Introduction to Carbohydrate Breakdown Pathways

Aerobic Respiration - Cellular respiration that requires oxygen

• Glucose and oxygen are converted to Carbon dioxide + water, yielding ATP

  • Consists of four metabolic pathways: glycolysis, acetyl-CoA formation, citric acid cycle, electron transfer phosphorylation

    • All involve electron transfer chains (ETCs) and share products/substrates

Fermentation - Glucose-breakdown pathways that make ATP w/o oxygen or ETC’s

• Many organisms use this as an alternative to cellular respiration due to a lack of available oxygen

Glycolysis - Sugar Breakdown Begins

Glycolysis - series of reactions that converts ATP to pyruvate, takes place in cytosol

• Pyruvate - Organic compound w/ 3 carbon backbone

Breaks one carbon-carbon bond of a glucose molecule, uses 2 ATP and produces 4

• 2 net atps

Investment (Energy-Requiring)

1. Phosphate group is transferred from ATP to glucose, forming a glucose-6-phosphate

2. phosphate group is transferred from 2nd ATP to an intermediate, forming a 6-carbon molecule w/ 2 phosphate groups

3. 6-carbon molecule splits in half, forms 2 G3P molecules

Energy-Harvesting

1. Redox reaction transfers e- and h+ from PGAL to NAD+ (coenzyme), reduces to NADH, and also attaches a phosphate group to 3-carbon intermediate

2. Transfers a phosphate group to ADP, makes ATP

3. Remaining phosphate group transfers from 1 carbon to another

4. Goes to another ADP, so another ATP forms, finally producing pyruvate

Acetyl-CoA Formation and the Citric Acid Cycle

• Acetyl-CoA and the citric acid cycle break both carbon-carbon bonds

  • Energy released from breaking is carried by NADH and phosphate bonds of ATP

Acetyl-CoA Formation

• Pyruvate is transported across 2 mitochondrial membranes, goes into inner compartment (filled with gel-like substance called matrix)

• Redox reaction splits a carbon from pyruvate which diffuses out of cell, produces NADH

• Carries 2 carbons into citric acid cycle

Citric Acid Cycle

• Harvests energy from Acetyl-CoA

  • Substrate of first reaction is product of last reaction

• 2 carbon atoms form citrate w/ oxaloacetate

• Redox reactions occur: NADH forms

  • NAD+ → NADH and FADH → FADH2

• Carbon is removed from intermediate, releasing e- and h+, forming NADH (NADP+ as well)

• Product: oxaloacetate

• 2 rounds of the citric acid cycle will harvest the energy

Aerobic Respiration’s Big Energy Payoff

Electron Transfer Phosphorylation is similar to the light-dependant reactions of photosynthesis.

• Excess energy transports H+ from matrix to intermembrane space

• Occurs w/ NADH and FADH2 delivering e- and h+ to ETCs in membrane

• Energy loss fuels H+ active transport H+ flow back to matrix through ATP synthase

  • drives ADP → ATP

• Oxygen accepts e- and h+, forming water (reverse photolysis)