Jurongville Secondary SchoolSecondary 3 Express Biology Notes

Syllabus 5100

Name: _________________________ ( )

Class: Sec 3___ Date: ____________

Chapter 6: Digestion

The Digestive System must accomplish the following tasks

Ingestion Digestion Physical + Chemical Digestion Absorption Assimilation Egestion

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Oesophagus

6.1 Mouth and Buccal Cavity (Ingestion Phase) Teeth

Function: grind and break down food expose food to a large surface area for enzyme action

Tongue: Function: Rolls food into bolus (a ball of solid mass of food) before swallowing

Salivary Glands secrete saliva into the buccal cavity Function of saliva:

Dilutes and moistens food mucus sticks food together form bolus Lubricates bolus for swallowing Contains Salivary Amylase

- Function: breaks down starch into sugars: Starch Maltose

6.2 Pharynx (Ingestion Phase)

A cavity that connects the mouth to the Oesophagus, it also connects the nose and mouth (via the larynx) with the trachea (windpipe)

Epiglottis flap-like cartilage (a soft bone)o Prevents food particles from entering tracheao Covers the larynx when food is being swallowed

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Salivary amylase

Physical Digestion

6.3 Oesophagus

Muscular tube takes food from the mouth and pushes it down through the neck into the stomach.

Moves food by waves of muscle contractionperistalsis.

When Circular muscle contracts Longitudinal muscle relax When Longitudinal muscle contracts Circular muscle relax When one muscle contracts the other relax (antagonistic action)

6.4 Stomach (Chemical Digestion)

Have thick muscles in its wall contract to mash the food into a sloppy soup. Mucous coat lining contain gastric pits

o Gastric pits contain gastric glands secrete gastric juices. o Function: These break down the food in a chemical way.

Two valves control the entrances of food into and out of the stomach

o Oesophageal Sphincter - opening between Oesophagus and stomach. o Function It stops the acid in the stomach from flowing back up (reflux)

into the Oesophagus. If there is damage or weakness to this valve, stomach contents, including hydrochloric acid, flow up into the Oesophagus and cause injury to the lining of the Oesophagus. This causes pain, commonly called "heartburn" or "acid reflux".

o Pyloric Sphincter – when relaxes, it opens the entrance to the small intestine. o A layer of mucus prevents the stomach from digesting itself.

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Circular muscle contracts Contracts

Pyloric sphincter

Oesophageal sphincter

o Food in the stomach is converted into a thick acidic liquid called chyme, which then moves into the small intestine (via peristalsis)

6.5 Small Intestines

o Length of 6 m in Man. o Consists of

o U-shaped duodenum,

o Jejunum

o Ileum

o Lining contains glandsproduce digestive enzymes that continue the chemical breakdown on the food. Finally the nutrients are small enough to pass through the lining of the small intestine, and into the bloodcarried away to the liver and other body parts to be processed, stored and distributed.

o The small intestine consists of minute fingerlike projections called villiincrease the surface area of the small intestines.

o Nutrients are absorbed across the villi and into the blood stream (via capillaries) as chyme travels down the small intestine.

 

6.6 Pancreas

o Produce pancreatic juices containing digestive enzymes

o Secretes the hormone insulin involved in the conversion of excess glucose into glycogen

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6.7 Gall Bladder

o Small baglike part tucked under the liver. o It stores fluid called bile, which is made in the

liver.

o As food from a meal arrives in the small intestine, bile flows from the gall bladder along the bile duct into the intestine.

o Bile emulsifies fats but does not contain enzyme cannot digest food

6.8 Liver

o Produces bile. o Also, blood from the intestines flows to the

liver, carrying nutrients, vitamins and minerals, and other products from digestion.

o The liver is like a food-processing factory with more than 200 different jobs.

o It stores some nutrients, changes them from one form to another, and releases them into the blood according to the activities and needs of the body

o It also serves to detoxify blood of harmful substances like alcohol and nicotine.

6.9 Large Intestine

o Any useful substances in the leftovers, such as water and minerals salts, are absorbed through the walls of the large intestine, back into the blood.

o Helps to remove bacteria during egestion

o The remains are formed into brown, semi-solid faeces removed from the body.

o Has two parts:

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o Colon

o Rectum

6.10 Chemical Digestion

Structure Digestion ActivityMouth Starch ---> maltose Chewing breaks up food into

smaller piecesOesophagus No Chemical Digestion Food travels down with the help

of Gravity Peristalsis

Oesophageal sphincter

No Chemical Digestion Relaxes allows food bolus to enter stomach

Stomach Hydrochloric acid (HCl)(pH = 2)

Protein Digestion

Proteins ---> Polypeptides

Caseinogen ---> Casein

Casein ---> Polypeptides

Stops action of salivary amylase

Pepsinogen ---> pepsinProrennin ---> rennin

Curdling of milk proteins

Food liquefies becomes chyme

Pyloric sphincter

Relaxes opens a channel to the small intestine

Small intestine (duodenum)

Carbohydrate Digestion

Starch ---> Maltose

Maltose ---> 2 x Glucose

Sucrose ---> glucose + fructose

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Salivary amylase

Pepsin

Rennin / Ca ions

Pepsin

HCl

Pancreatic amylase

Maltase

Sucrase

HCl

Lactose ---> glucose + galactose

Fat Digestion

Emulsified fats ---> Fatty acids + GlycerolProtein Digestion

Trypsinogen ---> Trypsin

Proteins ---> Polypeptides

Polypeptides ---> amino acids

Bile breaks fats into small fat molecules --> emulsified fats--> Increase SA: vol ratio of fats --> speed up fat digestion

6.11 Absorption (Adaptations for absorption)

Structure of small intestine Adaptation to functionHas lots of projections (villi) Increase SA --> faster rate of absorption of

digested foodVillus epithelial cells bear a lot of microvilli To further increase SAVillus contains

Blood capillaries

Lacteals

Transport sugar and amino acids; Transport fats away from small

intestine--> So that diffusion gradient maintained --> digested food continued to be absorbed.

Diagram of a structure of a villus:

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Lipase

Lactase

Enterokinase

Trypsin

Erepsin

6.12 Absorption Process

6.13 Assimilation

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Water & mineral salts

Absorbed by

Large intestine

Simple sugars + mineral salts + vitamins + amino acids

Pass through villi

Blood capillaries

Other parts of body

Fatty acids Glycerol

React with bile salts

Soluble soaps

Form

Diffuse into villi epithelium

Soluble soaps + glycerol --> small fat globules

Absorbed by lymphatic capillaries

Simple sugars + amino acids

Hepatic portal vein

In LiverExcess glucose --> glycogenExcess amino acids --> urea

Remaining

Hepatic vein Distributed round body & used

Simple sugars Amino acids

Tissue respiration Growth & repair of worn-out body parts

Used as building blocks of enzymes and hormones

6.14 Functions of Liver

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Fats

Enter

Lymphatic capillaries

Fats

Excess fats Glucose insufficient

Stored in adipose tissues Fat storage Insulation

Fats transported to liver --> converted to oxidizable forms

Fats + Lymph

Form

Chyle

Transported in Lymph vessels

Liver

DigestionBile production stored in

gall bladder

Assimilation

Regulation of blood glucose level

Deamination of excess amino acids urea + glycogen

Fasting (glucagon converts)

After heavy meal insulin converts

Glycogen Glucose enter into bloodstream

Glucose Glycogen stored

Breakdown of RBC in spleen

Release Hb back to

Protein synthesis

Detoxification Eg. Breakdown of alcohol acetaldehyde (harmless)Excessive

alcohol consumption

leads to

Stimulate HCl (acidic) secretion in the stomach

Liver cirrhosis liver cells killed replaced with fibrous tissue liver failure

Otherwise