Unit 6. Organ level of organization

An organ is a structural and functional unit of a plant or animal. It comprises a number of tissues which are coordinated to perform a variety of functions, although one major function often predominates. The majority of plants and animals are composed of organs. Examples include the leaf of plants, and the liver in mammals. Most organs do not function independently but in groups called organ systems. A typical organ system is the digestive system which comprises organs such as the stomach, duodenum, ileum, liver and pancreas. Certain organs may belong to more than one system. The pancreas, for example, forms part of the endocrine (hormone) system as well as the digestive system, because it produces the hormones insulin and glucagon, as well as the digestive enzymes amylase and trypsinogen.

Chapter 1. Nutrition and energy supply. Introduction

Did you know?

The energy released from one

peanut will keep the brain

active for one hour.

All organisms require a constant supply of essential nutrients. What these nutrients are and the amounts of each required by an organism varies from species to species. In mammals carbohydrates and fats are needed in relatively large quantities as sources of energy, and proteins are needed in large amounts for growth and repair. Vitamins and minerals are required in much smaller quantities for a variety of specific functions. Water is a vital constituent of the diet and is necessary for efficient digestion.

Food must be small enough to be ingested. This may involve the use of teeth or other organs designed to break up food into small pieces. This mechanical breakdown also has the effect of giving the food a larger surface area, which aids later digestion.

Food comprises relatively few building blocks, largely monosaccharides, amino acids, fatty acids and glycerol, arranged in a variety of macromolecules, which meet the needs of the organism they make up. What suits one organism, however, does not necessarily suit another. The food ingested must therefore be broken down further into its component parts so that they can be rebuilt into the macromolecules and structures of the organism. The food must be made small enough to pass across cell membranes. This breakdown is mainly achieved through hydrolysis reactions speeded up by enzymes and is termed chemical digestion.

1. Назовите существительные, соответствующие данным глаголам (обратите внимание на способ словообразования):

to achieve, to bleed, to chew, to decrease, to digest, to increase, to remove, to swallow, to treat.

2. Образуйте прилагательные, соответствующие данным существительным :

Abdomen, aorta, duodenum, gland, intestine, liver, lymph, mastication, mucus, oesophagus, saliva.

Аналитическое чтение

The stages of digestion

The alimentary canal begins above with the cavity of the mouth and terminates below at the anus, traversing in its course the length of the thoracic and abdominal cavities. After the entrance into the mouth the food undergoes a twofold reduction: a mechanical one and a chemical one. The former process is called mastication. The chemical action taking place in the mouth is accomplished by means of the first digestive secretion, the saliva.

The sight, smell, and thought of food may elicit salivary and gastric secretion. These juices cannot be effective, however, before food enters the body and therefore mastication may be considered the first step in digestion. After a mouthful has been taken, the teeth crush and break the food into fragments of proper size for swallowing. Particle size may vary up to 12 mm in diameter but generally it is less than 2 mm. After mastication the mass weighs from 3 to 6 grams, about one-fourth of this weight due to saliva.

The saliva assists in mastication in several ways: by dissolving some of the readily soluble food components; by partly digesting some of starch; by softening the mass of food; by covering the bolus with mucus so as to make it more easily moved about in the mouth.

In man the act of mastication has more significance than the mere grinding of food preparatory to swallowing. While we are chewing the food, it is moving about in the mouth so that the taste buds are being stimulated and odors are being released to stimulate the olfactory epithelium. On these stimuli depends much of the satisfaction and pleasure of eating, which in turn initiate the process of gastric secretion.

The food which reaches the stomach is mixed with saliva and is semisolid in consistency. It accumulates in the fundus. Gradually, however, due to muscular movements, the food is mixed with the gastric juice. The reaction of the gastric juice is acid, pH 0.9 to 1.5. The most important constituents are hydrochloric acid, mucin, pepsin, rennin and gastric lipase.

The first step in gastric digestion is the action of hydrochloric acid upon protein to form acid metaprotein. The acid causes the protein to swell to a gelatinous mass (acid metaprotein) which goes rapidly into solution in the acid gastric secretions. Pepsin then hydrolyzes acid metaprotein to proteoses and peptones. Protein digestion does not go beyond the peptone stage in the stomach; it is completed in the intestines. There is no carbohydrate enzyme in the gastric juice but some of the cane sugar (sucrose) is hydrolyzed by the hydrochloric acid.

As a result of muscular movements which mix the food with the gastric juice, various chemical and physical changes take place which reduce the gastric contents to a semifluid, more or less homogeneous, creamy mass called chyme. At intervals portions of the chyme are ejected through the pylorus into the duodenum.

In the duodenum the food is treated by the pancreatic juice and the bile, an important external secretion of the liver. The liver is the largest gland in the body which has the most powerful influence upon all the metabolic functions of the body.

The principal organ of absorption is the small intestine. The function of the small intestine is to separate the useful from the useless constituents of the food. The fats, in the form of a fine emulsion, are taken up by lymph vessels called "lacteals", and ultimately reach the blood, while sugars, salts, and amino-acids formed from proteins pass directly into the small blood-vessels of the intestine.

The process is facilitated by the extreme unevenness of the intestinal wall, which is folded into many ridges and pockets, while in microscopic structure the surface is covered by fine finger-like processes named "villi" which are bathed in the fluids passing down the intestine. Further absorption is probably assisted by the "leucocytes", or white cells of the blood which are enormously increased in numbers after a meal, and which have the power of wandering out of the blood stream and taking up particles into their substance.

Food materials are absorbed almost exclusively by the small intestine. The large intestine, or colon, absorbs water and salts. The food is passed down the intestine by the contractions of its muscular coat, and finally, the indigestible residue, together with various waste substance excreted from the liver and intestinal walls, is cast out of the body in the stools.

Grammar 1. Времена группы Continuous Active.