How Nutrients Get in, and Wastes Out.
By Faisal Premji
10 Assignment — Part B
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.
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.
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
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
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
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.
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
substances. 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.
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.
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.