The Digestive Processes

 Animal diets vary enormously, and so do the methods of feeding. Certain parasites, e.g. tapeworms that lack a mouth or digestive canal absorb food through their body surface. In contrast, the majority of the animals ingest their food by means of a mouth. After the food has been digested by mechanical and chemical means, it is absorbed and utilised. 

Heterotrophs and Autotrophs

Animal feeders can be divided into autotrophs and heterotrophs.

Autotrophs are primitive organisms that first appeared about 3.5 billion years ago and colonised the earth for the next 2 billion years. They are mainly bacteria and green algae (cyanobacteria).Some autotrophs manufacture their food from inorganic compounds while others do so by photosynthesis.

Heterotrophs consist of most of the bacteria, protozoa, fungi, and multicellular animals. They all feed on ready-made food substances from plant or animal sources.

Heterotrophs are divided into herbivores, carnivores and omnivores. Herbivore feed on plants while carnivores are meat eaters. The omnivores feed on plants and flesh. Man is a good example of an omnivore.

Saprophytic Digestion

The simplest heterotrophic digestion is that of the fungi and bacteria and is known as saprophytic digestion. In this type of digestion, food is digested outside the body with the aid of enzymes released by the saprophytes. The digested food is then absorbed into the body through the surface. The food is usually a decaying vegetable matter such as a loaf of bread or a rotting animal. Saprophytic digestion is extracellular because it occurs outside the cells.

Intracellular Digestion

Phagocytosis

Like the fungi, Amoeba lacks a gut although it has an intracellular digestion. Amoeba engulfs its food particles by means of extensions of the cytoplasm called pseudopodia. 

The engulfed particle ends up in a food vacuole within the cytoplasm where it is broken down by the actions of enzymes secreted by the Amoeba. The process of capturing food by the pseudopodia and engulfing it is called phagocytosis.

Incomplete Digestive System

The Gastrovascular Cavity (an incomplete digestive system)

The simplest form of a digestive organ is the gastrovascular cavity possessed by the Hydra. A gastrovascular cavity has only one opening which serves as a mouth and as an anus. The Hydra captures its prey using tentacles that ring its mouth region. 

The tentacles contain stinging cells called cnidocytes which are used for attack and defence. The cnidocytes contain a poisonous fluid that paralyses the prey.

Hydra shows an example of a combined intracellular and extracellular digestion in that food is first broken down to simpler particles by the action of enzymes within the lumen of the gastrointestinal lumen and then is absorbed into the cells where it is finally reduced to usable nutrients.

 Complete Digestive System

A complete digestive system consisting of a separate mouth and a separate anus is present in annelids, insects and vertebrates. The earthworm’s digestive system consists of a muscular pharynx, crop, gizzard and intestine. Ingested food, which is usually dirt and decaying matter, is stored in the crop and gizzard chambers while undergoing mechanical crushing by the contractions of the muscles of the gizzard.

Actual digestion involving enzymes and absorption takes place in the intestine. The surface area of the digestive system is greatly increased by the presence of a dorsal fold, the typhlosole that projects into the intestinal lumen.

 Insects such, as grasshoppers, use powerful mouthparts called mandibles to break up plant material, which is then digested mainly in the midgut with the aid of enzymes released by the gastric caeca. Water and other useful salts are reabsorbed in the rectum, leaving a whitish uric acid to be excreted as a waste product.

Bird gizzards are often packed with stones and pebbles that grind food. Birds and reptiles excrete uric acid through a common passage, the cloaca, that serves also for urine as well as reproduction.

Continuous and Discontinuous Feeders

Animals can be grouped into continuous feeders and discontinuous feeders depending on the habits of their feeding. Blood and tissue parasites do not require storage organs because they are surrounded by unlimited supply of nutrients and can feed continuously. Such animals are known as continuous feeders.

Other animals are not so lucky in having readily available food. They experience unreliable supply of food. These are the discontinuous feeders. Since they are not sure where the next meal will come from, they eat as much food as possible whenever it becomes available. They are therefore provided with stomachs, crops and gizzards which serve as storage organs. The storage organ enables the animal to perform other duties while the food is being digested.

Mammalian Dentition

The teeth are classified into incisors (in front of the mouth), canines (behind the incisors), premolars and molars (cheek teeth). Broadly speaking, the incisors are used for gripping and cutting; the canines for tearing and shredding; the premolars and molars for grinding and chewing.

Variation in teeth

Some vertebrates such as the reptiles have simple teeth of similar shape that are adapted to stopping the prey from escaping from the mouth. On the other hand, the mammalian teeth are of different shapes and sizes and are adapted to the nature of the diet of the animal. 

Structure of a tooth

The structure of a mammalian tooth consists of the crown, the part that projects out, and the root. The crown is made up of the enamel and the dentine. The enamel is the hardest material in animals. The bulk of the tooth is made up of the dentine, a material similar to the bone but much harder than the bone. The dentine consists mainly of mineral substances, with calcium making up 70%. 

 

A soft tissue within the tooth, called the pulp cavity, contains nerves and blood vessels that supply the tooth with nutrients. A cement substance that keeps it firmly attached to the gum surrounds the root of the tooth.

 

 Adaptation to Diet

(a) Herbivores e.g. goat

Herbivores lack incisors and canine teeth on the upper jaw. The incisors on the lower jaw bite against a horny pad with the upper jaw. A gap called the diastema separates the pad and the premolars. The presence of the diastema is a characteristic of herbivore dentition. The premolars and molars have large surfaces suitable for grinding plant material. During the course of time, these grinding teeth tend to wear out but because they have open roots, they grow throughout the life of the animal.

(b) Carnivores e.g. leopard

The dog’s canine teeth are large and pointed and are adapted for attack and defence. The carnassial teeth are particularly large and suited for cutting chunks of meat and splitting bones. The top and lower jaw teeth fit into each other like the blades of a pair of scissors.

(c) Gnawers e.g. beaver

The rabbit’s incisors are sharp and curved with thick enamel at the front. They are used to nibble at the vegetation. The incisors have open roots and grow throughout the animal’s life. 

Food Digestion in Humans

Human digestive system consists of the alimentary canal that extends from the mouth to the anus and associated glands. The digestive system is made up of different tissues doing different jobs. The lining of the alimentary canal appears different in different parts of the gut reflecting their different roles.

The Mouth

The teeth break up the food into small pieces and with the help of the tongue mix it with saliva produced by the salivary glands. The saliva contains mucus to lubricate food, water to dissolve undissolved substances, and lysosomes to kill bacteria. It also contains amylase enzyme that digests starch. The food is reduced to a paste and is swallowed as a food bolus by involuntary reflex action through the pharynx. The food is prevented from entering the windpipe during swallowing by a flap of tissue called the epiglottis. 

The Oesophagus

 The oesophagus or gullet is a tube that connects the mouth to the stomach. It is lined by a thin epithelium with no villi, a few mucus-secreting glands and a thick muscular tissue that contracts and relaxes propelling the food down the oesophagus to the stomach. This muscular contraction is known as peristalsis. It occurs throughout the digestive canal.

 The Stomach

 The stomach is an elastic bag that can store food for a few hours. Its wall consists of three layers of muscles that contract and mix up the food contents. The mixing up of the food by the stomach is known as churning and turns food into porridge-like liquid called chyme. The chyme is released gradually into the small intestine via a sphincter, a region of thick circular muscle that acts as a valve.

The stomach contains gastric glands that produce gastric juice. Hydrochloric acid produced by the oxynthic cells helps kill bacteria and provide the acidic medium for digestion in the stomach. The parietal cells produce pepsin, which digests proteins.  Mucus produced by the goblet cells lubricates the stomach and protects it from the corrosive effects of stomach enzymes.

The Duodenum

This is the first part of the small intestine and is about 30 cm long. Most of the digestion takes place in the duodenum due to the secretions of the pancreas. The pancreas through the pancreatic duct secretes enzymes that digest carbohydrates, lipids and proteins. Bile secreted by the liver and stored in the gall bladder is released through the bile duct into the duodenum. Bile is alkaline due to hydrogen bicarbonate and therefore neutralises hydrochloric acid produced in the stomach. It also helps digestion by emulsifying lipids. The bile duct and the pancreatic duct join just before they enter the duodenum.

The Ileum

 The longest part of the intestine is the small intestine, which is the site of final digestion and absorption. The small intestine contains numerous glands that secrete enzymes, sodium bicarbonate and mucus. Its surface is not only folded it also has microvilli and villi that increase its surface area enormously, providing ideal conditions for food absorption.   

The Large Intestine

The large intestine consists of the caecum, appendix, ascending colon, transverse colon, descending colon and rectum. The large intestine is mainly concerned with water absorption so that faeces is expelled in a semi-solid form. The faeces is made up of plant fibre (mainly cellulose), cholesterol, bile, mucus, cell debris, bacteria and water. It is ejected from the body by the anal sphincter.

Chemical Digestion

1. Carbohydrates 

By far the most abundant carbohydrate in human diet is starch but there may also be some sucrose and glycogen from meat. Salivary amylase acts on starch reducing it to maltose. Most of the amylase is, however, denatured in the stomach so that much of the starch remains undigested.

The pancreas produces pancreatic amylase, which completes the digestion of starch in the duodenum, converting it to maltose. Glycogen is also digested here. Disaccharidases produced by the small intestine break down disaccharides such as maltose, sucrose and lactose into monosaccharides. The major monosaccharides are glucose, fructose and galactose. These are absorbed by active transport in the ileum epithelium, where they diffuse into the blood capillaries of the villi. Other carbohydrates such as cellulose, hemicellulose and lignin that the human system cannot digest, are excreted in the faeces as fibre.

2. Proteins

 Rennin, produced by infant mammals and present in the gastric juice, converts the soluble milk protein casein into its insoluble calcium salt. This keeps longer in the stomach so that pepsin can digest it. Pepsin digests polypeptides to peptides (6-12 amino acids long). Pepsin is an endopeptidase, which hydrolyses peptide bonds in the middle of a polypeptide chain. Pepsin works best at an optimum pH of 2. Other pancreatic endopeptidases continue the digestion of peptides in the duodenum converting them to smaller molecules.

Exopeptidases produced by the epithelial cells in the ileum complete the digestion of the short peptides to individual amino acids. Exopeptidases remove amino acids one by one from the ends of peptide chains.  Carboxypeptidases work from the C-terminal end, aminopeptidases work from the N-terminal end, and dipeptidases cut dipeptides in half.

Amino acids are absorbed by active transport into the epithelial cells of the ileum where they diffuse into the capillary vessels of the villi. 

 3. Lipids

Fats are emulsified by bile salts into fat droplets called micelles that have large surface area. The pancreas produces enzyme lipase, which digests triglycerides to fatty acids and glycerols in the duodenum. Fatty acids and glycerols easily cross the epithelial membranes of the cells of the ileum where the triglycerides are resynthesised and combine with proteins to form tiny lipoprotein particles called chylomicrons. These chylomicrons diffuse into the lacteal vessel into the lacteal (lymph) vessel inside each villus.  They are then transported via the lymph and join the blood stream through the vena cava. They are stored in the body as adipose tissue.

4. Water

 Water is absorbed by osmosis in the large intestine.

Adaptation of the Digestive System to Diet

The length of the digestive system of an animal tells us something about the diet of that animal. Thus, herbivores have long digestive systems relative to their body size than carnivores. This is because they feed on plant material, which needs more time to digest and extract nutrients. Most herbivores have special chambers that host bacteria and protozoa, which convert cellulose to simple sugars.

On the other hand, carnivores have short digestive systems because they eat flesh, which is easy to digest.

Digestion in Caecum and Colon

 In some herbivores such as rabbits and rodents, the cellulose-digesting microbes are housed in the colon and caecum where absorption of nutrients also takes place. Digestion in this region of the digestive system is less efficient because some of the nutrients are excreted as faeces before they have been digested properly.  These animals eat their faces and digest it once more in order to reclaim some of the nutrients. 

 Digestion in Ruminants

Ruminants like goat, sheep and cows have four chambers in their stomachs. This is again an adaptation to the digestion of a predominantly cellulose diet. When the animal chews and swallows grass, the food goes to the rumen and then to the reticulum. In both chambers, the food is digested by symbiotic bacteria. The animal regurgitates some of the semi-digested food and chews it again. This chewing of the cud, as the process is known, softens the food and help break down the fibres,

Finally, the food passes into the omasum and enters the abomasum where the animal’s enzymes complete the digestion. The large bacterial population in the ruminant digestive system is a major source of protein to the host.

Control of Digestive Enzymes

Secretion of the digestive juices is controlled by a number of factors including smell, taste and hormones. The presence of food in the mouth will stimulate the production of saliva and gastric juices. The presence of food in the stomach will stimulate the production of a hormone called gastrin by the stomach. Similarly, the release of a hormone called cholecystokinin (CCK) by the duodenum is triggered by the presence of amino acids and fatty acids in the duodenum. Another hormone, secretin, produced by the duodenal cells, stimulates the pancreas to produce the pancreatic juice and NaHCO₃.