• How do we get the energy we need?
– Food
– What in food gives us the energy we need?
• Cellular Respiration- process that releases energy by breaking down food molecules in the presence of oxygen
– Made up of glycolysis, Krebs cycle, and the electron transport chain
• Equation for cellular respiration
– 6 O₂+ C₆H₁₂O₆→ 6 CO₂+ 6 H₂O + Energy

• Each stage captures some of the chemical energy available in food molecules and uses it to produce ATP
• Glycolysis- process in which one molecule of glucose is broken in half, producing two molecules of pyruvic acid- 3-carbon compound
• Glycolysis needs 2 ATP molecules to begin process
• What happens during glycolysis?
– 2 molecules of pyruvic acid, 2 molecules of ATP, and 2 molecules of
NADH are produced
• One of reactions of glycolysis removes 4 high energy electrons and passes them to NAD⁺-electron carrier
– Each NAD⁺ accepts a pair of high energy electrons and transfers them to other molecules
– Allows energy from glucose to be passed to other pathways in cell
• Cellular Respiration

• Releases energy from food molecules by producing ATP in the absence of oxygen
• What happens during fermentation?
– NADH is converted back to NAD⁺ by passing high energy electrons back to pyruvic acid
– Allows glycolysis to produce steady supply of ATP
• Anaerobic
• 2 main types of fermentation- alcoholic fermentation and lactic acid fermentation

• Alcoholic fermentation- uses pyruvic acid and
NADH to produce ethyl alcohol, carbon dioxide and NAD⁺
– Used by yeasts and few other microorganisms
• Lactic Acid- uses pyruvic acid and NADH to produced lactic acid and NAD⁺
– Produced in muscles during rapid exercise when body cannot supply enough oxygen to tissues
– Unicellular organisms produce lactic acid as waste, as result prokaryotes are used in array of food production

• 90% of chemical energy still available in glucose after glycolysis, locked up in high energy electrons of pyruvic acid
• Oxygen is required for final steps of cellular respiration- aerobic

• Oxygen must be present
• Also known as Citric Acid Cycle
• During cycle, pyruvic acid is broken down into carbon dioxide in a series of energy extracting reactions
• Citric Acid Production
– Pyruvic acid enters mitochondrion, carbon is removed forming CO₂, electrons are removed, changing NAD⁺ to
NADH
– Coenzyme A joins the 2 carbon molecule, forming acetyl-
CoA. Acetyl- CoA adds the 2 carbon acetyl group to a 4carbon compound forming citric acid

• Energy Extraction
– Citric acid is broken down into 5-carbon compound and then into 4-carbon compound
– 2 more molecules of CO₂ are released and electrons join NAD⁺ and FAD, forming NADH and
FADH₂, one molecule of ATP is generated
– Energy output from one molecule of pyruvic acid=
4 NADH, 1 FADH₂, and 1 molecule of ATP
• CO₂ released is source of all carbon dioxide we breathe
• ATP produced in Krebs cycle is used for cellular activities

• Krebs cycle generates high energy electrons that are passed to NADH and FADH₂
• Electrons are passed from carriers to electron transport chain
• Uses high energy electrons from Krebs cycle to convert
ADP to ATP
• Takes place in mitochondrion
• Steps of Electron Transport Chain
– High energy electrons passed along chain from one carrier protein to next. At end of chain, enzyme combines these electrons with hydrogen ions and oxygen to form water
– Oxygen serves as final acceptor, it is essential for getting rid of low energy electrons and hydrogen ions-the wastes of cellular respiration

• Every time 2 high energy electrons transport down chain, energy is used to transport hydrogen ions across the membrane
• Inner membrane of mitochondria contain protein spheres called ATP synthases, as H⁺ ions escape through channels, into these proteins, ATP synthases spin and grab a low energy ADP and attaches a phosphate, forming high energy ATP
• On average, each pair of high energy electrons produces 3 molecules of ATP from ADP

• Glycolysis produces 2
ATP molecules
• Krebs cycle and Electron
Transport Chain produce roughly 36 ATP molecules, 18 times more than glycolysis