Biology chapter 6 (6.4)

6.4 Food digestion

DIGESTION

The process that breaks down complex food substances into simpler, soluble

molecules that are small enough for the body to absorb.

Digestion in the mouth

• salivary gland saliva Salivary amylase

• starch + water maltose• bolus • epiglottis – closes the airway temporarily to

prevent food from entering the trachea• oesophagus (bolus move down by peristalsis)• mucus – lubricates the bolus & aids the

movement of bolus• cardiac sphincter relax- food enters the stomach

peristalsis

Digestion in the stomach

1. The epithelial lining of the stomach contain gastric glands that secrete gastric juice Gastric juice contains:(a) Hydrochloric acid

Creates an acidic condition (pH 1.5-2.0) for the enzymes in the stomach to function optimally.

Stops the activity of salivary amylaseKills bacteria in food


(b) Pepsin Starts the hydrolysis of large protein

molecules into smaller chains of polypeptides by breaking specific peptide bonds.

Proteins + water pepsin polypeptides


(c) Rennin (coagulates milk) Convert soluble milk protein, caseinogens,

into soluble casein. caseinogen + water rennin casein

(d) Mucus Protects the stomach from the action of

hydrochloric acid and digestive enzymes.


• Food stays in the stomach for a number of hours.

• The food is thoroughly churned and mixed with the gastric juice by the peristaltic contractions of the stomach wall.

• The contents of the stomach become a semi-fluid called chyme.

• Relaxation of pyloric sphincter allows the chyme to enter duodenum


• The small intestine consists of the duodenum, jejunum and ileum.

Digestion in the small intestine

Digestion in the duodenum

Duodenum receives:• chyme from the stomach• pancreatic juice from the pancreas • bile from the gall bladder

Bile is produced by the liver and stored in the gall bladder.Bile:• Doesn’t contain any digestive enzymes• Creates an alkaline environment (pH 7.6-8.6)• Helps to reduce the acidity of chyme• Bile salts emulsify lipids into tiny droplets to

increase the surface area for efficient digestion by enzyme lipase.


• Pancreatic juice contains pancreatic amylase, trypsin and lipase.

(a) Pancreatic amylaseStarch + water pancreatic amylase maltose

(b)Trypsinpolypeptides + water trypsin peptides

(c) LipaseLipid droplets+ water lipase glycerol + fatty acids


Digestion in the ileum• Intestinal glands secrete intestinal juice.Protein digestion

Peptides + water erepsin amino acids

Carhydrate digestionSucrose + water sucrase glucose +fructoseLactose + water lactase glucose + galactoseMaltose + water maltase glucose

The digestive system of Ruminants


• Ruminants and rodents feed on plants which contain a high percentage of cellulose.

• Ruminants obtain most of their energy from the breakdown of cellulose into simple sugar by the enzyme cellulase.

• Ruminants like cows and goats have stomach which are divided into four chambers: rumen, reticulum, omasum and abomasum

• This adaptation enables ruminants to carry out rumination, the process of regurgitating food and rechewing it.

• Chambers of rumen and reticulum have bacteria and protozoa which are able to secrete cellulase to digest cellulose.


Cellulose digestion of a cow

• Partially chewed food is passed to the rumen. Cellulose is broken down by the enzyme cellulase. Cellulase is produced by microorganisms such as bacteria and protozoa.

Food enters the reticulum and cellulose undergoes further hydrolysis. The content of the reticulum is called cud is regurgitated into the mouth to be thoroughly chewed again.


The cud is reswallowed and moves to the omasum. Here large particles of food are broken down into smaller pieces by peristalsis. Water is removed from the cud.


The food particles finally move into the abomasum, the true stomach. Gastric juices containing digestive enzymes complete the digestion of protein and other food substances. The food then passes through the small intestine to be digested and absorbed.


The Digestive Systems of Rodents

• In rodents like rats, the caecum and appendix are enlarged to store the cellulase-producing bacteria.

•In herbivores like rabbits, the breakdown products pass through the alimentary canal twice. The faeces in the first batch produced at night is watery and soft. These are eaten again to absorb the product of bacterial breakdown. The second bath of faeces becomes drier and harder.

Problems associated with food digestion

1. Incomplete digestion of food• Excessive intake of food, eating too much

oily food or eating too fast.• May cause severe pain the abdomen,

nausea, vomiting and bloated stomach.

2.Reduced production of specific digestive enzymes• Difficulty in digesting lactose in adult (lack

of lactase) • Damage of pancreas which reduce the

production of pancreatic amylase, trypsin and lipase. Digestion of food disrupted and body will not be able to obtain sufficient nutrients.

3. Formation of gallstones• Gallstones are formed through hardening

of cholesterol.• Excessive intake of fatty food• Gallstones block the bile duct, preventing

bile to be channel out. As a result, lipids cannot be emulsified.

6.5 ABSORPTION AND ASSIMILATION OF DIGESTED FOOD

How does villi improve the efficiency of absorption

MicrovilliNumerous villiThin walls (one-cell thick)Rich supply of blood capillariesHave lacteal

ABSORPTION OF DIGESTED FOOD

Glucose, amino acids, vitamin B, C and mineral Lumen epithelial cellsBlood capillaries

hepatic portal vein liver

Glycerol, fatty acids, vitamin A, D, E, K Lumen epithelial cellslacteal lymphatic

system blood stream

ASSIMILATION OF DIGESTED FOOD

The process of assimilation in the LiverAmino acidsGlucose

The process of assimilation in the cellAmino acidsGlucoseLipids

ASSIMILATION OF DIGESTED FOOD

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Biology chapter 6 (6.4)