Chapter 7. Metabolism
Chapter Review
IMPORTANT: Key Takeaways
- Metabolism is the sum of all chemical reactions required to support cellular function and hence the life of an organism. Metabolism can be categorized into either anabolism or catabolism.
- Anabolism refers to the energy-requiring metabolic processes that join smaller molecules into bigger molecules. Anabolic reactions combine monosaccharides to form polysaccharides, fatty acids to form triglycerides, amino acids to form proteins, and nucleotides to form nucleic acids.
- Catabolism refers to the energy-yielding metabolic processes that break down larger molecules into smaller molecules. Catabolic reactions break down proteins into amino acids, lipids into fatty acids, and polysaccharides into monosaccharides.
- The breakdown of glucose begins with glycolysis, which yields two ATP per glucose molecule and occurs in the cytosol. In the presence of oxygen (aerobic) and if the cell is in a negative-energy balance, pyruvate is transported into the mitochondria and converted into acetyl-CoA. In the citric acid cycle, electrons are generated in the form of hydrogen ions to pass to NADH and FADH2 to the inner mitochondrial membrane, where the electron transport chain takes place. In this metabolic pathway, a sequential transfer of electrons between multiple proteins occurs and a lot of ATP is produced. However, only 30-40% of the energy is captured, and the remaining is therefore released as heat to maintain body temperature.
- Once liberated from their storage sites, fatty acids are broken down in the catabolic pathway known as β-oxidation, which occurs in the mitochondria. β-oxidation produces the common intermediate, acetyl-CoA, which then enters the citric acid cycle and ETC (as seen in carbohydrate metabolism).
- Deamination removes nitrogen from the amino acid, allowing the carbon skeleton to be converted into pyruvate, acetyl-CoA, or another intermediate of the citric acid cycle, thus entering the same stages 2 and 3 as carbohydrate metabolism.
- Catabolic hormones such as cortisol, glucagon, adrenaline/epinephrine, and cytokines stimulate the breakdown of molecules and the production of energy. Anabolic hormones such as insulin-like growth factor, insulin, testosterone, and estrogen stimulate the synthesis of molecules.
Practice Questions
- How is energy classified? Provide examples of each type of energy being used in our daily lives.
- How do carbohydrates, proteins, and fats differ in their energy content per gram?
- What are nutrient-dense foods and do they relate to food quality?
- What is the difference between catabolism and anabolism?
- Compare fat and carbohydrate stores in humans.
- What would happen to chemical reactions without enzymes?
- How are metabolic pathways involved in catabolism and anabolism regulated by energy status?
- What happens to pyruvate in the presence and absence of oxygen?
- Why is some energy released as heat during nutrient catabolism?
