Chapter 6 Digestion - Lecture Notes

Jurongville Secondary School Secondary 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

Oesophagus

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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

Physical Digestion

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 Salivary amylase

6.2

Function: breaks down starch into sugars: Starch  Maltose 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 trachea o Covers the larynx when food is being swallowed

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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 contractionperistalsis.

Circular muscle contracts

• • •

Contracts

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)

Oesophageal sphincter

Pyloric sphincter

• •

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".

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Pyloric Sphincter – when relaxes, it opens the entrance to the small intestine. A layer of mucus prevents the stomach from digesting itself. o Food in the stomach is converted into a thick acidic liquid called chyme, which then moves into the small intestine (via peristalsis) o o

6.5 Small Intestines Length of 6 m in Man. Consists of o U-shaped duodenum, o Jejunum o Ileum o Lining contains glandsproduce 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 bloodcarried away to the liver and other body parts to be processed, stored and distributed. o The small intestine consists of minute fingerlike projections called villiincrease the surface area of the small intestines. o o

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 Small baglike part tucked under the liver. 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 o o

6.8 Liver o o o o

o

Produces bile. Also, blood from the intestines flows to the liver, carrying nutrients, vitamins and minerals, and other products from digestion. The liver is like a food-processing factory with more than 200 different jobs. 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 It also serves to detoxify blood of harmful substances like alcohol and nicotine. 6.9

Large Intestine

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: o Colon o

o

Rectum

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6.10

Chemical Digestion

Structure Mouth

Digestion Starch ---> maltose

Oesophagus

No Chemical Digestion

Oesophageal sphincter Stomach

No Chemical Digestion

Salivary amylase

Hydrochloric acid (HCl) (pH = 2)

Activity Chewing  breaks up food into smaller pieces Food travels down with the help of • Gravity • Peristalsis Relaxes  allows food bolus to enter stomach • Stops action of salivary amylase HCl

Pepsinogen ---> pepsin Prorennin ---> HCl rennin Protein Digestion Pepsin

Proteins ---> Polypeptides Rennin / Ca ions

Caseinogen ---> Casein Curdling of milk proteins

Pepsin

Casein ---> Polypeptides

Pyloric sphincter Small intestine (duodenum)

Food liquefies  becomes chyme Relaxes  opens a channel to the small intestine Carbohydrate Digestion Pancreatic amylase

Starch ---> Maltose Maltase

Maltose ---> 2 x Glucose Sucrase

Sucrose ---> glucose + fructose Lactase

Lactose ---> glucose + galactose Fat Digestion Lipase

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

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Emulsified fats ---> Fatty acids + Glycerol Protein Digestion

> speed up fat digestion

Enterokinase

Trypsinogen ---> Trypsin Trypsin

Proteins ---> Polypeptides Erepsin

Polypeptides ---> amino acids 6.11

Absorption (Adaptations for absorption)

Structure of small intestine Has lots of projections (villi) Villus epithelial cells bear a lot of microvilli Villus contains • Blood capillaries •

Lacteals

Adaptation to function Increase SA --> faster rate of absorption of digested food To further increase SA • •

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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6.12

Absorption Process

Water & mineral salts

Simple sugars + mineral salts + vitamins + amino acids

Fatty acids

Glycerol

Pass through villi

Absorbed by

React with bile salts Large intestine

Blood capillaries

Form

Soluble soaps

Other parts of body

Diffuse into villi epithelium Soluble soaps + glycerol --> small fat globules Absorbed by lymphatic capillaries 6.13

Assimilation

Simple sugars + amino acids Hepatic portal vein

Hepatic vein

Distributed round body & used

Remaining

In Liver Excess glucose --> glycogen Excess amino acids --> urea

Simple sugars

Amino acids

Tissue respiration

Growth & repair of worn-out body parts Used as building blocks of enzymes and hormones

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Fats Fats

Enter

Lymphatic capillaries

Excess fats

Fats + Lymph

Glucose insufficient

Stored in adipose tissues • Fat storage • Insulation

Form

Chyle

Fats transported to liver --> converted to oxidizable forms

Transported in Lymph vessels

6.14

Functions of Liver

Bile production  stored in gall bladder

Digestion Liver Release Hb back to

Deamination of excess amino acids  urea + glycogen

Assimilation

Regulation of blood glucose level

Breakdown of RBC in spleen Protein synthesis Detoxification

Otherwise

Eg. Breakdown of alcohol  acetaldehyde (harmless)

Fasting (glucagon converts)

Glycogen  Glucose  enter into bloodstream

After heavy meal insulin converts

Glucose  Glycogen  stored

Stimulate HCl (acidic) secretion in the stomach

Excessive alcohol consumption leads to Liver cirrhosis  liver cells killed  replaced with fibrous tissue  liver failure 9