Chapter 6
Energy Flow in the Life of a Cell

• 6.1 What Is Energy?
• 6.2 How Does Energy Flow in Chemical
Reactions?
• 6.3 How Is Cellular Energy Carried Between
Coupled Reactions?
• 6.4 How Do Cells Control Their Metabolic
Reactions?

• Energy is the ____________________
• Work is a force acting on an object that causes the object to _________
• _________________ is the energy that powers life
– The objects that move are __________, which reposition during chemical reactions

• The two fundamental types of energy
– Kinetic energy is the energy of __________
• e.g. light, heat, electricity, moving objects
– Potential energy is _____________ energy
• e.g. chemical energy in bonds, electrical charge in a battery, a rock at the top of a hill

• The laws of thermodynamics describe the availability and usefulness of energy
– ________________________________
( first law of thermodynamics )
– The total amount of energy within a given system remains __________ unless energy is added or removed from the system

• Availability and usefulness of energy:
– The amount of useful energy __________ when energy is converted from one form to another ( second law of thermodynamics )
– Entropy (disorder) ______________

• Living Things Use the Energy of Sunlight to
Create the _____________ Conditions
Characteristic of Life
• __________________ organisms use external solar energy to maintain orderly structure
• ____________________ organisms use stored chemical energy in other living things to counter increasing entropy

• 6.2 How Does Energy Flow in Chemical
Reactions?
– The Nature of Chemical Reactions
– Exergonic Reactions ___________ Energy
– Endergonic Reactions ___________ an Input of Energy
– Coupled Reactions _______Exergonic and
Endergonic Reactions

• Chemical reactions are processes that
____________________________ between atoms
• Chemical reactions convert _________ to products
Reactants Products

• Reactions can be categorized as exergonic or endergonic based on energy gain or loss

• Exergonic reactions __________ energy
• Reactants contain _______ energy than products in exergonic reactions

• Exergonic reaction example: the burning of glucose

• All chemical reactions require an initial energy input ( ____________________ ) to get started
– Molecules need to be moving with sufficient collision speed
– The electrons of an atom repel other atoms and inhibit bond formation

• Endergonic reactions are _______________
___________________________ of energy
• Products contain _______ energy than reactants in endergonic reactions

• Endergonic reaction example: photosynthesis

• ______________ reactions drive
_____________ reactions
– The product of an energy-yielding reaction fuels an energy-requiring reaction in a
_________________

• The exergonic and endergonic parts of coupled reactions often occur at different places within the cell
• ___________________________ are used to transfer the energy within cells

• 6.3 How Is Cellular Energy Carried
Between Coupled Reactions?
– Energy Carrier Molecules
– ______ Is the Principal Energy Carrier in
Cells
– ____________________ Also Transport
Energy Within Cells

• Food energy cannot be used _______ to power energy-requiring reactions (e.g. muscle contraction)
• Energy carrier molecules act as
______________ to carry energy between
__________ and _____________ reactions
• Energy carrier molecules are only used within cells because they are ________

• ______________________ ( ATP ) is the most common energy carrying molecule
• ATP is composed of an adenosine molecule and ______ phosphates

• Energy is stored in the high-energy bond extending to the _________________
• Heat is given off when ATP breaks into
_____ (adenosine diphosphate) and ___
(phosphate)

• The energy released when ATP is broken down into ADP + P is transferred to
______________ reactions through coupling

• Energy can be transferred to
__________ in glucose metabolism and photosynthesis
• Electron _________ transport highenergy electrons
• Two common electron carriers
1. Nicotinamide adenine dinucleotide ______
2. Flavin adenine dinucleotide _______

• 6.4 How Do Cells Control Their
Metabolic Reactions?
– Overview of Metabolism
– At Body Temperatures, Spontaneous Reactions
Proceed Too ________to Sustain Life
– Catalysts __________ Activation Energy
– Enzymes Are Biological ______________
– Cells Regulate Metabolism by __________
Enzymes
– Poisons, Drugs, and Environmental Conditions
Influence ___________________

• The sum of all the chemical reactions inside a cell is its __________________
• Many cellular reactions are linked through metabolic _______________

• Metabolic pathways proceed smoothly for three reasons:
1. Endergonic reactions are _______ with exergonic reactions
2. Energy-carrier molecules capture energy and ____________ between endergonic and exergonic reactions
3. Chemical reactions are regulated through protein _______________

• At body temperatures, spontaneous reactions proceed too _________ to sustain life
• Reaction speed is generally determined by the activation energy required
– Reactions with low activation energies proceed _________ at body temperature
– Reactions with high activation energies (e.g. sugar breakdown) move very _______ at body temperature, even if exergonic overall

• Enzyme molecules are employed to
___________ (speed up) chemical reactions in cells
• Catalysts ________________ of a chemical reaction without themselves being used up

• Catalytic converters in cars facilitate the conversion of carbon monoxide to carbon dioxide
Octane + oxygen carbon dioxide + water + energy + carbon monoxide
(poisonous)
• Catalyst in catalytic converter speeds carbon monoxide conversion
Carbon monoxide + oxygen carbon dioxide + energy

• Catalysts speed up spontaneous reactions by ____________________________

• Enzymes __________________________ molecules in the process of lowering activation energy
• Enzymes (proteins) differ from non-biological catalysts because:
1. Enzymes are very ___________ for the molecules they catalyze
2. Enzyme activity is often ____________________ by their reactants or products
• Some enzymes require helper ___________ molecules to function (e.g. certain B vitamins)

• Enzymes have a pocket called an _________
• Reactants (_________) bind to the active site
– Distinctive shape of active site is complementary and specific to the substrate
– Active site amino acids bind to the substrate and distort bonds to facilitate a reaction

• Three steps of enzyme catalysis
1. Substrates enter the
_________ in a specific orientation
2. Upon binding, the substrates and enzyme
_____________ to promote a reaction
3. Products of the reaction
________ the active site, leaving the enzyme ready for another catalysis

• A given enzyme usually catalyzes a single step in a chain of metabolic reactions
• Metabolic pathways are controlled in several ways
1. Control of _____________________ regulates availability
2. Some enzymes are inactive when synthesized and must be _____________
__________________

• Metabolic pathways are controlled in several ways (continued)
3. Small organic molecules can bind to enzymes and
___________________ activity ( allosteric regulation )

• Metabolic pathways are controlled in several ways (continued)
4. Adequate amounts of formed product inhibit enzyme activity ___________________

• Drugs and poisons often inhibit enzymes by ____________ with the natural substrate for the active site
• This process is known as
_______________________
• Some inhibitors bind permanently to the enzyme

• Three-dimensional structure of an enzyme is sensitive to ______________________
______________________________
• Most enzymes function optimally only within a very narrow range of these conditions

• Enzyme structure is
_________ and function is destroyed when pH is _________
_____________
• Salts in an enzyme’s environment can also destroy function by altering ___________

• Temperature also affects enzyme activity
– Low temperatures
______________ molecular movement
– High temperatures cause ___________ to be altered, destroying function