Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
11.3 The Kidney and Osmoregulation 11.3 The kidney and Osmoregulation Nature of science: Curiosity about particular phenomena—investigations were carried out to determine how desert animals prevent water loss in their wastes. (1.5) Understandings • Animals are either osmoregulators or osmoconformers • The Malpighian tubule system in insects and the kidney carry out osmoregulation and removal of nitrogenous wastes • The composition of blood in the renal artery is different from that in the renal vein • The ultrastructure of the glomerulus and Bowman’s capsule facilitate ultrafiltration • The proximal convoluted tubule selectively reabsorbs useful substances by active transport • The loop of Henle maintain hypertonic conditions in the medulla • ADH controls reabsorption of water in the collecting duct • The length of the loop of Henle is positively correlated with the need for water conservation in animals • The type of nitrogenous waste in animals in correlated with evolutionary history and habitat 11.3 The kidney and osmoregulation Applications and Skills • Application: Consequences of dehydration and overhydration • Application: Treatment of kidney failure by hemodialysis or kidney transplant • Application: Blood cells, glucose, proteins, and drugs are detected in urinary tests • Skill: Drawing and labeling a diagram of the human kidney • Skill: Annotation of diagrams of the nephron Retro 1.3 Membrane Structure Understandings • Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules • Membrane proteins are diverse in terms of structure, position in the membrane, and function • Cholesterol is a component of animal cell membranes Application and Skills • Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes • Skill: Drawing of the fluid mosaic model • Skill: Analysis of evidence from electron microscopy that led to the proposal of the DavsonDanielli model • Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model For flash cards… • You need to do • 11.3 Understandings • 11.3 Applications and Skills • 1.3 Understandings (you can reuse from SL if you still have them!) • 1.3 Applications and Skills (you can reuse from SL if you still have them!) • They need to be hand written! They can be on flash cards or on notebook paper What is excretion? • All body cells produce waste products that have to be removed because they can damage the body • These waste products are the results of metabolic activities • Waste products are different for different organisms • Excretion is the removal from the body of the waste products of metabolic pathways • Oxygen is a waste product associated with photosynthesis in green plants • Animals excrete carbon dioxide as a consequence of cellular respiration and nitrogen containing compounds that result from the breakdown of proteins and nucleic acids. • When these macromolecules are broken apart for energy or converted to carbohydrates or fats, enzymes remove nitrogen in the form of ammonia, a small and very toxic molecule • How animals excrete or get rid of ammonia correlates with their evolutionary history and habitat, especially the availability of water • Since most metabolic wastes must be dissolved in water when they are removed from the body (exception is the loss of carbon dioxide in air breathing animals) the type and quantity of waste products may have a large impact on water balance Osmoregulation • Osmoregulation is the control of the water balance of the blood, tissue, or cytoplasm of a living organism. • It is based largely on controlled movement of solutes between internal fluids and the external environment. • Osmolarity- total solute concentration expressed as molarity, or moles of solute per liter of solution, measured in milliOsmoles per liter (mOsm/L) • Osmolarity of human blood is about 300 mOsm/L, while seawater is about 1,000 mOsm/L • Osmosis: movement of water across a selectively permeable membrane. Moves from high osmolarity to low osmolarity Osmosis • Remember water moves from high to low concentration • Isoosmotic (isotonic): 2 solutions separated by a selectively permeable membrane have the same osmolarity. No net movement of H2O. • Hyperosmotic (hypertonic): solution with the greater concentration of solutes, water moves out of cell • Hypoosmotic (hypotonic): solution that is more dilute, water moves into cell • Water flows by osmosis from a hypoosmotic solution to a hyperosmotic one. Osmoconformers • Osmoconformer- tissues and cells are isosmotic with its surroundings. All marine animals. No tendency to gain or lose water. Live in water with stable composition. Squid. • Disadvantage- cells inside body may not contain the ideal solute concentration for body processes. Osmoregulators • Osmoregulator- controls its internal osmolarity independent of that of its environment. Freshwater and terrestrial animals like humans. • Disadvantage- energy has to be used to keep solute concentrations in the body constant. Nitrogenous Waste Products- Ammonia • The habitat an organism live in determines the type of nitrogenous waste product it will produce • In aquatic freshwater environment: • Freshwater fish and amphibian larvae: get rid of nitrogenous waste as ammonia (toxic because of its basicity= pH 11.6) because they have access to a lot of water • Ammonia is lost as ammonium ions (NH4+) across the epithelium of the gills, with kidneys excreting only minor amounts of nitrogenous wastes. The gill epithelium takes up Na+ from the water in exchange. • Advantage: ammonia takes little energy to make • Disadvantage: highly toxic Nitrogenous Waste Products- Urea • Mammals (including marine and terrestrial), marine fish, adult amphibians cannot excrete ammonia because it is so toxic (basic), and can only be transported and excreted in large volumes of very dilute solutions, and most mammals do not have access to enough water • Instead, mammals (like adult amphibians and many marine fishes) excrete mainly urea a substance produced in the vertebrate liver by a metabolic cycle that combines ammonia with CO2. • The circulatory system carries urea to the excretory organ= the kidney. • Urea has low toxicity, about 100,000 times less than that of ammonia. Urea can be safely carried and greatly reduces the amount of water required for nitrogen excretion. Nitrogenous Waste Products- Urea • Some desert mammals have an adaptation that allows them to produce a very concentrated urine (a very) long Loop of Henle • The Loop of Henle and the size of the medulla have a role in the nephron of the kidney in water absorption • The disadvantage of urea is that animals must expend energy to produce it from ammonia. Nitrogenous Waste Products- Uric Acid • Birds, as well as land snails, insects, and many reptiles, excrete uric acid as the major nitrogen containing waste. • It is relatively nontoxic like urea, but unlike urea or ammonia, it is largely insoluble in watery solutions such as blood and cytoplasm and can be excreted as a semisolid paste with very little loss of water. Good for animals with little access to water. • Cost: uric acid is even more energetically expensive to produce than urea, requiring lots of ATP to synthesize from ammonia. • Developmental history of these organisms gives us a clue as to why uric acid excretion of nitrogenous waste is advantageous, think of how they develop… • Embryonically, each has very little access to water. Nitrogenous Waste Excretion- Insects • Insects and terrestrial arthropods have Malpighian tubules that remove nitrogenous wastes and also function in osmoregulation • Works with their open circulatory system • Not all of the circulatory fluid, called hemolymph, (not blood) is contained in blood vessel. As a result, hemolymph is the same as interstitial fluid. • The heart pumps hemolymph through vessels into sinuses which are fluidfilled spaces where materials are exchanged between the hemolymph and cells. • Hemolymph returns to the heart through pores, which are equipped with valves that close when the heart contracts. • Branches of this blood vessel carry the hemolymph to different parts of the body and it is then released and is free to flow gradually through tissues until it is drawn back into the vessel for re-pumping. • Body cells are bathed in this fluid and release waste products into it Malpighian Tubules Malpighian Tubules • Located between midgut and hindgut • Structure: a ring of narrow blind-ended ducts which extend through the body cavity • Physiology: Cells in tubule walls extract waste products from the hemolymph and pass them into the lumen of the tubule. Ammonia is extracted and converted by the Malpighian tubule cells into uric acid • Cells in tubule wall transfer mineral ions (Na+, K+, Cl-) by active transport from the hemolymph to the lumen of the tubule and water follows passively by osmosis • This solution drains into the lumen of the hindgut where it mixes with semidigested food and this mixture is carried to the rectum, last section of the gut • Mineral ions (Na+, K+, Cl-) are pumped by cells in the wall of the rectum from the feces to the hemolymph and again water follows passively by osmosis • This process prevents dehydration and achieves osmoregulation Malpighian Tubules • Nitrogenous wastes, mainly insoluble uric acid, eliminated as nearly dry matter along with feces • Capable of conserving water very effectively and is a key adaptation contributing to these animals’ tremendous success on land Kidney • The kidney is central to homeostasis because it disposes of metabolic wastes and controls body fluid composition by adjusting the rates of loss of particular solutes • Like the excretory organs of most animal phyla, kidneys consist of tubules. These numerous tubules are arranged in a highly organized manner and closely associated with a network of capillaries. Skill: Drawing and labeling a diagram of the human kidney (6 labels) Renal Medulla When you label remember everything is RENAL!!! Blood flow through the kidney • Blood enters the kidney through the renal artery (a branch of the aorta) and exits the kidney via the renal vein. In fact 20% of resting cardiac output goes to the kidney • Urine exits the kidney through a duct called the ureter and both ureters drain into a common urinary bladder • During urination, urine is expelled from the urinary bladder through a tube called the urethra Renal Medulla Kidney • Mammalian kidney has two distinct regions: 1. an outer renal cortex 2. an inner renal medulla • A third area is a collecting region for all of the urine produced by the kidney’s unit of function, the nephron. This area is called the renal pelvis Renal Medulla Nephron • The functional unit of the vertebrate kidney is a microscopic excretory tubule called the nephron • It consists of a single long tubule and a ball of capillaries called the glomerulus • The blind end of the tubule forms a cupshaped swelling called Bowman’s capsule (also called the glomerular capsule), which surrounds the glomerulus • Each human kidney packs a million nephrons, with a total tubule length of 80 km Glomerulus Glomerulus • The glomerulus acts as a filter for the blood being carried by the afferent arteriole • The lining (endothelium) of the glomerulus is a single layer of cells which has pores called fenestrations (which comes from the word windows) • It restricts the passage of red blood cells and other formed elements Glomerulus • The basement membrane of the glomerulus lies between the endothelium and a layer of Bowman’s capsule • It consists of fibrils in a glycoprotein matrix and restricts the passage of larger proteins such as the plasma proteins • Podocyte specialized epithelial cells that form a layer of the glomerular (Bowman’s) capsule has filtration slits which restrict the passage of medium sized proteins • Extending from each podocyte are 1000’s of foot-like structures called pedicels that cover the basement membrane • Podocytes also provide support Basement membrane in general Ultrafiltration • The function of the glomerulus is the ultrafiltration of blood • Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule • Pressure is high because the vessel taking blood away from the glomerulus (efferent arteriole) is narrower than the vessel bringing blood • The porous capillaries (fenestrations), along with podocytes are permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins. Those exit through the efferent arteriole Ultrafiltration • Filtration of small molecules is non-selective and the filtrate in Bowman’s capsule contains salts, glucose, and vitamins, nitrogenous wastes such as urea and other small molecules. • The filtrate moves on into the next part of the nephron the proximal convoluted tubule. Proximal Convoluted Tubule • Note: Location of the various parts of the nephron is very important to what can happen in the different parts of the tubule • Proximal Convoluted Tubule, like the glomerulus, is located in the cortex of the kidney • Secretion and absorption in the proximal convoluted tubule change the volume and composition of the filtrate • The initial filtrate is about 1 liter every 10 minutes in the two kidneys • Substances are taken out of the plasma that are needed by the body • Process is called selective reabsorption Nephron • The nephron has to be able to reabsorb the necessary materials and put them back into the circulatory system • The nephron is composed of a single layer of cells • Cells of the proximal convoluted tubule have microvilli projecting into the lumen, giving a large surface area for absorption • Pumps in the membrane re-absorb useful substances by active transport using ATP produced by mitochondria in the cells • A constant pH is maintained in the nephron by the controlled secretion of hydrogen ions by active transport into the lumen • The epithelium synthesizes and secretes ammonia which neutralizes the acid and keeps the filtrate from becoming too acidic • It reabsorbs 90% of the important buffer, bicarbonate (HCO3-) • Drugs and other poisons that were processed by the liver pass from another capillary bed into interstitial fluid and then are secreted across the epithelium of the proximal tubule into the nephron’s lumen and get added to the filtrate • Valuable nutrients, including glucose, amino acids, and potassium are actively or passively transported from the filtrate to the interstitial fluid and then move into capillaries and are returned to the circulatory system. • Most of the NaCl and water are reabsorbed from the huge initial volume • Na+ is actively transported, Cl- follows by passive transport • As salt moves from the filtrate, water follows by osmosis • These also diffuse from the interstitial fluid back into the capillary bed to be returned to the body • In the end ALL of the glucose, 80% of the water, and 80% of the mineral ions are reabsorbed • The rest of the pathway for urine formation includes the Loop of Henle, which is located in the medulla, the distal convoluted tubule and the collecting duct Role of the Medulla • The interstitial fluid increases in osmolarity from about 300 to 1,200 mosm/L from the cortex to the inner medulla • Two solutes contribute to this gradient: 1. Urea 2. NaCl • So the medulla has a concentration gradient of salts. This will cause water to leave the nephron and go back into the interstitial fluid • Something has to create and something has to maintain this very important concentration gradient of salt in order for water to be reabsorbed… Role of the Loop of Henle • The Loop of Henle maintains the interstitial gradient of NaCl • Descending and ascending parts of the Loop of Henle have different permeability characteristics which will be discussed soon Role of ADH • Antidiuretic hormone (ADH) is also called vasopressin and is a hormone • This hormone originates from the posterior pituitary in response to detection of increased osmolarity by receptors in the hypothalamus and it increases the permeability of the collecting duct to water • It allows water to be conserved • ADH makes the cells of the conducting duct produce membrane channels called aquaporins that allows for the movement of water Overview of the role of the Loop of Henle • Overall it is to create an area of high solute concentration in the cells and tissue fluid of the medulla • Descending limb of the loop of Henle • Reabsorption of water continues. The epithelium is freely permeable to water but not very permeable to NaCl and other small solutes • Water moves out by osmosis because the interstitial fluid bathing the tubule is hyperosmotic to the filtrate. Osmolarity of the interstitial fluid becomes greater from the outer cortex to the inner medulla of the kidney. So filtrate continues to lose water. Overview of the role of the Loop of Henle • Ascending limb of the loop of Henle • The epithelium in the region is permeable to salt but not to water • In the thin segment of the ascending loop, NaCl leaves nephron through diffusion • In the thick segment NaCl is actively transported into the interstitial fluid. • By losing salt without giving up water, the filtrate becomes progressively more dilute as it moves up to the cortex in the ascending limb, but it has added to the high osmolarity of the interstitial fluid in the medulla. Summary • Descending limb • Water out passively • Ascending limb • Thin segment NaCl out passively • Thick segment NaCl out actively So what is the point… • A longer loop of Henle is an adaptation for water conservation • Amphibians have almost no Loop of Henle and are unable to conserve water. Thus their urine is always dilute. • Vertebrates in desert habitats such as the Kangaroo rate of desert Southwest US have long loop of Henle and nocturnal behavior • Also, a positive correlation between the thickness of the medulla compared to the overall size of the kidney and the need for water conservation. • A thicker medulla allows the loops of Henle and collecting ducts to be longer so that more water can be reabsorbed. Relationship between maximum solute concentration (MSC) and concentration factor of urine (CF) and habitat Species MSC (mOsm dm-3) CF Habitat Beaver 520 x2 Aquatic Human 1200 x4 Intermediate Brown rat 2900 x9 Intermediate Kangaroo rat 5500 x18 Desert Hopping mouse 9400 x25 Desert Distal Convoluted tubule • Distal convoluted tubule plays a key role in regulating potassium ion (K+) and NaCl concentrations of body fluid • It does so by varying the amounts of K+ that is secreted into the filtrate and the amount of NaCl reabsorbed from the filtrate • Like the proximal tubule, the distal tubule also contributes to pH regulation by the controlled secretion of H+ and reabsorption of bicarbonate Collecting Duct • Collecting Duct carries the filtrate through the medulla to the renal pelvis. It reabsorbs NaCl by active transport • Collecting duct plays a large role in determining how much salt is actually excreted in the urine • Epithelium is permeable to water but not to salt or (in the cortex) to urea • As filtrate goes down the collecting duct, it becomes more concentrated as it loses more and more water by osmosis • In the inner medulla, the duct becomes permeable to urea because of the high urea concentration in the filtrate at this point • Some urea diffuses out of the duct and into the interstitial fluid. • Along with NaCl, the interstitial urea is a major solute contributing to the high osmolarity of the interstitial fluid in the medulla. Skill: Annotation of diagrams of the nephron 1. Glomerulus and Bowman’s capsule- perform ultrafiltration 2. Proximal convoluted tubuletransfer substances from filtrate back to blood (selective reabsorption) 3. Loop of Henle- establishes a salt gradient, establishes high solute concentrations in medulla 4. Distal convoluted tubule- adjust individual solute concentrations and pH of blood 5. Collecting duct- osmoregulation Negative Feedback and ADH • When osmolarity in plasma rises above the set point of 300 mosm/L, more ADH is released and reaches the kidney • Main targets are distal tubule and mainly collecting duct • Alcohol inhibits the release of ADH, causing excessive urinary water loss and dehydration Comparing blood in glomerulus, filtrate and different parts of nephron and urine Concentration (mg per 100mL) Glucose Urea Blood in glomerulus 740 90 30 Glomerular filtrate 0 90 30 Filtrate at start of loop of Henle 0 0 90 Filtrate at end of loop of Henle 0 0 200 Urine with ADH 0 0 1800 Urine without ADH 0 0 180 Plasma Proteins • Glucose is not present in the urine but is present in the renal artery, glomerular filtrate and renal vein (less because of aerobic cell respiration) • Urea is increased from 30 mg to 1800 mg in urine and reduced to 24 mg in renal vein. • Proteins are not present in either the glomerular filtrate or the urine, but are present at the same level in the renal artery and vein (740 mg)- makes sense it goes in and comes out • Sodium ions and chloride ions are adjusted to normal levels in the renal vein by the kidney. • Oxygen levels are decreased in renal vein in comparison to renal artery and carbon dioxide levels are increased due to aerobic cell respiration. • Urine with ADH contains much more concentrated urea than urine without ADH. Comparison of substances in the Renal Artery vs Renal Vein Renal Artery Renal Vein Reason for difference a /v Oxygen Higher Lower Aerobic resp. to Carbon dioxide Lower Higher provide ATP for kidney function Glucose Slightly higher Slightly lower Use in cell aerobic respiration Urea Higher About 20% lower Excretion in urine Plasma proteins Equal Equal Not added or removed Na+/Cl- ions Variable Always at normal levels Kidney raises or lowers concentrations to normalize them Renal Medulla Application: Consequences of dehydration and overhydration • Dehydration- due to loss of water from the body but not an equivalent quantity of solutes, so body fluids become hypertonic • Consequences- thirst, small quantities of dark colored urine, lethargy, raised heart rate, low blood pressure, constipation, dry mouth and skin, dizziness, and headache. In severe cases, seizures, brain damage and DEATH! • Hypernatremia Application: Consequences of dehydration and overhydration • Overhydration is due to excessive intake of water • Bodily fluids become hypotonic • Consequences: behavior changes, confusion, drowsiness, delirium, blurred vision, muscle cramps, nausea, and vomiting, and in acute cases seizures, coma, and DEATH! • Hyponatremia Application: Blood cells, glucose, proteins, and drugs are detected in urinary tests 1. Glucose: indicator of diabetes – 100% should be reabsorbed in proximal convoluted tubules, but if high blood glucose levels, glucose levels in filtrate with overwhelm the pumps resulting in glucose in the urine 2. Blood cells: indicator of infections (white blood cells), kidney damage (red blood cells), and some cancers 3. Proteins: normally, only a small amount. Indication of kidney disease or damage 4. Drugs: metabolites of legal (antibiotics, anti-inflammatory drugs) and illegal drugs. Illegal drugs could have been used recreationally or for athletic advantage. Kidney Failure • Symptoms: build up of waste products in the body, weakness, shortness of breath, lethargy, and confusion • The inability to remove potassium from the bloodstream may lead to abnormal heart rhythms and sudden death • Causes: Diabetes, high blood pressure, autoimmune disease (Lupus, HIV), genetic disorders (polycystic kidney), injury, some medications, and drugs • Treatment: Kidney dialysis or hemodialysis or kidney transplant • Statistics (2011): Prevalence of chronic kidney disease is growing most rapidly in people ages 60 and older (24.5%). End-stage renal disease (ESRD) incident rates are more than 3X higher for African Americans than for Caucasians. At end of 2009, more than 871,000 people treated for ESRD. In 2009, 18,000 kidney transplants in US • Wolfgang Amadeus Mozart (1791) died of kidney failure Application: Treatment of kidney failure by hemodialysis or kidney transplant- History • 100 A.D. Dialysis began in the bathwaters of the Roman Empire surrounded by marble, heat, and steam, • Romans poisoned by the build up of urea in their bodies would surround themselves with marble, heat, and steam and attempted to sweat and soak as the toxins in their blood diffused through their skin into the swirling spa • Cleansed but totally exhausted Application: History • Patients usually died quietly at home of urea poisoning, sometimes called “dropsy” • 1850s: search for semipermeable membrane. They first used ox bladder and later used collodion- syrup made by dissolving cellulose nitrate in ether and alcohol. • Until present the material used to wrap cigarettes and the cellulose used as sausage skin were used • Choice of membrane critical: if it tore, patient could bleed to death • Clotting was a problem solved using heparin as an anticoagulant • Father of modern kidney dialyzer is Willem Kolff, a young physician who worked in the Netherlands during the Nazi occupation • Kolff brought his artificial kidney to Mt. Sinai Hospital in New York City and successfully dialyzed the first patient in 1948 Application: Early Hemodialysis Machine Application: Modern Hemodialysis Machine Application: Treatment of kidney failure by hemodialysis • Hemodialysis involves the diffusion of solutes from a higher to a lower concentration through a semi-permeable membrane 1. Dialysis membrane used in kidney machines is cellulose acetate or nitrate. It has large pores that let small solute particles pass through, but not large particles such as plasma proteins or blood cells 2. Blood flows on one side of the dialysis membrane and dialysis fluid on the other side. Application: Dialysis Fluid a) No urea or other excretory products, so these waste products diffuse into it from the blood. b) The same concentration of glucose, mineral ions, and other desirable substances as normal blood plasma so these substances do not diffuse unless the level in blood plasma is above or below normal. c) Dextran, a solute that cannot pass through the dialysis membrane and so causes excess water to move by osmosis from the blood to the dialysis fluid. d) High calcium and low potassium concentrations to extract potassium and add calcium to the blood e) Hydrogencarbonate (bicarbonate) ions (HCO3- ) to reduce the acidity of the blood f) A high solute concentration that will cause excess water to be removed from the blood by osmosis across the dialysis membrane Application: Process of Hemodialysis • Blood flows through tubes or between sheets of dialysis membrane. • The blood is taken from the patient (via vein) in and returned via needles inserted into a blood vessel in the arm • Blood passes through a kidney machine for 3-4 hours, 3 times per week. • Dialysis fluid has to be gradually replaced throughout a session to maintain the concentration gradient • A large volume of fluid is used, in contrast to the human kidney, which can excrete waste products with a very small loss of water Application: Treatment of kidney failure by kidney transplant • A new kidney from either a living donor or a person who has recently died is grafted in to the lower abdomen with the renal artery, renal vein, and ureter connected to the recipient’s blood vessels and bladder • It is essential that donor and recipient are in the same blood group and their tissues match as closely as possible to minimize the chance of rejection of the kidney by recipient’s immune system • Even with tissue matching, recipient will need immune-suppressing drugs for the rest of their life, because even well-matched kidneys are not a perfect tissue match • The immune system of the body does not recognize the new kidney and will try to attack it (we will discuss immunology later ) Retro 1.3 Cell Membrane 1.3.1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules • Phospholipid structure • Phosphate head- hydrophilic (water loving) • Two fatty acid tails- hydrophobic (water hating) • They are amphipathic • Due to their amphipathic nature when phospholipids are placed in water they form a bilayer with the hydrophobic tails facing away from the water and hydrophilic head facing toward the water- micelle Skill: Drawing of the fluid mosaic model • Integral proteins are embedded in the bilayer • Peripheral proteins are attached to one of the outer surfaces • Glycoproteins are proteins with a sugar unit attached 1.3.2 Membrane proteins are diverse in terms of structure, position in the membrane and function • Hormone binding sites • Has a specific shape that binds a hormone, causes a change in shape, allows for a message to be sent to the interior of cell • Ex: Insulin receptor • Enzymatic activity • Can be exterior or interior and catalyze reactions, sometimes working in metabolic pathways • Ex: Cytochrome oxidase • Cell adhesion • Can provide permeant or temporary connections between cells (junctions- tight junctions or gap junctions) • Ex: Cadherin • Cell-to-cell communication • Proteins can have carbohydrates attached that allow for cellular recognition • Ex: Glycoproteins • Passive transport channels • Proteins that span the membrane and allow for movement from high to low concentrations • Ex: Nicotinic acetylcholine receptor (receptor for a neurotransmitter) • Active transport pumps • Proteins that move molecules across the membrane with the use of ATP • Ex: Calcium pump 1.3.3 Cholesterol is a component of animal cell membrane • Cholesterol is a steroid • It is hydrophobic but had one hydrophilic end thus it can fit in the membrane • Cholesterol limit the movement of the phospholipids • Allows membranes to function at a wider temperature range Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutions • Since they limit the movement of phospholipids cholesterol reduces the fluidity of membranes • It also reduces permeability of the membrane to some hydrophilic particles like sodium and hydrogen ions • This allows animals to maintain concentration differences of these ions • Plant cells do not have cholesterol in their cell membranes • They depend on saturated and unsaturated fatty acids to maintain fluidity in their membranes