BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. ENERGY UTILIZATION

9.3. Cellular Respiration

9.3.1. Respiratory Substrates

Respiration is The oxidation of organic matter that serves as a respiratory substrate. CARBOHYDRATES, fats, and Proteins act as substrates for respiration.

CARBOHYDRATES. When carbohydrates are available, most Cells preferentially use them as substrates. Polysaccharides (starch in plants, and Glycogen in animals and Fungi) enter THE RESPIRATORY PROCESS only after being hydrolyzed into Monosaccharides.

Lipids (FATS OR OILS). Lipids constitute the "main reserve" and are generally utilized only when carbohydrate stores are depleted. First, they must be hydrolyzed into glycerol and Fatty acids. Fatty acids are energy-rich, and certain cells, such as Muscle cells, normally derive a portion of their required energy directly from them.

PROTEINS. Because proteins serve several other vital Functions, they are used for energy production only after all carbohydrate and fat reserves have been exhausted, such as during prolonged starvation (Section 8.9.3). Proteins are first hydrolyzed into Amino Acids, which are then deaminated (stripped of their amino groups). The resulting acid enters the Krebs cycle (Section 9.3.5) or is first converted into a fatty acid before undergoing oxidation (Section 9.3.5).