Animals -- Whether They Are Carnivorous, Omnivorous,

¶ … animals -- whether they are carnivorous, omnivorous, or herbivorous -- depend upon the mechanisms of photosynthesis as a source of food. Carnivorous and omnivorous animals eat other animals as a source of food, but prey animals such as herbivores consume plants as a source of energy. And oxygen, the by-product of plant photosynthesis, enables all animal life on land and in the water to breathe. The chlorophylls and carotenoids, the pigments present in the plant's cellular structure, absorb sunlight and convert it into energy, producing oxygen as a byproduct. Sunlight is transformed into ATP in plants. ATP enables the synthesizing of glucose from carbon dioxide and water within the plant's cellular membranes. "Glucose subunits are joined together, forming starch and other molecules," and producing oxygen for animal life as a by-product (Photosynthesis and aerobic respiration, 2010, Aquarium project website). Depending on where the plants are located, the oxygen is released into either the atmosphere or the water.

In aerobic respiration in animals, or respiration in the presence of oxygen, energy is released through the breakdown of glucose and other organic compounds. "Aerobic respiration produces the most ATP for each glucose molecule…In-between are three stages of reactions. The first stage is glycolysis, the second [is] the Krebs cycle, and the third [stage] is the electron transport phosphorylation reactions" (Photosynthesis and aerobic respiration, 2010, Aquarium project website). Through aerobic respiration, released energy in the form of ATP then becomes available within the animal's body for a variety of cellular tasks. Carbon dioxide and water are released as by-products, which are then used by plants in the processes of photosynthesis. The cycle of respiration between animals and plants is thus symbiotic and balanced, as one form of life produces the necessary products to sustain the other life form's respiratory and essential life activities.

Q2. Anaerobic respiration takes place in the absence of oxygen. Fermentation is a type of anaerobic respiration. The anaerobic respiration pathway breaks glucose into two molecules of pyruvate, a three-carbon compound. The process of breaking down glucose into pyurvate is called glycolosis. Pyruvate molecules can be converted into alcohol and carbon dioxide or into lactic acid, the substance that is also used to make yogurt. Animals' muscles, including human animals, also manufacture lactic acid during periods of "prolonged exercise" (Fermentation and anaerobic respiration, 2010, AP Study Guide). The build-up of lactic acid during exercise is most often experienced as 'cramping.' Energy from the sun is still involved in anaerobic respiration, as anaerobic respiration involves the breakdown of ATP through glycolosis. However, aerobic respiration is not as efficient as aerobic respiration, as energy molecules are still 'left over' at the end of the chemical conversion Fermentation and anaerobic respiration, 2010, AP Study Guide).

During anaerobic respiration "the organism invests 2 ATPs into the process and receives 4 ATPs back. The net gain is 2 ATPs" (Fermentation and anaerobic respiration, 2010, AP Study Guide). Anaerobic respiration was likely created, over the course of the evolution of life, with the earliest bacteria which used it as a way of generating energy when their ATP was depleted as a source of aerobic respiration. "In anaerobic respiration, there is a molecule called NAD that received 2 electrons to become NADH. The cell has only a limited supply of NAD and if it is all converted to NADH, the breakdown of glucose would stop. This is overcome by converting NADH back to NAD by giving the electrons to acetaldehyde to produce ethanol" or alcohol (Fermentation and anaerobic respiration, 2010, AP Study Guide).

Q3. Enzymes are catalysts, or agents that bind temporarily to reactants and, in doing so, lower the amount of activation energy needed to create the reaction. In the cycle of enzyme-substrate interactions, the initial phase is known as precursor activation, in which "the regulator molecule increases the affinity of the enzyme in the series for its substrate enzyme," then causing the enzyme to anchor or insert itself into the cell membrane during the second phase in a manner once described as a kind of 'lock and key' effect (Kimball 2010). In the final phase of feedback inhibition, affinity of the enzyme lowers and the catalytic enzyme detaches from the membrane (Kimball 2010).