Digestion

• All animals are heterotrophs• Different modes of feeding evolved • Modes of feeding (structure-function) can

be used as an evolutionary evidence• Compartmentalization

Intracellular vs. Extracellular

• Intracellular- occurs within food vacuoles– E.g. food vacuoles in Paramecium,

amoebocytes of sponge

• Extracellular- occurs within specialized compartments– Incomplete vs. complete

Incomplete

• A single opening is present for both entrance of food and exit of wastes

• Gastrovascular cavity of Cnidarians• Gastrodermis- has specialized cells that

secrete digestive enzymes• Hydrolysis of macromolecules is

intracellular

Incomplete

Obelia sp.

Complete

• Opening of food is different from exit of waste materials

• Specialized regions of the alimentary canal

• Complexity varies in each phylum• Extracellular hydrolysis of food

Complete

Bos taurus

Modes of feeding

• Herbivore• Carnivore• Omnivore

– Dentition– Alimentary canal

Modes of Feeding

• Suspension feeders

• Substrate feeders

• Fluid feeders

• Bulk feeders

Modes of Feeding

Symbiotic Relationship

• Vertebrates acquired symbiotic relationship with microscopic organisms in processing food

• E.g. Ruminants, Humans, termites

Four Main Stages of Food Processing

• Ingestion– Large oral cavity

• Digestion– Mechanical vs. Chemical

• Absorption• Elimination

Digestion in Humans• Primary organs of

digestion– Mouth, pharynx,

esophagus, stomach, small intestine, large intestine

• Accessory glands of digestion– Salivary glands,

pancreas, liver, gall bladder

Digestion in Humans

• Mouth- functions: ingestion and digestion• Digestion begins in the Oral Cavity

– Mechanical and Chemical digestion– Salivary amylase- breaks down

polysaccharide and glycogen– Saliva- also fxns as antibacterial agent and

neutralizes acidity of foods

Pharynx

• Upper portion of the throat• Where air and digested food meet• Digested ball (bolus) of food enters• Epiglottis- acts as a seal to prevent food

from entering the air passages

Esophagus

• Upper portion- striated muscle• Lower portion- smooth muscle• Connects the pharynx to the stomach• Peristalsis• Goblet cells- specialized cells that lines

the esophagus– Release mucus that covers the bolus

Peristalsis

• Wave-like motion responsible for the movement of digested food in the alimentary canal

Stomach

• Stores and digests food• Cardiac sphincter- prevents backflow • Pyloric sphincter- regulates entrance of

acidic chyme to the small intestine• Secretes digestive juice

– Mixed with food through churning of the stomach through smooth muscle contractions

Stomach

• Chief cells- secretes pepsinogen• Parietal cells- secrete HCl• HCl- disrupts the extracellular matrix of

plant and animal cell• Pepsin- active form of pepsinogen

– Hydrolyzes proteins– Works best in an acidic environment

Stomach

• Stomach lining is protected– First, pepsinogen is only activated when

secreted into the lumen due to the action of the acidic HCl (pepsinogen activation-positive feedback)

– Secondly, presence of goblet cells that secrete mucus

• product- acid chyme

Stomach

Small Intestine

• Major organ of digestion and absorption• Longest section of the alimentary canal• Divided into three sections:duodenum,

jejunum, ileum• Duodenum- site of mixing of acidic chyme

and other digestive juices (digestion)• Jejunum and Ileum- absorption

Small Intestine

• Carbohydrate digestion• Protein Digestion• Nucleic Acid Digestion• Fatty Acid Digestion

Carbohydrate digestion

• Starch, glycogen and other polysaccharides that were digested in the mouth is further digested

• Pancreatic amylase• Maltase- splits maltose into its glucose

units• Disaccharides- absorbed by intestinal

epithelium

Protein digestion

• Trypsin and Chymotrypsin- breakdown large polypeptide chain like pepsin

• Dipeptidase- split small peptides• Carboxypeptidase- breakdown

polypeptides in its carboxyl end• Aminopeptidase- breakdown peptidase in

its nitrogenous end• Enteropeptidase- activates pancreatic

enzymes

Protein digestion

• Intestinal enzymes- aminopeptidase, enteropeptidase

• Pancreatic enzymes- Trypsinogen, Procarboxypeptidase, Chymotrypsinogen– Enteropeptidase- activates Trypsinogen– Trypsin- activates Procarboxypeptidase,

Chymotrypsinogen

Nucleic Acid Digestion

• Nucleases- hydrolyze nucleic acids• Exonucleases- hydrolyze nucleic acids on

its terminal• Endonucleases- hydrolyze nucleic acids

within• Other enzymes hydrolyze the nucleotides

into its components

Fatty acid digestion

• Bile salts- emulsify undigested fats in the duodenum– Emulsification- inc SA of fat molecules

• Lipase- digests fat molecules

Absorption

• Villus- folds found in the small intestine

• Microvillus- microscopic fingerlike projections that increases the absorption of materials

• Each villus is connected to a capillary (BV) network and lacteals(Lymphatic system)

Transport of nutrients

• Passive Diffusion• Active transport

– Nutrients that were absorbed are transformed into what the body needs

– Chylomicron- small globule that is a combination of fats, cholesterol, coated with proteins

Hormones that regulate digestion

• Gastrin-stimulated by gastric juices– Inhibited by low pH– stimulates secretion of gastric juice

• Enterogastrones- group of enzymes found in the duodenum

• Cholecystokinin (CCK)- stimulated by fats and amino acids– Stimulates gall bladder to release bile

• Secretin- stimulated by the acidic chyme– Stimulates pancreas to release bicarbonates

– If chyme is rich in fats, it stimulates the duodenum to release other enzymes to slows down digestion in the stomach

Large Intestine

• Also called the colon• Cecum – small pouch that has different fxns• Appendix- small cecum found in man• Rectum- portion of the large intestine that

temporarily stores feces• Main fxn of colon is to reabsorb water• Feces- waste that was formed after digestion

• Compactness depends on water that was reabsorbed

Large Intestine

• Intestinal Bacteria- common example is E. coli

• Have mutualistic relationship with host

• Generate methane or hydrogen sulfide

• Some produce vitamins that are needed by the body

Nutrition

• Nutritionally adequate diet– composed of:

• fuel (chemical energy)• organic raw materials (carbon

skeletons)• essential nutrients (substances the

animal cannot make)

Balancing the fuel

• Homeostatic mechanism balances the animal’s fuel

• ATPs that were produced are budgeted depending on the energy requirements

• Fats have the highest amount of ATP• Glucose conversion is an example of

homeostatic mechanism

Caloric Imbalance

• Undernourishment– Calorie deficiency

• Overnourishment– Excessive calorie intake- results to obesity

• Malnourishment– Deficient in any essential nutrients

Essential Nutrients

• Essential Amino Acids• Essential Fatty Acids• Vitamins• Minerals

Essential Amino Acids

• Tryptophan, methionine, Valine, Threonine, Phenylalanine, Leucine, Isoleucine, Lysine, Histidine (infants)

• Deficiency in one of these may result into protein deficiency

Essential Fatty Acids

• Essential Fatty Acids belong to the unsaturated fatty acid groups

• An example is linoleic acid• Deficiencies are rare

Vitamins

• Organic molecules that are required relatively in small amount

• Two types:– Water-soluble

• B vits, C– Fat-soluble

• A, D, E, K

Minerals

• Inorganic nutrients that are required in small amounts

• E.g. Calcium, Phosphorous, Sodium, etc.

Balance diet

• what enters= what leaves• It is easier to take in calories in the body

than to burn it• The food pyramid serves as a guide in the kind of foods that should be taken in