Chapter 4: Energy, Enzymes, and Cellular Metabolism

Question 1

What is bioenergetics?

Answer 1

The flow of energy in living systems such as the conversion of light energy into glucose by plants.
Also obeys Laws of thermodynamics:

Question 2

What forms of work can energy carry out?

Answer 2

Chemical work

• Making and breaking of chemical bonds

Transport work

• Moving ions, molecules
• Concentration gradients

Mechanical work

• Moving organelles, changing cell shape, beating flagella and cilia
• Contracting muscles

Question 3

Explain Activation energy.

Answer 3

Energy required for the reactants to react.
Most molecules lack energy for a reaction.

• Heat increases the likelihood or rate of reaction
• But heat has some negative effects on cells
• Catalysts help reactions occur at lower temperatures

Question 4

What are Enzymes and what do they do?

Answer 4

A class of proteins that serve as catalysts
 Chemicals that:

• Increase the rate of a reaction
• Are not changed by the reaction
• Do not change nature of reaction

Question 5

Describe the mechanisms of enzymes.

Answer 5

• The function of an enzyme is determined by its structure
• Each enzyme has a characteristic 3D shape or conformation, with pockets that serve as active sites in the enzyme
• The reactants are called substrates, and they fit into this specific active site like a key to a lock

Question 6

How do Substrates interact with enzymes?

Answer 6

When a substrate binds to the active site of an enzyme, it forms temporary bonds, weakening the original bonds of the substrate

New bonds are formed between substrates as they are brought close together by the enzyme

• Bonding of enzyme to substrates forms a temporary enzyme-substrate complex
• This breaks to yield the products of the reaction
• The amount of enzyme in a sample determines the rate of product produced

Question 7

How are names for enzymes created?

Answer 7

• The first enzymes were given arbitrary names
• An international committee decided all enzyme names end with the suffix –ase
• Names apply to the function of the enzyme
• Phosphatases remove phosphate groups
• Synthetases catalyze dehydration synthesis
• Enzymes are often produced in an inactive form and activated when needed such as
• Pepsinogen -pepsin

Question 8

How is enzyme acticity measured and what influences it?

Answer 8

1. Measured by the rate at which substrate is converted to product
2. Influenced by:

• Temperature
• pH
• Concentration
• Cofactors and coenzymes
• Stimulatory or inhibitory substances

Question 9

What are the effects of temperature on enzymes?

Answer 9

Temperature will increase the rate of reaction to a point and then the enzyme will be denatured

Question 10

What are the effects of pH on enzymes?

Answer 10

Enzymes exhibit peak activity within a narrow pH range = pH optimum

• Due to changes in enzyme conformation
• Optimum pH reflects environment enzyme is found

Question 11

What are coenzymes?

Answer 11

• Most enzymes need additional small molecules to aid in a reaction
• Coenzymes are organic molecules derived from water-soluble vitamins
  Ex: NAD from vitamin niacin (B3)
• Coenzymes transport hydrogen atoms and other small molecules between enzymes
• Cofactors are metal ions such as: Ca2+ Mg2+ Mn2+ Cu2+ Zn2+
• Cofactors help form the active site or enzyme-substrate binding

Question 12

Explain the effects of substrate concentration on reaction rate.

Answer 12

As the substrate concentration increases, so will the rate of reaction until the enzyme is saturated = every enzyme is busy

Question 13

What are metabolic pathways?

Answer 13

• Metabolic pathways are series of chemical reactions occurring within a cell. In each pathway, a principal chemical is modified by a series of chemical reactions. Enzymes catalyze these reactions, and often require dietary minerals, vitamins, and other cofactors in order to function properly.
• Most reactions are linked together in a chain called a metabolic pathway

1. Begin with an initial substrate
2. End with a final product
3. Many enzymatic steps along the way

Question 14

How do cells regulate their metabolic pathways?

Answer 14

• Controlling enzyme concentrations
• Chemicals that change reaction rates
• Using different enzymes to catalyze reversible reactions
• Compartmentalizing enzymes within organelles
• Maintaining optimum ratio of ATP to ADP

Question 15

What are reversible reactions?

Answer 15

• A reversible reaction is a chemical reaction where the reactants form products that, in turn, react together to give the reactants back.
• Sometimes a single enzyme can drive a reaction in two directions, depending on the concentration of substrate vs product
• When one gets higher, the reaction reverses

Example: carbonic anhydrase
H2O + CO2 ↔ H2CO3 ↔ HCO3- + H+

Question 16

What are endergonic reactions?

Answer 16

• Chemical reactions that require input of energy
• Products contain more free energy than reactants
  Plants need the energy from light to turn carbon dioxide and water into glucose

Question 17

What are exergonic reactions?

Answer 17

• Chemical reactions that produce energy
• Products have less free energy than reactants
  Breaking glucose down into carbon dioxide and water produces energy

Question 18

What are coupled reactions and how do they work?

Answer 18

• A coupled biochemical reaction is one where the free energy of a thermodynamically favourable reaction (such as the hydrolysis of ATP) is used to ‘drive’ a thermodynamically unfavourable one, by coupling or ‘mechanistically joining’ the two reactions.
• To put it another way, two (or more) reactions may be combined by an enzyme (for example) such that a spontaneous reaction may be made ‘drive’ an unspontaneous one.
• So, Energy from the exergonic break down of food drives endergonic reactions in our bodies
• Energy must be converted into a usable form:
  The ATP molecule stores energy in its bonds

Question 19

What is Oxidation, Reduction, and Redox?

Answer 19

1. Reduction: atom or molecule gains electrons
2. Oxidation: atom or molecule loses electrons
3. Redox: for one molecule to lose an electron, it has to give it to another molecule

• Molecules can be both oxidizers and reducers in a chain reaction where e- are passed along

Question 20

Are free electrons or hydrogen atoms passed along during oxidation/reduction?

Answer 20

Usually, free electrons are not passed along, but hydrogen atoms carrying the electrons are

• A molecule that loses hydrogen is oxidized
• A molecule that gains hydrogen is reduced

Question 21

What are the oxidized/reduced states of NAD and FAD?

Answer 21

• Each NAD+ can accept 2 electrons and bind to 1 proton being reduced to NADH
• Each FAD can accept 2 electrons and bind to 2 protons being reduced to FADH2

Question 22

What are the two reactions of metabolism?

Answer 22

All reactions in the body can be divided into:

1. Anabolism: requires input of energy to synthesize large molecules from smaller ones
2. Catabolism: releases energy stored in bonds by breaking down large molecules into smaller ones

Question 23

How does the catabolism of nutrients work?

Answer 23

• Catabolic reactions that break down nutrients serve as energy sources for synthesis of ATP
  Glucose, fatty acids, and amino acids
• Complete breakdown of glucose requires
  Oxygen as the final electron acceptor, aerobic
  Many steps such as first ones are anaerobic

Question 24

Give a brief overview of energy metabolism

Answer 24

Question 25

What are the stages of cellular respiration?

Answer 25

Cellular respiration consists of:

1. Glycolysis
2. Pyruvate metabolism
3. Citric acid cycle (Krebs)
4. Electron transport

Question 26

Explain the process of glycolysis. What is the input and output?

Answer 26

1. Glucose, 2ATP and 2NADH are the input and 2 pyruvate are the output.
   6 carbon break into two groups of 3 carbon
2. Glycolysis is the first step used to breakdown glucose to produce ATP.

• Glucose, a 6 carbon sugar, is split into two 3 carbon sugars. The 3 carbon sugars are then oxidized and their remaining atoms reaarranged to form two molecules of pyruvate.
• Energy in glucose cannot be readily released unless energy from ATP is added first. In this phase, 2 ATP are added to the reaction, producing a glucose molecule with two phosphate groups. The phosphate groups make glucose less stable and ready for chemical breakdown.
• Investment of energy in prepatory phase is paid back with interest! 4 ATP and 2 NADH molecules are formed and as well as two molecules of pyruvate.

Question 27

Explain the process of the pyruvate metabolism. What are the inputs and outputs?

Answer 27

• 2 pyruvate are the input and 2 acetyl CoA along with 2NADH and 2 Co2 are the outputs.
• The end product of Glycolysis, pyruvate, is transported into the mitochondrion and converted to a compound called acetyl coenzyme A or acetyl CoA. This conversion of pyruvate to acetyl CoA also results in the transfer of electrons to NAD+, storing energy in the form of NADH.

Question 28

Explain the citric acid cycle. What are the inputs and outputs?

Answer 28

1. Input: 2 aceytl CoA, 6 NAD+, 2 FAD and output are: 4 CO2, 2 ATP, 6 NADH + H+, 2 FADH2

• Acetyl CoA binds a four carbon molecule (oxaloacetate) producing a six carbon molecule (citrate).
• Two carbons are removed as carbon dioxide.
• The four carbon starting material is regenerated.
• The Krebs Cycle generates ATP and many energized electrons (in the form of FADH2 and NADH) for the electron transport chain.

Question 29

Explain the Electron transport chain.

Answer 29

• Inputs of ETC: 10 NADH + H+, 2 FADH2
• Outputs of ETC: 10 NAD+,2 FAD, H2O, 34 ATP
• This energy is derived from the oxidation of NADH and FADH2 by the four protein complexes of the electron transport chain (ETC). The ten NADH that enter the electron transport originate from each of the earlier processes of respiration: two from glycolysis, two from the transformation of pyruvate into acetyl-CoA, and six from the citric acid cycle. The two FADH2 originate in the citric acid cycle.
• Further reading: The electron transport chain is a series of redox reactions( Oxidation-Reduction) in which electrons are passed from carrier molecules down the chain to ultimately form ATP. The electron transport chain is an energy converter, transforming the chemical energy to the energy of a H+ gradient. As electrons flow along the electrochemical gradient, some of the energy is used by each complex to pump H+ ions from the mitochondrial matrix to the inter-membrane space. These Hydrogen ions create the electrochemical proton gradient that drives them back across the inner membrane through the ATP synthase complex. ATP synthase uses energy of H+ flow(electrical energy) to synthesize ATP from ADP and P. Oxidation of each NADH + H to NAD+ yields 3 ATP. Remember that oxidation is the loss of an electron and reduction is the gain of an electron. Ultimately the purpose of Cellular respiration is to make ATP. The electron transport chain carries out oxidative phosphorylation, which accounts for most of the ATP generated by cellular respiration. A phosphate group is tacked onto ADP to make ATP, using the energy formed from the electrochemical gradient. This pathway has much detail, much more than I’ve spoke of. I hope this helps. Oh, and the krebs cycle( a simplified version) …During each turn of the cycle, 2 carbon atoms are removed from the substrates as CO2, 4 oxidations by removal of hydrogen atoms occur, producing four molecules of reduced coenzymes( 3 NADH + H+ and 1 FADH2,) and 1 ATP. The reduced coenzymes produced in the Krebs cylcle are the substrates for the Electron transport chain. These are the electron carriers.

Question 30

What is the overall “equation” for cellular respiration?

Answer 30

Inputs of Glycolysis glucose

2 ATP

2 NAD+

4ADP + P

Outputs of Glycolysis 2 pyruvates

4 ATP (2 net)

2 NADH + H+

2 H20

Inputs of Preparatory 2 pyruvate

2 CoA

2 NAD+

Outputs of Preparatory 2 aceytl CoA

2 CO2

2 NADH + H+

Inputs of Kreb 2 aceytl CoA

2 oxaloacetate

2 ADP + P

6 NAD+

2 FAD

Outputs of Kreb 4 CO2

2 ATP

6 NADH + H+

2 FADH2

Inputs of ETC 10 NADH + H+

2 FADH2

Outputs of ETC 10 NAD+

2 FAD

H2O

34 ATP