Glycolysis & Energy Metabolism — Quick Notes

Energy and Metabolism

Energy fuels life; carbohydrates (glucose) provide ATP. Metabolism is the sum of all chemical reactions in living cells, including catabolic (energy-releasing) and anabolic (energy-requiring) pathways.

Forms of Energy and the First Law

Energy exists in multiple forms; the First Law of Thermodynamics states that energy cannot be created or destroyed, only transferred or transformed.

Photosynthesis and Energy Flow

$6 \mathrm{H<em>2O} + 6 \mathrm{CO</em>2} \rightarrow \mathrm{C<em>6H</em>{12}O<em>6} + 6 \mathrm{O</em>2}$
Light energy is stored as chemical energy in glucose; energy flows from sun to producers and then to consumers; cellular respiration releases energy as ATP and heat.

Glucose: Primary Energy Source

Glucose is the monosaccharide used as the primary energy source for plants and animals (also called dextrose or blood sugar).

Glucose Uptake and Transport

Glucose enters cells via uptake processes regulated by hormones (e.g., insulin) and is delivered to tissues via circulation; the liver helps regulate blood glucose by storing or releasing glucose as glycogen.

Metabolic Pathways: Linear and Cyclic

Metabolic pathways can be linear or cyclic, with sequences of substrates, enzymes, and intermediates leading to end products or cyclic regeneration of intermediates.

Anabolism and Catabolism

Anabolic pathways build large molecules using energy; catabolic pathways break down large molecules to release energy.

Cellular Respiration Overview

Cellular respiration transfers energy from glucose to ATP, enabling work. Glycolysis occurs in the cytoplasm and can occur without oxygen; aerobic steps require oxygen.

Glycolysis: Overview

Glucose → 2 pyruvate in the cytoplasm. 2 ATP are consumed to start; 4 ATP and 2 NADH are produced; Net ATP = 2.

Equation: $\text{Net ATP} = -2 + 4 = 2.$
Equation: $2 \mathrm{NADH}$ produced.

The 10 Steps of Glycolysis

There are 10 steps from glucose to pyruvate; you do not need to memorize enzyme names, but you should recognize the biomolecules as they appear elsewhere.

Fermentation: Overview and Purpose

Under anaerobic conditions, fermentation regenerates NAD+ so glycolysis can continue.

Lactic Acid Fermentation

Pyruvate + NADH → Lactate + NAD^+. Occurs in muscle and some bacteria (e.g., yogurt).
Equation: $\text{pyruvate} + \text{NADH} \rightarrow \text{lactate} + \text{NAD}^+.$

Ethanol Fermentation

Pyruvate → Ethanol + CO2; NAD+ regenerated. Enzymes: pyruvate decarboxylase and alcohol dehydrogenase; occurs in yeast (wine, beer).
Equation: $\text{pyruvate} \rightarrow \text{ethanol} + \text{CO}_2.$

Cori Cycle

Lactate produced in muscle is transported to the liver, converted back to glucose, and returned to muscles for energy.

Why Fermentation

In the absence of oxygen, fermentation regenerates NAD+ so glycolysis can continue and ATP can be produced.

Quick Summary

• Energy is the capacity to do work; macronutrient metabolism provides chemical energy.
• Carbohydrates (glucose) provide primary energy; glycolysis in the cytoplasm yields net +2 ATP per glucose and produces 2 NADH.
• Fermentation enables ATP production under anaerobic conditions via lactic acid or ethanol pathways; the Cori cycle links lactate to glucose in the liver.