5 ENERGY AND LIFE
CHAPTER OUTLINE
Cells and Energy (p. 106)
5.1 The Flow of Energy in Living Things (p. 106; Fig. 5.1)
A. Most, if not all, of a cell's activities require energy.
B. Energy is defined as the capacity to do work.
1. Kinetic energy is the energy of motion.
2. Objects that are not actively moving but have the capacity to do so possess potential energy.
C. Energy can exist in a variety of forms (mechanical, heat, sound, electric current, light, or
radioactive radiation).
D. The study of energy is called thermodynamics.
E. Energy from the sun is captured by some organisms and used to make molecules.
1. These molecules then contain potential energy due to the arrangement of their atoms.
2. The potential energy in molecules is a form of chemical energy that can be used to do work in
cells.
3. Chemical reactions involve making and breaking bonds between atoms.
5.2 The Laws of Thermodynamics (p. 107; Figs. 5.2, 5.3)
A. The First Law of Thermodynamics
1. The First Law of Thermodynamics states that energy cannot be created or destroyed; it can
only undergo conversion from one form to another.
B. The Second Law of Thermodynamics
1. The Second Law of Thermodynamics states that disorder (entropy) in the universe is
increasing.
2. Life converts energy from the sun to other forms of energy that drive life processes; the
energy is never lost, but as it is used, more and more of it is converted to heat.
C. Entropy
1. Entropy increases and the universe is becoming increasingly disordered.
5.3 Chemical Reactions (p. 108; Fig. 5.4)
A. The molecules that react or combine are called reactants, or substrates, and the result of the
reaction is the products.
B. Reactions that tend to occur on their own are exergonic and release energy.
C. Reactions that require an input of energy are endergonic.
D. Activation Energy
1. Activation energy is the energy needed to destabilize bonds and cause a reaction to start.
E. Catalysis
1. Catalysis is the process of lowering activation energy, which helps both endergonic and
exergonic reactions proceed more rapidly.
Enzymes (p. 109)
5.4 How Enzymes Work (p. 109; Figs. 5.5, 5.6, 5.7)
A. Molecules called enzymes are the catalysts used by cells to touch off particular chemical reactions.
1. Cells use enzymes to catalyze chemical reactions.
2. Enzymes are specific for a particular reactant.
3. The place on the reactant that binds to the enzyme is the binding site.
4. The place on the enzyme where the reactant is destabilized is called the active site.
B. Biochemical Pathways
1. Chemical reactions in cells are often arranged in a sequence, so that the product of one
reaction becomes the reactant for the next.
2. Each reaction is catalyzed by its own specific enzyme.
3.
Often, the enzymes required for each step are located close together in sequence along a
membrane.
4. Biochemical pathways are key components of metabolism.
C. Factors Affecting Enzyme Activity
1. Temperature and pH both affect enzyme activity because they influence protein shape.
2. Each enzyme has a temperature range and a pH range for optimal activity.
3. High temperature and low pH can weaken protein structure and destroy an enzyme’s active
site in a process called denaturing.
5.5
How Cells Regulate Enzymes (p. 111; Fig. 5.8)
A. An enzyme is allosteric if its activity can be affected by signal molecules that bind to them,
changing their shape.
B. Enzymes may be inhibited by competitive and non-competitive inhibitors.
C. Many drugs and antibiotics work by inhibiting enzymes.
How Cells Use Energy (p. 112)
5.6
ATP: The Energy Currency of the Cell (p. 112; Fig. 5.9; Table 5.1)
A. Cells need energy for moving organelles around inside the cell, for cell motility, and for
driving endergonic reactions.
B. The molecule in cells that supplies such energy is adenosine triphosphate (ATP).
C. Structure of the ATP Molecule
1. ATP molecules are made up of a sugar, the base adenine, and three phosphate groups that
have high-energy bonds between them.
2. Most energy exchanges in the cell involve removing or adding on the terminal phosphate
group of the ATP molecule.
D. Oxidation-Reduction
1. In many chemical reactions, electrons, along with their energy, are passed between atoms
or molecules.
2. When an atom or molecule loses electrons, it has been oxidized (oxidation has occurred)
and has lost energy.
3. When an atom or molecule gains electrons, it has been reduced (reduction has occurred)
and has gained energy.
4. Energy is transferred from one molecule to the next via oxidation-reduction (redox)
reactions.