The digestive system in human beings is made of alimentary canal and some external glands like liver, pancreas and salivary glands which help in digestion of food. The alimentary canal is a long tube having a length of 8-10 meters. The various organs of human digestive system are: mouth, buccal cavity, oesophagus, stomach, small intestine (it consists of two parts duodenum and ileum), large intestine (it consists of colon and rectum) and anus.

Now, let’s discuss the various processes of nutrition taking place in the digestive system of human beings.

1. Ingestion

In human beings ingestion of food takes place through mouth, which is bounded by two movable lips.

2. Digestion

In human beings digestion starts in the mouth. The mouth consists of buccal cavity, teeth, tongue and salivary glands. Buccal cavity is the space between two jaws. Both the jaws have teeth which take part in cutting and grinding of food. Three pairs of salivary glands open into buccal cavity. Their names are parotid (situated below the ears), submaxillary (situated at the angles of lower jaw) and sublingual (situated below the tongue). The salivary glands produce saliva. The saliva contains enzymes ptyalin which converts starch present in the food to simple sugars. Saliva also helps in lubrication and softening of food.

Saliva is mixed with food by tongue present at the base of buccal cavity. Tongue also performs a number of other functions such as:

1) It helps in chewing of food
2) It helps in speech
3) It helps in perceiving taste of food
4) It helps in swallowing the food

The partially digested food in the mouth is swallowed by the tongue and goes into the stomach through oesophagus or food pipe. When the partially digested food enters oesophagus the walls of the oesophagus starts rhythmic contraction movements called peristalsis. By these peristaltic movements the partially digested food is pushed into the stomach. Oesophagus does not possess any digestive gland. However, it secretes mucus for further lubrication of food.

From the oesophagus the partially digested food enters the stomach which is the widest part of the alimentary canal. It is a J shaped organ that lies on the left side just behind the diaphragm. The food is further broken down in small pieces by churning movements of stomach. The stomach contains a number of gastric glands which secrete gastric juice.

The gastric juice consists of three substances: proenzyme pepsinogen, hydrochloric acid (HCl) and mucus. The presence of HCl in gastric juice makes the partially digested food acidic and prevents the growth of bacteria in food. HCl also converts the proenzyme pepsinogen present in the gastric juice into proteolytic enzyme pepsin. The enzyme pepsin digests the protein present in the food and covert it into small and simple molecules called peptones. In this way, digestion of proteins present in the food starts in stomach. It is to be noted, that the pepsin can digest proteins only in the presence of acidic medium. Thus, production of HCl in gastric juice is necessary for the working of pepsin. Walls of stomach are protected from the action of HCl by mucus. It also lubricates the food. The acidic partially digested food present at the end of its stay in stomach is called chyme. This partially digested food then passes into duodenum part of small intestine.

The small intestine is a narrow and coiled tube which constitutes about 75% of the whole length of alimentary canal. It is divided into two parts: duodenum and ileum. Duodenum is the first part of small intestine which is connected to stomach. It is about 25 cm in length. The remaining part of small intestine which is about 6.5 meters long in an adult man is called ileum. Duodenum receives the secretions of two glands: liver and pancreas. The secretions of liver and pancreas are received through a common duct. Liver is the largest gland present in the human body. The job of liver is to make a green liquid called bile which is alkaline in nature. So, bile neutralizes the acidity of food received from stomach. Bile also helps to break up the large molecules of fats present in food. Bile secreted from liver is normally stored in the gall bladder unless needed. Bile also helps in the absorption of fatty acids, fat soluble vitamins (vitamin A, D, E, and K), prevents putrefaction of food and helps in excretion of toxic materials out of body. Pancreas lies beneath the stomach. Pancreatic juice which contains digestive enzymes such as trypsin and pancreatic amylase are secreted by pancreas. Trypsin breaks up the proteins and peptones while the pancreatic amylase digests the starch present in food. The alkaline and partially digested food present in the duodenum of small intestine is called chyle.

From duodenum the partially digested food reaches ileum. In ileum the partially digested food is mixed with the colourless intestinal juice secreted by wall of ileum. The enzymes present in intestinal juice completely digest the partially digested food i.e. the intestinal juice breaks up the carbohydrates into glucose, proteins into amino acids and fats into fatty acids and glycerol. In this way, the digestion of food completes in the ileum part of small intestine.

3. Absorption

Absorption of digested food mainly takes place in small intestine. The wall of ileum contains lots of finger like structures called villi. The presence of villi on the inner wall of ileum increases its surface area to a large extent. This enlarged surface area helps in rapid absorption of digested food. Also, the walls of the villi are only one cell thick. The digested food can easily pass through them into tiny blood vessels present in villi, and mixes with blood. The diagram of villi is shown on next page.

4. Assimilation

The food absorbed by blood is then transported to all the parts of body and is used to obtain energy as well as for growth and repair of the body.

    

5. Egestion

The food not digested by human digestive enzymes goes from ileum to colon. Colon is the first part of large intestine. Here, water is absorbed from undigested food. Due to this, the undigested food becomes solid. This undigested solid food is stored temporarily in the rectum. Rectum is the last part of large intestine. From rectum the undigested solid food is finally removed out of the body through anus.

Test Your Understanding and Answer These Questions:

1. Explain various processes of nutrition taking place in the digestive system of human beings.
2. Give names of various digestive glands present in human beings. Also give their functions.
3. What is the function of small intestine and large intestine in digestion of food in human beings?
4. How food is absorbed in small intestine? Explain with the help of a diagram.
5. What are the functions of liver and pancreas?
6. What is the function of HCl in the process of digestion in stomach?
7. What is the importance of bile?
8. Explain digestion in mouth in human beings.
9. Give names of the salivary glands found in human beings and also give their location.
10. What are the functions of saliva?
11. What are functions of tongue in human beings?
12. What is the importance of HCl in digestion of proteins in human beings?
13. What are the functions of small intestine in humans?
14. Give difference between chyme and chyle.
15. What is the length of alimentary canal in humans?
16. Give the name of the organ in which bile is stored.
17. Name the largest gland present in human body.
18. Give the names of secretions of liver and pancreas.
19. Which organ of the alimentary canal absorbs most of water?
20. Give the names of two parts of small intestine.
21. Give the names of two parts of large intestine.

Grasshopper (or cockroach) is a multicellular organism. So, in grasshopper (or cockroach) the different processes of nutrition takes place in fully developed digestive system. The digestive system is a system of organs and glands concerned with obtaining, digesting and absorbing nutrients from food. The digestive system of grasshopper (or cockroach) is divided into three parts:

1) Foregut
2) Midgut
3) Hindgut

Foregut

The foregut of grasshopper (or cockroach) consists of following organs: mouth, oesophagus or food pipe, crop and gizzard.

Midgut

The midgut of grasshopper (or cockroach) consists of stomach and ileum.

Hindgut

Colon and rectum constitute the hindgut of a grasshopper (or cockroach). After discussing the digestive system let’s now discuss the different processes of nutrition in grasshopper.

 1. Ingestion

In grasshopper (or cockroach) food is ingested through the mouth with the help of forelegs and mouth parts.

2. Digestion

In grasshopper (or cockroach) the digestion of food starts in the mouth, where the saliva secreted by salivary glands mixes with the ingested food. The saliva contains some enzymes which digest starch present in the food. Saliva also helps in lubrication and softening of food. This partially digested food passes through the oesophagus into crop where it is stored temporarily. Then, the food passes into gizzard. Here, the food is finely ground by tough walls of gizzard. From gizzard, the food goes into stomach. In the stomach, the enzymes secreted by elongated glands called hepatic caeca get mixed with the food to digest it completely.

3. Absorption

The digested food is then absorbed by the walls of ileum when the digested good passes from the stomach into ileum.

4. Assimilation

The food absorbed by the walls of ileum is then supplied to all the parts of grasshopper (or cockroach) and is used to obtain energy, as well as for growth and repair of the body.

5. Egestion

The food not digested by grasshopper (or cockroach) goes from ileum to colon. Here water is absorbed from it. Finally, the undigested food is temporarily stored in the rectum. From the rectum the undigested food is removed out of the body through anus.

Test Your Understanding and Answer These Questions:

1. Explain the various processes of nutrition in grasshopper/cockroach.

 Amoeba is an important protozoa found in fresh water. It feeds on microscopic plants and animals present in water. The mode of nutrition in amoeba is Holozoic. And the process of obtaining food by amoeba is called phagocytosis. The different processes involved in the nutrition of amoeba are:

1. Ingestion

 Ingestion is the process of taking food in the body. Amoeba is a unicellular animal, so it doesn’t have a mouth for ingestion of food. Amoeba ingests the food by encircling it by forming pseudopodia. When the food is completely encircled , the food is engulfed in the form of a bag called food vacuole.

2. Digestion

 Digestion is the process of breaking the large and insoluble molecules in small and water-soluble molecules. In amoeba, several digestive enzymes react on the food present in the food vacuoles and break it down into simple and soluble molecules.

3. Absorption

The food digested by digestive enzymes is then absorbed in the cytoplasm by the process of diffusion. While the undigested food remains in the food vacuole. If a large amount of food is absorbed by amoeba, the excess food is stored in the cytoplasm in the form of glycogen and lipids.

4. Assimilation

During this step the food absorbed by the cytoplasm is used to obtain energy, growth and repair. This process of utilizing absorbed food for obtaining energy, repair and growth is called assimilation.

5. Egestion

When a sufficient amount of undigested food gets collected in the food vacuole, it is thrown out of the body by rupturing cell membrane. The process of removal of undigested food from the body is called egestion.

Test Your Understanding and Answer These Questions:

1. What is phagocytosis?
2. Define egestion?
3. Explain the various processes of nutrition in amoeba?

  The process of photosynthesis can be divided into two main phases:

1. Light reaction (or light dependent reaction)
2. Dark reaction (or light independent reaction)

Light reaction

It occurs in the grana present in chloroplast. During light reaction, chlorophyll absorbs light energy in the form of photons. Actually, light is emitted in the form of ‘packets of energy’ called photons. The light energy absorbed by chlorophyll is used to do following reactions:

1. Formation of ATP

ATP is the main energy source of a cell. Its full form is Adenosine Tri Phosphate. A molecule of ATP is formed by combining inorganic phosphate with ADP (Adenosine DiPhosphate) in the presence of energy.

ADP    +    phosphate    +    energy        ATP

This ATP is used as a source of energy during dark reaction.

2. Photolysis of water

The process of decomposition of water into hydrogen and oxygen by light energy absorbed by chlorophyll is called photolysis. The oxygen produced by the photolysis of water is released into the air as a byproduct of photosynthesis.

H₂O        2H⁺    +    1/2O₂    +    2e^–

It should be noted that oxygen released during photosynthesis comes from water.

3. Formation of NADPH (reduced Nicotinamide Adenine Dinucleotide Phosphate)

The hydrogen ions (H⁺) released during second reaction reduces the NADP (Nicotinamide Adenine Dinucleotide Phosphate) into NADPH in the presence of energy.

NADP    +    Hydrogen    +    Energy     NADPH

NADPH is also used during dark reaction.

Dark reactions (light independent reactions)

In dark reactions, the ATP molecule and NADPH produced during light reaction, are used in the matrix (or stroma) of chloroplast to make carbohydrates from carbon dioxide. In this process, carbon dioxide enters into a cycle of reactions in the presence of a substance called RuDP (Ribulose DiPhosphate) and enzymes. At the end of the cycle, carbohydrate is synthesized and ribulose diphosphate is regenerated.

RuDP + CO₂ + NADPH +ATP RuDP + Carbohydrate + NADP

                                                                  + ADP + phosphateThis cycle of reactions is called Calvin-Benson cycle because it was discovered by Melvin Calvin and Andy Benson.

Test Your Understanding and Answer These Questions:

1. What is a photon?
2. Explain the mechanism of photosynthesis.
3. What are the two phases of photosynthesis?
4. Give differences between light reactions and dark reactions.
5. Explain Calvin-Benson’s cycle.
6. From which raw material O2 is liberated during photosynthesis?
7. How water reaches the leaves from roots of a plant?

Site of photosynthesis

The main sites of photosynthesis in a plant leaf are small cell organelles called chloroplasts or plastids. Chloroplast is basically a sac like structure containing chlorophyll pigment. Chloroplast can be seen just below the upper epidermis in a tissue, called the palisade. Chloroplasts are usually spherical or ovoid vesicles bounded by an envelope of two membranes and filled with a fluid matrix or stroma. In the stroma of chloroplast many flattened, ovoid sacs called thylakoids are present. At places thylakoids are closely packed on one another to form stacks of discs called grana (singular granum). The thylakoids of one granum are connected to the thylakoids of another granum through stromal lamellae.

Test Your Understanding and Answer These Questions:

1. Why plants look green?
2. Write a short note on thylakoids?
3. In which organelle of plant chlorophyll is found?
4. Write the name of the organelle where photosynthesis occurs. Also, explain its structure?

 The process of photosynthesis is affected by two types of factors namely:

1. Internal factors
   1. Chlorophyll
   2. Leaf anatomy
   3. Accumulation of end products and
   4. Hormones
2. External factors
   1. Carbon dioxide
   2. Water
   3. Sunlight
   4. Oxygen and
   5. Temperature

Internal factors

1. Chlorophyll
The process of photosynthesis depends on the amount of chlorophyll present in the leaf. If chlorophyll is absent in a leaf, photosynthesis does not takes place.

2. Leaf anatomy
Leaf anatomy also influences the photosynthesis. The important anatomic structures which influence photosynthesis are size, shape, position and frequency of stomata.

3. Accumulation of end products
If the end products of photosynthesis are translocated at a slow rate, the rate of photosynthesis reduces.

4. Hormones
The rate of photosynthesis is also influenced by certain hormones present in a plant such as cytokinins, gibberellins and abscisic acid. Cytokinins and gibberellins increase the rate of photosynthesis, while the abscisic acid reduces it.

External factors

1.Carbon dioxide 
The amount of carbon dioxide in the atmosphere is 0.03%. If the amount of CO₂ increases up to 0.1%, the rate of photosynthesis also increases. But, the rate of photosynthesis decreases, if the amount of CO₂ increases beyond 0.1%.

2. Water
Water is a very important raw material for photosynthesis. A slight decrease in amount of water causes closing of stomata. This results in less absorption of CO₂ in leaves, hence the process of also slows down..

3. Sunlight
Photosynthesis does not take place in dark. If the intensity of sunlight is increased slowly, the rate of photosynthesis increases up to a certain point (called saturation point). If the intensity of sunlight increases beyond saturation point, then photosynthesis slows down.

4. Oxygen
It is observed that a small quantity of oxygen is essential for photosynthesis, whereas a high quantity of oxygen slow down the rate of photosynthesis.

5. Temperature
At low temperature the rate of photosynthesis is slow. As the temperature is increased, the rate of photosynthesis also increases up to a certain point. But a high increase in temperature slows down the rate of photosynthesis.

Test Your Understanding and Answer These Questions:

1. Explain the various factors which affect the rate of photosynthesis?

Every living organism requires food to fulfill the requirement of energy for their various activities. Green plants are autotrophic in nature. So, they synthesize their food by a very important process called photosynthesis. The term ‘photosynthesis’ consists of two words ‘photo’ means ‘light’and ‘synthesis’ means ‘to make’. Thus, photosynthesis is the process of making food with the help of sunlight.

It may be defined as the process of making food by green plants from carbon dioxide and water by chlorophyll in the presence of sunlight. In the process of photosynthesis oxygen gas is liberated. The chemical reaction for photosynthesis is as follows:

The process of photosynthesis takes place in leaves of a plant, because these contain the green colouring pigment chlorophyll which is necessary for synthesis of food in plants. Thus, leaves are the site of preparation of food in plants. That is why; leaves are also called food factories of a plant.

The carbon dioxide required for photosynthesis is obtained from the air which enters the leaves through very minute pores called stomata. But, aquatic plants use carbon dioxide dissolved in water. The water is absorbed from the soil by the roots of plants and transported to leaves by a network of tiny veins called xylem vessels. The sunlight energy is absorbed by chlorophyll. A chemical reaction using the trapped sunlight energy takes place in the chloroplasts (chlorophyll containing organelles of cells) to prepare food. The food prepared by the leaves of a plant is usually in the form of carbohydrates.

Significances of photosynthesis

1. Food is prepared by photosynthesis which is used by all the plants and animals
2. Photosynthesis decreases the amount of CO₂ present in atmosphere; hence it helps in reducing pollution.
3. During the process of photosynthesis O₂ gas is produced as a byproduct, which is used for respiration by animals.
4. During photosynthesis CO₂ is used by plants and O₂ is liberated. So, photosynthesis helps in maintaining the balance between CO₂ and O₂ present in air.
5. Many photosynthetic products such as coal, petroleum and natural gas are important sources of energy which are used to run our machines.

Test Your Understanding and Answer These Questions:

1. What is photosynthesis?
2. How do plants obtain food?
3. Which is main product of photosynthesis?
4. Which is by product of photosynthesis?
5. What is importance of photosynthesis?
6. Write the names of raw materials of photosynthesis.
7. From where do plants get CO2 for photosynthesis?
8. From where do aquatic plants get CO2 for photosynthesis?

In this type of nutrition, the organisms (called parasites) depend on the body of other living organisms (called their host) for getting their food. Many viruses, bacteria, fungi and animals have this mode of nutrition.

The organisms which depend on other living organisms (host) for food are called as parasites.

Parasites of two types:

• a) Ecto parasites
• b) Endo parasites

Ecto parasites

Ecto parasites are those parasites which obtain their food from their host by remaining outside the body of their host. For example, mosquitoes, ticks, lice and bed bug.

Endo parasites

Endo parasites are those parasites which obtain their food from their host by remaining inside the body of their host. For example, Ascaris, plasmodium vivax and tape worm.

Test Your Understanding and Answer These Questions:

1. Define parasites.
2. Explain types of parasites by giving two examples in each case.
3. Write names of two ectoparasites.
4. Write names of two endoparasites.

In saprophytic nutrition the organisms obtain their food from dead and decaying organic matter of dead plants, dead animals and other decomposing organic matter. And the organisms which obtain their food from dead and decaying organic matter of dead plants, dead animals and other organic matter are called saprophytes. Fungi, bread mould, some protists and many bacteria are saprophytic in nutrition.

In this type, the saprophytic organisms like fungi release digestive enzymes in their surrounding medium to convert the complex organic molecules such as sugars in simple forms such as glucose. This simple food is then absorbed through the body surface, and utilized for various activities by fungus.

Test Your Understanding and Answer These Questions:

1. What do you meant by saprophytic nutrition?
2. Define saprophytes. Give examples of saprophytes.
3. Explain nutrition in fungi.

The organisms having holozoic nutrition are further classified in three categories. These are:

1. Carnivores
2. Herbivores and
3. Omnivore  

Carnivores

The organisms which eat other animals or meat only are called carnivores. For example, lion, bear, snake, crocodile, lizard, eagle and wolf etc.

Herbivores

The organisms which consume or eat only plants as their source of food are called herbivores. For example, cow, rabbit, goat, deer, locust and camel etc.

Omnivores

The organisms which eat both plant food as well as meat of animals are called omnivores. The best examples of omnivores are human beings, crow, dog, sparrow and ants etc.

Test Your Understanding and Answer These Questions:

1. Explain different types of organisms based on their modes of nutrition.
2. Define herbivores. Give examples of herbivores.
3. Define carnivores. Give examples of carnivores.
4. What are omnivores. Give examples of omnivores.
5. Write differences between herbivores and carnivores.
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