Unit 3: Cellular Energetics

21 terms. Say what each one means, then open the card to check.

• Enzyme

  A biological catalyst, usually a protein, that speeds up one kind of reaction and comes out of it unchanged, ready to act again.

  Example. One catalase molecule in a liver cell can break down millions of hydrogen peroxide molecules into water and oxygen.

• Active site

  The pocket on an enzyme where the substrate binds and the reaction happens. Its shape and charges fit only certain substrates.

  Example. The active site of sucrase fits sucrose, so sucrase splits sucrose but leaves lactose untouched.

• Substrate

  The reactant that an enzyme binds and changes into product.

  Example. Lactose is the substrate of lactase, which splits it into glucose and galactose.

• Activation energy

  The energy reactants must take in before a reaction can start. Enzymes lower it, so more molecules react at body temperature.

  Example. Sucrose can sit in a sugar bowl for years without breaking down, but with sucrase in the gut it splits in seconds.

• Induced fit

  The small change in an enzyme's shape when the substrate binds, which wraps the active site more closely around the substrate.

  Example. When glucose enters the active site of hexokinase, the enzyme closes around it like a hand gripping a ball.

• Optimal temperature and pH

  The temperature and pH at which an enzyme works fastest. Away from them the rate drops, and far away the enzyme can denature.

  Example. Pepsin works best near pH 2 in the stomach, while trypsin works best near pH 8 in the small intestine.

• Competitive inhibitor

  A molecule shaped like the substrate that binds the active site and blocks the substrate. Raising the substrate level can overcome it.

  Example. Malonate fits the active site of succinate dehydrogenase and slows it, but adding much more succinate brings the rate back up.

• Noncompetitive inhibitor

  A molecule that binds an enzyme outside the active site and changes its shape, so it slows the enzyme even when substrate is plentiful.

  Example. In a fictional pathway, the end product binds the first enzyme at a second site and slows it, so the cell stops overmaking the product.

• Cofactor

  A non-protein helper, such as a metal ion or a vitamin-based coenzyme, that some enzymes need in order to work.

  Example. The enzymes that copy DNA need magnesium ions in their active sites to join nucleotides together.

• Exergonic reaction

  A reaction that releases free energy because its products hold less free energy than its reactants. Its free-energy change is negative.

  Example. Breaking glucose down to carbon dioxide and water in respiration releases a large amount of free energy.

• Endergonic reaction

  A reaction that needs free energy put in because its products hold more free energy than its reactants. Its free-energy change is positive.

  Example. Joining amino acids into a protein is endergonic, so the cell must supply energy to build each peptide bond.

• ATP hydrolysis

  Using water to remove the last phosphate of ATP, giving ADP and phosphate and releasing free energy the cell can use.

  Example. A muscle fiber uses ATP hydrolysis to power each stroke of the myosin heads that pull on actin.

• Energy coupling

  Linking an exergonic reaction to an endergonic one so the total free-energy change is negative and the pair can proceed.

  Example. The sodium-potassium pump uses the energy of ATP hydrolysis to move ions against their concentration gradients.

• Chlorophyll

  The main pigment of photosynthesis, held in the thylakoid membranes. It absorbs mostly red and blue light and reflects green light.

  Example. A maple leaf looks green in summer because its chlorophyll reflects the green part of sunlight.

• Light-dependent reactions

  The first stage of photosynthesis, in the thylakoid membranes. Light energy splits water, releases oxygen, and makes ATP and NADPH.

  Example. Bubbles rising from a pond plant on a sunny afternoon are oxygen released when its chloroplasts split water.

• Calvin cycle

  The second stage of photosynthesis, in the stroma. It uses ATP and NADPH to fix carbon from CO2 into the sugar G3P.

  Example. The carbon atoms in the starch of a corn kernel were once CO2 fixed by the Calvin cycle in the plant's leaves.

• Glycolysis

  The splitting of one glucose into two pyruvate in the cytosol, with a net gain of 2 ATP and 2 NADH. It needs no oxygen.

  Example. Mature human red blood cells have no mitochondria, so they make their ATP by glycolysis alone.

• Krebs cycle

  A cycle of reactions in the mitochondrial matrix that removes the carbons of food as CO2 and stores the energy mostly in NADH and FADH2.

  Example. Much of the carbon dioxide a jogger breathes out was released from food molecules in the Krebs cycle.

• Electron transport chain

  Proteins in the inner mitochondrial membrane that pass electrons from NADH and FADH2 to oxygen, using the energy to pump H+ across the membrane.

  Example. Cyanide blocks the last protein of the chain, so electrons can no longer reach oxygen and the chain stops.

• Chemiosmosis

  Making ATP with the energy of H+ flowing down its gradient through the enzyme ATP synthase.

  Example. In mitochondria, H+ that was pumped between the two membranes flows back into the matrix through ATP synthase, which makes ATP.

• Fermentation

  A way to turn NADH back into NAD+ without oxygen, by passing its electrons to pyruvate or a product of it, so glycolysis can continue.

  Example. Yeast in bread dough ferment sugar into ethanol and CO2, and the CO2 bubbles make the dough rise.