Endotherms are animals that maintain constant body temperatures that are usually higher than their surroundings. Endotherms generate considerable amounts of heat, have insulating structures (e.g. feathers, fur and fat layers), and use about 5 times more energy than ectotherms. The metabolic demands of endothermic animals are much higher than that of ectothermic animals. Examples of endotherms are mammals and birds.
Ectotherms are animals other than birds and mammals, and whose body temperatures fluctuate with that of the surroundings. Examples are fish, amphibians and reptiles.
Heterotherms are ectotherms that are able to maintain muscle temperatures higher than the surroundings. Examples are tuna and some sharks whose muscle temperature is 14 °C higher than the water, and insects that 'shiver' to warm their wing muscles.
Energy in Cells
The useable form of energy in cells is ATP (Adenosine Triphosphate), which is used as the activation energy for many cell chemical reactions. When ATP is broken down by losing one of its 3 phosphate groups, it forms ADP (Adenosine Diphosphate) and a phosphate group H2PO4–
The reaction is:
The Action of Enzymes
Metabolism is all the chemical reactions of cells that provide for growth, response to stimuli, movement, maintenance and repair, and reproduction.
A Chemical Reaction involves a change of chemical composition of the reactant chemicals to form different chemicals called products.
Activation Energy - Chemical reactions require a certain amount of energy called the activation energy. If an overall reaction such as photosynthesis requires energy to occur, it is called an endergonic reaction. If the overall reaction such as respiration releases energy, it is called an exergonic reaction.
Catalysts are substances that regulate the speed at which a chemical reaction occurs, without being used up in the reaction. Catalysts reduce the amount of activation energy required for a reaction to occur. They can be used over and over again.
Enzymes are catalysts in living organisms. They are large globular proteins produced by living cells. Many are dissolved in the cytoplasm, or are in mitochondria and other organelles. About 2000 enzymes are known (e.g. lipase breaks down lipids to fatty acids and glycerol).
Action of Enzymes - Enzymes have a specific shape. Part of the enzyme matches the shape of the molecule to be reacted called the substrate. The part of the enzyme that binds to the substrate is the active site. When the substrate and enzyme bind temporarily, an enzyme-substrate complex is formed. The activation energy needed for the reaction to occur is reduced. After the reaction is complete, the substrate has formed a new product and the enzyme is released to be reused.
Two Theories of Enzyme Action
Lock and Key Model - Substrate molecules already have the right shape to fit an enzyme.
Induced Fit Model - Interaction between the enzyme and the substrate induces or changes the shape of the molecules to produce a suitable fit.
Factors Affecting Enzyme Activity
Temperature - Enzymes are proteins that function optimally at certain temperatures. They are inactivated or denatured by temperatures above 50 °C to 60 °C. Enzymes are not inactivated by freezing, but either work slowly or do not work at all.
pH - Enzymes are sensitive to changes in acidity. For example, the high acidity of stomach juice is needed for the enzyme pepsin to function, whereas an enzyme called trypsin in the small intestine requires alkaline surroundings.
Concentration of Enzyme - If pH and temperature are kept constant, the rate of reaction is proportional to the amount of enzyme present. The more enzyme there is, the more reactions that occur.
Co-Enzymes - Some vitamins and minerals (e.g. copper, zinc, iron) in small quantities work with enzymes to speed up reactions.
Enzyme Inhibitors - A pesticide called DDT stops the action of enzymes in insects, thereby killing them.