Harvesting Energy: Glycolysis and Cellular Respiration

Harvesting Energy: Glycolysis and Cellular Respiration

Introduction to Energy Flow in Ecosystems

Energy flows through ecosystems while chemicals are recycled. The process of harvesting energy involves converting light energy into chemical energy (ATP and heat) through a series of metabolic pathways.

• Energy Flow: Light → organic molecules → ATP + heat

• Photosynthesis: Converts light to organic molecules

• Cellular Respiration: Converts organic molecules to ATP and heat

Overall equation for cellular respiration:

$\mathrm{C_6H_{12}O_6 + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O + \text{Energy (ATP + heat)}}$

Glycolysis

Overview of Glycolysis

Glycolysis is a catabolic pathway that breaks down glucose (a six-carbon sugar) into two molecules of pyruvic acid. This process occurs in the cytoplasm and can take place with or without oxygen.

• Multi-step pathway in the cytoplasm

• Splits glucose to pyruvate

• Occurs with or without O2

Phases of Glycolysis

The reactions of glycolysis occur in two main phases:

1. Glucose Activation Phase

   • Uses ATP to phosphorylate glycolysis intermediates

   • Costs two ATP molecules per glucose

2. Energy Harvest Phase

   • Produces ATP

   • Yields 4 ATP molecules per glucose

   • 2 molecules of NAD+ are reduced to NADH per glucose

Key Steps in Glycolysis

• Step 1: Phosphorylation of Glucose – Makes glucose more reactive and traps it in the cytoplasm.

• Step 2: Rearrangement – Shuffles some functional groups.

• Step 3: Second Phosphorylation – Regulatory step; makes fructose-6-phosphate more reactive.

• Step 4: Splitting – 6-carbon sugar is split into two 3-carbon sugars.

• Step 5: Rearrangement – Only one form proceeds through the rest of the pathway.

• Step 6: Oxidation and Phosphorylation

  • Step 6a: Rearrangement of 3-carbon sugar; 2 NADH produced per glucose

  • Step 6b: Phosphorylation creates a high-energy phosphate bond

• Step 7 & 10: Substrate-Level Phosphorylation – High-energy phosphate is transferred to ADP to produce ATP (2 ATP per glucose at each step).

• End Product: 2 pyruvic acid molecules per glucose

Cellular Respiration

Overview

Cellular respiration is the process by which cells harvest energy from organic molecules, typically in the presence of oxygen. It is a cumulative function of the Krebs cycle and electron transport.

• Overall equation: Organic compounds + O2 → CO2 + H2O + Energy (ATP)

• Results in the complete degradation of sugars

Major Stages of Cellular Respiration

1. Krebs Cycle (Citric Acid Cycle)

   • Occurs in the mitochondrial matrix

   • Completes the breakdown of glucose started by glycolysis

   • Discovered by Hans Krebs

2. Electron Transport Chain and Oxidative Phosphorylation

   • Occurs in the inner mitochondrial membrane and between the intermembrane space and the matrix

   • Couples electron transport to ATP synthesis

The Bridge Reaction

This reaction connects glycolysis to the Krebs cycle by converting pyruvic acid to acetyl CoA.

• Removal of CO2

• Production of NADH from NAD+

• Attachment of coenzyme A to form acetyl CoA

Krebs Cycle Details

• Each glucose yields two turns of the Krebs cycle (one per pyruvate)

• Produces NADH, FADH2, CO2, and a small amount of ATP by substrate-level phosphorylation

Summary Equation for Glycolysis and Krebs Cycle

$\mathrm{C_6H_{12}O_6 + 10\ NAD^+ + 4\ ADP + 2\ FAD \rightarrow 6\ CO_2 + 10\ NADH + 4\ ATP + 2\ FADH_2}$

Energy is stored in ATP, NADH, and FADH2.

Electron Transport Chain and Chemiosmosis

Electron Transport System

• Located in the inner mitochondrial membrane

• Accepts electrons from NADH and FADH2

• Uses energy from electron transfers to make ATP via oxidative phosphorylation

• Produces most (90%) of the ATP in cellular respiration

Chemiosmosis

Chemiosmosis is the energy-coupling mechanism that uses the proton gradient generated by the electron transport chain to drive ATP synthesis.

• Electron transport chain does not make ATP directly; it creates a proton gradient

• ATP synthase uses the proton gradient to synthesize ATP

• Enfoldings of the inner mitochondrial membrane increase surface area for chemiosmosis

ATP Yield from Electron Transport

• For every NADH: 3 protons are moved from the matrix to the intermembrane space

• For every FADH2: 2 protons are moved

• For every proton that crosses back into the matrix, one ATP is synthesized by ATP synthase

Overall Summary Equation

$\mathrm{C_6H_{12}O_6 + 36\ ADP + 6\ O_2 \rightarrow 6\ CO_2 + 36\ ATP + 42\ H_2O}$

ATP Yield Table

The following table summarizes ATP production from each stage of cellular respiration:

Metabolic Process	Substrate-Level Phosphorylation	Coenzyme Reduced	Oxidative Phosphorylation	Total ATP
Glycolysis	Net 2 ATP	2 NADH	4(6)-2	6
Oxidation of Pyruvate	-	2 NADH	6	6
Krebs Cycle	2 ATP	6 NADH, 2 FADH2	18, 4	24
Total				36

Other Fuels in Respiration

Glycolysis and the Krebs cycle are at a metabolic crossroads, accepting components from carbohydrates, fats, and proteins.

• Carbohydrates: Most polysaccharide breakdown products can be converted to glucose or fructose.

• Fats: Glycerol can enter glycolysis; fatty acids are oxidized to 2-carbon acetyl groups and enter at the Krebs cycle.

• Proteins: Amino acids are deaminated and enter at pyruvate or later steps.

Fermentation

Overview

If oxygen is not present, pyruvic acid from glycolysis undergoes fermentation. Fermentation is an anaerobic process (occurs without O2), does not produce ATP, and results in the partial degradation of sugars. Its main function is to regenerate NAD+.

Types of Fermentation

1. Lactic Acid Fermentation

   • Pyruvic acid is reduced to lactic acid

   • Equation: Glucose → pyruvic acid → lactic acid

   • Occurs in muscle cells under anaerobic conditions

2. Alcohol Fermentation

   • Pyruvic acid loses CO2 and is converted to ethanol

   • Equation: Glucose → pyruvic acid → ethanol + CO2

   • Carried out by many bacteria and yeast

Under anaerobic conditions, muscles carry out lactic acid fermentation instead of cellular respiration.