Cellular Respiration

• Living cells need energy from outside sources, like how chimpanzees eat plants, and other animals eat those plant-eating animals.
• Energy enters an ecosystem as sunlight and exits as heat.
• Photosynthesis makes oxygen and organic molecules, which are used in cellular respiration.
• Cells use the chemical energy stored in organic molecules to make ATP, which powers work.

Cellular Respiration

• Cellular respiration involves both aerobic and anaerobic respiration, but the term often refers to aerobic respiration.
• While carbohydrates, fats, and proteins can be used as fuel, tracking cellular respiration with glucose is useful.
• The breakdown of organic molecules is exergonic (releases energy).
• Fermentation is a partial breakdown of sugars without oxygen.
• Aerobic respiration uses organic molecules and oxygen to produce ATP.
• Anaerobic respiration is like aerobic respiration, but uses compounds other than oxygen.
• The formula for cellular respiration is: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)

Redox Reactions

• The transfer of electrons during chemical reactions releases energy stored in organic molecules, which is then used to synthesize ATP.
• Oxidation-reduction reactions (redox reactions) are chemical reactions that transfer electrons between reactants.
• Oxidation is when a substance loses electrons.
• Reduction is when a substance gains electrons.
• The electron donor is the reducing agent.
• The electron receptor is the oxidizing agent.
• Some redox reactions change electron sharing in covalent bonds instead of fully transferring electrons.
• During cellular respiration, the fuel (like glucose) is oxidized, and oxygen is reduced.

Electron Carriers

• During cellular respiration, glucose and other organic molecules are broken down in a series of steps.
• Electrons from organic compounds are first transferred to NAD+ (nicotinamide adenine dinucleotide), a coenzyme.
• NAD+ acts as an oxidizing agent during cellular respiration by accepting electrons.
• NADH (the reduced form of NAD+) represents stored energy used to synthesize ATP.
• NADH passes electrons to the electron transport chain.
• The electron transport chain passes electrons in a series of steps, preventing one explosive reaction.
• Oxygen pulls electrons down the chain, releasing energy to regenerate ATP.

Stages of Harvesting Energy

• There are three stages to harvesting energy from glucose:
  • Glycolysis: breaks down glucose into 2 molecules of pyruvate
  • Citric acid cycle: completes the breakdown of glucose
  • Oxidative phosphorylation: accounts for most of the ATP synthesis
• Oxidative phosphorylation generates the most ATP and is powered by redox reactions.
• Oxidative phosphorylation makes up almost 90% of the ATP generated by cellular respiration.
• A smaller amount of ATP is formed in glycolysis and the citric acid cycle through substrate-level phosphorylation.
• For each glucose molecule broken down to CO2 and water by respiration, the cell makes up to 32 ATP molecules.

Glycolysis

• Glycolysis ("splitting of sugar") breaks down glucose into 2 molecules of pyruvate.
• Glycolysis occurs whether or not oxygen is present.
• The process occurs in the cytoplasm and has two main phases:
  • Energy investment phase: the cell spends ATP.
  • Energy payoff phase: ATP is produced.
• In the presence of oxygen, pyruvate enters the mitochondrion (in eukaryotic cells) where glucose oxidation is completed.
• Before the citric acid cycle, pyruvate is converted to acetyl coenzyme A (acetyl CoA), linking glycolysis to the citric acid cycle.
• This conversion is carried out by a multienzyme complex that catalyzes 3 reactions.

Citric Acid Cycle

• The citric acid cycle is also known as the Krebs cycle or tricarboxylic acid cycle.
• This cycle completes the breakdown of pyruvate to CO2.
• For each turn, the cycle oxidizes organic fuel from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2.

Electron Transport Chain

• After glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food.
• These electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation.
• The electron transport chain is located in the inner membrane (cristae) of the mitochondrion.
• Most components of the chain are proteins in multi-protein complexes.
• Carriers alternate between reduced and oxidized states as they accept and donate electrons.
• Electrons lose free energy as they move down the chain and are finally passed to oxygen, forming water.
• Electrons are transferred from NADH or FADH2 to the electron transport chain.
• Electrons pass through proteins, including cytochromes (each with an iron atom), to oxygen.
• The electron transport chain does not directly generate ATP.
• It breaks the large free-energy drop from food to oxygen into smaller steps, releasing energy in manageable amounts.
• Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space.
• H+ then moves back across the membrane through ATP synthase.
• ATP synthase uses the exergonic flow of H+ to drive ATP phosphorylation.
• This is an example of chemiosmosis, where energy in an H+ gradient drives cellular work.
• The energy stored in the H+ gradient across the membrane couples the redox reactions of the electron transport chain to ATP synthesis.
• Energy flow: glucose → NADH → electron transport chain → proton-motive force → ATP.
• About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, producing about 32 ATP.

Anaerobic Respiration and Fermentation

• Most cellular respiration requires oxygen to produce ATP.
• Without oxygen, the electron transport chain stops.
• In this case, glycolysis couples with fermentation or anaerobic respiration to produce ATP.
• Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate).
• Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP.
• Fermentation involves glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis.
• Two common types of fermentation are:
  • Alcohol fermentation: pyruvate is converted to ethanol, releasing CO2.
  • Lactic acid fermentation: pyruvate forms lactate, with no release of CO2 (used by human muscle cells when oxygen is scarce).

Comparing Processes

• In all three processes (cellular respiration, anaerobic respiration, and fermentation):
  • All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy from food.
  • NAD+ is the oxidizing agent that accepts electrons during glycolysis.
• The processes differ in their final electron acceptors:
  • Fermentation: an organic molecule (pyruvate, acetaldehyde).
  • Cellular respiration: oxygen.
• ATP Production:
  • Cellular respiration produces 32 ATP per glucose molecule.
  • Fermentation produces 2 ATP per glucose molecule.

Summary of Processes

• Glycolysis
  • Anaerobic (does not need oxygen)
  • Occurs in the cytoplasm
  • Glucose → 2 pyruvate + 2 NADH + 2 ATP
• Krebs Cycle (Citric Acid Cycle)
  • Aerobic (needs oxygen)
  • Occurs in the mitochondrial matrix
  • 2 Acetyl-coenzymeA → 4 CO2 + 6 NADH + 2 ATP + 2 FADH2
• Electron Transport System
  • Aerobic (needs oxygen)
  • Occurs in the inner mitochondrial membrane (cristae)
  • Passage of electrons from NADH to O2 → H2O + H+ gradient
  • 26-28 ATP
• Overall Equation C6H12O6 + 6 O2 ⇔ 6 CO2 + 6 H20+ electrons → 30-32 ATP