Download Energy Flow in the Life of a Cell What is Energy? Laws of

Energy Flow in the
Life of a Cell
What is Energy?
• Energy is the capacity to do work.
– Work is a force acting upon an object that
causes the object to move.
– Chemical energy, electrons in atoms and their
interactions, powers all life on Earth.
Chapter 6
Laws of Thermodynamics
Describe the properties and
behavior of energy:
the quantity, or the total amount,
the quality, or the usefulness,
of a closed system, where no energy or
matter can enter or leave the system.
• Kinetic energy is the energy of movement.
• Potential energy is stored energy.
Laws of Thermodynamics
• 1st Law of Thermodynamics: energy can
neither be created nor destroyed.
– The Law of Conservation of Energy
– Energy can be converted, or change forms.
Laws of Thermodynamics
• 2nd Law of Thermodynamics: the entropy of a
system increases over time until equilibrium
is reached.
– Entropy: the degree of randomness or disorder in
a closed system.
– When energy is converted from one form to
another, the amount of useful energy decreases.
Energy Flow in
Chemical Reactions
• A chemical reaction is a process that
forms or breaks chemical bonds.
Reactants
Substrates
Starting Material
Endergonic Reaction
• Products contain more energy than the
reactants.
• Energy is absorbed by the reaction.
Products
Ending Material
Photosynthesis:
an endergonic reaction
• Plants use energy from the sun to create
high potential energy molecules, sugars.
Exergonic Reactions
• Reactants contain more energy than the
products.
• Energy is released.
Coupled Reactions
• An exergonic reaction provides the energy
needed for an endergonic reaction.
• Energy is released from the sun in nuclear
reactions as light and heat.
• Plants use energy from the sun for
photosynthesis.
Respiration:
an exergonic reaction
• Cells release potential energy by
breaking apart sugars.
Many Reactions Require Energy
• Activation energy is the amount of energy
needed for a chemical reaction to proceed.
– A little energy push.
Many Reactions Require Energy
• Catalysts reduce the activation energy needed.
– Speeds up the chemical reaction.
– Enzymes are biological catalysts.
Energy in ATP
Cellular Energy
• Energy-carrier molecules can pick up
energy released from exergonic reactions
and release it in endergonic reactions
elsewhere in the cell.
• Adenosine triphosphate (ATP) is the
primary energy-carrier molecule in cells.
Energy in ATP
• Energy is stored within highenergy phosphate bonds.
High Energy Bond
+
+
Enzymes Catalyze Specific Reactions
Electron Carriers
Also Transport Energy
• Electron carriers store energy from
energetic electrons, and bound hydrogen.
• NAD+ + H ! NADH
• FAD + 2H ! FADH2
NAD
H
(nicotinamide adenine dinucleotide)
(flavin adenine dicucleotide)
H
FAD
H
Chemical Reactions in Cells
• Metabolism is the sum of all of a cell’s
chemical reactions.
• Metabolic pathways are linked sequences
of reactions.
• Enzymes are
primarily protein.
• Each enzyme only
catalyzes one or a
few types of
chemical reactions.
– Determined by the
active site of the
enzyme, where the
substrate binds.
Cells Must Regulate Metabolism
• Enzymes can be controlled:
– Regulation of enzyme synthesis
• Commonly used enzymes are synthesized in
large numbers.
• Enzymes for rare reactions are only synthesized
when needed.
– Synthesis in inactive forms
• Active sites can be blocked until needed.
– Controlled by regulator molecules
Regulator Molecules
• Binding of regulatory molecules can
enhance or inhibit the enzyme reaction.
Allosteric Inhibition
• Allosteric regulators bind to a regulatory
site, separate from the active site.
• Non-Competitive Inhibition
Competitive Inhibition
• The inhibitor binds to the same active site
as the normal enzyme substrate, without
undergoing a reaction.
• Can be irreversible.
Inhibition as Regulation
• Competitive Inhibitors: molecules that
compete with the substrate for the active
site of an enzyme.
• Feedback Inhibitors: products that when
created at a high concentration prevent
the reaction from occurring.
Ammonia Monooxygenase
• Enzyme bacteria use to oxidize
ammonia, producing energy.
Ammonia Monooxygenase
• A non-specific enzyme that can also
oxidize similar-shaped molecules.
Competitive Inhibition
by methanol
Feedback Inhibition
• Regulation where the end product of a
metabolic pathway “turns off” the first
enzyme of the pathway.
Irreversible Inhibition
by acetylene
Quorum Sensing
• Method of feedback signaling for
communication by certain bacteria to
detect high population densities.
One bacteria sends out a signal.
Quorum Sensing
• Method of feedback signaling for
communication by certain bacteria to
detect high population densities.
A second bacteria senses the signal,
by binding to a receptor protein,
and sends a signal out in return.
Quorum Sensing
Quorum Sensing
Both bacteria continue to receive and send out signal.
Neighboring bacterial cells also recognize the signal,
and send out new signal in return.
A quorum has been reached.
A quorum is a specified minimum number
of individuals gathered from a population.
Quorum Sensing
• When a quorum is reached, Vibrio
fischeri produces enzymes to make light
for its host, the squid.
– The bacteria is housed in special light
organs on the underside of the squid.
– They provide camouflage against shadows
from the moon.
"Bacteria Talk", a profile of Bonnie Bassler
NOVA ScienceNow, January 2007
Optimal
Enzyme Activity
• Most enzymes have a
specific temperature
and pH range for
maximal activity.
Environmental Inhibition
•Environmental parameters can inhibit
enzymatic reactions.
– Protein structure is susceptible to temperature
and pH.
– High temperature and low pH can denature
proteins (breaking hydrogen bonds between
amino acids, disrupting their structure).
Ex: fried eggs and ceviche!
Bacterial metabolism slows down
at lower temperature.
Enzymes not needed
are down-regulated.
• Enzymes are expensive
to make.
• Cells will limit the
production of certain
enzymes if not actively
used by cells.
Long-time vegetarians
lack enzymes to break
down animal proteins.
Homework
Chapter 6
Thinking Through the Concepts
Review Questions 3 & 5.