DigestionDigestion

AP BiologyUnit 6

General Steps

• Ingestion = food is taken in (eating )

• Digestion = Food is broken down into smaller pieces /molecules

• Absorption = Nutrient molecules are absorbed into body cells

• Elimination = undigested material exits the body

Types of Digestion

• There are two kinds of digestion – mechanical and chemical

• Mechanical = Food is broken down into smaller pieces (chemical structure not changed)

• Chemical = Food is broken down into smaller molecules (chemical structure changed)

Intracellular Digestion

• In the animal kingdom, only sponges (Phyla Porifera) do this exclusively.

• Steps:– Cells engulf food via

phagocytosis or pinocytosis, forming food vacuole.

– Lysosomes fuse with food vacuoles; hydrolytic enzymes break down food.

Extracellular Digestion

• All animals (except sponges) perform this mode of digestion

• Digestion begins in a compartment continuous with the outside of the animal’s body (gut).

• What is the advantage of extracellular digestion?– animal can take in a lot of food at

once and slowly digest it

Extracellular Digestion

• Enzymes are secreted to break food into smaller molecules.

• After the food has been broken down, many animals continue digestion intracellularly.

• Animals that perform extracellular digestion can have a variety of different digestive systems.

Gastrovascular Cavity

• Gastrovascular cavity = digestive sac with a single opening

• Animals that have a gastrovascular cavity = Hydra – Phyla Cnidarian (also

includes jellyfish, anemones, corals)

Digestion in Gastrovascular Cavities

• Tentacles sting prey and stuff it into opening

• Digestive enzymes are secreted to allow for extracellular digestion

• Nutritive muscular cells then engulf food particles and complete digestion intracellularly

• Undigestible material leaves through mouth (no anus)

Complete Digestive Tracts

• Complete digestive tract = digestive tube running throughout body (alimentary canal)

• Organisms with a complete digestive tract have both a mouth and an anus

Question…

• Why would having a separate entry and exit point be beneficial?– digestion can be broken down into steps – there can be specialization of digestive tissues

for these steps

Specialized Digestive Structures

• Certain organisms have specialized digestive structures

• Crop– Function = Store and moisten

food – Who has it?

• Earthworms (Phylum Annelida)

• Grasshoppers (Phylum Arthropoda)

• Birds (Phylum Chordata)

Specialized Digestive Structures

• Gizzard– Function = Contains sand and

stones; As the muscles around it move, food is ground up (mechanical digestion)

– Who has it?• Earthworms (Phylum Annelida)

• Cockroaches (Phylum Arthropoda)

• Birds (Phylum Chordata)

Specialized Digestive Structures

• Gastric ceca– Function = Extend from midgut to help with

absorption – Who has it?

• Grasshoppers (Phyla Arthropoda)

Human Digestion

• Where do the four steps in food processing occur?

Human Digestion

• Ingestion = mouth

• Digestion = mouth, Stomach, Small Intestines

• Absorption = Small Intestines, Large Intestines

• Elimination = End of large intestines

Oral Cavity, Pharynx, Esophagus

• Participate in ingestion and digestion

• Mechanical Digestion – By teeth and tongue

(chewing)– Forms mass of food = bolus

• Chemical Digestion – Salivary amylase begins

digestion of carbohydrates (starch)

Oral Cavity, Pharynx, Esophagus• Epiglottis moves during swallowingto cover

the trachea, so food travels down “right pipe” to the esophagus

• Peristalsis (muscle contractions) will involuntarily continue movement of the bolus

Stomach

• Important in storage & digestion

• What advantage do folds in the stomach tissue provide? – Allows the stomach to expand to hold more

food– Tissue is also very elastic so that it can stretch

Stomach• Gastric glands produce

gastric juice from a combination of 3 cells:

• Mucus Cells secrete mucus (protects stomach lining)

• Chief Cells secrete pepsinogen (inactive enzyme)

• Parietal Cells secrete HCl

Stomach

• Gastric juice is churned with bolus to break down food and kill bacteria

• HCl converts pepsinogen into pepsin (active enzyme)

• Pepsin hydrolyzes (breaks down) protein.

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Stomach: Pepsin

• What kinds of bonds are broken by pepsin?– Peptide bonds

• What kind of reaction is this? – Hydrolysis

Protection of the stomach

• Stomach protects itself from self-digestion by – keeping pepsinogen

stores away from HCl until pepsin is needed

– lining stomach with mucus

• Food leaves stomach as acid chyme, squirting slowly through the pyloric sphincter

Small Intestine

• Participates in digestion and absorption

• Peristalsis allows for movement of chyme and digestive juices down the small intestine.

Digestion in the Small Intestine

• Digestion is usually completed in the duodenum (first section) with the help of digestive juices

Digestive Juices

• Digestive juices come from 4 sources, entering the duodenum:

• Pancreas – produces digestive

enzymes – produce basic

bicarbonate solution (buffer)

Digestive Juices

• Lining of duodenum – produces digestive enzymes

• Liver – Produces bile

• Gallbladder– Stores bile

Bile

• contains bile salts– breaks up fat droplets into

very small pieces called micelles

– micelles are then absorbed and taken up by the lymph vessel (lacteal)

Question…

• Does bile perform mechanical or chemical digestion?– Mechanical– breaking it up into smaller pieces

Protection of Small Intestine

• Small intestine and adjacent organs are protected from digestive enzymes by producing inactive forms that are only activated in the duodenum.

Absorption of Nutrients

• Occurs in the jejunum (mid-small intestine) and ileum (end-small intestine)

• The surface area in these regions is very large because of the villi and microvilli. – Villi and microvilli are projections of the lining

Absorption of Nutrients

• Some nutrients are absorbed into the lymph vessel (lacteals).

Absorption of Nutrients

• Most nutrients are absorbed into the blood vessels – capillaries hepatic portal vessel liver – the liver converts many nutrients and regulates

blood sugar and other nutrient levels

• Water is also absorbed here.

Digestion Summary

Biomolecule Where Digested Key Enzymes

Carbohydrates Mouth, Small Intestine

Amylase, Maltase, Lactase, Sucrase

Lipids Small Intestine Bile, Lipase

Proteins Stomach, Small Intestine

Pepsin, Peptidases, Trypsin

Nucleic Acids Small Intestine Nucleases

Large Intestine

• responsible for water recovery from digested material

• Feces = Waste of digestive tract

• Bacteria live here (including E. coli) that live on feces and produce vitamins B and K () and stinky gases

Large Intestine

• End of colon = rectum

• End of rectum = anus

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Ruminant Digestion

• Ruminants (ex. Cows) eat plants with tough-to-digest cellulose.

• In order to improve digestion efficiency, Ruminants have symbiotic relationships with microbes.

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Ruminant Digestion Steps

• Microbes in the rumen help digest the cellulose (products are fatty acids and “cud” (partially digested food)

• Cud is regurgitated and rechewed

• Water is removed in the omasum

• Digestive enzymes in the abomasum complete digestion

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Control of Digestive System

• The actions of the digestive system are regulated by hormones as well as it's own nervous system.

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Food in the stomach

Stomach cells release GASTRIN

Increased secretion of:HClPepsinIncreased movement in stomach

Faster delivery of chyme to small intestine

small intestine cells release CHOLECYSTOKININ (CCK)

small intestine cells release SECRETIN

gallbladder releases bile

pancreas releases digestive enzymes

pancreas releases sodium bicarbonate to neutralize acid

Digestion of food

CCK slows movements of stomach down

Lower pH inhibits gastrin release

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Diabetes

• Diabetes mellitus is a condition in which insulin is not produced or cells are insulin resistant

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