Grade 7, Science Olympiad (CBSE) - Nutrition in Animals 

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Grade 7  |   Science  |   Nutrition in Animals, Nutrition in Plants and Animals, Olympiad, CBSE, ICSE, SOF, ITO

Heterotrophic nutrition

Animals obtain their nourishment through this mode of nutrition as they cannot make their own food. But here is an exceptional case i. e Euglena which has chlorophyll and makes it own food in presence of light. But in the absence of light, it feeds on dead organic matter(saprophyte). 

Many animals are holozoic which means they ingest or swallow food. There are different types of holozoic animals which have been classified into the following categories-

1. Herbivores- grass and plant-eating animals like cow, goat, horse etc

2. Carnivores- Flesh-eating animals like Lion, Tiger Eagle etc

3. Omnivores- such animals feed on both plants and animals to fulfill their nutritional requirements like Man, Pigs, Bears etc.

4. Parasites- They live inside the body of another living animal. They are of two types -

1. Ectoparasites- living outside the host's body such as leech, lice etc.

2. Endoparasites- living inside the host's body such as tapeworm, malaria parasite etc.

5. Saprotroph- such animals feed upon dead and decaying organic matter like an earthworm.


Nutrition involves five basic steps in complex animals -

1. Ingestion- intake of food inside the body. This step may vary from one animal to another.

2. Digestion- breaking down of the complex components into simpler substances. The process is different in humans, ruminants, amoeba etc.

3. Absorption- Absorption of digested food substances into blood vessels through the intestine.

4. Assimilation- the absorbed food in the blood is converted into the substances required for growth and development like proteins and vitamins.

5. Egestion- The fecal material is removed from the body through anus from time to time in this process. 

Different ways of ingestion

1. An earthworm uses its pharynx to swallow its food.

2. Amoeba intakes food particles inside using pseudopodia which surrounds the food and makes a food vacuole to digest it inside.

3. Hydra uses the tentacles present around their mouth to catch its prey, killing it using the sting cells and pushing the food inside the body cavity.

4. Humming birds suck the nectar from flowers using its beak.

5. Humans use their hands to put food into their mouth and swallow the food after proper chewing. 

6. Snakes swallow their food as a whole without chewing it. 

7. Chameleon and frog use their long sticky tongue to catch their prey.

8. Earthworm uses its pharynx to swallow its food.

Digestion in Amoeba

 Amoeba is a unicellular microscopic organism which belongs to the group of protozoa. It is commonly found in fresh water. Amoeba contains jelly-like cytoplasm which contains cell organelles like the nucleus, food vacuoles, and contractile vacuole. The exchange of gases takes place through the cell membrane by the means of diffusion. It captures the prey using its locomotory organs (pseudopodia) which covers it from all sides and forms a food vacuole inside the cytoplasm. The prey captured in this vacuole is killed and digested with the help of digestive juices. The food vacuole gets fused with the lysosomes and all the digestive juices from the lysosomes enter the food vacuole which helps in digestion of the food. Two enzymes like amylase and proteinase have been found in the body of an amoeba. 

 Digestion in Humans

Humans have a well developed digestive system which consists of the alimentary canal in association with many digestive glands. The alimentary canal or the gut is divided into the mouth, esophagus, stomach, small intestine, large intestine, and rectum.

Mouth- The very first step of digestion starts from the mouth as food is ingested from here only. The mouth contains teeth, tongue and salivary glands. The teeth help in chewing food (Mastication). The chewed food is mixed well with saliva which is a watery fluid secreted by salivary amylase. Salivary amylase is responsible for converting starch into sugars. Humans have four types of teeth - Incisors, Canines, Premolars, and Molars. 

Incisors- Flat and chisel-shaped teeth, present in the front of the mouth, total eight in numbers, well adapted for cutting and biting.

Canines- Sharp and pointed teeth, adapted to hold and tear food, total four in number.

Premolars- adapted for crushing and grinding the food, total eight in number in an adult human.

Molars- flat surfaces with pointed edges, adapted for fine grinding of the food, a total of 12 molars are there including 4 wisdom teeth as well. 

Oesophagus- It is also known as food pipe which is a tube-like structure running from mouth to the stomach. Its length is about 30 cm and has powerful muscles which are adapted for gently pushing the food down to the stomach. It possesses a rhythmic fashion of contraction and relaxation which is known as peristalsis. This movement helps the food to pass through the esophagus to get into the stomach. 

Stomach- It is a J shaped organ which is considered the widest part of the alimentary canal. the stomach is well developed to perform the churning of food with the digestive juices. Here, the food becomes chyme which is semi-solid food substance. The stomach wall secretes gastric juices which contain enzymes and hydrochloric acid. The enzymes help in breaking down of food particles into proteins whereas the hydrochloric acid kills the harmful bacteria which entered our body through the food. Hydrochloric acid also provides a medium to activating another enzyme which helps in digestion of food. 

Small intestine - Now, the chyme enters the small intestine after the stomach has done its job. The small intestine is about 20 feet long in an adult human. It has been divided into three parts- Duodenum, Jejunum, and Ileum. The chyme from the stomach firstly enters duodenum where the liver and pancreas pour their secretions. The liver secretes bile juice which is responsible for modification of fats while the pancreas secretes its juices which contains several enzymes. These enzymes help in breaks down of starch into simple sugars and proteins into amino acids.

 After duodenum, the chyme enters jejunum and then Illeum. The intestinal wall absorbs the digested food with the help of finger-like projections called villi. The absorbed food enters the bloodstream which carries it to the cells of our body. Cells use this food to produce energy. 

Large intestine- The digested food which was not absorbed by the small intestine slowly enter the large intestine where the absorption of water takes place and all the unabsorbed food material is slowly pushed towards the rectum and eventually ejected out through the anus. 

Digestion in Ruminants

Herbivores feed upon grasses and various plants which are rich in cellulose. Cellulose is present in the cell wall of the plant cells. Animals like Cow, Buffalo, Ox, and Sheep swallow the food without chewing. After, they have filled their stomach with enough food they bring back the food into their mouth and keep chewing it. This is called rumination and such animals are called ruminants. Their stomach is divided into four chambers- the Rumen, Reticulum, Omasum, and Abomasum. The partially digested food present in the rumen is called cud. Many microbes which are present inside the stomach of the cow help in digestion of cellulose. Humans cannot digest cellulose. The process of digestion in ruminants starts with a mouth where half-chewed food from the mouth goes to the rumen. The rumen is the first chamber of the stomach. Bacteria present in this part helps in the digestion of cellulose. Now this partially digested food goes to the second chamber the reticulum. From here food is again sent back to the mouth as cud, to be chewed again. This re-chewed cud is swallowed again and now after passing first and second chamber, it enters the third chamber, the Omasum. Food is broken down to simpler substances here and finally gets entered into forth chamber, the abomasum. Here the digestion of food is completed and digested food is absorbed to send it to the cells of all the parts of the body.

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