Cellular Respiration & ATP

• Cellular respiration converts glucose to energy in the form of ATP
• This process provides the energy necessary for life activities like muscle contraction
• It also provides energy for nerve impulse transmission, active transport across cell membranes, and the biosynthesis of essential molecules.

Importance of Cellular Respiration

• Provides ATP, the energy currency for cells.
• Enables organisms to perform essential biological functions.
• Maintains homeostasis by regulating metabolic processes.

ATP

• Adenosine Triphosphate is a Nucleotide
• ATP is made up of Adenine, Ribose (5C-Sugar), and a Chain of 3 Phosphate groups.
• ATP stores energy in its phosphate bonds
• When the bond between the last two phosphates breaks during hydrolysis, ATP becomes ADP and releases energy.

Location of Cellular Respiration

• Cytoplasm: Glycolysis (initial breakdown of glucose).
• Mitochondria: Krebs cycle & Oxidative phosphorylation (energy extraction and ATP production).

Aerobic Respiration (With Oxygen)

• Requires oxygen as the final electron acceptor.
• More efficient, producing ≈38 ATP per glucose molecule.
• Overall equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP.

Anaerobic Respiration (Without Oxygen)

• Occurs when oxygen is unavailable.
• Less efficient, producing only 2 ATP per glucose molecule.
• Fermentation occurs instead of oxidative phosphorylation.

Lactic Acid Fermentation

• In muscle cells, converts pyruvate into lactic acid
• This causes muscle fatigue during intense exercise
• Certain bacteria, like Lactobacillus, used in the production of cheese, yogurt, sauerkraut, and kimchi.

Alcoholic Fermentation

• In yeast, converts pyruvate into ethanol and CO2
• Used in baking and brewing industries.

Glycolysis (Breaking Down Glucose)

• Location: Cytoplasm
• Glucose (C6) is split into two molecules of pyruvate (C3).
• ATP and NADH are produced.

Steps of Glycolysis: Energy Investment Phase

• Prepares glucose for breakdown

Phosphorylation

• Glucose → Glucose-6-phosphate (G6P) and uses 1 ATP.

Isomerization

• G6P → Fructose-6-phosphate (F6P).

Phosphorylation

• F6P → Fructose-1,6-bisphosphate (F1,6BP) and uses another ATP.

Cleavage

• F1,6BP splits into two molecules → G3P & DHAP.

Isomerization

• DHAP converts into another G3P.

Energy Payoff Phase

• Produces ATP and NADH

Oxidation & Phosphorylation

• G3P oxidized → NADH + high-energy compound

Substrate-Level Phosphorylation

• ADP → ATP (Direct ATP formation).

Isomerization

• Molecular rearrangement

Dehydration

• Creates phosphoenolpyruvate (PEP), and is a high-energy molecule

Substrate-Level Phosphorylation

• PEP donates phosphate to ADP → ATP.

Summary of Glycolysis

• ATP used: 2
• ATP produced: 4
• Net ATP gain: 2
• NADH produced: 2
• H2O released: 2
• Pyruvate (C3H4O3): 2

Formation of Acetyl CoA

• Link Reaction, also called Oxidative Decarboxylation or Pyruvate Oxidation
• Connects glycolysis to the Krebs cycle
• This process occurs in the Mitochondrial Matrix
• Each pyruvate (3C) is converted into Acetyl CoA (2C)

Step 3

• Acetyl group (2C) is unstable on its own
• CoA acts as a "carrier molecule"

Krebs Cycle or Citric Acid Cycle

• Named after Sir Hans Krebs, who discovered it
• Also called Tricarboxylic Acid Cycle (TCA) because citric acid has three carboxyl groups
• Occurs in the Mitochondrial Matrix
• Goal: Produce high-energy electron carriers (NADH and FADH2) for the Electron Transport Chain

Steps of Krebs Cycle

• Condensation: Acetyl-CoA donates its 2-carbon acetyl group to oxaloacetate (4C), forming citrate (6C) and releasing CoA.
• Isomerization: Citrate is rearranged into isocitrate through a reversible two-step process via an intermediate called cis-aconitate
• First Oxidative Decarboxylation: Isocitrate is oxidized, releasing one carbon as CO2. One molecule of NAD+ is reduced to NADH.
• Second Oxidative Decarboxylation: α-Ketoglutarate undergoes another decarboxylation, releasing CO2, where NAD+ is reduced to NADH, and CoA binds to form Succinyl-CoA.
• Substrate-level Phosphorylation: The high-energy bond in Succinyl-CoA is broken, releasing CoA and generating GTP (or ATP) through substrate-level phosphorylation.
• Oxidation: Succinate is oxidized to fumarate, and FAD is reduced to FADH2.
• Hydration: A water molecule (H2O) is added to fumarate, converting it into malate.
• Final Oxidation: Malate is oxidized to regenerate oxaloacetate, and NAD+ is reduced to NADH

Total Output per 1 Acetyl-CoA:

• 3 NADH
• 1 FADH2
• 1 ATP/GTP
• 2 CO2

Oxidative Phosphorylation (Electron Transport Chain + Chemiosmosis)

• Location: Inner mitochondrial membrane

Steps

• Electron Transport Chain of NADH and FADH2 through four protein complexes (I-IV).
• The energy from the ETC drives protons (H+) from the mitochondrial matrix to the intermembrane space.
• At the end of the ETC, oxygen accepts electrons and binds with H+ ions to form water (H2O).
• Chemiosmosis (from intermembrane space to matrix).
• ATP Synthesis.
• ATP produced in oxidative phosphorylation: ≈34

ATP Count in 4 stages.

• Glycolysis: 2 ATP and 2 NADH (x3) = 6 ATP
• Formation of Acetyl CoA: 2 NADH (x3) = 6 ATP
• Krebs Cycle: 2 GTP = 2 ATP, 6 NADH (x3) = 18 ATP, and 2 FADH (x2) = 4 ATP
• TOTAL: ≈ 38 ATP per 1 molecule of glucose

Summary

• Glycolysis: Occurs in Cytoplasm. Initial source(Input) Glucose. Main Molecule Product 2 Pyruvates. ATP yield 2 ATP
• Formation of Acetyl CoA: Occurs in Matrix. Initial source(Input) 2 Pyruvates. Main Molecule Product 2 Acetyl CoA. ATP yield 0
• Krebs Cycle: Occurs in Matrix. Initial source(Input) 2 Acetyl CoA. Main Molecule Product 6 NADH 2 FADH2. ATP yield 2 ATP
• Oxidative Phosphorylation: Occurs in Inner Membrane. Initial source(Input) 10 NADH 2 FADH2. Main Molecule Product 0. ATP yield 34 ATP

Anaerobic Respiration

• Glycolysis – first and last step
• Fermentation: Extension of glycolysis. By-product is either ethanol or lactic acid

Anaerobic Respiration Types

• Lactic Acid Fermentation: Bacteria (e.g., Lactobacillus), Muscle Cells. End product Lactic Acid and regenerates NAD+ for glycolysis in anaerobic conditions
• Alcoholic Fermentation: Yeast (e.g., Saccharomyces cerevisiae). End product Ethanol and Carbon Dioxide and produces ethanol as a byproduct of anaerobic respiration