Class 7 Science Nutrition in Animals Chapter Notes.

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Nutrition in animals explains how animals obtain and use food, covering herbivorous, carnivorous and omnivorous animals, parasitic, saprophytic and scavenging modes of nutrition, and the steps of holozoic nutrition: ingestion, digestion, absorption, assimilation and egestion. This Class 7 Biology chapter note also describes the human alimentary canal, teeth, tongue, liver, pancreas, digestive enzymes and the four-chambered stomach of ruminants.

ANIMAL NUTRITION

Animals are heterotrophs. They obtain food from green plants or from animals that feed on plants.

Herbivorous Animals: They feed only on plants or plant parts. Examples: Cow, buffalo, goat, deer, rabbit, horse and elephant.
Carnivorous Animals: They feed on the flesh of other animals. Examples: Lion, tiger and leopard.
Omnivorous Animals: They feed on both plants and animals. Examples: Pig, man and cockroach.

MODES OF NUTRITION IN ANIMALS

The mode of nutrition in herbivorous, carnivorous or omnivorous animals is called holozoic nutrition. However, some animals are parasites, saprophytes or scavengers.

i) Parasitic animals obtain food and shelter from the host. They may be endoparasites or ectoparasites.
Endoparasites live inside the host body and derive nutrients from the host's body fluid, blood or tissue. Examples: Malarial parasites live in RBCs, and flatworms and roundworms reside in the liver or intestine.
Ectoparasites such as ticks, mites, body louse and bed bugs cling to the body surface of their hosts and suck their blood.
ii) Saprophytic animals feed on dead and decaying organic matter. Earthworms are saprophytes.
iii) Scavengers feed on decaying animals. Vulture, crow and jackal are scavengers.

PROCESS OF HOLOZOIC NUTRITION

In holozoic nutrition, animals take in solid food. Their food contains insoluble complex nutrients. The body can use these nutrients only when they are digested and broken down into simple soluble molecules. The soluble molecules are then absorbed by the intestinal wall.

The process of holozoic nutrition involves the following steps: ingestion, digestion, absorption, assimilation and egestion.

Ingestion

Ingestion is the process of taking in food items from outside into the body through the mouth. Different organisms have different methods of food intake, and their food habits also vary. The organ involved in food intake, that is the mouth, varies from the lowest animals (protozoans) to the highest animals (mammals). The mouth parts of animals are modified to enable them to catch their prey or procure food. For example:

Unicellular Amoeba engulfs tiny particles of food by surrounding them with false feet or pseudopodia. The pseudopodia then join together to form a small cavity, a food vacuole.

[Figure: Fig. 1 Amoeba engulfing food by pseudopodia to form a food vacuole]

Paramoecium, another unicellular organism, has fine hair-like structures called cilia all over its body. The cilia sweep the food particles along with the water current into its mouth-like structure.

[Figure: Fig. 2 Paramoecium with labels: food residues being ejected, food vacuole, cilia, gullet, trichocysts (harpoons), contractile vacuole emptied, contractile vacuole filled, macronucleus and micronucleus]

In a multicellular organism like Hydra, there are numerous tentacles around its mouth which entangle small aquatic animals and kill them with their stinging cells. Then the food is pushed inside the mouth.

[Figure: Fig. 3 Feeding in Hydra, shown in four stages of tentacles catching and pushing prey into the mouth]

In insects like butterflies, houseflies, mosquitoes and bees, the mouth parts are modified to form a feeding tube to suck nectar from flowers. The feeding tube is called a proboscis.

[Figure: Fig. 4 Mouth parts of insects are modified to suit their feeding habits, showing the mouth parts of mosquito, housefly, butterfly and bumblebee, with the proboscis labelled]

A frog captures its prey with its long sticky tongue. The tongue is thrown out and the insect sticks to the tongue.

[Figure: Fig. 5 Ingestion of food in frog, shown in three stages]

Spider weaves a sticky web in which small insects get stuck.
An earthworm uses its muscular pharynx to swallow its food.
Human beings use their hands to put food into their mouths.

Digestion

The food that we take in is normally solid and complex. The cells of the body cannot use solid food. All living organisms, except parasites, must digest their food to get energy. Thus, the process of breakdown of complex food molecules into simpler forms is called digestion. Most animals use both physical and chemical methods for digesting food. Enzymes are biocatalysts, secreted by glands found in the digestive system. Enzymes help in the breakdown of complex molecules like carbohydrates, proteins and fats into simple molecules.

Digestion in lower animals like Amoeba and Paramoecium is generally intracellular (within the cell), as the food is digested inside the food vacuole and the digestive enzymes are secreted by the cell itself.

In higher animals like frogs and humans, the food passes through the alimentary canal and digestion occurs in its lumen. In animals with well-developed organs and organ systems, digestion is extracellular (outside the cell). Various digestive enzymes are mixed with the food as it passes through the alimentary canal. Examples are saliva in the mouth and gastric juices in the stomach.

Hydra is a multicellular organism where digestion is extracellular (outside the cell) as well as intracellular (in the cells of the inner layer).

Absorption

The digested food mixes in the body fluid, that is the blood of the animal, and gets absorbed. In unicellular organisms like Amoeba, absorption occurs in the cytoplasm. In Hydra, absorption of food occurs by diffusion of digested food through the cells lining the body cavity. In higher organisms (man), absorption of food occurs in the small intestine.

Assimilation

The process of conversion of absorbed food into living protoplasm is called assimilation. The food absorbed by the body is used as fuel to produce energy from glucose by respiration, for carrying out life processes such as growth and development. Proteins are the building blocks of our body and help in making new tissues as well as repairing worn-out tissues.

In the case of Amoeba and Paramoecium, assimilation of food occurs through streaming movements of protoplasm inside the body cavity. In Hydra, food is assimilated inside the cells.

Egestion

The process of removing undigested solid food parts from the body in the form of faeces is called egestion.

In Amoeba, when a sufficient amount of undigested food collects inside the body cavity, its cell membrane can rupture at any place to throw out the undigested food.

In Hydra, the waste materials are thrown out through the single body cavity, which works both as mouth and anus.

DIGESTION IN HUMANS

Humans have a well-developed digestive system. This consists of the gut or alimentary canal, along with the associated digestive glands.

[Figure: Fig. 10 Human digestive system with labels: Mouth, Tongue, Pharynx, Oesophagus, Pancreas, Liver, Gallbladder, Duodenum, Stomach, Transverse colon, Descending colon, Jejunum, Sigmoid colon, Ileum, Rectum, Anus, Ascending colon]

The food that we take in can provide energy only if it is digested, absorbed and assimilated to be distributed to all parts of the body.

During digestion, large insoluble pieces of food are broken down into soluble substances which are small enough to be absorbed through the walls of the intestine and into the bloodstream. This process involves certain physical and chemical reactions, as follows:

Physical Action
Chewing the food with teeth, so as to break the solid food into smaller pieces.
Churning by the stomach to mix the food with the gastric juices.

Chemical Action
Carbohydrates are changed to simple sugars.
Fats are changed to fatty acids and glycerol.
Proteins are changed to amino acids.

Enzymes are the digestive juices prepared by cells in various parts of the digestive system. Table 1 lists the source, substrate, enzyme and resultant products.

Table 1: Digestive enzymes: sources, action and resultant products

The alimentary canal

The alimentary canal is a long tube of varying diameter which begins at the mouth and ends at the anus. It consists of the mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus.

Mouth
Food is ingested through the mouth. The mouth contains the tongue, teeth and salivary glands. They work together in chewing (masticating) the food. The chewed food is mixed with saliva, a watery fluid secreted by the salivary glands. This process is called mastication. Saliva contains the enzyme amylase, which converts starch into sugar, so it tastes sweet to us.

Teeth
Our teeth cut, tear and grind food before we swallow it. We can feel the different shapes of our teeth with our tongue. There are four types of teeth in our mouth.
a) Incisors: These are the front 8 teeth, four on the upper and four on the lower jaw. They are flat, chisel-shaped teeth, well adapted for cutting and biting food items.
b) Canines: One canine is found on either side of each jaw. These round, sharp and pointed teeth are well adapted to hold and tear food.
c) Premolars: There are two premolars on each side of each jaw. They have flat surfaces bearing small grooves and ridges, and are well adapted to crush and grind food.
d) Molars: These are the last three teeth on both sides of both jaws. They have almost flat surfaces with small projections for effective grinding of food.

[Figure: Fig. 7 Teeth of an adult human showing incisors, canines, premolars and molars]

The white substance that covers our teeth is called enamel. It is the hardest substance in the body. All of us have two sets of teeth in our lifetime. The first set erupts when we are babies and lasts until we are about 8 years old. These are known as milk teeth or temporary teeth. A child has only 20 such teeth, 10 in each jaw. These teeth fall out one by one and are replaced by permanent teeth. A human adult has 32 teeth in all, 16 in each jaw.

Structure of a tooth
Each tooth remains buried in the jaw bone by its root, and the crown remains outside the gum. Incisors and canines have a single root, while premolars and molars have two roots each. The crown is formed of dentine and has a layer of enamel outside. Enamel forms the shining white part of the crown of each tooth. It is deposited on the surface of the crown. Enamel is the hardest substance in our body, even harder than bones. It is also chemically the most stable substance.

[Figure: Fig. 8 Structure of a tooth with labels: Enamel, Dentine, Pulp, Cementum, Periodontal Ligament, Bone, Nerves and Blood vessels]

Tongue
The tongue is a muscular organ that helps to mix saliva into the food. It pushes food towards our teeth. The tongue rolls the food into a bolus, which is easy to swallow. It is pushed down the food pipe or oesophagus by a series of rhythmic muscular contractions called peristalsis. The tongue is the main organ of taste. Different sets of taste buds located in specific areas of the tongue distinguish whether food is sweet, salty, sour, bitter, etc. Thus, taste buds help us to select the right quality of food. The tongue also helps us to speak.

[Figure: Fig. 9 Tongue showing location of different tastes, with the labels Bitter, Sour, Salt, Sweet, Taste region and Papillae]

Oesophagus
The oesophagus is the tube connecting the mouth and the stomach, and is about 30 cm long. It takes about six seconds for solid food to reach the stomach after it is swallowed. Liquids travel faster down the oesophagus. The powerful muscles in the oesophagus gently push food down to the stomach in a wave-like action, called peristalsis.

Stomach
The stomach is a J-shaped muscular bag. It can hold up to two litres of food at a time. Food stays in the stomach from a few minutes to a few hours, depending on the type of food. Gentle movements of the stomach wall churn the food up with digestive juices like hydrochloric acid and gastric juices, and change it into a semi-liquid state called chyme. The enzymes present in the gastric juices break down proteins. The hydrochloric acid kills harmful bacteria and also helps the enzymes to work. In the stomach, milk is changed to curd. Thus, food gets partly digested in the stomach.

Sometimes we experience vomiting. It is nothing but antiperistaltic movements of the stomach wall and oesophagus.

Small intestine
The food leaves the stomach at intervals and enters the small intestine, which is the longest part of the digestive system. It is about twenty feet or seven metres long and is a highly coiled tube, consisting of three parts: duodenum, jejunum and ileum.

In the duodenum, the liver and pancreas pour in their secretions, bile and pancreatic juice respectively. These juices contain salts and enzymes. The bile breaks up fat into tiny droplets, which are then acted upon by enzymes and split into fatty acids and glycerol. The pancreatic juice breaks down starch into simple sugars and proteins into amino acids.

Minerals and vitamins do not need to be changed. Cells are able to absorb them as they are. As the food moves further into the jejunum and ileum, the lower part of the small intestine, it is in simple soluble form and is readily absorbed into the bloodstream. Absorption occurs through numerous finger-like projections on the inner walls of the small intestine called villi, which have fine blood capillaries. After absorption, the food mixes in the bloodstream and is carried to all the cells of the body by the blood. The cells utilise this food and release energy.

Large intestine
Not all the food that we eat is digested and absorbed. The undigested food enters the large intestine, where water is absorbed from it. The semi-solid residue is stored in the last part of the large intestine, called the rectum, and is finally thrown out of the body through the anus. The large intestine is about 1.5 metres long and has a larger diameter than the small intestine.

LIVER AND PANCREAS

The liver is the largest gland of the body, weighing about 1.2 to 1.5 kg in an adult human. It is situated in the abdominal cavity, just below the diaphragm, and has two lobes. The hepatic lobules are the structural and functional units of the liver, containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called Glisson's capsule. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. The duct of the gall bladder (cystic duct), along with the hepatic duct from the liver, forms the common bile duct. The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct.

[Figure: Fig. 11 Human liver and pancreas with labels: Right hepatic duct, Left hepatic duct, Liver, Pancreas, Gallbladder, Stomach, Cystic duct, Common hepatic duct, Common bile duct, Pancreatic duct, Duodenum]

The pancreas is a compound (both exocrine and endocrine), elongated organ situated between the limbs of the 'U' shaped duodenum. The exocrine portion secretes an alkaline pancreatic juice containing enzymes, and the endocrine portion secretes the hormones insulin and glucagon.

DIGESTION IN GRASS EATING ANIMALS

Humans cannot digest the cellulose present in food, as the human system lacks the enzyme cellulase required to digest it. Ruminants are hooved, plant-eating animals that digest their food in two steps. Their stomachs are divided into four compartments: the rumen, reticulum, omasum and abomasum.

The half-chewed food is swallowed and first goes from the mouth to the rumen, the first chamber of the stomach. Here it is acted upon by bacteria and micro-organisms. This half-digested food then goes to the second muscular chamber, the reticulum, from where it is sent back to the mouth as cud to be chewed again. This action is known as ruminating. Grass eaters which chew the half-digested food, that is, ruminate, are called ruminants. Examples are cows, buffaloes, goats, sheep and bison.

The rechewed food is swallowed for the second time. Bypassing the first two chambers, it enters the third chamber, the omasum. Here the food is further broken down into still smaller pieces and finally enters the fourth chamber, the abomasum. Here, the enzymes act upon the food and digestion is completed as in the human stomach.

After this, the digested food is sent to the small intestine, where the nutrients are absorbed.

[Figure: Four-chambered stomach of cow (ruminant) with labels: Oesophagus, Abomasum, Omasum, Pancreas, Salivary gland, Mouth, Reticulum, Rumen, Liver, Small intestine, Large intestine, Anus]

EXERCISES

Answer the following in very brief:
1. Define holozoic nutrition.
2. Name the organ which produces bile juice.
3. Name the glands associated with digestion.
4. Name the stomach secretion which kills germs.
5. What is the role of large intestine?
6. What is the type of nutrition in Amoeba and Hydra?

Answer the following in brief:
1. What are the differences between milk teeth and permanent teeth?
2. What are the functions of gastric juice?
3. Name different types of teeth found in man.
4. How does Amoeba ingest its food?
5. What are enzymes? Give two examples.

Answer the following in detail:
1. Explain all the steps involved in holozoic nutrition.
2. With the help of a diagram, describe nutrition in Amoeba.
3. Describe an adult human dentition. Explain the role of the different teeth in digestion.

Table 1. Lists the details of the source, their substrates, affects and the resultant products.