In a human being, nutrients are necessary for survival.  But how are these nutrients obtained?  This report will go into depth on how the food we eat gets into our cells, and how the waste products that we produce get out of the body.  Also, the unicellular organism Paramecium will be compared with a human being, in terms of all of the above factors.

Dietary Nutrients

The chief nutrients in a diet are classified chemically in four groups:  carbohydrates, proteins, vitamins (Which do not require digestion) and fats.

Carbohydrates in the diet occur mainly in the form of starches.  These are converted by the digestive process to glucose, one of the main nutrients needed for cellular respiration to occur.  Starch is a large molecule, a polymer of glucose.  Dextrin and maltose are intermediate products in the digestion of starch.  Some foods contain carbohydrates in the form of sugars.  These are the simple sugars, such as sucrose (cane sugar) or lactose (milk sugar) that must be processed into smaller units.  Occasionally, the simplest form of sugar, a monosaccharide such as glucose, is present in food.  These monosaccharides do not require digestion.

Proteins are polymers composed of one or more amino acids.  When they are digested, they produce free amino acids and ammonia.

Vitamins are a vital part of our food that are absorbed through the small intestine.  There are two different types of vitamins, water-soluble (All the B vitamins, and vitamin C) and fat-soluble (vitamins A, D and K).

Neutral fats, or triglycerides, are the principal form of dietary fat.  They are simple compounds, and within digestion are broken down into glycerol and fatty acids, their component parts.

Ingestion

Intake of food in the Paramecium is controlled by the needs of the cell.  When food is sensed,  the organism guides itself towards the food, and guides it into the oral groove, then enclosing it in a vacuole.  Enzymes are then secreted to digest the food, which is then absorbed into the cytoplasm and made available to the various organelles.  But, a Paramecium has to be able to move to its food source, while a human cell has his food brought to it through the circulatory system.   In man, a much more complicated system exists than that of a unicellular organism, for the size of the animal and the fact that all of the cells within the animal must be able to absorb food and get rid of wastes, just like the Paramecium does.

Digestion in the Mouth

Upon entering the mouth, the food is mixed by mastication with saliva, which starts the digestive process by making contact with the food particles with the salivary enzyme ptyalin, dissolving some of the more soluble matter within the food. It also coats the food mass with mucin, to aid in swallowing.  The chemical phase of digestion in the mouth begins when the salivary amylase, ptyalin, attacks the cooked starch or dextrin, converting some of this starch into dextrin, and some of the dextrin into maltose.  The salivary glands can be activated when food is thought of, while the actual presence of food will produce a continuous flow.  Since food remains in the mouth for a very short period, very little of the digestive process actually occurs in the mouth.

Following digestion in the mouth, the semisolid food mass is passed by peristaltic movements of the esophagus, a long muscular tube that connects the mouth to the stomach.  The food then reaches the esophageal sphincter, a ring of muscle at the upper end of the stomach.  This sphincter then opens to let the food into the stomach.

Digestion in the Stomach

Here, salivary digestion continues until the acid of the gastric juice penetrates the food mass, and destroys the salivary amylase.  The food mass is then saturated with gastric juice, and the gastric phase of digestion is initiated.

The gastric phase of digestion is chiefly proteolytic, or protein-splitting.  Within this process, the gastric glands secrete the enzymes pepsin and rennin.  These enzymes, aided by gastric acid, converts a fairly large amount of the proteins to smaller forms, such as metaproteins, proteoses and peptones.  There also may be a small amount of fat digestion in the stomach, since a small amount of lipase is present in gastric juice.  This enzyme causes hydrolysis of the triglycerides into glycerol and fatty acids.

The digestive action of these enzymes, combined with the action of the gastric juice results in the solution of most of the food material.  In the final stages of gastric digestion, the fluid mass, propelled by peristaltic movements, passes into the small intestine through the pyloric sphincter.  Here, the chemical phase of digestion is initiated.

Digestion in the Small Intestine

The fluid product of gastric digestion mixes with the intestinal secretion, and two other fluids, namely the pancreatic juice (produced by the pancreas) and the bile (produced by the liver).  Both of these fluids are secreted near the pyloric valve, which separates the stomach from the intestine.  These secretions neutralize the acidic gastric juice, causing the gastric digestion phase to end.  The enzymes within the pancreatic juice, and those of the intestinal juice start the final digestion phase.  The pancreatic juice contains powerful amylase, protease, and lipase, that attack the starch, protein and fat that escapes the actions of the salivary and gastric phases of digestion.  The intestinal secretion contains enzymes that attack the intermediate products of proteolytic and amylolytic digestion, as well as some smaller food molecules.

The pancreatic amylase converts both the raw starch and the cooked starch that was not digested by the two previous phases.  Cooked starch is converted to dextrin, and the dextrin to maltose.  The pancreatic lipase hydrolyzes the neutral fat to glycerol and fatty acids.  The bile has an important role here, as it, along with the alkali content in the secretions, emulsifies the fat, producing many fat surfaces on which the lipase can act.  The pancreatic proteases convert any remaining protein to proteoses and peptones.  These intermediate products are then attacked by enzymes known as erepsins, and converted slowly into their individual amino acids.  The intestinal enzymes, maltase, sucrase, and lactase hydrolyze their respective disaccharides (maltose, sucrose and lactose) into their component monosaccharide units, and finally into glucose.

After all of these processes, carbohydrates have been broken down into glucose, proteins have been broken down into amino acids, and fats hydrolyzed into fatty acids and glycerol.  These nutrients are absorbed by the villi, finger-like microscopic projections that line the inside of the small intestine.  The sugars and amino acids take a direct route, and pass into the capillaries of the villi, taking them directly into the bloodstream.  Glycerol and fatty acids, however, are first resynthesized into triglycerides, they then enter the lymphatic system and then into the bloodstream.

Digestion in the Large Intestine

The last part of the digestive system, the large intestine is where all of the wastes enter.  It holds the wastes and reabsorbs some of the remaining undigested material.  The first part of the intestine is mainly responsible for reabsorbing.  The materials reabsorbed are water, bacterial vitamins, and sodium and chloride ions.  Within the last half of the intestine, wastes are stored.  These wastes are made up of undigested food, and dead bacteria.  The wastes then become feces, and are released through the anus.  The food is moved down the small and large intestine by peristalsis, much like how food moves down the esophagus.

Circulatory System

After the food molecules within the villi diffuse into the bloodstream, the blood carries the nutrients into the liver.  There, the sugar is removed from the blood and stored for later use as glycogen.  After the liver, the blood travels to the main provider of motive force, the heart.  It is then pumped out through the arteries into the body.  The blood vessels become smaller and narrower until they reach their ‘target’ tissue.  The blood is now within the smallest vessels called capillaries.  The capillary walls are only one cell think, enabling diffusion of the nutrients carried in the bloodstream into the individual cells, and diffusion of waste products back into the bloodstream from the cells.  At this level, the systems regulating and governing the maintenance of homeostasis are similar in both man and the Paramecium.  Absorption and excretion are basically governed by the concentration of fluids inside the cell, as compared with the fluid concentration outside the cell.

Excretion

When the blood takes the nutrients to the cells, it receives cellular waste products as well, such as carbon dioxide, urea, and surpluses of other chemicals, such as glucose.  From the cells, the blood (with the waste products) goes to the kidneys.  It enters the kidney through the renal artery, and branches out into many capillaries.  Here, there is a slowdown of circulation.  as a result of this, pressure increases, and much of the plasma is forced out of the blood.  Renal tubules (nephrons) number about one million in each kidney.  There tubules are responsible for the production of the fluid that is eventually eliminated as urine.  They filter out many of the chemicals, particularly urea (which is poisonous) and nitrates which are the by-products of protein digestion.  This process is called pressure filtration.  As the fluid moves down the tubule, many of the nutrients that escaped the cell such as sodium ions and glucose are reabsorbed so that the body can use them, and will not become short of these sub

Stances.  The fluid is now urine, and collects in a hollow region of the kidney.  From the kidney, the urine enters the urinary bladder, a storage container for the urine.  When this bag fills, a sphincter opens to the urethra, and the urine is let out of the body from an external opening.  Excretion also takes place in the lungs when people breathe out, and also though sweat.  But, these parts of the excretory system are not controlled as well as the kidney, and can lead to loss of salt.

Nervous System

The nervous system controls a large part of the activity of the digestive and excretory systems.  The control is exercised through the autonomic nervous system, of which there are two parts.  The first part, controlling increase in activity, is called the sympathetic system.  And second, controlling decrease in the level of activity is the parasympathetic system.  Both systems are unconscious, only chewing, swallowing and the anal and uretheral sphincters are under conscious control.

Endocrine System

The endocrine system deals with hormones, which regulate the metabolic rates of cells and organs.  They are much like nerves, but target only certain parts of the body.  They are essential to maintain homeostasis.  The gastrin hormone is found in the stomach, and controls the amount of gastric juice produced.  They also regulate excretions such as saliva.  The hormones controlling the digestive and excretory systems are located some distance away from the cells that they have to control; therefore some method of transport must be utilized.  This method is the bloodstream.  The hormonal glands secrete their hormones into the bloodstream.  These hormones then travel to the target organ or cell, and regulate the activity of that organ or cell.  This system is slower to respond than the nervous system.

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