Chapter Digestion 22

Chapter 6:Nutrition in Humans

IntroductionPeristalsisProcess of DigestionAbsorptionAssimilationThe Liver

NUTRITION is the intake of food and the processes that convert food substances into living matter.

Nutrition comprises of the following:-

FEEDING: 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

INTRODUCTION

PHYSICAL vs. CHEMICAL DIGESTION

Physical digestion:

Mechanical breakdown of food into small particles

Increases the surface area to volume ratio of the ingested food so that digestive enzymes can act on the food more efficiently.

Occurs in the mouth, when you chew food using your teeth and in the stomach, where churning of the stomach walls breaks up the food particles and mixes them with digestive enzymes.

Chemical digestion:

Enzymatic hydrolysis (recall Chap 4) of large food molecules into small soluble molecules which can be absorbed.

Achieved by the digestive enzymes found in the mouth,

stomach, duodenum and ileum.

The Human Alimentary Canal

You must be able to identify and label all structures!

Definition:

The rhythmic, wave-like contractions of the gut walls caused by the action of 2 antagonistic muscles.

Functions:

1. Moves the food along the gut

2. Mixes the food with digestive enzymes

Part of the gut wall

circular muscles

longitudinal muscles

What is Peristalsis?

The wall of the oesophagus is made up of 2 layers of muscles.

These muscles are present along the gut from the oesophagus to the rectum.


layers of muscles

Antagonistic Muscles

The two layers of muscles are:

1. Longitudinal muscles on the outside of the gut; and

2. Circular muscles on the inside of the gut.

They are antagonistic = 2 muscles opposes the action of one another to bring about movement. (When 1 muscle contracts, the other relaxes and vice versa.)

circular muscles


LABEL YOUR DIAGRAM NOW!

longitudinal muscles

Antagonistic Muscles

circular muscles constrict the lumen

longitudinal muscles dilates the lumen

Contraction of the circular muscles always constricts the lumen (becomes narrower and longer)

Contraction of the longitudinal muscles always dilates/ expands the lumen (becomes widen and shorter).

lumen

What is Peristalsis?

food mass is pushed forward

wall constricts to push food forward

circular muscles contract

longitudinal muscles relax

Contraction of Circular muscles

• When the circular muscles contract, the longitudinal muscles relax.

• The diameter of the gut decreases lumen constricts (becomes narrower and longer)

• Food is squeezed forward from original position.

food mass enters

circular muscles relax

longitudinal muscles contract

wall dilates to allow food to enter

Contraction of Longitudinal muscles

• When the circular muscles relax, the longitudinal muscles contract.

• The diameter of the gut increases lumen dilates/ expand (becomes widen and shorter)

• Food enters the lumen created.

Both food and air must pass through the pharynx when they enter the body. pharynx

What happens during breathing & swallowing?

pharynx

trachea (windpipe)

oesophagus

Normally, air passes into the trachea (windpipe) while food passes into the oesophagus.

air


trachea (windpipe)

glottis

During breathing, the larynx is lowered and the glottis is open.

pharynx

oesophagus

larynx (voice-box)

air


During swallowing, the larynx is raised and the glottis is covered by the epiglottis. This prevents food particles from entering the trachea.

pharynx

trachea (windpipe)

oesophagusglottis

epiglottis

food particles

larynx (voice-box)


trachea (windpipe)

Food particles then pass down the oesophagus by peristalsis.

pharynx

oesophagusglottis

epiglottisfood particles

larynx (voice-box)


Occasionally, small particles of food or water may get into the larynx or trachea.

trachea (windpipe)

larynx (voice-box)

food particles


This automatically induces violent coughing to force the food particles or water out and to prevent choking.

trachea (windpipe)

larynx (voice-box)

food particles


How is food digested in our body?

Stations: Mouth Oesophagus Stomach Small Intestines Large intestines Anus

Food enters the mouth.food

THE PROCESS OF DIGESTION

salivary gland

saliva

Food in the mouth stimulates the salivary glands to secrete saliva.

Saliva is mixed with the food. Mucin in saliva helps to soften the food.

Saliva contains an enzyme called salivary amylase which digests starch to maltose.

STATION 1: MOUTH

salivary gland

saliva

Chemical digestion in the mouth:

Action of salivary amylase:

Starch maltose

The pH of saliva is neutral (around pH 7). Salivary amylase is active at this pH.

Physical digestion in the mouth:

Chewing by teeth breaks the food up into smaller pieces. This increases the surface area to volume ratio for salivary amylase to work on.

STATION 1: MOUTH

The tongue rolls the food into small, slippery, round masses or boli (singular: bolus).

bolus

STATION 1: MOUTH

The boli are swallowed and pass down the oesophagus via the pharynx.

Peristalsis in the walls of the oesophagus pushes each bolus of food down into the stomach. Gravity also helps the food to pass down the oesophagus.

No digestion occurs here. bolus

STATION 2: OESOPHAGUS

Food passes into the stomach. bolus

stomach

STATION 3: STOMACH

The presence of food in the stomach stimulates the gastric glands to secrete gastric juice into the stomach cavity.

Peristalsis in the stomach wall churns and breaks up the food. Peristalsis also mixes the food well with gastric juice.

gastric juice

gastric gland

food

STATION 3: STOMACH

protein

Gastric juice is a dilute solution of hydrochloric acid (about pH 2) and two enzymes, pepsin and rennin.

curdled milk protein

milk protein pyloric sphincter

pepsin

polypeptide

rennin

STATION 3: STOMACH

protein

The dilute hydrochloric acid:1. stops the action of

salivary amylase by denaturing it;

2. changes the inactive forms of the enzymes, pepsinogen and prorennin, in the gastric juice, to the active forms, pepsin and rennin respectively;

3. provides a slightly acidic medium suitable for the action of the gastric enzymes; and

4. kills certain potentially harmful microorganisms in food.



pepsin

polypeptide

rennin

STATION 3: STOMACH

protein

The proteases pepsin and rennin act on proteins.

Action of Pepsin: proteins polypeptides

Action of Rennin: caseinogen casein

Rennin clots or curdles milk proteins by converting the soluble protein caseinogen into insoluble casein. This is necessary because milk proteins would pass through the the duodenum as easily as water and would not be digested by pepsin. Insoluble casein remains long enough in the stomach to be digested by pepsin.



pepsin

polypeptide

rennin

STATION 3: STOMACH

Food normally remains in the stomach for about three to four hours. The partly digested food becomes liquefied, forming chyme.

stomach

chyme

STATION 3: STOMACH

Chyme passes in small amounts into the duodenum when the pyloric sphincter relaxes and opens.

Structure:

Around 6m long!!!

It consists U-shaped duodenum (first part of small intestine), jejunum and the much coiled ileum.

pyloric sphincter

stomach

chyme

STATION 4: SMALL INTESTINES


3 types of digestion occurs here:

1. Carbohydrate digestion (continues from the mouth)

2. Protein digestion (continues from the stomach)

3. Fat digestion (1st time digestion!)

3 other organs associated with digestion here:

1. Liver – produces bile and store in gall bladder

2. Gall bladder – releases the bile

3. Pancreas – secretes pancreatic juice

Chyme enters the small intestine. It stimulates:

1. Intestinal glands to secrete intestinal juice.

2. Pancreas to secrete pancreatic juice. The pancreatic juice passes through the pancreatic duct into the duodenum.

3. Gall bladder to release bile. Bile does not contain enzymes. Bile passes through the bile duct into the duodenum.

bile

intestinal juice

pancreatic duct

pancreatic juicebile duct


During digestion in the small intestine,the chyme stimulates:

Intestinal glandsTo Secrete…

PancreasTo Secrete…

Gall bladderTo Release…

Intestinal juice:1. Enterokinase2. Erepsin3. Maltase4. Sucrase5. Lactase6. Intestinal lipase

Pancreatic juice:1. Pancreatic

amylase2. Pancreatic lipase3. Trypsinogen

Bile (passes through bile duct)



Intestinal juice is secreted by intestinal gland.

Intestinal juice consists of 6 digestive enzymes:

1. Enterokinase: inactive trypsinogen active trypsin

2. Maltase: maltose glucose + glucose

3. Lactase: lactose glucose + galactose

4. Sucrase: sucrose glucose + fructose

5. Erepsin: polypeptides amino acids

6. Intestinal lipase: fats fatty acids + glycerol


Pancreas secretes PANCREATIC JUICE.

Pancreatic juice consists of 3 digestive enzymes:

1. Pancreatic amylase: starch maltose

2. Pancreatic lipase: fats fatty acids + glycerol

3. Trypsin: protein polypeptides


Liver produces bile and stores in the gall bladder.

Gall bladder releases the bile via the bile duct.

Bile is not an enzyme!

Bile is an alkaline greenish-yellow liquid containing bile salts and bile pigment.

Function of bile:

Bile emulsifies fats breaks up fat into minute fat globules thereby increasing the surface area of the fat molecules for lipase to act on.

In the small intestine, bile salts emulsify fats.

They lower the surface tension of the fats, that is, they reduce the attractive forces between the fat molecules.

This causes the fats to break into tiny fat droplets suspended in water, forming an emulsion.

Note that this is just a physical break-up, but no chemical digestion of fat molecules has occurred.

big drop of fat

Bile salts emulsify fats into tiny fat droplets.

+

bile salts

tiny fat droplets

More about fat digestion…

Emulsification increases the surface area to volume ratio of the fats, speeding up their digestion by lipase.

Emulsified fats are digested by lipases (pancreatic and intestinal lipases) to fatty acids and glycerol.

fatty acids + glycerol

+

lipase

Bile salts emulsify fats into tiny fat droplets.

tiny fat droplets

More about fat digestion…

All three fluids (intestinal juice, pancreatic juice & bile) are alkaline.

The alkalis:

1. Neutralise the acidic chyme

2. Provide a suitable alkaline medium for the action of the pancreatic and intestinal enzymes.

bile

intestinal juice

pancreatic duct

pancreatic juicebile duct

STATION 4: SMALL INTESTINE

The digested food (simple sugars, amino acids, fatty acids & glycerol) are absorbed by the villi of the small intestine, especially of the jejunum & the ileum.

Water and mineral salts are absorbed from the undigested food material.


Structure:

• Consists of the colon & rectum (muscular tube).

Processes:

• No digestion occurs here.

• Water and mineral salts are absorbed from the undigested food material. (same as the small intestines!)

• Faeces are stored temporarily in the rectum.

STATION 5: LARGE INTESTINES

• Faeces are egested through the anus.

• This is the last station. All food particles, please alight via the anus. We thank you for traveling with us.

STATION 5: THE ANUS

How is food absorbed by our body?

Definition of absorptionEnd products of digestionAdaptation of the small intestinesStructure of the villusHow are nutrients absorbed?

Absorption is the process whereby digested food is taken into the body cell.

Definition of Absorption

Carbohydrates:

• Simple sugars (glucose, galactose & fructose)

Protein:

• Amino acids

Fats:

• Fatty acids + glycerol

End products of Digestion

1. INCREASES THE SURFACE AREA FOR ABSORPTION

The inner walls of the small intestine are thrown into numerous transverse folds and furrows.

The small intestine also has numerous minute finger-like projections called villi projecting into the intestinal cavity.

These villi further increase the surface area; the epithelial cells of the villi, in turn, bear numerous microvilli

2. REDUCES BARRIER TO DIFFUSION

Villi have thin walls epithelium is only ONE cell thick.

How is the small intestine adapted for absorption?

3. PROVIDES SUFFICIENT TIME FOR ABSORPTION

The length of the small intestine is around 6m which is long enough to provide sufficient time for absorption.

4. CARRIES AWAY ABSORBED FOOD SUBSTANCES + CONTINUAL REMOVAL MAINTAINS A CONCENTRATION

GRADIENT FOR ABSORPTION

The villi of the small intestine are richly supplied with blood capillaries and lymphatic capillaries (lacteal).

How is the small intestine adapted for absorption?

Structure of the Villi

Between the bases of the villi are the minute openings of the intestinal glands that secrete intestinal juice.

The intestinal wall and the villi are richly supplied with blood vessels & lymphatic vessels to carry away the absorbed substances.

In each villus is a lacteal or lymphatic capillary by blood capillaries.

The lymphatic capillaries of the villi transport fats while the blood vessels transport sugars and amino acids away from the intestine.

The Structure of the Villi

Adaptation of the villus for absorption

Minute/ small opening at the base of the villi helps in the production of intestinal juice that contains enzymes.

Each villi has its own blood capillary system aids in the absorption of amino acids and simple sugars

Each villi has its own lacteal system fatty acids and glycerol can be absorbed easily

Finger-like projections Allows for more efficient rate of absorption

Simple Sugars & Amino acids

Absorbed into blood capillaries of the villus

via diffusion or active transport

Glycerol & fatty acids

Absorbed into the lacteal (lymphatic capillaries) of the villus

via diffusion

Mineral salts

Absorbed into blood capillaries of the villus

via diffusion or active transport

How are nutrients absorbed into the body?

Assimilation

Definition of assimilation

How simple sugars, amino acids and fats are transported?

The Liver

Definition: 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.

Assimilation

The largest gland in the body.

Refer to Pg 5 of your notes :

Match the following functions of the liver to their respective descriptions and construct a mind map

titled “the Liver”.

Metabolism of glucose Deamination of amino acids Detoxification Protein synthesis Iron storage Heat production Bile production

The Liver (Group work)

Chapter Digestion 22

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