Chapter 6 Summary
Chapter 6.1
- Energy is defined as the ability to do work, and can broadly be categorized as kinetic (including heat) and potential (including chemical) energy.
- The total amount of energy in the universe is constant, so cells must transform and transfer existing energy to effectively function.
- The total entropy of the universe is always increasing, so cells must constantly use energy to avoid disorder.
- Gibb’s free energy (G) is a measure of energy that is available to do work.
- Exergonic (spontaneous) chemical reactions or processes have a negative change in free energy (∆G < 0), releasing energy that can be used to do work.
- Endergonic chemical reactions or processes have a positive change in free energy (∆G < 0), so energy input is required to make these reaction happen.
- All chemical reactions have an activation energy (EA), the amount of energy required to move the reaction through a high energy transition state, which can cause even exergonic (spontaneous) reactions to proceed slowly.
Chapter 6.2
- Metabolism consists of all of the biochemical reactions that occur in a cell or organisms.
- Biochemical reactions are chained together into metabolic pathways (multiple connected steps), which can either be catabolic (breaking down large molecules into small ones) or anabolic (synthesizing large molecules from smaller substrates)
- Cellular respiration, which primarily takes place in the mitochondria, is an example of a catabolic pathway. Glycogen breakdown is another.
- Photosynthesis, performed by the chloroplast, is a prime example of an anabolic pathway. Glycogen synthesis is another.
- The many metabolic pathways in cells often include reactants or products that are part of other metabolic pathways, so these pathways can “intersect.”
Chapter 6.3
- To overcome the energy barrier associated with endergonic reactions, energy coupling is often required, which often involves ATP.
- ATP, composed of adenosine and three phosphate groups, is a good temporary storage of chemical potential energy due to its phosphoanhydride bonds.
- ATP hydrolysis to ADP and Pi is exergonic, and can be coupled to many other chemical reaction and processes that are endergonic, allowing the free energy from ATP hydrolysis to be used by those endergonic processes.
- ATP synthesis is an endergonic process, so other exergonic processes are required to support ATP synthesis.
- In cellular respiration, the free energy released by oxidation of glucose (or other organic molecules) is transferred to electron carriers and then a proton gradient across the mitochondrial inner membrane to provide an energy sources for ATP synthesis from ADP and Pi.
Chapter 6.4
- To overcome the energy barrier associated with activation energy barrier, cells use enzymes to catalyze almost all biochemical reactions.
- Enzymes are (usually) proteins made of one or more polypeptide chains that increase chemical reaction rates by lowering the activation energy of those reactions.
- The active sites of enzymes provide an environment that helps lower activation energy by stabilizing (lowering the energy of) transition states during chemical reactions.
- To regulate the activity of metabolic pathways, enzyme activity is regulated by cellular conditions (e.g., temperature and pH), location (compartmentalization), covalent modification, and temporary binding to activators, inhibitors, cofactors, and coenzymes
- Most commonly, enzymes are regulated through feedback inhibition, in which the products of metabolic pathways act as inhibitors of the enzymes involved in the pathway.