Chapter 6.5

Metabolism- Organized, Enzyme Mediated Reactions

AP BiologyFall 2010

Bell Ringer

Objectives

• Explain the different types of metabolic pathways

• Know when chemical equilibrium is reached• Understand the principals of redox reactions

Types of Metabolic Pathways

• Metabolic pathways: enzyme-mediated sequences of reactions in cells

• Some pathways are linear– Reactant to product directly

• Others have cyclic components– Last step regenerates the reactant molecule of first

step • Others have branches– Reactants or intermediates are channeled into 2 or

more different sequences

Types of Metabolic Pathways

• Biosynthetic (anabolic): require net input of energy to produce large molecules from small molecules – Photosynthesis

• Degradative (catabolic): exergonic, end with a net release of usable energy, unstable molecules are broken down to smaller more stable products – Aerobic respiration, ATP as usable energy

Direction of Metabolic Reactions

• Metabolic reactions do not always run from reactants to products

• Chemical equilibrium: when reaction rate is about the same in either direction– Reversible reactions tend to run spontaneously

toward this

Redox Reactions in the Main Pathways

• Oxidation-reduction reactions (redox): are electron transfers between molecules– Oxidized: molecule that loses an electron (donor)– Reduced: molecule that gains an electron

(receptor)– LeO the lion says GeR

Redox Reactions in the Main Pathways

• Electron transfer chains: membrane bound arrays of enzymes and other molecules that accept and give up electrons in sequence– Control the release of energy

• Electrons are at a higher energy level when they enter the chain than when they leave

• Think of electrons as descending a staircase and losing some energy at each step

Review

1. When does a reaction reach chemical equilibrium?

2. A molecule that gains an electron is ____________ and a molecule that loses an electron is ____________.

Answers

1. A reaction reaches chemical equilibrium when reaction rate is about the same in either direction.

2. A molecule that gains an electron is reduced and a molecule that loses an electron is oxidized.