How Nutrients Get in, and Wastes Out.

In a human being, nutrients are necessary for survival. But how are thesenutrients obtained? This report will go into depth on how the food we eat getsinto 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

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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 occour mainly in the form of starches. These areconverted by the digestive process to glucose, one of the main nutrients neededfor cellular respiration to occour. Starch is a large molecule, a polymer ofglucose.

Dextrin and maltose are intermediate products in the digestion ofstarch. Some foods contain carbohydrates in the form of sugars. These are thesimple sugars, such as sucrose (cane sugar) or lactose (milk sugar), that mustbe processed into smaller units. Occasionally, the simplest form of sugar, amonosaccharide such as glucose, is present in food. These monosaccharides donot require digestion.

Proteins are polymers composed of one or more amino acids. When they aredigested, they produce free amino acids and ammonia. Vitamins are a vital part of our food that are absorbed through the smallintestine. There are two different types of vitamins, water-soluble (All the Bvitamins, and vitamin C) and fat-soluble (vitamins A, D and K). Neutral fats, or triglycerides, are the principal form of dietary fat. They aresimple compounds, and within digestion are broken down into glycerol and fattyacids, their component parts.


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

Digestion in the Mouth

Upon entering the mouth, the food is mixed by mastication with saliva, whichstarts the digestive process by making contact with the food particles with thesalivary enzyme ptyalin, dissolving some of the more soluble matter within thefood. It also coats the food mass with mucin, to aid in swallowing.

Thechemical phase of digestion in the mouth begins when the salivary amylase,ptyalin, attacks the cooked starch or dextrin, converting some of this starchinto dextrin, and some of the dextrin into maltose. The salivary glands can beactivated when food is thought of, while the actual presence of food willproduce a continuous flow. Since food remains in the mouth for a very shortperiod, very little of the digestive process actually occours in the mouth.

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

Digestion in the Stomach

Here, salivary digestion continues until the acid of the gastric juicepenetrates the food mass, and destroys the salivary amylase. The food mass isthen saturated with gastric juice, and the gastric phase of digestion isinitiated. 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 theproteins to smaller forms, such as metaproteins, proteoses and peptones. Therealso may be a small amount of fat digestion in the stomach, since a small amountof lipase is present in gastric juice. This enzyme causes hydrolysis of thetriglycerides into glycerol and fatty acids.

The digestive action of these enzymes, combined with the action of the gastricjuice results in the solution of most of the food material. In the final stagesof gastric digestion, the fluid mass, propelled by peristaltic movements, passesinto the small intestine through the pyloric sphincter. Here, the chemicalphase of digestion is initiated.

Digestion in the Small Intestine

The fluid product of gastric digestion mixes with the intestinal secretion, andtwo other fluids, namely the pancreatic juice (produced by the pancreas) and thebile (produced by the liver). Both of these fluids are secreted near thepyloric valve, which separates the stomach from the intestine. These secretionsneutralize the acidic gastric juice, causing the gastric digestion phase to end.

The enzymes within the pancreatic juice, and those of the intestinal juice startthe final digestion phase. The pancreatic juice contains powerful amylase,protease, and lipase, that attack the starch, protein and fat that escapes theactions of the salivary and gastric phases of digestion. The intestinalsecretion contains enzymes that attack the intermediate products of proteolyticand amylolytic digestion, as well as some smaller food molecules.

The pancreatic amylase converts both the raw starch and the cooked starch thatwas not digested by the two previous phases. Cooked starch is converted todextrin, and the dextrin to maltose. The pancreatic lipase hydrolyzes theneutral 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 pancreaticproteases convert any remaining protein to proteoses and peptones. Theseintermediate products are then attacked by enzymes known as erepsins, andconverted 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 intoglucose.

After all of these processes, carbohydrates have been broken down into glucose,proteins have been broken down into amino acids, and fats hydrolyzed into fattyacids and glycerol. These nutrients are absorbed by the villi, finger-likemicroscopic projections that line the inside of the small intestine. The sugarsand 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 systemand then into the bloodstream.

Digestion in the Large Intestine

The last part of the digestive system, the large intestine is where all of thewastes enter. It holds the wastes and reabsorbs some of the remainingundigested material. The first part of the intestine is mainly responsible forreabsorbing. The materials reabsorbed are water, bacterial vitamins, and sodiumand 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 thenbecome feces, and are released through the anus. The food is moved down thesmall and large intestine by peristalsis, much like how food moves down theesophagus.

Circulatory SystemAfter the food molecules within the villi diffuse into the bloodstream, theblood carries the nutrients into the liver. There, the sugar is removed fromthe blood and stored for later use as glycogen. After the liver, the bloodtravels to the main provider of motive force, the heart. It is then pumped outthrough the arteries into the body. The blood vessels become smaller andnarrower until they reach their target tissue.

The blood is now within thesmallest vessels called capillaries. The capillary walls are only one cellthink, enabling diffusion of the nutrients carried in the bloodstream into theindividual cells, and diffusion of waste products back into the bloodstream fromthe cells. At this level, the systems regulating and governing the maintenanceof homeostasis are similar in both man and the Paramecium. Absorption andexcretion are basically governed by the concentration of fluids inside the cell,as compared with the fluid concentration outside the cell.


When the blood takes the nutrients to the cells, it receives cellular wasteproducts 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 tothe kidneys. It enters the kidney through the renal artery, and branches outinto many capillaries. Here, there is a slowdown of circulation. as a resultof 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 tubulesare responsible for the production of the fluid that is eventually eliminated asurine. They filter out many of the chemicals, particularly urea (which ispoisonous) and nitrates which are the by-products of protein digestion. Thisprocess is called pressure filtration. As the fluid moves down the tubule, manyof the nutrients that escaped the cell such as sodium ions and glucose arereabsorbed so that the body can use them, and will not become short of thesesubstances.

The fluid is now urine, and collects in a hollow region of thekidney. From the kidney, the urine enters the urinary bladder, a storagecontainer 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 alsotakes place in the lungs when a person breathes 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 andexcretory systems. The control is exercised through the autonomic nervoussystem, of which there are two parts. The first part, controlling increase inactivity, is called the sympathetic system. And second, controlling decrease inthe level of activity is the parasympathetic system. Both systems areunconscious, only chewing, swallowing and the anal and uretheral sphincters areunder conscious control.

Endocrine System

The endocrine system deals with hormones, which regulate the metabolic rates ofcells and organs. They are much like nerves, but target only certain parts ofthe body. They are essential to maintain homeostasis. The gastrin hormone isfound in the stomach, and controls the amount of gastric juice produced. Theyalso regulate excretions such as saliva.

The hormones controlling the digestiveand excretory systems are located some distance away from the cells that theyhave to control, therefore some method of transport must be utilized. Thismethod is the bloodstream. The hormonal glands secrete their hormones into thebloodstream. These hormones then travel to the target organ or cell, andregulate the activity of that organ or cell. This system is slower to respondthan the nervous system.

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How Nutrients Get in, and Wastes Out.. (2019, Mar 19). Retrieved from