Food Processing and Digestion

Food Processing

• Ingestion is the act of eating or feeding.
• Animal species feeding mechanisms differ widely.
• Digestion breaks food down into molecules small enough to absorb.
• Mechanical digestion, like chewing, increases the surface area of food.
• Chemical digestion splits food into small molecules able to pass through membranes for building larger molecules.
• Enzymatic hydrolysis splits bonds in molecules with the addition of water in chemical digestion.
• Absorption is the uptake of small molecules by body cells.
• Elimination is the passage of undigested material out of the digestive system.

Digestive Compartments

• Most animals process food in specialized compartments.
• Compartments reduce the risk of animals digesting their own cells and tissues.
• Intracellular digestion sees food particles engulfed by phagocytosis, and liquids by pinocytosis.
• Food vacuoles, containing food, fuse with lysosomes containing hydrolytic enzymes.
• Sponges are examples of animals that digest food entirely through intracellular digestion.
• Extracellular digestion is the 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 digestion and nutrient distribution.
• More complex animals have a digestive tube with two openings, a mouth and an anus.
• This digestive tube is a complete digestive tract, or alimentary canal.
• It is capable of having specialized regions that carry out digestion and absorption in a stepwise fashion.

Mammalian Digestive System

• Mammals secrete digestive juices through ducts into the alimentary canal through a number of accessory glands.
• Mammalian accessory glands include salivary glands, the pancreas, the liver, and the gallbladder.
• Food processing begins in the oral cavity.
• Salivary glands deliver saliva to lubricate food, facilitate taste, and buffer acids to prevent tooth decay.
• Saliva contains mucus, which is a viscous mixture of water, salts, cells, and glycoproteins.
• Saliva also contains amylase, which breaks down starch and glycogen.
• Also present in saliva is lysozyme.
• Tongue movements shape food into a bolus to aid in 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.
• Swallowing causes the epiglottis to block entry to the trachea.
• The bolus is guided by the larynx, the upper part of the respiratory tract, while the esophageal sphincter relaxes.
• Coughing occurs when the swallowing reflex fails and food or liquids reach the windpipe.
• Peristalsis, rhythmic contractions of muscles in the wall of the canal, pushes food along from the pharynx to the stomach within the esophagus.
• Valves called sphincters regulate the movement of material between compartments.

Digestion in the Stomach

• The stomach stores up to 2 L of food and processes it into a liquid suspension.
• The stomach secretes gastric juice.
• The mixture of ingested food and gastric juice is called 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, which breaks peptide bonds to cleave 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.
• Cell division adds a new epithelial layer every three days.
• Gastric ulcers, lesions in the lining, are caused mainly by the bacterium Helicobacter pylori.
• Coordinated contraction and relaxation of stomach muscles churn the stomach's contents.
• Sphincters prevent chyme from entering the esophagus and regulate its entry into the small intestine one squirt at a time.
• Heartburn can result if the sphincter at the top of the stomach allows movement of chyme back to the lower end of the esophagus.

Digestion in the Small Intestine

• The small intestine is the longest compartment of the alimentary canal.
• Most enzymatic hydrolysis of macromolecules from food occurs here.
• The first portion of the small intestine is the duodenum.
• Here, chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and the small intestine itself.
• The pancreas produces the proteases trypsin and chymotrypsin, which are activated in the lumen of the duodenum.
• Its solution is alkaline and neutralizes the acidic chyme.
• Bile salts facilitate digestion of fats and are a major component of bile.
• Bile is made in the liver and stored in the gallbladder.
• Bile also destroys nonfunctional red blood cells.
• 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.
• Examples include fructose (facilitated diffusion), amino acids/glucose (active transport), and water (osmosis).
• The hepatic portal vein carries nutrient-rich blood from the capillaries of the villi to the liver, and then to the heart.
• The liver regulates nutrient distribution, interconverts many organic molecules, and detoxifies many organic molecules.
• Epithelial cells absorb fatty acids and monoglycerides, and then recombine them into triglycerides.
• These fats are coated with phospholipids, cholesterol, and proteins to form water-soluble chylomicrons.
• Chylomicrons are transported into a lacteal, a lymphatic vessel in each villus.
• Lymphatic vessels deliver chylomicron-containing lymph to large veins that return blood to the heart.

Processing in the Large Intestine

• The alimentary canal ends with the large intestine.
• It includes the colon, caecum, and rectum.
• The colon leads to the rectum and anus.
• The cecum aids in the fermentation of plant material, and connects the small and large intestines.
• The human cecum has an extension called the appendix, which plays a minor role in immunity.
• The colon completes the recovery of water that began in the small intestine.
• Feces, the wastes of the digestive system, become more solid as they move through the colon.
• Diarrhea and constipation are conditions that can arise pertaining to the large intestine.
• Feces are stored in the rectum until eliminated through the anus.
• Two sphincters exist between the rectum and anus that control bowel movements.

Evolutionary Adaptations of Vertebrate Digestive Systems

• Digestive systems of vertebrates are variations on a theme.
• There are adaptations associated with the animal's diet.
• Dentition, an animal's assortment of teeth, is one example of structural variation reflecting diet.
• The success of mammals is due in part to their dentition, which is specialized for different diets.
• Nonmammalian vertebrates have less specialized teeth, though exceptions exist.
• For example, the teeth of poisonous snakes are modified as fangs for injecting venom.
• Many carnivores have large, expandable stomachs.
• Herbivores and omnivores generally have longer alimentary canals than carnivores due to the longer time needed to digest vegetation.
• The coexistence of humans and many bacteria involves mutualistic symbiosis.
• Some intestinal bacteria produce vitamins such as vitamin K, biotin, and folic acid.
• They also regulate the development of the intestinal epithelium and the function of the innate immune system.
• The microbiome is the collection of microorganisms living on the body.
• Scientists use a DNA sequencing approach based on the polymerase chain reaction to study the microbiome, which contains around 400 bacterial species, and 100 times as many genes as the human genome.
• There are differences in the microbiome associated with diet, disease, and age.
• H. pylori can disrupt stomach health by eliminating other bacterial species from the stomach.
• Many herbivores have fermentation chambers, where mutualistic microorganisms digest cellulose.
• Horses, elephants, and koalas have large ceca.
• Rabbits and rodents have large ceca and large intestines.
• Rabbits and rodents pass food through their alimentary canal twice (coprophagy).
• Ruminants (cattle, sheep, deer) have the most elaborate adaptations for an herbivorous diet.

Feedback Circuits

• The processes that enable an animal to obtain nutrients are matched to the organism's circumstances and need for energy.
• Each step in the digestive system is activated as needed.
• The enteric division of the nervous system helps to regulate the digestive process, such as peristalsis.
• The endocrine system also regulates digestion through the release and transport of hormones.
• The body stores energy-rich molecules that are not needed for metabolism right away.
• In humans, energy is stored first in the liver and muscle cells in the polymer glycogen.
• Excess energy is stored in fat in adipose cells.
• When fewer calories are taken in than expended, the human body expends liver glycogen first, then muscle glycogen, and then fat.
• Synthesis and breakdown of glycogen are central to maintaining metabolic balance
• The hormones insulin and glucagon regulate the breakdown of glycogen into glucose.
• The liver is the site for glucose homeostasis.
• A carbohydrate-rich meal raises insulin levels, triggering the synthesis of glycogen.
• Low blood sugar causes glucagon to stimulate the breakdown of glycogen and release glucose.
• Insulin stimulates glucose uptake from blood in nearly all body cells.
• Brain cells are an exception; they can take up glucose whether or not insulin is present.
• Glucagon and insulin are both produced in the islets of the pancreas.
• Alpha cells make glucagon, and beta cells make insulin.
• Diabetes mellitus is caused by a deficiency of insulin or a decreased response to insulin in target tissues.
• Cells are unable to take up enough glucose to meet metabolic needs.
• The level of glucose in the blood may exceed the capacity of kidneys to reabsorb it
• Sugar present in the urine is a test for diabetes.
• Type 1 diabetes is an autoimmune disorder in which the immune system destroys the beta cells of the pancreas.
• It usually appears during childhood.
• Treatment consists of insulin injections, typically several times per day.
• Type 2 diabetes, or non-insulin-dependent diabetes, is characterized by a failure of target cells to respond normally to insulin.
• Insulin resistance is a factor.
• Excess body weight and lack of exercise significantly increase the risk of type 2 diabetes
• It generally appears after age 40, but may develop earlier in younger people who are sedentary.
• Overnourishment causes obesity, which results from excessive intake of food energy with the excess stored as fat.
• Obesity contributes to type 2 diabetes, cancer of the colon and breasts, heart attacks, and strokes.
• Researchers have discovered several mechanisms that help regulate body weight.
• Hormones regulate long-term and short-term appetite by affecting a "satiety center" in the brain.
• Ghrelin, a hormone secreted by the stomach wall, triggers feelings of hunger before meals.
• Insulin and PYY, which is a hormone secreted by the small intestine after meals, both suppress appetite.
• Leptin, produced by adipose (fat) tissue, suppresses appetite and regulates body fat levels.