Loading
Unit 3
12–16% of examLiving things need a steady supply of energy just to stay organized and alive. In this unit you'll see how enzymes speed up reactions, why heat, pH and inhibitors change how well they work, and how photosynthesis captures light energy while cellular respiration releases it as ATP. Together, these two processes link every cell to the flow of energy and carbon through ecosystems.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Enzymes are proteins that act as biological catalysts, speeding up reactions by lowering the activation energy without being used up. A substrate can bind only if its shape and charge fit the enzyme's active site, forming an enzyme-substrate complex.
Key terms
Warming things up speeds a reaction because enzymes and substrates collide more often, but past the optimum, heat or the wrong pH disrupts hydrogen bonds and can denature the enzyme, sometimes reversibly; the amounts of substrate and product also affect the rate. Competitive inhibitors block the active site, while noncompetitive inhibitors bind somewhere else (an allosteric site) and change the enzyme's activity.
Key terms
Cells need a constant input of energy to stay organized, and they pair energy-releasing reactions with energy-requiring ones, often using ATP. Metabolism runs as step-by-step pathways where each product feeds the next step, and core pathways like glycolysis are shared by all three domains of life, which points to common ancestry.
Key terms
In the chloroplast's thylakoid membranes, the light reactions use light energy to make ATP and NADPH, and water is split to replace the electrons that light knocks loose, which releases O₂. The Calvin cycle in the stroma then uses that ATP and NADPH to build sugars from CO₂. Photosynthesis first evolved in prokaryotes, and photosynthetic cyanobacteria are credited with giving Earth an oxygen-rich atmosphere.
Key terms
Glycolysis splits glucose into pyruvate in the cytosol, the Krebs cycle in the mitochondrial matrix releases CO₂ and loads electrons onto NADH and FADH₂, and the electron transport chain uses those electrons to pump protons so ATP synthase can make most of the cell's ATP, with oxygen as the final electron acceptor. Without oxygen, fermentation keeps glycolysis going and makes products such as lactic acid or alcohol.
Key terms