Biology 20

Digestive System

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DIGESTIVE SYSTEM

HETEROTROPHIC NUTRITION· Unable to manufacture food from inorganic

molecules. · Depend on autotrophs.

AUTOTROPHIC NUTRITION· Organisms capable of synthesizing organic

molecules from simple inorganic material. · Types: photosynthesizers and

chemosynthesizers.

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4 Steps of Heterotrophic Nutrition

1) Ingestion• eating or drinking• Heterotrophs need:

– Sugars– Amino Acids– Fatty Acids– Glycerol– Minerals– Vitamins

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4 Steps of Heterotrophic Nutrition

2) Digestion• Breakdown of macromolecules into smaller

ones that can be absorbed by the cells– Mechanical -- chewing– Chemical -- digestive enzymes

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4 Steps of Heterotrophic Nutrition

3) Absorption• Nutrients are absorbed into the blood

– Digested monomers– Water– Minerals– Vitamins

4) Egestion• Elimination of undigested material

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Parts

Organs• Mouth• Pharynx• Esophagus• Stomach• Small Intestine• Large Intestine• Rectum

Glands• Salivary Glands• Pancreas• Liver• Gall Bladder

Order of Events

Order of Events

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Digestive System

• Overview – Labeling of the Digestive system!

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Digestive System

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Mouth

Hard Palate • hard part of roof of mouth

Soft Palate • back of roof of mouth

Pharynx • chamber in throat where

– nasal cavity and mouth meet – esophagus and trachea meet

Mucus Membranes• lubricates walls of mouth for easy

passage of food

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Mouth

Purpose• Mechanical digestion by teeth and tongue

– increases surface area for action by enzymes– Mixes food thoroughly

• Chemical digestion by enzymes in saliva

Secretions• Saliva

– 1 to 2 liters per day– from 3 pairs of glands in the mouth

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Salivary Glands

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Mouth

Composition of Saliva• Water

– moistens food

• Amylase – begins STARCH digestion

• Mucin– lubricant

– binds food together for easier swallow

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Mouth

Control of Salivary Secretions

• Mechanical pressure

– food in the mouth

• Nervous system

– Brain thinking of food in the mouth

– Signal sent to salivary glands from the brain

– Mmmmmmm. Fooooood….

Order of Events

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Pharynx

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Pharynx

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Pharynx

Throat Common tube through which both air

and food pass Four openings

• Nasal Cavity Mouth Cavity

• Trachea Esophagus Contains a flap of tissue called the

EPIGLOTTIS• This prevents food from entering the

trachea

Order of Events

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Esophagus

Hollow muscular tube Connects pharynx to the stomach

What makes the food go to the stomach?

Movement of food by peristalsis • rhythmic waves of contraction and relaxation of

muscular walls• food is squeezed through esophagus into stomach

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Peristalsis

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Peristalsis

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Esophagus

Hollow muscular tube Connects pharynx to the stomach

What makes the food go to the stomach?

Movement of food by peristalsis • rhythmic waves of contraction and relaxation of

muscular walls• food is squeezed through esophagus into stomach

Epiglottis shunt food into esophagus during swallowing

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Esophagus

Structure

• thick walled and muscular

• from pharynx to stomach

• walls contain mucus glands which

secrete mucin

– mucin lubricates food for easy passage

Order of Events

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Stomach a hollow muscular pouch located high in the abdominal cavity,

just under the diaphragm The esophagus connects to the

stomach by the cardiac sphincter. Heartburn• muscular ring surround esophagus at this

point• acts like a valve to open and close the

tube• keeps the food from falling out of your

stomach when standing on your head.

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Stomach

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Stomach

FUNCTIONS

• temporary storage of food

• liquefaction of food

• beginning of protein digestion

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Secretions -- Gastric Juice

Secreted into the stomach cavity by stomach cells • churning of the stomach mixes juice with

food Contains

• Hydrochloric Acid – HCl(aq) – pH of 1-2 – destroys bacteria present in food – liquefies food.

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Secretions -- Gastric Juice

• Pepsin –an enzyme which begins protein digestion

• (proteins broken down into smaller chains)

But aren’t the stomach cells made of protein?

–Pepsin is produced and secreted in an inactive form called pepsinogen

• which is converted into the active form by the low pH.

• This protects the cell from self-digestion

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Stomach

stomach lined with mucus coating

• prevents acid and protein digesting

enzymes from damaging wall

when food is liquefied it is called

chyme.

squirted into small intestine through

the pyloric sphincter

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Stomach

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Small Intestine

About 6 meters long• Three sections

1) Duodenum • first 25 cm • digestion

2) Jejunum • next 2 meters • absorption

3) Ileum • next 4 or 5 meters • absorption

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Small Intestine

FUNCTION

• complete the digestion of food

• absorb the nutrients into the circulatory

system (blood)

• important secretions from the pancreas

and the liver.

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Secretions -- Pancreatic Fluid

Contains• NaHCO3 – sodium bicarbonate

– raises pH to 8

• Pancreatic Amylase – an enzyme which continues the digestion of

starch into maltose units

• Lipase – enzyme digesting lipids into fatty acids & glycerol

• Trypsin and Chymotrypsin – enzymes which continue protein digestion– also secreted in inactive forms– activated by alkaline pH

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Small Intestine

LIVER and GALLBLADDER• The liver produces bile

– brown fluid

• bile is stored in the gallbladder• when fats enter the small intestine, the

gallbladder contracts and squirts bile into the duodenum

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Small Intestine

• bile emulsifies fats–physically breaks them down into

smaller drops–Kaboom!!! Blows it up!!!

–greater surface area for action by lipase

• bile is NOT an enzyme because it does not cause a chemical change in fat.

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Small Intestine

All types of foods are acted upon.

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Small Intestine

digestion of disaccharides and dipeptides is completed by enzymes produced by the small intestine• sucrase

– digests sucrose

• maltase – digests maltose

• lactase – digests lactose

• aminopeptidase – breaks down dipeptides into amino acids

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Small Intestine

once all food molecules are at monomer stage, they are ready to be absorbed into the circulatory system

the inside of the S.I. is covered with millions of tiny finger-like projections called villi• each villus is actually covered with

millions of microvilli• the villi and microvilli increase surface

area for absorbing food molecules into blood.

Absorption in the Small IntestineDigestedNutrient

Product Method ofAbsorption

Site ofAbsorption

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Small Intestine

notice that glycerol recombines with 3 fatty acids to form a fat molecule which enters the lacteal instead of the capillary

Why is fat completely digested and then recombined to form a fat molecule again?

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Large Intestine

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Large Intestine

Material NOT absorbed by this point will enter the large intestine through another sphincter – the ileal-caecal sphincter

The caecum is a small pouch at the beginning of the large intestine.• contains a small projection called the

appendix– this is a vestigial organ which functions in

cellulose digestion in some herbivorous mammals

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Functions of the Large Intestine

absorb water into blood

absorb vitamins and minerals into blood

eliminate undigested material from

digestive tract

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Large Intestine

Some bacteria live in the large

intestine of mammals • they digest material that we are not able

to digest

a byproduct of this activity results in

the synthesis of vitamins • Vitamin K

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Rectum & Anus

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Rectum & Anus

last section of digestive tract rectum is a holding pouch for feces feces exits the digestive tract through

a sphincter muscle called the anus. rectal veins are found near opening

• if they get inflamed…– the inside diameter of anus decreases – passage of feces is difficult and painful – called hemorrhoids or piles.

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What About Fiber???

fiber in the diet serves to retain

water throughout the digestive

tract resulting in soft feces.

1 = Esophagus

2 = Stomach

3 = Small Intestine

4 = Pancreas

5 = Liver

6 = Large Intestine

7 =Large Intestine

8 =Large Intestine

9 = Large Intestine

10 = Rectum

Biology 20

Control of Digestive Secretions

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Saliva

Nervous

• Pavlov

• Sight, smell, presence, or even thought

of food stimulates vagus nerve

• results in the production of saliva

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Gastric Juice

Nervous

• Stimulation of vagus nerve also

stimulates cells of stomach to begin

producing gastrin

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Gastric Juice

Hormonal• Gastrin (a hormone) is released from

certain upper stomach cells into the bloodstream.

• gastrin circulates until it reaches the lower stomach cells resulting in the release of gastric juices.

• Once pH falls below 2, gastrin stops being secreted.

• If pH rises above 2.5 gastrin secretion begins again.

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Pancreatic Juice

Nervous• presence of food in mouth and stomach

stimulates pancreatic secretions Hormonal

• presence of chyme in the S.I. causes the cells of the duodenum to secrete the hormone secretin into the blood

• Secretin (a hormone) causes the pancreas to begin producing pancreatic juice

• Pancreatic Juice will enter the duodenum through the pancreatic duct.

• when acidic chyme becomes alkaline, production of secretin stops

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Liver & Gall Bladder

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Bile

Hormonal

• presence of chyme in the S.I. causes the

cells of the duodenum to secrete the

hormone cholesystekinin into the blood.

• This causes the gall bladder to contract,

propelling bile into the duodenum

through the bile duct

Biology 20

FUNCTIONS OF THE LIVER

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Liver

largest organ in the body receives blood from 2 supplies

• hepatic artery – oxygen rich blood from heart

• hepatic portal vein – nutrient rich blood from intestinal walls

liver acts as a “gatekeeper” between blood leaving intestines and blood entering general circulation• the liver removes excess nutrients

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Liver & Gall Bladder

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Functions of the Liver

1) Digestive Function• Production of bile which emulsifies fats

2) Regulation of Blood Glucose• removes excess glucose from blood after meal

and converts it to glycogen• once glycogen stores are full, it converts extra

glucose into fat which is released from liver into the blood and stored in adipose tissue

• when blood sugar levels fall, liver converts glycogen back into glucose and releases it into blood

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Functions of the Liver

3) Breakdown of Red Blood Cells• destroys old RBC’s • the pigment is excreted in bile• the iron is stored for future RBC’s

4) Synthesis of Plasma Proteins• manufactures important blood proteins

– fibrinogen– albumin– globulin

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Functions of the Liver Storage

• vitamin B12

• fat-soluble vitamins• iron

Detoxification• liver breaks down poisons and non-food

substances in blood• e.g.: alcohol, caffeine, nicotine, drugs, excess

hormones Excretory Function

• deamination (breakdown) of excess amino acids • production of urea

– excreted in urine