Digestion 8/28/10 1:39 PM6: Human Health and Physiology6.1 Digestion

Orange book: pg. 203-207Green book: pg. 95-97

6.1.1 Explain why digestion of large food molecules is essential (pg. 203, 95)

6.1.2 Explain the need for enzymes in digestion (pg. 204, 95)

6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase (pg. 204, 95)

6.1.4 Draw and label a diagram of the digestive system (pg. 205, 96)

6.1.5 Outline the function of the stomach, small intestine and large intestine (pg. 206, 96)

6.1.6 Distinguish between absorption and assimilation (pg. 207, 96)

6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion (pg. 97).

Role of Digestion 8/28/10 1:39 PM

6.1.1 Explain why digestion of large food molecules is essential

Orange book pg. 203Green book pg. 95

To do: Read and highlight the notes below, read the relevant sections in the green book. Discuss your findings Complete the table below Summary in Green book to include: list of events in digestive system compounds and their monomers sentence to explain the importance of digestion

Why do we digest food?When you eat a snack or meal, you begin a set of events that leads to your body cells being provided with needed nutrients. In very basic format, here is the series of events in order: Ingestion – eating food Digestion – series of chemical reactions, whereby large complex

insoluble molecules are broken down to small simple soluble molecules.

Absorption – small molecular forms are absorbed through cells of your digestive system and pass into nearby blood or lymphatic tissue.

Transport – your circulatory system delivers the small molecular nutrients to your body cells

Assimilation – the components of digestions are ‘reassembled’ by our cells into larger molecules that are needed by the body.

Digestion solves a problem of molecular size.Many of the foods that we ingest have very large molecules – too large to pass across the cell membrane. Yet to get into your bloodstream, molecules must pass through the cell membranes of your intestines and then through the cell membrane of a capillary. Any food that we eat must therefore be chemically digested to a suitable size.

Compound MonomersCarbohydrateProteinLipid

CarbohydratesNotes:

This symbol usually represents a single glucose molecule.

When two glucose molecules join together they form a disaccharde.

Carbohydrates are polymers – they are formed from lots of single glucose molecules joined together. Both starch and glycogen are formed from long chains of glucose molecules.

ProteinsProteins are complex molecules that are made from 20 basic monomers called amino acids.

The order in which the amino acids are arrange determines the function of the protein.

Each of these symbols represents a different amino acid (I have only used 6 instead of the 20 that are available):

The number of amino acids and the sequence in which they are arranged varies between different proteins.

LipidsLipids are composed of two different monomers called fatty acids and glycerol.

Key ImportanceDigestion is important because it breakdown large, complex, insoluble molecules into small, simple, soluble molecules.

Carbohydrates, proteins and lipids are too large to pass across the intestinal wall and into the blood. They must be digested in order to be absorbed into the blood.

GLYCEROL

FATTY ACID

FATTY ACID

FATTY ACID

Enzymes 8/28/10 1:39 PM

6.1.2 Explain the need for enzymes in digestion

Orange book pg. 204Green book pg. 95

To do: Read and highlight the notes below, read the relevant sections in the green book. Discuss your findings Summary in Green book to include: explanation of specificity function of enzymes in relation to activation energy sentence to explain the importance of digestion importance of enzymes being globular proteins explanation of bond weakening in active site

The role of enzymes during digestionAs food moves through your alimentary canal. Many digestive enzymes are added along the way. Each digestive enzyme is specific for a specific food type. For example, lipase is an enzyme specific for lipid molecules and amylase is specific for amylose (starch).

The real function of enzymes is to lower the activation energy of the reaction they catalyse. This means that reactions occurring with an enzyme can occur with a lower input of energy than the same reaction without the aid of an enzyme. The input of energy is typically in the form of heat. Enzyme-catalysed reactions proceed at higher reaction rates at a lower temperature than the same reaction without an enzyme. This is a tremendous advantage for living organisms. Many of the reactions which represent the digestive process would need far higher temperatures than we are able to maintain safely if enzymes were not involved. Humans maintain a stable body temperature of 37oC. This temperature is warm enough to maintain good molecular movement and, with the aid of enzymes, it provides enough activation energy for metabolic reactions including digestion.

Globular Proteins

Enzymes are globular proteins (spherical/ globe like) that function as biological catalysts (a catalyst speeds up a biological reaction without being altered or used up in the reaction itself). They permit biological reactions to occur very rapidly at normal body temperatures. The globular shape of the protein plays a key role in the action of the enzyme. Due to the specific globular shape the enzyme will have an active site, which will accommodate only one type of substrate and so carry out a very specific function.

Digestive enzymes all help to catalyse hydrolysis reactions. A typical example is the digestion of starch by amylase into glucose.

The role of amylase is to temporarily hold the starch in its active site and put stress on the covalent bonds that bind the glucose molecules together within the polysaccharide. When these bonds are stressed, it is more likely that the surrounding thermal energy (body temperature) will provide enough molecular motion to break the bonds.

Without enzymes it would take too long to try and totally digest our food mechanically (e.g. by chewing in the mouth and mixing/ churning in the stomach and small intestine).

Chewing and mixing will not break down food into small enough particles. Enzymes are need to break it down to small, simple, soluble molecules.

NamingThe modern system of naming enzymes involves identifying the substance the enzyme acts upon, called its substrate and adds the suffix –ase. Thus lipase digests lipids and protease digests protein.

Enzyme Examples 8/28/10 1:39 PM

6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase

Orange book pg. 204Green book pg. 95

To do: Read and the notes below, read the relevant sections in the green book.Use your green book to fill in the table below Copy the table into Green exercise bookAnswer the IB practice questions in your green exercise books

Examples of digestive enzymesThere are many enzymes that help us to digest food. Some of these are the individual enzymes that are specific to many of the types of carbohydrates we ingest. We also produce many different protease enzymes that collectively help us to digest proteins. Some of these protease enzymes work within a protein by recognizing specific amino acid pairs, and some digest proteins from the outer ends and work ‘inwards’.

Use your book to fill in the following table:

Enzyme Source Substrate Products Optimum pH

IB Practice Questions1. Describe the role of enzymes in the process of digestion of proteins, carbohydrates and lipids in humans. (6)

2. Which organ secretes enzymes that are active at a low pH? (1)A. MouthB. PancreasC. StomachD. Liver

3. State the sources, substrate, product, and optimum pH conditions for the enzyme amylase. (4)

4. Which of the following is correct regarding the enzymes listed in the table? (1)

EnzymeAmylase Lipase Protease

A. Substrate

polysaccharide emulsified fat dipeptide or polypeptide

B. Substrate

emulsified fat dipeptide or polypeptide

polysaccharide

C. Product amino acids small polysaccharides or monosaccharides

fatty acids and glycerol

D. Product small polysaccharides or monosaccharides

amino acids fatty acids and glycerol

5. Describe the role of enzymes in digestion with reference to two named examples. (5)

6. State the source, substrate, products and optimum pH for any two named digestive enzymes.

Digestive System 8/28/10 1:39 PM

6.1.4 Draw and label a diagram of the digestive system

Orange book pg. 205Green book pg. 96

To do:Study the diagrams of the digestive systemVisit each of the three websites Without using a book or your notes, on a piece of scrap paper draw a labeled diagram of the human digestive system. In your green exercise books draw a labeled diagram of the human digestive system

Much of the human digestive system is a tube called the alimentary canal. In order the alimentary canal consists of:MouthOesophagusStomachSmall intestineLarge intestine (colon)Rectum

Any food that you ingest must either be digested and absorbed for use by the body or remain undigested and eliminated as solid waste (faeces).

Study each of the diagrams of the digestive system:

Websites Animation: Organs of Digestionhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter26/animation__organs_of_digestion.html

Label the Digestive Systemhttp://www.mhhe.com/biosci/genbio/maderbiology7/graphics/mader07b/mader_labeling/mader_labeling_source/mi12-01b.dcr

http://www.mhhe.com/biosci/genbio/maderbiology7/graphics/mader07b/mader_labeling/mader_labeling_source/mi12-01c.dcr

Functions of Digestive System 8/28/10 1:39 PM

6.1.5 Outline the function of the stomach, small intestine and large intestine

Orange book pg. 206Green book pg. 96

To do:Visit each of the website Read and highlight the notes below, read the relevant sections in the green book. Complete the table at the end of the document Summary in Green book to include: movement of food in oesophagus composition and function of gastric juice churning and chyme pancreatic secretion bile increased surface area in small intestine absorption of water in large intestine mutualistic bacteria

Animation: Digestive Systemhttp://www.bioanim.com/CellTissueHumanBody6/index.html

Stomach

Food is brought to your stomach by a muscular tube called the oesophagus. When you swallow, the food is forced down to your stomach by a sequential series of muscular contractions called peristalsis. The stomach is an expandable bag where the food is stored for up to a few hours in order to mix it with a variety of secretions collectively known as gastric juice. The stomach wall has no villi, but numerous gastric pits, which secrete the gastric juice. Gastric juice contains: hydrochloric acid (pH 1) to kill bacteria (the acid does not help digestion, in fact it hinders it by denaturing most enzymes); however, it creates the pH necessary for pepsin to be active.mucus to lubricate the food and to line the epithelium to protect it from the acid;pepsin a protease enzyme digest proteins. It has a muscular wall which contracts and relaxes to churn the food into a liquid called chyme. This is gradually released in to the small intestine by a sphincter, a region of thick circular muscle that acts as a valve.

Small IntestineThis is about 6.5 m long, and can be divided into two sections:

The duodenum Although this is short, almost all the digestion takes place here, due to two secretions from different accessory glands.

Pancreatic juice is secreted by the pancreas through the pancreatic duct. This contains numerous carbohydrases (including maylase), protease (trypsin) and lipase enzymes and bicarbonate.

Bile, secreted by the liver, stored in the gall bladder, and released through the bile duct into the duodenum. Bile contains bile salts to aid lipid digestion, and the alkali sodium hydrogen carbonate to neutralise the stomach acid. Without this, the pancreatic enzymes would not work. The bile duct and the pancreatic duct join just before they enter the duodenum.

The ileum (4 m long). This is the site of final digestion and all absorption. There are numerous glands in the ileum lining which secrete enzymes. The digestion produces molecules, which are small enough to be absorbed into the blood.

The internal surface area is increased enormously by three levels of folding: large folds of the lining, villi, and microvilli. Don't confuse these: villi are large structures composed of many cells that can clearly be seen with a light microscope, while microvilli are small subcellular structures formed by the folding of the plasma membrane of individual cells. Microvilli can only be seen clearly with an electron microscope, and appear as a fuzzy brush border under the light microscope.

Circular and longitudinal muscles propel the liquid food by peristalsis, and mix the contents by pendular movements - bi-directional peristalsis. This also improves absorption.

Large IntestineThe vast majority of useful nutrients are absorbed while food is still inside the small intestine. What remains of the original food at the end of the small intestine in undigested (and therefore unabsorbed). The primary function of the large intestine is to absorb water. The lining contains villi but no microvilli, and there are numerous glands secreting mucus. Food can spend 36 hours in the large intestine, while water is absorbed to form semisolid faeces.

The large intestine is also home to a very large number of naturally occurring bacteria including Escherichia coli. These bacteria are examples of mutualistic organisms within us. We provide nutrients water, and a warm environment for them while they synthesize vitamin K and maintain a healthy overall environment for us in our large intestines.

Faeces is made up of plant fibre (cellulose mainly), cholesterol, bile, mucus, mucosa cells (250g of cells are lost each day), bacteria and water, and is released by the anal sphincter.

Look at the diagram below and fill in the table Keep functions very brief to act as an overview.

Name FunctionA

B

C

D

E

F

G

Absorption and Assimilation 8/28/10 1:39 PM

6.1.6 Distinguish between absorption and assimilation

Orange book pg. 207Green book pg. 96

To do:Do as it says in the objective Write a couple of sentences in your green exercise book explaining the difference between absorption and assimilation

Villi 8/28/10 1:39 PM

6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.

Orange book pg. 208Green book pg. 97

To do: Visit the website Read and highlight the notes below, read the relevant sections in the green book. Draw and label a diagram of a villus in vertical section. Summary in Green book to include: function of villi microvilli brush border enzymes

Label Villi Structurehttp://www.mhhe.com/biosci/genbio/maderbiology7/graphics/mader07b/mader_labeling/mader_labeling_source/mi12-06.dcr

If the lining of the small intestine is examined closely it appears fuzzy. This is due to finger like projections called villi, about 0.5 to 1.0mm high. Villi are very thin to allow for absorption of the soluble products of digestion. If the inner lining of your small intestine were smooth, you would have a fairly limited membrane surface for absorption. The function of the villi is to greatly increase the surface area for absorption of molecules such as glucose, amino acids and fatty acids.

The core of a villus contains an arteriole, a capillary network, a venule and a lymphatic capillary called a lacteal. Most nutrients are absorbed by the blood capillaries, but most fat is absorbed by the lacteal and gives its contents the milky appearance for which the lacteal is named. The core of the villus also has a few fibres of smooth muscle that contract periodically. This enhances mixing of the digested food in the intestine.

Each absorptive cell of a villus has a fuzzy brush border of microvilli about 1m high. The brush border increases the absorptive surface area of the small intestine and contains brush border enzymes, integral proteins of the plasma membrane. The enzymes carry out the final stages of digestion. They are not releases into the lumen; instead, the chime must contact the brush border for digestion to occur. This process, called contact digestion, is one reason that thorough mixing of the chime is so important.