Question

Which of the following organs does NOT require a constant supply of glucose from the blood for energy during a fast? (A) Red blood cells (B) Brain (C) Pancreas (D) Liver

Short answer

The liver (D).

Step-by-step solution

Identify the function of each organ

Determine the functions of red blood cells, the brain, the pancreas, and the liver, and their energy requirements, particularly in terms of glucose.

Understand red blood cells' energy requirements

Red blood cells (RBCs) rely solely on glucose for energy through glycolysis, as they lack mitochondria and cannot use fatty acids or ketone bodies.

Understand brain's energy requirements

The brain primarily depends on glucose for energy but can adapt to use ketone bodies during prolonged fasting. However, it still maintains a need for some glucose.

Understand pancreas' energy requirements

The pancreas helps regulate blood glucose levels by secreting insulin and glucagon. It does not primarily function on glucose for energy.

Understand liver's energy requirements

The liver can use a variety of energy sources, including fatty acids and amino acids, and has the ability to perform gluconeogenesis during fasting to maintain blood glucose levels.

Compare the organs' glucose dependence

Compare the energy requirements of the four organs, focusing on whether they require constant glucose supply even during fasting.

Conclusion

Red blood cells and the brain need continuous glucose. The pancreas helps regulate glucose but doesn't need it constantly for energy. The liver can use other sources and doesn't require constant glucose.

Key concepts

Red blood cells energy source

Red blood cells (RBCs) are unique in how they obtain energy. Unlike most cells in the body, RBCs lack mitochondria. This means they can't use fatty acids or ketone bodies for energy.
Without mitochondria, RBCs rely exclusively on glycolysis to produce ATP, the cellular energy currency.
Glycolysis is the process of breaking down glucose, so RBCs need a constant supply of glucose to function.
If glucose is not available, red blood cells cannot produce energy and would fail to do their essential job of carrying oxygen throughout the body.

Brain energy source

The brain is a highly active organ with significant energy demands. Primarily, it depends on glucose for energy to fuel its numerous functions, like thinking and controlling bodily movements. In fact, under normal conditions, the brain uses about 60% of the body's glucose.
During prolonged fasting or starvation, however, the brain can adapt slightly by using ketone bodies as an alternative energy source. But even then, it still requires some glucose to function fully. That's why maintaining blood glucose levels is critical for brain health and function.
Any drop in glucose can significantly affect cognitive abilities and other brain functions.

Pancreas function

The pancreas plays a crucial role in regulating blood glucose levels. It does this by secreting two main hormones: insulin and glucagon.
Insulin lowers blood glucose levels by helping cells absorb glucose from the bloodstream. Glucagon, on the other hand, increases blood glucose levels by promoting the release of glucose stored in the liver.
While the pancreas helps manage glucose levels, it does not consume large amounts of glucose for its energy needs. Instead, it primarily relies on other energy sources.
Therefore, the pancreas doesn't need a constant supply of glucose to function effectively.

Liver energy metabolism

The liver is a versatile organ when it comes to energy metabolism. It has the remarkable ability to use various energy sources such as fatty acids and amino acids.
During fasting, the liver performs gluconeogenesis, a process that produces glucose from non-carbohydrate sources. This maintains blood glucose levels to supply energy to other organs that depend on it.
Therefore, the liver doesn't necessarily need glucose for its energy during fasting periods. It is self-sufficient and adaptable, helping the body balance its energy needs no matter the conditions.
This ability to switch between different energy sources makes the liver essential for metabolism and energy regulation in the body.