Membrane Transport, Digestion, Absorption Flashcards
What are tight junctions?
Has 2 plasma membranes connected via junctional proteins with intercellular space.
Prevent substances from moving through the spaces between cells
Ex, cell lining the bladder have tight junctions so urine can not leak out into body cavity
Maintain distinct faces of a cell within a tissue by restricting the migration of membrane proteins over the cell surface from one face to another
How are epithelial sheets connected?
Connected by junctions: tight junctions, gap junctions, and desmosome
What are desmosomes?
Hold neighboring cells firmly together like spot welds or rivets
Material can still move around in the extra cellular matrix
Provides mechanical stability for tissues such as skin that receives physical stress
Plasma membranes held together via adhesion proteins with intercellular space and keratin fibers in proteins
What are gap junctions?
Plasma membranes connected via connexons which are a hydrophilic channel that all molecules to pass between intercellular space into other cells
Channels that run between membrane pores in adjacent cells, allowing substances to pass between cells
In heart gap junctions allow the rapid spread of electric current (mediated by ions) so the heart muscle cells beat in unison
What are syncytium?
Gap junctions that interconnect within epithelium
What are the different ways of membrane transport?
Simple diffusion (through lipid or through channel)- leads to uniform distribution of solutes (all mix together)
Facilitated diffusion (carrier proteins)
Secondary active transport (uses ion gradients as energy sources)
Primary active transport (pumps such as Na+/K+, ATPase
How does lipid solubility affect a molecules ability to pass through a membrane?
The more lipid soluble the molecule, the greater its ability to pass through the membrane
Gasses all diffuse readily through the membrane
The rate of diffusion of water does not seem to be related to its solubility in lipids
What is osmosis?
The spontaneous net movement of solvent molecules through a semi-permeable membrane into a region of high solute concentration, in the direction that tends to equalize the solute concentrations on the two sides
What does hypertonic, isotonic, and hypotonic mean?
Hypertonic- more salt outside the cell and less water outside, water moves out of cell causing it to shrink, high concentration outside the cell than inside causing the water to flow out
Isotonic- concentration same inside and outside the cell
Hypotonic- less salt outside the cell and more water outside, so no water moves into the cell, cells swell up and burst
How does simple diffusion differ from facilitated diffusion?
Unlike simple diffusion, facilitated diffusion saturates as the concentration of substrate is increased.
As rate of diffusion increases into the cell, the glucose concentration outside also increases
Glucose links to glucose carrier protein outside of cell, channel closes, and opens to diffuse into cell. Hexokinase within the cell rapidly converts transported glucose to glucose-6-phosphate
How does secondary active transport work?
Symports (both into cell, same direction) or Antiports (one out and one in, different directions) transport two or more ions or other substances across the plasma membrane in concert, using energy of ion concentration gradient
Na+-glucose co transport: both outside cell enter via channel and causes 2Na+ to 1 glucose
What is primary active transport?
The affinity of binding sites for Na and K changes with enzyme phosphorylation. ATP puts a phosphate directly onto an amino acid on the cytoplasmic side of protein and this drives conformational change that alters affinity of the protein. The combination of the change in conformation and the change in ion affinity moves the ions across the membrane
What is vectorial transport?
Basolateral (inside of body or blood)
Apical (outside of body or digestive tract)
Secretion- from basolateral to apical
Absorption- from apical to basolateral
How is glucose absorbed?
From the apical (lumen), glucose and Na transport glucose to the basolateral (blood)
Glucose absorption by brush border cells of gut
How does vectorial transport work in the kidney?
Principal cells of cortical collecting tubule of kidney. These cells simultaneously absorb sodium back into blood and secrete excess potassium out of the body into the urine
What is digestion and absorption?
Digestion: breaking down of complex food stuffs into component sugars, amino acids, and phospholipids
Absorption: transport of simple molecules, ions, and water from the lumen of the gut into cells lining the digestive tract and from these cells into the blood
What part does the mouth play in digestion?
1) chews the food
2) does some digestion of starch by breaking alph-1,4 linkages of component glucose molecules with the enzyme alpha-amylase
About 20%’of starch is broken down in the mouth (if you chew your food) and the rest is digested in the small intestine
What role does the stomach play in digestion?
1) it mashes up the food
2) it begins digestion of protein
3) it secretes acid to reduce growth of bacteria and aids in breaking down food stuffs
What are the three kinds of cells that make up the gastric glands?
Mucous cells: secrete mucous
Chief cells: secrete proteins called pepsins released from zymogen granules
Parietal cells: acid secreting cells, secrete H+
* note that the chief cells actually secrete pepsinogen which is then converted to pepsin, parietal cells secrete acid (HCl) into stomach lumen which then changes pH and causes the change
What causes pepsinogen to be converted to pepsin?
Pepsinogen molecule in low pH of stomach actually digests a part of its own structure, producing the active enzyme pepsin
Pepsinogen is an example of a zymogen or proenzyme
Inactive pepsinogen- loses masking sequence- active pepsin
What are parietal cells?
One billion in the human stomach
Collectively they secrete 1-1.5 liters per day of a 160 mM solution of HCl
They do this with an enzyme called the H+/K+ATPase, very similar in structure and function to the Na+/K+ATPase, which pumps H+ into the volume of the stomach
What is the process of acid secretion?
An enzyme in the cell called carbonic anhydrase makes bicarbonate and H+ from CO2 and H2O
Chloride enters basolateral membrane in exchange for bicarbonate
The H+/K+ATPase transport the H+ outward in exchange for K+
K+ and Cl- leave the cell passively through channels on the apical (lumen facing)!membrane
What is Omeprazole?
Sold as Prilosec, commonly used to treat gastrointestinal ulcers and gastroesophageal reflux disease (GERD characterized by frequent heartburn)
Acts by covalently attaching to a cysteine residue located in extra cellular domain of the H+/K+ATPase
This inhibits the enzyme and reduces acid transport
What role does he small intestine play in digestion?
1) neutralizes the acidity of the stomach with a high concentration of bicarbonate
2) finishes the digestion of starch and protein
3) emulsifies fat and digests complex lipid
4) absorbs most of the nutrients and water from the chyme
What is the role of the liver?
The liver synthesizes bile acid.
Bile is stored in the gall bladder and is released into the small intestine via the common bile duct
What does the exocrine pancreas do?
Synthesizes large number of digestive enzymes, secreted under hormonal control with HCO3- and water
What is the process of digestion from a starch to a sugar?
Pancreatic secretion contains alpha-amylase, finishes digestion of alpha-1,4 linkages of starch
This leaves only alpha-limit dextrans, consisting of 5-10 glucose molecules on either side of branch points
Rest of digestion accomplished by brush border cells. Alpha-limit dextranase breaks branch points, other enzymes reduce sugars to glucose, galactose, and fructose
Sugars are then transported into the blood
What does the brush border contain in the lumen of the intestine?
Brush border: microvilli containing digestive enzymes and transport proteins
What is the process of digestion of protein to amino acids?
Pancreas secretes proteases
Ex. Trypsin, secreted as trypsinogen, converted by enzyme enterokinase (in small intestine) into active form
Pancreatic proteases reduce proteins to polymers (2-6 amino acids long)
Brush border cells perform rest of digestion
Single amino acids are taken into blood by transporters like those for glucose.
How is the amino acid lysine transportered into the blood?
From the apical (lumen) side, lysine and Na+ become lysine as it enters the basolateral (blood) side
What is the process to digest fats?
Bile acids synthesized in liver are delivered to the small intestine Bile acids bind to large fat globules and act like soap, forming smaller fat micelles Pancreatic lipase (synthesized again by pancreas) attaches to micelles and breaks fat up into fatty acids and monoglycerides These then diffuse into brush border cells
What does a typical fatty acid look like after its liberated from head group?
All bonds between carbon atoms are single on a saturated fatty acids.
Carbon chain is straight
What is the process for fat to then be transported into the body?
Fatty acids and monoglycerides enter the brush border cells by diffusion and are re synthesized back into triglycerides
Triglycerides are packaged into large vesicles containing fat and protein called chylomicrons
The chylomicrons leave the brush border cells by exocytosis (pinched off vesicles from endoplasmic reticulum) and enter the circulatory system via lymphatic vessels to be transported to the cells of the body
What is the kidney comprised of and how does it work?
The basic unit of kidney function is the nephron
The nephron does two things: 1) It displaces a significant volume of the blood out into the lumen of the nephron by a process called ultrafiltration. 2) the rest of the nephron then re absorbs back into the blood nearly all of the water and useful ions and nutrients and fine tunes their concentrations
Everything else is exported (with urea) as urine
Where do the nephrons lie in the kidney?
The nephrons make up the renal pyramid in the renal medulla.
Each human kidney has one million nephrons
What are the principal parts of the nephron?
Bowman's capsule (BC) Proximal convoluted tubule (PCT) Loop of Henle (LH) Distal convoluted tubule (DCT) Collecting duct (CD)
What is ultrafiltration?
Fluid in the capillaries of the glomerulus(inside of the BC, tangled up)is pushed by the pressure of the blood out through fenestrae of the capillary wall and spaces between foot processes of podocytes into lumen of Bowman’s capsule
Water and small molecular weight molecules pass into the lumen but red blood cells and proteins do not
What part of the nephron is responsible for re absorption?
The proximal tubule is responsible for most of the re absorption of ions, nutrients and water
How does the proximal tubule use transporters to move glucose?
From inside the proximal tubule, glucose and Na+ enter. Then glucose diffuses inside of the blood vessel.
Proximal tubule uses transported just like those in the brush border cells of the small intestine
What is concurrent multiplication in the nephron?
Ascending limb of the loop of Henle is impermeable to water but transports NaCl out of the loop of lumen into the interstitial (extracellular) space of kidney inner medulla
Descending limb is permeable to water but impermeable to Na+ or Cl-. Water leaves, concentrating the NaCl concentration in the loop lumen
This increases the rate of transport of NaCl out of the ascending limb and multiplies the concentration accumulated in the medulla interstitial space
What is the distal convoluted tubule?
Continues transport of Na+ and Cl- out of tubule lumen reasoning additional 5% of NaCl
Na+ and Cl- transport blocked by thiazide-type diuretics that are often used to treat high blood pressure. Decrease in NaCl transport out of kidney lumen enhances excretion of salt and water out of body
Major site in kidney for Ca2+ reabsorption and regulation of blood Ca2+ concentration
What is the purpose of the Corticla Collecting Duct?
Three main functions occurring in two different cell types
Intercalated cells are primarily involved in the transport of H+ and HCO3- and in regulation of urine pH
Principal cells make fine adjustments in Na+ and K+ concentration and regulate Water permeability of collecting duct and thus water concentration of urine
How do principal cells regulate the permeability of collecting duct water?
Increase in plasma blood osmolarity
Release of vasopressin (ADH)
Vasopressin binds to V2 receptors on basolateral membrane of collecting duct principal cells
This causes increase in cytoplasmic cAMP
Which leads to fusion of aggrephores
And a delivery of water channels to apical membrane of collecting duct cells
Where is vasopressin secreted?
By the posterior pituitary in response to an increase in blood osmolarity
How does an increase in osmolarity affect the body?
An increase in the blood plasma osmolarity causes an increase in the blood title of ADH (vasopressin) and eventually the sensation of thirst
How does high blood osmolarity affect the membrane of the collecting duct?
When blood osmolarity is high, water channels are delivered to the apical membrane of the cells of the collecting duct.
Since osmolarity of the interstitial space is high from concurrent multiplication, water comes out of the collecting duct and is conserved by the body
