CHAPTER OUTLINE
6.1 Life and the Flow of Energy
Energy is the ability to do work or bring about a change. The majority of organisms get their
energy from organic nutrients produced by photosynthesizers.
Forms of Energy
Kinetic energy is the energy of motion. Potential energy is stored energy. Food is
specifically called chemical energy because it contains energy in the chemical bonds of
organic molecules.
Two Laws of Thermodynamics
The first law of thermodynamics, the law of conservation of energy, states that energy
cannot be created or destroyed, but it can be changed from one form to another. The
second law of thermodynamics states that energy cannot be changed from one form to
another without a loss of usable energy.
Cells and Entropy
The term entropy is used to indicate the relative amount of disorganization.
Because the processes that occur in cells are energy transformations, the second law
of thermodynamics means that every process that occurs in cells always does so
in a way that increases the total entropy of the universe.
6.2 Energy Transformations and Metabolism
Cellular metabolism is the sum of all the chemical reactions that occur in a cell. The term
catabolism is used to refer to the breaking down of molecules, and the term anabolism is used to
refer to the building up (synthesis) of molecules. Reactants are substances that participate in a
reaction, while products are substances that form as a result of a reaction. Free energy is the
amount of energy available after a chemical reaction has occurred. Exergonic reactions are
spontaneous and release energy, while endergonic reactions require an input of energy to occur.
ATP: Energy for Cells
ATP (adenosine triphosphate) is the common energy currency of cells. The amount of
ATP on hand at any one moment is minimal because ATP is constantly being generated
from ADP (adenosine diphosphate) and a molecule of inorganic phosphate.
Structure of ATP
ATP is a nucleotide composed of the nitrogen-containing base adenine and the 5carbon sugar ribose and three phosphate groups. ATP is called a “highenergy” compound because of the energy stored in the chemical bonds of the
phosphates.
Coupled Reactions
In coupled reactions, the energy released by an exergonic reaction is used to
drive an endergonic reaction.
Function of ATP
ATP can be used for:
Chemical Work
ATP supplies the energy to synthesize macromolecules.
Transport Work
ATP supplies the energy needed to pump substances across the plasma
membrane.
Mechanical Work
ATP supplies the energy needed to permit muscles to contract and cell
organelles to move.
6.3 Enzymes and Metabolic Pathways
Reactions in cells are usually part of a metabolic pathway, a series of linked reactions. An
enzyme is a protein that functions as a catalyst to speed a chemical reaction; the reactants are
called the substrates for that enzyme.
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Energy of Activation
Molecules frequently do not react with one another unless they are activated in some
way. This energy is called the energy of activation. Enzymes lower the amount of
energy required for activation.
How Enzymes Function
The active site of the enzyme undergoes a slight change in shape in order to
accommodate the substrate(s) and then complexes with it. This change in shape, called
the induced fit model, facilitates the reaction that now occurs. Every reaction in a cell
requires that its specific enzyme be present.
Factors Affecting Enzymatic Speed
Enzyme reactions proceed quite rapidly.
Substrate Concentration
Enzyme activity increases as substrate concentration increases until all the
enzyme’s active sites are filled almost continuously with substrate.
Temperature and pH
As the temperature rises, enzyme activity increases, until the temperature rises
beyond a certain point when the enzyme activity levels out and then declines
rapidly because the enzyme is denatured. Each enzyme also has a preferred pH
at which the rate of the reaction is highest.
Enzyme Activation
Enzymes can be present in the cell in an inactive form. Some enzymes are
covalently modified by the addition or removal of phosphate groups.
Enzyme Inhibition
Enzyme inhibition occurs when the substrate is unable to bind to the active site
of an enzyme. Most metabolic pathways are regulated by a complicated type of
feedback inhibition. Poisons are often enzyme inhibitors.
Enzyme Cofactors
Cofactors are inorganic ions that are required for an enzyme to function
properly. Organic, nonprotein helper molecules that are required for an enzyme
to function are called coenzymes. Vitamins are often components of coenzymes.
6.4 Oxidation-Reduction Reactions and Metabolism
Cellular respiration and photosynthesis are important metabolic pathways based on the use of
special enzymes to facilitate the movement of electrons, which plays a major role in the energyrelated reactions associated with these pathways.
Oxidation-Reduction Reactions
When discussing metabolic reactions, oxidation represents the loss of electrons and
reduction is the gain of electrons; reactions that involve the gain and loss of electrons are
collectively called oxidation-reduction reactions. Because they go hand-in-hand, the
entire reaction is called a redox reaction.
Chloroplasts and Photosynthesis
The chloroplasts in plants capture solar energy and use it to convert water and carbon
dioxide into a carbohydrate. The overall reaction for photosynthesis is: energy + 6 CO2 +
6 H2O goes to 1 C6H12O6 (glucose) + 6 O2. Hydrogen atoms are transferred from water to
carbon dioxide as glucose forms. Therefore, water has been oxidized and carbon dioxide
has been reduced. It takes energy to reduce CO2 to glucose, and this energy is supplied by
solar energy.
Mitochondria Cellular Respiration
Mitochondria, present in both plants and animals, oxidize carbohydrates and use the
released energy to build ATP molecules. The overall reaction for cellular respiration is
the opposite of the one used to represent photosynthesis. In this reaction, glucose has lost
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hydrogen atoms (been oxidized), and oxygen has gained hydrogen atoms (been reduced).
Cellular Respiration and Humans
Humans and other eukaryotic organisms are involved in the cycling of molecules
between chloroplasts and mitochondria. When we eat plants that photosynthesize and use
the oxygen they release, nutrients and oxygen enter our mitochondria, which produce
ATP. This allows a flow of energy from the sun through all living things.
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