Carbohydrate metabolism

We all need food to survive but most of us make mistakes while deciding the food intake. Some common questions that need to be addressed about nutrition are, what food types make the bulk of consumption? How much we need to consume them? What is the reason behind some specific food types need to be prefered over others? How do foods metabolise? To get answers to these questions, we need to learn in detail about various aspects of carbohydrates including their metabolism.

Daily requirement

A sizable quantity of our daily foods contains carbohydrates because around 60% of the total daily intake should come from carbs alone. Therefore, they are the primary nutrients placed at the bottom of the food pyramid.  Though we are active or not, our body definitely needs carbs for resting metabolism. Resting metabolism or metabolism at rest is the net calory demand required by every cell. As per the standard Dietary Guidelines, it is recommended to consume foods that contain at least 50% carbohydrates. To put it in another way, if we take 2,000 calories per day, an approximate 900 and 1,300 calories must come from foods that primarily contain carbohydrates; which weighs about 225 and 325 grams of pure carbohydrates a day.

Need for calories

All our body cells including neurons, nephrons, and muscle cells use glucose as a  fuel for their function. However, it again depends upon age, body weight, general health status, gender, activity levels, occupation, stress levels, height, body shape and some special physiological conditions like pregnancy and breastfeeding. Adequate glucose levels provide enough and a constant supply of energy to every cell. Our brain alone uses 20% of the total calory intake while heart, kidneys and other vital organs make up to another 30% totalling 50% of the net intake at rest. Digested carbohydrates, mainly glucose enter into the blood capillaries through the villi of the small intestine. Which are then transported to the liver via the portal circulation, where it is dealt in several ways to finally release energy necessary for all these organs

 Dietary carbs 

Dietary carbohydrates are carbohydrates present in our daily diet. Some foods that supply these include sugars, fruits, grains, pulses, juices, savouries, ice creams etc. Carbohydrates serve as a major energy source of animal diets. The calorific value of different foods vary, 1 gram of carbohydrate or a protein yields 4 kilocalories while the sam quantity of lipid gives 9 kilocalories. Though almost every food has some portion of calories, primary carbohydrates should be definitely included in the menu.  The tables below contain information on different types of dietary carbs and their components along with the most commonly used classification system of carbs.

 

Storage of energy

The glucose obtained from dietary carbohydrates is oxidized to provide the chemical energy, in the form of ATP which takes place in the liver. ATPs are necessary for day-to-day activities. Some glucose may remain in the circulating blood to maintain the normal blood glucose, which is about 3.5 to 8 millimoles per litre of blood (mmol/l) (63 to 144 mg/100 ml) but exceeding this is converted into glycogen which is an insoluble polysaccharide. Formation of glycogen takes place in the liver and cells of skeletal muscles in the presence of insulin. Glycogen formation is an adaptive mechanism to store carbohydrate without upsetting the osmotic equilibrium. Prior to its release into the blood or to provide ATP in the cells, it must be first broken down into its simpler units, glucose. Liver glycogen constitutes a store of glucose used for liver activity in addition to the required blood glucose levels while muscle glycogen stores provide the glucose requirement of muscle activity. Glucagon, adrenaline (epinephrine) and thyroxine help to breakdown glycogen to glucose. The picture below helps to explain the mechanism of production, storage and conversion of glucose into glycogen in the liver. It also mentions the role of glucagon and insulin in the process. When glucose stores in the muscles and liver drop, the pancreas is stimulated to release more glucagon. Similarly, when it rises, insulin is activated to suppress its levels. The liver acts in response to the pancreas and muscle tissues by holding or releasing the stores of glucose. Insulin and glucogon should complement each other for healthy glucose metabolism in our body. 

 

Glucose cycle in our body

The excess of carbohydrates after maintaining the blood glucose level and glycogen stores in the tissues are converted into fat and stored in the fat depots. the process is known as lipogenesis which can be reversed by lipolysis in case if the body goes hunger. Apart from day-to-day activities, different body cells require energy for different processes. Some of them include cell division and replacement of worn-out cells, contraction of muscle fibres, synthesis of secretions produced by glands, transportation of materials across cell membranes, etc. The oxidation of carbohydrate and fat provides most of the energy required by the body. When glycogen stores are low and more glucose is needed, the body can make glucose from non-carbohydrate sources, e.g. amino acids, glycerol. This is called gluconeogenesis (formation of new glucose). The glycogen stored in the liver is released when needed, this will eventually raise the blood glucose levels. If the blood glucose level shoots up beyond optimum, insulin activity is triggered to control the unnatural behaviour of glucose. 

Carbohydrate and energy release

Carbohydrate metabolism is a mix of aerobic and anaerobic processes in a phased manner. Some of the most common steps involved in the process are glycolysis, gluconeogenesis, citric acid cycle and anaerobic respiration. Carbohydrate metabolism is a process in which glucose is broken down in the body releasing energy, carbon dioxide and metabolic water hence it is a catabolic process. Catabolism of glucose occurs in a series of steps where a specific amount of energy is released in a phased manner following each step. The total number of ATP molecules generated from the complete breakdown of one molecule of glucose is 38. The whole process takes place aerobically as it requires oxygen. However, it is also possible by anaerobic means but the amount of energy release is too low. The paragraphs below briefs the aerobic and anaerobic mechanisms that release energy.

1. Aerobic catabolism or aerobic respiration

Glycolysis

Aerobic catabolism of glucose can occur only if the oxygen supply is adequate. The first stage of glucose catabolism is glycolysis. Glycolysis is an anaerobic process occur in the cytoplasm of the cell. It is a process of converting a glucose molecule into two molecules of pyruvic acid along with the releases of 2 molecules of ATP. Glycolysis is divided into 10 steps, which fall under 2 main phases. In the  first phase, breaks two ATP molecules while in the second phase, chemical energy from the intermediates is converted into ATP and NADH. A glucose molecule that is broken-down forms  2  molecules of pyruvate, which gives energy necessary for further processes. The process is mediated by many enzymes that help to upregulate, down regulate, and feedback regulates the process.

Citric acid cycle

The remainder of the considerable energy stores locked up in the original molecule of glucose is released only in the presence of adequate oxygen. During this stage, the pyruvic acid molecules enter into the biochemical roundabout called the citric acid cycle. The process takes place in the mitochondria of the cell and is purely aerobic. For every two molecules of pyruvic acid entering the citric acid cycle, a further two molecules of ATP are formed but this is still far short of the maximum possible 38 ATP molecules. The remaining 34 molecules of ATP come from the third energy-generating process, oxidative phosphorylation, a process dependent on hydrogen atoms released during earlier stages of glucose breakdown. Oxidative phosphorylation, like the citric acid cycle, can occur only in the presence of oxygen and takes place in the mitochondria.

Gluconeogenesis

It is the opposite of  glycolysis as it converts non-carbohydrate molecules into glucose that is required when the energy levels are not enough to continue the whole process of carbohydrate metabolism. non-carbohydrate molecules like pyruvate, lactate, glycerol, alanine, and glutamine become simple glucose molecules that can give instant energy. The process occurs in the liver and to some extent in kidney. The pathway is maintained by Glucagon, adrenocorticotropic hormone, and ATP itself.

 

Glycogenolysis

Glycogenolysis is the process of breaking down of glycogen to provide glucose where a single glucose molecule is broken from a branch of glycogen to transfer it to glucose-6-phosphate. It occurs in the liver, muscles, and the kidney, to provide glucose when necessary. Glucagon produced by the pancreases moves to liver where it stimulates Glycogenolysis when the blood glucose levels are dropped ( (hypoglycemia). Simultaneously, the adrenaline in the skeletal muscles stimulates the breakdown of glycogen during exercise.

2. Anaerobic catabolism

When oxygen levels in the cell are low, glucose molecule can still participate in glycolysis to split itself into two molecules of pyruvic acid, because glycolysis is an anaerobic process. However, the pyruvic acid does not enter the citric acid cycle or progress to oxidative phosphorylation; instead, it is converted anaerobically to lactic acid. 

End products of carbohydrate metabolism

  1. Lactic acid

Some of the lactic acid produced by anaerobic catabolism of glucose may be oxidized in the cells to carbon dioxide and water but first, it must be changed back to pyruvic acid. If complete oxidation does not take place, lactic acid passes to the liver in the circulating blood where it is converted to glucose and may then take any of the pathways open to glucose.

  1. Carbon dioxide

This is excreted from the body as a gas by the lungs.

  1. Metabolic water

This gets added to the considerable amount of water already present in the body while excess of which is excreted as urine by the kidneys.

Questions

  1. What are carbohydrates? Explain.

  2. Explain the classification of carbs.

  3. Differentiate between simple and complex sugars.

  4. What are the end products of carbohydrate metabolism?

  5. Explain the process of ATP formation from simple glucose.

  6. Explain the aerobic catabolism of glucose

  7. If we breakdown a molecule of glucose, how many ATPs are released? 



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