Chapter 7: Energy and Enzymes

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Chapter 7 Energy and Enzymes

• Living organisms must obey the universal laws of
  energy conversion and chemical change
  (thermodynamics).
• The sun is the primary source of energy for
  living organisms.
• Metabolism consists of the chemical reactions
  that produce and break down macromolecules such
  as sugars and proteins.
• Enzymes control the speed of chemical reactions
  in cells. Metabolic pathways are sequences of
  enzyme-controlled chemical reactions.
• Biosynthetic metabolic reactions create complex
  molecules from smaller compounds.
• Catabolic metabolic reactions break down complex
  molecules to produce energy.

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The laws of thermodynamics apply to living systems

• The first law of thermodynamics states that the
  total energy of a system always remains constant
  (Figure 7.1a).
• The second law of thermodynamics states that
  systems, such as a cell or even the whole
  universe, tend to become more disorderly (Figure
  7.1b).
• Because living organisms are highly ordered, they
  must transfer disorder by releasing heat into
  the environment to counteract the second law of
  thermodynamics (Figure 7.1c).

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The flow of energy and the cycling of carbon
connect living things with the environment

• Cells must collect energy from the environment.
• Photosynthetic organisms (producers) use light to
  make sugars from carbon dioxide and water.
• Nonphotosynthetic organisms (consumers) use
  energy in the form of chemical bonds consumed as
  food (for example, sugar and fat).
• The sun is the primary energy source for living
  organisms.
• Carbon atoms are cycled from carbon dioxide to
  sugars made by photosynthetic organisms and back
  to carbon dioxide by respiring produces and
  consumers (see Figure 7.2).

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Using Energy from the Controlled Burning
(Oxidation) of Food

• Living systems obtain energy from food by burning
  organic molecules such as sugars to form carbon
  dioxide and water.
• During the process of food burning, cells store
  the energy released in chemical bonds.

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Capturing energy from foods requires the transfer
of electrons

• Oxidation is the loss of electrons from one
  molecule or atom to another.
• Reduction is the gain of electrons by one
  molecule or atom from another.
• The combustion of an organic compound such as
  methane is an oxidation reaction (see Figure
  7.3a).
• Biological oxidation takes place in a series of
  steps, not all at once like combustion reactions.
• Each intermediate compound in a biological
  oxidation is slightly more oxidized than its
  precursor (see Figure 7.3b).

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• During a biological oxidation, energy is stored
  in the form of the chemical bonds in adenosine
  triphosphate (ATP).
• The energy produced when ATP is broken down to
  ADP and phosphate is used to power many processes
  in a cell.
• Almost every chemical reaction in the cell either
  consumes or produces ATP at some point.
• The catabolic reactions in the cell are tightly
  coupled to the biosynthetic reactions, forming
  the two sides of metabolism releasing energy by
  breaking things down and using energy to build
  things up.

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Chemical reactions are governed by simple energy
laws

• A generic chemical reaction can be represented as
  A B ? C D
• A and B are starting materials (reactants),
  whereas C and D are the products of the reaction.
• All chemical reactions tend to proceed in the
  direction that will result in products with
  greater stability and a lower energy state.
• Reactants need to be jump-started with an
  energy input before a chemical reaction can take
  place.
• The jump start is called the activation energy
  of the reaction (see Figure 7.4).
• Some chemical reactions in cells can acquire the
  activation energy they need from random
  collisions between molecules floating in the
  cytosol.
• Most reactions require enzymes (specialized
  proteins) to proceed.

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How Enzymes Speed Up Chemical Reactions

• Enzymes speed up chemical reactions and are
  specific for a single type of reaction.
• Enzymes bind to substrates (reactants) and lower
  their activation energy.
• Enzymes affect the rate at which reactions occur
  but remain unchanged by the reactions therefore,
  they can be referred to as catalysts.
• Carbonic anhydrase catalyzes the reactionH2O
  CO2 ? HCO3 H
• Without carbonic anhydrase, the reaction would be
  10 million times slower.

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The shape of an enzyme directly determines its
activity

• Enzymes have active sites that fit only
  substrates with the correct three-dimensional
  shape (Figure 7.5a).
• Carbonic anhydrase binds water and carbon dioxide
  in its active site (Figure 7.5b).
• The action of carbonic anhydrase demonstrates how
  specific binding of two substrates by an enzyme
  can push them together so that a chemical
  reaction takes place between them.

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Enzyme chain reactions have energetic advantages

• A chemical pathway is a series of reactions
  catalyzed by a group of enzymes to form a
  product.
• Multiple-step chemical pathways have advantages
  over single-step pathways.
• In a chemical pathway, the product of one enzyme
  is the substrate of the next enzyme in the
  pathway.
• To enhance the efficiency of chemical pathways,
  enzymes involved in common reactions are located
  in close physical proximity to one another (to
  get products of one enzyme to the active site of
  the next enzyme more quickly).
• At the cellular level, enzymes involved in the
  same pathway are located in the same organelle
  (Figure 7.6).
• On the molecular level, several enzymes can be
  physically connected in a single giant
  multienzyme complex.

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Enzymes and Energy in Use The Building of DNA

• Enzyme-catalyzed metabolic pathways are involved
  in the synthesis and breakdown of most complex
  molecules in the cell.
• DNA is replicated in a process that involves ATP
  and several enzymes (Figure 7.7).
• Other complex molecules such as fatty acids are
  produced by using ATP and specific enzymes.

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