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 occour 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 occour. 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 occours 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 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 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.
|