Textbook Chapter 15: Metabolism-Basic Concepts and Design

Question 1

As discussed, digestion renders the macromolecules in our meals into biochemically more manageable fragments. Proteins are hydrolyzed to the 20 amino acids, polysaccharides are hydrolyzed to simple sugars such as glucose, and fats are hydrolyzed to fatty acids. This stage is strictly ……..

Answer 1

• a preparation stage; no useful energy is captured at this point

Question 2

The generation of energy from the oxidation of food takes place in three stages (3):

Answer 2

1. In the first stage (top panel), large molecules in food are broken down into smaller units
2. In the second stage (middle panel), these numerous small molecules are degraded to a few simple units that play a central role in metabolism. In fact, most of them—sugars, fatty acids, glycerol, and several amino acids—are converted into acetyl CoA the activated two-carbon unit that is the fuel for the final stages of aerobic metabolism. Some adenosine triphosphate (ATP) is generated in the second stage, but the amount is small compared with that obtained in the third stage.
3. In the third stage (bottom panel), ATP is produced from the complete oxidation of acetyl CoA. The third stage consists of the citric acid cycle and oxidative phosphorylation, which are the final common pathways in the oxidation of fuel molecules.

Question 3

——– brings the breakdown products of proteins, sugars, and fats into the citric acid cycle [also called the tricarboxylic acid (TCA) cycle or Krebs cycle], where they are completely oxidized to ——

Answer 3

• Acetyl Co-A
• CO2

Question 4

Living organisms require a continual input of free energy for three major purposes:

Answer 4

(1) the performance of mechanical work in muscle contraction and cellular movements
(2) the active transport of molecules and ions
(3) the synthesis of macromolecules and other biomolecules from simple precursors.

Question 5

phototrophs

Answer 5

Photosynthetic organisms

Question 6

chemotrophs

Answer 6

obtain energy through the oxidation of carbon fuels

Question 7

Large molecules are constructed step by step in a series of ……

Answer 7

linked reactions called metabolic pathways

Question 8

The enzymes involved in metabolism are organized into
large complexes. The formation of metabolic enzymes into complexes (2):

Answer 8

• This organization allows for faster transfer of substrates and products between the individual enzymes within the complex. Instead of diffusing freely in the cellular environment, substrates are channeled directly from one enzyme to another, reducing the chance of side reactions and increasing the overall speed of the metabolic process.
• Some metabolic pathways involve the production or consumption of unstable or toxic intermediates. By forming complexes, cells can compartmentalize these reactions and efficiently process these intermediates without causing harm to the cell. The close proximity of enzymes within the complex allows for rapid conversion of these intermediates into more stable or less toxic forms.

Question 9

Metabolism

Answer 9

a linked series of chemical reactions that begins with a particular biomolecule and converts it into some other required biomolecule in a carefully defined fashion

Question 10

intermediary metabolism

Answer 10

These pathways are interdependent—a biochemical ecosystem—and their activities are coordinated by exquisitely sensitive means of communication in which allosteric enzymes are predominant.

Question 11

Glucose is metabolized to pyruvate in 10
linked reactions. Under anaerobic conditions, pyruvate is —– and, under aerobic conditions, to —. The glucose-derived carbon atoms of acetyl CoA are subsequently oxidized to —–

Answer 11

• metabolized to lactate
• acetyl CoA
• CO2

Question 12

catabolism

Answer 12

Those reactions that transform fuels into cellular energy are called catabolic reactions

Question 13

anabolism

Answer 13

Those reactions that require energy—such as the synthesis of glucose, fats, or DNA

Question 14

amphibolic pathways

Answer 14

When pathways can be either anabolic or catabolic, depending on the energy conditions in the cell

Question 15

A pathway must satisfy two criteria minimally:

Answer 15

(1) the individual reactions must be specific (the pathway functions as intended and doesn’t involve unwanted side reactions)
(2) the entire set of reactions that constitute the pathway must be thermodynamically favored

Question 16

A reaction that is specific will yield only —— or —— from its reactants. For example, ——

Answer 16

• one particular product
• set of products
• glucose can undergo step-by-step conversion to yield carbon dioxide and water as well as useful energy.

Question 17

A reaction can take place spontaneously only if the change in free energy, is —-

Answer 17

negative

Question 18

The ΔG of a reaction depends on (2)

Answer 18

• the nature of the reactants and products (expressed by the term ΔG°’, the standard free-energy change)
• Their concentrations (expressed by the second term).

Question 19

An important thermodynamic fact is that the overall free energy change for a chemically coupled series of reactions is equal to ——-

Answer 19

• the sum of the free-energy changes of the individual steps
• This principle is based on the fact that Gibbs free energy is a state function, meaning it depends only on the initial and final states of the system and not on the path taken to reach those states. Therefore, the overall change in Gibbs free energy for a series of coupled reactions can be calculated by summing the ΔG values for each individual reaction.

Question 20

Metabolic pathways are formed by the —— such that the overall free energy of the pathway is negative

Answer 20

• coupling of enzyme-catalyzed reactions

Question 21

Part of the free energy derived from the oxidation of carbon fuels and from light is transformed into this readily available molecule

Answer 21

adenosine triphosphate (ATP)

Question 22

Most of catabolism consists of reactions that …

Answer 22

extract energy from fuels such as carbohydrates and fats and convert it into ATP

Question 23

ATP consist of

Answer 23

is a nucleotide consisting of adenine, a ribose, and a triphosphate unit

Question 24

ATP is an energy-rich molecule because …

Answer 24

its triphosphate unit contains two phosphoanhydride linkages. Phosphoanhydride linkages are formed between two phosphoryl groups accompanied by the loss of a molecule of water. A large amount of free energy is liberated when ATP is hydrolyzed to adenosine diphosphate (ADP) and
orthophosphate (Pi) or when is hydrolyzed to
adenosine monophosphate (AMP) and pyrophosphate (PPi)

Question 25

The precise ΔG°' for these reactions (hydrolysis) depends on the (2)

Answer 25

- ionic strength of the medium and on the concentrations of Mg2+ and other metal ions in the medium

Question 26

endergonic chemical reaction

Answer 26

a reaction that would not occur without an input of free energy

Question 27

An otherwise unfavorable reaction can be made possible by

Answer 27

coupling to ATP hydrolysis

Question 28

In the cell, the hydrolysis of an ATP molecule in a coupled reaction changes the --------. Thus, a thermodynamically unfavorable reaction sequence can be converted into a favorable one by coupling it to the hydrolysis of ATP molecules in a new reaction.

Answer 28

- equilibrium ratio of products to reactants by a very large factor.

Question 29

phosphoryl-transfer potential

Answer 29

- refers to the energy released or required when a phosphate group is transferred from one molecule to another. This potential is often quantified by the standard free energy change (ΔG°) associated with the transfer of a phosphate group. - a means of comparing the tendency of organic molecules to transfer a phosphoryl group to an acceptor molecule

Question 30

The high phosphoryl-transfer potential of can be explained by features of the ATP structure, explain (4):

Answer 30

1. At PH 7 the triphosphate unit of ATP carries about four negative charges. These charges repel one another because they are in close proximity. The repulsion between them is reduced when ATP is hydrolyzed. 2. Orthophosphate (Pi) one of the products of ATP hydrolysis, has greater resonance stabilization than do any of the phosphates in ATP. Orthophosphate has a number of resonance forms of similar energy, whereas the gamma phosphoryl group of ATP has a smaller number. 3. The entropy of the products is greater in that there are now two molecules instead of a single ATP molecule. 4. Stabilization Due to Hydration. Water binds to ADP and Pi stabilizing these molecules and thereby rendering the reverse reaction, the synthesis of ATP less favorable.

Question 31

Explain why ATP has a central position in phosphoryl-transfer reactions.

Answer 31

Question 32

Creatine phosphate

Answer 32

A high-phosphoryltransfer-potential molecule in vertebrate muscle, serves as a reservoir of high-potential phosphoryl groups that can be readily transferred to ADP. This reaction is catalyzed by creatine kinase.

Question 33

After the creatine phosphate pool is depleted, ATP must be generated through:

Answer 33

metabolism

Question 34

How is it that phosphate came to play such a prominent role in biology (3)?

Answer 34

1. phosphate esters have the important property of being thermodynamically unstable while being kinetically stable 2. the stability of phosphate esters is due to the negative charges that make them resistant to hydrolysis in the absence of enzymes 3. because phosphate esters are so kinetically stable, they make ideal regulatory molecules, added to proteins by kinases and removed only by phosphatases.

Question 35

Hydrotropes

Answer 35

- amphipathic molecules, with a hydrophobic component and a hydrophilic component, but unlike fatty acids or lipids, the hydrophobic component is too small to self aggregate

Question 36

The generation of ATP is one of the primary roles of.....

Answer 36

catabolism

Question 37

The carbon in fuel molecules—such as glucose and fats—is oxidized to -------- and the energy released is used to regenerate -------- from ------ and ------

Answer 37

- CO2 - ATP - ADP/ Pi

Question 38

All oxidation reactions include the loss of electrons from the molecule being oxidized—----- in this case—and the gaining of those electrons by some other molecule, a process termed reduction

Answer 38

- carbon fuels

Question 39

In aerobic organisms, the ultimate electron acceptor in the oxidation of carbon is ---- and the oxidation product is -----

Answer 39

- O2 - CO2

Question 40

The more reduced a carbon is to begin with, the ------- is released by its oxidation

Answer 40

more free energy

Question 41

When a fuel is oxidized, the oxidation takes place ......

Answer 41

one carbon atom at a time

Question 42

Fats are a more efficient fuel source than carbohydrates such as glucose because -------

Answer 42

the carbon in fats is more reduced

Question 43

activated carriers

Answer 43

- Molecule carrying a group that can be readily donated to other molecules

Question 44

What may activated carriers carry (3)?

Answer 44

- Phosphoryl groups: ATP - Electrons for fuel oxidation: Nicotinamide adenine dinucleotide (NADH), flavin adenine dinucleotide (FADH2) - Two carbon units: acetyl-coenzyme A (aceytl-CoA)

Question 45

Nicotinamide adenine dinucleotide NAD+ (4): ## Footnote what it is made of+ what it is known as+ reactive part?+ what happens in oxidation?

Answer 45

- a pyridine nucleotide - a major electron carrier in the oxidation of fuel molecules - The reactive part of NAD+ is its nicotinamide ring, a pyridine derivative synthesized from the vitamin niacin - In the oxidation of a substrate, the nicotinamide ring of NAD+ accepts a hydrogen ion and two electrons

Question 46

Flavin adenine dinucleotide (FADH2) (2) | Reactive part+accept

Answer 46

- The reactive part of is its isoalloxazine ring, a derivative of the vitamin riboflavin - Can accept two electrons. In doing so takes up two protons

Question 47

NADPH is used almost exclusively for -----, whereas NADH is used primarily for the ------. The extra phosphoryl group on NADPH is a tag that enables enzymes to distinguish between high-potential electrons to be used in anabolism and those to be used in catabolism

Answer 47

- reductive biosyntheses - generation of ATP

Question 48

Coenzyme A another central molecule in metabolism, is a carrier of ----

Answer 48

acyl groups

Question 49

Coenzyme A reactive site

Answer 49

- The terminal sulfhydryl group in CoA is the reactive site, where other molecules attach themselves to -

Question 50

Acyl Group

Answer 50

Question 51

Acyl groups are linked to the sulfhydryl group of CoA by

Answer 51

thioester bonds

Question 52

Acetyl CoAhas a high acetyl-group-transfer potential because transfer of the acetyl group is -----.

Answer 52

exergonic

Question 53

The hydrolysis of a thioester is thermodynamically more favorable than that of an oxygen ester, such as those in fatty acids, because :

Answer 53

the electrons of the C=O bond form less stable resonance structures with the C-S bond than with the C-O bond.

Question 54

Metabolism is regulated through control of (3)

Answer 54

(1) the amounts of enzymes (2) their catalytic activities (3) the accessibility of substrates

Question 55

A high-energy charge .....

Answer 55

inhibits ATP-generating (catabolic) pathways because the cell has sufficient levels of ATP

Question 56

ATP-utilizing (anabolic) pathways, on the other hand, are stimulated by a high-energy charge because -----

Answer 56

ATP is available