Unit 7 - Digestion

The alimentary canal

Learning Outcomes

• identify the gross structure of the alimentary canal and associated organs (mouth, oesophagus, stomach, small intestine: duodenum and ileum, large intestine: colon and rectum, anus, pancreas, liver)

• define ingestion, digestion, absorption, assimilation and egestion

• https://www.youtube.com/watch?v=bFczvJp0bpU

• Digestion introduction

What happens to the food we eat?

The alimentary canal

• The alimentary canal is a long tube running from one end of the body to the other.

Types of digestion

• Mechanical digestion:– Large pieces of food are broken up by the teeth and by

movement in the alimentary canal.

• Chemical digestion– The large insoluble molecules are then broken up in to

small soluble ones.– It involves a chemical change from one type to another.– Enzymes are used in this process.

Coursebook

• Page 80 questions

What happens to the food we eat?

• Ingestion– Intake of food into the mouth

• Digestion– Breaking down large, insoluble food molecules

into smaller soluble ones using enzymes

• Absorption– Digested food molecules pass across the wall of

the small intestine into the blood or the lymph• Assimilation– Uptake of food molecules by cells

• Egestion– Passing out of undigested food, in the form of

faeces, from the anus

• Deamination– Removal of nitrogen containing part of an amino

acid as urea.

Ingestion

• Definition: the taking in of substances through the mouth.

• Teeth are important to this part of digestion.

• WHY?

Teeth

• Food is broken down into smaller pieces in the mouth by chewing. This is an example of mechanical digestion. The teeth cut and crush food, and the pieces are mixed with saliva to form a ball of food called a bolus.

Structure of a tooth

crown

root

enamel

dentinegum

Pulp cavity

cement

Types of teeth

• There are different types of human teeth, with a different structure and function:

• incisor – for biting and cutting• canine – for holding and cutting• premolar and molar - crushing and chewing

• Page 81 – figure 7.13 – types of teeth

Teeth problems

• Dental decay– Tooth decay:

• Caused by bacteria. They form a sticky film over your teeth called plaque.

• It is easy to remove at first but if it is left it will harden and become tartar, which cannot be removed by brushing.

– Gum Disease:• Caused by bacteria. The gums swell and may bleed.• The disease may spread to the root of the tooth.• The tooth will become loose and will need removing.

How can you avoid dental decay?Tooth decay can be prevented by:• avoiding foods with a high sugar content• using toothpaste and drinking water containing fluoride• regular, effective brushing to prevent the build-up of plaque (a

sticky layer on the teeth)

Fluoride compounds may be added to toothpaste and public water supplies. Fluoride reduces tooth decay by:• reducing the ability of bacteria on plaque to produce acid• helping to replace calcium ions and phosphate ions lost by

tooth enamel because of acid attack

Coursebook

• Page 82 Questions

Worksheet

• Worksheet 7.3 label alimentary canal

Salivary glands

stomach

trachea

tongue

mouth

anus

rectumcolon

pancreas

Large intestine

appendix

Small intestine ileum

Duodenum

Gall bladder

liver

oesophagus

Digestion in the mouth and oesophagus

• Chewed food is mixed with saliva in the mouth• Saliva contains– Amylase is an enzyme which starts to digest starch into

sugars (maltose)– Mucus helps soften the food making it easier to swallow

• The bolus travels down the oesophagus with the aid of peristalsis

Peristalsis in the gullet

Digestion

Digestion is the breakdown of large, insoluble food molecules into small, water-soluble molecules using mechanical and chemical processes. Mechanical digestion includes:• chewing in the mouth• churning in the stomachChemical digestion involves enzymes. These are proteins that function as biological catalysts.

Where enzymes are produced.

Enzyme Substrate End-products

Where produced

Salivary amylase Starch Maltose Salivary

glands

Protease Protein Amino acids Stomach, pancreas

Lipase Lipids (fats and oils)

Fatty acids and glycerol Pancreas

Pancreatic amylase Starch Maltose Pancreas

Maltase Maltose Glucose Small intestine

Where enzymes are produced

Where digestion happens

• Proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine

• Lipases catalyse the breakdown of fats and oils into fatty acids and glycerol in the small intestine

• Amylase catalyses the breakdown of starch into maltose in the mouth and small intestine

• Maltase catalyses the breakdown of maltose into glucose in the small intestine

Bile

Bile is secreted into the small intestine where it has two effects:• it neutralises the acid - providing the alkaline

conditions needed in the small intestine• it emulsifies fats - providing a larger surface

area over which the lipase enzymes can work

Absorption

• Once the molecules have been broken down, they are now small enough to pass through the wall of the small intestine and into the blood.

• The small intestine is the region where digested food is absorbed. Most absorption happens in the ileum. This is the longest part of the small intestine and is between 2-4 metres long. The small intestine has a large internal surface area for absorption to happen quickly and efficiently.

The Villi

The villi (one is called a villus) are tiny, finger-shaped structures that increase the surface area. They have several important features:• wall just one cell thick - ensures that there is only a short

distance for absorption to happen by diffusion and active transport

• network of blood capillaries - transports glucose and amino acids away from the small intestine in the blood

• internal structure called a lacteal - transports fatty acids and glycerol away from the small intestine in the lymph

• The hepatic portal vein transports absorbed food from the small intestine to the liver.

Assimilation

Assimilation is the movement of digested food molecules into the cells of the body where they are used. For example:• glucose is used in respiration to provide

energy• amino acids are used to build new proteins

The role of the Liver

• The liver is important in assimilation. For example, it converts glucose into glycogen (a complex carbohydrate used for storage) and amino acids into proteins.

• The liver is involved in the process of deamination. This is the removal of the nitrogen-containing part of amino acids, to form urea, followed by the release of energy from the remainder of the amino acid.

EgestionThe small intestine absorbs most of the water in the contents of the gut. By the time the contents reach the end of the small intestine, most of the digested food has also been absorbed.

The remaining material consists of:• water• bacteria (living and dead)• cells from the lining of the gut• indigestible substances - such as cellulose from plant cell walls

The colon is the first part of the large intestine. It absorbs most of the remaining water. This leaves semi-solid waste material calledfaeces. The faeces are stored in the rectum, the last part of the large intestine. Egestion happens when these faeces pass out of the body through the anus.

Coursebook

• Page 89 questions

Group TaskYour assignment:• Create a brochure or small booklet that will serve as a “travel

guide” of the gastrointestinal tract.• Your guide should include the following components:

– A hand-drawn picture of all 8 “locations” in the digestive tract.– A physical description of each location.– A creative explanation of interesting features, tour options,

restaurants, etc., that can be visited at each stop.• The emphasis on this assignment will be placed on creativity.

The goal is that you understand each part of the digestive tract well enough to create analogies between it and a real-life location that is known for tourism.

Once food enters the mouth, it moves through the following process:

1. Mouth - secrete saliva, which contains an enzyme that breaks down starch. Food is moistened to aid in chewing.

2. Esophagus – long tube that serves as the food delivery mechanism to the stomach. It allows us to breathe while chewing by spacing out the food matter.

3. Stomach - contains strong acids, enzymes and fluids that mix, toss and grind foods. The pepsin enzyme breaks down bonds found in proteins. Begins Digestion

4. Duodenum (Beginning of small intestine). Continues digestion.5. Remainder of small intestine Specifically, carbohydrates, lipids, and proteins are

broken into smaller units here and absorbed into the blood stream.6. Large intestine – reabsorbs water and some minerals into the blood. - Colon

– absorbs water and absorbs feces7. Rectum – Carries feces to anus.8. Anus – passes solid wastes out of the body

• Helpful organs:• -Pancreas – secretes pancreatic juice and a

bicarbonate that neutralizes stomach acid that makes its way into the small intestine..

• -Gallbladder – stores bile.• -Liver - makes bile and digests lipids.