Question

Which of the following options depicts the sequence of food materials broken down during starvation?
A.Carbohydrates->fats->proteins
B.Carbohydrates -> proteins -> fats
C.Proteins -> fats -> carbohydrates
D.Fats -> proteins -> carbohydrates

Answer 1

Hint:Some nutrients are broken down only during starvation or strenuous exercise. Respiration is the process in which energy and several important intermediates are produced as a result of oxidation of reduced substrates like carbohydrates, fats, and proteins.

Complete step by step answer:
-The carbohydrates are the fuel of choice during respiration. At times of starvation, or unavailability of carbohydrates, the living system metabolizes other reduced substrates- first lipids and then proteins.
-Carbohydrates are converted to maltose and then to glucose. Glucose enters the processes of glycolysis, link reaction, Krebs cycle, and electron transport chain (ETC) to form energy precursors like NADH and FADH2 which in the ETC form ATPs.
-Lipids are stored as free fatty acids (FFA) and triglycerides in adipose tissue and as cholesterol in the liver. These lipids enter the process of gluconeogenesis to form acetyl CoA and finally glucose.
-Proteins are converted to amino acids by proteases. These glucogenic amino acids like alanine enter the Cahil cycle to form intermediates of glycolysis.

Hence option A is correct.

Additional Information:
During starvation the brain uses the remaining glucose reserves (glycogen phosphorylase lyses individual glucose molecules. Ketone bodies are formed as a last resort by the ketogenic pathway from lipids, cardiac muscles and the brain are the main organs using ketone bodies for energy (ketolysis).
Depending on the type of exercise, the nutrients required varies. For aerobic exercises, lipids are the major energy sources.

Note:
-Alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, and glycine are some glucogenic amino acids.
-Ketogenic amino acids are lysine and leucine. They are degraded to form acetyl CoA directly
Phenylalanine, isoleucine, threonine, tryptophan, and tyrosine are both glucogenic and ketogenic.