Digestive System

Digestive System

Middle School

​

Learning Objectives

• Describe the three main jobs of the digestive system: digestion, absorption, and elimination.

• Follow the journey of food through the different organs of the digestive system.

• Explain the difference between mechanical and chemical digestion and where each one happens.

• Explain how the liver, gallbladder, and pancreas help the digestive system do its job.

Key Vocabulary

​

Mechanical Digestion

The physical breakdown of food into smaller pieces, occurring mostly in the mouth and stomach.

​

Chemical Digestion

The chemical breakdown of food into nutrient molecules, which happens mainly in the small intestine.

​

Peristalsis

Involuntary muscle contractions that create wave-like motions to move food through the digestive organs.

​

Absorption

The critical process where valuable nutrients pass from the small intestine into the bloodstream for distribution.

​

Villi

Tiny, finger-like projections that increase the surface area of the intestinal wall to maximize nutrient absorption.

​

​

Functions and Types of Digestion

Mechanical Digestion

• ​This is the physical process of breaking food into smaller pieces.

• ​​It involves actions like chewing with teeth and churning in the stomach.

• ​This process does not change the chemical composition of the food.

Chemical Digestion

• ​This process uses enzymes and acids to break down food chemically.

• ​​It changes food into small molecules that your body can absorb.

• ​This occurs mainly in the mouth, stomach, and small intestine.

Multiple Choice

What is the key difference between mechanical and chemical digestion?

• Mechanical digestion breaks food down physically, while chemical digestion breaks it down with enzymes and acids.

• Mechanical digestion happens in the small intestine, while chemical digestion happens in the stomach.

• Mechanical digestion absorbs nutrients, while chemical digestion eliminates waste.

• Mechanical digestion uses enzymes, while chemical digestion uses physical churning.

​

​

The Path of Digestion: GI Tract & Accessory Organs

Gastrointestinal (GI) Tract

• ​The gastrointestinal (GI) tract is the long tube that food travels directly through.

• ​​It begins at the mouth and includes the esophagus, stomach, and small intestine.

• ​The large intestine is the final section of the GI tract, ending at the anus.

Accessory Organs

• ​Food does not pass through the accessory organs, but they are vital for digestion.

• ​​They produce and release substances that help with the chemical digestion of food.

• ​The main accessory organs include the liver, gallbladder, and the pancreas.

Multiple Choice

Which of the following is part of the gastrointestinal (GI) tract, meaning food passes directly through it?

• Stomach

• Liver

• Gallbladder

• Pancreas

​

​

The Upper GI Tract: Mouth and Esophagus

In the Mouth

• Mechanical digestion begins by chewing food, also known as mastication, into smaller, manageable pieces.

• Saliva starts the chemical digestion process with an enzyme called amylase to break down carbohydrates.

• The mouth is where the entire process of digestion officially begins with these first steps.

The Esophagus

• This tube connects the mouth to the stomach, serving only as a path for food.

• It uses involuntary muscle movements called peristalsis to push food toward the stomach for digestion.

• A valve called a sphincter keeps stomach acid from flowing back up into the esophagus.

Multiple Choice

What are the two types of digestion that begin in the mouth?

• Mechanical digestion (mastication) and chemical digestion (amylase).

• Only mechanical digestion through chewing.

• Only chemical digestion through saliva.

• Peristalsis and absorption.

​

​

The Stomach's Role in Digestion

• The stomach is a muscular sac that digests food mechanically and chemically.

• ​Its strong muscles churn food, breaking it into smaller pieces.

• The stomach releases hydrochloric acid and an enzyme called pepsin.

• Acid kills bacteria and helps pepsin break down proteins.

​

Multiple Choice

What is the main purpose of hydrochloric acid in the stomach?

• To kill bacteria and create an acidic environment for enzymes.

• To physically churn and mix the food.

• To begin the digestion of carbohydrates.

• To absorb nutrients into the bloodstream.

​

​

The Small Intestine: Digestion and Absorption Hub

• Most chemical digestion and nutrient absorption occurs in the small intestine.

• ​In the duodenum, bile neutralizes acid and enzymes break down food.

• The jejunum’s walls have finger-like villi that increase its surface area.

• This large surface helps absorb the most nutrients into the blood.

​

Multiple Choice

What is the primary function of the villi in the small intestine?

• To increase the surface area for nutrient absorption.

• To produce bile and neutralize acid.

• To move food using wave-like muscle contractions.

• To break down proteins with powerful enzymes.

​

​

The Large Intestine: Water Absorption and Waste

• The large intestine connects the small intestine to the anus, starting with the cecum.

• ​From the cecum, waste moves into the colon where water is absorbed.

• This turns the liquid waste into a solid form called feces.

• Feces are stored in the rectum before being eliminated from the body.

Multiple Choice

What is the main function of the large intestine's colon?

• To absorb water from liquid waste.

• To absorb most of the body's nutrients.

• To digest proteins and fats with enzymes.

• To produce bile for the small intestine.

​

​

The Accessory Organs

• The liver produces bile, a substance that is essential for digesting fats.

• The gallbladder is a small sac that stores bile made by the liver.

• The pancreas creates enzymes to help digest carbohydrates, proteins, and fats.

Multiple Choice

Which accessory organ's main role is to store bile?

• Gallbladder

• Liver

• Pancreas

• Stomach

​

Common Misconceptions

Misconception	Correction
Digestion is completed in the stomach.	Most digestion and absorption occurs in the small intestine.
Food just falls down the esophagus.	Food is pushed down by muscle waves called peristalsis.
All digestive organs have food pass through them.	Food does not pass through the liver, gallbladder, and pancreas.
Stomach and esophagus are protected from acid the same way.	The stomach has a mucus lining; the esophagus does not.

Multiple Choice

How do the roles of the small intestine and large intestine differ in the digestive process?

• The small intestine absorbs nutrients, while the large intestine primarily absorbs water.

• The small intestine absorbs water, while the large intestine absorbs nutrients.

• The small intestine performs mechanical digestion, while the large intestine performs chemical digestion.

• The small intestine stores waste, while the large intestine digests proteins.

Multiple Choice

Why is the production of bile by the liver essential for digestion in the small intestine?

• Bile neutralizes stomach acid and breaks up fats, allowing enzymes to work effectively.

• Bile helps to digest complex carbohydrates into simple sugars.

• Bile triggers peristalsis to move food through the intestines.

• Bile is the primary enzyme responsible for breaking down proteins.

Multiple Choice

A person has a condition where the sphincter between the esophagus and stomach does not close properly. What is the most likely outcome?

• Acidic food from the stomach will flow back into the esophagus, causing tissue damage.

• Food will not be able to pass from the esophagus into the stomach.

• The stomach will not be able to churn and mechanically digest food.

• Bile will be prevented from entering the small intestine.

Multiple Choice

Imagine a carbohydrate molecule in a piece of bread. Analyze its journey from the mouth to the point of absorption, identifying the key locations and processes involved.

• It starts chemical digestion in the mouth via amylase, passes through the stomach, and is fully digested and absorbed in the small intestine.

• It is digested by pepsin in the stomach and absorbed by the large intestine.

• It is broken down by bile in the liver and absorbed in the stomach.

• It is mechanically digested only in the mouth and then absorbed by the esophagus.

​

Summary

• The digestive system has three main functions: digestion, absorption, and elimination.

• Digestion is both mechanical (chewing) and chemical (using enzymes and acid).

• Food travels the GI tract: mouth, esophagus, stomach, and intestines.

• The small intestine absorbs most nutrients with help from the liver and pancreas.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4