BioDigital Systems: Availble for free (individual) or for a fee (groups/businesses), this interactive system will first require you to sign in via your Facebook or Google account to gain access. Once inside the system, you can zoom and rotate your virtual skeleton. Eleven systems in total are able to view and examine. Note: this site allows you to repeatedly quiz yourself on all eleven systems that are covered within the site.
The AK Lectures are a series of lectures from a (external) educational platform designed to "promote collaboration between our users and help spread knowledge to every part of the world."
These lectures vary in length, and will open in a new window when you click on the provided link.
Types of Macromolecules: When we ingest food, we ingest several types of organic macromolecules that we use for nutrition and energy. These macromolecules are carbohydrates, proteins and lipids. Carbohydrates, also known as polysaccharides or sugars, are water-soluble polymers that consist of individual monomer units held together by glycosidic bonds. Our body only contains enzymes to break alpha-glycosidic linkages. In order to actual absorb these sugars into our cells, our body must digest (break down) these polysaccharides into their individual mononeric sugars. The body does this by using special proteolytic enzymes that catalyzes the hydrolysis of carbohydrates. The most common type of sugar monomer in the human body is glucose and most of the non-glucose sugars in our body are transformed into glucose in our liver and intestinal cells. The majority of the cells of our body transport glucose across the cell membrane via passive transport, which means down its electrochemical gradient and without using any energy. However, certain cells such as intestinal and kidney cells are capable of using active transport, which means that they move the glucose against its electrochemical gradient and they use ATP molecules. Proteins are yet another example of a macromolecule that is commonly ingested via food. Proteins are water-soluble polymers that consist of individual units called amino acids held together by peptide bonds. For this reason, proteins are also called polypeptides. Proteins have several stages of structure including primary, secondary, tertiary and quaternary. In order for our cells to actually absorb proteins, our body must first denature the proteins and then break them down into their amino acid form. In some cases, cells can also absorb dipeptides and tripeptides. Our body uses twenty different amino acids, all of which are alpha-amino acids. Ten of these amino acids are called essential amino acids because they cannot be manufactured by our body and must be obtained from our food. The final type of macromolecule that we ingest into our bodies are lipids, also known as fats. Lipids are not water-soluble and are not polymers. They can come in many different forms such as steroids, fatty acids, phospholipids, triglycerides, etc. Each of these types serves its own purpose. Since lipids are not water soluble, they cannot dissolve in our blood and must be carried by special protein carriers. For instance, fatty acids in our blood are carried by a protein carrier called albumin. The majority of the fat that we ingest into our body in food are triglycerides. Before triglycerides are ingest into our cells, they must be broken down by using special types of enzymes that break down these fats into fatty acids and glycerol. Fatty acids are the major form of fat that is ingested into our body. Our fat cells, called adipocytes, store fat in the form of triglycerides. The key component to break down all of these macromolecules quickly and efficiently is water and the appropriate enzymes.
Epithelial Tissue: Epithelial tissue, also known as the epithelium, is one of the four tissues found in the human body. It exists in various parts of the body, such as our digestive system. There are three important functions of the epithelium - to protect the cells underneath the layer of epithelium, to secrete specialized molecules into the body cavity (enzymes, etc) and to absorb and exchange nutrients and waste products. It is therefore no surprise that these cells are found in our digestive system since the role of the digestive system includes the secretion of specialized enzymes needed for digestion of food as well as the absorption of nutrients. Epithelial cells that compose epithelium can be categorized by their shape. Squamous cells are those that have a flatted shape, columnar cells are those that have a rectangular shape while cuboidal cells resemble a cube. These cells can form three types of epithelial tissue. Simple epithelium means that the epithelium layer consists of a single layer while stratified epithelium implies that it consists of many layers. Pseudostratified epithelium is epithelial tissue that looks as if it consists of many layers but only actually has one layer of cells. Epithelial cells are bound to a matrix of protein and other molecules called the basal membrane (also called the basal lamina or basement membrane). The basement membrane creates a foundation for attachment. The lumen side of the epithelial cells is called the apical side while the side attached to the basement membrane is called the basolateral side (or simply basal side).
Introduction to the Digestive System: Our digestive system has two important responsibilities - to digest the food and to absorb the nutrients obtained from the breakdown of that food. Food enters our body through the mouth, which begins both mechanical and chemical digestion. It then travels into the pharynx and then into the esophagus. The smooth muscles within the esophagus propel the food into the stomach, where the digestion of protein begins. The food then travels into the small intestine, where the food continues to be broken down and where absorption of the broken down nutrients begins. Once all the nutrients are absorbed, the food enters the large intestine. In the large intestine, water absorption takes place and anything that was not absorbed in the small intestine is absorbed into the body (things like calcium and other minerals).
Oral Cavity, Pharynx and Esophagus: The first three structures that move food along the alimentary canal are the oral cavity, the pharynx and the esophagus. The oral cavity, also known as the mouth, initiates two important processes, namely mechanical and chemical digestion. Mechanical digestion is the process by which the food is broken down into much small particles as to ensure that the proteolytic enzymes can act on a larger surface area. This process does not actually cleave any chemical bonds and in the mouth it is a result of mastication (chewing). Chemical digestion on the other hand is the actual break down of the chemical bonds that hold the macromolecules together via the process of hydrolysis, which is catalyzed by enzymes. Two proteolytic enzymes found in the mouth are amylase (also known as ptyalin), which breaks down starch into maltose and dextrin, as well as lingual lipase, which breaks down lipids into their constituents. Notice that proteins are not broken down in the mouth. The salivary gland releases saliva into the mouth, which acts to lubricate the food as well as acts as a disinfectant. The pharynx is the region that connects the oral and nasal cavity to the esophagus and the windpipe. A cartilaginous flap called the epiglottis blocks food from entering the windpipe in the pharynx. The esophagus is a narrow and relatively long cylindrical structure that connects the pharynx to the stomach. The upper portion consists of skeletal muscle while the rest of the esophagus consists of smooth muscle. This smooth muscle is involuntarily controlled and exhibits a wave-like contraction (called peristalsis) that propels food down the esophagus and eventually into the stomach. At the bottom of the esophagus is a circular muscle called the cardiac sphincter (also known as the lower esophageal sphincter) that opens up and allows the food bolus (a round mass of food) to travel into the stomach.
The Stomach: The stomach is a flexible sac that is the site of chemical and mechanical digestion, especially of proteins. Depending on how large the meal is and how much protein content is found within the meal, the stomach can store the food anywhere from several minutes to several hours. Although the stomach does not actually absorb the nutrients, it is capable of absorbing molecules such as alcohol, caffeine and aspirin. The stomach lining consists of millions of exocrine glands (gastric and pyloric glands) that secrete a special substance called gastric juice into the stomach lumen. These exocrine glands contain four types of specialized cells - mucous cells, chief cells, parietal cells and G cells. Mucous cells produce and secrete a sticky substance called mucus that plays a role in lubricating the stomach lining and protecting it from being damaged by the acidic environment. Chief cells secrete the principal zymogen called pepsinogen, which is activated by the acidic environment into pepsin. Pepsin is the proteolytic enzyme that cleaves proteins into smaller peptides. Parietal cells produce and release gastric acid (hydrochloric acid) into the lumen of the stomach. Hydrochloric acid (1) lowers the pH of the lumen and stimulates chief cells to release pepsinogen (2) activates pepsinogen into pepsin (3) denatures the protein's three-dimensional structure as to allow the pepsin to get close to the bonds (4) kills off bacterial cells that enter the stomach along with the food. Parietal cells also secrete a substance called the gastric intrinsic factor. This is a glycoprotein hormone that later assists the small intestine in absorbing vitamin B-12. G cells are cells that produce and secrete a peptide hormone called gastrin. Gastrin is released into the blood and stimulates parietal cells to secrete the hydrochloric acid. Another cell in the stomach that plays an important role is the enterochromaffin-like cell (ECL cell) that is responsible for releasing a molecule called histamine. Histamine plays a role in stimulating the parietal cells to secrete the gastric acid. Together, all these cells work together to produce gastric juice (a mixture of HCl and enzymes) that helps mechanically and chemically digest the food particles into smaller bits. The mixing of the gastric juice and the food produces a semi-fluid substance called chyme.
The Small Intestine: The small intestine is an organ where most of the digestion and almost all of the absorption takes place. It consists of three parts - the duodenum, the jejunum and the ileum. The duodenum is where the majority of the digestion occurs while the jejunum and ileum is where the absorption takes place. The small intestine contains a thick and thin layer of smooth muscle that creates a wave-like contraction called peristalsis, which allows the chyme to move along the small intestine. The inner layer of the small intestine contains epithelium along with projections called villi. Each villus consists of many enterocytes that each contain their own tiny hair-like projections called microvilli. This fuzzy-looking border of the villi is called the brush border and this is where digestion of the dipeptides, disaccharides and triglycerides takes place. Together, the villi and the microvilli greatly increase the surface area on which the digestive enzymes can act on, which makes digestion a much more efficient process. The small intestine can produce its own set of digestive enzymes that can break down the various macromolecules. In addition, accessory exocrine organs such as the pancreas produces its own set of pancreatic enzymes that help digestion in the small intestine. The liver can produce bile, which is stored in the gall bladder until it is released into the small intestine. Bile consists of phospholipids, cholesterol, bile salts, water, among other things and it helps mechanically digest and emulsify fat into smaller pieces. Emulsification greatly increases the efficiency and rate at which lipase breaks down the macromolecules. Besides digestion, absorption also takes place at the small intestine. Fatty acids can be easily absorbed into the cells via simple diffusion because they are hydrophobic. The cells then transfer these fatty acids into the lacteal found in the villus, which connects to the lymph system. The amino acids and simple sugars (i.e glucose) must be transported across the cell membrane via either active or passive transport, and are eventually transferred directly into the blood vessels found in the villi.
Digestive Enzymes of Small Intestine and Pancreas: The small intestine and the pancreas both produce a variety of digestive enzymes that are responsible for breaking down the many macromolecules found in the small intestine. At the brush border of the villi of the small intestine are many proteolytic enzymes, including disaccharidases (maltase, sucrace and lactase) and peptidases (especially dipeptidases that break down dipeptides). Many of these enzymes are attached to the membrane of the cells and can digest disaccharides and dipeptides directly on the membrane. The small intestine contains exocrine glands called crypts of Lieberkuhn which can produce an enzyme called enterokinase. Enterokinase is responsible for transforming the zymogen trypsinogen into trypsin. The small intestine can also produce several important hormones, including secretin, cholecystokinin (CCK) and enterogastrone. Secretin is a peptide hormone that stimulates the release of pancreatic juice, CCK is also a peptide hormone that stimulates the release the bile from the liver and enterogastrone slows down the movement of the chyme as to ensure that all the fat is digested. The pancreas produces several important proteolytic enzymes of its own along with a mixture of bicarbonate. This mixture is called the pancreatic juice and when stimulated, it empties into the pancreatic duct, which connects to the common bile duct and eventually makes its way into the small intestine. The pancreas produces amylase, which breaks down alpha glycosidic linkages found in starch and glycogen. The pancreas also produces lipase, which breaks down the triglycerides into fatty acids and glycerol. Finally, the pancreas also produces a set of peptidases which cleave peptide bonds. The three peptidases that you should be familiar with are trypsinogen, chymotrypsinogen and carboxytrypsinogen. Trypsinogen must be activated by enterokinase into trypsin, which then goes on to activate other digestive enzyme. Chymotrypsinogen is actived by trypsin into chymotrypsin, which cleaves peptides at aromatic amino acids. Carboxypeptidase cleaves peptide bonds at the carboxyl end of the peptide.
Emulsification of Fats: Fats are hydrophobic and as a result will not mix very well with the solution in the lumen of the small intestine nor with the chyme. Instead the fat molecules such as triglycerides and cholesterol will aggregate together to form large spherical bundles called fat globules. Due to the large size of the fat globule, pancreatic lipase (a water-soluble molecule) will have no way of actually reaching the inside portion of the fat globule. This means that the lipase can only cleave ester bonds of the triglycerides on the surface and it cannot access the inside portion, which makes the lipase very inefficient. To increase the efficiency and the rate at which lipase cleaves ester bonds, the liver produces and releases a fluid called bile. Bile is composed of amphipathic molecules such as phospholipids and bile salts. When bile enters the small intestine, it will mix with the fat globules and will cause them to break down into smaller units called emulsion droplets. This process is called emulsification. Emulsification greatly increases the surface area of the fat on which the lipase can actually act on. As a result, lipase is now in a position to begin digesting the ester bonds of the lipids efficiently. With the help of colipase, lipase binds onto the surface of these emulsion droplets and begins breaking them down. This is where digestion takes place. Eventually, the emulsion droplets are broken into fatty acids. Since fatty acids are hydrophobic, the bile phospholipids or bile salts can surround the fatty acids and form a tiny spherical structures called a micelles. The micelles are about two hundred times smaller than the emulsion droplets and can therefore easily cross the membrane of enterocytes and enter the cytoplasm of the cell.
Absorption of Carbohydrates by Small Intestine: Carbohydrates begin digestion in the mouth, where salivary amylase begins to break down the carbohydrates into smaller polysaccharides. These polysaccharides eventually end up in the small intestine. In the small intestine, pancreatic amylase begins to break down the polysaccharides into disaccharides. The three most common disaccharides in the human are maltose (combination of two glucose molecules), sucrose (combination of glucose and fructose) and lactose (combination of glucose and galactose). These disaccharides travel to the cell membrane (also known as the brush border) of enterocytes, where membrane-bound digestive enzymes act on the disaccharides and break them down into monomeric sugars. Fructose is transported across the cell membrane of enterocytes via passive transport, in which a membrane protein helps move the fructose without using any ATP molecules. However, both galactose and glucose are transported into the cell by using a sodium-linked secondary active transport system. This means that the cell uses a sodium-potassium ATPase to create an electrochemical gradient in which there is a lower concentration of sodium inside the cell than on the outside. As the sodium moves into the cell from the lumen of the small intestine, the glucose/galactose is brought into the cell with it. Most of the fructose in the cell is transformed into glucose, and these three sugars are transported across the basolateral membrane by using either a cotransport system or passive transport. They travel into the blood stream, which takes them to the liver. Inside the liver, the cells store glucose in the form of glycogen.
Absorption of Proteins in Small Intestine: Proteins begin chemical digestion in the stomach, where the digestive enzyme pepsin cleaves peptide bonds and transforms proteins into smaller polypeptides. The polypeptides eventually end up in the small intestine, where the pancreatic peptidases such as trypsin, chymotrypsin and carboxypeptidase cleave these polypeptides into smaller peptides. At the brush border of the enterocytes, membrane-bound digestive enzymes break down these small peptides even further into amino acids, dipeptides and tripeptides. Amino acids are absorbed by the enterocytes via a sodium dependent co-transport system. This is a secondary active transport system, which means the cell must utilize ATP to create an electrochemical gradient for sodium and use that electrochemical gradient to bring the amino acids into the cell. The dipeptides and tripeptides however use a hydrogen-ion dependent co-transport system. This means that they use a proton electrochemical gradient to bring the dipeptides and tripeptides into the cell. Once these dipeptides and tripeptides are in the cell, they are usually broken down into their constituent amino acids. These amino acids are eventually transported out of the basolateral side of the cell and into the blood system that takes the amino acids to the different cells of the body (especially liver cells). These cells utilize the amino acids to synthesize proteins.
Large Intestine: The large intestine follows the small intestine in the digestive tract. It consists of three segments - the cecum, the colon and the rectum. The cecum connects the ileum of the small intestine to the colon of the large intestine. It contains a small structure called the appendix. The colon itself can be subdivided into four segments - the ascending colon, transverse colon, descending colon and sigmoid colon. The function of the colon is to absorb the water, minerals and vitamins that have not been absorbed by other parts of the part. The colon also contains bacterial cells (E. coli) that are responsible for producing essential vitamins such as vitamin K, B-12 and thiamin. The rectum is a storage depot for the feces; it is capable of expanding when needed to hold more material. The anus contains the opening that allows the feces to travel out of the body. It contains an involuntary internal sphincter and a voluntary external sphincter. Feces consists predominately of water as well as roughage (composed of cellulose), dead bacterial cells, cells that have been scrapped off the walls of the intestine and stomach, enzymes, among other things.
Absorption of Fats in Small Intestine: The small intestine uses bile to emulsify and break down large fat globules into smaller pieces, which allows the lipase enzymes to break down the lipids into fatty acids. These fatty acids (and other lipids such as cholesterol) are packaged into micelles, which are taken up by the cells of the small intestine (called enterocytes). These enterocytes use the fatty acids to synthesize triglycerides within the smooth endoplasmic reticulum of the cell and within the lumen of the smooth ER, they combine many triglycerides, phospholipids and cholesterol molecules to form spherical chylomicrons. Chylomicrons also contain apoprotein components, which makes them lipoproteins. These chylomicrons are released from the basolateral side of the cell and move into the lacteal of the lymphatic system of the body. The lymphatic vessels carry the chylomicrons into the blood system via the thoracic duct, which connects to the blood vessels via the left subclavian vein. In the blood stream, the chylomicrons attach onto receptors on endothilial cells found on the wall lining of the blood capillaries. The membrane of these cells contain lipoprotein lipases that break down the triglycerides in the chylomicron, and the broken down components (fatty acids and glycerol) are then absorbed by the target cells, usually liver and fat cells. Chylomicrons are the largest type of lipoprotein. Several other smaller categories of lipoproteins exist, including low-density lipoproteins (LDL) and high-density lipoproteins (HDL).