Nutrition in Amoeba

1.0Introduction

The digestive system is different in various organisms since the food and the way it is obtained differs in these organisms. The mode of nutrition in amoeba is holozoic and it is omnivorous.

(i) When Amoeba comes in contact with food particles, it forms pseudopodia which engulf the prey by forming a food cup. This process is called ingestion or phagocytosis.

(ii) When the tips of encircling pseudopodia touch each other, the food is encaptured into a bag called food vacuole. The food vacuole serves as a temporary stomach secreting digestive juices. 

(iii) The digested food gets absorbed and diffuses into the cytoplasm and then assimilated.

(iv) Assimilation is the utilization of digested food in various anabolic and catabolic reactions and conversion of absorbed food into complex molecules for various purposes such as production of energy, growth, repair and storage.

(v) The egestion of undigested food takes place from any point of the surface of the body.

2.0Digestive System of Human

The human digestive system consists of the alimentary canal and digestive glands and it involves mastication, swallowing, digestion of food, absorption of nutrients (digestive matter) and elimination of undigested matter.

Alimentary Canal

The alimentary canal is basically a long tube extending from the mouth to the anus.

It is differentiated into following parts.

Mouth

It is a transverse slit bounded by two soft movable lips. The lips serve to close and open the mouth, holding the food in between and also help in speaking.

Buccal Cavity

The mouth leads into a large space known as the mouth/oral/buccal cavity. It accommodates muscular tongue, teeth and salivary glands.

(i) Tongue

The floor of the mouth cavity is occupied by a large, muscular, mobile tongue. It is the organ of taste with taste buds on its upper surface. In dogs, during panting, the tongue helps in thermoregulation (temperature regulation) by quick evaporation of water of saliva. In some mammals, tongue is used to clean skin by licking.

Functions of tongue

(i) It moves food in the buccal cavity for mixing of saliva.

(ii) It helps in swallowing food.

(iii) It plays a role in speech.

(iv) It helps to clean the teeth.

(ii) Teeth

Four different types of teeth are present in our buccal cavity which help in crushing of food. Four types of teeth found in humans are Incisors (biting and cutting the food), Canines (wearing and tearing of food), Premolars and Molars (crushing and grinding of food).

Salivary glands secrete saliva which contains a digestive enzyme called ptyalin or salivary amylase that breaks down starch. Enzymes are the biological catalysts which break down complex organic compounds into simpler forms.

$Starch\stackrel{salivaryamylase}{\to }Maltose+Dextrin$

Saliva is an antiseptic as it kills germs and bacteria due to the presence of an enzyme called lysozyme. Saliva makes the food soft and slippery and helps in digestion of starch. There are three pairs of salivary glands in our oral cavity : parotid gland, sublingual and submandibular gland.

Pharynx

The buccal cavity opens into a short narrow chamber called pharynx. It is a common passage for both food and air.

Oesophagus (Food Pipe)

The oesophagus is a collapsible muscular tube leading the food by peristalsis from pharynx to stomach. There are no digestive glands, only mucus glands are present.

Stomach

It is a thick, muscular and J shaped sac (when empty) present on the left side of upper part of the abdomen just below the diaphragm. Gastric glands are present in the wall of the stomach. These glands release gastric juice which contains mucus, hydrochloric acid and two inactive protein digesting enzymes called pepsinogen and prorennin. The muscular walls of the stomach help in mixing food thoroughly with digestive juices.

HCl activates pepsinogen to pepsin and prorennin to rennin. HCl inactivates salivary amylase.

$Pepsinogen\stackrel{HCl}{\to }Pepsin$

$Prorennin\stackrel{HCl}{\to }Rennin$

It produces an appropriate acidic medium (pH = 2). It facilitates the action of enzyme pepsin and kills the bacteria present in food. Pepsin breaks down proteins into peptides.

$Proteins\stackrel{Pepsin}{\to }Peptides$

Rennin is a milk protein digesting enzyme which is produced only in infants. Mucus lubricates the food and protects the inner lining of the stomach from the action of HCl. Sphincter muscles regulate the exit of food from stomach into intestine due to peristaltic waves of stomach.

An ulcer on the inner lining of the stomach is called peptic ulcer which is caused by the higher acidity of gastric juice secretions.

Intestine

It is the largest part of alimentary canal which is fitted into a compact space because of the extensive coiling. It is distinguished into small intestine and large intestine.

(i) Small intestine

The small intestine is the site of the complete digestion of carbohydrates, proteins and fats. With intestinal juice, it also receives the secretions of the liver and pancreas for digestion.

Digestion of proteins in man starts from stomach. In buccal cavity, there is no digestion of proteins because saliva does not contain proteolytic (protein digesting) enzyme.

Liver is lobed and reddish brown in colour. It is largest gland of body present in the right side of upper part of the abdomen. It synthesizes bile juice. Gall bladder is present below the right lobe of liver. It stores bile.

Emulsification is the phenomenon of physically breaking large sized fat globules into large numbers of small fat droplets by the bile-salts of the bile juice. This increases the surface area for digestion of fats by the lipase enzyme. 

Pancreas is a yellow-coloured gland present just behind the stomach. It secretes pancreatic juice. The pancreatic duct carries pancreatic juice to the small intestine. 

Food is mixed with three digestive juices (bile juice, pancreatic juice and intestinal juice).

Bile juice (from the liver) provides alkaline medium and emulsifies fats but it is a non-enzymatic digestive juice so has no chemical action on food. Pancreatic juice (from the pancreas) contains trypsin, pancreatic amylase and pancreatic lipase enzyme which digest the proteins, starch and emulsified fats into peptides, maltose, fatty acids and glycerol respectively.

$Proteins\stackrel{Trypsin}{\to }Peptides$

$Fats\stackrel{Pancreaticlipase}{\to }Glycerol+Fattyacids$

$Starch\stackrel{Pancreaticamylase}{\to }Maltose+Dextrin$

The wall of the intestine secretes intestinal juice which changes the acidic medium of chyme into alkaline medium. It contains enzymes for complete digestion of peptides into amino acids, maltose into glucose and fats into fatty acids and glycerol.

$Proteins\stackrel{Intestinalenzymes}{\to }Aminoacids$

$Maltose\stackrel{Intestinalenzymes}{\to }Glucose+Glucose$

$Fats\stackrel{Intestinalenzymes}{\to }Fattyacids+Glycerol$

The inner lining of the small intestine has numerous fingers like projections called villi which increase the surface area for absorption of digested food. 

The length of the small intestine differs in various animals depending on the food they eat. Herbivores eating grass need a longer small intestine to allow the cellulose to be digested. Meat is easier to digest hence carnivores like tigers have a shorter small intestine.

(ii) Large intestine

It is the site of absorption of water from indigestible food. It helps in formation and temporary storage of faeces.

Anus

The digestive wastes, solidifies into faeces and is removed/egested (defecation) from the body via anus. The exit of waste material is regulated by anal sphincter.

■ Digestion in Ruminants

● Grass eating animals like the buffalo, cow and sheep swallow the food without chewing. After swallowing, they bring the food from the rumen back into the mouth and chew it leisurely. This process is called rumination and such animals are called ruminants.

The stomach of a ruminant is divided into four chambers - the rumen, reticulum, omasum and abomasum.

1. Rumen - Rumen is the largest part of stomach of ruminants.

The food is partially chewed and mixed with saliva. This partially chewed food is sent to rumen. Here partial digestion of food takes place as it contains large population of anaerobic bacteria which help in fermentation of food.

Symbiotic microorganism (bacteria) present in it release cellulase enzyme that digest the cellulose.

2. Reticulum - The partially digested food in the rumen is called cud that enter the oesophagus through reticulum.

3. Omasum - In omasum absorption of fluid takes place.

4. Abomasum - Abomasum is the “true stomach” which secretes acid & enzymes for digestion and ultimately send to intestine for complete digestion.

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