Question

Write an essay on the following topic. Organize your thoughts before you start writing (the bonus points will primarily be given for presenting a coherent overview framework of this complex topic): Glycogen and Fatty Acids are used to store energy. Describe and contrast the physiological states that would result in breakdown vs. those that would favor synthesis of these energy stores (include what is going on in different organs). Glycogen can be synthesized from, or broken down to Glucose 1P. Similarly, Fatty acids can be synthesized from, or broken down to Acetyl CoA. Explain why, in each case, both the breakdown and synthesis pathways are energetically favorable. You do not need to write the whole pathway, just succinctly summarize key features that change the overall energetics of biosynthetic vs. the breakdown pathways to make both directions favorable.

Answer 1

There are two physiological states: Feeding and Fasting state.

Fasting: Voluntarily restraining from food and starvation refers to a state of severe and prolonged more than 10 days food deprivation. Insulin: Glucagon ratio falls in fasting state.

The main providers of energy in the body are hepatic and muscle glycogen, adipose tissue triglycerides and muscle proteins. Liver glycogen is the first main provider of energy during a short-term fast. Liver uses this stored glycogen as a ready-made source of free glucose. But this is useful only to a limited extent as the glycogen is exhausted in ab36 hours only. adipose tissue triglycerides and muscle proteins have much larger reservoirs and can provide energy for 1-3 months.

In initial stage or early fasting the glycogen stores of liver undergo glycogenolysis,and serve as the major source of blood glucose.After about 16 hours, there is gradual switch towards gluconeogenesis. After about 36 hours, liver glycogen is totally exhausted and gluconeogenesis becomes a major generator of glucose from noncarbohydrate sources. These are mainly lactate (comes from RBCs), glycerol (from adipocytes) and amino acids (from muscle proteins).

In intermediate stage of fasting alternate substrates utilization starts:

Beta-oxidation of fatty acids is speeded up and the fatty acids are obtained by lipolysis of triglyceride stores of adipose tissue.

Ketone bodies are generated and serve as important energy source for muscles, heart and kidneys.

In advanced stage of fasting liver continues to generate glucose and ketone bodies.Ketone bodies are used by brain but their utilization by other tissues such as muscles, heart and kidney decreases. These tissues are now dependent on fatty acids for their energy needs. So there is switch from glucose oxidation to fat oxidation.

Fed State Inthis state insulin level is high. Liver converts dietary carbohydrates to glycogen and Fat.