Download - 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!!