Every living cell runs on a constant flow of energy. Unit 3 traces that flow from the molecules that speed up chemical change (enzymes) to the universal energy currency (ATP) and finally to the two great pathways that move energy through the biosphere: photosynthesis, which stores light energy in the bonds of organic molecules, and cellular respiration, which releases that stored energy to power life. A recurring theme is that these processes are governed by the laws of thermodynamics and are exquisitely sensitive to environmental conditions such as temperature and pH.
Enzymes are biological catalysts, usually proteins, that speed up reactions without being consumed. Each enzyme has an active site whose three-dimensional shape is complementary to a specific substrate. Specificity comes from this shape and chemistry match. The favored model is induced fit: substrate binding causes the active site to change shape slightly, gripping the substrate and straining its bonds. Because enzyme structure determines function, anything that disrupts the active site's shape reduces or eliminates activity.
Even spontaneous (energetically favorable) reactions need an initial input of energy, the activation energy, to break existing bonds and reach the transition state. Enzymes work by lowering activation energy, allowing far more substrate molecules to react at biological temperatures. Enzymes do not change whether a reaction is exergonic or endergonic and do not alter the overall free-energy change (ΔG); they only change the rate by providing a lower-energy path.
Enzyme activity depends on conditions. Each enzyme has an optimal temperature and optimal pH; activity rises toward the optimum and falls beyond it. Excessive heat or extreme pH disrupts the bonds maintaining protein shape, causing denaturation, a loss of the functional 3-D structure. Increasing substrate concentration raises rate until enzymes are saturated, after which rate plateaus. Adding more enzyme (with ample substrate) also raises rate.
Competitive inhibitors resemble the substrate and bind the active site, blocking substrate entry; their effect can be overcome by adding more substrate. Noncompetitive (allosteric) inhibitors bind a site other than the active site, changing the enzyme's shape so the active site no longer functions well; adding substrate does not reverse this. Cells also use feedback inhibition, in which a pathway's end product binds an early enzyme to shut the pathway down once enough product accumulates.
ATP (adenosine triphosphate) is the cell's energy currency. Hydrolysis of its terminal phosphate bond releases energy and yields ADP + inorganic phosphate. This exergonic release is used to drive endergonic reactions through energy coupling, often by transferring the phosphate to another molecule (phosphorylation). ATP is continually regenerated from ADP using energy from cellular respiration, making it a renewable, short-term energy carrier rather than long-term storage.
Photosynthesis converts light energy into chemical energy in chloroplasts. In the light-dependent reactions on the thylakoid membranes, chlorophyll in photosystems II and I absorbs light, exciting electrons. Water is split (photolysis), releasing O₂ and supplying replacement electrons and protons. Electrons travel down an electron transport chain, pumping H⁺ into the thylakoid lumen; this proton gradient powers ATP synthase to make ATP (photophosphorylation), and NADP⁺ is reduced to NADPH.
The Calvin cycle (light-independent reactions) occurs in the stroma and uses the ATP and NADPH from the light reactions to build sugar. The enzyme rubisco fixes CO₂ onto RuBP (carbon fixation). The resulting molecules are reduced using NADPH and energized by ATP to form G3P, a three-carbon sugar; some G3P leaves to make glucose while the rest regenerates RuBP. Though called light-independent, the cycle depends on products supplied by the light reactions.
Glycolysis, in the cytoplasm, splits glucose into two pyruvate molecules, producing a net 2 ATP (by substrate-level phosphorylation) and 2 NADH; it requires no oxygen. With oxygen present, pyruvate is oxidized to acetyl-CoA (releasing CO₂ and making NADH). The Krebs (citric acid) cycle in the mitochondrial matrix fully oxidizes acetyl-CoA, releasing CO₂ and harvesting energy as NADH, FADH₂, and a small amount of ATP.
NADH and FADH₂ deliver high-energy electrons to the electron transport chain on the inner mitochondrial membrane. As electrons pass down the chain, energy is used to pump H⁺ into the intermembrane space, building an electrochemical gradient. In chemiosmosis, protons flow back through ATP synthase, driving oxidative phosphorylation to produce most of the cell's ATP. Oxygen is the final electron acceptor, combining with electrons and H⁺ to form water; without it, the chain backs up and stops.
When oxygen is absent, the electron transport chain cannot run, so NADH cannot be reoxidized there. Fermentation regenerates NAD⁺ so glycolysis can continue producing a small amount of ATP. In lactic acid fermentation (some animal and muscle cells), pyruvate is reduced to lactate. In alcohol fermentation (yeast, some microbes), pyruvate is converted to ethanol and CO₂. Fermentation yields far less ATP than aerobic respiration but allows survival without oxygen.
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