Glycolysis

Glycolysis is a cytoplasmic pathway which breaks down glucose into two three-carbon compounds and generates energy. The process produces two molecules of pyruvate, two molecules of ATP, two molecules of NADH, and two molecules of water.

The most common pathway of glycolysis was described by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas. Glycolysis is therefore known as the Embden–Meyerhof–Parnas (EMP) pathway.

Glycolysis takes place in the cytoplasm of a cell and does not require oxygen. Glycolysis is controlled by the properties of three regulatory enzymes: hexokinase, phosphofructokinase 1 and pyruvate kinase.

Glycolysis is followed by the Krebs cycle during aerobic respiration. Under aerobic conditions, pyruvate can diffuse into mitochondria, where it enters the citric acid cycle and generates reducing equivalents in the form of NADH and FADH2.

In the absence of oxygen, the cells make small amounts of ATP as glycolysis is followed by fermentation. Glycolysis produces 2 ATP per glucose molecule, and thus provides a direct means of producing energy in the absence of oxygen.
Glycolysis

Adenosine triphosphate (ATP)

Adenosine 5′-triphosphate, abbreviated ATP is a coenzyme that works with enzymes such as ATP triphosphatase to transfer energy to cells by releasing its phosphate groups. It is produced by the catabolism of proteins, carbohydrates, and fats.

ATP was discovered in 1929 by two independent sets of researchers: Karl Lohmann and also Cyrus Fiske/Yellapragada Subbarow. Later in the year 1948, Scottish biochemist Alexander Todd was the first person to synthesized the ATP molecule.

The molecule consists of three components: an adenine bicyclic system, a furanose ring, and a triphosphate chain. These molecules provide energy for various biochemical processes in the body.

It is soluble in water and has a high energy content, which is primarily due to the presence of two phosphoanhydride bonds connected to the three phosphate groups (alpha, beta and gamma). The bonds between the beta and gamma phosphates are particularly high in energy. When these bonds break, they release enough energy to trigger a range of cellular responses and mechanisms.

ATP synthesized in mitochondria is the primary energy source for important biological functions, such as muscle contraction, and protein synthesis. In addition to metabolic functions, ATP is involved in signal transduction. It is believed to be the neurotransmitter responsible for the sensation of taste.

ATP synthesis utilizes energy obtained from multiple catabolic mechanisms, including cellular respiration, beta-oxidation, and ketosis. Whenever a cell needs energy, it breaks the beta-gamma phosphate bond to create adenosine diphosphate (ADP) and a free phosphate molecule. The key to energy production lies ​with the phosphate groups. Breaking the phosphate bond is an exothermic reaction.

A cell stores excess energy by combining ADP and phosphate to make ATP. Cells get energy in the form of ATP through a process called respiration, a series of chemical reactions oxidizing six-carbon glucose to form carbon dioxide.
Adenosine triphosphate (ATP)

How the human body burns calories

Each day the average adult human body burns roughly 2,300 calories. This is considered Basal Metabolic Rate (BMR) BMR is the amount energy the body expend, while at rest on a daily basis.

If a man consume 3,000 calories for example, the body will burn the 2,3000 calories it needs and stores the remaining 700 as fat.

Homeostasis, body temperature regulation, central nervous system function, hormone and chemical production and electrolyte balance, as swell as every day function needed for life, are part of the resting metabolic rate.

The higher the metabolic rate the more calories the body burns and the easier it is to maintain a healthy bodyweigth and composition. The human body burns calories in two ways.

The first way is through involuntary activate, such as the beating of the heart, the inhaling of oxygen and maintanance of body temperature.

The second is through voluntary activities such as exercising and moving around throughout the day.

Another larger user energy is the thermal effect of feeding (TEF), or the amount of calories needed to eat, digest and distribute food fuels.

This requirements varies with the type of food digested as well as with the body composition of then individual.
How the human body burns calories

Fat cells and adipose tissue

Fat cells are normally present in loose connective tissue either singly or in small groups. Fat cells are created from stem cells in fatty tissue that develop into mature, long-lived fat cells that signal immature fat cells to divide and reproduce as needed throughout life to store excess calories as fat and to disappear when no longer needed. When large numbers of these cells are organized, the resulting lobules constitute adipose tissue.

Adipose tissue is sometimes referred to simply as fat. Adipose tissue may be partitioned by connective tissue septa into lobules. While in the subsequent developmental phases the lobules continuously increase in size.

The special capability of fat cells is the storage of a fuel reserve, which may vary in extent from an approximate 40-day reserve in the average person to one sufficient for a year or more in some obese individuals.

Brown adipose tissue

At first, fat cells were thought to be placid repositories of excess fuel which, when organized into subcutaneous layers, served as insulting material to prevent loss of body heat.

When the individual is on reducing diet, adipose tissue undergoes a process called lipolysis. Lipolysis is the separation of fatty acids from the stored form of fat, and release of the fatty acids into the blood so that they can be used by tissues and organs as a source of energy.

Another recognized function of adipose tissue was it ability to cushion, support abdominal organs and shapes the body.
Fat cells and adipose tissue

Marasmus - Protein energy malnutrition

Appropriately named form the Greek word meaning ‘dying away’, marasmus reflects a severe deprivation of food over a long time.

Marasmus is the state which virtually all available body fat stores have been exhausted because of starvation. It results in a reduction in body weight adjusted for age and size.

It is the commonest severe form of PEM (protein energy malnutrition) and tends to occur earlier in life than kwashiorkor, usually within the first year. It is common in all overpopulated and impoverished areas of the world.

Children living in poverty simply do not have enough to eat. They subsist on diluted cereal drinks that supply scant energy and protein of low quality; such food can barely sustain life, much less support growth.

Illnesses that produce marasmus is developed countries are chronic and indolent, such as cancer, acquired immunodeficiency syndrome and chronic pulmonary disease.

Marasmus is easy to detect because of the patient’s emaciated appearance: marasmic children look like little old people – just ‘skin and bones’.
Marasmus - Protein energy malnutrition

Fat is a reserve form of fuel

The lipids in foods and in the human body, though many in number and diverse in function, generally fall I three classes: triglycerides, phospholipids, sterols.

When the people speak off at, they are usually talking about triglycerides. Most animal fats are saturated and hence, solidify at low temperatures.

Most vegetables oils are unsaturated and, hence remain fluid.

Oxidative breakdown of fats yields more than double the amount of energy yielded by glucose, because of their poor oxygen contents.

These can be stored in an almost pure unhydrated form in large amounts in lesser space. Fat is stored with a very minimal amount of water, so fat stores are more compact and thus store significantly great amounts of energy than carbohydrate reserves.

Hence, fats serve as the best storage of spare energy in the form of ‘reserve stored food’. Gram for gram, fats provide more than twice the energy of carbohydrate or protein, making fat the most efficient storage form of energy.

The body fat found on a normal-weight person contains more than enough energy to fuel an entire marathon run or to battle disease should the person become ill and stop eating for a while.

For the typical 125 pound woman, roughly 20 percent of body weight – about twenty five pounds – is fat. 

Statistically, most of people carry more than that. Excessive fuel storage over the long haul taxes the body, precipitates disease and shortens life.
Fat is a reserve form of fuel

Classification of Lipids

Classification of Lipids
Lipids include all substances that are extractable from biological materials with the usual fat solvents (ether, chloroform, benzene, carbon tetrachloride, acetone, etc). Certain lipids are an energy source for the cell, others are structural compounds (particularly important are the lipid constituents of cell and organelle membranes), and still other function as hormones. This diversity of function recalls the not too remote past when lipids were considered to be inert constituents of adipose cells.

Of the many compounds classified as lipids, only fraction is significant in the diet, or in the structure and function of the human cell. The following classification is limited to lipids of importance in animal nutrition and excludes the fat soluble vitamins that, although properly classified as lipids, following the classic practice of nutritionist.

A. Simple Lipids
Fatty acids
Neutral fats (mono-, di- and triacyl glycerols)
Wax
*Sterol ester
*Nonsterol ester

B. Compound Lipids
Phospholipids
*Phosphatidic acids, lecithin, cephalins, etc
*Plasmalogens
*Sphinghomyelins
Glycolipid (carbohydrate containing)
Lipoproteins (lipids in combination with protein)

C. Derived Lipids, alcohol (including sterols and hydrocarbons)
Classification of Lipids