Food is required by all organisms. The intake of food is called ingestion. The ingested food is broken down into simpler substances in the process called digestion.
Different Ways of Taking Food
- Each species or type of organism has their own way of taking in food.
- Bees and hummingbirds suck plant-nectars, infant of humans and some animals feed on mother’s milk, Snakes such as python consume the animals upon which they prey.
- Aquatic animals filter small food particles floating close by and feed on them.
Digestion in Humans
The human digestive system contains alimentary canal and associated glands. The digestive system of man consists of Mouth, Buccal Cavity, Oesophagus, Pharynx, Stomach Duodenum, Small Intestine, Large Intestine (colon), Rectum and Anus.
Human Digestive SystemThe Mouth and Buccal Cavity
- Mouth and Buccal Cavity is bounded by the upper lip and lower lip.
- Food is entered into the Buccal Cavity through mouth is called ingestion.
- The food is crushed and chewed in the Buccal Cavity with the help of the teeth and tongue.
- The Buccal Cavity leads into pharynx.
- The different teeth present in the Buccal Cavity are incisors, canines, premolars and molars.
- Salivary glands are present inside the Buccal Cavity. They secrete saliva. This saliva plays an important role in breaking down complex components like starch,which is further simplified into sugars.
- A flap-like valve called the epiglottis closes the windpipe. It prevents the entry of food particles into tracheae.
- Movement of food inside the esophagus in the stomach is by peristalsis movement.
- Stomach is u-shaped and it is the widest part of the alimentary canal.
- Food is digested inside the stomach with the help of gastric juice secreted by the gastric glands present in the stomach.
- Liver is the largest gland in our body. It produces bile juice. Bile plays an important role in the digestion of fats.
- Pancreas is the Mixed gland, it acts as both an endocrine and an exocrine gland.
- The pancreas secretes the pancreatic juice that helps to digest carbohydrates, proteins and fats. The pancreatic juice converts carbohydrates into simple sugars and glucose, proteins into amino acids, and the fats to fatty acids and glycerol.
The inner walls of the small intestine have millions of small finger-like projections called the Villi. Due to their presence the surface area for digestion as well as absorption of digested food increases by eight times.
- Energy needed for various activities is obtained from glucose.
- This process of utilisation of absorbed food, such as glucose, amino acids, fatty acids and glycerol is called as Assimilation.
The undigested and unabsorbed food then enters the large intestine. The food then travels upward in the ascending colon of the large intestine. The function of the large intestine is to absorb the water and salts from the undigested food material. The undigested semi-solid waste that passes into the Rectum is called faeces
It is then removed through the anus at intervals in a process called Egestion.
The Mouth and Buccal Cavity
- The internal part of the mouth contains the teeth and the tongue. Each tooth is rooted in a separate socket in the gums.
- The mouth, oesophagus, stomach, small intestine and large intestine from the alimentary canal, also known as the digestive tract.
- The glands that secrete juices are the salivary glands, the gastric glands, the liver, the pancreas and the intestinal glands.
- The digestive tract, along with the associated glands, forms the digestive system.
- Bacteria in the mouth break down the sugars from leftover food, and release acids. The acids damage the teeth, causing tooth decay.
- The buccal cavity is bounded in the front by the upper and the lower lips, at the top by the roof of the mouth, at the bottom by the tongue, and on the sides by the cheeks.
- Food is taken in through the mouth, a process known as ingestion.
- Incisors act like scissors,and are used for biting food. Canines are sharp and pointed, and are used to pierce or tear food. Premolars and molars, or the principal grinders, help in chewing and grinding food.
- The salivary glands secrete saliva, a transparent fluid that helps break down complex components into simpler forms.
Fig. Buccal cavity
- The taste buds, located in different areas of the tongue, help detect different tastes.
- The oesophagus is the food pipe that carries food from the throat to the stomach. The oesophagus is also known as the gullet, and is about 25 centimetres long.
- When you swallow food, a flap-like valve, called the epiglottis, closes the windpipe.
- The stomach is the widest part of the alimentary canal. Its walls are thick and very muscular. It is closed off at each end by a ring of muscular valves.
- Digestion in the stomach occurs with the aid of hydrochloric acid, mucous and digestive juices.
The Small Intestine
- The 'C' shaped tunnel, called the duodenum, is the upper part of the small intestine.
- The liver secretes a yellowish-green watery fluid called bile. It is temporarily stored in a sac called the gall bladder. It has a narrow tube-like structure called the bile duct, which opens into the duodenum.
- The pancreas secretes pancreatic juice that helps digest carbohydrates, proteins and fats. The pancreatic juice converts carbohydrates into simple sugars, glucose and proteins into amino acids, and fats into fatty acids and glycerol.
- Bile breaks the larger fat molecules into tiny droplets, thereby increasing their surface area, which helps digest fats easily.
- The pancreatic duct that arises from the pancreas joins the bile duct.
Absorption in the Small Intestine
- The inner walls of the small intestine have millions of small finger-like projections, called the villi. The surface of the villi allows amino acids and glucose to reach the blood capillaries, and allow fatty acids and glycerol to reach the lymph vessels, a process called absorption.
- The process of utilisation of absorbed food such as glucose, amino acids, fatty acids and glycerol is known as assimilation.
- Undigested food, which contains a lot of water and salts, is absorbed by the walls of the large intestine.
- Food travels upwards in the ascending colon of the large intestine, then moves towards the transverse colon, and then back downwards towards the descending colon.
- The undigested semi-solid waste that passes into the rectum is called faeces. Faeces are temporarily stored in the rectum, and are removed through the anus at intervals by a process called egestion.
Digestion in Green Eating Animals
Cellulose is an important component in the diet of herbivorous animals. It is present in the cell wall of plant cells. Humans cannot digest cellulose. Grass eating animals like the cow, ox, buffalo and sheep swallow the food without chewing. After feeding, they bring the food from the stomach back into the mouth and chew it leisurely. This process is called rumination, and such animals are called ruminants. Fig. Digestive System in Cow
- The stomach of a ruminant is divided into four chambers - the rumen, reticulum, omasum and abomasums. Among these, the rumen is the largest.
- The partially digested food in the rumen is called cud.
- Micro-organisms present in the stomach of the cow help digest the cellulose.
- Digestion in ruminants
Grass eating animals like cows, sheep, buffaloes and goats do not chew their food properly. Instead, they swallow it quickly and store it in the rumen. In the rumen, the digestive juices partially digest the food. The partially digested food in the rumen is called cud.
Partially chewed food is pushed down through the oesophagus into the rumen.
The cud is brought back to the mouth to be chewed properly. The process of chewing cud is called rumination. Rumination is also called second chewing. Animals that partially digest food in the rumen and bring it back to the mouth for additional chewing of the cud are called ruminants.
A large sac-like structure, called the caecum, lies between the small and large intestines.
Ruminants have green plants as their food. These plants contain a type of complex carbohydrate, called cellulose. In the caecum, a kind of symbiotic bacteria helps digest cellulose. In ruminants, a major part of all carbohydrates, including the complex carbohydrates such as cellulose and hemicellulose, is digested by bacterial action.
These symbiotic bacteria are not present in the human digestive system. That is why humans cannot digest cellulose.
Feeding and Digestion in Amoeba
The amoeba is a microscopic unicellular organism which belongs to the group protozoa. The name comes from the Greek word amoibe, meaning change. The habitat of the amoeba is freshwater. The amoeba contains jelly-like cytoplasm. Inside the cytoplasm are cell organelles like the nucleus, food vacuoles and contractile vacuole. An amoeba takes in oxygen gives off carbon dioxide through the cell membrane by a mechanism of diffusion. It takes in oxygen dissolved in water. An amoeba can move all directions and can change its shape with the help of pseudopodia as a locomotory organ.Fig. Steps involved in digestion in amoeba
An amoeba engulfs its prey along with a droplet of water with the help of pseudopodia and then forms a food vacuole inside the cytoplasm. The prey can be killed and digested with the help of the digestive juice secreted by the food vacuole. The undigested food is thrown out by changing the shape of the body. Amoeba forms a cyst in unfavorable conditions.
The term amoeba is derived from the Greek word amoibe, meaning change.
The amoeba is one of the simplest jelly-like animals. the ultimate shapeshifter. An amoeba is so minute that it can be seen only under a microscope.
Amoebae are found at the bottom of freshwater bodies like ponds and lakes, even in a gutter or muddy water. Some of them are also found in damp soil and food.
The amoeba is a single-celled animal. The amoeba has a jelly-like substance, called the cytoplasm, inside a flexible boundary called the cell membrane. Floating in the cytoplasm are various structures like the nucleus, food vacuoles and contractile vacuoles.
An amoeba takes in dissolved oxygen from the surrounding water, and gives off carbon dioxide into the water through the cell membrane.
An amoeba can move in all directions using temporary feet-like structures, called pseudopodia. An amoeba, with the help of pseudopodia, captures and engulfs its prey along with a droplet of water.
In an amoeba, digestive juices convert food particles into simpler substances. The cytoplasm absorbs digested food directly, and uses it for growth, maintenance and multiplication.
In the amoeba, undigested remains of food are thrown out of the body. An amoeba roll into a tiny ball called cyst during unfavorable conditions.