Biology 203 (Exam 2) Flashcards
Free-Energy Change △G
Biologists:
－Want to know which reactions occur spontaneously and which require input of energy – To do so, Biologists need to determine energy changes that occur in chemical reactions
Free-Energy Change △G
The free-energy change of a reaction
Tells us whether or not the reaction
occurs spontaneously
A living system’s free energy
Energy that can do work when temperature and pressure are uniform • As in a living cell • △G must have a negative value for a process to be spontaneous
△G = Gfinal state – Ginitial state
Free energy is a measure of a system’s instability
Its tendency to change to a more stable state
During a spontaneous change
－Free energy decreases and the stability of a system
increases
– Unless something prevents it, each system will move
toward greater stability
• Diver on a top of a platform
• Drop of concentrated dye
• Sugar molecule
Equilibrium is a state of maximum stability
A process is spontaneous and can perform work only when it is moving toward equilibrium.
The change in free energy (△G) during a process
Related to the change in enthalpy or change in total energy (△H), change in entropy (△S), and temperature in Kelvin (T)
△G = △H - T△S
Free energy extra
Only processes with a negative ΔG are spontaneous
• Spontaneous processes can be harnessed to perform
work
Equilibrium and Metabolism
The concept of free energy :
Can be applied to the chemistry of life’s processes
Exergonic and endergonic reactions in metabolism
An exergonic reaction
■Proceeds with a net release of free energy and is spontaneous
An endergonic reaction
▪ Absorbs free energy
from its surroundings
and is nonspontaneous
Reactions in a closed system
Eventually reach equilibrium and then do no work
Cells are not in equilibrium
They are open systems experiencing a constant flow of material
A catabolic pathway in a cell
Releases free energy in a series of reactions
Closed and open hydroelectric systems
Can serve as analogies.
A defining feature of life
Metabolism is never at equilibrium
ATP
powers cellular work by coupling exergonic reactions to endergonic reactions
A cell does three main kinds of work:
– Chemical
• Coupling energy from ATP (△G 0)
– Transport
• Pumping ions and molecules across
membranes against concentration
gradient
– Mechanical
• muscle contraction, vesicle,
flagella and cilia movement
To do work, cells manage energy resources by energy coupling
The use of an exergonic process to drive an endergonic one
Most energy coupling in cells is mediated by ATP
－Cell’s energy shuttle
– Composed of ribose (a sugar), adenine
(a nitrogenous base), and three
phosphate groups
– The bonds between the phosphate
groups of ATP’s tail can be broken by
hydrolysis
– Energy is released from ATP when the
terminal phosphate bond is broken
– This release of energy comes from the
chemical change to a state of lower
free energy
• Not from the phosphate bonds
themselves
How ATP performs work
The three types of cellular work are powered by the hydrolysis of ATP
－Mechanical
– Transport
– Chemical
In the cell
– Energy from the exergonic
reaction of ATP hydrolysis
• Can be used to drive an endergonic reaction
Overall, the coupled reactions
are exergonic
ATP drives endergonic
reactions by phosphorylation
Transferring a phosphate group to some other molecule
• Such as a reactant
The recipient molecule becomes phosphorylated
ATP is a renewable resource
Regenerated by addition of a phosphate group to adenosine diphosphate (ADP)
The energy to phosphorylate ADP
Comes from catabolic reactions in the cell
The chemical potential energy
Temporarily stored in ATP drives most cellular work
A catalyst
A chemical agent that speeds up a reaction
• Without being consumed by the reaction
An enzyme
– A catalytic protein
Hydrolysis of sucrose by the
enzyme sucrase
An example of an enzymecatalyzed
reaction
Every chemical reaction
between molecules
Involves bond breaking and
bond forming
The initial energy needed to
start a chemical reaction
Activation energy (EA)
Activation energy
Often supplied in the form of heat from the
surroundings
Enzymes catalyze reactions
by
lowering the EA barrier
Enzymes do not affect the change
in free energy (ΔG)
Instead, they hasten reactions that would occur
eventually
The reactant that an enzyme acts on
– Called the enzyme’s substrate
The enzyme binds to its substrate
Forming an enzyme-substrate complex
The active site
The region on the enzyme where the substrate binds
Induced fit of a substrate
－Brings chemical groups of the active site into positions
– Enhance their ability to catalyze the reaction
In an enzymatic reaction
Substrate binds to the active
site of the enzyme
The active site can lower an EA barrier by
－Orienting substrates correctly – Straining substrate bonds – Providing a favorable microenvironment – Covalently bonding to the substrate
An enzyme’s activity can be
affected by
－General environmental factors,
such as temperature and pH
– Chemicals that specifically
influence the enzyme
Effects of Temperature and pH
－Each enzyme has an optimal temperature in which it can function – Each enzyme has an optimal pH in which it can function
Cofactors
Cofactors are nonprotein enzyme helpers – May be inorganic • metal in ionic form – May be organic • coenzyme
Coenzymes include vitamins • Vitamin C – Ascorbic acid – Scurvy – Required for the synthesis of collagen – Bleeding from mucous membranes, spots on skin
Enzyme Inhibitors
Competitive inhibitors
– Bind to the active site of an enzyme
– Competing with the substrate
Noncompetitive inhibitors
– Bind to another part of an enzyme
– Causing the enzyme to change shape
– Making the active site less effective
Regulation of enzyme activity
Helps control metabolism
Living cells require energy
from outside sources
－Some animals, such as the giant panda, obtain energy by eating plants – Some animals feed on other organisms that eat plants
Energy flows into an ecosystem as sunlight
Leaves as heat
Photosynthesis generates O2 and organic molecules
Used in cellular respiration
Cells use chemical energy stored in organic molecules
－Regenerate ATP
• Powers work
Stages of Cellular Repsiration
A Preview
Cellular respiration has three stages:
- Glycolysis
- Breaks down glucose into two molecules of pyruvate
- The citric acid cycle
- Completesthe breakdown of glucose
- Oxidative Phosphorylation
- Accounts for most of the ATP synthesis
Cellular Respiration
Includes both aerobic and
anaerobic respiration
– Often used to refer to aerobic respiration • Although carbohydrates, fats, and proteins are all consumed as fuel
– Helpful to trace cellular respiration with the sugar glucose: Cellular Respiration C6H12O6 + 6 O2 >>>> 6 CO2 + 6 H2O + Energy (ATP + heat)
Redox Reactions: Oxidation and Reduction
- The transfer of electrons during
chemical reactions
– Releases energy stored in organic
molecules
• This released energy is ultimately
used to synthesize ATP
The Principle of Redox (slide 6, ch10)
--Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions • Redox reactions – In oxidation • A substance loses electrons – Oxidized – In reduction • A substance gains electrons – Reduced – The amount of positive charge is reduced
Reducing Agents and Oxidizing Agents
*The electron donor
– Called the reducing agent
• The electron acceptor
– Called the oxidizing agent
• Some redox reactions do not transfer electrons
– But change the electron sharing in covalent bonds
– An example is the reaction between methane and O2
During cellular respiration
The fuel (such as glucose) is oxidized –O2 is reduced
Electron transport chain
Passes electrons in a series of steps • Breaks the fall of e- – Unlike an uncontrolled, explosive reaction
*O2 pulls electrons down the chain
in an energy-yielding tumble
• The energy yielded is used to
regenerate ATP
Nicotinamide adenine
dinucleotide (NADH)
passes the
electrons to the electron
transport chain
Niacin
Vitamin B3
Pellagra
Vitamin deficiency disease
Tradition food preparation of corn
Treatment with lime • An alkali • Makes niacin nutritionally available • Corn dependence in Spain, American South (1900s) • No lime treatment • Pellagra first described • Aggression • Skin lesions • Dilated cardiomyopathy • Dementia • Death in 5 years
In cellular respiration:
Glucose and other organic molecules are broken down in a series of steps
Electrons from organic compounds are usually first transferred to NAD+
– A coenzyme
NAD+ functions as an oxidizing agent during cellular respiration
NAD+ is an electron acceptor
Each NADH (the reduced form of NAD+)
Represents stored energy
– Tapped to synthesize ATP
Glycolysis
“Splitting of sugar” – Breaks down glucose into two molecules of pyruvate – Occurs in the cytoplasm – Two major phases • Energy investment phase • Energy payoff phase
Energy Investment
Phase
Glucose enters the cell • Phosphorylated by hexokinase – Sugar trapped in cell – More chemically reactive – Transfer of a phosphate group • Investment of energy
Glycolisis: Energy Investment
Phase
Glucose-6-phosphate converted to its isomer • Fructose-6- phosphate
aerobic respiration takes place in about 20 steps, grouped into three stages:
1) Glycolysis
2) Formation of acetyl coenzyme A and the citric acid cycle (Krebs cycle)
3) The electron transport chain and chemiosmosis
Cellular respiration has three stages:
– Glycolysis • Breaks down glucose into two molecules of pyruvate – The citric acid cycle • Completes the breakdown of glucose – Oxidative phosphorylation • Accounts for most of the ATP synthesis
Oxidative Phosphorylation
• Process that generates most of the ATP
– Powered by redox reactions
• Oxidative phosphorylation
– Accounts for almost 90% of the ATP generated by cellular respiration
• A smaller amount of ATP is formed in glycolysis and the citric acid cycle
– Substrate-level phosphorylation
During Oxidative Phosphorylation
Chemiosmosis
Couples Electron Transport To ATP Synthesis
Following glycolysis and the citric acid cycle,
• NADH and FADH
2 account for most of the energy extracted from food
• These two electron carriers donate electrons to the electron transport chain
• Powers ATP synthesis via oxidative phosphorylation
Chemiosmosis
Diffusion of ions across a selectively-permeable membrane
• Generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration
The Pathway of Electron Transport
Electron transport chain – In the cristae of the mitochondrion • Most of the chain’s components are proteins – Exist in multiprotein complexes – Flavoprotein (FMN) – Iron-sulfur protein (Fe-S) – Ubiquinones (Q) – Cytochromes (Cyt) • The carriers alternate reduced and oxidized states – As they accept and donate electrons • Electrons drop in free energy as they go down the chain – Finally passed to O2 • Forming H2O
Chemiosmosis Couples The Electron Transport
Chain to ATP Synthesis
Energy stored in a H+ gradient across a membrane
– Couples the redox reactions of the electron transport chain to ATP synthesis
• The H+ gradient
– Referred to as a proton-motive force
• Emphasizes its capacity to do work
NADH and FADH2 shuttle high-energy electrons to an
electron transport chain
-Built into the inner mitochondrial membrane
– Electrons extracted from food
• During glycolysis and the citric acid cycle
Gold Arrow
Trace the transport of electrons
• Finally pass to oxygen at the “downhill” end of
the chain
• Forming water
• Most of the electrons
– Grouped into 4 complexes • Two mobile carriers – Ubiquinone (Q) – Cytochrome C (Cyt C) – Move electrons between the large complexes
Complexes I, III, and IV accept and then donate
elctrons
– Pump protons from the mitochondrial matrix into the
intermembrane space
– FADH2 deposits its electrons via complex II
• Fewer protons being pumped compared to
NADH
Chemical energy
–Transformed into a proton-motive force
• A gradient of H+ across the membrane
Electron transfer in the electron
transport chain
Causes proteins to pump H+ from the mitochondrial matrix to the
intermembrane space
H+ then moves back across the
membrane
Pass through channels in ATP synthase
ATP synthase
Uses the exergonic flow of H+ to drive phosphorylation of ATP
