Digestion & Nutrition

Chapter 41

We need to eat!

 • Since we as animals cannot produce our own food, we must EAT it.

• Classifying organisms by what they eat… – Herbivores: eat autotrophs – Carnivores: eat other animals – Omnivores: eat both autotrophs and animals –

What are we??

•

Classifying Animals by

HOW

They Get Their Food

Suspension feeders

from the water – Examples: clams, oysters, whales : – Sift small food particles •

Substrate feeders

source leaves : – Live on (or in) their food – Example: leaf miners, eat their way through

Classifying Animals by

HOW

They Get Their Food

• •

Fluid-feeders

– Suck nutrient-rich fluids from a living host – Example: mosquitoes, hummingbirds

Bulk-feeders

– Eat relatively large pieces of food – That’s us! 

The 4 Stages of Food Processing

1. Ingestion

The act of eating

2. Digestion

The process of breaking down food into molecules small enough for the body to absorb (physical & chemical) This involves

hydrolysis

, which you should know well  Polymers  Monomers

This has to occur in a specialized compartment… why??

The 4 Stages of Food Processing

3.

Absorption

– The animal’s cells absorb the small molecules broken down during digestion – Only

monomers

(monosaccharides, amino acids, glycerol & fatty acids) can be absorbed 4.

Elimination

– Undigested material passes out of the digestive compartment

Gastrovascular Cavities

• •

Gastrovascular cavities

are the digestive systems in simple animals (Cnidarians, Platyhelminthes)

Incomplete Digestive System-

one opening

Digestion in Alimentary Canals

• Complex animals – An “Entrance” and an “Exit” – This is a

complete digestive system

.

• Mouth = site of ingestion • Anus = site of elimination • What phyla have complete digestive tracts?

• (Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata, Chordata)

Alimentary Canals/Complete Digestive System

The Mammalian Digestive System

The Mammalian Digestive System

• Consists of: – Oral cavity (mouth) – Pharynx – Esophagus – Stomach – Small Intestine – Large Intestine • Food moves through the digestive system by

peristalsis

, rhythmic waves of contraction by the smooth muscles

Accessory Organs

• Salivary glands – Secrete saliva • Liver – Secretes bile • Gallbladder – Stores and concentrates bile • Pancreas – Secretes digestive enzymes

Oral Cavity

• The mouth • Chewing cuts, smashes, and grinds food to facilitate swallowing • Saliva is secreted into the mouth by the salivary glands – Saliva contains salivary amylase – This enzyme breaks down starch and glycogen into smaller pieces

Specialized Teeth

• Structure of teeth reflects feeding behavior • Antelope brush teeth against dirt as they eat; wear down crown.

ANTELOPE MOLAR crown root HUMAN MOLAR crown root

Pharynx

• The pharynx is the intersection that leads to both the esophagus (digestive system) and the trachea (respiratory system) • The epiglottis makes sure that food doesn’t enter the trachea

• The esophagus carries food from the pharynx into the stomach • Where is your stomach?

Esophagus

Stomach

• Our stomachs are stretchy – they expand to fit our food!  • The inside of the stomach contains

gastric juice

, which has a pH of about 2 –

HCl

– kills most bacteria swallowed with food -activates the enzyme pepsinogen –

Pepsin

(active form) – breaks down proteins to smaller polypeptides –

Why must pepsinogen be secreted in an inactive form?????????

Stomach

• The

pyloric sphincter

closes off the stomach from the small intestine, the next stop on our tour • The pyloric sphincter lets in small amounts of

acid chyme

at a time

Small Intestine

• Most digestion and absorption occurs in the small intestine • In humans, the small intestine is about 6 meters long – It’s called the “small intestine” because it has a small diameter

Small Intestine

• First section of the small intestine is the

duodenum

 responsible for digestion – Acid chyme enters from the stomach – Pancreatic enzymes and bile from the liver are mixed with the food here.

Pancreatic Enzymes

• • • • • • Secreted into duodenum

Pancreatic amylase-

disaccharides breaks down polysaccharides to

Trypsin and chymotrypsin-

smaller polypeptides breaks down proteins to

Carboxypeptidase Lipase

-breaks down lipids to

Pancreatic nucleases

nucleotides - breaks down proteins to amino acids fatty acids and glycerol -breaks down DNA & RNA to .

Fat Digestion

• •

Liver

produces bile

Bile

is stored in

gallbladder

, then secreted into duodenum • Bile emulsifies fats; breaks them into small droplets • This gives enzymes a greater surface area to work on

Small Intestine

• • The digestion occurs in the

duodenum Disaccharidases

- break disaccharides down into monosaccharides .

•

Aminopeptidases

-break polypeptides down into amino acids.

•

Intestinal nucleases-

break down nucleotides into nucleotide bases and monosaccharides .

• The absorption of nutrients occurs in the

jejunum

and

ileum (sections 2 & 3 of small intestine)

Small Intestine

• The

jejunum border

and

ileum

have a

brush

– The jejunum and ileum have

villi

(that look like fingers) and

microvilli. WHY??

– This gives them more

surface area

, which facilitates absorption of nutrients • Classic example of structure & function (form follows function)!

The Small Intestine

Absorption of Nutrients

•

What diffuses where?

• All nutrients except glycerol & fatty acids diffuse directly into capillaries.

• Glycerol & Fatty Acids (from fats) diffuse into

LACTEALS

(part of lymphatic system) and eventually dump into the circulatory system at ducts under the clavicles.

INTESTINAL LUMEN

Absorption Mechanisms

Monosaccharides & amino acids are actively transported across plasma membrane of epithelial cells, then from cell into internal environment

carbohydrates proteins EPITHELIAL CELL

Figure 41.11

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INTERNAL ENVIRONMENT monosaccharides amino acids

Fat Absorption

bile salts fat globules (triglycerides) emulsification droplets bile salts + fatty acids, monoglycerides micelles EPITHELIAL CELL

Figure 41.11

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INTERNAL ENVIRONMENT triglycerides + proteins chylomicrons

Hormones and Digestion

•

Gastrin

–secreted by stomach lining; causes acid secretion. •

Secretin

- causes pancreas to secrete bicarbonate •

Cholecystokinin (CCK)-

controls secretion of pancreatic enzymes & bile release from gall bladder •

GIP

(glucose insulinotropic peptide)-causes cells to take in more glucose by stimulating insulin secretion

Large Intestine (Colon)

• The major function of the colon is to

reabsorb water

• • The wastes of the digestive tracts are called

feces

, and they become more solid as they are moved along the colon – If it moves through too quickly, not enough water is reabsorbed is reabsorbed  

diarrhea

– If it moves through too slowly, too much water

constipation

• The waste is stored in the rectum until it can be eliminated through the anus

Fecies are not considered “excretion.” Why not?

Evolutionary Adaptations

• Evolutionary adaptations of the digestive system exist among animals, primarily based on their

diet

• • Herbivores have less specialized teeth than do carnivores

Herbivores have longer alimentary canals than carnivores…why??

Evolutionary Adaptations

The Mammalian Digestive System

Food Pyramid

added fats and simple sugars milk, yogurt, cheese group legume, nut, poultry, fish, meat group vegetable group fruit group bread, cereal, rice, pasta group

Carbohydrates

• Body’s main energy source • Foods high in complex carbohydrates are usually high in fiber; promote colon health • Simple sugars lack fiber as well as minerals and vitamins of whole foods; intake should be minimized • 4 Cal/g

Proteins

• 20 Total Amino Acids • 8 Essential Amino Acids (Body can’t make these, so they must be ingested in foods) • Plant proteins are incomplete; must be combined to get all 8 essential a. acids.

• 4 Cal/g

Lipids

• Most fats can be synthesized • Essential fatty acids must be obtained from food • Fats should be about 30 percent of diet • Excess saturated fats can raise cholesterol level and contribute to heart disease • • 9 Cal/g

So why do we want to cut lipids if we want to lose weight?

Dietary Essentials

•

Vitamins

(Coenzymes: helps enzymes function) – Essential organic substances •

Minerals

(Cofactors: help enzymes function) – Essential inorganic substances