Digestive System Chapter Notes | Biology for GCSE/IGCSE - Class 10 PDF Download

Introduction

The digestive system is a vital process in our body that breaks down food into simpler, absorbable forms to provide energy and nutrients. It involves various organs, glands, and enzymes that work together to convert complex food into substances that can be absorbed by the blood and utilized by body cells. This chapter explores the structure and function of teeth, the alimentary canal, digestive glands, and the processes of digestion, absorption, and assimilation, along with the role of enzymes and peristalsis.

Why the Digestive System

• Food consumed cannot be used directly by the body; it needs to be broken down into soluble, absorbable forms.
• Digestion converts complex food into simpler substances that can pass through cell membranes.
• Some foods, like cane sugar, are water-soluble but require molecular breakdown for absorption.
• Digestion involves breaking down foodstuffs into diffusible forms suitable for absorption through living membranes.

Enzymes

Enzymes are crucial proteins that facilitate food digestion.

Characteristics of enzymes:

• They are proteins and get destroyed by heat.
• Specific to one substrate, acting only on it.
• Produce consistent end-products from the substrate.
• Speed up chemical reactions without being consumed.
• Reusable and act as catalysts.
• Function best at a specific pH (acidic or alkaline).
• Work optimally within a temperature range of 35–40°C.

The Digestive System

• Comprises the alimentary canal and digestive glands.
• Alimentary canal: A 9-meter-long muscular tube from mouth to anus, coiled in the small intestine.
• Digestive glands: Include salivary glands, liver, pancreas, and glands in the alimentary canal lining.

The Mouth

• Mouth cavity is where food is chewed and mixed with saliva.
• Lips: Help close the mouth, suck/sip liquids, speak, and sense touch/heat.
• Tongue: Manipulates food during chewing, aids tasting, cleans teeth, and assists in speaking.

The Teeth - Dentition

• Teeth break food into smaller pieces for better enzyme action and aid in speaking and facial appearance.
• Adult humans have 32 teeth, categorized as:
  • Incisors: Four front teeth per jaw, broad and sharp for biting/cutting.
  • Canines: One on each side of incisors per jaw, pointed for holding/tearing.
  • Premolars: Two per side per jaw, with two cusps for grinding/crushing.
  • Molars: Three per side per jaw, larger surface for grinding; last molar is the wisdom tooth (appears at 17–20 years).
• Human teeth are heterodont (different shapes) unlike homodont teeth in some animals (e.g., lizards, frogs).
• Premolars are sometimes called molars by dentists, but scientifically distinct from permanent molars.
• Two sets of teeth in humans:
  • Milk teeth: 20 temporary teeth (no molars) appear at 7–8 months, complete by 2 years, replaced by 12 years.
  • Permanent teeth: 32 teeth, including wisdom teeth by 17–20 years.
• Dental formulas:
  • Child (up to 2 years): 2,1,2,0 / 2,1,2,0 = 20 teeth.
  • Adolescent (up to 17–20 years): 2,1,2,2 / 2,1,2,2 = 28 teeth.
  • Adult: 2,1,2,3 / 2,1,2,3 = 32 teeth.

Structure of a Tooth

Parts of a tooth:

• Crown: Exposed part above the gum.
• Root: Embedded in the jawbone socket; single root in incisors/canines, two in premolars/lower molars, three in upper molars.
• Neck: Slight constriction between crown and root.

Internal structure:

• Enamel: Hardest body substance, covers the crown.
• Dentine: Forms tooth bulk, harder than bone, contains cytoplasmic strands from pulp cavity cells.
• Cement: Bone-like, fixes root in place.
• Pulp: Soft tissue in pulp cavity with blood capillaries, lymph vessels, and nerves, connected to the body through root base opening.

The Salivary Glands

Three pairs of salivary glands:

• Parotid: In front of and below each ear.
• Submandibular: Inner side of lower jaw.
• Sublingual: Below the tongue.

Saliva production: 1000–1500 ml daily, secreted via ducts, increases during eating or with food stimuli. Saliva composition: 99% water, salts, mucus, and salivary amylase (ptyalin), slightly acidic (pH 6.8).

Functions of saliva:

• Moistens mouth and tongue for speaking/swallowing.
• Lubricates food for swallowing.
• Dissolves food to stimulate taste buds.
• Forms food bolus for swallowing.
• Digests starch into maltose via ptyalin.
• Cleans mouth, destroys germs to prevent tooth decay.
• Signals thirst to maintain body water balance.

Swallowing and Peristalsis

Swallowing involves:

• Tongue pushing food bolus into pharynx.
• Soft palate closing nasal passage.
• Larynx rising, epiglottis closing windpipe to direct food to oesophagus.
• Peristalsis: Wave-like muscle contractions in the gut to move food, aided by mucus lubrication.
• Coughing occurs if food enters the windpipe.

Oesophagus

• A tube conducting food from throat to stomach, passing through the diaphragm.
• No digestive enzymes produced.

Stomach

An elastic, muscular bag below the diaphragm, holds 2–3 liters of food.
Churns food with gastric juice; has sphincters:

• Cardiac sphincter: Prevents backflow to oesophagus.
• Pyloric sphincter: Controls food passage to duodenum, prevents regurgitation.

Vomiting: Reverse peristalsis (antiperistalsis) expels stomach contents through the mouth when overloaded.
Gastric juice: Acidic, contains water, salts, hydrochloric acid, and pepsin.

• Hydrochloric acid: Kills germs, activates pepsinogen to pepsin.
• Pepsin: Digests proteins (20%) into peptides.
• Rennin (in children): Curdles milk protein (caseinogen) into casein.

The Small Intestine

The small intestine is a coiled, 7-meter-long tube, approximately 2.5 cm wide, located in the abdomen. It consists of three sections:

1. Duodenum: The short, uppermost section, about 12 finger-widths long (hence "duodenum," from Latin for twelve). The common bile duct empties into this region.
2. Jejunum: A roughly 2-meter-long section, often found empty during autopsies (hence "jejunum," meaning empty).
3. Ileum: The longest part, about 4 meters, named for its twisting movements (from Latin "ileum," meaning to twist).

The ileum’s inner lining features numerous tiny, finger-like projections called villi, which greatly increase the surface area for absorption (about eight times the body’s external surface area). Between the villi, glandular cells secrete intestinal juice into the intestinal lumen. Each villus is covered by a single layer of epithelial cells and contains an artery, vein, blood capillaries, and a lymph vessel called a lacteal.

The small intestine is crucial for both digestion and absorption. It receives bile and pancreatic juice in the duodenum and secretes intestinal juice in the ileum.

Adaptations of the Ileum for Absorption:

1. Long length, increasing surface area.
2. Numerous villi, further enhancing surface area.
3. Thin, single-cell epithelium for efficient absorption.
4. Narrow diameter, slowing food movement to allow absorption.

Digestive Juices:
(i) Bile: Produced by the liver, bile travels through the hepatic duct, often stored in the gall bladder, and enters the duodenum via the common bile duct. Its yellowish-green color comes from pigments (biliverdin and bilirubin) formed from broken-down red blood cells. Bile contains sodium bicarbonate, which neutralizes acidic stomach contents, creating an alkaline environment for pancreatic and intestinal enzymes. Bile salts emulsify fats, breaking them into tiny droplets to increase their surface area for digestion.

Functions:

• Converts fats into emulsified fat.
• Neutralizes acidic chyme to alkaline chyme.

(ii) Pancreatic Juice: Secreted by the pancreas, a gland behind the stomach, it enters the duodenum through the pancreatic duct. It contains:

• Amylopsin (pancreatic amylase): Converts leftover starch into maltose.
• Trypsin: Breaks down proteins and polypeptides into smaller peptides and amino acids. Trypsin is secreted as inactive trypsinogen, activated by enterokinase from the duodenal lining.
• Steapsin: Splits emulsified fats into fatty acids and glycerol.
• Functions:
  • Trypsinogen → Trypsin (via enterokinase).
  • Proteins/Peptides → Smaller peptides + Amino acids.
  • Leftover starch → Maltose.
  • Emulsified fat → Fatty acids + Glycerol.

Intestinal Juice: Secreted by the ileum’s glandular cells, it contains:

• Erepsin (peptidases): Converts peptides into amino acids.
• Maltase: Breaks down maltose into glucose.
• Lactase: Converts lactose into glucose and galactose.
• Sucrase (invertase): Splits sucrose into glucose and fructose.
• Lipase: Digests fats into fatty acids and glycerol.
• Functions:
  • Peptides → Amino acids.
  • Maltose → Glucose.
  • Sucrose → Glucose + Fructose.
  • Lactose → Glucose + Galactose.
  • Emulsified fat → Fatty acids + Glycerol.

Absorption in the Small Intestine: The small intestine absorbs most final digestion products:

• Amino acids and simple sugars (glucose, fructose, galactose) pass through the villi’s thin epithelium into blood capillaries, traveling via the hepatic portal vein to the liver.
• Fatty acids and glycerol enter the lacteals, joining the lymphatic system before entering the bloodstream. Food moves slowly through the small intestine, taking about four hours to reach the large intestine.

The Large Intestine

The large intestine, about 1.5 meters long, consists of three parts:

1. Caecum: A small pouch at the junction of the small and large intestines, with a vestigial, worm-shaped vermiform appendix projecting from it (inflammation causes appendicitis).
2. Colon: Wider than the ileum, about 1 meter long, it includes the ascending colon (right side), transverse colon (crossing below the stomach), and descending colon (left side).
3. Rectum: The final 15 cm, ending at the anus, which is controlled by sphincter muscles to regulate defecation.

Functions: The large intestine secretes no enzymes and absorbs minimal digested food, primarily absorbing water to form semi-solid faeces. These are stored in the rectum and expelled via defecation. 
Faeces consist of:
~75% water.
~25% solids, including:

• 30% dead bacteria.
• 10-20% fat.
• 2-3% proteins.
• 30% roughage (undigested material). The foul odor of odur of faeces results from bacterial action. Composition varies based on diet and individual differences.

Assimilation of Food

Assimilation involves converting absorbed, digested food into body tissues. After digestion in the gut, nutrients are transported via two systems: the hepatic portal system and the lymphatic system.

Hepatic Portal System: Simple sugars (e.g., glucose), amino acids, vitamins, and minerals are carried by the hepatic portal vein to the liver. The liver regulates nutrient levels:

• Excess glucose (or other simple sugars) is converted into glycogen for temporary storage. When needed, glycogen is reconverted into glucose and released into the bloodstream.
• Glucose serves as an energy source for cellular respiration and is used in synthesizing certain compounds.
• Amino acids circulate as building blocks for proteins. They cannot be stored. Excess amino acids undergo deamination in the liver, where the nitrogen-containing amino group is removed and converted into urea for excretion. The remaining portion forms glucose for energy use.

Lymphatic System: Fatty acids and glycerol are primarily absorbed through the villi in the small intestine into the lacteals of the lymphatic system. The thoracic ductempties these nutrients into large veins leading to the heart.

• Some fats are used to synthesize compounds in body cells.
• Excess fats are stored as subcutaneous fat beneath the skin or, in smaller amounts, around visceral organs.

Liver

The liver, the body's largest gland, weighs approximately 1,500 grams on average. This reddish-brown organ is situated in the upper right quadrant of the abdomen.

Functions (beyond bile production):

• Regulates blood sugar by storing/releasing glucose as glycogen.
• Deaminates excess amino acids into urea and glucose.
• Produces fetal red blood cells (in embryos).
• Synthesizes fibrinogen and prothrombin for blood clotting.
• Produces heparin (anticoagulant).
• Regulates blood volume by storing excess water.
• Destroys dead red blood cells.
• Stores iron, copper, and vitamins (A, D).
• Excretes toxic and metallic poisons.
• Produces heat via cellular metabolism.
• Detoxifies substances like drugs and alcohol.

Certain Experiments on Digestion

Action of saliva on starch:

• Prepare starch suspension (1 g in 100 ml water).
• Test for starch (iodine) and sugar in separate tubes.
• Mix starch with saliva, warm at 38°C for 15 minutes, test again; shows sugar presence, no starch.

Action of pepsin on egg-white (protein):

• Prepare egg-white suspension, distribute in four test-tubes (A: pepsin, B: HCl, C: pepsin + HCl, D: acid + boiled pepsin).

Warm at 35°C for 10 minutes; only C clears, proving:

• Pepsin digests protein in acidic conditions.
• Acid alone does not digest protein.
• Boiled pepsin is ineffective (destroyed by heat).

Effect of pH on pepsin:

• Add pepsin and sodium bicarbonate (alkaline) to egg-white suspension; no digestion occurs, proving pepsin needs acidic medium.

Practical Work on Food Tests

Test for Glucose (Sugar)

• A little glucose is added to a blue-green chemical called Fehling's solution in a test-tube. 
• Tilt the test-tube away from you and heat it over a flame until bubbles begin to appear but not allowing it to boil over.
• The colour would change from blue-green to a deep brick red with the appearance of a precipitate.
• A small piece of apple or grape will give the same result.
• A similar test is performed to test sugar in urine of the suspected cases of diabetes (diabetes mellitus).
• Sucrose (cane-sugar) does not give the above test until it has been boiled with dilute hydrochloric acid and neutralized with sodium bicarbonate. 

Test for Starch

• A little starch powder is shaken in a test tube with some water and then boiled to make a solution.
• When the solution is cold, add a few drops of iodine solution. (Iodine solution can be prepared by dissolving 1 g potassium iodide in 100 mL of distilled water. The solution should be diluted before using).
• The starch solution would turn blue-black indicating the presence of starch.
• Foods like potato and rice give a positive test of starch.

Test for Protein

•  Place a piece of hard-boiled egg-white in a test-tube. 
•  Add a few drops of dilute nitric acid just to cover the food (one should be very cautious in using acids). 
•  Heat the test-tube gently, then rinse off the acid with water and add ammonium hydroxide. 
•  You will note a colour change—first from white to yellow and then (after adding ammonium hydroxide) from yellow to orange. 

Test for Fats and Oils

• Rub a piece of groundnut, walnut, a piece of butter on a plane paper.
• The spot rubbed turns translucent especially when viewed against light.

Test for the Presence of Water

Take some cobalt chloride paper which is blue when dry but turns pink on exposure to moisture. The dry cobalt chloride paper can be directly touched with the cut surface of a food like potato or banana to observe the colour change.

Test for the Presence of Minerals

Place a piece of banana or potato in a crucible. Heat till it burns completely. An ash is left behind which does not burn. The ash indicates mineral substance.

Points to Remember

• Enzymes facilitate digestion with specific properties.
• Teeth chew food, tongue aids swallowing, and oesophagus conducts food to stomach.
• Carnivores (e.g., dogs) have sharp canines and projection-bearing premolars/molars.
• Herbivores (e.g., sheep) lack canines, have broad grinding premolars/molars.
• Saliva digests starch and performs other functions like lubrication.
• Gastric juice kills bacteria and digests proteins partially.
• Small intestine is the primary site for digestion and absorption.
• Bile and pancreatic juice aid in fat and starch/protein/fat digestion, respectively.
• Pancreatic juice contains amylopsin, trypsin, and steapsin.
• Colon absorbs water and some food remnants.
• Rectum stores waste for defecation via anus.
• Liver, beyond bile, performs multiple metabolic functions.
• Food tests include iodine for starch, Fehling’s for glucose, etc.