Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 9.6 Glycolysis and the Citric Acid Cycle Connect to Many Other Metabolic Pathways
Cellular respiration is not just for breaking down glucose—it also connects to a wide network of metabolic pathways, both catabolic and anabolic, that process various types of food molecules.
Catabolic Versatility
Glycolysis and the citric acid cycle are central hubs that process carbohydrates, proteins, and fats
Starch and glycogen are broken into glucose for glycolysis
Disaccharides (e.g., sucrose) provide monosaccharides used in respiration
Proteins are broken into amino acids, which are deaminated and enter glycolysis or the citric acid cycle
Fats are digested into glycerol (→ glycolysis) and fatty acids (→ beta oxidation → acetyl CoA)
Fats yield more than twice as much ATP per gram as carbohydrates
Biosynthesis (Anabolic Pathways)
Not all nutrients are broken down for ATP; some are used to build needed molecules
Intermediates from glycolysis and the citric acid cycle serve as precursors for biosynthesis
Humans synthesize about half of required amino acids; the rest are “essential” and must be obtained from food
Fatty acids can be synthesized from acetyl CoA; glucose can be made from pyruvate
Regulation of Cellular Respiration
Cells regulate ATP production based on demand
If ATP levels drop → respiration speeds up; if ATP is abundant → respiration slows down
Key control point: phosphofructokinase (enzyme in glycolysis)
Inhibited by ATP and citrate
Stimulated by AMP (from ADP)
This feedback ensures efficient use of resources and synchronizes glycolysis with the citric acid cycle
In a Nutshell
Glycolysis and the citric acid cycle are key crossroads in metabolism. They allow cells to break down or build up carbohydrates, proteins, and fats, depending on cellular needs. Enzyme-based regulation ensures energy balance and efficient use of resources.