to acetaldehyde, which is then reduced to ethanol

The Krebs Cycle

• The Krebs cycle, also known as the citric acid cycle, is the second stage of cellular respiration
• Occurs in the mitochondrial matrix
• Oxidizes acetyl-CoA (derived from pyruvate) to carbon dioxide, generating high-energy electrons carried by NADH and FADH2
• Steps of the Krebs cycle:
  1. Acetyl-CoA combines with oxaloacetate to form citrate
  2. Citrate is isomerized to isocitrate
  3. Isocitrate is oxidized and decarboxylated to form α-ketoglutarate, generating NADH
  4. α-ketoglutarate is oxidized and decarboxylated to form succinyl-CoA, generating NADH
  5. Succinyl-CoA is converted to succinate, generating GTP (which is converted to ATP)
  6. Succinate is oxidized to fumarate, generating FADH2
  7. Fumarate is hydrated to malate
  8. Malate is oxidized to oxaloacetate, generating NADH
• Each turn of the cycle produces 3 NADH, 1 FADH2, and 1 GTP (or ATP) per acetyl-CoA

Electron Transport Chain

• The electron transport chain (ETC) is the third and final stage of cellular respiration
• Occurs in the inner mitochondrial membrane
• Consists of a series of protein complexes (I, II, III, and IV) and mobile electron carriers (ubiquinone and cytochrome c)
• High-energy electrons from NADH and FADH2 are transferred through the ETC, releasing energy used to pump protons (H+) from the mitochondrial matrix into the intermembrane space
• Proton gradient generated by the ETC is used to power ATP synthase, which produces ATP through chemiosmosis
• Oxygen serves as the final electron acceptor, combining with protons to form water
• Each NADH yields approximately 2.5 ATP, while each FADH2 yields approximately 1.5 ATP
• In total, the ETC generates about 26-28 ATP per glucose molecule (assuming 10 NADH and 2 FADH2 from glycolysis and the Krebs cycle)

Photosynthesis Overview

• Photosynthesis is the process by which plants, algae, and some bacteria capture light energy to synthesize organic compounds (primarily glucose) from carbon dioxide and water
• Occurs in the chloroplasts of eukaryotic cells and the thylakoid membranes of cyanobacteria
• Overall equation: $6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$
  • Carbon dioxide and water are converted into glucose and oxygen using light energy
• Consists of two main stages: light-dependent reactions and the Calvin cycle (light-independent reactions)
• Pigments such as chlorophyll a, chlorophyll b, and carotenoids absorb light energy for photosynthesis
• Photosystems I and II are protein complexes that contain chlorophyll and other pigments that capture light energy

Light-Dependent Reactions

• Light-dependent reactions occur in the thylakoid membranes of chloroplasts
• Capture light energy to generate ATP and NADPH, which are used in the Calvin cycle
• Photosystem II absorbs light energy, exciting electrons that are transferred through an electron transport chain to photosystem I
• Electron transport chain pumps protons (H+) from the stroma into the thylakoid lumen, creating a proton gradient
• ATP synthase uses the proton gradient to generate ATP through chemiosmosis (photophosphorylation)
• Photosystem I absorbs light energy, exciting electrons that are transferred to NADP+ to form NADPH
• Splitting of water (photolysis) by photosystem II replaces the electrons lost from the photosystems and releases oxygen as a byproduct

Calvin Cycle (Light-Independent Reactions)

• The Calvin cycle, also known as the light-independent reactions, occurs in the stroma of chloroplasts
• Uses ATP and NADPH generated by the light-dependent reactions to fix carbon dioxide into organic compounds (glucose)
• Steps of the Calvin cycle:
  1. Carbon fixation: Ribulose bisphosphate (RuBP) combines with CO2 to form two molecules of 3-phosphoglycerate (3-PGA)
  2. Reduction: 3-PGA is reduced to glyceraldehyde 3-phosphate (G3P) using ATP and NADPH
  3. Regeneration: Some G3P is used to regenerate RuBP, while the rest is used to synthesize glucose and other organic compounds
• Key enzyme: Ribulose bisphosphate carboxylase-oxygenase (RuBisCO) catalyzes the carbon fixation step
• Three turns of the Calvin cycle are required to produce one molecule of G3P (half of a glucose molecule), consuming 9 ATP and 6 NADPH

Energy Efficiency and Yield

• Cellular respiration is highly efficient, with about 30-32 ATP produced per glucose molecule during aerobic respiration
  • Glycolysis: 2 ATP
  • Krebs cycle: 2 ATP (directly) + 6 NADH (15 ATP) + 2 FADH2 (3 ATP)
  • Electron transport chain: 26-28 ATP (from NADH and FADH2)
• Photosynthesis has a lower energy yield, with about 1 glucose molecule produced per 18 ATP and 12 NADPH consumed in the Calvin cycle
• Factors affecting photosynthetic efficiency:
  • Light intensity and wavelength
  • Carbon dioxide concentration
  • Temperature
  • Water and nutrient availability
• C4 and CAM plants have adaptations to increase photosynthetic efficiency in hot, dry environments
  • C4 plants (corn, sugarcane) concentrate CO2 in bundle sheath cells to minimize photorespiration
  • CAM plants (cacti, pineapple) open stomata at night to minimize water loss

Real-World Applications

• Biofuels: Fermentation of plant-derived sugars to produce ethanol as a renewable fuel source
• Algal bioreactors: Cultivation of microalgae for biofuel production, carbon sequestration, and wastewater treatment
• Crop improvement: Genetic engineering and selective breeding to enhance photosynthetic efficiency and yield in agricultural crops (rice, wheat)
• Biohydrogen production: Using microorganisms or artificial photosynthetic systems to generate hydrogen gas as a clean energy source
• Artificial photosynthesis: Developing synthetic systems that mimic natural photosynthesis to produce renewable fuels and chemicals
• Bioremediation: Using photosynthetic organisms to remove pollutants and heavy metals from contaminated sites (constructed wetlands)
• Space exploration: Designing closed-loop life support systems that incorporate photosynthesis for oxygen production and carbon dioxide removal (Biosphere 2)