Cells need vitamins to use energy. They are used as the building blocks of other cell parts and with enzymes for chemical processes.
"Food is of no use to our body until we have allowed the cells of our body to convert the food energy (organic energy) into chemical energy through respiration. Cell respiration is when organic material (the food we eat) is converted into chemical energy within the cells to provide the energy we use to perform our everyday activities. Chemical energy is stored within the bonds between carbon and hydrogen. Every time a bond is broken energy is released due to the exothermic reaction that takes place, that is, energy is given to the body. Glucose is a good energy store because of the six carbon-hydrogen bonds. However, the main source of energy is one that is produced within our body. It is the universal energy carrier, ATP, formally known as adenosine triphosphate".
Digestion is a complex process. The cells that line the digestive tract secrete into the lumen of the gut a variety of substances, such as hydrochloric acid and digestive enzymes, to break down food molecules into simpler nutrients. The cells absorb these nutrients from the gut lumen, process them, and then release them into the blood for utilization by other cells of the body. All of these activities are adjusted according to the composition of the food consumed and the levels of metabolites in the circulation.
The first step in the metabolism of digestible carbohydrate is the conversion of the higher polymers to simpler, soluble forms that can be transported across the intestinal wall and delivered to the tissues. The breakdown of sugars begins in the mouth. Saliva is slightly acidic and contains lingual amylase that begins the digestion of carbohydrates. Once the food has arrived in the stomach, acid hydrolysis contributes to its degradation; specific gastric proteases and lipases aid this process for proteins and fats, respectively. The mixture of gastric secretions, saliva, and food, known collectively as chyme, moves to the small intestine. The resultant glucose and other simple carbohydrates are transported across the intestinal wall to the hepatic portal vein and then to parenchymal liver cells and other tissues. There they are converted to fatty acids, amino acids, and glycogen, or else oxidized by the various catabolic pathways of cells. Most of these pathways are in the mitochondria, whose outer membrane forms an aqueous channel through which proteins up to 10,000 daltons can pass and go into the inter-membrane space. The average person's body contains enough glycogen to provide energy for 6-12 hours. In contrast to this, it contains enough fat to provide energy for up to 40 days. An adult man produces enough heat every day during the metabolism of energy, to boil almost 40l of water
Energy is also required to enable these digestive and metabolic processes. Between 5% and 10% of the energy that is available in the body is required for metabolising food. Various factors play a role in the generation, storage and utilization of energy, and include the body's surface-area, age, gender, thyroid hormones, dopamine, serotonin, adrenaline, body temperature and women's menstrual cycle
Major pathways of energy metabolism
Glucose is oxidised by all tissues to synthesise ATP. The first pathway which begins the complete oxidation of glucose is called glycolysis. The normal pathways are briefly described
- The Pyruvate Dehydrogenase Complex
- The Citric Acid Cycle (Krebs cycle)
- Electron Transport and Oxidative Phosphorylation
- The Pentose Phosphate Pathway
- Beta-Oxidation of Fatty Acids