Chapter 9: Cellular Respiration and Fermentation

Marlie Gantt Remdation Assignment

How does energy flow into and out of an ecosystem?

Energy enters an ecosystem as sunlight and exits as heat. Matter and chemical elements are recycled.

What is the general chemical equation for cellular respiration

C₆H₁₂O₆ + 6O₂ to 6CO₂ + 6H₂O + Energy (ATP + heat)

What does “catabolic” mean in cellular respiration

It refers to metabolic pathways that break down complex organic molecules into simpler ones, which is an exergonic (energy-releasing) process.

What is fermentation

A catabolic process that partially breaks down sugars to produce ATP without using oxygen

Whta is aerobic respiration

A catabolic pathway that completely breaks down organic fuels by consuming oxygen to yield ATP

What is anaerobic respiration?

A pathway similar to aerobic respiration that uses an electron transport chain, but uses a final electron acceptor other than oxygen.

What is a redox (oxidation-reduction) reaction?

A chemical reaction involving the transfer of one or more electrons from one reactant to another.

What happens during oxidation?

A substance loses electrons (hint: LEO – Loss of Electrons is Oxidation)

What happens during reduction?

A substance gains electrons, reducing its total amount of positive charge (hint: GER – Gain of Electrons is Reduction).

What is a reducing agent?

The electron donor; it loses electrons and causes another substance to be reduced.

What is an oxidizing agent?

The electron acceptor; it gains electrons and causes another substance to be oxidized.

In cellular respiration, which molecule gets oxidized and which gets reduced?

Glucose (C₆H₁₂O₆) is oxidized to CO₂, and oxygen (O₂) is reduced to (H₂O).

Why are organic molecules with lots of hydrogen (like glucose) good energy fuels?

Their bonds contain an abundance of high-energy electrons that release potential energy when transferred toward electronegative oxygen.

What is the role of NAD+ in cellular respiration?

It acts as an oxidizing agent and mobile electron carrier that traps high-energy electrons from fuel molecules.

What do dehydrogenase enzymes do during cellular respiration?

They remove two hydrogen atoms (2 electrons and 2 protons) from fuel, transfer 2 electrons and 1 proton to NAD⁺ to form NADH, and release 1 proton (H⁺) into solution.

What is NADH

The reduced form of NAD⁺ carrying two high energy electrons and one proton, representing stored energy used to make ATP.

Why doesn't cell respiration transfer electrons directly from NADH to oxygen in one big step?

Direct transfer releases energy in an uncontrolled, explosive burst of heat. Respiration uses a chain to release energy gradually and safely.

What are the three main stages of cellular respiration?

• Glycolysis

• Pyruvate oxidation and the citric acid cycle

• Oxidative phosphorylation (ETC and chemiosmosis)

Where in the cell does glycolysis occur?

In the cytosol (cytoplasm)

Where do pyruvate oxidation and the citric acid cycle occur in eukaryotes?

Inside the mitochondrial matrix

What is substrate-level phosphorylation?

Direct enzymatic transfer of a phosphate group from a phosphorylated organic substrate directly to ADP to generate ATP.

Which stage produces roughly 90% of a cell's total ATP?

Oxidative phosphorylation, powered by redox reactions across the mitochondrial membrane.

What is the primary purpose of glycolysis?

To split one 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

What are the two major phases of glycolysis?

The Energy Investment Phase and the Energy Payoff Phase.

How many ATP are invested in the first phase of glycolysis?

2 ATP molecules are used to phosphorylate glucose derivatives.

What does the enzyme hexokinase do in step 1 of glycolysis?

It transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate and trapping it inside the cell.

What does phosphofructokinase (PFK) do in step 3 of glycolysis?

It transfers a phosphate from ATP to fructose-6-phosphate, making fructose-1,6-bisphosphate (a major committed regulatory step).

What occurs during step 4 and 5 of glycolysis?

The 6-carbon sugar is cleaved into two 3-carbon isomers: G3P and DHAP; DHAP is rapidly converted into G3P.

What occurs in step 6 of glycolysis?

G3P is oxidized by triose phosphate dehydrogenase, transferring electrons to NAD⁺ to create NADH while attaching an inorganic phosphate.

How is ATP generated in step 7 of glycolysis?

Phosphoglycerokinase transfers a high-energy phosphate group from 1,3-bisphosphoglycerate directly to ADP to form ATP.

What is the product of enolase in step 9 of glycolysis

Enolase removes a water molecule to create phosphoenolpyruvate (PEP), which has a high-energy phosphate bond.

What occurs in step 10 of glycolysis?

Pyruvate kinase transfers the phosphate group from PEP to ADP, making ATP and pyruvate.

What is the gross vs. net yield of ATP in glycolysis?

Gross yield is 4 ATP; net yield is 2 ATP (since 2 ATP were invested upfront).

What electron carrier molecules are generated in glycolysis?

2 molecules of NADH (from 2 NAD⁺)

Does glycolysis require oxygen (O₂)?

No, glycolysis occurs whether oxygen is present or absent.

What is the net chemical equation for glycolysis?

Glucose+2NAD⁺+2ADP+2P_i - 2 Pyruvate+2NADH+2H⁺+2ATP+2H₂O

How does pyruvate enter the mitochondrion?

It crosses the mitochondrial membranes via active transport through a transport protein.

What enzyme complex converts pyruvate to acetyl CoA?

Pyruvate dehydrogenase.

What three reactions take place during pyruvate oxidation?

• Carboxyl group is removed as CO₂

• Remaining 2-carbon fragment is oxidized to reduce NAD⁺ to NADH

• Coenzyme A binds the 2-carbon fragment to form acetyl CoA

What are the total products of pyruvate oxidation per glucose molecule?

2 acetyl CoA, 2 NADH, and 2CO₂ (because 1 glucose yields 2 pyruvates).

What combines with acetyl CoA in step 1 of the citric acid cycle?

Oxaloacetate (4 carbons) joins with the 2-carbon acetyl group to form citrate (6 carbons).

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

Because oxaloacetate is regenerated in the final step to accept another acetyl group.

How many CO₂ molecules are released per turn of the citric acid cycle?

2 molecules of CO₂ per turn (4 CO₂ per original glucose).

What energy products are made in a single turn of the citric acid cycle?

1 ATP (via GTP), 3 NADH, and 1FADH₂

What are the total citric acid cycle products for one full glucose molecule?

2 ATP, 6 NADH, 2FADH₂, and 4CO₂ (cycle turns twice per glucose).

What happens in step 3 and 4 of the citric acid cycle?

Successive oxidations release two CO₂ molecules and generate two NADH molecules (𝛼-ketoglutarate then succinyl CoA)

What type of phosphorylation produces ATP in step 5 of the citric acid cycle?

Substrate-level phosphorylation (succinyl CoA transfers phosphate to GDP to make GTP, then ATP)

What occurs in step 6 of the citric acid cycle?

Succinate is oxidized to fumarate, transferring electrons and hydrogens to FAD to form FADH₂

What occurs in the final step (step 8) of the citric acid cycle?

Malate is oxidized to regenerate oxaloacetate, producing the cycle's third NADH.

Where is most of the chemical energy from the original glucose stored after the citric acid cycle?

In elextrons carriers: NADH and FADH₂

Where are the electron transport chain (ETC) proteins located in eukaryotes?

In the inner mitochondrial membrane (folded into cristae to maximize surface area).

What are cytochromes?

Iron-containing proteins in the ETC that transfer electrons down the energy gradient.

At which complex does NADH enter the electron transport chain?

Complex I (transferring electrons to FMN)

At which complex does FADH₂ enter the electron transport chain?

Complex II (delivering electrons at a lower free-energy level than Complex I)

What is the final electron acceptor at the end of the ETC?

Oxygen (O₂)

What byproduct forms when oxygen accepts electrons and protons at Complex IV?

Water (H₂O)

Does the electron transport chain itself make ATP directly?

No; it uses electron energy strictly to pump protons (H⁺) across the membrane.

Where are protons (H⁺) pumped during electron transport?

From the mitochondrial matrix into the intermembrane space.

What is the proton-motive force?

The electrochemical gradient created by high H⁺ concentration in the intermembrane space versus low in the matrix.

What enzyme complex synthesizes ATP using the proton gradient?

ATP synthase

What is chemiosmosis?

The process where energy stored in an ion (H⁺) gradient is used to drive cellular work, specifically ATP synthesis

How does ATP synthase work like a molecular rotary motor?

Protons stream down their gradient through the rotor, causing it to spin and catalyzing the attachment of P_i to A

What is the overall sequence of energy flow during aerobic respiration?

Glucose → NADH → ETC → Proton-motive force → ATP

How much of the chemical energy in glucose is converted to ATP?

Roughly 34% (about 30 to 32 ATP total); the remaining ~66% is released as heat.

Why is the exact number of ATP produced per glucose not a fixed single number?

Phosphorylation and redox steps aren't directly coupled (NADH:ATP ratio is fractional), shuttle types vary (NAD⁺ vs FAD), and proton gradient energy drives other cell transport.

How do cyanide and lack of oxygen affect oxidative phosphorylation?

They block electron flow along the ETC, halting proton pumping and stopping ATP synthesis.

What do uncoupling proteins do to oxidative phosphorylation?

They let protons leak back across the membrane without turning ATP synthase, releasing energy as pure heat instead of making ATP.

What is the primary purpose of fermentation?

To oxidize NADH back to NAD⁺ so glycolysis can keep running and generating 2 ATP without oxygen.

What are the two steps of alcohol fermentation?

1. Pyruvate releases CO₂ to form acetaldehyde.

2. Acetaldehyde is reduced by NADH to form ethanol, regenerating NAD⁺

What happens during lactic acid fermentation?

Pyruvate is reduced directly by NADH to form lactate (no CO₂ is released), regenerating NAD⁺

What causes burning/fatigue during intense anaerobic exercise in human muscle cells?

When oxygen runs low, muscle cells switch to lactic acid fermentation to keep glycolysis producing ATP.

Why is glycolysis considered an evolutionary heirloom?

Ancient prokaryotes used it before oxygen existed in the atmosphere; it is widespread and occurs entirely in the cytoplasm without needing organelles.

How do proteins and fats enter cellular respiration?

Proteins are broken into amino acids that enter at various stages; fats break into glycerol (enters glycolysis) and fatty acids (enter as acetyl CoA).

How is cellular respiration controlled by feedback inhibition?

When ATP levels are high, respiration slows down; when ATP drops and AMP rises, respiration speeds up.

How does phosphofructokinase (PFK) act as the "pacemaker" of respiration?

It is stimulated by AMP (signaling low energy) and inhibited by ATP and citrate (signaling excess fuel/energy)