6-1 Chapter 6 Metabolism: Energy and Enzymes

6-2 Cells and the Flow of Energy Energy is the ability to do work. Living things need to acquire energy; this is a characteristic of life. Cells use acquired energy to: Maintain their organization Carry out reactions that allow cells to develop, grow, and reproduce

6-3 Two Basic* Forms of Energy Kinetic energy is the energy of motion. Potential energy is stored energy (eventually transferred to kinetic energy). Potential energy in foods is chemical energy. Organisms: convert chemical  mechanical energy for motion. * Other forms include solar, heat, and electrical energy

6-4 The flow of energy in ecosystems occurs in one direction; energy does not cycle. The two laws of thermodynamics explain this phenomenon.

6-5 Two Laws of Thermodynamics First Law: Energy cannot be created or destroyed, but it can be changed from one form to another. Second Law: Energy cannot be changed from one form to another without loss of usable energy.

6-6 First law Second law When energy transformations occur, energy is neither created nor destroyed (1 st Law) but there is always loss of usable energy, usually as heat (2 nd Law). Flow of energy

6-7 Due to the two laws of thermodynamics, living things depend on an outside source of energy. Energy exists in several different forms and the ultimate source of energy for ecosystems is the sun.

6-8 Cells and Entropy Entropy is a measure of the relative state of disorganization of a system. Low entropy = highly organized. High entropy = highly disorganized. Systems (such as cells) tend toward disorganization and therefore cells need a constant supply of energy to maintain their internal organization. Energy is used to “fight entropy”.

6-9 Cells and entropy Low entropy High entropy Fig. 6.2 Breakdown of glucose results in a loss of potential energy and an increase in entropy

6-10 Metabolic Reactions and Energy Transformations Metabolism is the sum of all the chemical reactions that occur in a cell. A reaction will occur spontaneously if it increases entropy. Biologists use the term “free energy” instead of entropy for cells.

6-11 A + B C + D ReactantsProducts Chemical Reactions Reactants are substances that participate in a reaction; products are substances that form as a result of a reaction.

6-12 Free energy, G, is the amount of energy available to do work after a reaction has occurred. ΔG (change in free energy) is calculated by subtracting the free energy of reactants from that of products. A negative ΔG means the products have less free energy than the reactants, and the reaction will occur spontaneously.

6-13 Respiration Photosynthesis Positive  G Negative  G Endergonic reaction - requires energy Exergonic reaction - releases energy

6-14 ATP: Energy for Cells ATP (adenosine triphosphate) is the energy currency of cells. ATP is constantly regenerated from ADP (adenosine diphosphate) after energy is expended by the cell.

6-15 Use of ATP by the cell has advantages: 1) It can be used in many types of reactions. 2) When ATP → ADP + P, energy released is sufficient for cellular needs and little energy is wasted. 3) ATP breakdown (i.e., exergonic rxn) is coupled to endergonic reactions in such a way that it minimizes energy loss.

6-16 The ATP cycle Fig. 6.3 ATP is a nucleotide made of adenine and ribose and three phosphate groups. ATP is called a “high- energy” compound because a phosphate group is easily removed.

6-17 Coupled Reactions In coupled reactions, energy released by an exergonic reaction drives an endergonic reaction. Exergonic Endergonic Page 105

6-18 Coupled reactions

6-19 Function of ATP Cells make use of ATP for: Chemical work – ATP supplies energy to synthesize macromolecules, and therefore the organism (ex: amino acids into proteins) Transport work – ATP supplies energy needed to pump substances across the plasma membrane (ex: active transport) Mechanical work – ATP supplies energy for cellular movements (ex: muscle contraction)

6-20 Metabolic Pathways and Enzymes Enzyme – protein molecule that acts as an organic catalyst Catalyst – substance that increases the rate of a chemical rxn, but is unchanged by the rxn.

6-21 An enzyme brings together particular molecules and causes them to react to produce a product. Substrates - The reactants in an enzymatic reaction. Enzyme + SubstrateEnzyme-Substrate Complex Enzyme + Product

6-22 Cellular reactions are usually part of a metabolic pathway: E 1 E 2 E 3 E 4 E 5 E 6 A → B → C → D → E → F → G A-F are reactants or substrates B-G are the products E 1 -E 6 are enzymes.

6-23 Energy of Activation Energy of activation (E a ) - Energy that must be added to cause molecules to react with one another. An enzyme lowers the energy of activation. The addition of an enzyme does not change the free energy of the reaction.

6-24 With EnzymeWithout Enzyme Energy of activation (E a ) *Notice:  G does not change – just energy of activation

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6-26 Enzyme-Substrate Complexes Every reaction in a cell requires a specific enzyme. Enzymes are named for their substrates: Substrate Enzyme Lipid Lipase Urea Urease Maltose Maltase Ribonucleic acid Ribonuclease

6-27 -Only one small part of an enzyme, called the active site, complexes with the substrate(s). -The enzyme is not changed by the reaction, and it is free to act again. Fig 6.6

6-28 Enzymes can either change, split (degradation), or form (synthesis) molecules.

6-29 Induced fit model The active site may undergo a slight change in shape, called induced fit, in order to accommodate the substrate(s). Fig 6.7

6-30 Factors Affecting Enzymatic Speed Substrate Concentration – Speed increases with substrate concentration Temperature – Above or below effective range, enzymes become denatured pH – Each enzyme has an optimal pH

6-31 Rate of an enzymatic reaction as a function of temperature and pH Fig 6.8

6-32 Cells regulate enzyme quantity and activity by either turning genes on or off. Another way to control enzyme activity is to activate or deactivate the enzyme. -Phosphorylation is one way to activate an enzyme.

6-33 Enzyme Inhibition Enzyme inhibition occurs when an active enzyme is prevented from combining with its substrate. When the product of a metabolic pathway is in abundance, it binds competitively with the enzyme’s active site, a simple form of feedback inhibition.

6-34 Feedback inhibition Shape change Fig 6.9

6-35 Enzyme Cofactors Presence of enzyme cofactors may be necessary for some enzymes to carry out their functions. Examples: -Inorganic metal ions, such as copper, zinc, or iron -Some organic molecules, termed coenzymes, such as vitamins.

6-36 Oxidation-Reduction and the Flow of Energy Oxidation is the loss of electrons and reduction is the gain of electrons. In covalent rxn’s, oxidation also refers to the loss of hydrogen atoms, and reduction refers to the gain of hydrogen atoms. These types of oxidation-reduction (redox) reactions are exemplified by the overall reactions of photosynthesis and cellular respiration.

6-37 Photosynthesis The overall reaction for photosynthesis can be written: 6CO 2 + 6H 2 O + energy → C 6 H 12 O 6 + 6O 2 Hydrogen is removed from water Energy comes from the sun. Glucose is formed OxidizedReduced

6-38 Cellular Respiration The overall equation for cellular respiration is opposite that of photosynthesis: C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + Energy An energy source is now available for endergonic cellular reactions OxidizedReduced In the form of ATP

6-39 Relationship of chloroplasts to mitochondria