3.4 Cellular Energetics: Respiration and Photosynthesis

Cellular energetics is the powerhouse of life, driving everything from muscle movement to brain function. Respiration breaks down nutrients for energy, while photosynthesis captures sunlight to make food. These processes are the foundation of life's energy flow.

Understanding cellular energetics is crucial for grasping how cells function. It explains how organisms get energy, grow, and interact with their environment. This knowledge connects to broader concepts of cell structure, metabolism, and ecological relationships.

Cellular Respiration and Energy Production

Overview of Cellular Respiration

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• Cellular respiration breaks down glucose and other organic molecules to release energy in the form of ATP (adenosine triphosphate)
• The overall equation for cellular respiration: $C_6H_{12}O_6$ (glucose) + $6O_2$ (oxygen) → $6CO_2$ (carbon dioxide) + $6H_2O$ (water) + energy (ATP)
• Cellular respiration occurs in three main stages glycolysis, the Krebs cycle (citric acid cycle), and the electron transport chain

Stages of Cellular Respiration

• Glycolysis takes place in the cytoplasm breaks down glucose into two pyruvate molecules, producing a small amount of ATP and NADH
• The Krebs cycle occurs in the matrix of the mitochondria oxidizes pyruvate to CO2, generating high-energy molecules like NADH and FADH2
• The electron transport chain is located in the inner mitochondrial membrane uses the high-energy molecules from the Krebs cycle to create a proton gradient, which drives ATP synthesis through chemiosmosis (oxidative phosphorylation)
• Examples of cellular processes powered by ATP include biosynthesis (protein synthesis), active transport (sodium-potassium pump), and muscle contraction (myosin-actin interactions)

Mitochondria's Role in Respiration

Mitochondrial Structure and Function

• Mitochondria are double-membrane organelles known as the "powerhouses" of the cell due to their central role in cellular respiration and ATP production
• The inner mitochondrial membrane contains the electron transport chain complexes (Complex I, II, III, and IV) and ATP synthase, which are essential for the final stages of cellular respiration
• The matrix, the space enclosed by the inner mitochondrial membrane, contains enzymes involved in the Krebs cycle (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase)

Mitochondrial Genetics and Endosymbiotic Theory

• Mitochondria have their own DNA, ribosomes, and the ability to synthesize proteins, suggesting an endosymbiotic origin (ancient bacteria engulfed by eukaryotic cells)
• The number of mitochondria in a cell varies depending on the cell's energy requirements cells with high energy demands, such as muscle cells (skeletal muscle) and neurons (brain cells), have more mitochondria compared to less metabolically active cells (skin cells)

Aerobic vs Anaerobic Respiration

Aerobic Respiration

• Aerobic respiration requires oxygen includes glycolysis, the Krebs cycle, and the electron transport chain, producing a large amount of ATP (around 30-32 ATP per glucose molecule)
• Aerobic respiration is more efficient in terms of ATP production but requires the presence of oxygen
• Examples of organisms that primarily use aerobic respiration include animals (humans), plants, and many microorganisms (aerobic bacteria)

Anaerobic Respiration and Fermentation

• Anaerobic respiration occurs in the absence of oxygen only involves glycolysis and fermentation, producing a small amount of ATP (2 ATP per glucose molecule)
• In animals, anaerobic respiration results in the formation of lactic acid, leading to muscle fatigue (during intense exercise)
• In yeast and some bacteria, anaerobic respiration produces ethanol and CO2 through alcoholic fermentation (beer and wine production)
• Organisms may switch between aerobic and anaerobic respiration depending on the availability of oxygen and their energy requirements (facultative anaerobes like Escherichia coli)

Photosynthesis and Energy Capture

Overview of Photosynthesis

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in the bonds of glucose and other organic compounds
• The overall equation for photosynthesis: $6CO_2$ (carbon dioxide) + $6H_2O$ (water) + light energy → $C_6H_{12}O_6$ (glucose) + $6O_2$ (oxygen)
• Photosynthesis occurs in two stages the light-dependent reactions and the light-independent reactions (Calvin cycle)

Light-Dependent and Light-Independent Reactions

• Light-dependent reactions take place in the thylakoid membranes of chloroplasts convert light energy into chemical energy (ATP and NADPH) through the process of photophosphorylation (cyclic and noncyclic)
• Light-independent reactions occur in the stroma of chloroplasts use the ATP and NADPH from the light-dependent reactions to fix CO2 into organic compounds through the Calvin cycle (carbon fixation, reduction, and regeneration)
• Photosynthetic pigments, such as chlorophyll a, chlorophyll b, and carotenoids (beta-carotene and xanthophylls), absorb light energy and transfer it to reaction centers for photosynthesis (Photosystem I and II)
• The glucose produced during photosynthesis is used by plants for growth, development, and storage in the form of starch (amylose and amylopectin) or other polysaccharides (cellulose)

Cellular Respiration and Photosynthesis in the Carbon Cycle

Complementary Nature of Photosynthesis and Cellular Respiration

• Photosynthesis and cellular respiration are complementary processes that play a crucial role in the global carbon cycle
• Photosynthesis removes CO2 from the atmosphere and incorporates it into organic compounds, while cellular respiration releases CO2 back into the atmosphere
• The oxygen released by photosynthesis is used by organisms for cellular respiration, while the CO2 released by cellular respiration is used by plants for photosynthesis

Carbon Cycle Disruptions and Environmental Impact

• The balance between photosynthesis and cellular respiration helps regulate the levels of atmospheric CO2 and oxygen, maintaining the stability of Earth's ecosystems
• Disruptions in the carbon cycle, such as increased CO2 emissions from human activities, can lead to climate change and other environmental issues (global warming, ocean acidification)
• Examples of human activities that disrupt the carbon cycle include fossil fuel combustion (coal, oil, and natural gas) and deforestation (Amazon rainforest)