Cells obtain energy from more than glucose alone. Carbohydrates, proteins, and lipids can all connect with the pathways of glucose catabolism, especially glycolysis and the citric acid cycle. This lesson explains how nutrients enter shared metabolic pathways and how the history of photosynthesis and respiration shaped cellular energy use.
Metabolic pathways are porous networks: substances can enter from other pathways, and pathway products can become reactants elsewhere. This interconnectedness lets cells use different foods to supply shared intermediates and energy-producing processes.
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What does it mean to say metabolic pathways are porous?
Pathways exchange substances with other pathways. A compound made in one pathway may become a reactant in another, and compounds from other pathways can enter.
What is glycogen, and when is it used?
Glycogen is a polymer of glucose used for short-term energy storage in animals. It is stored in the liver and muscles and can be broken down when glucose is needed.
How does the body handle nitrogen from excess amino acids?
The amino group is removed and converted to ammonia. In mammals, the liver uses ammonia and carbon dioxide to synthesize urea, which is excreted in urine.
How do glycerol and fatty acids from triglycerides enter energy metabolism?
Glycerol can be phosphorylated and proceed through glycolysis. Fatty acids are broken into two-carbon units that feed into the citric acid cycle.
How did oxygen-releasing photosynthesis affect cellular energy use?
It released oxygen that accumulated in the atmosphere. Organisms later evolved oxygen-using respiration, enabling more energy to be extracted from sugar.
Quiz questions
Why are metabolic pathways described as porous rather than closed systems?Answer: Substances can enter from other pathways, and a pathway's products can become another pathway's reactants.
What happens to sucrose during digestion?Answer: It is split into glucose and fructose, which are absorbed separately.
Compared with carbohydrates, how much energy do triglycerides store?Answer: About twice as much energy per amount stored.
Which statement describes cholesterol synthesis?Answer: It begins with acetyl CoA, proceeds in one direction, and does not produce ATP.
Which sequence best describes the proposed development of cellular energy metabolism?Answer: Glycolysis arose early, followed by anoxygenic photosynthesis; later, oxygen-releasing photosynthesis enabled oxygen-using respiration.
Based on Concepts of Biology by Samantha Fowler, Rebecca Roush, James Wise (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.