Question

1. How is the digestion similar and different between a mammal, such as a pig, and a smaller multi-cellular organism like a Hydra? You must be specific (BUT BRIEF) with your answer. I expect you to tell me the similarities and differences between the two, with specific mentioning of the types of digestion they have, particular cells, and other processes to discuss how their overall digestion takes place.

Answer 1

Pigs are omnivores, hence have a monogastric digestive system similar to that of humans. Since they maximum feed on plant materials they have great capacity to digest enzyme degradable carbohydrates in the upper part of GI tract.

The digestive tract is made of:

1. Mouth: The pig has 44 permanent teeth which help in chewing. The broken and ground food is mixed with saliva secreted from 3 salivary glands i.e. parotid, madibular and sun lingual glands which is a mixture of water(99%), mucin, inorganic salts, lysozyme and -amylases. It moistens food and oesophages and marks the beginning of digestion of starch.
2. Oesophagus: It is a tube that carries food from the mouth to the stomach.
3. Stomach: The Gastric juice secreted in the stomach is viscous. It constitutes, HCl secreted by the parietal cells regulating pH, mucin secreted by mucous cells for lubrication, Pepsin A and Gastricin secreted by chief cells for protein digestion, Chymosin and Pepsin B secreted by chief cells for clotting milk and digests proteins and Lipase secreted by the chief cells for Triglyceride digestion. HCl activates pepsinogen into pepsin and ensure low pH for its activity.

The pancreatic secretions are clear consisting of salts, bicarbonaes adn enzymeswitha pH of 8.2. It neutralizes the acidic chyme entering the duodenum. Pancreatic -amylases hydrolyse starch and glycogen forming maltose, isomaltose, maltotriose and other simple sugars. Pancreatic juice contains three lipolytic enzymes i.e. lipase(to digest fats producing freefatty acids and monoglycerides), phospholipase A2(splits fatty acids from phspholipids after being activated by trypsin) and caboxyl ester hydrolase(hydrolyses mono, di and triglycerides, cholesterol and retinol esters) and a cofactor-colipase. Lipase is inhibited by bile salts which is counteracted by colipase. Colipase in order to become active should be cleaved by trypsin.

The proteolytic enzymes of pancreas are secreted as inactive zymogens to protect the gland from autodigestion, There are 5 enzymes: Trypsin which is an endopeptidase secreted as trypsinogen activated by trypsin and enterokinase, Chymotrypsin is an endopeptidase secreted as chymotrypsinogen,activated by trypsin, Elastase is an endopeptidase secreted as proelastase activated by trypsin.

Bile is secreted by liver with a pH 7.4- 7.8 containing bile salts, phospholipids and cholesterol. They help in emulsification and absorption of lipids. Peptidases digests proteins in the jejunum and ileum, the free aminoacids being absorbed by enterocytes. Lipid absorption occurs in jejunum.

4. Small intestine: Brunner's gland in the submucosa secrete alkaline secretions containing bicarbonates. The goblet cells secrete mucin. The wall of SI has villi increasing the surface area for absorption.

5. LargeIntestine: There are no digestive enzymes. It is an active site of microbial digestion. The undigested food passes through the caecum and expelled through the anus.

HYDRA:

Cnidarians do not possess digestive organs. In Hydra, the food which are generally aquatic animals like Daphnia are captured by stinging cells on the tentacles called Cnidoblasts. The gut is a hollow cavity with a single opening i.e. mouth. The food is trapped in mucous and moved to the gastrovascular cavity by the action of cilia. Proteinases are the digestive enzymes which are secreted by the endodermal gland cells into the cavity. Hydra possess two kinds of gland cells for the secretion of mucous and enzymes which breakdown the food into smaller particles. Long hair like flagellated cells lining the endodermis keep the contents mixing and other amoebic cells engulf smaller particles. Digestion is completed inside these cells in food vacuoles, diffusing the products all over the body. Digestion in Hydra is divided into two phases. A preliminary digestion which is extracellular i.e. outside the cells of endodermins and a secondary phase inside the cells i.e. intracellular.Undigested food is passes out of the cells into the gut cavity and expelled from the mouth.