Glycolysis (Chapter 2)

- a 10 step oxidation of glucose occurring in the cytoplasm

- anaerobic (but does occur if oxygen is present)

1. energy-investment phase

Two ATP molecules phosphorylate glycolytic intermediates making them unstable and easier to break their chemical bonds

At the end of this phase, two molecules of glyceraldehyde 3-phosphate (G3P - a 3C sugar) have been generated from the initial molecule of glucose

2. energy-yielding phase, an important redox reaction occurs (review redox)

Two H are taken from each G3P molecule to reduce a molecule of NAD⁺

The NADH is a dinucleotide coenzyme that serves as a high-energy electron carrier.

The G3P is oxidized while the NAD+ is reduced.

Four ATP are generated

NET YIELD

2 pyruvate 2ATP 2 NADH

NOTE Pyruvate cannot be the final product of respiration

There’s the problem of regenerating NAD⁺

Limiting reagents

-limiting reagent in a chemical pathway is the key arbiter of the entire process

In glycolysis, if all NAD⁺ is reduced to NADH, then glycolysis cannot continue

Without an adequate pool of NAD⁺ molecules to capture the high-energy electrons from G3P, the cell will die from lack of energy

- no oxygen available to the cell - - > two possible fermentation pathways

- the cell must sacrifice the energy in pyruvate in order to oxidize NADH

- if oxygen is present, then the pyruvate will enter the mitochondrion (in eukaryotic cells) to be further oxidized

- cytoplasmic NADH (from glycolysis) donates its electrons to the electron transfer chain, thus solving the NAD⁺ dilemma.

- in animals, muscles may enter a state of oxygen debt

- to continue glycolysis, the NADH donates H to pyruvate in one simple redox reaction

- NAD+ is replenished while pyruvate is reduced to lactate (3C ionized acid)

No additional energy is gained through fermentation, so the ATP generated during glycolysis is the only ATP generated

- two step process after glycolysis

- in certain bacteria and fungi (yeasts), pyruvate (3C acid) loses a carboxyl group and carbon dioxide is released

- acetaldehyde (2C aldehyde) is then reduced to ethanol (2C alcohol) while NADH is oxidized to NAD⁺

No additional energy is gained through fermentation, so the ATP generated during glycolysis is the only ATP generated

Fermentation equations

glucose - > 2 alcohol + 2 ccarbon dioxide + 2ATP + heat

glucose - > 2 lactate + 2ATTP + heat

All the ATP synthesized in fermentation is by way of substrate-level phosphorylation