Metabolism and Digestion

The use of chemical energy is a fundamental characteristic of living animals. It is necessary to maintain cellular order and is vital to almost all physiological processes. Catabolic metabolism breaks down macromolecules for production of usable energy by cellular processes such as active transport, muscle contraction, ciliary movement, and production of heat, electricity, or light. Most cellular reactions need 20-40 kJ of energy per mole of reactants, which is much less than the energy yield of the complete oxidation of a typical metabolic substrate. Therefore high-energy phosphate compounds (phosphagens) are used as intermediary chemical energy stores. ATP is the most common phosphagen. Free energy is released by the hydrolysis of its terminal phosphate to form adenosine diphosphate (ADP) and inorganic phosphate (Pi) that is, ATP $ ADP + Pi + 30.5 kJ moP1. There is a cyclic formation of ATP from ADP (by cellular metabolism) and subsequent breakdown of ATP by energy-requiring processes.

Animals are heterotrophs, and as such are unable to synthesize their own organic compounds from inorganic molecules and so rely on other organisms for nutrients. Energy is obtained from nutrients such as carbohydrates, lipids, and sometimes proteins (amino acids are required for protein systhesis but also produce energy when oxidized). Essential vitamins, minerals, and fatty acids are also needed for proper cell functioning and must also be obtained via the diet. Single-celled animals and sponges ingest food particles by phagocytosis. These are chemically and enzymatically reduced within a food vacuole to a few constituent substances (e.g., monosaccharides, fatty acids, and amino acids) that are transported into the cytoplasm. Most multicellular animals have a digestive system specialized for extracellular digestion. Food particles enter the digestive system where a series of physical and chemical digestive processes break down food particles into constituent molecules that are absorbed and distributed to the cells. These molecules can then be used for energy metabolism, or for cell maintenance or growth.

Metabolism may be aerobic or anaerobic. Aerobic metabolism is the oxidation of carbohydrates, lipids, and proteins by oxygen to provide energy in the form of ATP. There are three major steps in the aerobic process: glycolysis, where glucose is converted to pyruvate with a net gain of 2 ATP (and 2 NADH/H+), the citric acid (or Kreb's) cycle where pyruvate is converted to acetyl-CoA before undergoing a cycle of chemical reactions resulting in a further net gain of 2 ATP (and 6 NADH/H+ and 2 FADH2), and finally the mitochondrial electron transfer system. Ninety-five percent of the ATP is generated by electron transfer, where electrons from NADH/H+ and FADH 2 are transferred to electron carrier proteins, passing through several protein complexes and generating 34 ATP. Oxygen is the final electron receptor in the chain, and water is formed as the end product.

Anaerobic metabolism is an alternative to aerobic metabolism, but it is very inefficient by comparison, forming as little as 2 ATP per glucose molecule. Consequently most large and complex animals rely on aerobic metabolism to meet their resting requirements, but they may use anaerobic metabolism for supplemental energy, for example, during intense activity or anoxia. Build-up of lactate as an anaerobic end product of glycolysis is a major inhibitory factor in the long-term use of anaerobic metabolism in tetrapod vertebrates. However some (e.g., carp) can convert pyruvate to ethanol as the end product, which can be easily excreted to the environment and therefore does not inhibit glycolysis.

Many factors affect the metabolic rate (MR) of animals, including temperature, developmental stage, diet, photoperiod, taxonomy, habit, environment, activity, and circadian rhythm. Body size is a major determinant of MR and is probably the best studied but least understood topic in animal physiology. Larger animals have a higher overall MR than small animals but have a lower MR per gram of body mass, so the relationship (eqn [1]) between mass (M) and MR

does not scale isometrically (i.e., b ^ 1). Rather, b < 1 since small animals use proportionally more energy (i.e., per gram) than larger animals. This relationship is remarkably uniform for all animals, from single-celled protists to birds and mammals. Although there is some debate as to what the scaling coefficient actually is (and why), b appears to generally fall between 0.67 (the value expected if MR scales with surface area) and 1 (the value if MR is proportional to mass); b is typically about 0.75. The intercept of the scaling relationship (a) is lowest for unicellular organisms, higher for ectothermic animals, and highest for endothermic animals, but the slope is consistently about 0.75 (Figure 8).

10"

Figure 8 Scaling of metabolic rate for unicellular organisms, and ectothermic animals (at 20 °C) and endothermic mammals and birds (at 39 °C). Modified from Hemmingsen AM (1950) The relation of standard (basal) energy metabolism to total fresh weight of living organisms. Reports of the Steno Memorial Hospital and Nordic Insulin Laboratory 4: 7-58.

10"

Figure 8 Scaling of metabolic rate for unicellular organisms, and ectothermic animals (at 20 °C) and endothermic mammals and birds (at 39 °C). Modified from Hemmingsen AM (1950) The relation of standard (basal) energy metabolism to total fresh weight of living organisms. Reports of the Steno Memorial Hospital and Nordic Insulin Laboratory 4: 7-58.

Solar Power Sensation V2

Solar Power Sensation V2

This is a product all about solar power. Within this product you will get 24 videos, 5 guides, reviews and much more. This product is great for affiliate marketers who is trying to market products all about alternative energy.

Get My Free Ebook


Post a comment