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Metabolism: Carbohydrates

Introduction

Carbohydrate metabolism refers to the production, storage and use of carbohydrates within organisms. It is essentially the metabolism of sugars and the control of blood sugar levels - we need to maintain these levels in an internal homeostasis. Carbohydrate metabolism is highly conserved and can be observed even within bacteria- the prime example being the Lac Operon of E.coli.

Carbohydrates all consist of carbon, hydrogen and oxygen (CnH2nOn), yet not all forms of carbohydrate can be metabolised. The primary example of a carbohydrate that is used almost universally within living organisms is glucose, which is a monosaccharide. However, larger molecules (disaccharides, oligosaccharides and polysaccharides) are also metabolised within our bodies. Other examples include fructose and galactose.

Numerous processes within the body are dependent upon carbohydrate metabolism: Cellular respiration to produce ATP involves catabolic reactions that often rely upon the use of sugars, for example. In particular, the brain relies heavily on the metabolism of glucose.

After we have taken in carbohydrates (the main sources being foods such as breads, cereals, pastas, potatoes, etc.) metabolism begins within the gastrointestinal tract - namely the small intestine. This is where simple monosaccharides are absorbed through the thin epithelium into the bloodstream. Larger molecules are broken down by enzymes called glycoside hydrolases. These enzymes include amylase, which is produced by the pancreas and generally converts starch into disaccharides and trisaccharides, to later be hydrolysed into glucose. Not all carbohydrates are digested at this stage (particularly not polysaccharides) and many pass straight into the large intestine.

While alpha-glucose (which makes up starch) is metabolised in a relatively straight-forward manner, beta-glucose cannot be digested and this is the reason why some polysaccharides such as cellulose are never digested in humans. Ruminants, such as cattle, are able to digest cellulose.

What does carbohydrate metabolism involve?

 

As monosaccharides are continually passing into the bloodstream (at much larger concentrations after eating) the levels of these monosaccharides must be carefully maintained. Failure of this regulation can result in metabolic diseases such as diabetes mellitus. There are three main hormones involved in this glucoregulation - epinephrine, glucagon and insulin.

Insulin, produced in the pancreas, stimulates the uptake of excess digested glucose into fat tissue, the liver and muscles from the bloodstream. In the liver and muscle, it is stored as glycogen in a process called glycogenesis. This stored glycogen is only used when the concentration of glucose in the blood falls - preventing the progression of hypoglycaemia, clinical low blood sugar. This is done through a process called glycogenolysis, the production of glucose through the breakdown of glycogen, which is triggered by epinephrine and glucogon. Carbohydrates can also be stored for structural support within cells.

These processes are important for the storage and later use of carbohydrates. However, often the monosaccharides are needed to produce energy immediately upon digestion and, at this point, a third process becomes paramount: Glycolysis.

Glycolysis converts glucose into pyruvate, through a series of intermediates, in a process which releases energy as ATP. Two molecules of pyruvate can be made from just one glucose monosaccharide. As the intermediates (such as 3-carbon sugar phosphates) are formed, electrons are transferred to the enzyme NAD+ and ATP is produced. Pyruvate can be converted back into glucose through gluconeogenesis, particularly during extensive exercise that causes us to respire anaerobically. During aerobic respiration, glucose and oxygen are metabolised to create energy, releasing carbon dioxide and water as waste products.

Carbohydrate metabolism is relatively efficient at providing energy quickly for the short-term: Even polysaccharides are easier to hydrolyse than the fats and proteins that we use for energy.

Summary

  • Glucose is the main monosaccharide used in carbohydrate metabolism
  • The maintenance of glucose levels is referred to as glucoregulation
  • Glucose is stored as glycogen in a process called glycogenesis
  • Glycogen is convered back into glucose in a process called glycogenolysis
  • Glycolysis is used to convert glucose into pyruvate, which is used to produce energy
  • Pyruvate can be converted back into glucose in a process called gluconeogenesis

Glycogenesis

Glycogenesis

Gluconeogenesis

Gluconeogenesis

Glycogenolysis

Glycogenolysis

Glycolysis

Glycolysis

Gluconeogenesis

Gluconeogenesis

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