Samacheer Class 11 Bio Zoology - Digestion and Absorption

Food serves multiple essential functions in the human body. The primary use of food is to provide energy in the form of calories that fuel all metabolic activities and physical work. Beyond energy provision, food supplies organic substances including proteins, carbohydrates, lipids, vitamins, and minerals that are necessary for growth and development, particularly during childhood and adolescence. Food also provides the raw materials required for the replacement of worn-out and damaged tissues throughout life, enabling tissue repair and maintenance. Additionally, food regulates and coordinates the various physiological activities that take place in the body through the action of vitamins, minerals, and other bioactive compounds. These regulatory functions include maintaining proper enzyme function, hormone production, immune system function, and overall metabolic balance. Therefore, food is not merely a source of energy but a comprehensive source of nutrients essential for survival, growth, repair, and optimal functioning of all body systems.

• There is an increase in the small intestine surface area.
• The villi are present in the inner walls of the intestine.
• The villi is the absorbtive unit
• The microvilli present in the villi increase the absorptive surface.

The digestive system performs several critical functions essential for maintaining life and health. The primary function is to ingest food and break it down through mechanical and chemical processes into smaller, absorbable molecules. The digestive system mechanically breaks down food through chewing and churning in the stomach, while chemically breaking it down using various enzymes secreted by different organs including salivary glands, stomach, pancreas, and small intestine. Another crucial function is to absorb the digested nutrients, water, and electrolytes from the food across the intestinal epithelium into the bloodstream. Once absorbed, these nutrients are transported via the circulatory system to every cell in the body where they are utilized for energy production, growth, tissue repair, and maintenance of various physiological functions. The digestive system also eliminates undigested food residues and waste products through defecation. Additionally, the digestive system plays an important role in immune function, as the intestinal mucosa contains lymphoid tissue that helps protect against pathogenic microorganisms. Therefore, the digestive system serves as the gateway through which all external nutrients enter the internal environment and are distributed to support all cellular activities and maintain homeostasis.

• The HCI in the stomach coverts the inactivated pepsinogen into active pepsin.
• The activated pepsin acts on protein and converts them into proteases and peptones
• HCI provides an acidic medium which is optimum for pepsin action.

Protein digestion is a complex process that begins in the stomach and continues through the small intestine, ultimately resulting in the absorption of amino acids that are incorporated into cytoplasmic components. In the stomach, gastric juice containing the enzyme pepsin initiates protein digestion by breaking down proteins into smaller polypeptides and peptones. Rennin, present in the gastric juice of infants, helps in the digestion of caseinogen and converts it into casein for better absorption. As the partially digested protein moves into the small intestine, the pancreas secretes trypsin which hydrolyzes proteins into polypeptides and peptones, and chymotrypsin which hydrolyzes peptide bonds associated with specific amino acids. The intestinal juice, known as succus entericus, contains peptidases that further break down the remaining di and polypeptides into individual amino acids. These amino acids are the end products of protein digestion and are absorbed by the intestinal villi through active transport and diffusion. Once absorbed, the amino acids enter the bloodstream and are transported to various cells throughout the body where they are utilized for the synthesis of new proteins that become integral parts of the cytoplasm, including structural proteins, enzymes, and other functional molecules necessary for cellular activities.

Diphyodont dentition refers to the condition in which an organism develops two sets of teeth during its lifetime. Human beings and many other mammals exhibit this type of dentition. The first set consists of twenty temporary milk teeth or deciduous teeth that erupt during infancy and early childhood. As the individual grows, these milk teeth are gradually shed and replaced by a second set of thirty-two permanent teeth or adult teeth. This replacement occurs over several years during childhood and adolescence. The term diphyodont is derived from the Greek words 'di' meaning two and 'phyodont' meaning tooth-bearing. This type of dentition is advantageous because it allows the organism to have appropriately sized teeth for different life stages, with smaller milk teeth suited to the smaller jaws of young individuals and larger permanent teeth suited to the adult jaw size. Diphyodont dentition is characteristic of most mammals and is considered an evolutionary adaptation for efficient feeding at different ages.

Food prepared at home without artificial enhancers and preservatives is nutritionally superior and healthier compared to commercially prepared food containing these additives. Artificial preservatives and chemical enhancers used in commercial food production can accumulate in the body over time and cause numerous health problems. Consuming food with artificial additives has been linked to heart problems and cardiovascular diseases due to increased sodium content and unhealthy fats. Hypertension or high blood pressure is commonly associated with excessive consumption of preserved foods that contain high levels of salt and other chemical additives. Stomach disorders including gastritis, ulcers, and digestive disturbances can result from the irritating effects of artificial chemicals on the gastrointestinal lining. Some artificial additives and hormones in processed foods have been implicated in causing sterility and reproductive problems in both males and females. Additionally, certain food additives and preservatives may disrupt hormonal balance and lead to attainment of early puberty in girl children, affecting their normal development. Home-prepared food, on the other hand, uses fresh natural ingredients without chemical additives, provides better nutritional value, and is free from harmful preservatives, making it a healthier choice for maintaining overall health and preventing diet-related diseases.

The dental formula of human beings is a standardized notation that represents the arrangement and types of teeth in each half of the upper and lower jaw. The dental formula is written as 2123/2123, where each number represents the number of teeth of a particular type in one half of each jaw. The first number 2 represents two incisors, the second number 1 represents one canine, the third number 2 represents two premolars, and the fourth number 3 represents three molars. This arrangement is present in both the upper and lower jaws, and on both the left and right sides. Therefore, the total number of permanent teeth in an adult human is 32, with 16 teeth in the upper jaw and 16 teeth in the lower jaw. This dental formula indicates that humans are heterodont, meaning they possess different types of teeth with different shapes and functions suited for cutting, tearing, and grinding food.

• We have to take healthy foods.
• We have to take plenty of water.
• We have to regulate our stress.
• We have to take probiotics daily.
• We have to do exercise daily.

Dietary fibres are classified into two main types based on their solubility in water, and each type serves distinct functions in the digestive system. Soluble fibres dissolve in water to form a gel-like substance in the digestive tract. These fibres help regulate blood glucose levels by slowing down the absorption of sugars, and they also help reduce cholesterol levels in the blood. Additionally, soluble fibres act as prebiotics, promoting the growth of beneficial bacteria in the colon, and they help soak up toxins and waste materials in the digestive system, facilitating their elimination from the body. Insoluble fibres, also known as roughage, do not dissolve in water and remain largely intact as they pass through the digestive system. These fibres add bulk to the faecal matter and stimulate peristaltic movements of the intestines, promoting regular and healthy bowel movements. Insoluble fibres help move bulk through the intestine efficiently, preventing constipation and maintaining proper digestive health. Both types of fibres are essential components of a balanced diet and contribute significantly to overall gastrointestinal health and general well-being.

The tongue is a muscular organ that performs multiple important functions in the digestive and speech systems. It plays a crucial role in the intake of food by manipulating and positioning food particles within the buccal cavity. During mastication, the tongue helps in the mechanical breakdown of food by moving it around and mixing it thoroughly with saliva secreted by the salivary glands, which initiates chemical digestion of carbohydrates. The tongue also facilitates the formation of a bolus, a rounded mass of chewed food mixed with saliva, which is essential for the process of swallowing. Additionally, the tongue aids in propelling the bolus towards the pharynx during deglutition. Beyond its digestive functions, the tongue is instrumental in speech production by modulating the flow of air and shaping sounds. The upper surface of the tongue is covered with small projections called papillae, which contain taste buds or taste receptors. These taste buds enable the sensation of taste, allowing discrimination between different flavours such as sweet, salty, sour, bitter, and umami, thereby enhancing the sensory experience of eating and contributing to appetite regulation.

Gastro-oesophageal reflux disorder, commonly abbreviated as GERD, is a condition that occurs when the cardiac sphincter, the muscular valve located at the junction between the oesophagus and the stomach, fails to contract properly. During the churning action of the stomach, which is necessary for mechanical digestion and mixing of food with gastric juices, the weakened or incompetent cardiac sphincter allows gastric contents including acidic gastric juice to flow backward into the oesophagus. This retrograde movement of acidic stomach contents into the oesophagus causes irritation and inflammation of the oesophageal lining, which is not adapted to withstand such acidic conditions. This results in a burning sensation in the chest region, commonly referred to as heartburn or acid reflux. Prolonged or frequent episodes of GERD can lead to complications such as oesophagitis, Barrett's oesophagus, and oesophageal ulcers. The condition may be triggered by various factors including obesity, pregnancy, certain medications, smoking, and consumption of spicy or fatty foods. Management typically involves lifestyle modifications and, if necessary, pharmacological interventions to reduce gastric acid production.

• It is a digestive disorder in which the food is not properly digested leading to a feeling of fullness of the stomach.
• It may be due to in adequate enzyme secretion anxiety food poisoning overeating and spicy food.

Vomiting is a complex physiological reflex that involves the forceful expulsion of stomach contents through the mouth. It is essentially a reversal of the normal peristaltic movement, known as reverse peristalsis, in which the normal downward wave-like contractions of the alimentary canal are reversed, causing the contents to move upward and outward. Vomiting serves as a protective mechanism by which harmful substances, toxins, irritants, or indigestible materials are ejected from the body through the mouth, thereby preventing their absorption and potential harm to the organism. The vomiting reflex is controlled by a specialized region in the brain called the vomit centre or chemoreceptor trigger zone, which is located in the medulla oblongata of the brainstem. This centre receives sensory inputs from various sources including the gastrointestinal tract, the vestibular system, and higher brain centres. Vomiting is typically preceded by a sensation of nausea, which is an unpleasant feeling of queasiness and discomfort. The act of vomiting involves coordinated contractions of the abdominal muscles, diaphragm, and muscles of the pharynx and oesophagus. Vomiting can be triggered by various stimuli including infections, food poisoning, motion sickness, psychological factors, certain medications, and increased intracranial pressure.

Digestion is the biochemical process of breaking down large macromolecules of food such as carbohydrates, proteins, and lipids into smaller micromolecules that can be easily absorbed and utilized by the body. This process involves both mechanical and chemical breakdown of food. The different processes involved in digestion and nutrition are ingestion, which is the intake of food into the mouth; digestion, which is the breakdown of food into smaller molecules through mechanical and enzymatic action; absorption, which is the uptake of digested food molecules through the intestinal epithelium into the bloodstream; assimilation, which is the incorporation of absorbed nutrients into the body cells and tissues for growth, repair, and energy production; and elimination of undigested substances, which is the removal of waste materials that cannot be digested or absorbed, such as fiber and other indigestible components, through defecation. These stages collectively constitute the complete process of nutrition in the human body.

• There are two sphincter muscles namely the cardiac sphincter and pyloric sphincter present in the stomach.
• If the sphineter does not contract properly during the churning action of the stomach of the gastric juice with acid may flow back into the oesophagus and cause heart bum resulting in GERD.

• The opening of the stomach into the duodenum is guarded by the Pyloric Sphincter.
• It periodically allows partially digested food to enter the duodenum and also prevents regurgitation of food.

A person suffering from digestion problems may have various underlying causes. One common condition is constipation, which occurs when faeces are retained within the rectum due to irregular bowel movements. Constipation is often caused by poor intake of dietary fibre, inadequate water consumption, and lack of physical activity. Other possible reasons for digestion problems include indigestion due to inadequate enzyme secretion, anxiety, food poisoning, overeating, or consumption of spicy foods. Additionally, infections, inflammatory bowel conditions, hormonal imbalances, or structural abnormalities of the digestive tract may also cause digestion problems. A proper diagnosis requires medical evaluation to determine the specific cause and appropriate treatment.

Oral hydration therapy is a medical treatment used to manage dehydration caused by excessive loss of water and electrolytes, particularly during episodes of diarrhea or vomiting. This therapy involves drinking plenty of fluids, typically water and electrolyte solutions, in small amounts at regular intervals to gradually rehydrate the body. The key principle is to consume fluids slowly and frequently rather than in large quantities at once, which helps the body absorb water more effectively and prevents further fluid loss. Oral hydration therapy is a simple, cost-effective, and non-invasive method that helps restore the body's fluid and electrolyte balance, preventing serious complications of dehydration such as organ failure and shock. It is particularly important in cases of acute diarrhea and is often recommended as the first line of treatment before considering intravenous rehydration.

BMI or Body Mass Index is a numerical measure of body fat based on height and weight that applies to adult men and women. BMI is calculated as body weight in kilograms divided by the square of height in meters, expressed by the formula BMI = Body Weight in Kg / (Body Height)² in meter. For example, if a person weighs 50 kilograms and has a height of 1.6 meters, the BMI would be calculated as 50 divided by 1.6 squared, which equals 50 divided by 2.56, resulting in a BMI of 19.5. BMI values are used to categorize individuals into different weight status categories: a BMI below 18.5 indicates underweight, a BMI between 18.5 and 24.9 indicates normal weight, a BMI between 25 and 29.9 indicates overweight, and a BMI of 30 or above indicates obesity. This simple calculation provides a quick assessment of whether a person's weight is appropriate for their height and helps identify potential health risks associated with being underweight or overweight.

Gingivitis is an inflammatory condition of the gums caused by the accumulation and spread of plaque on the teeth. When plaque is not removed regularly through proper oral hygiene, it spreads down the tooth into the narrow gap between the gums and tooth enamel, causing inflammation of the gingival tissues. The primary symptoms of gingivitis include redness and swelling of the gums, bleeding during brushing or flossing, and bad breath or foul odour from the mouth. If left untreated, gingivitis can progress to more severe periodontal disease, leading to damage of the supporting structures of the teeth, including the periodontal ligament and alveolar bone. Regular brushing, flossing, and professional dental cleaning are essential preventive measures to avoid gingivitis and maintain healthy gums.

Heterodont refers to the condition in which an organism possesses teeth of different types and shapes, each specialized for different functions. In humans, the permanent teeth are heterodont, meaning they are of four different types. Incisors are chisel-like cutting teeth located at the front of the mouth, used for cutting and biting food. Canines are dagger-shaped tearing teeth, one on each side of the incisors, used for tearing and piercing food. Premolars are grinding teeth located behind the canines, used for crushing and grinding food. Molars are large grinding and crushing teeth located at the back of the mouth, used for grinding and crushing food into smaller pieces. The dental formula of humans is 2123/2123, which means there are 2 incisors, 1 canine, 2 premolars, and 3 molars in each half of both the upper and lower jaws. Multiplying this by 2 (for both sides) gives a total of 16 teeth in the upper jaw and 16 teeth in the lower jaw, for a total of 32 permanent teeth in an adult human. This heterodont arrangement of different tooth types allows humans to efficiently process a variety of foods with different textures and consistencies.

• Destroy aging and defective blood cells.
• Stored glucose in the form of glycogen or disperses glucose into the bloodstream with the help of pancreatic hormones.
• Stores fat-soluble vitamins and iron.
• Detoxifies toxic substances.
• Involves in the synthesis of non – essential aminoacids and urea.

Protein deficiency diseases result from inadequate protein intake and are serious nutritional disorders, particularly affecting children in developing countries. The two major forms of protein energy malnutrition (PEM) are marasmus and kwashiorkor. Marasmus occurs when there is severe deficiency of both protein and calories, typically affecting children suffering from chronic diarrhea and malabsorption. In marasmus, the body becomes lean and weak with severely reduced fat and muscle tissue, and the skin becomes thin and folded, giving the appearance of an old person. The child experiences stunted growth and developmental delays. Kwashiorkor, on the other hand, occurs primarily due to protein deficiency while caloric intake may be relatively adequate. Children with kwashiorkor develop characteristic symptoms including dry and scaly skin, a prominent potbelly caused by abdominal edema and liver enlargement, edema or swelling in the legs and face due to low plasma protein levels, stunted growth and developmental retardation, changes in hair color from normal dark color to reddish or blonde, and general weakness and irritability. Both conditions are serious and can lead to permanent physical and mental damage if not treated promptly with adequate nutrition and medical care.

There are three pairs of salivary glands in the human body, each with distinct locations and associated ducts. The parotid glands are located in the cheeks and secrete saliva through Stenson's duct. The submandibular glands are located in the lower jaw region and their secretion passes through Wharton's duct. The sublingual glands are situated beneath the tongue and secrete saliva through Bartholin's ducts, also known as the ducts of Rivinis. These glands collectively produce saliva continuously, with the daily secretion ranging from 1000 to 1500 millilitres. Saliva contains enzymes like salivary amylase and lipase, along with mucus and antimicrobial compounds, which aid in the initial digestion of food and maintain oral health.

The gastric glands of the stomach contain several types of specialized cells, each secreting different substances essential for digestion. Chief cells, also called peptic cells or zymogen cells, secrete gastric enzymes including pepsinogen, which is converted to pepsin in the acidic environment to digest proteins. Goblet cells secrete mucus, which protects the stomach lining from the corrosive effects of gastric acid and enzymes. Parietal cells, also known as oxyntic cells, secrete hydrochloric acid (HCl) which provides the acidic environment necessary for enzyme activation and protein digestion. Parietal cells also secrete the intrinsic factor, a glycoprotein essential for the absorption of vitamin B12 in the terminal ileum, a condition known as Castle's intrinsic factor. Together, these secretions create an optimal environment for the chemical and mechanical breakdown of food in the stomach.

Growing children require adequate amounts of protein for proper growth, development, and maintenance of body tissues. A protein-deficient diet during early childhood can lead to serious conditions of protein-energy malnutrition, such as marasmus and kwashiorkor. Both conditions present with characteristic symptoms including dry and scaly skin, a distended abdomen or pot-belly appearance, oedema in the legs and face, stunted growth and reduced height, changes in hair colour often becoming reddish or blonde, general weakness, and irritability. Marasmus is an acute form of protein-energy malnutrition resulting from a diet inadequate in both carbohydrates and proteins. Children suffering from marasmus often experience chronic diarrhea, and their bodies become severely emaciated with significant loss of fat and muscle tissue, resulting in thin and folded skin that hangs loosely on the body. Kwashiorkor, on the other hand, results primarily from protein deficiency with relatively adequate carbohydrate intake. These conditions can cause permanent damage to physical and mental development if not addressed promptly through nutritional rehabilitation and medical intervention.

• Food adulteration causes harmful effects in the form of head ache palpitations allergies, cancers.
• It reduces the food quality common adulteration are addict onto citric acid to lemon juice.
• Papaya seeds to pepper melamine to milk.

The person is suffering from jaundice and follows a specific diet to support liver recovery and reduce the burden on the affected organ. Jaundice is a condition in which the liver is damaged and fails to break down hemoglobin and remove bile pigments from the blood. The deposition of these bile pigments in the skin and sclera of the eyes causes yellowing of these tissues. Jaundice is commonly caused by viral hepatitis infection, though it can also result from other liver diseases or biliary obstruction. The recommended diet of rice, curd, buttermilk, and onion is light and easily digestible, providing essential nutrients while minimizing stress on the liver. Rice is a bland carbohydrate source, curd and buttermilk provide probiotics and easily digestible proteins, and onion has antimicrobial properties. This diet helps maintain nutrition during recovery while allowing the liver to gradually restore its normal function.

The effects of crystallized cholesterol result in the formation of gallstones. Gallstones form when any alteration occurs in the composition of bile, particularly when cholesterol crystallizes within the gallbladder. These stones, mostly composed of crystallized cholesterol, can cause serious complications by obstructing the cystic duct, hepatic duct, and hepatopancreatic duct. Such obstruction leads to multiple clinical manifestations including severe pain in the upper right abdomen, jaundice due to blocked bile flow and accumulation of bile pigments in the blood, and pancreatitis when the pancreatic duct becomes blocked, preventing pancreatic secretions from reaching the duodenum. The severity of symptoms depends on the size and location of the stones and whether they cause complete or partial obstruction of the ducts.

Indigestion is a common digestive disorder characterized by improper digestion of food, resulting in a feeling of fullness, discomfort, or heaviness in the stomach. The condition may manifest as bloating, heartburn, nausea, or abdominal pain. Indigestion can result from multiple causes including inadequate secretion of digestive enzymes, which impairs the breakdown of food components. Psychological factors such as anxiety and stress can inhibit digestive function and enzyme secretion. Food poisoning from contaminated food can irritate the digestive tract and impair normal digestion. Overeating causes the digestive system to become overwhelmed and unable to process food efficiently. Consumption of spicy, fatty, or heavy foods can irritate the stomach lining and slow down digestion. Other contributing factors include irregular eating habits, insufficient chewing of food, and certain medical conditions. Management typically involves dietary modifications, stress reduction, and in some cases, medication to enhance enzyme secretion or reduce stomach acid.

• It refers to an eroded area of the tissue lining (Mucosa) in the stomach or duodenum.
• A duodenal ulcer occurs in people in the age group of 25-45 years.
• Gastric ulcer is more common in person above the age of 5üyears.
• Ulcer mostly due to infections caused by the bacterium Helicobacter pylon.
• It may be due to uncontrolled usage of aspirin or certain anti-inflammatory drugs.
• It is caused due to smoking, alcohol, caffeine, and psychological stress.

A hiatus hernia, also known as a diaphragmatic hernia, is a structural abnormality in which the superior or upper part of the stomach protrudes slightly above the diaphragm through the oesophageal hiatus. This condition allows part of the stomach to enter the thoracic cavity, disrupting the normal anatomical relationship between the stomach and oesophagus. The exact cause of hiatus hernias is not completely understood, but several factors can contribute to their development. Weakening of the muscle tissue surrounding the stomach and the diaphragm can predispose individuals to this condition. Repeated application of excessive pressure on the muscles around the stomach can damage and weaken the supporting structures. Common activities that increase intra-abdominal pressure include chronic coughing, frequent vomiting, straining during bowel movements, and lifting heavy objects. Obesity, pregnancy, and aging can also increase the risk of developing hiatus hernias. Symptoms may include heartburn, regurgitation of food or acid, chest pain, and difficulty swallowing, though some individuals may be asymptomatic.

• Stomach functions as the temporary storage organ for food.
• It consists of three parts cardiac fundic and pyloric stomach.
• The oesophagus opens into a cardiac stomach and guarded by cardiac sphincter.
• The pyloric stomach opens into duodenum and is guarded by the pyloric sphincter.
• It allows partially digested food to enter the duodenum and prevents regurgitation of food.
• The inner walls of stomach has many folds which unfolds to accommodate a large meal.

• The ileal mucosa has numerous vascular projections called villi which are involved in the process of absorption.
• The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance and increase the surface area enormously.
• Along with villi the clear mucosa contain mucous secreting goblet cell and peyer patches which produce lymphocytes.
• The wall of the small intestine bears crypts between the base of villi called crypts of leiberkuhn.

Peptic ulcers refer to eroded areas of the tissue lining (mucosa) in the stomach or duodenum. Duodenal ulcers occur more frequently in people in the age group of 25 to 45 years, while gastric ulcers are more common in persons above the age of 50 years. The primary cause of peptic ulcers is infection by the bacterium Helicobacter pylori, which damages the protective mucosa and increases acid secretion. However, ulcers may also develop due to uncontrolled usage of aspirin or certain anti-inflammatory drugs like NSAIDs, which inhibit prostaglandin synthesis and reduce the protective mucus layer. Additional risk factors include smoking, which impairs mucosal healing and increases acid production; alcohol consumption, which irritates the mucosa; caffeine, which stimulates acid secretion; and psychological stress, which increases gastric acid production. The combination of these factors can lead to ulcer formation, and treatment typically involves eliminating the causative agent and protecting the mucosa.

• The Pancreatic is the second-largest gland in the digestive system which is a yellow coloured compound organ.
• It consists of exocrine and endocrine cells.
• It is situated between the limbs of the ‘U’ shaped duodenum.
• The exocrine portion secretes trypsin, pancreatic lipase, amylase.
• The islets of Langerhans cells of the pancreas secrete insulin and glucogen hormone.

The alimentary canal consists of several organs where different substances are absorbed. In the mouth, water and simple sugars begin to be absorbed. The stomach absorbs water, alcohol, medicines, and simple sugars. The small intestine is the primary site of absorption where simple sugars such as glucose, amino acids, fatty acids, and glycerol are absorbed into the bloodstream. The colon absorbs additional water, minerals, vitamins, and some medicines. Each region of the alimentary canal is specialized for both digestion and absorption of specific nutrients. The small intestine, in particular, has a highly developed structure with villi and microvilli that greatly increase its surface area, making it the most efficient organ for nutrient absorption. This sequential absorption of different substances throughout the alimentary canal ensures that all useful nutrients are extracted from food before the remaining waste material is eliminated from the body.

Villi are present in the small intestine and not in the stomach because the two organs have different functions in the digestive process. The stomach serves as a temporary storage organ where food is mechanically churned and chemically broken down by enzymes such as pepsin, renin, and lipase, as well as hydrochloric acid. The primary function of the stomach is digestion rather than absorption. In contrast, the small intestine is where digestion is completed and the major absorption of digested food materials occurs. Glucose, amino acids, fatty acids, and glycerol are absorbed through the intestinal wall into the bloodstream. To facilitate this absorption, the small intestine has numerous finger-like projections called villi that dramatically increase the surface area available for nutrient absorption. This increased surface area allows for more efficient and rapid absorption of digested nutrients. The stomach's smooth lining is adequate for its digestive function, but the small intestine requires the specialized structure of villi to maximize the absorption of nutrients before the remaining material passes into the colon.

• The pile contains bile pigments (bilirubin and biliverdin)
• The pile pigments are broken down products of hemoglobin of dead RBC’s
• Bile salts, cholesterol, and phospholipids.
• Bile has no enzyme.
• Bile helps in the emulsification of fats.
• Bile salts reduce the surface tension of fat droplets and break them into small globules.
• Bile also activates lipase to digest lipids.

The saliva is secreted by the salivary gland in the mouth Saliva
* The saliva contains water.
* Electrolytes – Na +, K +, Cl –, HCo 3 –
* Salivary amylase (ptyalin)
* Mucus (a glycoprotein)
* Polysaccharides, starch is hydrolyzed by the salivary amylase enzyme into disaccharides (maltose)
Stomach
* The gastric j uice contains HCI and proenzymes
* Proenzyme pepsinogen on exposure to HCI gets converted into active enzyme pepsin.
* The HCI provides an acidic medium (pH=1.8) which is optimum for pepsin, kills bacteria and other harmful organisms and avoids putrification.
* Proteolytic enzyme found in gastric juice of Infants is rennin helps in the digestion of milk protein caseinogen to casein in the presence of calcium.
Small Intestine
Pancreatic juice:
Enzymes: Trypsinogen, Chymotrypsinogen, Carboxypeptidases, Pancreatic, Amalyse, Pancreatic Lipase, and Nucleases.
Trypsinogen is activated by an enzyme enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the enzyme chymotrypsinogen in the pancreatic juice.
Bile Juice:
The bile contains bile pigment (Bilirubin, and biliverdin) as the breakdown product of heamoglobin of dead RBCs, Bile salts, Cholesterol, and phospholipids. But has no enzymes. Bile helps in the emulsification of fats. Bile salts reduce the surface tension of fat droplets and break them into small globules, bile also activates lipases to digest lipids.
Pancreatic juice action:
Trypsin hydrolyses protein into polypeptides and peptones. While chymotrypsin hydrolyses peptide bonds associated with specific amino acids.
Succus enterius:
The secretions of the Brunner’s gland along with the secretions of the intestinal glands constitute the intestinal juice or succus entericus.
Enzymes: Maltase, lactase, sucrase (invertase), dipeptidases, lipases, nucleosidases.
Bicarbonate ions from the Pancreas provide an alkaline medium (pH=7.8) for the enzymatic action.
All macromolecules → Micromolecules
Carbohydrate → Monosaccharides
Protein → Aminoacid
Lipids → Fatty acids and Glycerol

A – Right hepatic duct of the liver
B – Common bile duct
C – Pancreatic duct (duct of wirsung)
D – Hepatopancreatic duct
E – Cystic duct
Part II
11th Bio Zoology Guide Digestion and Absorption Additional Important Questions and Answers
I. Choose The Best Option.

1. The Caecum:
* It is a small blind pouch-like structure that opens into the colon and it possesses a narrow finger-like tubular projection called vermi form appendix.
* Caecumand vermiform appendix is large in herbivorous animals and act as an important site for cellulose digestion with the help of symbiotic bacteria.
2. The Colon:
The colon is divided in to four regions an ascending transverse a descending part and a sigmoid Colon. The Colon is lined by dilations called haustra.
3. Sigmoid Colon:
* ‘S’ shaped sigmoid colon opensinto the rectum.
* The anus is guarded by two anal sphincter muscles. The anal mucosa is folded in to several vertical folds and contains arteries and veins called anal column.
* Anal colomn may get enlarged and causes piles.

• The liver is the largest gland in our body.
• It is situated below the diaphragm.
• The liver consists of two major left and right lobes and two minor lobes.
• Each lobe has many hepatic lobules called a functional unit of liver and is covered by a thin connective sheath called Glisson’s capsule.
• Liver cells secrete and are stored in the gall bladder. The duct of gall bladder and the hepatic duct form the common bile duct.
• The bile duct and the pancreatic duct joined to gether formed a common duct and opens into the duodenum and is guarded by a sphincter of Oddi.
• Liver has high power of regeneration and liver cells are replaced by new ones every 3-4 weeks.

• The smell the sight and taste as well as the mechanical stimulation of food in the mouth trigger a reflex action that results in the secretion of saliva.
• The mechanical digestion starts in the mouth by grinding and chewing of good.
• The saliva contains water electrolytes like Na, K, Cl, HCO3 salivary amylase or ptyalin antibacterial agent lysozyme and a lubrication agent mucus.
• The saliva moistening lubricating and adhering the masticated food into a bolus.
• The ptyalin in the saliva hydrolyzes 30% of the poly saccharide into disaccharides.
• The bolus is passed into the pharynx and then into the oesophagus by swallowing or deglutition.
• The bolus reaches the stomach by successive waves of muscular contraction called peristalsis.

• The secretion of gastric juice begins when the food is in the mouth.
• The gastric juice contains HCI pepsinogen renin etc.
• The HCI changes the pepsinogen into pepsin.
• Pepsin acts on protein and converts into proteoses and peptones.
• The HCI provides an acidic medium that is optimum for pepsin kills bacteria and other harmful organisms and avoids putrefaction.
• The mucous and bicarbonates protect the stomach from acidic HCl.
• The rennin converts the milk protein carcinogen to casein in the presence of calcium ions.

The bile pancreatic juice and intestinal juice the secretions released into the small intestine.
Bile:
* The bile contains bile pigments bilirubin and biliverdin as the breakdown products of haemoglobin of dead RBCs bile salts.
* Bile helps in the emulsification of fats Bile salts reduce the surface tension of the fat droplets and break them into small globules.
* Bile also activates lipases to digest lipids.
Pancreas:
* The pancreatic juice contains enzymes such as trypsinogen, Chymotrypsinogen.
* Trypsinogen is activated by an enzyme enterokinase into active trypsin.
* Trypsin activates the chymotrypsinogen into chymotrypsin.
* Trypsin hydrolyses protein into polypeptides and peptones.
Chymotrypsin hydrolyses peptide bonds associated with specific aminoacids.
The amylase converts glycogen and starch into maltose.
Lipase acts on tri glycerides and hydrolyes them into free fatty acid and mono glycerides.
Succtts entricus:

Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood and lymph for transport to cells throughout the body. The digested food is absorbed through several mechanisms depending on the nature of the nutrient. Simple diffusion allows small amounts of glucose, amino acids, and chloride ions to move across the intestinal epithelium down their concentration gradient without requiring energy. Facilitated transport enables fructose to be absorbed with the help of carrier proteins and ions like sodium, which assist in moving molecules across the membrane. Active transport is responsible for the absorption of amino acids, glucose, and sodium ions, requiring energy in the form of ATP to move these substances against their concentration gradient. Passive transport through lacteals of the villi allows fatty acids and fat-soluble vitamins to be absorbed into the lymphatic system. The small intestine, particularly the ileum, is the primary site of absorption due to its large surface area provided by villi and microvilli, which maximize contact between the intestinal lining and digested nutrients.

The caloric values of macronutrients are essential for understanding energy requirements and nutritional balance. Carbohydrates have a caloric value of 4.1 calories per gram and a physiological fuel value of 4 kilocalories per gram. Lipids (fats) have a caloric value of 9.45 kilocalories per gram and a physiological fuel value of 9 kilocalories per gram, making them the most energy-dense macronutrient. Proteins have a caloric value of 5.65 kilocalories per gram and a physiological fuel value of 4 kilocalories per gram. In a balanced diet, approximately 50 percent of energy should come from carbohydrates, 35 percent from fats, and 15 percent from proteins. Daily requirements include approximately 400 to 500 grams of carbohydrates, 60 to 70 grams of fat, and 65 to 75 grams of proteins per day for an average adult, though these values may vary based on age, activity level, and individual metabolic needs.

• It is caused due to the storage of excess body fat in adipose tissue.
• Obesity may be genetic or due to excess intake of food endocrine and metabolic disorders.
• The degree of obesity is assessed by body mass index (BMI).
• A Normal BMI range for adults is 19 – 25 above 25 is obese.
• BMI is calculated as body weight in Kg divided by the square of body height in meters.
• For example, a 50 Kg person with a height of 160 Cms would have a BMI of 19.5.
• That is BMI \(=50 / 1.6^{2}\) = 19.5