Biology 2201

Unit 3 – Section 5

“Human Digestive System”

The Chemical Foundation of Digestion

All organisms, regardless of size or complexity, have some

method to obtain the essential nutrients they need for

survival.

Heterotrophs: Organisms that depend on organic molecules

manufactured by other living things. Humans are ingestive

heterotrophs.

Ingestive = nutrients are taken in through the mouth.

Autotrophs : Organisms that nourish themselves using

inorganic material (examples: water and carbon dioxide).

Nutrients

Nutrient: Any substance used by the human body to maintain

homeostasis.

Essential Nutrients

The basic raw materials organisms need to make their own structures,

perform functions, and obtain energy for survival.

Essential = Needed and must be obtained from the organism’s diet.

6 Essential Nutrients:

1.Carbohydrates

2.Fats/Lipids

3.Proteins

4.Minerals

5.Vitamins

6.Water

Your body functions best when these essential nutrients

are present in correct proportions. A diet that satisfies

this is called a balanced diet.

1. Carbohydrates

• They consist of atoms of carbon, hydrogen, and oxygen

(CHO).

• Carbohydrates are broken down to simple sugars during

digestion such as glucose, fructose and galactose.

• Their main function is to provide a source of energy.

• The excess carbohydrates are converted to fat and

stored.

• Some food sources would be glucose, rice, bread,

pasta, etc.

Carbohydrate Structure”

2. Fats/Lipids

• Their functions include providing a source of energy, insulating the body from the cold, and protecting organs from injury.

• They can also be used as a building material for cell membranes and hormones.

• They are broken down to fatty acids and glycerol by the digestive system.

• Some food sources would be margarine, butter, meat, cheese, nuts, eggs, etc.

Cholesterol

• A soft waxy lipid found in body cells and among the fats in

arteries and veins. Everyone needs cholesterol.

• Functions to form cell membranes insulate nerves,

produce vitamin D, bile acids and hormones.

• Meat, shellfish, whole milk, and egg yolks are high

sources of cholesterol.

• Too much cholesterol can be life-threatening.

Structure of Fats:

3. Protein

• Function: include, cell growth and repair,

enzymes in your body are all proteins (act in

metabolism), and can supply energy.

• They are made up of peptides which consist of

strings of amino acids (8 essential amino acids).

• Food sources: meat, milk, poultry, fish, eggs,

cheese, etc.

Structure of a Protein:

4. Vitamins

• Our bodies require very small amounts in

diet.

• They act as coenzymes, chemicals needed

to make enzymes function.

• Examples would be vitamins: A, B, C, D, E,

K.

• See table 11-1 page 358.

5. Minerals

• They are the inorganic compounds that your

body needs in small amounts.

• Their functions include helping to build bones

and cartilage, enabling certain chemical

reactions, and helping to maintain acid-base

balance within the body.

• They are readily absorbed into the bloodstream.

• Examples: Iron, calcium, sodium, magnesium,

potassium, phosphorous, zinc, sulfur, nickel,

copper plus other trace minerals.

6. Water

• Its major role in the body is to act as

a solvent.

• Most chemical reactions (body

metabolism) that occur in the body

require water.

• The main source would be food and

liquids.

Human Digestive System

Purpose:

It breaks down food masses into small useful

substances that can be absorbed into the

circulatory system where nutrients are

transported to individual cells and are again

absorbed.

Where:

This occurs in the alimentary canal, a continuous

tube beginning at the mouth and ending at the

anus.

Types of Digestion:

Types:1. Mechanical Digestion : It is the initial stage of physically

breaking down food into smaller pieces. It occurs mainly in the mouth where teeth chew food and tongue manipulates the food.

2. Chemical Digestion : It is the separation of food into molecular components by chemical means.

• Mechanical and chemical digestion act together to speed up the digestionprocess.

• Mechanical digestion aids chemical digestion by exposing more food particles to digestive enzymes

Chemical Digestion and Enzymes:The chemical break down of food occurs by the process of hydrolysis.In a hydrolysis reaction, the bonds between subunits in macromolecules are broken down by the addition of a water molecule. The water molecule is added at the point where a link is being broken.Enzymes serve to speed up the hydrolysis reaction.

Three types of enzymes are associated with digestion: carbohydrase, lipases, and proteinases.

Carbohydrases- speed up the hydrolysis of complex carbohydrates into simple sugars.

Lipases- speed up the hydrolysis of fats into glycerol and fatty acids.

Proteinases- speed up the hydrolysis of proteins into amino acids.

Parts of the Human Digestive System

1. Mouth

2. Esophagus

3. Stomach

4. Small Intestine

5. Large Intestine

6. Rectum

7. Liver

8. Pancreas

9. Gall Bladder

1. The Mouth

Digestion begins Here

• Mouth contains:

a) Teeth,

b) Tongue

c) Salivary Glands.

A) Teeth (32) -- Grind and chew food. Mechanically break food apart.

a) Tongue – Made of skeletal muscle. Contains taste buds for taste.Manipulates food into position.

b) Salivary Glands (3 pairs) – Secrete Saliva

Saliva – Liquid in the mouth that lubricates and moistens food.

Contains enzyme Salivary Amylase that converts Starch into Maltose.

Salivary Amylase

Starch ------------------------------------> Maltose

Digestion in the Mouth

1. Food enters the mouth where teeth begin breaking it down.

2. The salivary glands secrete mucin which makes the food slippery and helpsprepare it for further digestion.

3. Salivary Amylase breaks down starches into maltose.

4. At the same time the tongue shapes the food into a round ball called a

bolus.

5. Swallowing (an involuntary response) is initiated when food touches theback of the tongue.

6. Food is passed into the esophagus.

2. Esophagus•

•

Mechanical digestion occurs here also.

This is the pathway for food from the mouth to the stomach.

Parts:

A. Muscles: Smooth muscles undergo a series of muscle contractionscalled Peristalsis.

B. Glottis – Opening into the trachea

C. Epiglottis – Flap of tissue that covers the trachea

D. Esophageal Sphincter – Ring of muscle at the bottom of theesophagus where it joins with the stomach.

– Controls amount of food entering the stomach

Peristalsis

Peristalsis:

Wave-like muscle contractions

that move food along the

alimentary canal.

Occurs in the esophagus, small

intestine and Colon.

3. Stomach

The Stomach

3. Stomach

-- Protein digestion begins here.

J-shaped

Has 3 layers of muscle (smooth muscle)

Has two portions

A. Cardiac Portion –Toward the heart. Contains the Cardiac orEsophageal Sphincter.

B. Pyloric Portion – Toward the feet. Contains the Pyloric Sphincter.– Controls food leaving the stomach and entering the small

intestine.

- Contains Gastric glands which secrete gastric juices ( HCl, water

and enzymes including Pepsinogen and Proteases (enzymes that

break down proteins).

•

- Highly folded (folds are called rugae) to increase surface areafor digestion.

Structure:

Digestion in the Stomach

1. A bolus enters the stomach via the cardiac sphincter.

2. The food is further broken down by the movement of muscles in thestomach called churning.

3. Gastric glands secrete gastric juice containing HCl and pepsinogen thataid in the digestion of proteins.

– Functions of HCl in gastric juice.

• The HCl gives a pH of 1 - 2 which

– kill harmful bacteria,

– Breaks down cellulose and cell membranes in food.– changes pepsinogen (an inactive enzyme) into Pepsin (an

active enzyme).

» Pepsin changes proteins into long polypeptides.

Pepsin

Proteins ---------------- peptides

4. When digestion in the stomach is finished, an acidic soupy liquid remainscalled chyme

4. The Small Intestine• Site of Food Absorption

• Site of End stage digestion reactions

• Structure:

– Composed of 3 sections

• A) Duodenum – 1st section (attached to stomach)

– Bile duct and Pancreatic Duct attaches here.

• B) Jejunum – 2nd section (about 2.5 m long)

• C) Ileum – 3rd section (connects to large intestine – 3m)

– Contains Millions of Villi (singular= Villus)

• Villi – Fingerlike projections in the small intestine that increases the

surface area for absorption.

– Made of smooth muscle

4. The Small Intestine

Small intestine

Structure and Function of a Villus

A. Duodenum• Final digestion of Carbohydrates, Lipids and Fats occurs here.

• Chyme from the stomach enters the duodenum via the pyloric sphincter.

• It is immediately mixed with Bile from the liver/gallbladder and pancreaticjuices and bicarbonate ions from the pancreas.

– Bile and Pancreatic juices help to finish digestion.

Let’s have a look at digestion so far!

• http://www.uclan.ac.uk/facs/health/nursing/sonic/scenarios/uclananim/whole

bodyzoom.swf

– Click on the speaker button before beginning. (turns on sound)

– Click on the “start” button to begin.

Bile• Green liquid produced in the liver.

• Stored in the Gall Bladder.

• Sent to duodenum via the bile duct.

– Bile duct: Tube connecting the gall bladder/Liver to the duodenum.

• Function of Bile:

– Emulsifies (breaks up) large fat molecules into fine droplets called

micelles.

• This increases the surface area of the fat molecules making themeasier to digest.

Pancreas• Created in the Pancreas

• Contains Enzymes that digest Fats, Proteins and Carbohydrates.

• Released into the duodenum via the pancreatic duct.

Pancreatic Juices/Enzyme Reactions

1. Trypsin and Chymotrypsin

– Breaks polypeptides into shorter polypeptides.

2. Pancreatic Lipase

– Breaks fats into fatty acids and glycerol.

3. Pancreatic Amylase

– Breaks starch into maltose

4. Bicarbonate Ions

– Serve to increase pH in the duodenum so enzymes can work. (Chyme fromstomach has a very low pH so enzymes would not work to further digestion).

Other Enzymes found in the Duodenum

• NOTE: These enzymes are released by glands in the intestine.

1. Peptidases

– Breaks peptides into amino acids.

2. Maltase

– Breaks maltose into simple sugars (monosaccharides)

3. Sucrase

– Breaks sucrose into simple sugars (monosaccharides)

4. Lactase

– Breaks Lactose into simple sugars (monosaccharides)

B. Jejunum

• Function:

– Finish breakdown of carbohydrates and proteins so they can be

absorbed by the villi.

C. Ileum– Function:

– Absorb nutrients.

– Push remaining undigested materials into the large intestine.

5. Large Intestine

Structure of Large Intestine• Composed of the following Sections

1. Caecum

– The blind end of the large intestine

2. Appendix

– Small organ attached to caecum. Contains bacteria that help to fightinfection

3. Ascending/Transverse/Descending Colon

4. Rectum

– Final 20 cm of Colon

– Holding area for waste (Feces) to be excreted.

5. Anus

– Opening through which feces passes to the outside.

– Contains the Anal Sphincter

– Anal Sphincter controls movement of feces to the outside.

Function of Large Intestine

• Function

– Remove water and minerals from undigested materials.

– Bacteria produce Vitamins K and B-12.

Questions about Large Intestine

1. What is feces?

2. What is fiber and what role does it play in the large intestine?

3. What is diarrhea? Why is this a problem?

4. What is a sigmoidoscopy and why might it be done?

5. What causes feces to have the colour it does?

Other Organs Related to Digestion1. Liver (4 lobed structure)

Functions

•

•

•

•

•

Creates bile from cholesterol.

Stores carbohydrates as Glycogen

Stores the Fat soluble vitamins (A,D,E,K)

Deaminizes (breaks down) amino acids

Detoxifies various poisons

2. Pancreas (located under stomach)

Functions

•

•

Creates enzymes for digestion

Creates and stores Insulin

– Controls sugar levels in the blood.

• Creates bicarbonate ions

• Counteracts acidity of chyme entering the small intestine, and

gives a basic pH for the intestinal enzymes to work.

3. Gall Bladder (located under liver)

Functions

• Stores Bile

Other Organs Related to Digestion

3. Gall Bladder (located under liver)

Functions

• Stores Bile