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Unit 2: Digestion IB Biology Why do we digest food? • The series of events in digestion include: – – – – Ingestion – you eat the food Digestion – a series of chemical reactions where food is converted into smaller and smaller molecular forms Absorption – small molecular forms are absorbed through the cells of the digestive system and pass into nearby blood vessels and lymphatic vessels Transport – your circulatory system delivers the small molecular nutrients to your body cells Why do we digest food? • Digestion solves a problem of molecular size – – – – Many of the foods that we eat have large molecules that are too large to pass across any cell membrane. Molecules must pass through the cell membranes of your intestines to get to your blood stream Then they must pass through the cell membranes of a capillary Food we eat must therefore be chemically digested to a suitable size Molecule Type Molecular form ingested Molecular form after digestion Protein Protein Amino acids Lipids Triglycerides Glycerol and fatty acids Carbohydrates Polysaccharides, disaccharides, monosaccharides Monosaccharides Nucleic acids DNA, RNA Nucleotides Why do we digest food? • Digestion allows you to turn molecules into ‘your own’ – – – – All foods are composed of either plant cells or animal cells Each contain molecules characteristic of a living organism that is not a human being Plant cells store excess carbohydrates in the form of starch Animals store excess carbohydrates as glycogen Why do we digest food? • Each type of organism has its own set of proteins – – • • Cow muscle (steak) is similar to human muscle but the amino acid sequences are different Nucleic acids (DNA and RNA) of other organisms are also ingested When molecules are digested they are hydrolyzed (broken down) into their smallest components The components can then be reassembled into the larger macromolecules that are useful to you The role of enzymes during digestion • • As food move through the alimentary canal, many digestive enzymes are added. Each enzyme is specific for specific food types. • • • Lipase: enzyme specific for lipid molecules Amylase: enzyme specific for amylose (starch) Enzymes are protein molecules which act as catalysts for chemical reactions. The role of enzymes during digestion • Enzymes lower the activation energy for the reactions they catalyze. – – – Reactions occurring with an enzyme will require a lower input of energy than a reaction without an enzyme. Heat is the most common input of energy Reactions catalyzed with enzymes occur at a higher reaction rate and lower temperature than one without an enzyme The role of enzymes during digestion • Many digestive reactions need higher temperatures than would be safe for a living being. – – • Stable body temperature for a human is 37°C (98.6°F) This temperature provides high enough activation energy for metabolic reactions such as digestion to occur with the aid of enzymes. Digestive enzymes all help to catalyze hydrolysis reactions. The role of enzymes during digestion • The role of amylase is to hold starch in its active site and put stress on the covalent bonds that bind the glucose molecule together – – With enough stress the surrounding thermal energy will likely be enough to provide molecular motion to break the bonds. Enzymes do not necessarily cause a chemical reaction; they just make them more likely to occur at a given temperature. Examples of digestive enzymes • There are many enzymes specific to the types of carbohydrates we ingest. There are also many protease enzymes that help our bodies digest proteins. • – – Some protease enzymes work within a protein by recognizing specific amino acid pairs. Some digest proteins from the outer ends and work inward. Salivary amylase Pepsin (a protease) Pancreatic lipase Source Salivary glands Stomach cells Pancreas cells Substrate Amylose (starch) Proteins (polypeptides) Lipids Products Maltose and glucose Amino acids Glycerol and fatty acids Optimum pH Neutral (pH 7) Acidic (pH 3) Neutral (pH 7) The Human Digestive System • Most of the human digestive system consists of a tube called the alimentary canal. The alimentary canal consists of: • – – – – – – Mouth Esophagus Stomach Small intestine Large intestine (colon) Rectum • Food that you eat must either be digested and absorbed by the body or remain undigested and be eliminated as solid waste (feces). Role of stomach, small intestine and large intestine • Stomach – – – Food is brought to the stomach by the esophagus (muscular tube) After swallowing the food is forced down to the stomach by a sequence of muscle contractions called peristalsis. Once food enters the stomach it is held for a period of time to mix with gastric juices Role of the stomach • Gastric juice is a mixture of three secretions from the cells of the stomach inner lining: – – – • • Pepsin – a protease enzyme active in an acidic pH Hydrochloric acid – helps degrade and break down foods and creates the acidic environment necessary for pepsin to be active Mucus – lines the inside of the stomach wall to prevent damage from the hydrochloric acid The muscular wall of the stomach creates a churning motion in order to mix food with gastric juices. After a specific period of time the valve at the lower end of the stomach opens to allow food to enter into the small intestine Role the small intestine • Small Intestine – The duodenum is the first portion of the small intestine In the duodenum three accessory organs secrete juices into the small intestine to continue digestion – • • bile: from the liver and the gall bladder trypsin (a protease), lipase, amylase, and bicarbonate from the pancreas Role of the small intestine • In the small intestine molecules are produced that are small enough to be absorbed The inner wall of the small intestine is made of villi (small finger-like extensions. • – – – – – – Each villus contains a capillary bed (small vessel of the circulatory system) And a lacteal (a small vessel of the lymphatic system) If the wall of the small intestine were smooth, there would be very little surface area for absorption to take place The function of the villi is to increase surface area for absorption of molecules such as glucose, amino acids, and fatty acids. Most absorbed molecules are taken in through capillary beds within each villus Fatty acids however are taken in through the lacteal Role of the small intestine • • • All absorbed molecules are taken to a variety of body cells by the circulatory system. Within the body cells, molecules may be used for energy (ex. Glucose) or may be used to help build larger molecules within the cell (ex. Amino acids) Assimilation is the process of bringing the nutrient molecule to the body cells when they are used for building larger molecules. Role of the Large Intestine • Large intestine – – – – The majority of useful nutrients are absorbed while food is inside the small intestine The remnants of the original food at the end of the small intestine is undigested (and unabsorbed) Most of the water that we drink or that is part of the food we eat is also still present Water in the alimentary canal is beneficial because it keeps the moving food in a fluid environment Role of the Large Intestine • A very large number of naturally occurring Escherichia coli bacteria live in the large intestine – – – These bacteria are mutualistic organisms We provide nutrients, water, and a warm environment and the bacteria synthesize vitamin K and maintain the healthy environment Any undigested food by the bacteria is eliminated as feces The process of digestion requires “juice” • Salivary glands – – • Saliva is secreted into the mouth Saliva includes the first of the many digestive enzymes, salivary amylase. Gastric glands – – Located in the inner lining of the stomach. Secrete mucus, hydrochloric acid, and pepsinogen (the precursor to pepsin – an enzyme that begins protein digestion) The process of digestion requires “juice” • The pancreas – – – Pancreatic juice is sent to the small intestine by the pancreas through a duct. This juice contains another protease enzyme, additional amylase, and lipase. A form of hydrogen carbonate is present to neutralize the acid fluid of the stomach The process of digestion requires “juice” • The liver – – – The liver secretes bile into the small intestine Bile can come straight from the liver or from the gall bladder (where it is stored) Bile emulsifies lipids and increases the surface area of lipids for lipase to break down The process of digestion requires “juice” • Intestinal glandular cells – – – – Some of the cells that line the small intestine are called glandular cells. Glandular cells secrete a variety of digestive enzymes Some enzymes are added to the partially digested fluid within the small intestine and some enzymes remain attached to the villi cells. The attached enzymes are also referred to as “membrane-bound” enzymes and they catalyze digestive reactions as the undigested food flow past them in the lumen of the small intestine The cells of the exocrine glands • • Exocrine glands are a collection of cells that produce and secrete a product which is sent to another part of the body by way of ducts. Often the secretion is a protein, which many of the digestive enzymes are proteins. The cells of the exocrine glands • Some of the major steps of protein synthesis and secretion are outlined below: – – – – – – – mRNA is transcribed from a gene of DNA mRNA is translated at a ribosome in the cytoplasm of the cell ribosomes are often attached to endoplasmic reticulum making rough ER synthesized proteins at ribosomes move through the channels of the ER and reach a golgi body Golgi bodies package protein into a vesicle Vesicles fuse with the plasma membrane to release their contents outside the cell in a process called exocytosis (or secretion) Several of the steps of protein synthesis and secretion require ATP, therefore these cells typically contain a large number of mitochondria The cells of the exocrine glands • Exocrine glands typically contain a large number of ribosomes, a large amount of endoplasmic reticulum, and many golgi bodies, mitochondria, and vesicles. The cells of the exocrine glands • The arrangement of exocrine gland cells into acini and ducts – – – – Exocrine glands secrete a product into a duct to be transported to a specific location throughout the body Exocrine glands surround the end of every small branch of the pancreatic duct All the cells surround the small branches (ductule) secrete digestive enzymes into it. The ductule takes the secretion into increasingly larger ducts until the pancreatic duct is reached Components of saliva, gastric juice, and pancreatic juice • Saliva: – – – • Solvent is water Amylase Mucus Gastric juice: – – – – Solvent is water Mucus Hydrochloric acid Pepsin (secreted as pepsinogen) • Pancreatic juice: – – – – Solvent is water Amylase Bicarbonate Trypsin (secreted as trypsinogen) – Lipase Control of gastric juice secretion • • Digestive juices are not secreted at all times, only as needed Specific juices need to be secreted at the right times to help hydrolyze the molecule that is in need of digestion Control of gastric juice secretion • Ivan Pavlov experiments showed that seeing and smelling food can begin the process of digestion (salivation) – – – Drops of saliva were measured from dogs that were being prepared to be fed The dogs nervous system influenced the secretion from the salivary glands The same applies to gastric juices from the stomach, the sight or smell of food can initiate gastric juice secretion Control of gastric juice secretion • • • • After food enters the stomach, receptors within the stomach lining are stimulated and will send sensory signals to the brain. The brain will respond by causing the stomach to secrete more gastric juice Distension (swelling) of the stomach results from the production of the hormone gastrin The hormone maintains the release of gastric fluid, and the hydrochloric acid Membrane-bound digestive enzymes • • Some digestive enzymes do not mix with the food to catalyze hydrolytic reactions and digest the food Instead these enzymes remain in the membranes of the cells that line the small intestine Membrane-bound digestive enzymes • Maltase is an example of a membrane-bound enzyme. – – – – Maltase hydrolyzes the disaccharide maltose into two glucose molecules Maltase remains embedded in the inner epithelial cell membranes of villi and microvilli Maltose floats to the active site on Maltase and the reaction is catalyzed The advantage of membrane-bound enzymes is that these enzymes remain in the small intestine for longer than free-floating enzymes, and the product (glucose) is already in place for absorption Humans cannot digest cellulose • • Although a large number of mammals are herbivores, not a single mammal produces an enzyme to digest cellulose Cellulose is a polysaccharide carbohydrate composed of thousands of glucose monosaccharides Humans cannot digest cellulose • Herbivorous mammals contain a large colony of mutualistic microorganisms which produce cellulase (the enzyme necessary to hydrolyze cellulose into glucose – – Even with the help from bacteria the energy yield from plant material is small These animals must ingest a large amount of plant material Humans cannot digest cellulose • • Humans do not have a mutualistic relationship with bacteria that produce cellulase The large majority of plant material that humans eat makes its way to the alimentary canal and exits the body in feces Why doesn’t the alimentary canal digest itself? • • • • Protease enzymes such as pepsin and trypsin hydrolyze peptide bonds within proteins A fully active protease enzyme cannot distinguish between the proteins in food and the proteins that makeup the human body In order to prevent the hydrolysis of human proteins, pepsin and trypsin are initially synthesized in a form that is not chemically active The inactive forms of these enzymes are called zymogens Why doesn’t the alimentary canal digest itself? • The inactive structure of pepsin contains 44 additional amino acids and is known as pepsinogen – – – Pepsinogen is produced in the lining of the stomach wall and remains in the zymogen form until it reaches the stomach cavity When pepsinogen encounters the hydrochloric acid of the stomach the additional 44 amino acids are removed and the enzyme becomes pepsin The inner lining of the stomach are protected from the hydrochloric acid by mucus Why doesn’t the alimentary canal digest itself? • Trypsinogen is the inactive structure of trypsin and is synthesized in the pancreas and is one of the components of pancreatic juice – – – Pancreatic juice is brought to the duodenum of the small intestine by the pancreatic duct When partially digested food enters the duodenum, an enzyme known as enterokinase (or enteropeptidase) is produced. This enzyme converts trypsinogen into trypsin How do we digest lipids? • Lipids have many functions for the human body – • • • Ex. Glycerol and fatty acids help synthesize phospholipids for cell membrane structure Lipids are relatively insoluble in water which makes up the aqueous liquid in the alimentary canal In the aqueous environment lipid molecules “stick together” or coalesce The coalesced globules have relatively little surface area compared to their overall volume How do we digest lipids? • • • • Lipase is the enzyme that hydrolyzes triglyceride lipids The partially digested lipids are added to the rest of the partially digested molecules in the duodenum of the small intestine When lipase encounters the coalesced globules of it catalyzes the hydrolysis of the lipid molecules on the outside The interior molecules will rarely encounter a lipase molecule How do we digest lipids? • Bile produced by the liver is added to the partially digested food in the duodenum – Bile molecules contain both hydrophobic and hydrophilic ends, and thus are partially soluble in both lipids and in water Bile molecules insert themselves between lipid molecules and prevent them from coalescing – • • • This process is known as emulsification Emulsification doesn’t change the molecular structure of lipids but converts them into smaller pieces The same volume of lipid now has a much greater surface area for lipase to attach to How do we digest lipids? • The overall structure of lipase is hydrophilic because many amino acids are polar – – – This allows lipase to be soluble in the aqueous environment of the alimentary canal However, the amino acids at the active site of lipase are predominately non-polar which accepts the hydrophobic substrate (lipid) into the relatively hydrophobic active site This solves yet another problem of digestion of lipid molecules What causes stomach ulcers? • • • Until fairly recently, scientists believed that nothing could live in the highly acidic environment of our stomachs (~pH 2) In 1982-1983 Dr. Barry J. Marshall and Dr. J. Robin Warren isolated living bacterial cells from the stomach lining of patients suffering from stomach ulcers and gastritis (inflammation of the stomach lining) At that time most scientists believed stomach ulcers and gastritis were caused by excess hydrochloric acid, possibly brought on by stress What causes stomach ulcers? • • Marshall and Warren have since shown the following – Helicobacter pylori is a bacterium that survives when introduced into the stomach, it then burrows itself beneath the mucus layer and infects the stomach lining cells – Helicobacter pylori employs the enzyme urease the create ammonia to help neutralize the stomach acid – Helicobacter pylori infection of the stomach lining can cause ulcers and gastritis – Patients with antibiotics tend to respond well to treatment – Patients with gastritis (infected with H. pylori) for many years (~20-30 years) are much more prone to stomach cancer than the general population – H. pylori may be the most common bacterial infection in the world, over 3 billion people are estimated to be infected Dr. Marshall and Dr. Warren were awarded the Nobel Prize in medicine in 2005 for their work Overview of small intestine structure • The small intestine is composed of three sections – – – The short duodenum where secretions from the pancreas and liver are added to the partially digested contents The jejunum The ileum Overview of small intestine structure • Partially digested food passes through the small intestine because of contractions of two layers of smooth muscle within the walls – – These layers are the longitudinal and circular muscle layers The contraction of these muscle layers is known as peristalsis, this keeps the food moving throughout the alimentary canal Overview of small intestine structure • The innermost cellular lining of the small intestine is known as the intestinal mucosa – – This tissue is in direct contact with the partially digested food and is responsible for absorption The mucosa forms the villi of the small intestine to increase the surface area for absorption Adaptations of villi epithelial cells for efficient absorption • Digested molecules must pass through the villi cells and are absorbed by either a capillary bed or a lacteal Microvilli • – – The surface of each villus cell that faces into the lumen (cavity) of the small intestine contains many microscopic finger-like projections known as microvilli The function of microvilli is to further increase the surface area for absorption Adaptations of villi epithelial cells for efficient absorption • Mitochondria and pinocytotic vesicles – – – Some molecules absorbed through the villi are absorbed using an active transport mechanism The epithelial villi cells require ATP for active transport which explains why they contain mitochondria Pinocytotic vesicles are visible near the plasma membrane of the villi cells, pinocytosis is another active transport mechanism used to absorb molecules Adaptations of villi epithelial cells for efficient absorption • Tight junctions – – – Most cells in the body are surrounded by interstitial fluid which allows molecules to pass between them Epithelial villi cells have to provide a selective barrier to prevent pass of intercellular fluid and dissolved molecules between adjoining cells Epithelial villi cells are sealed to each other by membrane-to-membrane ‘seals’ called tight junctions Transport mechanisms used to absorb foods • Facilitated diffusion – Many digested molecules are small enough to fit through the membrane, their concentration gradients permit diffusion, but their polarity prevents passage through the hydrophobic interior of the plasma membrane • • • Protein channels within the microvilli membranes solve this problem These channels have relatively non-polar amino acids that make up the outer perimeter of the channel and polar amino acids forming the interior of the channel This interior channel allows the appropriately sized, polar molecules to diffuse from the lumen into the cytoplasm of the villi cells Transport mechanisms used to absorb foods • Active transport – – – – The plasma membrane of the villi cells also contain membrane proteins that require ATP to transport molecules across the membrane Many molecules being absorbed do not have a concentration gradient across the plasma membrane Active transport allows transportation across the membrane regardless of molecular concentrations The mitochondria within the villi cells produces the ATP for this process Transport mechanisms used to absorb foods • Pinocytosis – – – Microvilli membranes also perform endocytosis (pinocytosis) Small droplets of fluids within the lumen are surrounded by membrane and form pinocytotic vesicles These vesicles are taken into the cytoplasm of the villus and later the contents are released into the cytoplasm Substances remaining after digestion • Some substances cannot be digested and are never absorbed, these substances continue to the large intestine and are released as feces – – – – – Cellulose – in the cell walls of food from plants Lignin – another component of plant cells Bile pigments – give characteristic color of feces Bacteria – normal inhabitants of the digestive system Intestinal cells – break off as foods move through the lumen Functions of the Liver • Circulation of blood to and from the liver – – – The liver receives blood from major blood vessels and is drained by one major blood vessel. The hepatic artery is a branch of the aorta and carries oxygenated blood to the liver The two blood vessels carry blood into the ‘capillaries’ of the liver which are called sinusoids Sinusoids are then drained by the hepatic vein (the only blood vessel that takes blood away from the liver) Functions of the Liver • • The hepatic portal vein receives the blood from the capillaries within the villi of the small intestine The blood within the hepatic portal vein varies in two ways from the blood that enters into the liver from the hepatic artery • • it is low pressure, deoxygenated blood because it has been through a capillary bed it varies in quantity of nutrients (i.e. glucose) depending on the types of food ingested and the timing of ingestion, digestion, and absorption of foods within the small intestine Functions of the Liver • • Blood within the hepatic vein is also low pressure, deoxygenated blood but it doesn’t vary in nutrients as much as the hepatic portal vein Stabilization of nutrients within the hepatic vein is one of the major functions of the liver especially storage of nutrients Functions of the Liver • Sinusoids are the capillaries of the liver – – – – The function of the liver is to remove some substances from the blood and add others Hepatocytes (liver cells) are responsible for the removal or addition of substances Oxygen-rich blood from the hepatic artery and nutrient rich blood from the hepatic portal vein both flow into the sinusoids of the liver Exchanges occur between the blood and the hepatocytes within the sinusoids Functions of the Liver • Sinusoids differ from typical capillary beds: – – – – sinusoids are wider than capillaries sinusoids are lined by endothelial cells with gaps between them the gaps allow large molecules (proteins for example) to be exchanged between hepatocytes and the bloodstream hepatocytes are in direct contact with blood components making all exchanges more efficient Functions of the Liver • • sinusoids contain Kupffer cells that help break down older red blood cells for recycling their components sinusoids receive a mixture of oxygenated blood (from hepatic artery) and nutrient rich blood (from hepatic portal vein). This mixture drains into small branches of the hepatic vein Functions of the Liver • Regulation of nutrients in the blood – Solutes that are dissolved in blood plasma vary a little in concentration, but each type of solute has a normal range they must maintain for homeostasis. – Any concentration below or above this range creates physiological problems within the body Functions of the Liver • Glucose for example: – – – – For most people, glucose levels in the blood are lowest in the morning and highest after meals When a meal is digested that is high in carbohydrates (starch for example) the hepatic portal vein contains blood with a very high concentration of glucose When this blood enters the sinusoids of the liver, some excess glucose is taken in by the surrounding hepatocytes and converted into the polysaccharide glycogen This keeps glucose levels within the normal range Functions of the Liver • • • • • If you haven’t eaten carbohydrates for a long time blood glucose levels decrease as cells are using glucose for cellular respiration To keep glucose levels within the normal range, the stored glycogen is reconverted into glucose and added into the bloodstream of the sinusoids The homeostatic mechanisms that work to maintain glucose levels are regulated by the hormones insulin and glucagon from the pancreas When blood glucose levels are in the upper end of the normal range, insulin is produced to stimulate hepatocytes to take glucose in and convert it to glycogen When blood glucose levels are in the lower end of the normal range, the pancreas produces glucagon to stimulate hepatocytes to convert glycogen back into glucose Nutrient storage Nutrient Relevant Information Glycogen Polysaccharide carbohydrate Iron Iron is removed and stored following breakdown of erythrocytes and hemoglobin molecules Vitamin A Associated with good vision, one of the first signs of deficiency is ‘night blindness’ Vitamin D Vitamin D is often added to milk and milk products in some countries Synthesis of plasma proteins and cholesterol Molecule(s) Relevant Information Plasma proteins Albumin – helps regulate osmotic pressure of fluids in the body Fibrinogen – soluble form of blood clotting protein which is converted to fibrin when clot is needed Globulins – widely diverse group of blood proteins not all of which are produced in the liver Cholesterol Some cholesterol is ingested and absorbed in foods, some is synthesized in the liver Some cholesterol is used to produce bile, while some is carried in the bloodstream to be used for cell membranes Detoxification Molecule Source Ethanol Alcoholic drinks Food preservatives Added to foods to retard spoiling Pesticides Often used on produce Herbicides Also used on produce Functions of the Liver • Alcohol consumption damages liver cells – – – – Drinking often and heavily damages liver cells The hepatic portal vein brings absorbed alcohol first to the liver Any alcohol not removed is eventually brought back through the liver sinusoids many times by way of the portal artery (within the bloodstream) Each time the blood passes through the liver, hepatocytes attempt to remove the alcohol from the bloodstream Functions of the Liver • Alcohol has a magnified effect on liver tissue compared to other tissues in the body Long term alcohol abuse results in three primary effects on the liver: • – – – Cirrhosis – This is scar tissue left when areas of hepatocytes, blood vessels, and ducts have been destroyed by exposure to alcohol. Areas of the liver that show cirrhosis no longer function. Fat accumulation – Damaged areas of the liver will build up fat in place of normal liver tissue Inflammation – This is the swelling of damaged liver tissue due to alcohol exposure; sometimes referred to as alcoholic hepatitis http://www.tuhc.com/CPM/cirrhosis-liver.gif http://www.herbalprovider.com/imgs/yahoo/fatty-liver-cirrhosis.jpg Functions of the Liver • • • • Frequency and volume of alcohol consumption are both positively correlated with liver damage Data shows that females are more susceptible to liver damage than males Liver damage that is not too severe is at least partially reversible (the liver can regenerate some of its damaged areas if the person ceases to consume excessive amounts of alcohol) Liver damage can be fatal when the damage becomes too severe Functions of the Liver • The liver recycles components of erythrocytes and hemoglobin – – – Erythrocytes have a typical life span of about 4 months Every red blood cell needs to be replaced every 120 days or so by the tissue of the bone marrow This is because erythrocytes are anucleate (have no nucleus) and cannot undergo mitosis to form new blood cells Functions of the Liver • • • • • As erythrocytes approach their life span the cell membrane becomes weak and eventually ruptures More often this occurs in the spleen or bone marrow, but it can also happen anywhere in the bloodstream When the red blood cell ruptures it releases hemoglobin molecules As the blood circulates through the sinusoids of the liver the circulating hemoglobin molecules are ingested by Kupffer cells within the sinusoids This ingestion is by phagocytosis because hemoglobin molecules are very large proteins Functions of the Liver • • Hemoglobin consists of four polypeptides (globin) and a non-protein molecular component at the center of each globin called a heme group (at the center of each heme group is an iron atom Hemoglobin consists of four globins, four heme groups, and four iron atoms http://www.sciencecases.org/tazswana/hemoglobin.gif Functions of the Liver • Within the Kupffer cells hemoglobin is disassembled into its component parts – – – The four globin proteins are hydrolyzed into amino acids. The amino acids are released back into the bloodstream to become available for any cell for protein synthesis The iron atom is removed from each heme group. Some iron is stored within the liver and some is sent to the bone marrow to be used in production of new erythrocytes Once iron is removed from hemoglobin, the remnants of the molecule are called bilirubin or bile pigment. This is absorbed by the hepatocytes and becomes a key component of bile.