CHAPTER 6: NUTRITION IN HUMANS

CHAPTER 6: NUTRITION IN HUMANS

(A)  INTRODUCTION

Feeding/ ingestion	Is the intake of food into the body
Digestion	The process whereby large food molecules are broken down into soluble and diffusible molecules that can be absorbed into the body cells
Absorption	The process whereby digested food materials are taken into the body cells
Assimilation	The process whereby some of the absorbed food materials are converted into new protoplasm or used to provide energy

·               Digestion involves 2 components: physical and chemical digestion

 

Physical Digestion	·         Mechanical breakdown of food into small particles  which increases the surface area to volume ratio so that digestive enzymes can act on the food more efficiently ·         Achieved by teeth, tongue, stomach walls
Chemical Digestion	·         Enzymatic hydrolysis of large food molecules such as proteins, starch and fats, into small soluble molecules, which can be absorbed. ·         Achieved by digestive enzymes in the mouth, stomach, duodenum and ileum.

(A)  WHAT IS PERISTALSIS?

 

·               Definition: Peristalsis is the rhythmic wave-like contractions of the gut walls caused by the action of two antagonistic muscles (circular and longitudinal muscles). It moves the food along the gut and mixes it with digestive juices.

 

·               Antagonistic muscles = a set of 2 muscles that opposes the action of one another to bring about movement. (When 1 muscle contracts, the other relaxes and vice versa.)

·               When the circular muscles contract, the longitudinal muscles relax à the diameter of the gut decreases (lumen constricts) à food is squeezed forward from original position.

 

·               When the circular muscles relax, the longitudinal muscles contract à the diameter of the gut increases (lumen expands) à food enters the lumen created.

HOW FOOD IS DIGESTED IN OUR BODY? ä

 

Mouth – Carbohydrate digestion only	·         Chewing by teeth cuts up the food particles into smaller pieces. This increases the surface area to volume ratio for salivary amylase to work on.   ·         Action of salivary amylase (from salivary gland): starch à maltose   ·         Tongue rolls food into bolus for easier swallowing.
Oesophagus	·         Bolus of food passes down by peristalsis   ·         No digestion occurs here.
Stomach   Protein digestion only	·         Peristalsis of the stomach walls mixes food with gastric juice to form chyme. ·         Gastric glands stimulated to produce gastric juice (contains hydrochloric acid, enzyme pepsin and enzyme rennin)   Functions of hydrochloric acid (HCl):   1.    Stops the action of salivary amylase (neutral pH)   2.    Converts enzyme pepsinogen (inactive form) à Pepsin (active form)   3.    Converts enzyme prorennin (inactive form) à Rennin (active form)   4.    Kills certain potentially harmful microorganisms present   5.    Provides an acidic medium for the action of gastric enzymes   Protein digestion:   ·         Action of pepsin: proteins à polypeptides   ·         Action of rennin: caseinogen (soluble milk proteins) à casein (insoluble)   ·         Casein à polypeptides by pepsin
Small intestine   Duodenum ▼ Jejunum ▼ ileum   Protein, Carbohydrates & Fats digestion	Intestinal glands: Intestinal juice (alkaline)   Intestinal glands secrete intestinal juice (contains 6 enzymes).   ·         Action of enterokinase:                               trypsinogen (inactive form) à trypsin (active)   ·         Action of maltase: maltose à glucose   ·         Action of lactase:  lactose à glucose and galactose   ·         Action of sucrase: sucrose à glucose and fructose   ·         Action of erepsin:  polypeptides à amino acids   ·         Action of intestinal lipase: fats à fatty acids + glycerol   Pancreas: Pancreatic juice (alkaline)   ·            Pancreas secretes pancreatic juice (contains 3 enzymes).   ·            Action of pancreatic amylase: starch à maltose   ·            Action of trypsin:                     protein à polypeptides   ·            Action of pancreatic lipase:     fats à fatty acids + glycerol   Liver and Gall Bladder: Bile (alkaline)   ·            Liver produces bile and stores in gall bladder. The gall bladder releases bile via the bile duct. Bile is an alkaline greenish-yellow liquid containing bile salt and bile pigments.   ·            Bile emulsifies fats (breaks up fat into minute fat globules, which increases surface area: volume ratio for lipase to act on).   The 3 alkaline liquids:   1.    Neutralise the acidic chyme           2.    Provides a suitable alkaline medium for the action of pancreatic and intestinal enzymes   Absorption in the small intestines:   ·            Digested foods (simple sugars, amino acids and fatty acids and glycerol) are absorbed by the villi of the small intestine, especially of the jejunum and ileum. Refer to Sect. E of the notes. ·      ·         Water and mineral salts are absorbed from undigested food material. Most of the water is absorbed by the ileum.   ·Water and mineral salts are absorbed from undigested food material. Most of the water is absorbed by the ileum.
Large intestine	·         No digestion occurs here.   ·         Water and mineral salts are absorbed from undigested food material.   ·         Faeces are stored temporarily in the rectum.
Anus	·         Faeces are egested through the anus. €

ABSORPTION OF DIGESTED FOOD

 

·               Definition of absorption: Process whereby digested food is taken into the body cells.

 

·               End products of digestions are:

 

1	2	3
Simple sugars: glucose, fructose & galactose	Amino acids	Fatty acids + glycerol

 

·               HOW IS THE SMALL INTESTINE ADAPTED FOR ABSORPTION? (Very important!!!)

 

	Features	How does this help absorption?
1	Inner walls have numerous folds and lined with numerous finger-like projections called villi. The epithelium of the villi, in turn, has numerous microvilli.	Increases the surface area for absorption
2	Villi have thin walls. (epithelium is only one cell thick)	Reduces barrier to diffusion
3	Length is long.	Provides sufficient time for absorption.
4	Richly supplied with blood capillaries and lymphatic capillaries	1.  Carries away absorbed food substances.   2.  Continual removal maintains a concentration gradient for absorption.

 

 

 

 

 

·               HOW ARE NUTRIENTS ABSORBED?

 

Food Substance	Absorbed into	Mechanism
Simple sugars & Amino acids	Blood capillaries of the villus	·         Diffusion & active transport if the concentration of the digested food is lower in the lumen of the small intestines than in the blood capillaries
Glycerol &  Fatty acids	Lacteal (lymphatic capillaries) of the villus	·         By diffusion into the epithelium of the villi and combined to form minute fat globules which enters the lacteal
Mineral salts	Blood capillaries of the villus	·         Diffusion & active transport

ASSIMILATION

 

·               Definition of assimilation: The transport, modification and utilization of absorbed food.

 

·               Simple sugars and amino acids are transported by the hepatic portal vein to the liver.

 

·               Fats are transported by the lymphatic system to empty directly into the heart.

       THE LIVER

 

·               The liver is the largest gland in the body.

TAS

 

FUNCTIONS OF THE LIVER:

Metabolism of Glucose/ Deamination of amino acids/ Detoxification/ Protein synthesis/ Heat Production/ Iron storage/ Bile production

Description 1:

·               Liver detects and maintains blood sugar level at 70-90mg of glucose/100cm³ of blood.

·               When blood sugar rises (after a meal): Excess glucose is converted to glycogen and stored in the liver. This conversion is regulated by pancreatic hormone insulin.

·               When blood sugar drops (after exercise): Glycogen converted back to glucose in the liver and re-enters circulation. This conversion is regulated by hormone glucagon.

Description 2:

·               Excess amino acids are deaminated in the liver.

·               The amino group of the amino acid is removed and converted to urea.

·               Remains of amino acid converted to glucose.

Description 3:

·               Bile is produced by the liver and stored in the gall bladder before release. Bile emulsifies fats.

Description 4:

·               The process of converting harmful substances into harmless products

·               Alcohol is broken down by enzyme alcohol dehydrogenase produced by the liver to form acetaldehyde.

Description 5:

·               Red blood cells are destroyed in the spleen and their haemoglobin is brought to the liver.

·               Liver breaks down the haemoglobin and stores the iron released in the process. Bile pigments are also formed from the breakdown of haemoglobin.

Description 6:

·               Plasma protein is synthesized by the liver. Eg of synthesized proteins include albumins, globulins and fibrinogen.

Description 7:

·               Heat is produced by the numerous chemical reactions in the liver. This heat is distributed by the blood to other parts of the body, thus maintaining the body temperature.