Energy and Enzymes Almost all energy for life is derived from the sun. Life requires energy. A “factoid” - The sun’s energy that strikes Earth each day is equivalent to one million Hiroshima-sized atomic bombs. Photosynthesis harnesses about 1% of that energy – 10,000 “atomic bomb equivalents” per day.

Chemistry and Life One unromantic yet productive way of viewing life is to see it as a set of coordinated chemical reactions. This leads to an obvious question – What determines what chemical reactions are possible?

Chemical Reactions Whether a chemical reaction will or won’t occur under particular conditions is determined by the laws of thermodynamics. If the overall amount of order is decreased by a reaction, the reaction is favored. Keeping it simple - If the overall amount of order is increased by a reaction, the reaction is not favored.

The Direction of Spontaneous Reactions (and what it takes to go the other way) Going this way is possible, but it requires the input of energy.

Life Requires Lots of Unfavorable Reactions – How Is This Possible? By coupling favorable, energy- releasing reactions to unfavorable, energy- requiring reactions. favorable reactions unfavorable reactions

The “Nature of Life” – Coupling Favorable, Energy-Yielding Reactions to Unfavorable, Energy-Requiring Reactions

Even When Cells Build Complex Molecules and Structures, the Second Law of Thermodynamics (Entropy or the “kids bedroom principle”) Isn’t Violated

ATP - Life’s Energy Currency Energy is released when ATP is hydrolyzed (broken down) to ADP. ATP is restored from ADP and an input of energy. ATP’s energy is used to drive endergonic (energy-requiring) reactions.

The Way ATP Often Works

Enzymes Speed Biochemical Reactions Enzymes are biological catalysts – substances that speed a reaction without being altered in the reaction. Most enzymes are proteins, some are RNA. Enzymes are essential for life. Model of the surface of an enzyme.

Enzymes Lower a Reaction’s Activation Energy

Enzyme Action

The Fit Between Enzyme and Substrate is Critical and Precise Hexokinase, an enzyme (blue), binding its substrate, glucose (yellow).

The active site of an enzyme is where substrate is bound. Many Enzymes Work by Altering the Shape of Their Substrates

The synthesis of biological molecules often requires many enzyme- catalyzed steps. The entire set of steps is a metabolic pathway. Metabolic Pathways

Enzyme Activity is Often Regulated Feedback inhibition - a common form of enzyme regulation in which the product inhibits the enzyme.