Cellular Respiration Overview and Pathways

Cellular Respiration Overview

• Living cells obtain energy from organic molecules and release waste products
• The primary aim is to make ATP
• Aerobic respiration requires oxygen: O2 consumed and CO2 released
• When glucose is broken down via oxidation, a tremendous amount of free energy is released (-685kcal/mol)
• Glucose metabolism equation: C6H12O6 + 6O2 → 6CO2 + 6H2O
• Some energy from glucose breakdown is lost as heat, but much is used to make 3 energy intermediates: ATP, NADH, FADH2

Four Metabolic Pathways of Cellular Respiration

• Glycolysis: Glucose is broken down to two pyruvate molecules, producing 2 ATP and 2 NADH molecules.
• Breakdown of Pyruvate: Occurs in the mitochondrial matrix in eukaryotes. Each pyruvate is broken down to an acetyl group and CO2, generating one NADH per pyruvate.
• Citric Acid Cycle: Occurs in the mitochondrial matrix. Acetyl groups are incorporated into an organic molecule, later oxidized to liberate two CO2 molecules. Total yield: 4 CO2, 2 ATP, 6 NADH, 2 FADH2
• Oxidative phosphorylation: Occurs in the cristae in eukaryotes. NADH and FADH2 contain high-energy electrons, which are harnessed to produce an H+ electrochemical gradient, ultimately generating 30-34 ATP molecules.

Glycolysis

• Can occur with or without oxygen.
• Nearly identical in all living species.
• Ten enzyme steps in three phases:
  • Energy investment: Steps 1 to 3, 2 ATP hydrolyzed (ATP to ADP) to create fructose-1,6 bisphosphate
  • Cleavage: Steps 4 to 5, 6 carbon molecules broken into two 3 carbon molecules of glyceraldehyde-3-phosphate
  • Energy liberation: Steps 6 to 10, Two glyceraldehyde-3-phosphate molecules are broken down into two pyruvate molecules, yielding 2 NADH and 4 ATP (net yield = 2 ATP).
• The rate of glycolysis is regulated by the availability of substrates and feedback inhibition.
• Phosphofructokinase catalyzes the third step (the rate-limiting step).
• High ATP concentrations bind to phosphofructokinase, inhibiting the rate.

Breakdown of Pyruvate

• In eukaryotes, pyruvate is transported into the mitochondrial matrix.
• Broken down by pyruvate dehydrogenase.
• CO2 is removed from each pyruvate.
• The remaining acetyl group is attached to CoA (coenzyme A) to make acetyl CoA.

Citric Acid Cycle

• A metabolic cycle where some molecules enter while others leave, and the series of organic molecules is regenerated in each cycle.
• Acetyl is removed from Acetyl CoA and attached to oxaloacetate to form citrate (aka citric acid).
• Each cycle releases: 2 CO2, 1 ATP, 3 NADH, and 1 FADH2.
• Oxaloacetate is regenerated to start the cycle again.
• The rate of the citric acid cycle is regulated by the availability of substrates and feedback inhibition.
• Three steps are considered rate-limiting, catalyzed by: Citrate synthase, Isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase.

Oxidative Phosphorylation

• First three stages of glucose metabolism yield 6 CO2, 4 ATP, 10 NADH, 2 FADH2.
• High-energy electrons are removed from NADH and FADH2 to make ATP.
• Requires oxygen, and the oxidative process involves an electron transport chain (ETC).
• Phosphorylation occurs by ATP synthase.
• The ETC consists of protein complexes and small organic molecules embedded in the inner mitochondrial membrane.
• Electrons transfer in a linear manner in a series of redox reactions.
• Electrons from NADH or FADH2 are transferred to components of the ETC, each having an increasingly higher electronegativity.
• Oxygen is the final acceptor of electrons in the ETC, which is also called the respiratory chain.
• Energy released during electron movement is used to pump protons (H+) across the mitochondrial membrane into the intermembrane space, forming a proton gradient.
• The proton gradient provides the energy for synthesizing ATP.
• ATP synthase is a rotary machine that allows protons to pass through the membrane, using the free energy difference to convert ADP to ATP.

Connections Among Carbohydrate, Protein, and Fat Metabolism

• Many molecules are used for energy: carbohydrates, proteins, and fats.
• They enter glycolysis or the citric acid cycle at different points.
• Utilizing the same pathways for breakdown increases efficiency.

Anaerobic Respiration and Fermentation

• Anaerobic Respiration: Occurs in the absence of oxygen. Uses a substance other than O2 as the final electron acceptor in the electron transport chain.
• Fermentation: Produces ATP only via substrate-level phosphorylation.
• Fermentation is the breakdown of organic molecules without net oxidation.
• Many organisms can only use O2 as the final electron acceptor, so they need alternative ways to produce ATP under anaerobic conditions.

Lactic Acid Fermentation

• Muscle cells convert pyruvate into lactate to solve NADH buildup under anaerobic conditions.
• Lactic acid fermentation produces much less ATP than oxidative phosphorylation.

Alcohol Fermentation

• Yeast use fermentation to produce ethanol, also solving NADH buildup.
• Fermentation produces far less ATP than oxidative phosphorylation.