Function: Coenzyme in the production of nucleic acids
Source: Red meats, dairy products
Deficiency Symptoms: Pernicious anemia
Biotin
Function: Coenzyme in fat synthesis and amino acid metabolism
Source: Meat, vegetables
Deficiency Symptoms: Rare: depression, nausea
Folic acid
Function: Coenzyme in amino acid and nucleic acid metabolism
Source: Green vegetables
Deficiency Symptoms: Anemia, diarrhea
Vitamin C
Function: Important in forming collagen, cementum of bone, teeth, connective tissue of blood vessels; may help maintain resistance to infection
Source: Fruit, green leafy vegetables
Deficiency Symptoms: Scurvy, breakdown of skin, blood vessels
Vitamin D (calciferol)
Function: Increases absorption of calcium and promotes bone formation
Source: Dairy products, cod liver oil
Deficiency Symptoms: Rickets, bone deformities
Vitamin E (tocopherol)
Function: Protects fatty acids and cell membranes from oxidation
Source: Margarine, seeds, green leafy vegetables
Deficiency Symptoms: Rare
Vitamin K
Function: Essential to blood clotting
Source: Green leafy vegetables
Deficiency Symptoms: Severe bleeding
The Main Stages of Food Processing
Ingestion
Digestion
Mechanical digestion
Chemical digestion (enzymatic hydrolysis)
Absorption
Elimination
Undigested material
Nutrient molecules enter body cells
Digestion
Breaking food down into molecules small enough to absorb
Mechanical digestion
Chemical digestion è enzymatic hydrolysis splits bonds in molecules with the addition of water.
Absorption
Elimination
Digestive Compartments
Most animals process food in specialized compartments.
These compartments reduce the risk of an animal digesting its own cells and tissues.
Intracellular Digestion
Food particles are engulfed by phagocytosis.
Digestion in a hydra
Food vacuoles, containing food, fuse with lysosomes containing hydrolytic enzymes.
Digestive enzymes released
Food particles broken down
Food particles engulfed and digested
Extracellular Digestion
Breakdown of food particles outside of cells.
It occurs in compartments that are continuous with the outside of the animal’s body.
Animals with simple body plans have a gastrovascular cavity that functions in both digestion and distribution of nutrients.
Complete Digestive Tract
More complex animals have a digestive tube with two openings, a mouth and an anus.
complete digestive tract = alimentary canal
It can have specialized regions that carry out digestion and absorption in a stepwise fashion.
Mammalian Digestive System
Organs specialized for sequential stages of food processing form the mammalian digestive system
Alimentary canal and accessory glands that secrete digestive juices through ducts:
Salivary glands
Pancreas
Liver
Gallbladder
The human digestive system
Peristalsis: rhythmic contractions
The Oral Cavity, Pharynx, and Esophagus
Oral cavity
Salivary glands deliver saliva to lubricate food.
Teeth chew food into smaller particles that are exposed to salivary amylase, initiating breakdown of glucose polymers.
Saliva also contains mucus, a viscous mixture of water, salts, cells, and glycoproteins.
The tongue shapes food into a bolus and provides help with swallowing.
The throat, or pharynx, is the junction that opens to both the esophagus and the trachea.
The esophagus connects to the stomach. The trachea (windpipe) leads to the lungs.
The esophagus conducts food from the pharynx down to the stomach by peristalsis.
Swallowing Reflex and Esophageal Peristalsis
From mouth to stomach: the swallowing reflex and esophageal peristalsis
Chemical Digestion in the Stomach
The stomach stores food and secretes gastric juice, which converts a meal to acid chyme.
Gastric juice has a low pH of about 2, which kills bacteria and denatures proteins.
Gastric juice is made up of hydrochloric acid (HCl) and pepsin.
Pepsin is a protease, or protein-digesting enzyme, that cleaves proteins into smaller peptides.
Parietal cells secrete hydrogen and chloride ions separately into the lumen (cavity) of the stomach.
Chief cells secrete inactive pepsinogen, which is activated to pepsin when mixed with hydrochloric acid in the stomach.
Mucus protects the stomach lining from gastric juice.
The Stomach and its Secretions
Gastric pits on interior surface of stomach
Folds of epithelial tissue
Stomach Dynamics
Coordinated contraction and relaxation of stomach muscle churn the stomach’s contents.
Sphincters prevent chyme from entering the esophagus and regulate its entry into the small intestine.
Digestion in the Small Intestine
The small intestine is the longest section of the alimentary canal.
It is the major organ of digestion and absorption.
Chemical Digestion in the Human Digestive System
Oral cavity, pharynx, esophagus:
Carbohydrate digestion: Polysaccharides to Disaccharides via Salivary amylase
Stomach:
Protein digestion: Proteins to Small polypeptides via Pepsin
Small intestine (enzymes from pancreas):
Carbohydrate digestion: Polysaccharides to Disaccharides via Pancreatic amylases
Protein digestion: Small polypeptides to Smaller polypeptides via Pancreatic trypsin and chymotrypsin; Smaller polypeptides to Small peptides via Pancreatic carboxypeptidase
Nucleic acid digestion: DNA, RNA to Nucleotides via Pancreatic nucleases
Fat digestion: Fat (triglycerides) to Glycerol, fatty acids, monoglycerides via Pancreatic lipase
Small intestine (enzymes from epithelium):
Carbohydrate digestion: Disaccharides to Monosaccharides via Disaccharidases
Protein digestion: Small peptides to Amino acids via Dipeptidases, carboxypeptidase, and aminopeptidase
Nucleic acid digestion: Nucleotides to Nucleosides via Nucleotidases; Nucleosides to Nitrogenous bases, sugars, phosphates via Nucleosidases and phosphatases
Pancreatic Secretions
The pancreas produces proteases trypsin and chymotrypsin that are activated in the lumen of the duodenum.
Its solution is alkaline and neutralizes the acidic chyme.
Bile Production by the Liver
In the small intestine, bile aids in digestion and absorption of fats
Bile is made in the liver and stored in the gallbladder
Bile also destroys nonfunctional red blood cells
Secretions of the Small Intestine
The epithelial lining of the duodenum produces several digestive enzymes
Enzymatic digestion is completed as peristalsis moves the chyme and digestive juices along the small intestine
Most digestion occurs in the duodenum; the jejunum and ileum function mainly in absorption of nutrients and water
Absorption in the Small Intestine
The small intestine has a huge surface area, due to villi and microvilli that are exposed to the intestinal lumen
The enormous microvillar surface creates a brush border that greatly increases the rate of nutrient absorption
Transport across the epithelial cells can be passive or active depending on the nutrient
Nutrient Absorption in the Small Intestine
Villi
Microvilli (brush border) at apical (lumenal) surface
Epithelial cells
Absorption in the Large Intestine
The colon of the large intestine is connected to the small intestine
The cecum aids in the fermentation of plant material and connects where the small and large intestines meet
The human cecum has an extension called the appendix, which plays a very minor role in immunity
Function of the Colon
A major function of the colon is to recover water that has entered the alimentary canal
The colon houses bacteria (e.g., Escherichia coli) that live on unabsorbed organic material; some produce vitamins
Feces, including undigested material and bacteria, become more solid as they move through the colon
Feces are stored in the rectum until they can be eliminated through the anus
Two sphincters between the rectum and anus control bowel movements
The Digestive System
Breaks Down Food to Small Molecules for Absorption