Digestive System Evaluation

The purpose of this essay is to research and evaluate the digestive system effectively and extensively. Then describe the differences and indifferences of three different organisms in relation to their digestive system. The three organisms that have been chosen are the human, for comparative purposes, the Goliath Bird Eating Spider and the Cobra Lily carnivorous plant.
The Human Digestive System
Digestion is the breaking down of substance in the body, into an easier form of which it can be absorbed or excreted. It is also the process of which the body breaks down components so that they can be absorbed into the blood stream easier.
The digestive system involves many of the body’s organs and structures, such as: * The oral cavity (mouth); teeth, saliva, tongue * The pharynx * The oesophagus * The stomach * The small intestine * The large intestine * The rectum
Digestion starts at the oral cavity, where teeth grind the food to enhance surface area, and grind the food to a more digestible pulp. Saliva is excreted by three pairs of exocrine salivary glands; the parotid, the submandibular and the sublingual as shown in Figure 1. The saliva neutralises acidic foods and lubricates food with mucin. Saliva also contains antibacterial agents which help fight off bacteria that may be on the food. The tongue then manipulates and tastes the food before it is swallowed.
The food is then passed on to the oesophagus, a narrow, muscular tube about 30 centimetres in length. The oesophagus is a tube that leads from the pharynx, passing through the larynx and diaphragm, to the stomach. Food is transported through the oesophagus by a process called peristalsis. Peristalsis is a series of wave like contractions from the sphincter muscles located on the top and the bottom of the wall of the oesophagus. When this process occurs the lining of the oesophagus excretes mucus to lubricate the ball of food.
Figure 1 Human Digestive System Diagram
Figure [ 1 ] Human Digestive System Diagram
The food leaves the oesophagus and goes into the stomach, a j-shaped expandable sack normally located at the top left side of the abdomen. The stomach contains hydrochloric acid that is strong enough to break down metal with a pH of about 1.5 to 3. The HCL helps kill bacteria and aids in the breaking down of proteins to make them more susceptible to pepsin, an enzyme that digests protein.
The food then travels from the stomach to the small intestine via the pyloric sphincter. The small intestine is broken up into three sections; the duodenum (the first), the jejunum (the second) and the ileum (the last). Most of the enzyme digestion takes place in the small intestine, as most of the digestive enzymes are secreted either by the small intestine itself or by other organs such as the pancreas or liver and gall bladder. The main enzyme used in digestion is pepsin, an enzyme that breaks down the proteins into amino acids.
The food then passes through the ileocecal valve into the large intestines. At the start of the large intestines the food begins further digestion in a blind pouch called the cecum. The large intestines function is to reabsorb or resorb water and continue in the process of digestion by absorbing further nutrients. As it does this the food travels through the colon and into the rectum. The waste then stays in the rectum until the process of excretion via the anus takes place.
Goliath Bird Eating Spider
To process their liquid meals, tarantulas have developed an extensive digestive tract consisting of; * Oesophagus * Sucking stomach * Midgut * Excretory organs.
Figure 2 Goliath Bird Eating Spider anatomy
Figure [ 2 ] Goliath Bird Eating Spider anatomy
Prey with large amounts of solid parts such as rats must pre-digested, which is accomplished by spraying the prey with digestive enzymes that are excreted from openings in the chelicerae, the sharp points at their mouths as labelled in figure 2. The food is then sucked into the oesophagus by a straw shaped mouth. The liquidized food travels through the oesophagus, and into the suck stomach.

The sucking stomach consists of powerful contracting muscles. When this contraction occurs, the stomach in increased in a cross section thus creating a strong sucking action that allows the spider to suck its liquidized prey up through the mouth and into the intestines.

Once the fluid has entered the intestines, it is then broken down into small enough particles thus that it can pass through the intestines walls into the blood stream (haemolymph). The waste product is then moved into and excreted by and excretory organ.

The Cobra Lily

The California Pitcher Plant or “Cobra Lily” is a carnivorous plant. As the name suggests the pitcher plant has modified leaves that form pitfall traps as shown in figure 3. These pit traps are filled with enzymes to break down the plants food into a solution of amino acids, peptides, phosphates, ammonium and urea, from which the plant obtains its mineral nutrition (particularly nitrogen and phosphorus).These enzymes are deep within the pitcher and when prey falls into the pitcher and cannot escape; it soon becomes tired and falls into the enzyme rich fluid at the base.

Figure 3 Pitcher Plant anatomy
Comparisons

All three of the discussed organisms use digestive enzymes in one way or another to digest their food. A similarity that all three possess is the fact that they all have use of an oesophagus in their digestive tract. Both the human and the goliath bird eating spider have a stomach although they are used for different purposes. Also these two excrete the waste that is not absorbed. The Pitcher Plant is the only organism that does not have any intestines as it is a plant. The plant only has use of one digestive organ. The human and the bird eater both absorb the nutrients in the intestines via the intestinal wall. The pitcher plant is the only one out of the three that do not have an oral cavity to consume their food.