activation energy extra energy required to destabilize existing bonds and initiate a chemical...
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Activation energy
• Extra energy required to destabilize existing bonds and initiate a chemical reaction
• Exergonic reaction’s rate depends on the activation energy required– Larger activation energy proceeds more slowly
• Rate can be increased 2 ways1. Increasing energy of reacting molecules (heating)2. Lowering activation energy
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ΔG
Ener
gy R
elea
sed
Ener
gy S
uppl
ied
Free
Ene
rgy
(G) Activation
energy
Activationenergy0
uncatalyzedcatalyzed
Course of Reaction
Product
Reactant
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Catalysts
• Substances that influence chemical bonds in a way that lowers activation energy
• Cannot violate laws of thermodynamics– Cannot make an endergonic reaction spontaneous
• Do not alter the proportion of reactant turned into product
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ΔG
Ener
gy R
elea
sed
Ener
gy S
uppl
ied
Free
Ene
rgy
(G) Activation
energy
Activationenergy0
uncatalyzedcatalyzed
Course of Reaction
Product
Reactant
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ATP
• Adenosine triphosphate• Chief “currency” all cells use• Composed of– Ribose – 5 carbon sugar– Adenine– Chain of 3 phosphates• Key to energy storage• Bonds are unstable• ADP – 2 phosphates• AMP – 1 phosphate – lowest energy form
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AMP
CORE
O
O–
O
O
O
HH H
H
O
CC
NN
N
C
N
C
CHH
P O–
O P
O P O
ADP
ATP
Triphosphategroup
O–
CH2
High-energybonds
a.
Adenine NH2
Ribose
OHOH
b.
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ATP cycle
• ATP hydrolysis drives endergonic reactions– Coupled reaction results in net –ΔG (exergonic and
spontaneous)• ATP not suitable for long-term energy storage– Fats and carbohydrates better– Cells store only a few seconds worth of ATP
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+
+
Pi
Energy fromexergoniccellularreactions
ATP H2O
ADP
Energy forendergoniccellularprocesses
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Enzymes: Biological Catalysts• Most enzymes are protein– Some are RNA
• Shape of enzyme stabilizes a temporary association between substrates
• Enzyme not changed or consumed in reaction• Carbonic anhydrase– 200 molecules of carbonic acid per hour made without
enzyme– 600,000 molecules formed per second with enzyme
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Active site
a. b.Enzyme Enzyme–substrate complex
Substrate
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Active site
• Pockets or clefts for substrate binding• Forms enzyme–substrate complex• Precise fit of substrate into active site• Applies stress to distort particular bond to
lower activation energy– Induced fit
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1. The substrate, sucrose, consists of glucose and fructose bonded together.
2. The substrate binds to the active site of the enzyme, forming an enzyme– substrate complex.
3. The binding of the substrate and enzyme places stress on the glucose– fructose bond, and the bond breaks.
4. Products arereleased, andthe enzyme isfree to bind othersubstrates.
BondGlucose
Fructose
Active site
Enzymesucrase
H2O
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ATP• Cells use ATP to drive endergonic reactions– ΔG (free energy) = –7.3 kcal/mol
• 2 mechanisms for synthesis1. Substrate-level phosphorylation• Transfer phosphate group directly to ADP• During glycolysis
2. Oxidative phosphorylation• ATP synthase uses energy from a proton gradient
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PEP
– ADP– ADP
Enzyme Enzyme
– ATP– ATP
Adenosine
Pyruvate
PP
PPPP
PP
PP
Adenosine
PP
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Oxidation of Glucose
The complete oxidation of glucose proceeds in stages:
1. Glycolysis2. Pyruvate oxidation3. Krebs cycle4. Electron transport chain & chemiosmosis
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Outermitochondrial
membrane
Intermembranespace
Mitochondrialmatrix
FAD O2
Innermitochondrial
membrane
ElectronTransport Chain
ChemiosmosisATP Synthase
NAD+
Glycolysis
Pyruvate
Glucose
PyruvateOxidation
Acetyl-CoA
KrebsCycle
CO2
ATPH2O
ATP
e–
e–
NADH
NADH
CO2
ATP
NADH
FADH2
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H+
e–
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Glycolysis
• Converts 1 glucose (6 carbons) to 2 pyruvate (3 carbons)
• 10-step biochemical pathway• Occurs in the cytoplasm• Net production of 2 ATP molecules by
substrate-level phosphorylation• 2 NADH produced by the reduction of NAD+
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NADH must be recycled
• For glycolysis to continue, NADH must be recycled to NAD+ by either:
1. Aerobic respiration– Oxygen is available as the final electron acceptor– Produces significant amount of ATP
2. Fermentation– Occurs when oxygen is not available– Organic molecule is the final electron acceptor
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Krebs Cycle
• For each Acetyl-CoA entering:– Release 2 molecules of CO2 – Reduce 3 NAD+ to 3 NADH– Reduce 1 FAD (electron carrier) to FADH2 – Produce 1 ATP– Regenerate oxaloacetate
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Electron Transport Chain (ETC)
• ETC is a series of membrane-bound electron carriers
• Embedded in the inner mitochondrial membrane
• Electrons from NADH and FADH2 are transferred to complexes of the ETC
• Each complex– A proton pump creating proton gradient– Transfers electrons to next carrier
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Mitochondrial matrix
NADH + H+
ADP + PiH2O
H+ H+
2H+ + 1/2 O2
Glycolysi s
Pyruvate Oxidatio n
2
KrebsCycle ATP
Electron Transport ChainChemiosmosis
NADH dehydrogenase bc1 complexCytochrome
oxidase complex
Innermitochondrial membrane
Intermembrane space
a. The electron transport chain
ATPsynthase
b. Chemiosmosis
NAD+
Q
C
e–
FADH2
H+H+
H+H+
e–22 e–22
ATP
FAD
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Chemiosmosis
• Accumulation of protons in the intermembrane space drives protons into the matrix via diffusion
• Membrane relatively impermeable to ions• Most protons can only reenter matrix through
ATP synthase– Uses energy of gradient to make ATP from ADP + Pi
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ADP + Pi
Catalytic head
Stalk
Rotor
H+
H+
Mitochondrialmatrix
Intermembranespace
H+ H+
H+
H+H+
ATP
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H2O
CO2
CO2
H+
H+
2H+
+1/2 O2
H+
e–
H+
32 ATPKrebsCycle
2 ATP
NADH
NADH
FADH2
NADH
PyruvateOxidation
Acetyl-CoA
e–
QC
e–
Glycolysis
Glucose
Pyruvate
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Photosynthesis
Chapter 8
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Photosynthesis Overview
• Energy for all life on Earth ultimately comes from photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
• Oxygenic photosynthesis is carried out by– Cyanobacteria– 7 groups of algae– All land plants – chloroplasts
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Chloroplast
• Thylakoid membrane – internal membrane– Contains chlorophyll and other photosynthetic
pigments– Pigments clustered into photosystems
• Grana – stacks of flattened sacs of thylakoid membrane
• Stroma lamella – connect grana• Stroma – semiliquid surrounding thylakoid
membranes27
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Vascular bundle Stoma
Cuticle
Epidermis
Mesophyll
Chloroplast
Inner membraneOuter membrane
Cell wall
1.58 mm
Vacuole
Courtesy Dr. Kenneth Miller, Brown University
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Stages
• Light-dependent reactions– Require light1.Capture energy from sunlight2.Make ATP and reduce NADP+ to NADPH
• Carbon fixation reactions or light-independent reactions– Does not require light3.Use ATP and NADPH to synthesize organic
molecules from CO2
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O2
Stroma
Photosystem
Thylakoid
NADP+ADP + Pi
CO2
Sunlight
PhotosystemPhotosystem
Light-DependentReactions
CalvinCycle
Organicmolecules
O2
ATP NADPH
H2O
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Pigments
• Molecules that absorb light energy in the visible range
• Light is a form of energy• Photon – particle of light– Acts as a discrete bundle of energy– Energy content of a photon is inversely
proportional to the wavelength of the light• Photoelectric effect – removal of an electron
from a molecule by light
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Light-Dependent Reactions
1. Primary photoevent– Photon of light is captured by a pigment molecule
2. Charge separation – Energy is transferred to the reaction center; an excited
electron is transferred to an acceptor molecule3. Electron transport– Electrons move through carriers to reduce NADP+
4. Chemiosmosis– Produces ATP
Capt
ure
of li
ght e
nerg
y
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Chloroplasts have two connected photosystems
• Oxygenic photosynthesis• Photosystem I (P700)– Functions like sulfur bacteria
• Photosystem II (P680)– Can generate an oxidation potential high enough to oxidize
water
• Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH
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Chemiosmosis
• Electrochemical gradient can be used to synthesize ATP
• Chloroplast has ATP synthase enzymes in the thylakoid membrane– Allows protons back into stroma
• Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions
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Production of additional ATP
• Noncyclic photophosphorylation generates– NADPH– ATP
• Building organic molecules takes more energy than that alone
• Cyclic photophosphorylation used to produce additional ATP– Short-circuit photosystem I to make a larger
proton gradient to make more ATP35
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Carbon Fixation – Calvin Cycle
• To build carbohydrates cells use• Energy– ATP from light-dependent reactions– Cyclic and noncyclic photophosphorylation– Drives endergonic reaction
• Reduction potential– NADPH from photosystem I– Source of protons and energetic electrons
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Calvin cycle
• Named after Melvin Calvin (1911–1997)• Also called C3 photosynthesis
• Key step is attachment of CO2 to RuBP to form PGA
• Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco
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• There are four basic mechanisms for cellular communication1. Direct contact2. Paracrine signaling3. Endocrine signaling4. Synaptic signaling
• Some cells send signals to themselves (autocrine signaling)
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Signal transduction• Events within the cell that occur in response to a
signal• When a ligand binds to a receptor protein, the cell
has a response• Different cell types can have similar response to the
same signal– Glucagon example
• Different cell types can respond differently to the same signal– Epinephrine example
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Kinase cascade
• Mitogen-activated protein (MAP) kinases– Important class of cytoplasmic kinases– Mitogens stimulate cell division– Activated by a signaling module called a
phosphorylation cascade or kinase cascade– Series of protein kinases that phosphorylate each
other in succession– Amplifies the signal because a few signal
molecules can elicit a large cell response
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Response
Signal
Receptor
Activator
ActiveInactive
Inactive
Inactive
Active
Active
MKKMKK
MKMK
MKKKMKKK
Ras
PP
P
PP
P
MAP kinase cascade
Firstkinase
Secondkinase
MAPkinase
Responseproteins
Cellularresponse
ResponseproteinsResponseproteins
MKKK
MKK
MK
MKKK
MKK
MK
a.
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Signal amplification
Signal
Receptor
Activator
Cellular responses
MKK MKK MKK
MK MK MK MK
MKKK MKKK
Response proteins
b.
MKKK
MKK
MK
MKKK
MKK MKK
MK MK MK
Response proteins
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