Question

Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule? (a) Glucose 6 -phosphate (b) Dihydroxyacetone phosphate (c) Glyceraldehyde 3 -phosphate (d) Fructose (e) Sucrose

Short answer

(a) 3 ATP, (b) 1 ATP, (c) 1 ATP, (d) 2 ATP, (e) 4 ATP.

Step-by-step solution

Understanding Glycolysis

Glycolysis is a series of reactions that convert glucose into pyruvate with the generation of ATP. Under anaerobic conditions, pyruvate can be further reduced to lactate. The conversion of one molecule of glucose to two molecules of lactate results in the net production of 2 ATP molecules.

Calculate ATP from Glucose 6-Phosphate

Glucose 6-phosphate enters the second energy phase of glycolysis, bypassing the initial phosphorylation step, which consumes 1 ATP. Thus, the conversion to lactate generates a net gain of 3 ATP per molecule (since 2 ATP are produced per glucose molecule with no initial consumption in this case).

Calculate ATP from Dihydroxyacetone Phosphate

Dihydroxyacetone phosphate is readily converted to glyceraldehyde 3-phosphate, which is processed through glycolysis. Since this enters just before the payoff phase, each molecule results in 1 ATP net gain because it covers half of one glucose molecule that generates 2 ATP when fully metabolized to lactate.

Calculate ATP from Glyceraldehyde 3-Phosphate

Glyceraldehyde 3-phosphate is processed through the payoff phase of glycolysis. Since this is essentially half of one glucose molecule, it results in a net production of 1 ATP when converted to lactate.

Calculate ATP from Fructose

Fructose can enter glycolysis either as fructose-6-phosphate or as glyceraldehyde 3-phosphate by different pathways in the liver. We'll consider the usual pathway where it is metabolized to fructose-6-phosphate, which does not require the consumption of additional ATP. Thus, it results in a net gain of 2 ATP when converted to lactate.

Calculate ATP from Sucrose

Sucrose is hydrolyzed into one molecule of glucose and one molecule of fructose. The glucose molecule can produce 2 ATP when converted to lactate, and fructose can also enter glycolysis as previously explained, giving 2 ATP. Therefore, the total net ATP produced from sucrose is 4 ATP (2 from glucose and 2 from fructose).

Key concepts

ATP production

ATP, or adenosine triphosphate, is the energy currency of the cell. During glycolysis, the process by which glucose is broken down, a small but immediate payoff occurs in the form of ATP production. Glycolysis starts with one molecule of glucose and results in the production of two molecules of pyruvate.
Under normal oxygen conditions, cells will further process pyruvate to extract more energy. However, in certain conditions, cells need energy quickly and can't afford the slower, oxygen-requiring processes. This is where ATP produced during glycolysis becomes critical.

• Glycolysis produces net 2 ATP molecules per molecule of glucose.
• Despite being less efficient than aerobic respiration (which can yield up to 36-38 ATP per glucose molecule), glycolysis is faster.
• The immediate ATP generated is especially important under intense activity or oxygen-limited situations.

This initial ATP production is vital for maintaining cellular processes, even when oxygen isn't readily available.

Anaerobic conditions

Anaerobic conditions refer to environments where oxygen is limited or absent. During such conditions, cells rely on glycolysis to fulfill their energy needs, since glycolysis does not require oxygen.
When human muscles are highly active, such as during vigorous exercise, the oxygen demand can surpass what the supply can provide. Under these circumstances, cells switch to anaerobic glycolysis to quickly produce their required energy in the form of ATP. Key points about anaerobic conditions include:

• Glycolysis is the primary pathway for ATP generation in the absence of oxygen.
• Although this process generates less ATP, it serves as a rapid means to create energy needed by the cell.
• The conversion of glucose to lactate instead of to further oxidized products like carbon dioxide happens under these conditions.

The anaerobic pathways ensure that energy production continues without interruption, which is crucial for survival in low-oxygen environments.

Lactate formation

Lactate formation occurs when pyruvate, the end product of glycolysis, is converted into lactate, especially under anaerobic conditions. This transformation is facilitated by the enzyme lactate dehydrogenase.
This shift allows glycolysis to continue by regenerating NAD+, an essential cofactor for glycolysis to proceed.

• Under aerobic conditions, pyruvate typically enters the mitochondria for further energy extraction.
• In anaerobic conditions, consistent conversion into lactate is vital to keep ATP production ongoing.
• The accumulation of lactate can lead to muscle fatigue but is also rapidly cleared when oxygen becomes available, as it gets converted back to pyruvate.

Lactate formation is not just a byproduct; it is a crucial step allowing cells to sustain energy production during periods when oxygen availability is limited.

Glucose metabolism

Glucose metabolism is a comprehensive term describing the sequence of biochemical processes that break down glucose for energy. The first stage of glucose metabolism is glycolysis, where each molecule of glucose undergoes a transformation into two pyruvate molecules while generating ATP and NADH. Glycolysis is a central pathway and occurs in the cytoplasm of cells. Its efficiency helps in generating immediate energy but does not rely on oxygen, making it independent of aerobic respiration at this stage.

• The process begins with glucose entering the cell and being phosphorylated.
• Through a series of enzymatic reactions, glucose is broken down to pyruvate.
• The glycolysis pathway yields a net gain of 2 ATP molecules per glucose molecule.
• It also contributes to the cellular pool of NADH, which can be used in other metabolic pathways.

Understanding glucose metabolism, especially glycolysis' role, is fundamental for comprehending how cells manage their energy needs, whether through aerobic or anaerobic means.