5.2 Catabolic Pathways for ATP Production (Part 1)

Which molecule is the energy currency of the cell?

ATP

Is the production of ATP from ADP and Pi (inorganic phosphate) endergonic or exergonic?

Endergonic

What is the importance of coupling endergonic and exergonic reactions?

Allows energy released from exergonic reactions to power endergonic reactions that require energy

What is transferred during an oxidation-reduction (redox) reaction?

Electrons

Oxidation is the …

Loss of electrons - may also involve loss of hydrogen or addition of oxygen

Reduction is the…

Gain of electrons - may also involve gain of hydrogen or loss of oxygen

What pneumonic device can you use to remember the definitions of oxidation and reduction?

OIL RIG or LEO GER

Do reduced substances gain or lose potential energy?

Gain

Why do reduced substances gain potential energy?

They gain electrons that carry energy

How are electron carriers (such as NAD+ and FAD) used in metabolic reactions?

They accept & transport high-energy electrons during metabolic reactions becoming NADH & FADH2

What are the three mechanisms for synthesizing ATP?

Substrate level phosphorylation (SLP), oxidative phosphorylation, photophosphorylation

Substrate level phosphorylation (SLP)

Transfer of a phosphate group from a substrate to ADP, making ATP

What is oxidative phosphorylation (OxPhos)

The final stage of aerobic and anaerobic respiration that produces ATP using energy from the electron transport chain (ETC) to power ATP synthase.

Photophosphorylation

ATP is produced using light energy during photosynthesis

What are the three major catabolic mechanisms for ATP production?

Aerobic & anerobic respiration, fermentation

Aerobic Respiration

Breaks down glucose to produce ATP using oxygen

Anaerobic Respiration

Produces ATP without oxygen, using another final electron acceptor

Fermentation

Produces ATP without an electron transport chain, using substrate-level phosphorylation

What process is common to all three metabolic pathways (aerobic & anaerobic resp, fermentation?

Glycolysis & produces ATP, sequence of multi-step processes, enzymes required

Why is oxygen necessary for aerobic cellular respiration?

Its the final electron acceptor in the electron transport chain

Why is oxygen necessary for anaerobic respiration?

Uses nitrate or sulfate as final electron acceptor other than oxygen

Compare and contrast aerobic respiration, anaerobic respiration, and fermentation (this could be essay, folks!)

All three produce ATP and require enzymes.

• Aerobic: Uses oxygen + ETC; produces the most ATP.

• Anaerobic: No oxygen, but uses ETC with another final electron acceptor.

• Fermentation: No ETC; produces the least ATP through substrate-level phosphorylation (SLP).

Steps for Aerobic Respiration

1. Glycolysis 2. Pyruvate oxidation 3. Krebs cycle (citric acid cycle) 4. Electron Transport Chain & Chemiosmosis (O2 final e- acceptor)

Glycolysis

Breaks glucose into 2 pyruvate → producing 2 ATP & NADH

Pyruvate oxidation

Converts pyruvate into acetyl-CoA, releasing CO2 and producing NADH

Krebs cycle (Citric acid cycle)

Breaks down acetyl-CoA, releasing CO2, and producing ATP, NADH, and FADH2

Electron Transport Chain & Chemiosmosis

Uses electrons from NADH & FADH2 to produce most ATP

Final electron acceptor (s) of Aerobic Respiration

Oxygen

Is oxidation of glucose complete or incomplete in Aerobic Respiration?

Complete oxidation

Efficiency (i.e., how much ATP is made) for Aerobic Respiration

Very efficient

Steps for Anaerobic Respiration

1. Glycolysis 2. Pyruvate oxidation 3. Krebs cycle (citric acid cycle) 4. Electron Transport Chain & Chemiosmosis (nitrate or sulfate as final e- acceptor)

Final electron acceptor(s) for Anaerobic Respiration

Nitrate, sulfate, or carbon dioxide

Is oxidation of glucose complete or incomplete in Anaerobic Respiration?

Complete or incomplete oxidation - depends on organism

Efficiency (i.e., how much ATP is made) in Anaerobic Respiration

Less efficient - less than aerobic but more than fermentation

Steps for Fermentation

1. Glycolysis 2. Fermentation

Final electron acceptor(s) for Fermentation

Organic compounds

Is oxidation of glucose complete or incomplete in Fermentation?

Incomplete

Efficiency (i.e., how much ATP is made) in Fermentation?

Low efficiency

Is glucose oxidized or reduced during aerobic cellular respiration?

Oxidized

Is oxygen oxidized or reduced during aerobic cellular respiration?

Neither

Where does glycolysis occur?

In the cytoplasm (cytosol) of both prokaryotic and eukaryotic cells

Is glycolysis aerobic or anaerobic?

Anaerobic (no O2 needed)

What are the inputs of glycolysis?

1 glucose

2 ATP

2 NAD+

What does glycolysis produce, and where do the products go?

• 2 pyruvate → Continue into respiration or fermentation.

• 2 NADH → electrons to the ETC (respiration) or help regenerate NAD⁺ (fermentation).

• 2 net ATP → Used by the cell for energy.

Where does the transition reaction occur?

• Prokaryotes: Cytoplasm (cytosol)

• Eukaryotes: Mitochondrial matrix

Is the transition reaction aerobic or anaerobic?

Aerobic

What does the transition reaction start with?

2 pyruvate from glycolysis

2NAD+

2 Coenzyme A (CoA)

What does the transition reaction produce, and where do the products go?

• 2 acetyl-CoA: Enter the Krebs cycle.

• 2 NADH: Carry electrons to the ETC.

• 2 CO₂: Released as waste.

• 0 ATP: No ATP produced.

Lab connection: What is the citrate utilization test?

Tests whether bacteria can use citrate as their sole carbon source for growth

Lab connection: Where is citrate involved in the process of aerobic/anaerobic cellular respiration?

Krebs cycle (Citric Acid Cycle) — Citrate is formed when acetyl-CoA combines with oxaloacetate during aerobic or anaerobic respiration that uses the Krebs cycle.

Lab connection: What is a positive citrate test?

Blue = Postive (+)

• Bacteria can use citrate as their sole carbon source.

• Alkaline products raise the pH, turning the medium blue.

Lab connection: What is a negative citrate test?

Green = Negative (−)

• Bacteria cannot grow using citrate as their sole carbon source under the test conditions.

• No growth or color change; medium stays green.

Where does oxidative phosphorylation occur?

• Prokaryotes: Cell (plasma) membrane

• Eukaryotes: Inner mitochondrial membrane

• Oxygen: Used in aerobic respiration, but not anaerobic respiration.

Where does the Citric Acid Cycle (Krebs Cycle) occur?

• Prokaryotes: Cytoplasm (cytosol)

• Eukaryotes: Mitochondrial matrix

Is the Citric Acid Cycle aerobic or anaerobic?

Aerobic

What are the inputs of the Citric Acid Cycle?

2 Acetyl CoA → from transition reaction

6 NAD+

2 FAD

Oxaloacetate is also required to start each turn but is regenerated, so it is not a net input.

What does the Citric Acid Cycle produce, and where do the products go?

6NADH → carry e- to ETC

2FADH2 → carry e- to ETC

2ATP → used by cell for energy

4CO2 → released

Chemiosmosis

Uses H+ gradient to generate more ATP

As H+ pass through, turbine spins

ADP & Pi combined to form ATP → highly efficient

Note: it is called oxidative phosphorylation for aerobic cellular respiration, but is often just referred to as the “electron transport chain” for other processes.

Is OxPhos aerobic or anaerobic?

Aerobic

What are the two steps of Oxidative phosphorylation (OxPhos)?

Electron transport chain & chemiosmosis

Describe the structure of the electron transport chain (don’t get too detailed – where is it and what are its main components?).

Carrier molecules chain in bacterial cytoplasmic membrane & eukaryotic inner mitochondrial membranes

Pass electrons from one to another & to final electron acceptor

*proteins embedded in a membrane

Lab Connection: Oxidase test

Tests if bacteria produces cytochrome c oxidase (enzyme in ETC during aerobic resp)

What are positive results of oxidase test and what do they tell you?

Dark purple/blue = bacteria have the enzyme cytochrome c oxidase

What are negative results of oxidase test and what do they tell you?

No color change = bacteria lacks cytochrome c oxidase

What organisms is cytochrome c oxidase found in and considered oxidase positive?

Neisseria & Pseudomonas

What organisms lack cytochrome c oxidase and are considered oxidase negative?

Escherichia, salmonella, & proteus

What do NADH and FADH2 release at the ETC?

High-energy electrons

What kinds of reactions are involved in passing hydrogen ions and electrons in the electron transport chain?

Oxidation-reduction (redox) reactions

As the electrons are passed to some of the complexes, what happens to the protein complexes? What is pumped by them?

Changes shape and pumps hydrogen ions (H⁺) across the membrane creating a proton gradient

Explain how the ETC creates an H+ concentration gradient.

ETC uses energy from electrons to pump H⁺ ions across the membrane, creating a high concentration of H⁺ on one side and a low concentration on the other

What is the final acceptor of H+ and electrons at the end of the ETC for aerobic cellular respiration?

Oxygen

Is oxygen being oxidized or reduced when it accepts the H+ and electrons at the end of ETC for aerobic cellular respiration?

Reduced as it gains e- and H+ to form water

What is formed when oxygen substance binds with H+ and electrons?

Water at the end of the ETC

What is the ATP synthase?

Protein complex that acts as a molecular turbine

Explain how the energy from the H+ gradient produced by the ETC is used to power ATP synthesis in chemiosmosis.

H⁺ ions flow from high to low concentration through ATP synthase, providing energy to combine ADP + Pi to make ATP

What types of substances are used as final electron acceptors for anaerobic cellular respiration?

Inorganic substances other than oxygen like nitrate, sulfate, & carbon dioxide

What does Nitrate Test test for?

If bacteria can reduce nitrate (NO₃⁻) to nitrite (NO₂⁻) or other nitrogen compounds.

What are positive results of nitrate test and what do they tell you?

Red after reagents A&B and no color after zinc = bacteria reduced nitrate to nitrite

What are negative results of nitrate test and what do they tell you?

Red after zinc = bacteria didn’t reduce nitrate

What happens if there is no final electron acceptor for aerobic or anaerobic cellular respiration? (possible essay question!)

ETC stops because e- have nowhere to go

As a result:

1. NADH and FADH₂ cannot release their electrons to the ETC.

2. NAD⁺ and FAD are not regenerated by the ETC.

3. H⁺ pumping stops, so the proton gradient cannot be maintained.

4. ATP production through oxidative phosphorylation stops.

Endergonic

Requires energy

Products have more energy than reactants

Anabolic (small → big)

ATP Synthesis: ADP + Pi + energy → ATP

Exergonic

Releases energy

Reactants have more energy than products

Catabolic (big → small)

ATP hydrolysis: ATP → ADP + Pi + energy

What can cells do if there is no final electron acceptor for cellular respiration?

Some cells switch to fermentation to regenerate NAD⁺, allowing glycolysis to continue producing 2 net ATP per glucose.