Due to presence of specialized digestive tract/alimentary canal the process of nutrition involves the following steps:
Ingestion means intake of food. Feeding mechanisms varied in range of animals. In different animals ingestions occurs by different process.
(a) In parasitic protozoans (e.g. Trypanosoma), ingestion occur by diffusion.
(b) In sanguivorous animals (e.g. leech, mosquitoes) ingestion occur by pinocytosis or cell drinking and by blood sucking.
(c) In aphids, ingestion occurs by sucking sap of plant.
Microphagous feeders are those animals which take small sized food particles.
Ingestion occurs by maintaining water current which brings microscopic organisms like bacteria, diatom, protozoa etc.
Filter feeder strain small particles of organic matter from water.
Types – Pseudopodial feeder (Amoeba), Ciliary feeder (Paramecium), Flagellar feeder (Sponges) Mucoid feeder (Neries), Tentacular feeder (Hydra)
(a) Feed on large sized food particles
(b) These animals have different structure to capture and ingest the food.
E.g. (i) Tentacles with batteries of cnidoblasts in coelenterates
(ii) Eversible and sticky tongue (eg. frog, toad, wall lizard) to capture insects with differently adapted teeth, like well developed incisors in rabbit and well developed canines (tearing teeth) in carnivores (eg. lion, tiger etc.)
Mechanical digestion comprises mastication or chewing, liquefaction of food by digestive juices, swallowing and peristalsis.
Deglutition or Swallowing of food occurs in three stages after mastication is complete and the bolus has been formed. The first two stages of swallowing are deliberate and voluntary.
Subsequently, the individual has no further control and it becomes involuntary (a reflex action). The three stages are as follows:
(i) The food bolus is pushed backwards into the pharynx by the upward movement of the tongue. The soft palate rises and shuts off the nasopharynx. At the same time the larynx is raised and its opening is occluded by the overhanging epiglottis.
(ii) The bolus is grasped into the pharynx by the contraction of the constrictor muscles and forced into the oesophagus.
(iii) The bolus of food is carried down the oesophagus by peristalsis, and finally reaches the stomach.
(Dysphagia = Difficulty in swallowing)
The digestive process is regulated by the enteroendocrine system. The lower gastrointestinal tract contains many types of endocrine cells. The secretions of these cells control the motility and secretion of gastrointestinal tract components. Many enteroendocrine cells are part of the APUD (amine precursor uptake and decarboxylation) system.
Enteroendocrine cells usually are wedge-shaped cells tucked in among the other epithelial cells. Some enteroendocrine cells contact the lumen (using their surface microvilli); other cells do not. All cells rest on the basement membrane and contain prominent granules.
Absorption is the process by which nutrients produced after digestion are circulated throughout the body by blood and lymph and supplied to all body cells according to their requirements.
Almost no absorption takes place in mouth and oesophagus. In the stomach water, alcohol, simple salts and glucose are absorbed.
Mainly absorption occur through the villi of the ileum (small intestine) due to following reasons –
1. Internal folds of intestinal wall increases its absorptive surface area many folds.
2. Existence of microvilli on the free surface of the intestinal epithelial cells also enhances the absorption.
3. The chyme stays for a considerable time in small intestine.
4. The absorptive cells of intestinal mucous membrane are specialized for these functions. Absorption of dipeptides, monosaccharides and aminoacids across the plasma membrane of intestinal cells into blood present in the blood capillaries of villi, depends on 2 types of process –
It is a vital process in which nutrients are absorbed through the intestinal mucosa against concentration gradient. It occurs more rapidly as it is energy dependent. During this the nutrients are completely absorbed from the intestinal lumen.
It can stop when the cells are poisoned with cyanides or depressed by cold. It can also occur by two processes – active transport and endocytosis.
It is the movement of molecules from a low concentration area to a high concentration area by means of carrier molecules or proteins and the expenditure of energy. Glucose, galactose, amino acids, etc. are actively absorbed nutrients.
These are absorbed by a common sodium dependent pump in the intestinal cells.
During this large sized liquid or solid nutrients are taken in some vesicles through the plasma membrane.
It is a physical process in which nutrients are absorbed along the concentration gradient. Basically it depends on diffusion, osmosis, facilitated diffusion for absorption. It is a slow process as it does not require energy.
Water and their soluble substances (like water soluble vitamins C and B) and fructose are absorbed by passive absorption.
Absorption of fats:
It is already discussed that small intestine is the most important absorptive organ. Most of the end products of fat digestion are actively absorbed from the intestine. But some mono-, triglycerides and fatty acids are insoluble in water and cannot be directly absorbed from the intestinal contents. So they must first be made soluble in water.
For their solubilization bile salts are required (because bile salts have considerable ability to lower surface tension) and the process is called emulsification of fats.
First of all the emulsified products of lipid digestion must form micelles along with bile salts, phospholipids and lecithin in the intestinal lumen. Micelles are water soluble particles whose structure is similar to that of lipid emulsion except that these are smaller in size (about 5 min diameter).
Micelle has bile salts on their outer surface where as the fatty acids and cholesterol molecule forms the internal core. Action of bile salt is called hydrotrophic action. Formation of micelle helps absorption of free fatty acids, cholesterol and monoacyl glycerols from the intestinal lumen into the epithelial cells by simple diffusion.
In these cells, with the aid of bile salts many fatty acids and glycerols enter epithelial cells of intestinal mucosa resulting in the resynthesis of triacyl glycerides by combining together.
The resynthesised fats then pass into the lacteals (lymph vessels) of the intestinal villi, primarily in the form of small lipid droplets known as chylomicron. These small lipid droplets contain about 90% triglycerides and small amounts of phospholipid, cholesterol and free fatty acids and proteins.
By the lacteals, the fat is carried to the receiver of the chyle and then by the thoracic (lymph) duct to the left branchiocephalic vein, where it enters the blood [chylomicron are transported out of cells by exocytosis]. The lymph reaching the thoracic duct from the intestines contains an excess of fat giving it a milky appearance.
It is called chyle. In this way fatty acids and glycerol are eventually brought into the blood stream and so by a circuitous route, to the liver where they are reorganised and recombined to form human fat.
It is an anabolic process by which absorbed nutrients enters into the cells and involved in metabolic processes to resynthesize the complex biomolecules like proteins, carbohydrates, lipids and nucleic acid etc. According to their utility in the body the various nutrients are divided into four categories –
Energy producers – Carbohydrates, fats
Body builders – Proteins
Metabolic regulators – Vitamins and water minerals
Heredity substances – Nucleic acids
It is the elimination of faecal matter which is formed by undigested food along with water and excess of digestive enzyme.
This undigested food is absorbed in colon and changes into faeces which is temporarily stored in rectum where more water is absorbed. The faeces are finally expelled out through the anus. Faeces contain number of substances mentioned below
i. Undigested food which mainly consists of cellulose and other plant fibres
ii. Dead bacteria. Intestine harbours many symbiotic bacteria which synthesize amino acids and some vitamins like vitamin K.
iii. Mucus and dead mucosal cells from the gut wall. The gut lining is constantly shed off or wears off and is replaced by new cells.
iv. Bile pigments and its derivatives give colour to faeces.