Edexcel iGCSE Biology-2.27 structure and function of the human alimentary canal- Study Notes- New Syllabus

Human Alimentary Canal – Structure & Function

📝 Introduction

• The alimentary canal is a long, continuous tube that runs from the mouth to the anus.
• It is also called the digestive tract.
• Its main job is to break down food into small, soluble molecules so that they can be absorbed and used by the body.
• Digestion takes place in different organs, and each organ has a specific role.
• Some organs carry out mechanical digestion (breaking food into smaller pieces), while others carry out chemical digestion using enzymes.
• After digestion, useful nutrients are absorbed into the blood, while undigested material leaves the body as faeces.

📍 Pathway of Food

Mouth → Oesophagus → Stomach → Duodenum → Ileum → Colon → Rectum → Anus

👄 1. Mouth

Structure

• First part of the alimentary canal.
• Contains teeth, tongue and salivary glands.

Functions

• Teeth break food into smaller pieces by chewing.
• This is called mechanical digestion.
• Smaller pieces have a larger surface area, making enzymes work faster.
• Salivary glands produce saliva.
• Saliva moistens food and makes it easier to swallow.
• Saliva contains the enzyme amylase.
• Amylase begins chemical digestion.
• It breaks starch into maltose.
• Tongue mixes food with saliva.
• It rolls the food into a soft ball called a bolus.
• The bolus is then pushed into the oesophagus during swallowing.

Important

• Digestion starts in the mouth.
• Both mechanical and chemical digestion occur here.

🛤️ 2. Oesophagus

Structure

• Long muscular tube connecting the mouth to the stomach.
• Does not produce digestive enzymes.

Functions

• Moves food from the mouth to the stomach.
• Uses peristalsis to push food downward.

Peristalsis

• Alternate contraction and relaxation of circular and longitudinal muscles.
• Pushes food along the digestive tract even if a person is upside down.

Important

• No digestion takes place here.
• Only transport of food occurs.

🍲 3. Stomach

Structure

• Thick muscular sac.
• Inner lining contains glands that release gastric juice.

Functions

Mechanical Digestion

• Strong muscles churn and mix food with gastric juice.
• Food becomes a semi-liquid mixture called chyme.

Chemical Digestion

• Gastric juice contains hydrochloric acid (HCl) and pepsin.

Hydrochloric Acid (HCl)

• Creates a strongly acidic environment.
• Kills many harmful bacteria present in food.
• Provides the correct pH for pepsin to work.

Pepsin

• A protease enzyme.
• Digests proteins into shorter peptide chains.

Important

• Starch digestion stops here because the acidic pH inactivates salivary amylase.
• Protein digestion begins in the stomach.

🧪 4. Small Intestine

• Longest part of the alimentary canal.
• Divided into two main parts:
  • Duodenum
  • Ileum

A. Duodenum

Structure

• First section of the small intestine.
• Receives digestive juices from the pancreas and bile from the liver.

Functions

• Completes most chemical digestion.

Pancreatic Enzymes

Amylase

• Breaks starch into maltose.

Protease

• Breaks proteins into amino acids.

Lipase

• Breaks lipids into fatty acids and glycerol.

Role of Bile

• Produced by the liver.
• Stored in the gall bladder.
• Released into the duodenum.

Bile does not contain enzymes.

Instead, it:

• Neutralises acidic chyme from the stomach.
• Creates an alkaline pH for intestinal enzymes.
• Emulsifies fats into tiny droplets.
• Smaller droplets provide a larger surface area for lipase.

Important

• Digestion of carbohydrates, proteins and fats is completed mainly in the duodenum.

B. Ileum

Structure

• Second and longest part of the small intestine.
• Inner surface contains millions of villi.
• Each villus has even smaller projections called microvilli.

Function

• Main site of nutrient absorption.

Adaptations for Absorption

Long length

• Gives more time for absorption.

Villi and microvilli

• Greatly increase surface area.

Thin epithelium

• Only one cell thick.
• Provides a short diffusion distance.

Rich blood supply

• Quickly carries absorbed nutrients away.
• Maintains a steep concentration gradient.

Lacteals inside villi

• Absorb fatty acids and glycerol into the lymphatic system.

Nutrients Absorbed

• Glucose
• Amino acids
• Fatty acids
• Glycerol
• Vitamins
• Mineral ions
• Water (some)

Important

• Most digestion is completed in the duodenum.
• Most absorption takes place in the ileum.

🚰 5. Large Intestine

The large intestine consists mainly of the:

• Colon
• Rectum

A. Colon

Structure

• Wider and shorter than the small intestine.

Functions

• Absorbs most of the remaining water.
• Absorbs some mineral ions.
• Helps convert undigested food into solid faeces.

Important

• Prevents excessive water loss from the body.

B. Rectum

Structure

• Final section of the large intestine.

Functions

• Stores faeces before removal.
• Stretch receptors signal when the rectum is full.
• Faeces leave the body through the anus during egestion.

Important

• Egestion is the removal of undigested food.
• It is not the same as excretion.

🧫 6. Pancreas

Structure

• Large gland located below the stomach.
• Connected to the duodenum by the pancreatic duct.
• Not part of the alimentary canal, but an important accessory digestive organ.

Functions

• Produces pancreatic juice containing digestive enzymes.

Pancreatic Amylase

• Digests starch into maltose.

Pancreatic Protease

• Digests proteins into amino acids.

Pancreatic Lipase

• Digests fats into fatty acids and glycerol.

Additional Function

• Produces alkaline secretions that help neutralise acidic chyme entering the duodenum.

🔄 Summary of Digestion Along the Alimentary Canal

📌 Important Points

• The alimentary canal is a continuous tube from mouth to anus.
• Mechanical digestion occurs mainly in the mouth and stomach.
• Chemical digestion is carried out by enzymes.
• Peristalsis moves food through the digestive system.
• The stomach starts protein digestion using pepsin.
• The duodenum receives bile and pancreatic enzymes.
• Bile is not an enzyme. It neutralises acid and emulsifies fats.
• The ileum is the main site of nutrient absorption because of its villi and microvilli.
• The colon absorbs water.
• The rectum stores faeces before egestion.
• The pancreas produces digestive enzymes but food never passes through it.

⚡ Quick Recap
Mouth → Chewing + amylase starts starch digestion.
Oesophagus → Peristalsis moves food.
Stomach → Churning + pepsin digests proteins.
Duodenum → Bile + pancreatic enzymes complete digestion.
Ileum → Villi absorb nutrients.
Colon → Absorbs water and forms faeces.
Rectum → Stores faeces before egestion.
Pancreas → Produces amylase, protease and lipase.