Chapter 10: Cellular Respiration

Cellular Respiration: Overview

• Organisms get energy from food, which is stored in units of calories (1 calorie = amount of energy needed to raise 1 gram of water 1°C)
• Cells use different molecules for food, such as fats, proteins, and carbs, with varying energy storage
• Cellular respiration is the process of energy conversion that releases energy from food in the presence of oxygen
• The overall equation for cellular respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + chemical energy in food (ATP)
• The cell reduces the chemical energy in food molecules a little at a time, trapping the energy in the form of ATP

Stages of Cellular Respiration

• There are three main stages of cellular respiration: glycolysis, Krebs cycle, and electron transport chain
• Glycolysis: 10% of glucose energy is captured to produce ATP, leaving 90% of chemical energy still available
• Krebs cycle: More energy is captured, with reactants from other stages of the process
• Electron transport chain: Extracts energy from reactants using oxygen, requiring reactants from other stages of the process

Oxygen and Energy

• Oxygen is required at the end of the electron transport chain
• When cell demand for energy increases, oxygen use also increases
• Most energy-releasing pathways within cells require oxygen, which is why we need to breathe oxygen

Aerobic Respiration

• Aerobic respiration is a process that requires oxygen
• It occurs in two stages: the Krebs cycle and the electron transport chain
• The Krebs cycle is a series of chemical reactions that occur in the mitochondria of cells, requiring an electron transport chain to function
• The electron transport chain is a series of molecules that use electrons from the Krebs cycle to generate ATP, requiring oxygen

Anaerobic Respiration

• Anaerobic respiration is a process that does not require oxygen
• It occurs in the cytoplasm of cells
• The most common type of anaerobic respiration is glycolysis
• Glycolysis breaks down glucose into two molecules of pyruvate, not requiring oxygen

The Relationship between Photosynthesis and Cellular Respiration

• Photosynthesis and cellular respiration are two complementary processes
• Photosynthesis produces glucose and oxygen, while cellular respiration uses glucose and oxygen to produce carbon dioxide, water, and ATP
• The products of photosynthesis are the reactants of cellular respiration, and vice versa

Glycolysis

• Glycolysis breaks down glucose into two molecules of pyruvate, releasing a small amount of energy
• The energy from glycolysis is used to produce ATP and NADH

The Krebs Cycle

• The Krebs cycle is a series of chemical reactions that occur in the mitochondria of cells
• It begins with the breakdown of acetyl-CoA, producing citric acid
• The cycle releases energy, producing ATP, NADH, and FADH₂
• The energy from the Krebs cycle is used to produce ATP in the electron transport chain

Electron Transport and ATP Synthesis

• The electron transport chain uses high-energy electrons from the Krebs cycle to synthesize ATP from ADP
• The electron transport chain is a series of electron carriers located in the inner membrane of the mitochondria
• High-energy electrons are passed from carrier to carrier, releasing energy used to produce ATP

ATP Production

• The cell uses chemiosmosis to produce ATP
• The inner mitochondrial membrane contains enzymes called ATP synthases
• The charge difference across the membrane forces hydrogen ions through channels in these enzymes, producing ATP
• On average, each pair of high-energy electrons that moves down the full length of the chain provides enough energy to produce 3 molecules of ATP

Fermentation

• Fermentation is an anaerobic process that occurs in the absence of oxygen
• It releases energy from food molecules by producing ATP
• There are two types of fermentation: alcoholic and lactic acid

Alcoholic Fermentation

• Carried out by yeast, producing ethyl alcohol and carbon dioxide
• Pyruvic acid + NADH → alcohol + CO₂ + NAD+

Lactic Acid Fermentation

• Chemical reaction that converts pyruvic acid to lactic acid
• Does not give off CO₂
• Regenerates NAD+ so that glycolysis can continue
• Pyruvic acid + NADH → lactic acid + NAD+
• Humans are also lactic acid fermenters, particularly during brief periods without enough oxygen

Energy and Exercise

• During short, intense bursts of energy, the body uses ATP already in muscles and ATP made by lactic acid fermentation
• For longer periods of exercise, cellular respiration is the only way to continue generating a supply of ATP
• Aerobic forms of exercise are beneficial for weight control because they increase the efficiency of the respiratory and circulatory systems to supply oxygen to muscles for aerobic respiration

Learn about cellular respiration, the process of generating energy from food, including the importance of calories and different macromolecules in cells.