Bio Week 11 PDF

The Digestive System The functions of the digestion system include: Ingestion Occurs when food/liquids enter the digestive tract via the oral cavity An active process involving conscious choice and decision making Secretion Release of water, acids, enzymes, buffers, and...

The Digestive System The functions of the digestion system include: Ingestion Occurs when food/liquids enter the digestive tract via the oral cavity An active process involving conscious choice and decision making Secretion Release of water, acids, enzymes, buffers, and salts from the epithelium of the digestive tract and glandular organs into the digestive tract Secretions aid in digestion and absorption Secretions provide a nonspecific defense against the corrosive effects of digestive acids and enzymes, mechanical stresses and bacteria that are either swallowed with food or live in the digestive tract Mechanical processing Physical breakdown of food into smaller particles that makes materials easier to propel along the digestive tract Mixes the food with secretions and increases the surface area of the food, both of which make it more susceptible to attack by enzymes Digestion Chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium Absorption The movement of organic molecules, electrolytes (inorganic ions), vitamins, and water across the digestive epithelium, into the interstitial fluid of the digestive tract, then into the blood or lymph Defecation Elimination of wastes, indigestible substances, bacteria, dead cells from the body The four major layers of the digestive tract are the: (TLO T8A2); 1. Mucosa Inner lining of the digestive tract Consists of an epithelium, moistened by glandular secretions Epithelium varies with location, function and the stresses placed on it. The locations that encounter severe mechanical stresses, e.g., the oral cavity, pharynx and oesophagus, are lined with stratified squamous epithelium. The locations where absorption occurs, e.g., stomach, small intestine and large intestine, are lined with simple columnar epithelium with goblet cells. 2. Submucosa Dense, irregular connective tissue beneath the mucosa Has numerous blood vessels and lymphatic vessels and a network of sensory neurons In some locations, it contains glands that secrete buffers and enzymes into the lumen of the digestive tract Binds the mucosa to the muscularis externa 3. Muscularis externa Dominated by smooth muscle cells The inner circular layer is essential for agitation and in the formation of valves. The outer longitudinal layers of muscle is essential for mechanical processing and in moving materials along the digestive tract. 4. Serosa Covers the muscularis externa along most portions of the digestive tract. The digestive system consists of a muscular tube, the digestive tract also called the gastrointestinal tract, and various accessory organs. The digestive tract is the passageway the food travels upon entry into the body to exit. It begins at the oral cavity, continues through the pharynx, oesophagus, stomach, small intestine, and large intestine, which opens to the exterior at the anus. Accessory organs secrete their products into ducts that empty into the digestive tract. These secretions prepare organic and inorganic nutrients for absorption across the epithelium of the digestive tract. Accessory digestive organs include teeth, tongue, and various glandular organs, such as the salivary glands, liver, gallbladder, and pancreas. ORGANS: Oral cavity The major digestive functions of the oral cavity are; ingestion sensory analysis of food before swallowing lubrication by mixing with mucus and saliva mechanical digestion through the actions of the teeth, tongue and palatal surfaces initiation of carbohydrate digestion by salivary amylase initiation of lipid digestion by lingual lipase Liver All blood leaving the absorptive surfaces of the digestive tract enters the hepatic portal system and flows into the liver. The liver carries out more than 200 functions. They fall into 3 general categories; 1. Metabolic regulation; modifies and stores excess glucose, amino acids, and fatty acids therefore stabilising blood glucose levels converts ammonia to urea which can be excreted by the kidney removes other waste products, circulating toxins, and drugs from the blood for inactivation, storage, or excretion absorbs fat-soluble vitamins and vitamin B12 absorbs and breaks down antibodies, releasing amino acids for recycling absorbs and recycles hormones including adrenaline, noradrenaline, insulin, thyroid hormone, and steroid hormones 2. Haematological regulation; synthesis and release of the plasma proteins, including albumin, transport proteins, clotting proteins, and complement proteins. 3. Bile production; bile salts break lipid droplets apart in a process called emulsification, which dramatically increases the surface area accessible to the water-soluble enzyme, lipase, that digests lipid. Gall bladder The gall bladder stores and concentrates bile prior to its excretion into the small intestine. When full the gallbladder contains 40-70 mL of bile. The composition of bile gradually changes as it remains in the gallbladder. Much of the water is absorbed and the bile salts and other components of bile become more concentrated. Stomach The stomach is a distensible organ that connects the oesophagus to the duodenum and temporarily stores ingested food. It is shaped like an expanded J. The shape and size vary from one meal to the next and from individual to individual. When the stomach is empty the wall lies in folds called rugae, these allow the stomach to expand when full. The epithelial cells of the stomach produces an alkaline mucous layer that protects the epithelial cells against the acid and enzymes in the gastric lumen. The gastric glands are dominated by parietal and chief cells. Parietal cells; secrete intrinsic factor - required for the absorption of vitamin B12 in the small intestine secrete hydrochloric acid (HCl) - pH 1.5-2, kills most microorganisms, denatures proteins, breaks down connective tissue, activates pepsinogen. Chief cells; secrete pepsinogen - becomes pepsin when activated by HCl, digests protein. Longitudinal and circular muscle layers in the wall of the stomach allows for mechanical digestion. When the bolus combines with the secretions of the glands of the stomach, it produces a viscous highly acidic mixture of partially digested food called chyme Pancreas The pancreas is both and endocrine and exocrine organ. Endocrine - because it secreted the hormones, insulin and glucagon into the bloodstream. Exocrine - because it secretes pancreatic juice into the duodenum. In pancreatic juice contains; an alkaline solution containing bicarbonate - neutralises the HCl that enters the duodenum from the stomach enzymes for chemical digestion, including lipase, amylase and proteases. There are a number of proteases made in the pancreas, these are all produced and secreted as inactive enzymes. If the enzyme has an 'ogen' at the end like trypsinogen its inactive, or if it has a 'pro' at the start like proelastase its inactive. You do not need to know all of the proteases made in the pancreas. Small intestine The small intestine begins at the pyloric sphincter and ends at the large intestine. It has a large surface area due to its length, but also circular folds, villi and microvilli which are epithelial adaptations to increase its function in absorbing nutrients. The small intestine is divided into three sections; 1. Duodenum - first and shortest segment of small intestine, receives chyme from the stomach and digestive secretions from the pancreas and liver. The secretions from the pancreas includes a buffer, bicarbonate, that neutralises the stomach acid that enters the duodenum. This prevents the stomach acid damaging the absorptive surfaces of the small intestine. 2. Jejunum - middle segment of small intestine, is the location where the most chemical digestion and nutrient absorption occurs. 3. Ileum - final segment of small intestine that ends at the ileocecal valve which is a sphincter that controls the flow of material from the ileum into the cecum of the large intestine. Large intestine The large intestine beings at the end of the ileum and ends at the anus. The large intestine can be divided into three parts; the cecum, colon and rectum. 1. Caecum - an expandable pouch that receives the chyme from the ileum, starting the process of compaction. 2. Colon - the largest portion of the large intestines. It has a larger diameter and a thinner wall than the small intestine. The wall of the colon forms a series of pouches (haustra) that permit the colon to expand and elongate. The colon can be divided into four regions, the ascending, transverse, descending and sigmoid colon. 3. Rectum - last part of the digestive tract. It is an expandable organ for the temporary storage of feces. The last portion of the rectum is the anal canal. The large intestine; compacts and stores feacal material prior to defecation produces mucous for lubrication of faecal material reabsorbs water and other useful substances such as electrolytes, vitamins and bile salts It has no role in the production of enzymes for chemical digestion. Any chemical digestion that occurs in the large intestine results from enzymes from the small intestines or bacterial action. Bacteria in the large intestine produce vitamins (K, B5, biotin) and break down indigestible carbohydrates causing the release of gases.  Mechanical Digestion Mechanical Digestion is also known as Mechanical Processing. This may or may not be required after digestion. You can swallow liquids immediately but you must chew most solid food first. MOUTH - Mastication - teeth (tearing and mashing), tongue (squashing and comparing) and palate OESOPHAGUS - Peristalsis (occurs all around the digestive tract) STOMACH - Mixing, Churning SMALL INTESTINE - Segmentation The major digestive functions of the oral cavity are ingestion, sensory analysis, mastication or chewing lubrication. By mixing mucus and saliva and initiation of chemical digestion, food in the mouth stimulates receptors that trigger an involuntary twine reflex. The chewing motion by teeth breaks down large food particles into smaller pieces that can be easily swallowed. This increases the surface area of the food particles that can come into contact with digestive enzymes in the saliva allowing chemical digestion to occur. The tongue which is bulky and muscular, is extremely agile and sensitive. It can push food against the teeth to be chewed but avoids being bitten itself. It is sensitive enough to feel a stray hair in your food. The palate makes it possible to breathe through the nose whilst chewing the food. Peristalsis consists of waves of muscular contractions that move the food along the length of the digestive tract. During Peristaltic movement, the circular muscles contract behind the bolus while the circular muscles ahead of the bolus relax and the longitudinal muscles ahead of the bolus contract and shorten the segment. This moves the bolus forward. The arrival of the bolus in the stomach stretches the stomach and triggers a receptive relaxation response. Pacemaker cells in the stomach, particularly in the smooth muscle of the stomach initiates parasal contractions that sweep towards the small intestines every 20 seconds when a small amount of the bolus is forced through the sphincter into the duodenum. The pyloric sphincter then contracts, After 30 minutes, the pace picks up and the smooth muscles of the stomach contract more forcefully, churning and mixing the bolus with gastric secretions. Motility of the small intestines serves three functions. It mixes chime with the intestinal juices and the bile as well as the pancreatic juice. It turns the chime. Originally food is food and then it becomes bolus. Once it is in the small intestine it becomes chime. It is mixing the chime to increase contact of the chime with the mucosa of the small intestine for absorption and also for contact with digestive enzymes for digestion. Mechanical Digestion in the small intestine is moving the residue into the large intestine. Motility begins with segmentation so it is similar to peristalsis, however with segmentation it is not moving material along the digestive tract. Instead the rhythmic timing of the muscle contractions force the chime backwards and forwards so it is mixing the contents in the chime with the intestina fluid and allowing more time for digestion and absorption. As the nutrients are absorbed and the content decreases with the small intestine, there is a decrease in stretch and this signifies the beginning of peristalsis which moves the residue towards the large intestine. Most ingested organic materials (carbohydrates, lipids, protein) are complex chains of simpler molecules. The digestive system first breaks down the physical structure of the ingested material, then disassembles the component molecules into smaller fragments. Digestive enzymes break the bonds between the component molecules of carbohydrates, lipids and proteins. In a typical dietary carbohydrate, the component molecules are monosaccharides. In proteins, they are amino acids. In lipids, they are fatty acids. In the diagram below the food being broken down is the substrate. The substrate binds to the enzyme at the active site, forming an enzyme-substrate complex. The classes of digestive enzymes differ with respect to their targets. An enzymes specificity is due to the ability of its active site to bind only to substrates with particular shapes and charges. Substrate binding typically produces a temporary reversible change in shape of the enzyme that may place physical stresses on the substrate molecules, leading to product formation. Product release frees the enzyme, which can then repeat the process.  Digestive enzymes act outside cells in the lumen of the digestive tract. Digestive enzymes are act in secreted by Salivary glands Tongue Mouth Stomach Stomach Pancreas Small intesti Small intestine (microvilli) This is a simplified table of digestive enzymes and digestive aids that are discussed in the above video. Please note that salivary amylase can break polysaccharides into trisaccharides and disaccharides and pancreatic amylase can digest any undigested polysaccharides and trisaccharides to disaccharides. Also, proteases can digest proteins and polypeptides. I have simplified this detail in the table for ease of discussion. A digestive aid is a substance that enhances digestion but is not an enzyme so does not breakdown specific bonds between molecules (e.g., bile). Chemical Digestion: THE MOUTH: This is where we ingest food. The food then travels through the oesophagus and into the stomach. From the stomach, it then travels into the small intestine and then into the large intestine and then out through the rectum and anus. The three organs make digestive enzymes. There are three classes of organic molecules that are proteins, carbohydrates and lipids. Fats, protein digestion starts in the stomach and that is because there are no enzymes in the mouth that can work on protein. Protein digestion starts in the stomach and that is because there is no enzymes in the mouth that can work on protein. In the stomach - the protein is broken down to polypeptide and that occurs by the enzyme pepsin. Pepsin is actually produced as a precursor protein. Pepsinogen Pepsin is very active. If it was present all the time and there was no protein in your stomach to break it down, then it would start to break down the epithelial lining of your stomach so it is produced as a precursor protein and is activated when there is protein detected for digestion. Pepsinogen is converted into pepsin by Hydrochloric Acid, which is a digestive aid produced by the stomach. It is also providing the optimal pH for Pepsin to work. Proteins are broken down into polypeptides and then two peptides and this predominantly occurs in the small intensities due to pancreatic proteases like pepsin. The pancreatic proteases are very active and re also produced as inactive precursor proteins which need to be activated when there is presence of proteins. Each of the major pancreatic proteases will work slightly differently so they cleave the peptide bond after different amino acids. So you need all of the enzymes to be able to break down the proteins or the polypeptides into small peptides. The final step is converting peptides to amino acids, so amino acids are the smallest form of protein, and these are absorbed across the small intestine into the blood, and then they go to the liver. Now, the family of enzymes that convert peptides to amino acids are pepti AES So peptidases are, um also known as brush border enzymes. They are made by the epithelial lining of the small intestine. And that epithelial lining looks like a brush border with the micro vili. Um, which is where they, um uh, released from. So Proteins are broken down to polypeptides by pepsin polypeptides are broken down to peptides by pancreatic proteases and peptides are broken down to amino acids. Pancreatic proteases can work on protein. Carbohydrate digestion starts in the mouth, so complex carbohydrates or polysaccharides are broken down to try saccharide in the mouth by Salivary amylase. So salivary amylase s is active in the 6 to 7 p h range. Food's not in the mouth for a very long time. So this step in carbohydrate digestion is quite short. The salivary amylase will be active while it's in the mouth, but once it gets to the stomach, it's quickly inactivated by the P H of the stomach. If hydrochloric acid is being produced by the stomach that hydrochloric acid is going to in a inactivate the sliva amylase. So there's really little digestion of carbohydrates in the stomach. Most digestion of carbohydrates is gonna occur in the small intestine. So in the small intestine, trisaccharide are converted to disaccharide. Then finally monosaccharide. Now the step where trisaccharide are converted to disaccharide is mostly occurring by pancreatic amylase. There are three; Maltase, sucrose and lactose and each of these disaccharide are broken down by a specific enzyme. The three different monosaccharide are fructose, glucose and galactose. So sucrose is broken down to fructose and glucose. Maltose is broken down to glucose and lactose is broken down to galactose and glucose. Sucrose is broken down by sucres, maltose by Maltese and lactose by lactase. So where it's OS e. It is the substrate where it's a S E. It is the enzyme. These monosaccharide fructose, glucose and galactose are absorbed across the epithelial lining of the digestive tract by facilitated diffusion, where they enter the blood and then travel to the liver to then be converted to glucose. So fructose and glutose are converted to to glucose and glucose can then be used for energy or it can be stored in the form of glycogen. Fat digestion starts in the mouth. Lipid or fat or triglycerides are broken down to mono glycerides and fatty acids. So the triglycerides are broken down to mono this right and fatty acids. So unlike protein and carbohydrate, there's only one conversion. But there are still a number of different enzymes that are involved. So in the mouth it is lingual lipes. So lingual means made by the tongue. So the tongue makes lingual lipes, which starts the digestion of fat light carbohydrates. Food isn't in the mouth for very long. So lingual lipas is only active for a short amount of time. Once it moves into the stomach with the food. It will be inactivated. The inactivation isn't as fast as what it is with the carbohydrates with the amylase. Because Lipase has a larger PH range and is active at a Ph of three up to six, so it will be active a little bit longer, then amylase in the stomach. But the stomach acid will eventually inactivate lipase. So again, there's not a lot of digestion of fat in the stomach. If you read your textbook, you'll see that there is also a stomach lipase. We're not gonna talk about that in this unit because it's predominantly active in Children, and we don't need to go to that level of detail in this unit. Most of the fat digestion is broken down or occurs in the small intestine by pancreatic lipase. So of the two enzymes, pancreatic lipase is the most important one again. Lingual lipase isn't active for very long because the food is not present in your mouth for very long. Whereas pancreatic amylase sorry, pancreatic lipase is present or in contact with food for a lot longer because the food is in the small intestines for a lot longer. The other reason it's more important is because also in the small intestine there will be bile. So bile is made by the liver and stored in the small intestine there will be bile. So bile is made by the liver and stored in the gallbladder. So I'm just gonna have a look at the structure of fat. Fat is a lipid soluble droplet, whereas your lipase is water soluble. So lipas can't get into the lipid droplet because water and um, fat don't mix, so lipas can only break down what is on the surface. So what bile does is it emulsifies the fat. So instead of having one large droplet, you've got lots of small little droplets. Therefore, you have a much larger surface area for the to be able to come and, um, break down that fat molecule. Just to recap lingual lipase, it's only present for a very short amount of time. It's quickly inactivated by the stomach. Pancreatic lipase is in the presence of food for a lot longer, because foods in the small intestine for a lot longer, but also biles in the small intestine and bile emulsifies the fat increase in the surface area for low pace to be able to break down the fat. So pancreatic lipase is the main enzyme involved of converting triglycerides to mono glycerides and fatty acids.  Defecation: The last portion of the rectum is the anal canal, which contains two types of anal sphincters. The internal anal sphincter is a circular layer of smooth muscle. Being smooth muscle it is under involuntary control. The external anal sphincter encircles the distal portion of the anal canal. It is a ring of skeletal muscle fibres. Being skeletal muscle it is under voluntary control. The process of defecation begins when mass movements force faeces from the colon into the rectum, stretching the rectal wall causing the defecation reflex, which eliminates faeces from the rectum. This parasympathetic reflex triggers contraction of the sigmoid colon and rectum, and relaxation the internal anal sphincter. The presence of faeces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed. If you decide to delay defecation, it takes a few seconds for the reflex contractions to stop and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate. Rectal pressures of 15 mmHg will stimulate the urge to defecate but this can be voluntarily overridden. When rectal pressure reaches 55 mmHg, internal and external sphincters will relax, forcing elimination. This is how babies and paraplegics eliminate wastes. If defecation is delayed for an extended time, additional water is absorbed, making the faeces firmer and potentially leading to constipation. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and diarrhea can result. Metabolism Metabolism is the sum of all biochemical reactions that occur in the body. Catabolism Catabolic reations: Breakdown larger/complex organic molecules into smaller ones Release energy i.e., produce more energy than they use (exergonic) E.g., used in glycolysis, Krebs cycle, electron transport chain and digestion of food Anabolism Anabolic reactions: Use simple molecules/monomers to make larger organic molecules Consume energy i.e., use more energy than they produce (endergonic) E.g., make protein from amino acids and phospholipids from fatty acids The figure below shows examples of catabolic and anabolic reactions. Basal metabolic rate (BMR) is the minimum resting energy expenditure of an awake, alert person. (TLO T8B2) A direct method of determining the BMR involves monitoring respiratory activity. In resting individuals, energy use is proportional to oxygen consumption. The BMR estimates the rate of energy use by the body. The energy that cells do not capture and harness is released as heat. Heat is measured in calories (kilojoules). kCal (kJ) = unit of energy in foods = amount of energy needed to raise 1 kg of water by 1 degree Celsius. Male BMR Female BMR 1,700 kcal/day 1,400 kcal/day 7,100 kJ/day 5,900 kJ/day Influences of BMR Total lean mass, including muscle mass is largely responsible for the BMR. Anything that reduces lean mass will reduce BMR. Therefore it is important to preserve muscle mass when losing weight Sex - BMR is lower in females (except in pregnancy) because in general women have a higher percentage of body fat compared to males Age - BMR is faster in children and decreases with age Body temperature - increased temperatures speed up BMR (10% with one degree) e.g., an infection with a fever to 42°C increases BMR by 50% Diet/Food intake - eating speeds up BMR. Highest with protein, lower with carbohydrates and lipids. Calorie starvation can reduce BMR by 30% Exercise - increases BMR by 15-20x, exercise also increases muscle therefore BMR Hormones - the sympathetic nervous system increases rate of metabolism e.g., adrenaline, noradrenaline (stress) increases BMR, thyroid hormone is a regulator of metabolism Adenosine triphosphate (ATP) The energy currency of cells i.e., it is a molecule of stored energy which the cells can break down. Structurally, ATP molecules consist of an adenine, a ribose, and three phosphate groups. The chemical bond between the second and third phosphate groups represents the greatest source of energy in a cell. Dephosphorylated (i.e. ATP → ADP + Pi) Releases energy Cells use this energy to carry out anabolic reactions including, building new tissue and repairing damaged tissue Phosphorylated (i.e. ADP + Pi → ATP) Energy is stored in the ATP molecule Cells can use this energy for future cellular functions During digestion, carbohydrates are broken down to monosaccharides (glucose, fructose and galactose) that can be transported across the intestinal wall into the circulatory system to be transported to the liver. The liver then converts fructose and galactose into glucose. Glucose (C6H12O6) is used for: Catabolism to yield ATP Amino acid synthesis Glycogen synthesis Triglyceride synthesis (liver converts glucose to glycerol and fatty acids) Glucose Catabolism - Glucose is catabolised to yield ATP. It is also known as cellular respiration. Cellular respiration summary: Glycolysis: 2 ATP (net gain) Kreb’s cycle: 2 ATP Electron transfer: 26-28 ATP Total yield: 30-32 ATP Provides 16 kJ per gram Can be summed up with the following equation: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 32 ATP + heat 2. Glycogenolysis (breakdown of glycogen): When blood glucose levels drop, stored glycogen in hepatocytes release glucose into blood Note: skeletal muscle cells do not release glucose from glycogenolysis into blood, it is kept for their own use Triggered by glucagon and adrenaline Glucose Anabolism: Glucose cannot be stored as a monosaccharide. 1. Glycogenesis (synthesis of glycogen): Glycogen: glucose molecules joined together Allows for storage of glucose in the liver and skeletal muscle cells Formed when not needed to produce ATP inside cells or when blood glucose levels are high Triggered by insulin 2. Gluconeogenesis (synthesis of glucose from new (non-carbohydrate) sources): Production of glucose when blood glucose level is low Generation of ATP from non-carbohydrate sources e.g., amino acids, lactic acid, glycerol 60% of the body’s amino acids can be used for gluconeogenesis Triggered by cortisol and glucagon Also catabolic Lipids (triglycerides) within the body are ingested as food or synthesized by adipocytes or hepatocytes. Lipids function as an energy reserve, form phospholipid bilayers, form chemical messengers, speed up nerve impulses, cushion vital organs, and transport fat-soluble nutrients. Lipid catabolism Lipolysis - lipid breakdown Lipids are important energy reserves (37 kJ per gram) because they can provide large amounts of ATP e.g., a 18 C fatty acid can yield 144 ATP. Lipids can be stored in compact droplets in the cytosol. Because they are insoluble in water. This method saves space, but when the lipid droplets are large, it is difficult for water-soluble enzymes to get at them. For this reason, lipid reserves are more difficult to access than carbohydrate reserves. During lipolysis, triglycerides are broken down to; 1 Glycerol - enzymes in the cytosol convert glycerol to pyruvate, which then enters the Kreb's cycle 3 Fatty acids - converted to acetyl-CoA and enters Kreb's cycle Lipid anabolism If not needed, lipids are primarily stored as triglycerides in adipose tissue and liver (why excess food can lead to weight gain, even if fat content is low). Lipogenesis = lipid synthesis Occurs in the liver Glucose or amino acids can be converted into glycerol and fatty acids and assembled into triglycerides Stimulated by insulin Some fatty acids cannot be synthesised in the body (essential fatty acids) Much of the body is made of proteins, of which there are over 140,000 different types. Proteins include cell signaling receptors, signaling molecules (hormones and neurotransmitters), structural proteins, enzymes, intracellular trafficking components, extracellular matrix scaffolds, ion pumps, ion channels, O2 and CO2 transporters (hemoglobin), antibodies, clotting factors and many more. Ingested proteins are broken down to smaller peptides and then amino acids (proteolysis), by enzymes in the lumen of the digestive tract. The amino acids are transported across the intestinal mucosa, into blood, then travel to the liver to be used to create new proteins (proteogenesis). Only free amino acids are used to create protein i.e., excess amino acids are not stored for later use as the body has no capacity or mechanism for their storage. Excess amino acids are converted into glucose or triglycerides. Protein catabolism Proteolysis (protein breakdown) Proteins only broken down for energy (17 kJ per gram) under special circumstances. Worn out cells are broken down to release amino acids which are recycled into new proteins Liver cells can convert amino acids into fatty acids or glucose During starvation the body can break down protein e.g. in muscle is broken down to amino acids which can then be used to produce glucose The processing of amino acids results in the creation of metabolic intermediates, including pyruvate and acetyl CoA therefore amino acids can serve as a source of energy production through the Krebs cycle. However protein catabolism produces toxic ammonium ions, which are metabolised to urea in the liver. Extensive protein catabolism threatens homeostasis by destroying structural and functional components of cells. Protein anabolism Proteogenesis (protein synthesis). Formation of peptide bonds between amino acids to produce proteins Occurs in the ribosomes Requires energy Under direction of DNA and RNA Adequate protein is essential for growth Metabolism is controlled by the thyroid gland. It produces thyroid hormone which affects almost all cells. It binds to mitochondria to increase ATP production. It also influences genes that increase metabolic rate. The tables below include some additional catabolic and anabolic hormones in the body help regulate metabolic processes. Produced Hormone Function in Released when blood glucose levels (BGL) are low. It stimulates the Alpha breakdown of glycogen in cells in Glucagon the liver (glycogenolysis) to the increase BGL. It also pancreas stimulates gluconeogenesis and lipolysis. Released in response to Adrenal Cortisol stress. It increases BGL by gland gluconeogenesis Released in response to activation of the sympathetic nervous system. It stimulates Adrenal Adrenaline gluconeogenesis and gland lipolysis, mobilising stored energy reserves to make them available for demands of tissue repair. Catabolic hormones Produced Hormone Function in Released when BGL are high. It promotes the uptake of glucose into muscle, Beta cells adipose tissue and the liver. Insulin in the The liver and muscle store pancreas glucose as glycogen (glycogenesis). It stimulates lipogenesis in adipose tissue. Anabolic hormone Read: Metabolic states of the body You eat periodically throughout the day; however, your organs, especially the brain, need a continuous supply of glucose. Your body processes the food you eat both to use immediately and, importantly, to store as energy for later demands. The absorptive state, or the fed state, occurs after a meal when your body is digesting the food and absorbing the nutrients (anabolism exceeds catabolism). Depending on the amounts and types of nutrients ingested, the absorptive state can last for up to 4 hours. The increase in BGL stimulates the release of insulin into the bloodstream, where it initiates the absorption of blood glucose by liver hepatocytes, and by adipose and muscle cells. Insulin also promotes the synthesis of protein in muscle. If energy is exerted shortly after eating, glucose will be processed and used immediately for energy. If not, the excess glucose is stored as glycogen in the liver and muscle cells. Excess lipids are stored as triglycerides in adipose tissues. The post-absorptive state, or the fasting state, occurs when the food has been digested, absorbed, and stored. During this state, the body must rely initially on stored glycogen. Glucose levels in the blood begin to drop as it is absorbed and used by the cells. However, due to the demands of the tissues and organs, blood glucose levels must be maintained in the normal range of 80–120 mg/dL. Glucagon is released in response to a decrease in BGL. Glucagon acts upon the liver cells, where it inhibits the synthesis of glycogen and stimulates the breakdown of stored glycogen back into glucose. This glucose is released from the liver to be used by the peripheral tissues and the brain. As a result, blood glucose levels begin to rise. Gluconeogenesis will also begin in the liver to replace the glucose that has been used by the peripheral tissues.