Cellular Respiration Overview

1. Definition:
   • Cellular respiration is a series of biochemical steps that enable cells to produce energy.
   • The process overall can be summarized by a simple chemical equation:
     • Chemical reaction:
     • Organic compounds (sugars, fats, proteins) + Oxygen → Carbon Dioxide + Water + Energy (ATP + Heat).

Key Concepts of Cellular Respiration

2. Step-wise Process:
   • Cellular respiration is composed of many individual steps that ensure the efficient transfer of energy from organic compounds into ATP.
   • This is different from releasing energy all at once, as it increases efficiency.
   • Enzymatic Control:
     • Each step is regulated by specific enzymes.
     • These enzymes facilitate redox reactions, which are processes that involve the transfer of electrons.
3. Redox Reactions:
   • Redox reactions help in the efficient transfer of electrons from organic compounds to coenzymes (NAD+ or FAD).
   • Definitions:
     • Oxidized: A substance losing electrons.
     • Reduced: A substance gaining electrons.
     • Mnemonic: Remember that electrons are negatively charged. Adding electrons (reducing) makes a substance more negative.
4. Coenzymes - NAD+ and FAD:
   • NAD+: Initially has one positive charge, gets reduced to NADH when it gains two electrons and one H+ ion.
   • FAD: Starts neutral, becomes FADH2 upon gaining two electrons and two H+ ions.
   • Function:
     • NAD+ and FAD act as carriers for electrons (called electron wheelbarrows), transporting them for ATP production in mitochondria.

Steps in Cellular Respiration

5. Overview of Process:
   • The metabolism of one glucose molecule with 6 carbons will be examined through four main steps:
     • Glycolysis
     • Intermediate step
     • Citric acid cycle
     • Oxidative phosphorylation
   • Each step will indicate what is input, what comes out, and the location of occurrence in the cell.
   • The end goal is to see how glucose is oxidized to produce ATP.

Citric Acid Cycle

6. Function:
   • The citric acid cycle processes the remaining carbons from the glucose molecule (after glycolysis).
   • Occurs in the mitochondrial matrix and requires oxygen (aerobic process).
   • Outputs from 1 Acetyl CoA:
     • 6 NADH
     • 2 FADH2
     • 2 ATP
     • 4 CO2
   • Note:
     • For every glucose molecule, there are 2 acetyl CoA generated, thus doubling these outputs.
   • Important to note that all carbon atoms from glucose are utilized.

Oxidative Phosphorylation

7. Final Stage:
   • Consists of two interconnected parts: the Electron Transport Chain (ETC) and Chemiosmosis.
8. Electron Transport Chain:
   • Comprised of 4 multiprotein complexes (Complex 1, 2, 3, 4).
   • Key Points:
     • Pumps protons (H+) across the inner mitochondrial membrane creating a gradient.
     • Complex Actions:
     • Complex 1, 3, and 4 pump H+; Complex 2 does not.
     • NADH is oxidized back to NAD+ at Complex 1; FADH2 is oxidized at Complex 2.
     • As electrons flow through, they ultimately reduce oxygen to water.
   • Electron Pathway:
     • From NADH → 1 → Q → 3 → CytC → 4 → O2
     • From FADH2 → 2 → Q → 3 → CytC → 4 → O2

Glycolysis

9. Overview:
   • The first step of cellular respiration, taking place in the cytoplasm and does not require oxygen (anaerobic).
   • Starting Point:
     • One glucose molecule (6 carbons) enters glycolysis but requires 2 ATP to initiate the process (Energy investment phase).
   • Splitting of Glucose:
     • Glucose is split into two 3-carbon pyruvate molecules during glycolysis.
   • Final Outputs:
     • Produces 2 NADH, 4 ATP (net gain of 2 ATP due to initial investment), and 2 pyruvate.
     • All enzymatic reactions occur without oxygen.

Intermediate Step

10. Transition to Mitochondria:
    • Connects glycolysis products to the citric acid cycle.
    • Pyruvate (3 carbons) loses a carbon as CO2, is converted into acetyl CoA.
    • Important Enzymatic Action:
      • Requires NAD+ to reduce to NADH.
      • Process requires oxygen, thus marking it as aerobic.
    • The carbon released represents carbon from organic food, which becomes part of exhaled CO2.
    • Importantly, the O2 in CO2 comes from glucose, not directly from inhaled O2 in respiration (which eventually reduces to water).