Transport proteins carbohydrate digestion

Malabsorption is defined as an inadequate assimilation of dietary substances due to defects in digestion, absorption or transport. Malabsorption can affect macronutrients (proteins, carbohydrates, fats), micronutrients (vitamins, minerals) or both, causing excessive faecal excretion and producing nutritional deficiencies and GI symptoms. Digestion and absorption occur in three phases, namely (i) the intra-lumen hydrolysis of fats, proteins and carbohydrates by enzymes, and emulsification by bile salts, (ii) digestion by brush-border enzymes and uptake of end-products and (iii) lymphatic transport of nutrients. Malabsorption can occur when any of these phases is impaired. 

An interesting and important phenomenon related to the micellar solubilization, and mixed micelles in particular, is that of the digestion and absorption of fatty nutrients by the body. Since fat is a major source of energy ( 9 vs. 6 cal g for proteins and carbohydrates), its transport through the digestive track and ultimate transfer through the intestinal walls is obviously of great importance. 

Chylomicrons leave the absorptive cell by way of exocytosis. Because they are unable to cross the basement membrane of the blood capillaries, the chylomicrons enter the lacteals, which are part of the lymphatic system. The vessels of the lymphatic system converge to form the thoracic duct that drains into the venous system near the heart. Therefore, unlike products of carbohydrate and protein digestion that are transported directly to the liver by way of the hepatic portal vein, absorbed lipids are diluted in the blood... 

In addition to their plasma membrane eukaryotic cells also contain internal membranes that define a variety of organelles (fig. 17.2). Each of these organelles is specialized for particular functions The nucleus synthesizes nucleic acids, mitochondria oxidize carbohydrates and lipids and make ATP, chloroplasts carry out photosynthesis, the endoplasmic reticulum and the Golgi apparatus synthesize and secrete proteins, and lysosomes digest proteins. Additional membranes divide mitochondria and chloroplasts into even finer, more specialized subcompartments. Like the plasma membrane, organellar membranes act as barriers to the leakage of proteins, metabolites, and ions they contain transport systems for import and export of materials, and they are the sites of enzymatic activities as diverse as cholesterol biosynthesis and oxidative phosphorylation. 

The carbohydrates, proteins, and lipids in foods are mostly in complex forms. For example, the carbohydrates are present as disaccharides, such as sucrose, or polysaccharides, such as starch. The first step in digestion is the breakdown of the larger, insoluble molecules into smaller, soluble forms that can be transported across the intestinal wall into the blood for delivery to the tissues. 

Oral administration is the most common route of drug administration. Major physiologic processes in the GI system include secretion, digestion, and absorption. Secretion includes the transport of fluid, electrolytes, peptides, and proteins into the lumen of the alimentary canal. Enzymes in saliva and pancreatic secretions are involved in the digestion of carbohydrates and proteins. Other secretions such as mucus protect the linings of the lumen of the GI tract. Digestion is the breakdown of food constituents into smaller structures in preparation for absorption. Both drug and food constituents are mostly absorbed in the proximal area (duodenum) of the small intestinal. The process of absorption is the entry of constituents from the lumen of the gut into the body. Absorption may be considered as the net result of both lumen-to-blood and blood-to-lumen transport movements. 

Salivary a-amylase is a protein that contributes to the enamel pellicle (Sect. 12.1.3). More importantly, it attaches bacteria, especially streptococci, to teeth surfaces. Thus, following a meal rich in carbohydrates, amylopectin, amylase, and glycogen are digested to maltose at the surface of many oral bacteria. The maltose is taken into the cytosol by a phosphoenolpyruvate transporter homologous to the fructose transporter of S. mutans. Within these bacteria, the maltose is digested to two molecules of glucose 6-phosphate and metabolized to lactic acid. Thus, twice as much acid is produced per mole maltose than per mole sucrose and it contributes to tooth demineralization even if less sucrose is consumed. 

The small intestinal tract is the major place of digestion and absorption. Protein digestion reaches approximately 15% in the stomach, and in the lower intestine, the hydrolyzed fraction amounts to about 60%. Seventy percent of fats, 60% of carbohydrates, and about 30% of protein and peptide are absorbed within the duodenum. Within enterocytes, triglycerides are resynthesized from long-chain fatty acids and transformed into chylomicrons, which are transported via the lymphatic system, whereas short- and medium-chain fatty acids are directly transported into mesenteric and portal vein system. 

The most obvious role of the small intestine is the digestion of nutrients such as carbohydrates, lipids, and proteins into molecules that are small enough to be absorbed (sugars, fatty acids, glycerol, and amino acids). Nutrient absorption by the enterocytes of the small intestine is an extremely vital and complicated process that involves numerous enzymes and transport mechanisms. As described, (p. 374) enterocytes then transport these molecules (and water, minerals, vitamins, and other substances) into the blood and lymph, which carry them throughout the body. 

The Fed State During a meal, we ingest carbohydrates, lipids, and proteins, which are subsequently digested and absorbed Some of this food is oxidized to meet the immediate energy needs of the body. The amount consumed in excess of the body s energy needs is transported to the fuel depots, where it is stored. During the period from the start of absorption until absorption is completed, we are in the fed, or absorptive, state. Whether a fuel is oxidized or stored in the fed state is determined principally by the concentration of two endocrine hormones in the blood, insulin and glucagon... 

Aggravation of disorders of metabolism—When people obtain their nourishment via the digestive tract the absorbed nutrients are conveyed by the portal system of blood vessels to the liver, where excess sugars are converted into fats and glycogen (a complex carbohydrate that is stored in the liver and muscles), fats are combined with proteins for safe transport in the blood, and amino acids are converted into proteins and waste products such as urea. 

Fats have one property that distinguishes them from other macronutrients, i.e., carbohydrates and proteins they are not water soluble but rather fat or Upid soluble. This property means that the body must have specific mechanisms to handle the fat in digestion, transport across membranes, and transport in blood serum. Many different types of molecules in cells and body fluids must contain fatty acids in order to function. A few definitions are necessary first to elucidate the importance of the dietary fats. 

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