Nutrient metabolism and cellular biology

FNN200

Metabolism

• the sum of all chemical processes that occur within an living organism in order to maintain life 

Energy sources in the body

• macronutrients 

  • carbs, proteins, fats 

Catabolism 

• The breaking down of larger molecules to release energy trapped in chemical bonds 

  • EXOTHERMIC -deltaG

    • this process produces energy

• uses glucose, amino acids and fatty acids as fuel 

• requires ATP 

Anabolism

• simpler molecules combining to create more complex compounds

  • ENDOTHERMIC +DeltaG

  • this requires energy

Chemical energy

• stored in the chemical bonds of atoms and molecules

• measured usign a bomb calorimeter

Food energy

• measured in a bomb calorimeter

  • unit is calories (kcal)

  • 1cal is 4.18kJ

• The energy from food combustion is transfered to weater as heat which allows for the machine to measure the potential caloric energy of the food

Palmitic acid

• a fatty acid 

Biological energy from nutrient oxidation

• cellular oxidation involves the production of ATP as well as the production of heat 

  • this means that there is still a heat produced but there is 40% ATP for your cells to use as energy 

ATP

• the source of cellular energy 

• Adenosine (Adenine and ribose)

• Triphosphate 

  • where the high energy bonds are 

    • energy usage involves the breaking of the high energy bonds and therefore the release of energy 

Macronutrient energy values

• Carbs = 4kcal/g

• Lipids 9kcal/g

• Protein = 4kcal/g 

• Alcohol = 7kcal/g 

• MULTIPLY BY 4.16 TO GET KJOULES/G

Catalytic proteins

• speed up the reaciton

• take part in the reaction - these are not a part of teh final product 

Characteristics of catalytic proteins

• Specificity (lock and key)

  • require certain substrates in order tobind 

• Regulation of chemical reactions and enzyme function 

  • Covalent modification 

  • allosteric modulation

  • induction 

  • compartmentalization 

  • reversability

Enzymes involved in nutrient metabolism

• Oxidoreductases 

• transferases

• hydrolases

• lyases

• isomerases

• ligases

Lysosomes

• contain acid hydroxylases for digesting most biomolecule types 

Endoplasmic reticulum

• synthesizes proteins and lipid substances destined to be exported from the cell

Golgi

• processes molecules synthesized in the ER, packaging for exocytosis molecules and synthesizes some carbohydrates

Peroxisomes

• contain oxidases, participate in ethanol metabolism

Endosomes

• structures produced by the inagination of the cell membrane or golgi body for degradation or recycling

Oxireductases

• catalyze reactions in which one compound is oxidized and the other is reduced 

  • eg electron transport chain on mitochondria 

Transferases

• enzymes that catalyze reactions involving oxidation and reduction in which a functional group is transferred from one substrate to another

• Transaminases located in mitochondrial matrix

Hydrolases

• catalyze cleavage of bonds between carbon atoms and other atoms by adding water to them 

• eg. digestive enxymes and those in lysosomes 

Lyases

• enzymes which catalyze cleavage of carbon-carbon, carbon-sulfur and certain carbon-nitrogen bonds 

  • without hydrolysis or oxidation reduction 

• Eg. Citrate lyase freeing acetyl-CoA for fatty acid synthesis

Ligases

• enzymes which catalyze the formation of bonds between carbon and a variety of other atoms like oxygen, sulfur and nitrogen 

Isomerases

• enzyme that catalyzes the rearrangement of an atom into an isomer 

Free energy

• energy when on release is capable of doign work at a constant temperature and pressure - conditions met within the cell 

• G is free energy so delta G is the change in free energy 

Digestive system

• inter-related with other organ systems

Digestive tract organs

• Mouth-oral Cavity - Entry

• Pharynx

• esophagus - brings food to stomach

• stomach 

• small intestine

• large intestine 

• rectum 

Stomach 

• J-shaped curve to the right 

• just beneath the diaphragm and 

• connects to small intestine with the pyloric sphincter 

• sibdivided into

  • fundus, cardia and antrum

• Secretions from oxyntic glands

Oxyntic glands

• primary gastric gland 

• secrete HCl solution, pepsinoegen, intrinsic factos, mucis etc 

Esophagus

• muscular tube for food to travel

• ends with a thickened muscular ring called the lower esophageal sphincter

Mouth and pharynx 

• entry to GI tract 

• secretory glands in teh mouth release saliva to begin the chemical digestion of food, chewing and swallowing occur

Accessory organs for digestive system

• provide or store secretions delivered ultimately to the lumen

Small intestine

• divided into Duodenum (closest to eh stomach)

  • secretions from liver and gullbladder collect in the common bile duct which empty into the duodenum 

  • Secretions from the pancreas flow into the terminal aspects of the common bile duct 

• Jejunum and ileum are the distal segments of the small intestine which combine for approx 2.75 in length

• Primary site of digestion and absorption

Rugae, villi and microvilli

• Rugae are the folds in teh wal which enhance the surface area for absorption 

  • Folds are called folds of kerckring 

  • contain enterochromaffin cells for endocrine activities

• Villi are the small fingerlike projections 

• Enterocytes are columnar epithelial cells which are end-to-end in areas called tight junctions 

  • regulated by zonulin proteins 

• Microvilli are fine envaginations on luminal surface of enterocytes 

Digestion

• the process of breaking food into component parts small enough to be absorbed into the body 

  • exterior process 

Absorption

• movement of nutrients in to the blood or lymphatic (interior)

Large intestine (colon)

• segmented into cecum, colon, rectum and anal canal 

• responsible for absorbing water and some electrolytes as well as in the activities involved in defecation 

Pancreas

• secretions flow into the terminal aspect of common bile duct

Liver and gallbladder

• secretions combine into common bile duct 

Peristalsis

• basic propulsive movement 

• ring of muscular constriction encircling the gut is initiated and then begins to move forward by pushing the intestinal matter in front of the ring forward 

Distension

• strong stimulus for the origin of a peristaltic wave

  • eg if intestinal matter stretches the gut wall, a contractile ring is begin behind that point of distension 

  • also the gut can relax several centimeters on teh anus side of the distension to ease the transit of paristalsis 

Small intestinal structure

• Serosa 

  • connective tissue 

  • outer cover protecting GI tract 

• Muscularis externa 

  • 2 layers of smooth muscles (longitudinal muscle and circular muscle)

  • responsible for GI motility 

• Submucosa

  • connective tissue

  • contains BV, lymphatic vessels, nerves and lymph

• Mucosa

  • innermost mucous membrane layer 

  • produces and releases secretions for digestion 

  • lymphoid tissue protects the body 

Cephalic phase

• digestion begins in the brain with sensory inputs 

• vagal activation of gastric motility and gastric juice secretion 

Gastrin and histamine function cephalic phase

• increase gastric acid secretion

• signal that food is coming so that the body begins to prepare the stomach 

Hormone

• chemical messanger released into blood that triggers a response in a distant target organ or tissue 

Gastric phase

• Ingested food reaches the stomach 

  • food enhances gastric secretion + motility through neural stimulation 

Gastrin

• produce in G cells -Gastric atria and duodenum

• release controlled by 

  • Gastrin releasing peptide, via vagal -stimulation and the presence of small peptides and calcium 

• Function 

  • stimulate gastric acid secretion via oxyntic glands 

  • mediates pepsinogen and intrinsic factor secretion

Cholecytokinin (CCK)

• secreted from cells in duodenal mucosa

• stimulated by 

  • Phe and Tryp, intraluminal fatty acids with 9+ carbons, and partially digested proteins 

• Function 

  • stimulates the release of pancreatic enzymes 

  • stimulates gallbladder contraction 

  • allows for the relaxation of sphincter of oddi (enterence to small intestine)

Secretin

• duodenum and jejunum

• released when 

  • linterluminal pH is acidic and there are fatty acids in the lumen 

• Function 

  • stimulates the pancreatic biliary system to release bicarbonate-rich alkaine solution 

  • inhibits gastric emptying 

  • inhibits gastric acid secretion and release of peosinogen in stomach

Somatostatin

• mucosal cells through GI tract, Delta cells in pancreatic islets and nerve fibers in enteric and centeral nervous systems 

• Stimulated by 

  • fat and proteinin the intestine and acid pH in stomach antrum and duodenum 

• Function 

  • inhibit gastrin, pancreatic release and secretion of stomach acid 

  • released from G cells 

  • known as inhibitory hormone 

GIP (Gastric inhibitory)

• duodenum and jejunim

• stimulated by

  • intraluminal presence of glucose, amino accids and hydrolyzed triglycerides 

• Function 

  • intensifying glucose-stimulating release of insulin 

  • may be referred to as glucose-dependent insulinotropic peptide

Histamine

• secreted by gastric mass cells 

• function 

  • increase gastric acid secretion 

Peptide YY

• pancreas, ileum, colon cells

• Function 

  • reduce appetite

  • binds neuropeptide Y receptors in Autonomic 

  • inhibits gastric motility

  • increases water and elecytrolyte absorption

  • supresses pancreatic secretions 

Apetite regulatory hormones

• CCK, PPY, ghrelin, GIP and GLP-1

GIP

• produced and released in duodenum and jejunum 

• released in response to intraluminal presence of glucose, amino acids and hydrolzyed triglycerides as well as being released in response to an increase in diodenal hydrogen ion concentration 

• regulates gastric acid secrition 

• intensifies the release of insulil 

Phases of digestion

• Cephalic phase: when food stimulates the mechanoreceptors in the mouth and chemoreceptors in the mouth and nasal cavity

  • can be triggered by the thought of food through central pathways that relay impulses to vagal efferent nerves

• Gastric phase: begins when distention of the stomach wall stimulates mechanoreceptors

  • stimulates vagovagal and intramural reflexes whcih stimulare gastrin and other hormones to increase stomach secretions

• Intestinal Phase: as a result of both mechanical and chemical events

  • duodenal luminal distention leads to the release of entero-oxyntin

Saliva

• lubricates food for easier swallowing

• solubilizes food for taste perception

• contains blood group substances

• Main proteins are enzymes and mucus

Salival enzymes and mucus

• serous type contains a-amylase and lingual lipase

  • A-amylase begins the digestion of starches by cleaving the a1-4 links between glucose momoners

  • Lingual lipase hydrolyzes the ester bonds of triglycerides

    • important in infants 

  • Kallikrein supports the digestive process throughthe conversion of plasma protein into bradykinin 

• Mucus type contains glycoprotein mucin

Salival electrolytes

• contains sodium and potassiuum, chloride, bicarbonate, calium, magnesium and phosphate

• at rest it is hypotonic but the potassium is higher than the plasma levels 

• when the flow of saliva is stimulated the concentration of potassium decreases and the levels of others will increase 

• increasing the salivary pH to 7.8 from resting of 6.0-7.0

Esophagus

• swallowing is voluntary and involuntary 

• 3 layers of striated muscle 

  • superior, middle and interiour constrictors make up the muscular wall fo the pharynx 

• Continues until 1/3 the length ofthe esophagus and then becomes longitudinal smooth muscle 

Gerd

• reflux of stomach acid because of an issue in the esophageal lining 

Esophogeal sphincter 

• contraction associated with the peristaltic wave propelling food along the esophagus 

• food can travel in 6-9 seconds

Stomach

• depot for ingested food to regulate its release into the small intestine 

• acidic environment good for digestion and bacterial activities 

• stomach can relax to allow for contents 

• musculature of the wall is able to

Stomach glands

• contain a variety of exocrine cells 

• secrete HCl and intrinsic factor

• gastric secretions are released under control of acetylcholine, gasrtrin and histamine

Gastric juice and HCl

• create an acidic environment in the stomach

• pH  is low and is important for denaturing complex proteins, activating pepsin, liberating nutrients from organic complexes and destroying microbe

• stimulation of gastric acid release is divided into 3 phases

  • cephalic = 30%, 

  • Gastric = 60-% 

  • intestinal = 10%

Pepsin

• endopeptidase manufactured and stored in inactive pepsinogen proenzyme

Intrinsic factor

• required for the absorption of vitamin b12

Gastric emptying

• result fo peristaltic contractions in the antrum

• weak and function as mixing mechanism

• Release of gastric contents to small int3estine is regulated by inhibitory factors 

• presenc of fats stimulates CCK inhibiting emptying 

Small intestine

• chyme is mixed with pancreatic secretions and peristaltic waves propel food onwards through small intestine 

• low pH of entering chyme netrilized by the bicarbonate in the small intestine 

• cells in crypts of leiberkuhn secrete fluids with alkaline pH 

Liver hepatocytes

• breakdown sugars like galactose and fructose into glucose

• glycogen is syntehsized to store glucose in the liver 

• Gluconeogyenesis also occurs 

Pancreatic secretions

• proteins are the main part fo juice 

• trypsin inhibitor is nto a main oart of the digestive enzyme but are active in pancreatic secretory vesicles 

Pancreatic release in cephalic phase

• vagal efferents to the pancrease release acetylcholine at both ductule and acinar cells with stronger responses in acinar cells 

• intestinal phase accounts for most of the secretory response because of the presence of protein and fat in the acidic chyme mixture which elicits the release of secretin and CCK 

• products of protein and fat 

Digestive enzymes of the small intestine

• a-1-6dexrtrinase is responsible for digesting carbohydrates in the small intestine 

Small intestine absorption 

• glucose is absorbed but requires Na+ transport system in the enterocyte 

Large intestine

• responsible for absorbing water and electrolytes from the entering content

  • function which occurs predominantly in the proximal half 

• also stores fecal matter until poop 

Gallbladder 

• during fasting half of th ebiole enters the galbladder 

• bile is produced by hepatocytes 

• most of the buile acids emptied into the small intestine are reabsorbed in the distal ileum 

Bile

• contains acids, billrubin, cholesterol, fatty acids, phospholipids, electrolytes and bicarbonate 

• concentrating efforts of the wall mucosal cells allows 450 ml of bile to be stored 

• pH is isotonic and the pH is lowered