Characteristics of enzymes
1. Proteins that lower the activation energy needed to initiate a reaction
2. Influence rates only - do not affect the equilibrium rate or overall energy profile
3. Exist in small quanities - efficiency dependent on concentration
4. Form reversible complex with substrate
5. Not consumed in reaction - used over and over till degraded
6. Show specificity for the substrate
7. Fooled by inhibitors (mimics) - medicine capitalizes on this trait
8. Each enzyme is subject to environmental factors (optimal conditions)
9. Many enzymes require cofactors or coenzymes (vitamins)
10. Two to three thousand kinds of enzymes in each cell. Different cells have different enzyme complements
11. Often involved in multi-step pathways
12. Subject to regulation
13. Production directly controlled by the genes
14. Many genetic diseases caused by faulty enzymes
Energy flow
Most ecosystems, but not all, depend on sunlight as an energy source.
Photoautotrophic organisms trap light energy and transform it into chemical bonds. Inorganic molecules (carbon dioxide and water) are reduced to organic molecules (carbohydrates, e.g.) and oxygen is released as a by-product.
The cells of autotrophs and heterotrophs burn these organic molecules in the presence of oxygen to build up ATP reserves. Carbon dioxide and water are released as by-products.
Only some of the initial energy is stored in the high-energy bonds of ATP; most is released into the environment as heat. Thus, materials are recycled, but energy is not.
The products of respiration serve as the raw materials for photosynthesis. The products of photosynthesis serve as the raw materials for cellular respiration.
Redox reactions
Oxidation-reduction (redox) reactions involve the complete or partial transfer of electrons from one reactant to another. There cannot be a loss of electrons without a gain of electrons, and vice versa.
When an electron is transferred, its energy is transferred as well. Oxidation, then, can be looked at as a loss of energy, and reduction as a gain in energy.
Respiration
External respiration is the exchange of gases with the environment. Internal or cellular respiration is a catabolic process in which the energy stored in fuel molecules (carbohydrates, fats, proteins) is transferred to ATP.
There are two pathways of respiration. Both begin with glycolysis. Glycolysis is the breakdown of sugars. Glyco = sweet; lysis = to split
One pathway is fermentation. This is often an anaerobic process occurring in the cytoplasm. In this pathway, both the electron donors and acceptors are organic molecules. Fermentation results in the partial degradation of sugars, so much of the energy is unharvested. In cellular respiration, the electron acceptor is an inorganic molecule, such as oxygen. It is the most prevalent and efficient pathway, resulting in the complete combustion of fuel molecules.
Overall, the oxidation of glucose (the most common fuel) can be described as
C6H12O6 + 6O2 goes to 6 CO2 + 6 H20 + energy (ATP and heat)