MOLECULAR BIOLOGY: METABOLISM

Question 1

Metabolism consists of two parts:

Answer 1

Catabolism and anabolism.

Question 1

Catabolism is

Answer 1

breaking stuff down for energy. This is the part that the MCAT focuses on.

Question 2

Anabolism is

Answer 2

using energy to build stuff for storage.

Question 3

Another name for metabolism is

Answer 3

cellular respiration.

Question 4

Steps of aerobic metabolism (needs oxygen)

Answer 4

Glycolysis, Oxidative decarboxylation, Krebs cycle, Electron transport chain.

Question 5

Steps of anaerobic metabolism (don’t need oxygen)

Answer 5

Glycolysis, Alcohol or lactic acid fermentation

Question 6

Aerobic metabolism of glucose

Answer 6

Complete oxidation of metabolite (glucose) to carbon dioxide. ~ 36 ATP produced per glucose. C6H12O6 + 6O2 => 6CO2 + 6H2O C6H12O6: this is glucose. You get it from your diet. 6O2: this is molecular oxygen that you breath in. 6CO2: this is carbon dioxide produced by the Krebs cycle. Both the carbon and oxygen in this CO2comes from the metabolite (glucose). 6H2O: this is water produced in the electron transport chain. The oxygen comes completely from the molecular oxygen that you breath in. If we were to follow the carbon in the metabolite (glucose), it will end up in carbon dioxide. If we were to follow the oxygen in the metabolite (glucose), it will end up in carbon dioxide. If we were to follow the oxygen you breath in, it will end up in water. As for the hydrogens, they’ll either be in water, exist as protons in solution, or be transferred to some other entity. As we can see, the total reaction involves complete oxidation of the metabolite (glucose) and complete reduction of molecular oxygen. When electrons pass from the metabolite (glucose) to molecular oxygen, energy is released. The electron transport chain harnesses this energy.

Question 7

Anaerobic metabolism of glucose

Answer 7

Partial oxidation of metabolite (glucose) to pyruvate. 2 net ATP produced per glucose. Pyruvate is then reduced to either alcohol or lactate. Bacteria reduce pyruvate to alcohol in a process called alcohol fermentation. Humans reduce pyruvate to lactate in a process called lactic acid fermentation.

Question 8

Glycolysis, anaerobic and aerobic, substrates and products

Answer 8

Glycolysis = convert glucose (6 carbons) to 2 molecules of pyruvate (3 carbons). Location: cytosol. 2 net ATP made for every glucose (2 input ATP, 4 output ATP). 2 NADH made for every glucose. Occurs under both aerobic and anaerobic conditions. Glycolysis is inhibited by ATP.

Question 9

Aerobic decarboxylation = convert pyruvate (3 carbons) to an acetyl group (2 carbons).

Answer 9

1 NADH made for every pyruvate. Only occurs in the presence of oxygen. Acetyl group attaches to Coenzyme A to make acetyl CoA.

Question 10

Anaerobic fermentation = redox reaction: reduce pyruvate, oxidize NADH.

Answer 10

1 NAD+ made for every pyruvate. Alcohol fermentation = pyruvate reduced to ethanol. Lactic acid fermentation = pyruvate reduced to lactate. The purpose of anaerobic fermentation is to regenerate NAD+, which is needed for glycolysis.

Question 11

Krebs cycle, substrates and products, general features of the pathway

Answer 11

Location: matrix of mitochondria. Acetyl CoA feeds into the cycle. 3 NADH made per acetyl CoA. 1 FADH2 made per acetyl CoA. 1 ATP (GTP) made per acetyl CoA. Coenzyme A is regenerated (during the first step of the cycle). Krebs cycle, TCA, Tricarboxylic acid cycle, citric acid cycle all mean the same thing. Krebs cycle is Inhibited by ATP and NADH.

Question 12

Electron transport chain and oxidative phosphorylation, substrates and products, general features of the pathway

Answer 12

Location: the cristae (inner membrane of mitochondria). Input NADH Proton gradient The electron transport chain (ETC) is essentially a series of redox reactions, where NADH gets oxidized to NAD+ and O2 gets reduced to H2O. The series of redox reactions consists of electrons passing from NADH to FMN, to Coenzyme Q, iron-sulfur complexes, and cytochromes (cytochrome b, c and aa3) before finally being used to reduce oxygen. NADH is highest in energy, while O2 is lowest in energy. When electrons are passed from NADH down a series of proteins and finally to O2, energy is released. FADH2 is lower in energy than NADH, that’s why it releases less energy when it gets oxidized. FADH2 skips FMN and passes its electrons to Coenzyme Q. The energy released from these reactions generates a proton gradient, which drives ATP synthase to make ATP. This is called oxidative phosphorylation.

Question 13

Proton gradient

Answer 13

The energy released from passing electrons down the ETC is used to pump protons into the intermembrane space of the mitochondria. H+ concentration is very high in the intermembrane space (higher than those in the matrix). Thus, this establishes an electrochemical gradient called the proton gradient. H+ wants to migrate down the proton gradient (from the intermembrane space back into the matrix), but it can only do this by going through the ATP synthase. Like a water mill, ATP synthase harnesses the energy of the falling protons to convert ADP into ATP.

Question 14

The ETC is inhibited by

Answer 14

certain antibiotics, by cyanide, azide, and carbon monoxide.

Question 15

Fat metabolism

Answer 15

Location: beta-oxidation occurs in the matrix of the mitochondria. Ester hydrolysis occurs in the cytosol. Fatty esters gets hydrolyzed into free fatty acids by lipases. For example, triacylglycerol gets hydrolyzed into free fatty acids and glycerol. With the help of ATP, the fatty acid is “activated” at the acid end by CoA (to be precise, it turns into a thioester). A process called beta-oxidation breaks down the fatty-CoA, 2 carbons at a time, to make acetyl CoA. β-oxidation produces acetyl CoA and also FADH2 and NADH. The acetyl CoA feeds into the Krebs cycle, and the FADH2 and NADH feed into the ETC. On a per gram basis, fats give more energy than any other food source.

Question 16

Protein metabolism

Answer 16

Proteins are broken down into amino acids by peptidases. The nitrogen in the amino acid is converted to urea (for desert animals, birds and reptiles, it is uric acid). The carbon in the amino acid is converted to pyruvate or acetyl-CoA, (or other metabolical intermediates such as oxaloacetate), depending on what amino acid it is. The carbon products from amino acid metabolisms can either feed to the Krebs cycle, or be the starting material for gluconeogenesis.