CHAPTER 9 Flashcards
how do cells get energy
cells get energy from glucose in a series of metabolic pathways
what are the five principles of metabolic pathways
-Complex transformations occur in a series of separate reactions.
-Each reaction is catalyzed by a specific enzyme.
-Many metabolic pathways are similar in all organisms.
-In eukaryotes, metabolic pathways are compartmentalized in specific organelles.
-Key enzymes can be inhibited or activated to alter the rate of the pathway.
burning/metabolism of glucose
C6H12O6 + 6O2 –> 6CO2 + 6H2O + free energy
ΔG = - 686 kcal/mol
-this is highly exergonic (releasing energy); which drives endergonic formation of many ATP molecules
what are the three catabolic processes that harvest energy from glucose?
-glycolysis (anaerobic)
-cellular respiration (aerobic)
-fermentation (anaerobic)
oxidation-reduction (redox) reactions
-one substance transfers electrons to another substance
-reduction: gain of electrons (oxidizing agent)
-oxidation: loss of electrons (reducing agent)
-always occur together
-partial transfer also happens
what is oxidized/reduced in glucose metabolism
-glucose is the reducing agent meaning it gains and electron
-O2 is the oxidizing agent meaning it losses and electron
-the more reduced a molecule is the more energy it has
transfer of electrons is often associated with what?
-with transfer of hydrogen ions: H = H+ + e-
-when a molecules loses H atom it becomes oxidized
what happens to coenzyme NAD+ in redox reactions
-it is reduced if it is changing from NAD+ to NADH
-it is oxidized if it is changing from NADH to NAD+
In aerobic conditions, _______?
-O2 is available as the final electron acceptor
in anaerobic conditions,_______?
-the pyruvate produced by glycolysis is metabolized by fermentation
glycolysis and cellular respiration
-glycolysis –> pyruvate oxidation –> citric acid cycle –> electron transport/ ATP synthesis –> products are CO2 and H2O
glycolysis and fermentation
-glycolysis –> fermentation –> products are lactate or alcohol
Glycolysis
-takes place in the cytoplasm
-converts glucose into 2 molecules of pyruvate
-produces 2 ATP and 2 NADH
-occurs in 10 steps
what happens in steps 1-5 and steps 6-10 of glycolysis
-steps 1-5 require ATP
-steps 6-10 produces NADH and ATP
two types of reactions occur repeatedly in many metabolic pathways:
-oxidation-reduction: energy released by glucose oxidation is trapped via the reduction of NAD+ to NADH
-substrate-level phosphorylation: energy released transfers a phosphate from the substrate to ADP, forming ATP
pyruvate oxidation
-occurs in the mitochondrial matrix
-pyruvate is oxidized to acetate and CO2
-acetate binds to coenzyme A to form acetyl CoA
-exergonic; one NAD+ is reduced to NADH
what does Acetyl CoA do?
-acetyl CoA donates its acetyl group to oxaloacetate, forming citrate. this initiates the critic acid cycle
citric acid cycle
-acetyl CoA is the starting point
-eight reactions completely oxidizes the acetyl group to 2 molecules of CO2
-energy released is captured by GDP, NAD+, and FAD
-oxaloacetate is regenerated in the last step
citric acid cycle steps (important step 8)
what can GTP do?
-it can transfer its high-energy phosphate to form ATP
overall, the oxidation of one glucose molecules produces what?
-6 CO2
-10 NADH
-2 FADH2
-4 ATP
for the citric acid cycle to continue what must happen?
-the starting molecules (acetyl CoA and oxidized electron carriers) must be replenished
oxidative phosphorylation
-ATP is synthesized by deoxidation of electron carriers in the presence of O2
what are the two components of oxidation phosphorylation?
-electron transport
-chemiosmosis
electron transport
-electrons from NADH and FADH2 pass through the respiratory chain of membrane-associated carriers
-electron flow results in a proton concentration gradient across the inner mitochondrial membrane
-the respiratory chain is located in the folded inner mitochondrial membrane
-energy is released as electrons are passed between carriers
chemiosmosis
-electrons flow back across the membrane through a channel protein, ATP synthase, which couples the diffusion with ATP synthesis
why does the electron transport chain have so many steps?
-a single reaction would release too much free energy all at once which would be damaging to organelles and cells
-in a series of reactions, each releases a small amount of energy that can be captured by an endergonic reaction
what happens to protons during electron transport? what does this create?
-protons (H+) are actively transported into the inter membrane space during electron transport
-this creates a concentration gradient and charge different
proton-motion force
-this is potential energy
-diffusion of protons back across the membrane is coupled to ATP synthesis (chemiosmosis)
what can ATP synthase act as?
-it can also act as ATPase, hydrolyzing ATP to ADP and P
why is ATP synthesis important?
-ATP leaves the metric as soon as it is made, keeping ATP concentration in the matrix low
-H+ gradient is maintained by active transport
what was the first experimental evidence of chemiosmosis?
-it came from studies on isolated chloroplast thylakoid membranes
brown fat cells
-in them, a protein called UPC1 inserts into the mitochondrial membrane and makes it permeable to protons.
-This uncouples electron transport and chemiosmosis, and energy is released as heat rather than being trapped in ATP.
-EX: babies have more brown fat cells so they can regulate their temperature
ATP synthase in all living organisms
-its all the same in all living organisms
what organism use anaerobic respiration? what electron acceptors do they use?
-many bacteria and archaea
-they use alternate electron acceptors such as SO4 -2, Fe3 +, and CO2
-this allows them to exist where O2 is scarce or absent
without O2 how is ATP made?
-made by glycolysis and fermentation
glycolysis and fermentation
-occurs in the cytoplasm
-glucose is only partially oxidized
-2 ATP: 1 glucose are produced by substrate-level phosphorylation
-NAD+ is regenerated to keep glycolysis going
lactic acid fermentation
-pyruvate is the electron acceptor; lactate is the product
-microorganisms and some complex organisms
-lactate dehydrogenase catalyzes fermentation; in presence of O2, it catalyzes oxidation of lactate to pyruvate
other than organisms what also uses lactic acid fermentation?
-muscle cells break down glycogen and cary out lactic acid fermentation if O2 cannot be delivered fast enough for aerobic respiration
-causes muscle pain and lowers pH
alcoholic fermentation
-yeasts and some plant cells
-requires two enzymes to metabolize pyruvate to ethanol
-reactions are reversible
-used to produce alcoholic beverages
energy produced difference between cellular respiration and fermentation
-glycolysis and fermentation: 2ATP; glucose is only partially oxidized in fermentation which means more energy remains in the products than in CO2
-glycolysis plus cellular respiration: 32 ATP
how do metabolic pathways operate?
-there is an interchange of molecules in and out of the pathways all the time
catabolic interconversions
-polysaccharides are hydrolyzed to glucose –> enters glycolysis
-lipids are broken down to
1. glycerol -> DHAP -> glycolysis
2. fatty acids -> acetyl CoA -> citric acid cycle
-proteins are hydrolyzed to amino acids, –> glycolysis or citric acid cycle
anabolic interconversions
-most catabolic reactions are reversible
-gluconeogenesis: citric acid cycle and glycolysis intermediates are reduced to form glucose “going backward”
-acetyl CoA can be used to form fatty acids
what can the critic acid cycle intermediates be used for
-can be used to synthesize nucleic acid components
how do cells “decide” which pathway to use?
-the levels of substances in the metabolic pool are quite constant “balanced”
-organisms regulate enzymes to maintain balance between catabolism and anabolism
mechanisms that regulate rates of each steps in a metabolic pathways:
-regulating gene expression
-phosphorylation
-feedback inhibition by allosteric enzymes
-substrate availability
how is glycolysis and the citric acid cycle regulated?
-allosteric regulation of key enzymes
-a high concentration of the final product an inhibit an enzyme making it slow down to stop
-an excess of product of one pathway can activate an enzyme in another pathway
