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.