IB Bio: Digestion

Esophagus

The esophagus receives food from the mouth when you swallow. Through peristalsis (a series of muscular contractions), the esophagus delivers food to the stomach.

Gall Bladder

The gall bladder stores and concentrates bile, and releases it into the duodenum to help absorb/digest fats.

Exocrine Gland Acinus

A cluster of cells form a hollow sphere. One acinus is the fundamental unit of a gland. Multiple acini (plural) are found in one gland.

Peristalsis

A series of contractions called peristalsis that push the chyme through the small intestine tube.

Secretory Cell

A single layer of cells that synthesize molecules for secretion (i.e. digestive enzymes that are released from the cell).

Duct

A tube structure for passage of the secreted material.

Absorptive Cell

A.k.a. epithelial cells, these cells line villi (small finger-like projections that protrude from the epithelial lining of the intestinal wall) in the small intestine. They have lots of microvilli (microscopic cellular membrane protrusions that increase surface area for absorption and are involved in secretion and cellular adhesion), mitochondria (provide ATP to fuel the active transport of molecules into the cell), pinocytotic vesicles (form when small particles enter the cell membrane and create a vesicle that fuses with lysosomes to hydrolyze the particles), and tight junctions (closely associated area of two cells whose membranes join together, forming a virtually impermeable barrier to fluid that gives the sheet mechanical strength and prevents small molecules from seeping through).

Parietal Cells

Acid-secreting cells. Membrane transport proteins both take in and let out potassium ions while releasing hydrogen ions and chlorine ions. The net movement is H+ and Cl- out. Thus, HCl is released.

Absorption vs. Assimilation

After enzymatic digestion of food, absorption of digested food occurs in the small intestine. Absorption is the transport of molecules from the intestine to the bloodstream. Assimilation is the incorporation of molecules from the bloodstream into cellular structures.

Amylase

Amylase digests carbohydrates. Salivary amylase comes from the salivary glands. Its substrate is starch and the product is smaller polysaccharides and maltose. Its optimal pH is 7. Pancreatic amylase comes from the pancreas. Its substrate is oligosaccharides and its product is maltose and other disaccharides. It optimal pH is 6.7

Gland

An organ in an animal's body that synthesizes a substance for release.

Brush Border Enzymes (Membrane-Bound Digestive Enzymes)

Brush border enzymes are immobilized in the membrane of the small intestine. Chyme passes by the enzymes (with polypeptides, nucleotides, and disaccharides) and gets digested by the enzymes into monosaccharides, nitrogenous bases, and amino acids. With enzymes embedded in the cell membrane, the substrate can come to the enzyme, the enzyme can be reused and it doesn't need to keep making and secreting, the body can avoid self-digestion, and the enzyme doesn't flush away.

Mouth

By chewing, the mouth breaks the food into pieces that are more easily digested. Saliva mixes with the food and breaks it down partially (comes from the salivary glands).

William Beaumont

By putting foods attached to string inside a boy's stomach (the boy had a hole into the stomach because of a gunshot wound), Beaumont was able to pull out the food and observe the effects of the gastric juices in the stomach. He saw that the gastric juices digested the food over several hours (broke up the food). Beaumont was also able to extract the gastric juices from the stomach and observe its effects in a test tube. The juices still digested the food, and Beaumont showed that there was no essential life force required to perform the processes of life (gastric juice can digest outside of the body).

Dietary Fiber

Cellulose, lignin, and pectin are all dietary fiber. Dietary fiber increases satiety, decreases obesity, lowers the risk of colon cancer, lowers the risk of hemorrhoids, lowers the risk of appendicitis, lowers the rate of sugar absorption which lowers the risk of diabetes, decreases constipation, and increases "regularity" of excretion. It also increases the rate of transit of material through the large intestine.

Facilitated Diffusion

Fructose and some other molecules (small, charged) that are hydrophilic are absorbed by facilitated diffusion, aided by channel proteins (passive, follow the concentration gradient).

Endocytosis

Digested food can be absorbed via endocytosis of vesicles into the cytoplasm.

Endocrine Gland

Endocrine glands secrete hormones directly into the bloodstream.

Chief Cells

Enzyme-secreting cells. Vesicles release pepsinogen via exocytosis.

Exocrine Gland

Exocrine glands secrete stuff other than hormones (sweat, oil, wax, enzymes, etc.) into ducts (a pipe or tube).

Active Transport

Glucose, amino acids, Na, Ca, and Fe ions are absorbed by active transport (against the concentration gradient, requires energy).

Helicobacter Bacteria

H. pylori bacteria secrete mucase which eats away at the mucus layer in the stomach, causing a particular region to become exposed to pepsin and HCl (causing an ulcer).

Chemical Digestion (Stomach)

Hydrochloric acid (HCl) kills microorganisms, helps chemically digest food, and converts pepsinogen to pepsin. Pepsinogen becomes pepsin when HCl is present. Pepsin digests proteins into smaller polypeptides. Mucus is made up of glycoproteins. "Glyco" is sugar, "protein" is protein. It forms a protective layer over the stomach (protects from acid and enzymes). It also lubricates food to aids its movement.

Starch Digestion

In the mouth, starch is digested mechanically (with jaws, teeth, tongue) and chemically (salivary amylase). The mouth only partially breaks down starch (hydrolyzes it into oligosaccharides by breaking alpha 1-4 glycosidic bonds in amylose and amylopectin). In the stomach, the oligosaccharides are not broken up; they are only mixed up. The acidic pH of the stomach denatures salivary amylase. In the small intestine, pancreatic amylase from the pancreas breaks down oligosaccharides into monosaccharides by hydrolyzing alpha 1-4 glycosidic bonds. Brush border enzymes like maltase (1-4 bonds) and sucrase/isomaltase (1-4 and 1-6) hydrolyze corresponding bonds to further break up the oligosaccharides. Finally, in the intestinal lining, the active transport of glucose into absorptive cells via sodium-glucose co-transporters (brings in 2 Na and 1 glucose) occurs. Glucose in the bloodstream is used by tissues for respiration and is stored by the liver (conversion to glycogen).

Lipase

Lipase digests fats. An example is pancreatic lipase which comes from the pancreas. The substrate is lipids and the product is glycerol and fatty acids. The optimal pH is 7.5.

Simple Diffusion

Lipids and other molecules (small and non-charged) that can easily pass through the hydrophobic center of the epithelial cell membrane are absorbed by simple diffusion (follow the concentration gradient, passive).

Salivary Gland

Located in the mouth, it secretes a mixture of water, salts, amylase, and mucus.

Gastric Glands

Located in the stomach, they secrete a mixture of water, mucus, HCl, and enzymes (i.e. pepsin).

Mucosal Cells

Mucus-secreting cells. The vesicles in mucosal cells release mucus (glycoproteins).

Mechanical Digestion (Stomach)

Muscular contractions that swish around the food in the stomach to break it up and expose all the food to the gastric juices.

Control of Gastric Gland Secretion

Once food enters the stomach, the stomach stretches. Cells with stretch receptors detect the stretching and send a neural signal from the stomach to the brain (medulla oblongata). The medulla oblongata sends a neural signal back to the stomach that causes endocrine cells to release a hormone called gastrin. Gastrin enters the blood through circulation and circulates throughout the body until it returns back to the stomach where it can bind to gastrin receptors in chief and parietal cells. Gastrin triggers the chief and parietal cells to secrete pepsinogen and HCl, both of which combine to make gastric juice.

Treatment of Ulcers

One can treat ulcers by inhibiting the H+/K+ ATPase enzyme with a competitive inhibitor molecule called PPI (proton pump inhibitor). It reduces the production of HCl, which allows ulcers to heal. Examples include Prilosec, Zantac, and Prevacid.

Small Intestine Wall

Order of layers (outward to inward): Serosa, longitudinal muscle, circular muscle, submucosa, and mucosa with circular folds. Serosa is the outer covering. Longitudinal and circular smooth muscle create peristalsis. Submucosa contains blood vessels and lymph vessels. Mucosa lines the lumen and absorbs nutrients.

Pancreas (Composition of "Juice")

Pancreatic juice is composed of water, bicarbonate (buffers acid from stomach), and enzymes (i.e. amylase, lipase).

Carbohydrate Digestion

Polysaccharides and disaccharides are first digested in the oral cavity, pharynx, and esophagus by salivary amylase into smaller polysaccharides and maltose. There is no carbohydrate digestion in the stomach. Then, in the lumen of the small intestine, polysaccharides are digested by pancreatic amylase into maltose and other disaccharides. Finally, in the epithelium of the small intestine (brush border), the disaccharides are digested into monosaccharides by disaccharidases. There is no further digestion after this.

Protease

Protease digests proteins. An example is pepsin which comes from the chief cells in the stomach. The substrate is proteins and the product is polypeptides. The optimal pH is 1.5. Another example is trypsin which comes from the exocrine pancreas. The substrate is ingested proteins that are already partially digested by pepsin. The products are polypeptide fragments (di/tri-peptides). The optimal pH is 8.

Exocytosis

Secreted molecules are released from the cell by fusion of the vesicle membrane and cell membrane.

Vesicle

Small, membrane-bound sacs in the cell that store and transport the molecule that will be secreted.

Ulcers

The acidic environment of the stomach is good because it kills microorganisms and helps to chemically digest food. The stomach mucus is good because it forms a protective layer over the stomach (protects from acid and enzymes) and lubricates food to aid its movement. Stomach ulcers are areas where the mucus layer has eroded, leaving the stomach muscle layers unprotected and exposed to gastric acids and digestive enzymes. Stomach cancer can develop from ulcers. However, having an ulcer does not necessarily indicate stomach cancer. Heliobacter bacteria are associated with ulcers and cancer.

Structures in an Exocrine Gland Cell

The are many large secretory vesicles for storage of substances being transported and secreted through the plasma membrane. They have a high concentration of proteins. The vesicles are close to the lumen so they can excrete materials easier (space to transport molecules, all on one side). The vesicles are amongst many golgi apparatus that process proteins (the proteins that are produced can enter the nearby vesicles faster). There is also an extensive network of rough ER for protein synthesis (these proteins get sent to the golgi for processing). Finally, there is a large number of mitochondria for production of ATP for protein synthesis and other cell activities.

Basement Membrane

The basement membrane is a thin, fibrous, extracellular layer that provides support to the cells, limits contact between cells and the other cell types in the tissue and acts as a filter allowing only water and small molecules to pass through.

Blood Vessel in an Acinus

The blood vessel supplies the cells of the acinus with the nutrients for their functioning and takes away their waste.

Lumen

The hollow space inside a gland or tube.

Large Intestine

The large intestine processes waste left over from digestion (stool). Stool passes through the large intestine via peristalsis and starts out as liquid and ends up solid (water is removed as it passes through). Stool is stored in the sigmoid until it is emptied into the rectum.

Liver

The liver processes the nutrients absorbed from the small intestine and makes all the chemicals the body needs. It also secretes bile into the small intestine and detoxifies harmful chemicals. The bile is stored in the gall bladder.

Villus Structure and Function

The small intestine has large folds which increase the surface area for absorption. Each villus has a capillary bed that absorbs small molecules from the small intestine and transports the molecules in the bloodstream. Epithelial cells that line the villi have microvilli that also increase the surface area for absorption. There are goblet cells between the epithelial cells that secrete protective mucus.

Pancreas

The pancreas secretes digestive enzymes (pancreatic juice) into the duodenum (first section of the small intestine). These enzymes break down proteins, fats, and carbohydrates. It also make insulin.

Egestion

The process by which undigested and unabsorbed materials that travel through the alimentary canal leave the body. Materials that are egested are water, cellulose, lignin, pectin, bacteria, bilirubin, intestinal cells, mucus, and enzymes (mucus and enzymes that have been secreted into the lumen).

Apical Surface

The side of the absorptive cell that faces the lumen. Surface proteins transport molecules from the small intestine lumen into the cell.

Basal Surface

The side of the absorptive cell that faces the submucosal tissue. Surface proteins transport molecules from the cell into the capillary or lacteal.

Small Intestine

The small intestine breaks down food using enzymes from the pancreas and bile. Peristalsis (muscle contractions) moves the food through the organ. Products leave the small intestine in liquid form (nutrients all absorbed).

Stomach

The stomach holds food while it is being mixed with enzymes that continue to break down the food. Stomach cells secrete strong acid and enzymes to break down food (gastric juice). Food comes into the stomach and chyme comes out.

Alimentary Canal

The whole passage along which food passes through the body from mouth to anus. It includes the esophagus, stomach, and intestines.

Fat Digestion

There is no fat digestion in the mouth or stomach. In the lumen of the small intestine, fat globules are digested by bile salts into fat droplets. The fat droplets are then digested by pancreatic lipase into glycerol, fatty acids, and glycerides. There is no further digestion in the epithelium of the small intestine (brush border).

Nucleic Acid Digestion

There is no nucleic acid digestion in the mouth or in the stomach. Then, in the lumen of the small intestine, DNA and RNA are digested by pancreatic nucleases into nucleotides. Finally, in the epithelium of the small intestine (brush border), nucleotidases digest the DNA and RNA and nucleotides into nucleosides. Nucleosidases and phosphatases digest the nucleosides into nitrogenous bases, sugars, and phosphates.

Protein Digestion

There is no protein digestion in the mouth. In the stomach, proteins are digested by pepsin (pepsinogen-HCl) into smaller polypeptides. Then, in the lumen of the small intestine, the polypeptides are digested by pancreatic trypsin and chymotrypsin into smaller polypeptides. All of the smaller polypeptides are digested by pancreatic carboxypeptidase into amino acids. Finally, in the epithelium of the small intestine (brush border), the amino acids and small peptides are digested by dipeptidases, carboxypeptidase, and aminopeptidase into amino acids. There is no further digestion after this.

Absorption

Transport of molecules from the intestine to the bloodstream. To get to the circulatory system which will then transport nutrients to the rest of the body, the nutrient subunits must first pass through the cells of the small intestine to then enter capillaries which are embedded within the villi.

Cholera

Vibrio cholera bacteria enters the body through the fecal-oral route, travels to the large intestine, releases a toxin which binds to protein receptors on the cell membranes of cells and triggers endocytosis of the toxin into the digestive cells. This causes a mass release of Cl-, H2O, and HCO3- out of the cell. The person suffers from severe dehydration and eventually dies.