Download CHAPTER 3: CELL STRUCTURE AND FUNCTION

CHAPTER 6: METABOLISM: ENERGY AND ENZYMES
LECTURE OUTLINE
6.1 Cells 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.
Since 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 Metabolic Reactions and Energy Transformations
Cellular metabolism is the sum of all the chemical reactions that occur in a cell. 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.
In exergonic reactions, the change in free energy is negative and energy is released. In
endergonic reactions, the change in free energy is positive and the products have more free
energy than the reactants.
ATP: Energy for Cells
ATP (adenosine triphosphate) is the common energy currency of cells.
Structure of ATP
ATP is a nucleotide containing 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 is 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 necessary for muscles to contract, cell
organelles to move, and so on.
6.3 Metabolic Pathways and Enzymes
Enzymatic reactions are usually part of a metabolic pathway, a series of linked reactions. An
enzyme is a protein (or sometimes an RNA molecule) that functions to speed a chemical reaction.
It is a catalyst.
Energy of Activation
Molecules do not react with one another unless energy is added to cause the molecules to
react with one another. This energy is called the energy of activation. Enzymes lower
the amount of energy required for activation.
How Enzymes Function
The substrate binds with the active site of the enzyme. The active site undergoes a slight
change in shape in order to accommodate the substrate. This change in shape 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 is denatured and activity ceases. 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
activated by covalent modifications such as the addition or removal of
phosphate groups.
Enzyme Inhibition
Enzyme inhibition occurs when the substrate is unable to bind to the active site of
the 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 molecules that are required for an enzyme to function are
called coenzymes. Vitamins are often components of coenzymes.
6.4 Oxidation-Reduction and the Flow of Energy
Oxidation is the loss of electrons, and reduction is the gain of electrons.
Photosynthesis
The overall reaction for photosynthesis is: energy + 6 carbon dioxide + 6 water goes to 1
glucose + 6 oxygen. Hydrogen atoms are transferred from water to carbon dioxide, and
glucose is formed. Therefore, water has been oxidized, and carbon dioxide has been
reduced. It takes energy to form glucose, and this energy is supplied by solar energy.
Cellular Respiration
The overall reaction for cellular respiration is the opposite of the one used to represent
photosynthesis. In this reaction, glucose has lost hydrogen atoms (been oxidized), and
oxygen has gained hydrogen atoms (been reduced).
Organelles and the Flow of Energy
It is the cycling of molecules between chloroplasts and mitochondria that allows a flow of
energy from the sun through all living things.