Question

Giardia intestinalis and Entamoeba histolytica are protozoa that live in the colons of mammals and can cause life-threatening diarrhea. Interestingly, these microbes lack mitochondria. What kind of pathway must they have for carbohydrate catabolism?

Short answer

Giardia intestinalis and Entamoeba histolytica must primarily rely on glycolysis for carbohydrate catabolism.

Step-by-step solution

Understanding the Role of Mitochondria in Metabolism

In most eukaryotic cells, mitochondria play a vital role in metabolism, particularly in the catabolism of carbohydrates. This is where the aerobic respiration processes, including the Krebs cycle and oxidative phosphorylation, occur. These processes break down glucose, a carbohydrate, into ATP molecules which are used for energy. When a cell lacks mitochondria, other metabolic pathways must be utilized.

Identifying Alternative Metabolic Pathways

In the absence of mitochondria, organisms have to rely on metabolic pathways that don't depend on the presence of these organelles. One such pathway is glycolysis, a process that occurs in the cytoplasm and does not require mitochondria nor oxygen.

Concluding the Type of Carbohydrate Catabolism Pathway

Given that Giardia intestinalis and Entamoeba histolytica lack mitochondria, they must primarily depend on glycolysis for carbohydrate catabolism, a universal process wherein glucose is converted into pyruvate, generating a small amount of ATP.

Key concepts

Mitochondria

Mitochondria are often referred to as the powerhouses of the cell. They are double-membraned organelles found in most eukaryotic cells. Their primary function is to produce ATP, the energy currency of the cell, through processes known as aerobic respiration.
In mitochondria, aerobic respiration can be broken down into two major processes: the Krebs cycle and oxidative phosphorylation.

• The **Krebs cycle**, also known as the citric acid cycle, occurs in the mitochondrial matrix. Here, acetyl-CoA derived from carbohydrates, fats, and proteins is oxidized, releasing energy.
• During **oxidative phosphorylation**, the energy from electrons, transferred through the electron transport chain, is used to pump protons across the mitochondrial membrane, creating a gradient that powers ATP synthase to produce ATP.

Without mitochondria, cells must rely on other pathways to generate energy, particularly if they lack the capability to perform aerobic respiration.

Glycolysis

Glycolysis is a fundamental metabolic pathway that is crucial for energy production, especially in cells that lack mitochondria, such as some protozoa. This pathway occurs in the cytoplasm and is anaerobic, meaning it does not require oxygen. Glycolysis involves the breakdown of glucose, one of the simplest carbohydrates, into two molecules of pyruvate.
During this multistep process, a net gain of two ATP molecules per glucose molecule is achieved, along with two molecules of NADH. Here's a breakdown of the glycolysis process:

• **Investment Phase**: Initially, two ATP molecules are consumed to phosphorylate glucose, making it more reactive.
• **Cleavage Phase**: The six-carbon sugar is split into two three-carbon molecules.
• **Energy Payoff Phase**: These three-carbon molecules are further processed to produce four ATPs and two NADHs, resulting in a net gain of two ATP molecules.

Glycolysis is a universal pathway present in almost all organisms, providing essential energy output especially when mitochondria, and thus aerobic respiration, is not an option.

Carbohydrate Catabolism

Carbohydrate catabolism refers to the breakdown of carbohydrates to release energy. In cells with mitochondria, the complete oxidation of glucose through the Krebs cycle and oxidative phosphorylation provides the maximum yield of ATP. However, in cells like those of Giardia intestinalis and Entamoeba histolytica, which lack mitochondria, carbohydrate catabolism is limited to pathways such as glycolysis.
These protozoa utilize glycolysis for the breakdown of carbohydrates into pyruvate, providing a modest amount of energy in the form of ATP. Since glycolysis does not exhaust all the potential energy stored in glucose, such cells often have various fermentation processes in place to further metabolize pyruvate. These processes help regenerate NAD+ necessary for the continuation of glycolysis in anaerobic conditions.

• **Energy Yield**: Without mitochondria, the energy yield from carbohydrate catabolism is lower, as they cannot utilize the efficient oxidative phosphorylation pathway.
• **Adaptations**: Protozoan adaptations allow them to survive in anaerobic environments, relying on glycolysis and fermentation for energy.

Thus, understanding the unique ways protozoa metabolize carbohydrates helps us comprehend how these organisms manage to thrive despite lacking mitochondria.