CHAPTER 9

Question 1

What are the five principles of metabolic pathways?

Answer 1

• 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

Question 2

What are the three catabolic processes that harvest energy from glucose?

Answer 2

• GLYCOLYSIS (anaerobic)
• CELLULAR RESPIRATION (aerobic)
• FERMENTATION (anaerobic)

Question 3

oxidation-reduction (redox)

Answer 3

reaction where one substance transfers electrons to another substance

Question 4

reduction

Answer 4

gain of electrons

Question 5

oxidation

Answer 5

loss of electrons

Question 6

What is the relationship between redox reactions and metabolic pathways?

Answer 6

in glucose metabolism, glucose gets oxidized and O2 gets reduced; the more reduced a molecule is, the more energy it has; in a redox reaction, some energy is transferred from the reducing agent (glucose) to the reduced product

Question 7

NAD+

Answer 7

coenzyme that is a key electron carrier in redox reactions

Question 8

glycolysis

Answer 8

• takes place in the CYTOPLASM
• CONVERTS GLUCOSE into 2 molecules of PYRUVATE
• PRODUCES 2 ATP and 2 NADH
• OCCURS in 10 STEPS
  [STEPS 1-5 REQUIRE ATP (ENERGY-INVESTING REACTIONS)]
  [STEPS 6-10 YILED NADH AND ATP (ENERGY-HARVESTING REACTIONS)]

Question 9

How do oxidation-reduction reactions behave in metabolic pathways?

Answer 9

energy released by glucose oxidation is trapped via the reduction of NAD+ to NADH

Question 10

How does substrate-level phosphorylation behave in metabolic pathways?

Answer 10

energy released transfers a phosphate from the substrate to ADP, forming ATP

Question 11

pyruvate oxidation

Answer 11

• occurs in the MITOCHONDRIAL MATRIX
• PYRUVATE is OXIDIZED to ACETATE and CO2
• ACETATE BINDS to COENZYME A to form ACETYL CoA (it donates its acetyl group to oxaloacetate, forming citrate = initiates the citric acid cycle)
• EXERGONIC; ONE NAD+ is REDUCED to NADH
  catalyzed by pyruvate dehydrogenase complex (three enzymes that catalyze the three intermediate steps in the process)

Question 12

citric acid cycle

Answer 12

• 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

Question 13

What yields when one glucose molecule gets oxidized?

Answer 13

• 6 CO2
• 10 NADH
• 2 FADH2
• 4 ATP

Question 14

oxidative phosphorylation

Answer 14

ATP is synthesized b y reoxidation of electron carriers in the presence of O2
TWO COMPONENTS:
- ELECTRON TRANSPORT
- CHEMIOSMOSIS

Question 15

respiratory chain

Answer 15

where electrons from NADH and FADH2 pass through; electron flow results in a proton concentration gradient across the inner mitochondrial membrane; located in the folded inner mitochondrial membrane; energy is released as electrons are passed between carriers

Question 16

chemiosmosis

Answer 16

electrons flow back across the membrane through a channel protein, ATP synthase, which couples the diffusion with ATP synthesis

Question 17

proton-motive force

Answer 17

protons (H+) are actively transported into the intermembrane space during electron transport which creates a concentration gradient and charge difference = potential energy

Question 18

ATP synthase

Answer 18

the same in all living organisms; molecular motor with TWO parts:
- F0 unit - a transmembrane H+ channel
- F1 unit - projects into the matrix; rotates to expose active sites for ATP synthesis

Question 19

anaerobic respiration

Answer 19

many bacteria and archaea use alternate electron acceptors such as SO4-2, Fe3+, and CO2; this allows them to exist where O2 is scarce or absent

Question 20

Anaerobic glycolysis and fermentation

Answer 20

• occurs in the CYTOPLASM
• GLUCOSE is only PARTIALLY OXIDIZED
• 2 ATP per GLUCOSE are produced by SUBSTRATE-LEVEL PHOSPHORYLATION
• in ALL types, NAD+ is REGENERATED to keep GLYCOLYSIS going

Question 21

lactic acid fermentation

Answer 21

• PYRUVATE is the ELECTRON ACCEPTOR; LACTATE is the PRODUCT
• MICROORGANISMS and some COMPLEX organisms
• LACTATE DEHYDROGENSASE CATALYSES FERMENTATION; in presence of O2, it CATALYZES OXIDATION of LACTATE to PYRUVATE

Question 22

alcoholic fermentation

Answer 22

• YEASTS and some PLANT CELLS
• REQUIRES TWO ENZYMES to METABOLIZE PYRUVATE to ETHANOL
• reactions are REVERSIBLE
• used to produce ALCOHOLIC beverages

Question 23

catabolic interconversions

Answer 23

• POLYSACCHARIDES are HYDROLYZED to GLUCOSE -> enters GLYCOLYSIS
• LIPIDS are BROKEN DOWN to
• glycerol -> DHAP ->
  glycolysis
• fatty acids -> acetyl CoA
  -> citric acid cycle
• PROTEINS are HYDROLYZED to AMINO ACIDS -> glycolysis or citric acid cycle

Question 24

anabolic interconversions

Answer 24

• most CATABOLIC reactions are REVERSIBLE
• GLUCONEOGENESIS
• ACETYL CoA can be used to form FATTY ACIDS

Question 25

gluconeogenesis

Answer 25

citric acid cycle and glycolysis intermediates are reduced to form glucose

Question 26

For what can citric acid cycle intermediates be used?

Answer 26

to synthesize nucleic acid components: - α-ketoglutarate -> purines - Oxaloacetate -> pyrimidines (α-ketoglutarate is also a starting point for synthesis of chlorophyll and the amino acid glutamate)

Question 27

How do cells "decide" which pathways to use?

Answer 27

- levels of substances in the metabolic pool are quite constant - organisms regulate enzymes to maintain balance between catabolism and anabolism

Question 28

What are the mechanisms that regulate rates of each step in a metabolic pathway?

Answer 28

- CHANGE the AMOUNT of ACTIVE ENZYME by REGULATING GENE EXPRESSION - CHANGE ENZYME ACTIVITY by COVALENT modifications, such as PHOSPHORYLATION - feedback INHIBITION by ALLOSTERIC ENZYMES - SUBSTRATE availability

Question 29

substrate availability

Answer 29

if the substrate of a particular enzyme is used up by another pathway, the first enzyme can no longer function and the pathway shuts down

Question 30

What are the main control points in metabolic pathways?

Answer 30

- in GLYCOLYSIS is PHOSPHOFRUCTOKINASE (step 3), which is INHIBITED by ATP - in FERMENTATION, PHOSPHOFRUCTOKINASE operates at a high rate to produce ATP; if O2 is present, more ATP is produced, which inhibits the enzyme and slows glycolysis - in the CITRIC ACID CYCLE is ISOCITRATE DEHYDROGENASE (step 3); it is INHIBITED by NADH and ATP; if too much of either accumulates, the citric acid cycle shuts down - ACETYL CoA is another control point - if ATP levels are high and the citric acid cycle shuts down, the accumulation of citrate activates fatty acid synthesis from acetyl CoA, diverting it to storage - fatty acids may be metabolized later to produce more acetyl CoA