Bioenergetics and Metabolism

• Bioenergetics describes the concept of energy flow through living systems.
• Cellular processes, such as building and breaking down complex molecules, occur through stepwise chemical reactions.
• Metabolism includes all chemical reactions inside cells, including those using and releasing energy.

Carbohydrate Metabolism

• Living things consume sugar as a major energy source due to the energy stored within its bonds.
• The breakdown of glucose is described by the equation: C6H12O6+6O2→6CO2+6H2O+energy
• Plants use the energy of sunlight to convert carbon dioxide gas (CO2) into sugar molecules, like glucose (C6H12O6) through photosynthesis.
• The synthesis of glucose is described by the equation: 6CO2+6H2O+energy→C6H12O6+6O2
• During photosynthesis, energy is in the form of ATP, or adenosine triphosphate, which is the primary energy currency of all cells.
• Sugar (glucose) is stored as starch or glycogen, and these energy-storing polymers break down into glucose to supply ATP molecules.
• Light energy from the sun initially transforms into chemical energy that temporally stores itself in the energy carrier molecules ATP and NADPH (nicotinamide adenine dinucleotide phosphate).
• The stored energy in ATP and NADPH is used to build one glucose molecule from six molecules of CO2 during photosynthesis.
• Energy from 18 molecules of ATP is required to synthesize one glucose molecule during photosynthesis reactions.
• When an organism consumes sugars, glucose molecules eventually break down through a complex series of chemical reactions inside the cell to harvest the energy stored inside.
• The harvested energy makes high-energy ATP molecules, which perform work, powering many chemical reactions in the cell.
• The amount of energy needed to make one glucose molecule from six carbon dioxide molecules is 18 ATP molecules and 12 NADPH molecules, or a total of 54 molecule equivalents.

Metabolic Pathways

• A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product or products.
• Anabolic pathways synthesize sugar from smaller molecules and require energy.
• Catabolic pathways break sugar down into smaller molecules and produce energy.
• Building (anabolism) and degradation (catabolism) comprise metabolism.

Evolution of Metabolic Pathways

• Metabolic complexity varies from organism to organism.
• Photosynthesis is the primary pathway in which photosynthetic organisms harvest the sun’s energy and convert it into carbohydrates, with oxygen as a by-product.
• During cellular respiration, oxygen aids in the catabolic breakdown of carbon compounds, like carbohydrates, producing CO2 and ATP.
• Some eukaryotes perform catabolic processes without oxygen (fermentation); that is, they perform or use anaerobic metabolism.
• Organisms probably evolved anaerobic metabolism to survive when the atmosphere lacked oxygen.
• All branches of life share some of the same metabolic pathways, suggesting that all organisms evolved from the same ancient common ancestor.
• Over time, the pathways diverged, adding specialized enzymes to allow organisms to better adapt to their environment.
• All organisms must harvest energy from their environment and convert it to ATP to carry out cellular functions.

Anabolic and Catabolic Pathways

• Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones.
• Examples include synthesizing sugar from CO2, large proteins from amino acid building blocks, and new DNA strands from nucleic acid building blocks.
• These biosynthetic processes are critical to the cell's life, take place constantly, and demand energy that ATP and other high-energy molecules like NADH and NADPH provide.
• The breakdown of sugars illustrates how a single glucose molecule can store enough energy to make 36 to 38 molecules of ATP.
• Catabolic pathways involve degrading (or breaking down) complex molecules into simpler ones.
• Molecular energy stored in the bonds of complex molecules is released in catabolic pathways and harvested in such a way that it can produce ATP.
• Other energy-storing molecules, such as fats, also break down through similar catabolic reactions to release energy and make ATP.
• Metabolic pathway chemical reactions do not take place spontaneously; instead, a protein called an enzyme facilitates or catalyzes each reaction step.
• Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.