5.1 The Flow of Energy in Living Things Energy is defined as the ability to do work It exists in two states: kinetic and potential Fig. 5.1 Potential Energy = stored energy Kinetic Energy = energy of motion

5.1 The Flow of Energy in Living Things Energy exists in many forms Mechanical, sound, light, electric, heat, etc. All other forms can be converted into heat Thermodynamics is the study of energy Or heat changes

5.1 The Flow of Energy in Living Things Energy flows from the sun into the earth Photosynthetic organisms capture part of it They then use it to generate sugars These sugars have potential energy due to the arrangement of their atoms into bonds A chemical reaction is the making or breaking of chemical bonds

5.2 The Laws of Thermodynamics A set of universal laws governing all energy changes in the universe The First Law Energy cannot be created or destroyed However, it can change forms During each conversion, some energy is lost into the environment as heat energy The energy of random molecular motion

The Second Law of Thermodynamics Disorder in closed systems is continuously increasing Entropy is a measure of the disorder of a system Fig. 5.3 Simply put, the Second Law states: “entropy increases”

5.3 Chemical Reactions Chemical reactions begin with reactants (substrates) which are converted to products Exergonic reactions Products contain less energy than the reactants Endergonic reactions Products contain more energy than the reactants

Fig. 5.4 Energy of Activation Extra energy required to destabilize chemical bonds and so initiate a chemical reaction

However, they cannot make an endergonic reaction exergonic Fig. 5.4 Catalysts lower the activation energy of a reaction, and thus increase its rate However, they cannot make an endergonic reaction exergonic

5.4 How Enzymes Work Enzymes are proteins that serve as catalysts They speed up chemical reactions within cells Enzymes bind a specific molecule and stress bonds to make a particular reaction more likely Active site Site on enzyme surface where reactant fits Binding site Site on reactant where enzyme binds

Fig. 5.5 Enzyme shapes determines its activity Lysozyme Changes upon binding of the substrate The substrate is now bound more intimately

Fig. 5.6 How enzymes work

Fig. 5.7 The catalytic cycle of an enzyme 1 The substrate, sucrose, consists of glucose and fructose bonded together. Bond Enzyme Active site Glucose Fructose Products are released, and the enzyme is free to bind other substrates. 4 The substrate binds to the enzyme, forming an enzyme-substrate complex. 2 H2O The binding of the substrate and enzyme places stress on the glucose-fructose bond, and the bond breaks. 3

5.5 Factors Affecting Enzyme Activity Enzyme activity is affected by any change in condition that alters the enzyme’s 3-D shape The structural bonds of enzymes are sensitive to changes in temperature and pH Therefore, a temperature or pH beyond the optimal range will cause the enzyme to denature

Fig. 5.8 Enzymes are sensitive to their environment Digests proteins in the stomach Digests proteins in the intestine

5.6 How Cells Regulate Enzymes A cell can control the activity of an enzyme by altering its shape Allosteric enzymes have shapes that can be altered by the binding of signal molecules These molecules bind to the allosteric site Repressors bind and repress enzyme activity Activators bind and restore or increase enzyme activity

Fig. 5.9 How cells control enzymes

Enzyme inhibition occurs in two ways: Competitive inhibition - Inhibitor binds at the enzyme’s active site Noncompetitive inhibition - Inhibitor binds at the enzyme’s allosteric site Fig. 5.10

5.7 ATP: The Energy Currency of the Cell Fig. 5.11 Due to electrostatic repulsion between phosphate groups Adenosine triphosphate (ATP) is the main energy currency of the cell Each ATP molecule is composed of three parts:

5.7 ATP: The Energy Currency of the Cell Most energy exchanges in the cell involve cleavage of only the outermost bond in ATP Fig. 5.12 Used to power many of the cell’s activities Provided through photosynthesis or aerobic respiration

Oxidation reduction reactions always take place together Many reactions involve the passage of electrons from one atom/molecule to another Oxidation is the loss of electrons Reduction is the gain of electrons Oxidation reduction reactions always take place together Note that the transfer of electrons through these redox reactions also transfers energy

Loss of electron (oxidation) Fig. 5.13 Redox reactions Loss of electron (oxidation) A* + A B o + B* _ Gain of electron (reduction) e– B A Low energy High energy