digestion

DIGESTION

Digestion is the process by which

insoluble food, consisting of large molecules is broken down into soluble compounds

What are starch, proteins & fats broken down into?

enzyme

enzyme

enzyme

STARCH

PROTEIN

FAT

Glucose

Amino acids

Fatty acids & glycerol

Why must food be digested?

Blood Blood

Blood

Food in intestine

Food in intestine

Food in intestine

1

3

2

Wall of intestine

To be small enough to be absorbed

through the wall of intestine.

enzyme

Digestion starts at

the mouth and ends

at the

……………….anus

Food is broken down by two actions:

1. PHYSICAL /MECHANICAL

2. CHEMICAL

1. PHYSICAL DIGESTION by: teeth peristalsis of the alimentary

canal

Chewing (mastication)

Why is it important to break down food physically?

To increase the surface area for enzyme action.

Question: SEP, 2009

Give TWO reasons why food must be masticated before swallowed. (4)1. Increased surface area for enzymes to act

quickly.2. Easier to manipulate by tongue to form a

bolus.

2. CHEMICAL DIGESTION by: enzymes

Digestive enzymes are the chemicals that break large insoluble food molecules into smaller soluble molecules.

Four stages in feeding & digestion:

1. Ingestion

2. Digestion

3. Absorption into the bloodstream

4. Egestion

Digested food is absorbed & then assimilated by the cells

Digestive system

Circulatorysystem

Body cell

Absorption

Assimilation

Digestive tract is also called:

Alimentary canal / tract Gut

Mucus is produced throughout the gut:

For lubrication To protect gut wall

from digestive enzymes

Fig. 1 The human digestive system.

The 1st stage of nutrition:

2. Digestion

1. Ingestion

3. Absorption4. Egestion

DIGESTION IN THE MOUTH

salivary glands produce SALIVA (pH = 7-8)

Salivation is a reflex action which is triggered by:

Sight

Smell Thinking

of food

mucus water sodium hydrogen

carbonate salivary amylase

(ptyalin [old term])

Saliva contains:

Functions of mucus in saliva:1. lubricates the food2. sticks the food particles together

bolus

Salivary amylase

In swallowing, food is directed into the gullet / oesophagus

A person cannot breathe while swallowing. Why?

1. The soft palate closes the nasal cavity .

2. The epiglottis closes the trachea.

Swallowing

Peristalsis: wave-like motions of the alimentary

canal which move the food forwards

1

2

BolusDigestive tract

Wave ofcontraction

Wave ofrelaxation

Bolusmoves

2

Peristalsis:

Peristalsis is possible as gut wall is muscular

contraction of circular & longitudinal muscles alternate to push food

Food is pushed when:

What does the longitudinal muscle

do at the same time?

Circular muscle above bolus CONTRACTS

Relaxes

Food reaches the stomach

The stomach stores food for some time & releases it slowly

Food stays for 2-6 hours in the stomach

Chyme: creamy liquid leavingthe stomach

Food remains inside stomach due to contraction of two sphincters

Cardiac sphincter

Oesophagus

Duodenum

Pyloric sphincter

Layers of muscleChurning due to:

Lining of mucus

Which sphincter is contracted?

Cardiac sphincter:Pyloric sphincter: contracte

drelaxes to let chyme into the duodenum

Gastric glands

secrete gastric juice

What happens to food inside the stomach?

Gastric juice contains:1. HYDROCHLORIC ACID2. PEPSIN 3. RENNIN

What is the pH in the stomach?

1-2

Functions of hydrochloric acid:

1. creates the optimum pH for pepsin

Pepsinogen(inactive form)

Pepsin(active enzyme)

3. kills bacteria taken in with the food

2. HCl

Excessive production of gastric juice: burning sensation

Gastric juice enters oesophagus: not enough mucus to protect wall

Remedy for burning sensation in throat and chest:

1. Drink milk2. Take antacids

Doctors can see the stomach lining by using an:

endoscope

oesophagus

Stomach ulcers due to: little mucus in stomach

Pepsin is a protease: breaks:

Polypeptides

Proteins

Which conditions are needed for a piece of meat to be digested?

Pepsin in acidic conditions

Question: MAY,2009

The following diagram represents the protein found in egg white. Each circle represents an amino acid. Complete the diagram below to show how pepsin breaks down the protein found in egg white.

[Idea is that short chains representing polypeptides are made.] (Do NOT accept individual amino acids represented as single circles or dipeptides represented as a pair of circles)

Question: MAY, 2012

List TWO reasons why amylase does not act on starch in the stomach. (2)i. pH in stomach is not optimum for amylaseii. there is no enzyme in the stomach to break

starch

Some substances that are absorbed by the stomach:

ethanol vitamins certain salts and drugs some water glucose

How did I get drunk so

quickly?

Rennin is an enzyme found in the stomach of young

children Function:

Soluble protein in milk

Changes into insoluble protein

Pepsin then acts on the insoluble

protein

In cheese-making, rennet [contains rennin] is added to milk

THE INTESTINES:

Small intestine

Large intestine

1.5 m long

7 m long

The small intestine is composed of:

duodenum & ileum

Two functions of the small intestine:

1. digestion is completed2. digested food is absorbed

Bloodstream

Products of digestion

Duodenum U-shaped first part of small intestine (most digestion

occurs here) about 25 cm long

The duodenum receives secretions from the:

Pancreas[pancreatic juice via the pancreatic duct]

Liver[bile via the bile duct]

DIGESTION IN THE DUODENUM

Where is bile:i) made? ii)stored?

Made in liver

Stored in gall

bladder

Bile:

has no enzymes consists of:

water sodium chloride bile salts bile pigments

pH = 8

Functions of Bile: dilutes contents from stomach creates optimum pH for pancreatic enzymes

to work in duodenum contains bile salts which emulsify fats

Bile salts

Bile salts emulsify fats i.e.: break large fat globules into smaller globules

Emulsificationof fats

What is the benefit of emulsification?The surface area where lipase can act is increased.

A person had his gall bladder removed. Explain why the doctor told him to limit fats in the diet.

Fats cannot be emulsified; difficult to digest fats by lipase.

Three enzymes in pancreatic

juice:

Pancreatic amylaseTrypsin

Pancreas

Lipase

Action of each

enzyme:

lipase +

fat moleculeglycerol fatty

acids

starch molecule

amylasemaltose

proteins & polypeptides

trypsin

peptides

Question: MAY, 2012

Bile and pancreatic juice are involved in digestion of fats.a) Describe the role of both secretions in this

process. (4) b) Explain why bile must be released before

pancreatic juice for the efficient digestion of fats. (2)

DIGESTION IN THE ILEUM

The wall of the ileum has

intestinal glands that produce

intestinal juice

Enzymes in intestinal juice

SUBSTRATE ENZYME END PRODUCT

Peptides Peptidase Amino acidsLipids Lipase Fatty acids &

glycerolMaltose Maltase Glucose + glucoseSucrose Sucrase Glucose + fructoseLactose Lactase Glucose + galactose

Remember:

Pepsin

Amino acids

Peptides

Polypeptides

Proteins

Trypsin

Peptidase

Remember: Digestion of starch

Starch

Maltose

Maltose(in ileum)

Glucose

Pancreatic Amylase

Salivary Amylase(In Mouth)

No digestion of starch in stomach (amylase is denatured)

Maltase

Digestion is complete in the ileum. What happens to the

digested food?

Model of the ileum showing numerous villi

Villus: a finger-like projection

Villi contain blood & lymph vessels

Structure of a villus

A villus is adapted for absorption

The ileum is adapted to absorb digested food

by providing a LARGE surface area: Folded walls Has villi Has microvilli

Ileum is adapted for absorption:

1. Ileum is long: more time for digestion a greater surface area for absorption

2. Villi: increase surface area for absorption

3. Microvilli in epithelium: increase surface area for absorption

4. Epithelium is very thin: soluble products of digestion pass through quickly

5. Dense capillary network: removes the digested food

THE LARGE INTESTINE

The large intestine is composed of:

1.Caecum

Anus

2. Appendix

4. Colon

3. Rectum

Material in the large intestine consists of:

water mucus dead cells undigested matter

Most of the undigested matter is cellulose. Why is this so?

Function of the:

Colon: absorbs water

Rectum: stores FAECES before being expelled through the ANUS

The 4th stage of feeding & digestion:

4. Egesting

Egestion / Defaecation

the process of removing faeces

I really have to go to the bathroom!

Caecum & Appendix are

Large in herbivores Vestigial in humans:[small & no function]

Caecum

Appendix

Question: SEP, 2011

Humans are omnivores.Do not depend only upon vegetation for nutrients.A large caecum is found in herbivores and is full of bacteria that produce cellulase to digest cellulose.

Explain why the caecum / appendix in humans is small and non-functional. (4) Human Rabbit

Ingestion(2 L) Salivary gland secretions

(1 L)

Gastric secretions(2 L)

Pancreatic secretions(1.2 L)

1% infaeces

Small intestinesecretions(2 L)

Bile(0.7 L)

Although 8.9 L of water are ingested or secreted daily,

the faeces are dry.Explain.

Ingestion(2 L) Salivary gland secretions

(1 L)

Gastric secretions(2 L)

Pancreatic secretions(1.2 L)

Absorbed in the small intestine &

colon.

1% infaeces

Small intestinesecretions(2 L)

Bile(0.7 L)

ASSIMILATION OF DIGESTED FOOD

FOOD USE STOREGlucose

Respiration Glycogen or Fat

Fats

Respiration, to build cell structure, as a store of energy

Fat

Amino acids

Build proteins for cell structures, enzymes

CANNOT BE STORED – ARE DEAMINATED & USED IN RESPIRATION

Deamination occurs in the liver

AMMONIA

AMINO ACIDis broken into two:

The rest is used for energy

Part with nitrogen forms:

UREA

Question: MAY, 2007

Write the correct term for each of the following processes:i) the breakdown of excess amino acids

Deamination

ii) rhythmical muscular contractions that push food along the digestive systemPeristalsis

iii) the elimination of indigestible food from the bodyEgestion / Defaecation

iv) the passage of digested food through the gut wall in the blood stream.

Absorption

BLOOD SUPPLY OF THE LIVER

Oxygenated blood from heartBlood from

digestive system

HEPATIC PORTAL VEIN

HEPATIC ARTERY

LIVER

HEART

HEPATIC VEINto heart

GUT

Blood supply to and away from the liver

Write an account of a controlled experiment you would perform to show the action of the enzyme amylase on starch. (6) [MAY, 1997]

Two spotting tiles are prepared with a drop of iodine solution in each hole.

An equal volume of starch is placed into two separate test tubes, labelled A and B.

12

An equal volume of amylase is placed into two separate test tubes, labelled C and D. Test tube C is boiled for 5 minutes to denature the enzyme and so act as a control.

All four test tubes are placed in a water bath at 37C for 5 minutes to acclimatise.

3

4

Starch in test tube A is poured into amylase in test tube D, mixed and the stop watch is started.

5

After 30 seconds, a drop of the mixture is taken and added to one of the drops of iodine solution on the spotting tile. This is repeated until a yellow colour appears.

6

Starch in test tube B is poured into amylase in test tube C, mixed and the same method is repeated.

7

8Contents in test tube:i) D give a yellow colour with

iodine solution after a few minutes

ii) C continues to give a blue-black colour after 30 minutes.

9 The results show that amylase breaks down starch.

10If Benedict’s solution is added to the contents of test tubes C and D and heated for a few minutes, a brick red colour is obtained only in test tube D [unboiled amylase]. This shows that when starch breaks down, reducing sugars are formed.

C D

THE END

I HAVE SO

MUCH TO

DIGEST!!