Chap 8 An Introduction to Metabolism

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The effect of enzymes on reaction rate. Without affecting the free–energy change (ΔG) for a reaction, an enzyme speeds the reaction by reducing its activation energy (EA).

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Hydrolysis of ATP drives endergonic reactions that consume energy (anabolism). Energy released by exergonic reactions (catabolism) is used to phosphorylate ADP, regenerating ATP in a process called cellular (aerobic) respiration.

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The hydrolysis of ATP. The hydrolysis of ATP yields inorganic phosphate (Pi) and ADP, with the release of energy.

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How ATP drives cellular work. Phosphate group transfer is the mechanism responsible for most types of cellular work.

  1. ATP drives active transport by phosphorylating membrane proteins.
  2. ATP drives mechanical work by phosphorylating motor proteins, such as the ones that move organelles along cytoskeletal “tracks” in the cell.
The phosphorylated molecules lose the phosphate groups as work is performed, leaving ADP and inorganic phosphate (Pi) as products.

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The active site and catalytic cycle of an enzyme. An enzyme can convert reactant (substrate) molecules to product molecules. In an enzymatic reaction, the substrates bind to the active site of the enzyme. The active site can lower an EA barrier by orienting substrates correctly and providing a favorable microenvironment.

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First law of thermodynamics: Energy can be transferred or transformed but neither created nor destroyed. For example, the chemical (potential) energy in food will be converted to the kinetic energy of the cheetah’s movement.

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Second law of thermodynamics: Every energy transfer or transformation increases the disorder (entropy) of the universe. For example, disorder is added to the cheetah’s surroundings in the form of heat and the small molecules that are the by-products of metabolism.

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First law of thermodynamics: energy can be converted from one form to another.
Here potential energy is converted to kinetic energy.

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A concentration gradient across a membrane represents an ordered state; diffusion of solute molecules down the gradient increases the disorder (entropy) of a system.

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Induced fit between an enzyme and its substrate.

  1. The active site of this enzyme (hexokinase) forms a groove on its surface.
  2. When the substrate (glucose) enters the active site, it induces a change in the protein's shape.
This change allows more weak bonds to form, causing the active site to embrace the substrate and hold it in place.

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Environmental factors affecting enzyme activity. Each enzyme has an optimal temperature and pH that favor the most active conformation of the protein molecule.

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Energy profile of an exergonic reaction. Thermodynamically, this is an exergonic reaction, with a negative ΔG, and the reaction occurs spontaneously. However, the activation energy (EA) provides a barrier that determines the rate of the reaction.

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Catabolic reactions are exergonic: they release energy from the break down of complex molecules into simpler compounds.
Anabolic reactions are endergonic: they consume energy to build large molecules from simpler ones.

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Order as a characteristic of life.
Order is evident in this cross section of root tissue from a buttercup plant. As open systems, organisms can increase their order as long as the order of their surroundings decreases.

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A metabolic pathway begins with a specific molecule and ends with a product.
Each step is catalyzed by a specific enzyme.

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Example of an enzyme-catalyzed reaction. The enzyme sucrase catalyzes the hydrolysis of sucrose. The starting molecules in a chemical reaction are called reactants or substrates and the molecules produced by the reaction are called products.