Chapter 3: Energy and Cellular Respiration

What is the net ATP yield from one glucose molecule during aerobic cellular respiration, and why is it lower than the theoretical maximum of 38 ATP?

30 net ATP; the difference arises from energy costs to transport NADH and other molecules into mitochondria and to transport ADP and Pi for ATP synthesis.

Where does glycolysis occur and what are the net products per glucose?

Occurs in the cytosol; nets 2 ATP and 2 NADH per glucose and produces 2 pyruvate molecules.

What happens to pyruvate when oxygen is insufficient, and which enzyme catalyzes this reaction?

Pyruvate is reduced to lactate by lactate dehydrogenase to regenerate NAD+, allowing glycolysis to continue.

What theory explains how ATP is synthesized via a proton gradient in mitochondria, and who proposed it?

Chemiosmotic theory proposed by Peter Mitchell; a proton (H+) gradient across the inner mitochondrial membrane drives ATP synthase to convert ADP and Pi into ATP.

How do fatty acids enter cellular respiration, and why must they be oxidized aerobically?

Fatty acids undergo beta-oxidation in mitochondria to form acetyl-CoA, which enters the citric acid cycle; this process requires oxygen (aerobic) because it occurs in mitochondria.

What is the Cori cycle and how is lactate processed during it?

Lactate produced by muscles is transported to the liver, where it is converted to glucose (gluconeogenesis); glucose can then be released into the blood or stored as glycogen, cycling between muscle and liver.

Where does glycolysis occur in the cell?

Cytosol (cytoplasm); does not require oxygen.

Does glycolysis require oxygen?

No; it is anaerobic.

What are the net energy outcomes of glycolysis per glucose?

Net production of 2 ATP and 2 NADH.

What are the end products of glycolysis per glucose molecule?

Two pyruvate molecules, 2 NADH, and a net 2 ATP.

How is the fate of pyruvate determined?

By oxygen availability: with oxygen, pyruvate enters mitochondria for aerobic breakdown; without oxygen, it is converted to lactate.

Where does the intermediate stage occur and what enzyme complex is involved?

In the mitochondrial matrix; catalyzed by the pyruvate dehydrogenase multienzyme complex.

What are the products of the intermediate stage per glucose?

2 NADH, 2 Acetyl CoA, and 2 CO2 (1 CO2 per pyruvate).

Where does the citric acid cycle occur and does it require oxygen?

Mitochondrial matrix; aerobic (requires oxygen).

What are the outputs of one turn of the citric acid cycle?

1 ATP, 3 NADH, 1 FADH2, and 2 CO2.

How many ATP, NADH, and FADH2 are produced by the citric acid cycle per glucose?

2 ATP, 6 NADH, and 2 FADH2 (since the cycle runs twice per glucose).

Where is the electron transport system located and what are its key players?

Located in the inner mitochondrial membrane (cristae); includes H+ pumps, NADH/FADH2 electron carriers (ubiquinone Q and cytochrome c), and ATP synthase.

What is the final electron acceptor in the electron transport system?

Oxygen (O2) is the final electron acceptor, forming water.

What is oxidative phosphorylation?

The process by which ATP is formed as electrons are transferred through the electron transport chain and protons flow back through ATP synthase.

How many ATP molecules are produced per NADH and per FADH2 in the electron transport system?

NADH yields about 3 ATP; FADH2 yields about 2 ATP.

What is the role of ATP synthase?

An enzyme that uses the flow of H+ down its gradient to synthesize ATP from ADP and Pi.

What is substrate-level phosphorylation, and where does it occur in cellular respiration?

Direct synthesis of ATP from a substrate; occurs in glycolysis (steps 7 and 10) and in the citric acid cycle (step 6).

What is the total ATP yield from glucose oxidation according to these notes?

38 ATP per glucose: 4 ATP from substrate-level phosphorylation and 34 ATP from oxidative phosphorylation.

Why is the citric acid cycle called a 'cycle'?

Oxaloacetate is used in step 1 and regenerated in step 9/10, allowing two turns per glucose (two Acetyl CoA enter per glucose).

What is the effect of cyanide poisoning on cellular respiration?

Cyanide binds to cytochrome c oxidase, inhibiting the electron transport chain and ATP production despite the presence of oxygen.

What is a decomposition reaction and provide an example from the notes?

A large molecule is broken down into smaller structures (AB → A + B). Example: sucrose → glucose + fructose.

What is a synthesis reaction and what is an example in the notes?

Two or more atoms, ions, or molecules are combined to form a larger structure (A + B → AB). Example: amino acids → dipeptide.

What is an exchange reaction and what is an example in the notes?

Atoms, molecules, ions, or electrons are exchanged between two structures (AB + C → A + BC). Example: creatine phosphate + ADP → creatine + ATP.

What is an exchange reaction and its significance in ATP production?

An exchange reaction has both decomposition and synthesis components; ATP production example: creatine phosphate + ADP → creatine + ATP.

What is oxidation-reduction (redox) and the direction of electron movement?

A reaction involving movement of electrons; oxidation is loss of electrons, reduction is gain of electrons.

In the NAD+/NADH system, which form gains electrons and becomes reduced?

NAD+ gains electrons (and a proton) to become NADH; NADH is the reduced form.

What memory aid explains electron movement in redox reactions?

LEO says GER: Loses Electrons is Oxidized; Gains Electrons is Reduced.

What is the overall chemical reaction for glucose oxidation in cellular respiration?

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.

How are exergonic and endergonic reactions defined?

Exergonic: reactants have more energy than products; energy is released. Endergonic: products have more energy than reactants; energy is required.

What molecule serves as the cell's 'energy currency' and what does ATP cycling involve?

ATP is the cell’s energy currency; ATP cycling is the continuous formation and breakdown of ATP linking fuel oxidation to energy-using processes.

What is activation energy (Ea) and how do enzymes affect it?

Ea is the energy barrier to start a reaction; enzymes lower Ea to increase the reaction rate.

What are cofactors and how do inorganic cofactors differ from organic cofactors?

Cofactors are nonprotein helpers; inorganic cofactors are ions; organic cofactors are molecules (coenzymes) like NAD+.

Describe the induced-fit model of enzyme action.

Substrate binds to active site; enzyme undergoes conformational change to fit more tightly; stresses bonds; lowers Ea; product released.

What is the active site and its role in enzyme specificity?

A grooved region on the enzyme where the substrate binds to form the enzyme-substrate complex, determining which substrate can bind.

What are the six major enzyme classes and a representative example for each?

Oxidoreductase (dehydrogenase), Transferase (kinase), Hydrolase (protease), Isomerase, Ligase (synthase), Lyase (decarboxylase).

How do competitive inhibitors regulate enzyme activity?

Competitive inhibitors resemble the substrate and compete for the active site; higher substrate reduces inhibition; effect depends on substrate:inhibitor ratio.

How do noncompetitive (allosteric) inhibitors regulate enzyme activity?

Bind to an allosteric site causing a conformational change that alters the active site; not influenced by substrate concentration.

What is negative feedback regulation in metabolic pathways?

Product acts as an allosteric inhibitor of an enzyme early in the pathway, reducing production; as product decreases, activity increases again, maintaining steady state.

What is a metabolic pathway vs a multienzyme complex?

Metabolic pathway: many enzymes in sequence converting substrate to product; multienzyme complex: enzymes physically attached forming a complex enabling substrate channeling and coordinated regulation.

What are phosphorylation and dephosphorylation in enzyme regulation?

Phosphorylation adds phosphate groups; dephosphorylation removes them; effects can activate or inhibit depending on the enzyme.

What is ATP cycling schematic: formation and splitting steps?

ATP formation: ADP + P forms ATP using energy from fuel oxidation; ATP splitting: ATP → ADP + P releases energy for cellular work.

Where are enzymes located and give examples?

Some stay inside the cell (e.g., RNA polymerase), some are membrane-bound (e.g., lactase), and some are secreted (e.g., pancreatic amylase).

What is the relationship between temperature and enzyme activity?

Enzymes have an optimum temperature around 37°C; activity rises to about 40°C; higher temperatures denature enzymes.

What is the relationship between pH and enzyme activity?

Optimum pH for most human enzymes is around 6–8; deviations denature enzymes by disrupting electrostatic interactions.

What is substrate saturation in enzyme kinetics?

At high substrate concentrations, all enzyme active sites are occupied, and the reaction rate plateaus.

What is lactose intolerance and where is lactase located?

Lactase, the enzyme that digests lactose, is located in the plasma membranes of cells lining the small intestine.

What is the reaction catalyzed by carbonic anhydrase and its significance?

CO2 + H2O ⇌ H+ + HCO3-; reversible; important for CO2 transport and acid-base balance.

How are enzyme names formed and what are exceptions?

Most enzyme names end with -ase and reflect substrate or reaction; exceptions include pepsin, trypsin, and chymotrypsin which do not clearly reveal activity.

What are ribozymes and why are they mentioned?

RNA molecules that function as enzymes; ribozymes catalyze certain biochemical reactions and illustrate RNA-based catalysis.

What analogy is used to describe the induced-fit model of enzyme action in the notes?

The interaction is like giving someone a hug; the enzyme changes shape to better fit the substrate.

What are the two classes of energy?

Potential energy and kinetic energy.

Define potential energy.

The energy of position or stored energy.

Define kinetic energy.

The energy of motion.

What is chemical energy?

Energy stored in chemical bonds of molecules (a form of potential energy).

Name some forms of usable energy in the body.

Electrical energy, mechanical energy, radiant energy, sound energy, and chemical energy (stored in bonds); heat is considered nonusable energy.

What does the First Law of Thermodynamics state?

Energy cannot be created or destroyed; it can only be transformed from one form to another.

What does the Second Law of Thermodynamics state?

Every energy transformation produces some heat; no conversion is 100% efficient.

Give an example of potential energy converting to kinetic energy with water.

Water behind a dam has potential energy; when it falls, it becomes kinetic energy and can do work.

Give an example of potential energy converting to kinetic energy with a bow and arrow.

Pulling back the bow stores potential energy; releasing converts it to kinetic energy propelling the arrow.

What is the role of a concentration gradient in potential and kinetic energy?

A concentration gradient (e.g., Na+ outside vs inside the cell) represents potential energy; movement down the gradient is kinetic energy.

What is a redox (oxidation-reduction) reaction?

A reaction involving the movement of electrons from one structure to another; oxidation is loss of electrons, reduction is gain of electrons.

What does the mnemonic LEO says GER stand for?

Losing Electrons Is Oxidized; Gaining Electrons is Reduced.

In NAD becoming NADH, is NAD oxidized or reduced?

NAD+ is reduced to NADH (it gains electrons).

What are the three categories of chemical reactions by changes in chemical structure?

Decomposition, synthesis, and exchange reactions.

Define a decomposition reaction.

A large molecule is broken down into smaller structures (AB → A + B).

Define a synthesis reaction.

Two or more atoms, ions, or molecules combine to form a larger structure (A + B → AB).

Define an exchange reaction.

Atoms, molecules, ions, or electrons are exchanged between two structures (AB + CD → AD + CB).

What is ATP cycling?

A process involving both exergonic and endergonic reactions; energy is released to do work (ATP → ADP + Pi) and later replenished (ADP + Pi → ATP).

What is an oxidation-reduction (redox) reaction and why is it paired?

Movement of electrons between structures; oxidation loses electrons, reduction gains electrons; redox reactions always occur together.

What are exergonic and endergonic reactions?

Exergonic reactions release energy; endergonic reactions require energy input.

What are the three high-energy molecules stored for chemical energy in the body?

Triglycerides, glucose (as glycogen in liver/muscle), and ATP.

Where are triglycerides stored and where is glycogen stored?

Triglycerides in adipose tissue; glycogen in liver and muscle tissue.

What is radiant energy?

Energy from electromagnetic waves; part of the kinetic energy forms and ranges from gamma rays to radio waves.

What is heat energy?

Kinetic energy of random motion of atoms and molecules; generally considered nonusable for work.

What is metabolism?

The sum of all biochemical reactions in the body, including catabolic and anabolic processes.

What are reactants and products in a chemical equation?

Reactants are substances present before the reaction (left side); products are formed after the reaction (right side); a balanced equation has equal numbers of each element on both sides.

What is the difference between irreversible and reversible chemical reactions?

Irreversible reactions proceed predominantly in one direction; reversible reactions can proceed in both directions depending on conditions.