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Chapter 8: An Introduction to Metabolism and Chapter 9: Cellular…
Chapter 8: An Introduction to Metabolism and Chapter 9: Cellular Respiration and Fermentation
Concept 8.5 Regulation of enzyme activity helps control metabolism
Allosteric Regulation of Enzymes
Allosteric Activation & Inhibition
most enzymes allosterically regulated are constructed w/2 + subunits, polypeptide chains with its own active site.
bind to regulatory site
an
activator
binds to a regulatory site
inhibitor
stabilizes the inactive form of the enzyme
Ex: ATP binding to several catabolic enzymes allosterically
ADP activates the enzymes to speed up catabolism to produce more ATP.
cooperativity
amplifies the response of enzymes to substrates
One substrate molecule primes an enzyme to act on addition substrate molecules.
allosteric, bc its binding of active site affects catalysis in another active site.
Ex: hemoglobin; the oxygen binding sites increase the affinity for oxygen of remaining binding sites.
Feedback Inhibition
Defined as
: a metabolic pathway is halted by the inhibitory binding of its end product of an enzyme that acts early in the pathway.
see
Figure 8.21
defined as:
any case in which a protein's function at one site is affected by the binding of a regulatory molecule to a separate site.
Localization of Enzymes Within the Cell
Some enzymes have fixed locations within cell & act as structural components of a membrane
Others are in solution within particular membrane-enclosed eukaryotic organelles, w/own internal chem. environment.
Ex: Enzymes for 2nd & 3rd stages of cell respiration reside in specific locations of the mitochondria.
Concept 8.4 Enzymes speed up metabolic reactions by lowering energy barriers
Enzyme
catalyst
which acts as a chemical agent that speeds up a rxn w/o being consumed
The Activation Energy Barrier
contorting the starting molecule into a highly unstable state before rxn can proceed.
reactant molecules must absorb energy from surroundings
New bonds form as heat is release, & molecules return to stable shapes w/ lower energy than contorted state.
Activation Energy
is the initiation of energy needed to start a reaction.
transition state
: unstable condition, bonds are breaking
How Enzymes Speed Up Reactions
Heat can increase the rate of a reaction by allowing reactants to attain the transition state more often.
Organisms carry out
catalysis
, process by which a catalyst speeds up a reaction w/o consumed.
An enzyme catalyzes a rxn by lowering
E
a barrier enabling reactants to absorb enough energy to reach the transition state at moderation.
Substrate Specificity of Enzymes
Substrate
reactant an enzyme acts on
Enzyme+Substrate <-->Enzyme-SubComplex<-->Enzyme+Prod.
enzyme-substrate complex
Forms whenever an enzyme is bonded to its substrate.
Ex: Sucrase+Sucro.+H2O <->Sucrase-Sucro-H2OCompl.<->Sucrase+Glucose+Fructose
Active Site
pocket or groove on the enzyme where catalysis occurs
formed from amino acids
Induced fit
the tightening of the bind after intital contact; like a clasping handshake.
brings chemical groups of the active site into positions that enhance their ability to catalyze the chem rxn.
Catalysis in the Enzyme's Active Site
substrates are held by weak interactions (hydrogen & ionic bonds)
R groups of a few amino acids that make up the active site catalyze the conversion to product, & the product departs from the active site.
metabolic rxns are reversible, either forward or reverse depending on which direction has a negative △G.
net effect is always in direction of equilibrium.
Effect of Local Conditions on Enzyme Activity
Effects of Temperature & pH
Each enzyme works better under other conditions, due to
optimal conditions
factoring the most active shape for the enzyme
Important factors for
E
a: Temperature & pH
Thermal agitation disrupts hydrogen, ionic bonds, & other weak interactions that stabilize the shape of the enzyme causing the protein to denature.
The optimal temp allows the reaction rate at its greatest, allowing more molecular collisions, and fast conversions (R<--->P)
optimal temperature in humans: 35 ℃- 40℃
pH ranges: 6-8
Exceptions of pH ranges, human stomach (pepsin low pH) or trypsin (high pH), digestive enzyme
Cofactors
nonprotein helpers of enzyme for catalytic activity
used for electrons transfers that can't be carried out by AA in protein.
may be bound tightly or loosely and reversibly on enzyme along w/ substrate.
inorganic & IONIC; like zinc, iron, copper in ionic form.
coenzyme
cofactor that is
organic
made of vitamins
Enzyme Inhibitors
competitive inhibitors
reduce the productivity of enzymes by blocking substrates from entering active sites.
can be overcome by increasing substrate conct. so active site becomes available.
noncompetitive inhibitors
do NOT compete w/substrate to bind to the enzyme at the active site.
bind to other parts of enzyme
causes enzyme to change shape which make active site less effective at catalyzing the substrate to product.
The Evolution of Enzymes
enzymes are proteins, proteins are genes
mutation
is permanent change in a gene
natural selection would favor the mutated form of the gene, causing to persist.
if changed AA are in the active site, enzyme might bind to a diff substrate.
Concept 8.3 ATP powers cellular work by coupling exergonic reactions to endergonic reactions
A cell does 3 main kinds of work:
Transport Work
: pumping of substances across membranes against direction of spontaneous movement.
Mechanical Work
: beating of cilia, contraction of muscle cells, & mvmt of chromosomes during cell production
Chemical work
: pushing of endergonic rxns that don't occur spontaneous.
Energy coupling
is the use of an exergonic process to drive an endergonic one; ATP is used for energy & powers cell work in coupling.
The Structure & Hydrolysis of ATP
ATP
contains sugar ribose, nitrogenous base, & triphosphate group
bonds between phosphate group can be broken by hydrolysis.
ATP to ADP is exergonic & releases 7.3 kcal of energy per mole
ATP + H2O --> ADP + P
The release of energy during hydrolysis of ATP comes from chemical change of the system to a state of lower free energy, not from phosphate bonds.
Hydrolysis releases so much energy during the break down of ATP b/c all 3 phosphates are (-) charge.
triphosphate tail of ATP chemical = a compressed spring.
How the Hydrolysis of ATP Performs Work
if change in
G
of an endergonic rxn is less than the amt of energy release by ATP hydrolysis, then the two rxns can be coupled so that, the coupled rxns are exergonic.
Involves phosphorylation, (ATP transfer to another molecule, such as a reactant.
Phosphorylated intermediate
is the recipient molecule w/ the phosphate grp
covalently bonded
to it; key to coupling exergonic & endergonic rxns, most reactive.
Transport Work
: ATP phosphorylates transport proteins causing a shape in change to allow solute transports.
Mech. Work
ATP binds noncovalently to motor proteins then hydrolyze, causes shape change that walks the motor protein.
Figure 8.12 The ATP cycle: energy released by catabolism in cell is used to phosphorylate ADP, regenerating ATP. Chem potential energy stored in ATP drives most cell work.
The Regeneration of ATP
ATP cycle: free energy required to phosphorylate ADP comes from exergonic breakdown (catabolism) in cell.
couples the cell's energy yielding processes to the energy consuming ones.
EX: a working muscle cell recycles its ATP in less than a minute.
ADP + P = ATP + H2O nonspontaneous/endergonic
Concept 8.1 An organism's metabolism transforms matter and energy, subject to the laws of thermodynamics
Metabolism
Totality of an organism's chemical reactions; emergent property of life from orderly interactions b/w molecules
Organization of the Chemistry of Life into Metabolic Pathways
Metabolic Pathway
1) starts w/specific molecule 2)alters in a series of defined steps 3)results in a certain product.
Mechanisms balance supply & demand
analogous to red, yellow & green stop light to control flow.
Energy released from "downhill" rxns of catabolic pathways can be stored and used to drive the "uphill" rxns of anabolic pathways.
Catabolic Pathways
Break down complex molecules to simple compounds
Major Pathway: Cellular Respiration
Energy that was from organic molecules become available to work in the cell;
EX:
ciliary beating or membrane transport
Anabolic Pathways
consume energy to build molecules
Biosynthetic Pathways
Ex:
synthesis of amino acids from simple compounds or synthesis of proteins from AA.
Bioenergetics
is the study of how energy flows through living organisms.
Forms of Energy
Energy
exists in various forms and is the ability to do work.
Kinetic Energy
is energy in motion.
moving objects can perform work by imparting motion to other matter
EX: a pool player uses cue sticks to push cue balls; water gushing through a dam turns turbines
Thermal Energy
kinetic energy associated w/ random movement of atoms or molecules.
heat
is TE from one object to another
Photosynthesis uses light as a type of energy to perform work in plants.
Potential Energy
it is energy that matter possesses b/c of its location or structure.
Molecules possess energy due to arrangement of electrons in the bond b/w their atoms.
EX: water behind dam; possess energy b/c its above sea level.
Chemical Energy
Potential Energy available for release in a chemical rxn.
during catabolic rxns, bonds are broken or formed, release energy & result in lower-energy breakdown products.
EX: food molecules w/ oxygen provide chemical energy in biological systems, & produce CO2 & H2O as waste products.
The Laws of Energy Transformation
The First Law of Thermodynamics
Energy is constant.
Energy can be transferred & transformed, but never created or destroyed
EX:
by converting sunlight to chem energy, a plant is an energy transformer, not a producer.
The Second Law of Thermodynamics
most energy is converted to thermal energy then released as heat.
a system can put energy to work only when there's a temp difference resulting in thermal energy from a warmer to cooler location.
Entropy
is a measure of molecular disorder; randomness.
The more randomly arranged a collection of matter is, the greater the entropy.
Every energy transfer or transformation increases the entropy of the universe
Spontaneous Process
"Energetically favorable"
A given process, leads to an increase in entropy, can proceed w/o requiring input of energy.
EX
: explosion (instantaneous), or rusting of an old car (much slower sp)
Nonspontaneous Process
leads to a decrease in entropy and will only happen if energy is supplied.
Ex: machine pumps the water against gravity.
usage of energy means that a np also leads to an increase in entropy of universe as a whole.
Biological Order and Disorder
cells create ordered structures from less organized materials.
In organismal levels, complex structures that result from biological processes that use simpler starting materials
organisms take in organized forms of matter & energy from the surrounding & replaces them w/less ordered forms.
Thermodynamics
is the study of energy transformations that occur in a collection of matter.
isolated system
such as liquid in a thermos bottle, unable to exchange energy or matter outside of the thermos.
Open system
, energy & matter CAN be transferred b/w system & its surroundings;
EX:
Organisms
Concept 8.2 The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously
Free-Energy Change, △G
Gibbs free energy of a system ; referred to free energy
Free energy is portion of a system's energy that can perform work when temp & pressure are uniform throughout system, as in a
living cell
△G = △H - T△S ; H is
enthalpy
, S is
system's entropy
; T is temperature in Kelvin; △ is change
for △G to be negative, enthalpy must be negative (decrease in H) or T△S is positive (system gives up order and S increases) or both.
G has a negative value for all spontaneous processes.
spontaneous processes decrease a system's free energy, & process that are positive (or zero G) are nonspontaneous.
Free Energy, Stability, and Equilibrium
△G = G(
final state
) - G(
init. state
)
G can only be negative when energy is lost during the change from initial to final state.
system's instability- it's tendency to change to a more stable state.
unstable systems (higher G) change in such a way they become stable (lower G);
EX:
a diver is most likely to fall from a platform than floating in water.
as a rxn proceeds toward equilibrium, free energy of the mixture of reactants and products decreases.
a spontaneous process can perform work only when it is moving toward equilibrium; non spontaneous is away from equilibrium.
Free Energy & Metabolism
Exergonic and Endergonic Reactions in Metabolism
Exergonic Rxn
with a net release of free energy
△G is (-) for an exergonic rxn.
occur spontaneously (energetically favorable); rapid
the greater the decrease in free energy, the greater amt of work done.
"downhill" releasing energy
catabolic
Endergonic Rxn
absorbs free energy from its surroundings
stores
free energy in molecules (G increases) △G is positive
nonspontaneous; g is the quantity of energy required to drive the rxn.
"uphill" using energy
anabolic
Equilibrium and Metabolism
chemical rxns of metabolism are reversible & would reach equilibrium if they occurred in isolation of a test tube.
the fact that metabolism as a whole is never at equilibrium is one of the defining feature of life
cells are never in equilibrium; constant flow of materials in & out cause metabolic pathways from reaching equilibrium.
Ex: cellular respiration; reversible reactions prevent equilibrium by creating products then become reactants then wastes are expelled from the cell.
Concept 9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules
Oxidation of Pyruvate to Acetyl CoA
This step is carried out by a multienzyme complex that catalyzes 3 rxns:
1) Pyruvate's carboxyl grp, given off as CO2
2) 2-Carbon Fragment is oxidized & electrons transferred to NAD+, storing energy in the form of NADH
3) CoA (sulfur cmpd derived from a B vitamin) is attached via its sulfur atom ro the two-carbon intermediate, forming Acetyl CoA.
The Citric Acid Cycle
(Krebs Cycle)
1) two carbons enter in the form of an acetyl group
Steps 3 & 4: Two different carbons leave in the completely oxidized form of CO2 molecules
Step 1) the acetyl group of acetyl CoA joins the cycle by combining oxaloacetate, forming citrate.
Steps 3,4, & 8: for each acetyl group that enter the cycle, 3 NAD+ are reduced to NADH
Step 6: Electrons are transferred to FAD, which accepts 2 electrons & 2 protons to become FADH2.
Step 5: rxn produces GTP molecule by substrate-level phosphorylation. GTP is similar to ATP in structure & function.
Step 5: in cells of plants, bacteria, & some animal tissues, step 5 forms an aTP molecule directly by substrate-level phosphorylation.
The output from
Step 5
represents the only ATP generated during the citric acid cycle.
The total yield per glucose from the citric acid cycle turns out to be
6
NADH,
2
FADH, and the equivalent of 2 ATP.
Location: Mitochondrial Matrix; No O2 required.
Krebs Cycle Reactants and Products
Reactants: 1) 2 Acetyl CoA 2) 6 NAD+ from ETC 3) 2 ADP + 2P 4) 2 FAD from ETC.
Products: 1) 4 CO2 2) 6 NADH go to ETC 3) 2 FADH2 got to ETC 4) 2
ATP
Concept 9.4 During oxidative phosphorylation chemiosmosis couples electron transport to ATP synthesis
The Pathway of Electron Transport
ETC is located in the inner membrane of the mitochondrion in eukaryotic cells.
The folding of the inner membrane to form cristae increases SA, providing space for 1000s of each component of ETC in a mitochondrion.
most components are made of proteins & exist in complex I to IV.
prosthetic groups
are nonproteins bound to proteins such as cofactors and coenzymes essential for catalytic function of certain enzymes.
Each component becomes reduced when it accepts electrons from its "uphill' neighbor, which is least electronegative.
then returns to oxidized form as it passes electrons to its "downhill" electronegative neighbor.
Electrons from glucose by NAD+ during glycolysis & citric acid are transferred from NADH ETC complex
I
; flavoprotein
Ubiquinone
Most electron carries b/w ubiquinone & oxygen are protein
cytochromes
the last cytochrome passes its electrons to oxygen; oxygen atoms pick up hydrogen and form water.2
FADH2
adds electrons from complex II; lower than NADH
Location: Inner mitochondrial membrane;
Aerobic
ETC Reactants and Products: 10 NADH = H2O; 2FADH2 = 32 or 34ATP, O2 = NAD+; H+ = FAD; ADP, P
Chemiosmosis: The Energy-Coupling Mechanism
ATP Synthase
protein; enzyme that makes ATP from ADP & inorganic phosphate; lie in the inner membrane of mitochondrion; prokaryote plasma membrane.
works like ion pump
uses the energy of an existing ion gradient to power ATP synthesis.
Chemiosmosis
flow of H+ across a membrane
ATP synthase are the sites that provide a route through membrane for H+.
use exergonic flow of H+ to drive phosphorylation of ADP.
H+ gradient couples redox rxns of ETC to ATP synthesis.
made of multiple polypeptides
The chain is an energy converter that uses exergonic flow of electrons from NADH & FADH2 to pump H+ across the membrane, from mitochondrial MATRIX into the inter membrane space.
Proton-motive force
emphasizing the capacity of the gradient to perform work.
the force drives H+ back across membrane thru the H+ channels provided by ATP synthases.
chemisosmosis is energy-coupling mech. that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work.
An Accounting of ATP by Cellular Respiration
Based on experimental date, the most accurate # is 4H+
a single molecule of NADH generates enough proton-motive force for synthesis of 2.5 ATP.
ATP yield varies slightly depending on the type of shuttle used to transport electrons from the cytology into mitochondrion.
the mitochondrial inner membrane is impermeable to NADH, NADH in cytosol is segregated from oxidative phosphorylation.
The electrons are passed to NAD+ or to FAD in mitochondrial matrix depending on cell type.
Ex: Electrons --> FAD results to about 1.5 ATP; Electrons -->NAD+, yield about 2.5 ATP per NADH.
Use of Proton-motive force may reduce yielding of ATP
Ex: PMF powers the mitochondrion's uptake of pyruvate from the cytosol.
34% of the potential chemical energy in glucose has been transferred to ATP.
Concept 9.5 Fermentation & anaerobic respiration enable cells to produce ATP without the use of oxygen
Types of Fermentation
fermentations includes glycolysis + reactions that regenerate NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate.
Alcohol Fermentation
pyruvate is converted to ethanol in two steps.
1) CO2 released from pyruvate; converted to the two-carbon cmpd acetaldehyde.
2) Acetaldehyde is reduced by NADH to ethanol; this regenerates the supply of NAD+ needed for glycolysis continuation.
Ex: Yeast, in addition to aerobic respiration, carries out alcohol fermentation.
Lactic Acid Fermentation
pyruvate is directly reduced by NADH to form lactate as an end product, regeneration NAD+ w. no release of CO2.
EX: human muscle cells make ATP by lactic acid fermentation when O2 is scarce; occurs when sugar catabolism for ATP production outpaces muscle's supply of O2 from the blood.
Used in the dairy industry by certain fungi & bacteria to make cheese and yogurt.
Comparing Fermentation with Anaerobic & Aerobic Respiration
All involve glycolysis to oxidize glucose & other fuels to pyruvate, w a net prod. of 2 ATP by substrate-level phosphorylation.
NAD+ is the oxidizing agent that accepts electrons from four during glycolysis.
Key Difference on mechanisms for oxidizing NADH back to NAD+, required to sustain glycolysis.
In Fermentation:
final electron acceptor is an organic molecule, pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation).
In Cellular Respiration:
Electrons carried by NADH are transferred to an ETC, which generates the NAD+ for glycolysis.
Another major difference
Amount of ATP produced.
Fermentation yields 2 ATP molecules, produced by substrate-level phosphorylation.
Energy stored in pyruvate is absent.
Cellular Respiration: chemical energy shuttled by NADH & FADH2 formed in electrons to the ETC.
pyruvate is completely oxidized in mitochondrion.
series of redox reactions until it gets to final electron acceptor.
Aerobic: Oxygen; yields up to 32 molecules of ATP per glucose molecule- 16 times as much as fermentation.
Anaerobic: Other molecule that is electronegative; not oxygen.
obligate anaerobes
carry out only fermentation or anaerobic respiration; cannot survey in oxygen.
facultative anaerobes
make enough ATP to survive using either fermentation or respiration. Ex: Muscle Cells
The Evolutionary Significance of Glycolysis
prokaryotes have used glycolysis to make ATP before the presence of oxygen.
Cyanobacteria produced O2 as a by-product of photosynthesis.
Fermentation is a way of harvesting chemical energy w/o the uses of oxygen or any electron transport chain (w/o cellular respiration); consists of glycolysis.
Concept 9.1 Catabolic pathways yield energy by oxidizing organic fuels
Catabolic Pathways & Production of ATP
fermentation
partial degradation of sugars or other organic fuel w/o the use of oxygen.
catabolic process
aerobic respiration
most efficient catabolic pathway
oxygen is consumed as a reactant along w/the organic fuel
most eukaryotic & many prokaryotic carry out aerobic respiration.
anaerobic respiration
prokaryotes use substances other than oxygen as reactants in similar process that harvest chem energy w/o oxygen
Cellular Respiration
includes both aerobic & anaerobic processes.
organic cmpds + O2 --> CO2 + water + Energy
breakdown of glucose is exergonic, having free-energy; happen spontaneously, w/o energy input
Redox Reactions: Oxidation and Reduction
The Principle of Redox
electron transfers(known as oxidation-reduction rxns or redox rxns)
oxidation
the loss of electrons
reduction
gain of electrons
adding negatively charged electrons to an atom reduces the amount of positive charge of that atom.
reducing agent
electron donor (oxidized electron)
oxidizing agent
electron acceptor (removing/gaining electron)
Not all redox reactions require complete transfer of electrons; Ex: Methane (CH4) combustion - covalent bonding b/w H+ & C- are about equally electronegative & same affinity; methane is oxidized b/c it lost shared electrons b/c O2 is very electronegative.
O2 is one of the most powerful oxidizing agents causing more energy to pull an electron off; releasing chemical energy.
Oxidation of Organic Fuel Molecules During Cellular Respiration
In cellular respiration, fuel (glucose) is oxidized & oxygen is reduced.
the electrons lose potential energy along the way, & energy is released.
the more hydrogen bonds, the more energy they contain; hydrogenous transferred from glucose to oxygen.
the energy state changes as hydrogen is transferred to oxygen.
Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
Glucose is broken down in a series of steps, each one catalyzed by an enzyme; at key steps, electrons are leaving from glucose;
hydrogen is then passed to coenzyme
nicotinamide adenine dinucleotide
the coenzyme is an electron carrier b/c it can cycle its oxidized form NAD+ , and its reduced form NADH.
NAD+ functions as an oxidizing agent during respiration.
Dehydrogenases remove a pair of H+ atoms from the substrate (glucose), therefore oxidizing it; it delivers 2 electrons & 1 proton to NAD+, form NADH.
Each NADH represents stored energy; can be tapped to make ATP when electrons complete the steps from NADH to oxygen.
Electron Transport Chain
molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells (and PM of some prokaryotes)
electrons removed from glucose are shuttled by NADH to "top" higher-energy end of the chain.
At the "bottom" lower-energy end, O2 captures electrons along w/ H+, forming water.
electron transfer from NADH to O2 is exergonic.
electrons cascade in a series of redox rxns, losing energy as they reach O2 (electron acceptor)
e- transferred from sugar to NAD+, reduced to NADH, fall down energy gradient in ETC, to electronegative O2 atom (more stable location).
The Stages of Cellular Respiration: A Preview
1) Glycolysis
occurs in cytosol
begins degradation process glucose --> 2 molecules of pyruvate
in eukaryotes, pyruvate enters mitochondrion & is oxidized to acetyl CoA which enters
citric acid cycle
2) Pyruvate Oxidation & Citric Acid Cycle
glucose --> CO2 is broken down & completed
takes part in cytosol for prokaryotes
CO2 produced by respiration contains oxidized organic molecules.
3) Oxidative Phosphorylation
ETC accepts electrons from NADH or FADH2, generated during first two stages.
at the end of ea. chain, electrons are combined w/ molecular oxygen & H+ to form water.
oxidative phosphorylation
ATP synthesis is powered by the redox rxns of the ETC.
inner membrane of mitochondrion in eukaryotes is site of electron transport and chemiosmosis, making OP.
accounts for 90% of ATP generated by respiration.
substrate-level phosphorylation
known as ATP synthesis in glucose & citric acid cycle (sml amount of ATP made)
occurs when an enzyme transfers a phosphate group from substrate molecule to ADP, rather than adding inorganic phosphate to ADP in OP.
Concept 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis can be divided into two phases:
Energy Investment Phase
The cell spends ATP.
Energy Payoff Phase
investment repaid.
ATP is produced by substrate-level phosphorylation & NAD+ is reduced to NADH by electrons released from oxidation of glucose.
Net Energy from glycolysis PER glucose molecule = 2 ATP plus 2 NADH
no carbon is released during stage.
occurs whether or not O2 is present.
if O2 is present, it'll be extracted by pyruvate oxidation, the citric acid cycle, & OP.
Glycolysis Reactants and Products: 1 Glucose --> 2 ATP; 2 NAD+ from ETS --> 2 Pyruvic Acid; 2 ADP+2P--> 2H+
Location: Occurs in cytoplasm of cell near mitochondria; no O2 required.
Concept 9.6 Glycolysis and the citric acid cycle connect to many other metabolic pathways
The Versatility of Catabolism
Most calories are from other fuels such as fats, proteins, and carbs such as sucrose, disaccharides, and starch, a polysaccharide- all these molecules can be used by cellular respiration to make ATP.
Glycolysis can accept a wide range of carbs for catabolism.
Proteins, must be digested to their constituent amino acids.
deamination
: process when amino acids must be removed so they can feed into glycolysis or the citric acid cycle.
Most of the energy of a fat is stored in the fatty acids.
Beta oxidation
breaks the fatty acids down to two-carbon fragments which enter the citric acid cycle as acetyl CoA.
NADH & FADH2- generated during beta oxidation; enter ETC, leading to further ATP production.
Biosynthesis (Anabolic Pathways)
Food must also provide carbon skeletons that cells require to make their own molecules.
Ex: amino acids from the hydrolysis of proteins in food may be incorporated into organism's own protein.
compounds in glycolysis/krebs cycle can divert into anabolic pathways from which the cell can synthesize the molecules it requires.
Glucose can be made from pyruvate; fatty acids from acetyl CoA; Anabolic or Biosynthetic do not generate ATP, but is is consumed.
Regulation of Cellular Respiration via Feedback Mechanisms
Feedback inhibition
The end product of the anabolic pathway inhibits the enzyme that catalyzes an early step of the pathway.
The Cell controls its catabolism.
If the cell works hard, ATP concentration will drop, and respiration speeds up.
Plenty of ATP slows down respiration, sparing valuable organic molecules for other functions.
Control is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway.
Ex: Phosphofructokinase, enzyme that catalyzes step 2 of glycolysis.
First step commits the substrate irreversibly to the glycolytic pathway.
The cell can speed up or slow down the catabolic process.
Phosphofructokinase can be considered the pacemaker of respiration.
Phosphofructokinase is an allosteric enzyme w/ receptor sites specific inhibitors & activators.
Inhibited by ATP & stimulated by AMP (cell is derived from ADP)