Topic 6.1 – Digestion & Absorption

Digestion of Large Food Molecules

• Many of the food molecules that humans consume are much too large to be absorbed by the cells of the body, particularly the villi of the small intestine where absorption of molecules takes place.

• As you will recall from topic 2 on the biological molecules, they are large molecules that need to be broken down into simpler components. For example, all complex carbohydrates are digested or broken down into their smallest component, glucose (sugar).

• All large molecules have to broken down into molecules that are small enough to diffuse across the plasma membranes of the body’s cells.

Events in Digestion• Ingestion – taking food in • Digestion – a series of chemical reactions that break

down the ingested food into molecules that are small enough to pass across plasma membranes.

• Absorption – uptake of small nutrient molecules by the body’s cells.

• Transport & Assimilation –nutrients are delivered to all cells in your body via the circulatory system. They converted into fluid or solid parts of the organism or used in biochemical reactions of cells in the tissues of the body.

Figure 22.6

Accessory organs

Salivary glands

Liver

Gallbladder

Pancreas

Alimentary canal

Oral cavity

Tongue

Pharynx

Esophagus

Stomach

Small intestine

Colon

AppendixRectumAnus

Large intestine

6.1 S1 Production of an annotated diagram of the digestive system

(GI tract)

IN your drawing book:

Annotate with the functions of each part

A person produces over a litre of saliva a day!

The Stomach• large j-shaped organ

attached to the esophagus at one end and the small intestine at the other.

• food is temporarily stored while it mixes with gastric juices consisting of pepsin, hydrochloric acid and mucus.

• It can hold about 1.5 L of food (digested into semi-solid mixture called chyme)

• Gastric ulcers

– Are erosions of the stomach lining

– Are often caused by a bacterium named Helicobacter pylori

Figure 22.11

6.1 U1 The contraction of circular and longitudinal muscle of the small intestine mixes food with enzymes and moves it along the gut

- Smooth muscle (short cells not elongated)

- Often has continuous force with short period of vigorous contraction

- Peristalsis occurs here as well and moves the food through the intestines

- Circular muscles behind the food prevent the food from backing up

- Longitudinal muscle contraction (at site of food) move it along the intestines

https://www.youtube.com/watch?v=Ujr0UAbyPS4

-From esophagus to small intestine peristalsis moves food forward (vomiting is abdominal muscles)

- Moves slow through small intestine (cm at a time) to give time for gastric juice to mix with food leading to faster absorption

6.1 U2 The pancreas secretes enzymes into the lumen of the small intestine.

• Pancreas is composed of two types of gland tissue: cells that produce hormones (insulin & glucagon) and cells that produce digestive enzymes.

• Hormones made and released by the stomach regulate the production of enzymes by the pancreas (responds to food contents).

• Enteric nervous system plays a role as well (stomach has a mind of its own!)

• The pancreas contains ducts which are small tubes which all lead to the main pancreatic duct.

• Hydrolases released by the pancreas include:

• Amylase (digests starches)• Lipase (digests triglycerides and

phospholipids)• Proteases (or endopeptidase –

digests proteins and peptides)

Source: [Frank, A., Deng, Sh. et al. 2004, Transplantation for type I diabetes: comparison of vascularized whole-organ pancreas with isolated pancreatic islets. 240: 631-643.]

6.1 U3 Enzymes digest most macromolecules in food into monomers in the small intestine.

• The pancreas secretes enzymes into the lumen of the small intestine and the following chart summarizes the effect of the hydrolases:

Biological Molecule Enzyme Digested Form

Starch amylase maltose

Proteins & polypeptides

protease Shorter peptides

Phospholipids phospholipase Fatty acids, glycerol & phosphate

Triglycerides lipase Fatty acids & glycerol or fatty acids

monoglycerides

Figure 22.4b

Protein Hydrolase (pepsin)

Amino acid

Hydrolase (amylase)

Hydrolase (lipase)

Glycerol

Fatty acid

(b) Hydrolases

Sugar

Enzymes and Digestion• Digestion is completed in the small intestine by enzymes

produced in gland cells of the walls of the small intestine.

Molecule Enzyme Digested Form

DNA & RNA Nucleases Nucleotides

Maltose Maltase Glucose

Lactose Lactase Glucose & glalactose

Sucrose Sucrase Glucose & fructose

Peptides Exopeptidases dipeptides

Dipeptides dipeptidases Amino acids

Small Intestine• The small intestine has

three different portion or sections; the duodenum, jejunum and ileum.

6.1 S2 Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph

First…without the EM picture

Goblet cells

Lymph vessel also called lacteal

E-epithelium

SM-submucosa

M-muscosa

IM- inner circular muscle

OM-outer longitudinal muscle

Examine these!

Now try and label the parts on the diagram on page 281 of your text

Digestion and the Small Intestine

• Digestion of the macromolecules is completed in the small intestine and the end products of nutrient digestion are also absorbed here. Food spends hours here due to the length of the small intestine and to allow for completion of digestion and absorption. 90% of absorption takes place here.

• Once the food has been digested and the nutrients have been absorbed the only thing left to empty into the large intestine is waste products and water. Some substances such as cellulose (main component of dietary fibre) remain undigested because humans lack the enzymes required to break it down.

6.1 U4 Villi increase the surface area of epithelium over which absorption is carried out.

• The villi that line the small intestine greatly increase the surface area for absorption. They contain capillaries and a lacteal for the transport of nutrients.

• The villi are only one cell layer thick so it is easy for nutrients to diffuse across the cells. This layer is called the epithelium layer.

• The surface area greatly influences the rate of absorption. An adult small intestine is about 7m in length and 25-30 mm in width. The inner surface is highly folded and contains the villi which further increase the surface area.

• Each villus measures between 0.5 and 1.5 mm in length and there are approximately 40 villus per square mm of small intestine making the surface 10x larger.

6.1 U5 Villi absorb monomers formed by digestion as well as mineral ions and vitamins

• Epithelium of the villi needs to protect the body (from harmful substances) but at the same time be permeable to nutrients.

Other absorbed substances needed (but no digesting needed):

-Mineral ions (ie. calcium, potassium, sodium)

- vitamins (ie. Vitamin C)

Nutrients absorbed are:

- glucose, fructose, amino acids and other monosaccharides- Any of the 20 amino acids used to make proteins- Fatty acids, monoglyceride and glycerol- Nucleotide (DNA/RNA) bases

Other absorbed substances not needed:

-harmful substances (liver detoxifies)

-food color and flavour chemicals

-Small amounts of bacteria (wbc’s in liver destroy)

6.1 U6 Different methods of membrane transport are required to absorb different nutrients.

Large Intestine• The large intestine, also

known as the colon receives the waste products of digestion from the small intestine.

• The material that passes into the colon contains water and any unabsorbed or undigested food.

• There is an ascending, transverse and descending portion of the colon.

Function of the Large Intestine• The major function of the large intestine is the

absorption of water. Water is reabsorbed through the walls of the colon along with any small molecules that may be dissolved in the water.

• The colon also hosts a large number of naturally occurring E. coli bacteria with whom we have a symbiotic relationship. Our colon provides the bacteria with nutrients, water and a warm environment and in turn they synthesize vitamin K and keep our colon healthy.

• Any food that remains undigested or unused by us or the bacteria is eliminated by the body as feces.

6.1 A1 Processes occurring in the small intestine that result in the digestion of starch and transport of the products of

digestion to the liver

• See assignment completed in class.

6.1 A2 Use of dialysis tubing to model absorption of digested food in the intestine

• Read pages 287-88 in text to prepare for the next lab day.