Cellular Respiration Stages

• Cellular respiration occurs in the presence or absence of oxygen.
• In the presence of oxygen, aerobic cellular respiration occurs.
• In the absence of oxygen, alcoholic fermentation (in yeast) or lactic acid fermentation (in humans) occurs.
• Cellular respiration has four major stages: glycolysis, pyruvate oxidation, citric acid cycle, and electron transport chain & oxidative phosphorylation.
• Glycolysis occurs in the cytoplasm.
• Pyruvate oxidation occurs in the mitochondrial matrix.
• Citric acid cycle occurs in the mitochondrial matrix.
• Electron Transport Chain & Oxidative Phosphorylation occurs in the inner mitochondrial membrane.

Stages of Respiration

• Glycolysis: Glucose turns into pyruvate.
• Krebs Cycle.
• Electron Transport Chain and oxidative phosphorylation.
• ATP (substrate-level phosphorylation) occurs during glycolysis and during the Krebs cycle.
• ATP (oxidative phosphorylation) occurs during the Electron Transport Chain and oxidative phosphorylation.
• Electrons are carried during Glycolysis via NADH
• Electrons are also carried during the Krebs Cycle via NADH and FADH2

Glycolysis Overview

• Glycolysis involves 10 steps.
• It occurs in the cytoplasm.
• Enzymes break down a 6-carbon glucose molecule into two 3-carbon pyruvate molecules, releasing ATP.
• Glycolysis does not require oxygen and uses anaerobic conditions.

Goals of Aerobic Respiration

• Break down 6-carbon glucose and release 6 molecules of CO2.
• Move glucose electrons to O2, combine with H+ ions to form 6 molecules of H2O.
• Collect energy in the form of ATP.

Glycolysis Step-by-Step

• Glycolysis involves 10 steps, each with specific reactions, enzymes, and energy changes.
• Step 1: Carbon 6 of glucose is phosphorylated by ATP, preventing glucose from leaving the cell; reaction type: phosphorylation; enzyme: kinase; energy: absorbed.
• Step 2: Atoms of the molecule are rearranged (isomerization); reaction type: isomerization; enzyme: isomerase; energy: equivalent.
• Step 3: Carbon 1 is phosphorylated to produce an energetically unstable product; reaction type: phosphorylation; enzyme: kinase; energy: absorbed.
• Step 4: A high-energy molecule is split into two molecules; reaction type: lysis; enzyme: lyase; energy: equivalent.
• Step 5: DHAP and G3P are isomers, and G3P is used in other metabolic pathways; reaction type: isomerization; enzyme: isomerase; energy: equivalent.
• Step 6: NADH (energy molecule) is created; reaction type: redox phosphorylation; enzyme: dehydrogenase; energy: released.
• Step 7: ADP phosphorylation creates ATP; reaction type: substrate-level phosphorylation; enzyme: kinase; energy: released.
• Step 8: Phosphate moves from carbon 3 to carbon 2; reaction type: isomerization; enzyme: isomerase; energy: equivalent.
• Step 9: Water is removed to set up the next reaction; reaction type: dehydration; enzyme: lyase; energy: released.
• Step 10: ADP phosphorylation creates ATP; reaction type: substrate-level phosphorylation; enzyme: kinase; energy: released.

NAD+/NADH

• NAD+ (nicotinamide adenine dinucleotide) is the oxidized form.
• NADH (nicotinamide adenine dinucleotide) is the reduced form.

Substrate-Level Phosphorylation

• Direct ATP formation through phosphate transfer from a molecule to ADP occurs.
• This is utilized in Glycolysis and the Krebs cycle.

Glycolysis - Key Parts of Process

• 2 ATPs are used to prime glucose for splitting (glucose molecules are phosphorylated) in the energy investment phase.
• At the end of the energy investment phase, there are two molecules of G3P.
• 2 NADH are formed when G3P reduces NAD+ by adding an H.
• 4 ATP are formed by substrate-level phosphorylation in the energy-harvest phase, removing 2Pi from each 3-carbon intermediate.
• 2 pyruvates are produced at the end of glycolysis, which move on to the next phase of cellular respiration.

Energy Yield

• 4 ATP produced - 2 ATP used = 2 net ATP.
• 2 NADH and 2 pyruvates will be processed further in aerobic cellular respiration if oxygen is available.