Unit 3 (Cellular Respiration)

Cellular Respiration

• Cells harvest chemical energy stored in organic molecules and use it to generate ATP.

• Starch is the main source of fuel for animals and breaks down into glucose.

• The oxidation of glucose transfers e- to a lower energy state, releasing energy to be used in ATP synthesis.

• Catabolic

Cellular Respiration Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

• Glucose is oxidized into carbon dioxide

• Oxygen is reduced to water

Path of Electrons in Energy Harvest

Glucose → NADH → ETC → oxygen

Energy Harvest

• Each e- taken travels with a proton (H+)

• Dehydrogenases: enzymes that take 2 e- and 2 protons from glucose

• Transfers 2e- and 1 proton to the coenzyme NAD+ and is reduced to NADH (stores energy)

• Other proton is released into surrounding solution as H+

• NADH carries e- to the ETC.

Stages of Cellular Respiration

1. Glycolysis

2. Pyruvate oxidation and the Citric acid cycle

3. Oxidative phosphorylation (ETC and chemiosmosis)

Glycolysis

• Starting point

• Cytosol

• Splits glucose (6C) into 2 pyruvates (3C)

• Net: 2 ATP, 2 NADH, 2H+, 2 Pyruvates

• Stages: energy investment and energy payoff

Energy investment stage

• The cell uses ATP to phosphorylate compounds of glucose

• 2 ATP → 2 ADP + P

Energy payoff stage

• 2 pyruvates are formed

• Energy is produced by substrate-level phosphorylation

• 4 ADP + P → 4 ATP (net 2)

• 2NAD+ + 4e- + 4H+ → 2 NADH + 2H+

Pyruvate Oxidation

• If oxygen is present, the pyruvate enters the mitochondria (eukaryotic cells)

• Mitochondrial Matrix

• Pyruvate is oxidized into acetyl-CoA

• Outputs: 2CO₂, 2NADH

Citric Acid Cycle

• AKA Krebs cycle

• Mitochondrial matrix

• Turns Acetyl CoA into citrate

• Releases 2 CO₂

• ATP synthesized

• Electrons are transferred to NADH and FADH₂

• Outputs: 4 CO₂, 2 ATP, 6 NADH, 2 FADH₂ (after 2 turns of the cycle)

• The cell makes more NAD⁺ when NADH gives its electrons to the etc in the mitochondria.

Oxidative Phosphorylation

• Electron Transport Chain

• Chemiosmosis

Electron Transport Chain (ETC)

• Inner membrane of the mitochondria

• As electrons “fall,” proteins alternate between reduced (accepts e-) and oxidized (donates e-) states.

• Cristae increases surface area for reactions to occur

• Does not directly produce ATP

• Final electron acceptor: oxygen

• Each oxygen pairs with 2H+ and 2e- to form H₂O

• Creates a proton gradient by pumping out H+ (exergonic flow powered by NADH and FADH₂)

• Powers cellular work

Chemiosmosis

• ATP synthase: The enzyme that makes ATP from ADP + P

• Uses energy from the H+ gradient across the membrane (from ETC)

• H+ ions flow down their gradient through ATP synthase

• When H+ binds, ATP synthase activates catalytic sites to turn ADP + P to ATP

• Output: 26-28 ATP per glucose

Total ATP from Aerobic Cellular Respiration

30-32 ATP

Anaerobic Respiration

• Generates ATP using an ETC in the absence of oxygen

• Takes place in prokaryotic organisms that live in environments with no oxygen

• The final electron acceptors: sulfates or nitrates

Fermentation

• Generates ATP without an ETC

• Extension of glycolysis

• Recycles NAD+

• 2 ATP is produced

• Cytosol

• No Oxygen

• Types: Alcohol fermentation and lactic acid fermentation

Alcohol Fermentation

• 2 pyruvate is converted (reduced) into 2 ethanol

• 2 pyruvate → 2 acetaldehyde → 2 ethanol

• Ex. Bacteria and Yeast

• Can be seen in baking bread or alcohol making

Lactic Acid Fermentation

• Pyruvate is reduced directly by NADH to form lactate

• Ex. muscle cells use this to produce ATP when they run out of oxygen

• Causes the burning sensation you may feel when exercising (lactic acid building up)

• Muscles produce lactate, which goes into the blood, and is broken down into glucose in the liver

• When lactate is in the blood, it lowers the pH

• If lactate builds up and is unable to be broken down, it can lead to lactic acidosis (excessively low blood pH)