Essentials of Biology
Sylvia S. Mader
Chapter 5
Lecture Outline
Prepared by: Dr. Stephen Ebbs
Southern Illinois University Carbondale
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5.1 What Is Energy
• Energy is the capacity to do work.
• There are two basic forms of energy.
– Potential energy is stored energy.
– Kinetic energy is energy of motion.
• Energy is constantly being exchanged
between these two forms.
5.1 What Is Energy (cont.)
Measuring Energy
• Energy can be measured in different units.
– Most forms of energy are measured in joules.
– Food energy is measured in calories.
• A calorie is the amount of heat required to raise
the temperature of 1 gram of water 1 degree
Celsius.
• Food labels list the caloric value of food in
kilocalories (1,000 calories).
Two Energy Laws
• Energy laws describe the principles of
energy flow and energy conversion.
• The law of conservation of energy says
that energy cannot be created or
destroyed, but can change from one form
to another.
Two Energy Laws (cont.)
• The second energy law says that energy
cannot be changed from one form to
another without a loss of usable energy.
• Most of the energy lost during energy
interconversions is lost as heat.
Entropy
• Another interpretation of the second energy law
says that every energy transformation leads to
more disorder.
• The degree of disorder or disorganization is
referred to as entropy.
• All energy transformations, including those in
cells, increase the entropy of the universe.
Entropy (cont.)
Entropy (cont.)
5.2 ATP: Energy for Cells
• ATP (adenosine triphosphate) is the
energy currency of cells.
• ATP is used to drive nearly all cellular
activities.
Structure of ATP
• ATP is a nucleotide, similar to the monomers of
DNA and RNA.
• The ATP molecule contains three parts.
– The sugar ribose
– The nitrogenous base adenine
– Three phosphate groups
• The energy of ATP is stored in the phosphate
groups.
Structure of ATP (cont.)
Use and Production of ATP
• The continual breakdown and
regeneration of ATP is the ATP cycle.
• Because of its instability, ATP provides
only short term storage of energy.
• Carbohydrates and fats are more stable
energy storage molecules that, when
degraded, are used to generate ATP.
Use and Production of ATP
(cont.)
Use and Production of ATP
(cont.)
• The production of ATP has several
benefits for cells.
– ATP can be used for many different types of
chemical reactions.
– When ATP is split to release energy, the
amount of energy released is sufficient for
most reactions without being wasteful.
– The breakdown of ATP can be coupled to
energy-requiring reactions.
Coupled Reactions
• Coupled reactions occur in the same place
at the same time.
• The energy-releasing reaction provides
the energy to drive the energy-requiring
reaction, as in the example below.
Coupled Reactions (cont.)
The Flow of Energy
• The cycling of molecules between the
chloroplasts and mitochondria is responsible for
the flow of energy through the biosphere.
• Chloroplasts use solar energy to convert water
and carbon dioxide to carbohydrates.
• Cellular respiration in the mitochondria breaks
down carbohydrates to yield energy (ATP),
releasing carbon dioxide and water.
The Flow of Energy (cont.)
The Flow of Energy (cont.)
• Humans also contribute to the flow of energy
from the sun and through the biosphere.
• Humans release carbon dioxide and water that
plants can use for photosynthesis.
• The carbohydrates and nutrients in foods are
broken down in the mitochondria of human cells
to produce ATP needed for cellular activities.
The Flow of Energy (cont.)
5.3 Metabolic Pathways and
Enzymes
• In living organisms, chemical reactions are often
linked together in series to form metabolic
pathways.
• The reactants, or substrates, are the chemicals
that enter the metabolic pathway.
• Enzymes are protein molecules that function as
organic catalysts to speed up a chemical
reaction.
5.3 Metabolic Pathways and
Enzymes (cont.)
• A metabolic pathway can be represented
by a simple diagram.
E1
E2
E3
E4
E5
E6
A  B  C  D  E  F  G
– The letters A-G indicate substrates.
– The letters E1-E6 represent enzymes.
– A is the substrate for E1, B is the substrate for
E2, and so on.
Energy of Activation
• Molecules often must be activated before a
chemical reaction can occur.
• The energy needed to cause a molecule to react
with another molecule is called the energy of
activation (Ea).
• Enzymes help catalyze reactions by lowering the
energy of activation for a reaction.
Energy of Activation (cont.)
An Enzyme’s Active Site
• The active site of an enzyme is the point where
a substrate binds like a key in a lock.
• According to the induced fit model, the active
site may undergo a slight change to
accommodate a substrate.
• Once bound to the active site, the enzyme
facilitates the conversion of substrate to product.
• The product is then released from the active site.
An Enzyme’s Active Site (cont.)
Enzyme Inhibition
• Enzyme inhibition occurs when an active
enzyme is prevented from binding to a
substrate by an inhibitor.
• Some inhibitors are poisonous to living
organisms.
– Cyanide is an inhibitor that blocks ATP
synthesis.
– Penicillin inhibits a specific bacterial enzyme.
Enzyme Inhibition
• Another type of inhibition, called feedback
inhibition, is used to control metabolic pathways.
• In feedback inhibition, production of sufficient
product shuts the synthesis pathway off.
• There are several other complex mechanisms
by which products provide feedback inhibition to
pathways.
Enzyme Inhibition (cont.)
5.4 Cell Transport
• The plasma membrane regulates the
transport of molecules into and out of the
cell.
• The plasma membrane is differentially
permeable, which means that some
substances move freely across the
membrane but others are restricted.
5.4 Cell Transport (cont.)
• Substances can enter cells in three ways.
– Passive transport
– Active transport
– Bulk transport
Passive Transport: No Energy
Required
• Simple diffusion occurs when the solute (a
substance dissolved in a liquid solvent) moves
from a higher concentration to a lower
concentration.
• Simple diffusion occurs until equilibrium is
reached.
• Simple diffusion is passive because it does not
require energy.
Passive Transport: No Energy
Required (cont.)
• Small, uncharged molecules such as
oxygen, carbon dioxide, and water cross
membranes by simple diffusion.
• Ions and polar molecules cross
membranes by facilitated diffusion.
– Facilitated diffusion is also passive transport.
– Membrane proteins assist the movement of
the molecule across the membrane.
Passive Transport: No Energy
Required (cont.)
Osmosis
• Diffusion of water across a differentially
permeable membrane is called osmosis.
• Osmosis is a type of passive diffusion
where the solvent (water) moves across
the membrane, rather than the solute.
Osmosis (cont.)
The Effect of Osmosis on Cells
• Osmosis can affect the size and shape of
cells, depending on differences in water
concentration across the membrane.
• Cells placed in an isotonic solution do not
change because the concentration of
water on both sides of the membrane is
the same.
The Effect of Osmosis on Cells
(cont.)
• Cells placed in a hypotonic solution gain water
(and may lyse) because the concentration of
water is higher outside the cell and water rushes
in.
• Cells placed in a hypertonic solution lose water
because the concentration of water is higher
inside the cell and water rushes out.
– An animal cell in a hypertonic solution shrinks.
– A plant cell in a hypertonic solution undergoes
plasmolysis (shrinking of the cytoplasm).
The Effect of Osmosis on Cells
(cont.)
Active Transport: Energy Required
• During active transport, molecules move
against their concentration gradient.
• Active transport requires a membrane
protein and energy to move the molecule.
• The energy for active transport is generally
provided by the mitochondria.
Active Transport: Energy Required
(cont.)
• Proteins engaged in active transport are
often called pumps.
• The sodium-potassium pump is an
example of an active transport process
critical to nerve conduction.
Active Transport: Energy Required
(cont.)
Bulk Transport
• Macromolecules are too large to move with
membrane proteins and must be
transported across membranes in vesicles.
• The transport of macromolecules out of a
cell in a vesicle is called exocytosis.
• The transport of macromolecules into a cell
in a vesicle is called endocytosis.
Bulk Transport (cont.)
Bulk Transport (cont.)
Bulk Transport (cont.)
• If the material taken up by endocytosis is a large
particle it is called phagocytosis.
• If the material taken up by endocytosis is a liquid
or small particle it is called pinocytosis.
• Receptor-mediated endocytosis is a selective,
highly efficient form of endocytosis.
Bulk Transport (cont.)