Unit 2 – Water of Life Name ______

Chapter 3: Water and Life

Introduction

3.1 Polar covalent bonds in water molecules result in hydrogen bonding

Bonding in Water Molecules

 Because oxygen is more electronegative than hydrogen, a water molecule is a polar molecule in which opposite ends of the molecule have opposite charges. o The oxygen region of the molecule has a partial negative charge (-), and the hydrogen regions have a partial positive charge (+).

3.2 Various properties of water contribute to Earth’s suitability for life

1) Cohesive behavior  Collectively, hydrogen bonds hold water together, a phenomenon called cohesion.  Adhesion, the clinging of one substance to another, also contributes, as water adheres to the walls of the vessels.  Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. Water molecules and surface tension Water transport in plants

2) Ability to moderate temperature  Water moderates air temperatures by absorbing heat from warmer air and releasing the stored heat to cooler air.  Atoms and molecules have kinetic energy, the energy of motion, because they are always moving. o The faster a molecule moves, the more kinetic energy it has. o The kinetic energy associated with the random movement of atoms or molecules is called thermal energy.  Heat is a measure of the total quantity of kinetic energy due to molecular motion in a body of matter.  Temperature measures the intensity of heat in a body of matter due to the average kinetic energy of molecules.  In most biological settings, temperature is measured on the Celsius scale (°C). o At sea level, water freezes at 0°C and boils at 100°C. o Human body temperature is typically 37°C.  Although there are several ways to measure heat energy, one convenient unit is the calorie (cal). o One calorie is the amount of heat energy necessary to raise the temperature of 1 gram of water by 1°C. o A calorie is released when 1 g of water cools by 1°C. 2

 In many biological processes, the kilocalorie (kcal) is a more convenient unit. o One kilocalorie is the amount of heat energy necessary to raise the temperature of 1000 g (1 kg) of water by 1°C.  Another common energy unit, the joule (J), is equivalent to 0.239 cal.  Water stabilizes temperature because it has a high specific heat.  The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1°C. Water’s high heat of vaporization has many effects.  The transformation of a molecule from a liquid to a gas is called vaporization, or evaporation. o Heating a liquid increases the average kinetic energy and increases the rate of evaporation.  Heat of vaporization is the quantity of heat that a liquid must absorb for 1 g of it to be converted from liquid to gas. o As moist tropical air moves to the poles, water vapor condenses to form rain, releasing heat.  As a liquid evaporates, the surface of the liquid that remains behind cools, a phenomenon called evaporative cooling.

3) Expansion upon freezing  Water is unusual because it is less dense as a solid than as a cold liquid. o Most materials contract as they solidify, but water expands. o At temperatures higher than 4°C, water behaves like other liquids, expanding as it warms and contracting as it cools.

4) Versatility as a solvent.  A liquid that is a completely homogeneous mixture of two or more substances is called a solution.  The dissolving agent is the solvent, and the substance that is dissolved is the solute.  In an aqueous solution, water is the solvent.  Water is not a universal solvent, but it is very versatile because of the polarity of water molecules. o Water is an effective solvent because it readily forms hydrogen bonds with charged and polar covalent molecules. o Each dissolved ion is surrounded by a sphere of water molecules, a hydration shell.

 Polar molecules are soluble in water because they form hydrogen bonds with water.  Even large molecules, like proteins, can dissolve in water if they have ionic and polar regions.  A substance that has an affinity for water is hydrophilic (water-loving).  Some hydrophilic substances do not dissolve because their molecules are too large.  Substances that have no affinity for water are hydrophobic (water-fearing). Aqueous Chemistry  When carrying out experiments, we use mass to calculate the number of molecules.  The actual number of molecules in a mole is called Avogadro’s number, 6.02 × 1023.

 To illustrate, how can we measure 1 mole of table sugar—sucrose (C12H22O11)? o A mole of sucrose contains 6.02 × 1023 molecules by definition o A mole of sucrose is also the molecular weight in grams from the periodic table. . Carbon:12 x 12.0g + Hydrogen: 22 x 1.0g + Oxygen:11 x 16.0g = 342 grams o So, 1 mole of sucrose = 342 grams of sucrose = 6.02 X 1023 molecules of sucrose. 23  A mole of ethyl alcohol (C2H6O) also contains 6.02 × 10 molecules but weighs only 46 g because the molecules are smaller, have less atoms.  In “wet” chemistry, we typically combine solutions or measure the quantities of materials in aqueous solutions. o The concentration of a material in solution is called its molarity. o A one-molar (1 M) solution has 1 mole of a substance dissolved in 1 liter of a solvent, typically water. o To make a 1 M solution of sucrose, we would slowly add water to 342 g of sucrose until the total volume was 1 liter and all the sugar was dissolved.

Application Two students are both attempting to make a 1.0M sucrose solution. 1) Circle which flask is the correct 1.0M sucrose solution Flask A Flask B

1)Measure 1L of dH2O, 1)Measure 500mL of add to flask dH2O, add to flask

2)Add 342 g sucrose 2)Add 360 g sucrose

3)Mix 3)Add enough dH2O to make 1L volume

2) Explain your rationale why you picked that flask.

Outline the steps to…. A. Make a 0.5M sucrose solution.

B. Determine the grams of solute in a 1.0M stock glucose solution.

3.3 Acidic and basic conditions affect living organisms +   When water forms ions it dissociates into a hydrogen ion and a hydroxide ion: H2O  H + OH .  The pH scale is used to describe how acidic or basic a solution is.  An acid is a substance that increases the hydrogen ion concentration in a solution.  Any substance that reduces the hydrogen ion concentration in a solution is a base.  Some bases reduce the H+ concentration directly by accepting hydrogen ions.  Other bases reduce the H+ concentration indirectly by dissociating to OH−, which then combines with H+ to form water. The pH scale measures the H+ concentration of a solution.  In any aqueous solution at 25°C, the product of the H+ and OH- concentrations is constant at 10-14.  Brackets ([H+] and [OH−]) indicate the molar concentration of the enclosed substance. o [H+] [OH−] = 10−14 o In a neutral solution at room temperature, [H+] = 10−7 M and [OH−] = 10−7 M.  Formula for pH = -log [H+]

Summary of Acids, Bases and pH

Organisms are sensitive to changes in pH.  The chemical processes in the cell can be disrupted by changes in the H+ and OH− concentrations away from their normal values, usually near pH 7. o The pH of human blood is close to 7.4. A person cannot survive for more than a few minutes if the blood pH drops to 7 or rises to 7.8.  Buffers resist changes in the pH of a solution when H+ or OH− is added to the solution. o Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted. o Buffers typically consist of a weak acid and its corresponding base. o One important buffer in human blood and other biological solutions is carbonic acid (H2CO3), formed when CO2 reacts with water in blood plasma. - + . Carbonic acid dissociates to yield a bicarbonate ion (HCO3 ) and a hydrogen ion (H ). . The chemical equilibrium between carbonic acid and bicarbonate acts as a pH regulator. Acidification of rivers, lakes, seas, and rain threatens the environment.  Human activities threaten water quality.  The burning of fossil fuels releases gaseous compounds into the atmosphere, which react with water to increases the acidity of the water.

 About 25% of human-generated CO2 is absorbed by the oceans.  When CO2 dissolves in seawater, it can react with water (H20) to form carbonic acid (H2CO3). o This weak acid lowers the pH of seawater, a process known as ocean acidification. 2-  As seawater acidifies, the extra hydrogen ions combine with carbonate ions (CO3 ) to form - bicarbonate ions (HCO3 ). o Scientists predict that ocean acidification will cause carbonate concentrations in the oceans to decrease by 40% over this century.

 Calcification, the production of calcium carbonate (CaCO3) by corals and other organisms, is directly 2- affected by the concentration of CO3 .  The burning of fossil fuels is also a major source of sulfur oxides and nitrous oxides. o These oxides react with water to form strong acids, which fall to Earth with rain or snow.  Acid precipitation is rain, snow, or fog with a pH below 5.2. o Acid precipitation can damage aquatic life and also adversely affects land plants by changing soil chemistry. o The United States passed the Clean Air Act in 1990, mandating improved industrial technologies to reduce the release of harmful chemical pollutants. Application Coniferous trees (pines) thrive in acidic soil in the Northeast of the United States. Provide an explanation for their success in this area.

Chapter 7: Membrane Structure and Function

Overview

7.1 Cellular membranes are fluid mosaics of lipids and proteins  The plasma membrane separates the living cell from its surroundings.  Like all biological membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others.  The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important.  The most abundant lipids are phospholipids.  Phospholipids and most other membrane constituents are amphipathic molecules, which have both hydrophobic and hydrophilic regions. Phospholipid bilayer

Membrane models have evolved to fit new data.  The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model. o In this model, the membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) of phospholipids. 7

Animal cell plasma membrane (cutaway view)

 Historically, membrane models did not address two problems. 1. Not all membranes are alike. . Membranes with different functions differ in chemical composition and structure. 2. Measurements showed that membrane proteins are not very soluble in water. . Membrane proteins are amphipathic, with both hydrophobic and hydrophilic regions. . If membrane proteins were at the membrane surface, their hydrophobic regions would be in contact with water.  In 1972, S. J. Singer and G. L. Nicolson proposed that membrane proteins reside in the phospholipid bilayer with their hydrophilic regions protruding into the cytosol. o In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water, and the hydrophobic regions are in a nonaqueous environment within the membrane.  A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholipid bilayer. o When a freeze-fracture preparation is viewed with an electron microscope, protein particles are interspersed in a smooth matrix, thus supporting the fluid mosaic model.

Membranes are fluid.  Membrane molecules are held in place by relatively weak hydrophobic interactions.  Most of the lipids and some proteins drift laterally in the plane of the membrane but rarely flip-flop from one phospholipid layer to the other.  Some large membrane proteins drift within the phospholipid bilayer, although they move more slowly than the phospholipids.  Some proteins move in a very directed manner, perhaps guided or driven by motor proteins attached to the cytoskeleton.  Other proteins never move and are anchored to the cytoskeleton or to the extracellular matrix.  Membrane fluidity is influenced by temperature. o As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids pack more closely.  Membrane fluidity is also influenced by the components of the membrane. o Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids because kinks in the unsaturated fatty acid tails at the locations of the double bonds prevent tight packing.

 The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells. o At warm temperatures (such as 37°C), cholesterol restrains the movement of phospholipids and reduces fluidity. o At cool temperatures, cholesterol maintains fluidity by preventing tight packing. o Thus, cholesterol acts as a “fluidity buffer” for the membrane, resisting changes in membrane fluidity as temperature changes. Factors that affect membrane fluidity

 To work properly with active enzymes and appropriate permeability, membranes must be about as fluid as salad oil. o Enzymes in the membrane may become inactive if their activity requires them to move within the membrane.  Variations in the lipid composition of cell membranes of many species are evolutionary adaptations to maintain membrane fluidity under specific environmental conditions. o The membranes of fishes that live in extreme cold have a high proportion of unsaturated hydrocarbon tails, enabling them to stay fluid. o The membranes of bacteria and archaea living in thermal hot springs and geysers include unusual lipids that prevent excessive fluidity at such high temperatures. Membranes are mosaics of structure and function.  A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.  Proteins determine most of the membrane’s specific functions.  The plasma membrane and the membranes of the various organelles each have unique collections of proteins.  There are two major populations of membrane proteins: integral and peripheral. Transmembrane protein  Integral proteins penetrate the hydrophobic interior of the lipid bilayer, usually completely spanning the membrane as transmembrane proteins. o Other integral proteins extend partway into the hydrophobic interior. o The hydrophobic regions embedded in the membrane’s interior consist of stretches of nonpolar amino acids, usually coiled into alpha helices. o The hydrophilic regions of integral proteins are in contact with the aqueous environment. o Some integral proteins have a hydrophilic channel through their center that allows passage of hydrophilic substances.

 Peripheral proteins are not embedded in the lipid bilayer at all. o Instead, peripheral proteins are loosely bound to the surface of the membrane, often to integral proteins.  On the cytoplasmic side of the membrane, some membrane proteins are attached to the cytoskeleton.  On the extracellular side of the membrane, some membrane proteins attach to the fibers of the extracellular matrix.  These attachments combine to give animal cells a stronger framework than the plasma membrane itself could provide.  The proteins of the plasma membrane have six major functions:

Transport of specific solutes Cell-cell recognition, allowing other into or out of cells proteins to attach two cells together

Enzymatic activity, catalyzing Intercellular joining of adjacent step of a metabolic pathway cells with gap or tight junctions

Signal transduction, relaying Attachment to the cytoskeleton hormonal messages to the cell and extracellular matrix

 Proteins present on the surface of a cell may help outside agents invade the cell.

The genetic basis for HIV resistance

Membrane carbohydrates are important for cell-cell recognition.  Cell-cell recognition, the ability of a cell to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. o Cell-cell recognition is important in the sorting and organizing of cells into tissues and organs during development. o Recognition is also the basis for the rejection of foreign cells by the immune system. o Cells recognize other cells by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane.  Membrane carbohydrates are usually branched chains with fewer than 15 sugar units.  Membrane carbohydrates may be covalently bonded to lipids, forming glycolipids, or more commonly to proteins, forming glycoproteins.  The carbohydrates on the extracellular side of the plasma membrane vary from species to species, from individual to individual, and even from cell type to cell type within an individual. o The four human blood groups (A, B, AB, and O) differ in the carbohydrate part of glycoproteins on the surface of red blood cells.

Membranes have distinct inside and outside faces.  The inside and outside faces of membranes may differ in lipid composition.  Each protein in the membrane has a directional orientation in the membrane. Synthesis of membrane components and their orientation in the membrane

7.2 Membrane structure results in selective permeability  The fluid mosaic model helps explain how membranes regulate the cell’s molecular traffic.  A steady traffic of small molecules and ions moves across the plasma membrane in both directions.  Substances do not move across the barrier indiscriminately; membranes are selectively permeable. o The cell is able to take up many varieties of small molecules and ions and exclude others. o Substances that move through the membrane do so at different rates.  Movement of a molecule through a membrane depends on the interaction of the molecule with the hydrophobic interior of the membrane. o Nonpolar molecules, such as hydrocarbons, CO2, and O2, are hydrophobic and can dissolve in the lipid bilayer and cross easily, without the assistance of membrane proteins. o The hydrophobic interior of the membrane impedes the direct passage of ions and polar molecules, which are hydrophilic. o Polar molecules, such as glucose and other sugars, and even water, an extremely small polar molecule, cross the lipid bilayer slowly. o An ion, whether a charged atom or a molecule, and its surrounding shell of water also have difficulty penetrating the hydrophobic interior of the membrane. Permeability of the Plasma Membrane

 Proteins assist and regulate the transport of ions and polar molecules.  Cell membranes are permeable to specific ions and a variety of polar molecules, which can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane.  Some transport proteins called channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane.  The passage of water through the membrane can be greatly facilitated by channel proteins known as aquaporins.  Some transport proteins called carrier proteins bind to molecules and change shape to shuttle them across the membrane 7.3 Passive transport is diffusion of a substance across a membrane with no energy investment  Molecules have thermal energy or heat, due to their constant motion. o One result of thermal motion is diffusion, the movement of molecules of any substance to spread out in the available space.  The movements of individual molecules are random. However, the movement of a population of molecules may be directional.  In the absence of other forces, a substance diffuses from where it is more concentrated to where it is less concentrated, down its concentration gradient.

 No work must be done to move substances down the concentration gradient; diffusion is a spontaneous process, needing no input of energy.  Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances.  The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen.

Diffusion of solutes across a synthetic membrane

Osmosis is the passive transport of water.  Imagine that two sugar solutions differing in concentration are separated by a membrane that allows water through, but not sugar. How does this affect the water concentration?  In a dilute solution like most biological fluids, solutes do not affect the water concentration significantly.  However, the clustering of water molecules around the hydrophilic solute molecules makes some of the water unavailable to cross the membrane.  It is the difference in the free water concentration that is important.  In the end, the effect is the same: Water diffuses across the membrane from the region of lower solute concentration (higher free water concentration) to the region of higher solute concentration (lower free water concentration) until the solute concentrations on both sides of the membrane are equal.  The diffusion of water across a selectively permeable membrane is called osmosis.  The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms.

Osmosis

 Both solute concentration and membrane permeability affect tonicity, the ability of a surrounding solution to cause a cell to gain or lose water. o The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrating solutes) relative to the concentration of solutes in the cell itself.  If a cell without a cell wall, such as an animal cell, is immersed in an environment that is isotonic to the cell, there is no net movement of water across the plasma membrane. o Water diffuses across the membrane, but at the same rate in both directions.  If the cell is immersed in a solution that is hypertonic to the cell (containing nonpenetrating solutes), the cell loses water to its environment, shrivels, and probably dies. o For example, an increase in the salinity (saltiness) of a lake can kill aquatic animals. o If the lake water becomes hypertonic to the animals’ cells, the cells may shrivel and die.  If the cell is immersed in a solution that is hypotonic to the cell, water enters the cell faster than it leaves, and the cell swells and lyses (bursts) like an overfilled water balloon.

Water balance of living cells

Cell survival depends on the balance between water uptake and loss.  Water balance is not a problem if such a cell lives in isotonic surroundings. o Seawater is isotonic to many marine invertebrates. o The cells of most terrestrial animals are bathed in extracellular fluid that is isotonic to the cells.  Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance. o The protist Paramecium is hypertonic to the pond water in which it lives. o In spite of a plasma membrane that is less permeable to water than other cells, water continually enters the Paramecium cell. o To solve this problem, Paramecium cells have a specialized organelle, the contractile vacuole, which functions as a bilge pump to force water out of the cell.  The cells of plants, prokaryotes, fungi, and some protists are surrounded by walls.  A plant cell in a solution hypotonic to the cell contents swells due to osmosis until the elastic cell wall exerts turgor pressure on the cell that opposes further water uptake. o At this point the cell is turgid (very firm), a healthy state for most plant cells.  If a plant cell and its surroundings are isotonic, there is no movement of water into the cell. The cell becomes flaccid (limp), and the plant may wilt.  The cell wall provides no advantages when a plant cell is immersed in a hypertonic solution.  As the plant cell loses water, its volume shrinks. Eventually, the plasma membrane pulls away from the wall. This plasmolysis is usually lethal.  The walled cells of bacteria and fungi also plasmolyze in hypertonic environments.

Specific proteins facilitate the passive transport of water and selected solutes.  Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane.  The passive movement of molecules down their concentration gradient with the help of transport proteins is called facilitated diffusion. o Most transport proteins are very specific: They transport some substances but not others.  Two types of transport proteins facilitate the movement of molecules or ions across membranes: channel proteins and carrier proteins.

 Channel proteins provide hydrophilic corridors for the passage of specific molecules or ions. o For example, water channel proteins, aquaporins, greatly facilitate the diffusion of water. o Kidney cells have a high number of aquaporins, allowing them to take up water from urine before it is excreted.  Carrier proteins appear to actually translocate the solute-binding site and the solute across the membrane as the transport protein changes shape. o These shape changes may be triggered by the binding and release of the transported molecule.

Proteins that carry out facilitated diffusion Channel protein Carrier protein

 Many ion channels function as gated channels. o These channels open or close depending on the presence or absence of an electrical, chemical, or physical stimulus. o If chemical, the stimulus is a substance other than the one to be transported.

7.4 Active transport uses energy to move solutes against their gradients  Some transport proteins can move solutes across membranes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated. o The transport proteins that move solutes against a concentration gradient are all carrier proteins, rather than channel proteins.  This active transport requires the cell to expend metabolic energy and enables a cell to maintain internal concentrations of small molecules that would otherwise diffuse across the membrane. o Compared with its surroundings, an animal cell has a much higher concentration of potassium ions and a much lower concentration of sodium ions. o The plasma membrane helps maintain these steep gradients by pumping sodium out of the cell and potassium into the cell.  ATP supplies the energy for most active transport by transferring its terminal phosphate group directly to the transport protein. o This process may induce a conformational change in the transport protein, translocating the bound solute across the membrane. o The sodium-potassium pump works this way in exchanging sodium ions (Na+) for potassium ions (K+) across the plasma membrane of animal cells.

The sodium-potassium pump (active transport)

Some ion pumps generate voltage across membranes.  All cells maintain a voltage across their plasma membranes.  Voltage is electrical potential energy resulting from the separation of opposite charges.  The cytoplasm of a cell is negative in charge relative to the extracellular fluid because of an unequal distribution of cations and anions on the two sides of the membrane.  The voltage across a membrane is called a membrane potential and ranges from −50 to −200 millivolts (mV). The inside of the cell is negative compared to the outside.  The membrane potential acts like a battery. Because the inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell.  Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane. o One is a chemical force based on an ion’s concentration gradient. o The other is an electrical force based on the effect of the membrane potential on the ion’s movement.  An ion does not simply diffuse down its concentration gradient but diffuses down its electrochemical gradient. o For example, there is a higher concentration of Na+ outside a resting nerve cell than inside. o When the neuron is stimulated, gated channels open and Na+ diffuses into the cell down the electrochemical gradient. o The diffusion of Na+ is driven by the concentration gradient and by the attraction of cations to the negative side (inside) of the membrane.  Special transport proteins, called electrogenic pumps, generate the voltage gradient across a membrane.  The sodium-potassium pump, the major electrogenic pump in animals, restores the electrochemical gradient not only by the active transport of Na+ and K+, setting up a concentration gradient, but also because it pumps two K+ inside for every three Na+ that it moves out, setting up a voltage across the membrane.  In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting protons out of the cell and transferring positive charge from the cytoplasm to the extracellular solution. A proton pump

 By generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work.

In cotransport, a membrane protein couples the transport of two solutes.  A single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes in a mechanism called cotransport.  As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration (or electrochemical) gradient. o Plants commonly use the gradient of H+ generated by proton pumps, which are not technically part of the co-transport process, to drive the active transport of amino acids, sugars, and other nutrients into the cell. o One specific transport protein couples the diffusion of H+ out of the cell and the transport of sucrose into the cell. o Plants use the mechanism of sucrose-proton cotransport to load sucrose into specialized cells in the veins of leaves for distribution to non-photosynthetic organs such as roots. Cotransport: active transport driven by a concentration gradient

 An understanding of cotransport proteins, osmosis, and water balance in animal cells has helped scientists develop effective treatments for the dehydration that results from diarrhea, a serious problem in developing countries where intestinal parasites are prevalent. o Patients are given a solution to drink that contains a high concentration of glucose and salt. o The solutes are taken up by cotransport proteins on the intestinal cell surface and passed through the cells into the blood. o The resulting increase in the solute concentration of the blood causes a flow of water from the intestine through the intestinal cells into the blood, rehydrating the patient.

7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis  Small solutes and water enter or leave the cell through the lipid bilayer or by transport proteins.  Particles and large molecules, such as polysaccharides and proteins, cross the membrane via packaging in vesicles.  Like active transport, these processes require energy.  In exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane. o When the two membranes come in contact, the bilayers fuse and spill the contents to the outside.  Many secretory cells use exocytosis to export products. o Pancreatic cells secrete insulin into the blood by exocytosis. o Neurons use exocytosis to release neurotransmitters that signal other neurons or muscle cells. 19

o When plant cells are making walls, exocytosis delivers proteins and certain carbohydrates from Golgi vesicles to the outside of the cell. Exocytosis

 During endocytosis, a cell brings in biological molecules and particulate matter by forming new vesicles from the plasma membrane. ○ Endocytosis is a reversal of exocytosis, although different proteins are involved in the two processes.  In endocytosis, a small area of the plasma membrane sinks inward to form a pocket. o As the pocket deepens, it pinches in to form a vesicle containing the material that had been outside the cell.  There are three types of endocytosis: phagocytosis (“cellular eating”), pinocytosis (“cellular drinking”), and receptor-mediated endocytosis.

 Receptor-mediated endocytosis enables a cell to acquire bulk quantities of specific materials that may be in low concentrations in the environment.

 Human cells use this process to take in cholesterol for use in the synthesis of membranes and as a precursor for the synthesis of steroids. o Cholesterol travels in the blood in low-density lipoproteins (LDL), complexes of protein and lipid. o These lipoproteins act as ligands by binding to LDL receptors on membranes and entering the cell by endocytosis.

36.2 Different mechanisms transport substances over short or long distances.

Solute transport across plant cell plasma membranes

Differences in water potential drive water transport in plant cells.  The survival of plant cells depends on their ability to balance water uptake and loss.  The absorption or loss of water by a cell occurs by osmosis, the diffusion of water across a membrane.  The physical property that predicts the direction in which water will flow is called water potential (), a quantity that includes the effects of solute concentration and physical pressure. o Free water moves from regions of higher water potential to regions of lower water potential if there is no barrier to its flow. o For example, if a plant cell is immersed in a solution with higher water potential than the cell, osmotic uptake of water causes the cell to swell.  As it moves, water can perform work, such as cell expansion. o The word potential in the term water potential refers to water’s potential energy—water’s capacity to perform work when it moves from a region of higher water potential to a region of lower water potential. 21

 Water potential is represented by the Greek letter .  Plant biologists measure  in units called megapascals (MPa), where 1 MPa is equal to about 10 atmospheres of pressure. o An atmosphere is the pressure exerted at sea level by a volume of air extending though the height of the atmosphere—about 1 kg of pressure per square centimeter. o The internal pressure of a plant cell is approximately 0.5 MPa, twice the air pressure inside a tire.

Both pressure and solute concentration affect water potential.  Both solute concentration and physical pressure can effect water potential, as expressed in the water potential equation, where P is the pressure potential and S is the solute potential (or osmotic potential):  = S + P  The solute potential (S) of a solution is directly proportional to its molarity. o Solute potential is also called osmotic potential because solutes affect the direction of osmosis.

 By definition, the S of pure water is 0.  Solutes bind water molecules, reducing the number of free water molecules and lowering the capacity of water to move and do work.

 Adding solutes always lowers water potential; the S of a solution is always negative.  Pressure potential (P) is the physical pressure on a solution and can be positive or negative relative to atmospheric pressure. o The water in the hollow, nonliving xylem cells (vessel elements and tracheids) may be under negative pressure of less than −2 MPa.  Water in living cells is usually under positive pressure. o The cell contents press the plasma membrane against the cell wall, and the cell wall then presses against the protoplast, producing turgor pressure. o This internal pressure is critical for plant function because it helps maintain the stiffness of plant tissues and serves as the driving force for cell elongation.

Water potential affects the uptake and loss of water in plant cells.  Remember: Water moves from regions of higher water potential to regions of lower water potential.

 In a flaccid cell, P = 0 MPa and the cell is limp.  If this cell is placed in a solution with a higher solute concentration (and, therefore, a lower ), water will leave the cell by osmosis. o The cell’s protoplast undergoes plasmolysis by shrinking and pulling away from its wall.  If a flaccid cell is placed in pure water (= 0 MPa), the cell will have lower water potential than pure water due to the presence of solutes, and water will enter the cell by osmosis.

 As the cell begins to swell, it will push against the cell wall, which exerts turgor pressure.  The partially elastic wall will push back and confine the pressurized protoplast until this pressure is great enough to offset the tendency for water to enter the cell because of solutes.

 When P and S are equal in magnitude but opposite in sign,  = 0, and the cell has reached a dynamic equilibrium with the environment, with no further net movement of water.  A walled cell with a greater solute concentration than its surroundings is turgid, or firm. o You can see the effects of turgor loss in wilting, the drooping of leaves and stems as plant cells lose water.

Aquaporins affect the rate of water transport across membranes.  A difference in water potential determines the direction of water movement across membranes, but how do water molecules actually cross the membrane?  Both plant and animal membranes have specific transport proteins, aquaporins, which facilitate the passive movement of water across a membrane.  Aquaporins do not affect the water potential gradient or the direction of water flow, but rather increase the rate at which water moves osmotically across the membrane.  Aquaporin channels are highly dynamic: their permeability is decreased by increases in cytoplasmic calcium ions or increases in cytoplasmic pH.

44.2 An animal’s nitrogenous wastes reflect its phylogeny and habitat.  Because most metabolic wastes must be dissolved in water when they are removed from the body, the type and quantity of waste products may have a large impact on water balance.  The nitrogenous breakdown products of proteins and nucleic acids are among the most important wastes in terms of their effect on osmoregulation.

 During their breakdown, enzymes remove nitrogen in the form of ammonia (NH3), a small and very toxic molecule. o Some animals excrete ammonia directly, but many species first convert ammonia to other compounds that are less toxic but more costly to produce.

 Animals excrete nitrogenous wastes in different forms that vary in toxicity and energy cost.  Animals that excrete nitrogenous wastes as ammonia need access to lots of water, so ammonia excretion is most common in aquatic species. o This is because ammonia is very soluble and can be tolerated at only very low concentrations. o Many invertebrates release ammonia across the whole body surface.

+ o In fishes, most of the ammonia is lost as ammonium ions (NH4 ) at the gill epithelium.  Ammonia excretion is much less suitable for land animals. o Because ammonia is so toxic, it can be transported and excreted only in large volumes of very dilute solutions. o Most terrestrial animals and many marine organisms (which tend to lose water to their environment by osmosis) do not have access to sufficient water for ammonia excretion.  Instead, mammals, most adult amphibians, sharks, and some marine bony fishes and turtles excrete mainly urea. o Urea is synthesized in the liver by combining ammonia with carbon dioxide and is excreted by the kidneys.  The main advantage of urea is its low toxicity, so it can be transported in the circulatory system and stored safely at high concentrations. o Urea’s low toxicity reduces the amount of water needed for nitrogen excretion when a concentrated solution of urea rather than a dilute solution of ammonia is released.  The main disadvantage of urea is that animals must expend energy to produce it from ammonia.  Many amphibians excrete mainly ammonia (saving energy) when they are aquatic tadpoles, and then switch to urea (reducing excretory water loss) as land-dwelling adults.  Land snails, insects, birds, and many reptiles excrete uric acid as the main nitrogenous waste. o Bird droppings, or guano, are a mixture of white uric acid and brown feces.  Like urea, uric acid is relatively nontoxic.  Unlike either ammonia or urea, however, uric acid is largely insoluble in water and can be excreted as a semisolid paste with very little water loss.  While saving more water than urea, uric acid is more energetically expensive to produce.  Uric acid and urea represent different adaptations for excreting nitrogenous wastes with minimal water loss.

Nitrogenous waste variations in animals

 Mode of reproduction appears to have been important in determining the type of nitrogenous wastes an animal excretes. o Soluble wastes can diffuse out of a shell-less amphibian egg or be carried away by the mother’s blood in a mammalian embryo. o Uric acid precipitates out of solution and can be stored within the egg as a harmless solid left behind when the animal hatches.  The type of nitrogenous waste reflects habitat as well as evolutionary lineage. o For example, terrestrial turtles (which often live in dry areas) excrete mainly uric acid, whereas aquatic turtles excrete both urea and ammonia.  The amount of nitrogenous waste produced is coupled to the energy budget and depends on how much and what kind of food an animal eats. o Because they use energy at high rates, endotherms eat more food—and thus produce more nitrogenous wastes—per unit volume than ectotherms. o Carnivores (which derive much of their energy from dietary proteins) excrete more nitrogen than animals that obtain most of their energy from lipids or carbohydrates.

44.3 Diverse excretory systems are variations on a tubular theme.  Much of this regulation is handled by excretory systems, which are central to homeostasis because they dispose of metabolic wastes and control body fluid composition by adjusting the rates of loss of particular solutes. Most excretory systems produce urine by refining a filtrate derived from body fluids.  Although excretory systems are diverse, nearly all produce urine in a process that involves several steps.  First, body fluid (blood, coelomic fluid, or hemolymph) is collected.  The initial fluid collection usually involves 1) filtration, driven by hydrostatic pressure, through selectively permeable membranes consisting of a single layer of transport epithelium, driven by hydrostatic pressure.  Water and small solutes, such as salts, sugars, amino acids, and nitrogenous wastes, form a solution called the filtrate.  It is important to recover useful molecules from the filtrate and return them to the body fluids.  Excretory systems specifically transport materials into or out of the filtrate in a process of selective 2) reabsorption. o Valuable solutes—including glucose, certain salts, vitamins, hormones, and amino acids—are reabsorbed by active transport in excretory systems.  Nonessential solutes and wastes are left in the filtrate or added to it by selective 3) secretion, which also uses active transport. o The pumping of various solutes also adjusts the osmotic movement of water into or out of the filtrate.  The processed filtrate containing nitrogenous wastes is 4) excreted as urine. Key steps of excretory system

The systems that perform basic excretory functions vary widely among animal groups.  In all animals, excretory systems are built of a complex network of tubules that provide a large surface area for the exchange of water and solutes, including nitrogenous wastes.  Flatworms have an excretory system called protonephridia, consisting of a branching network of dead-end tubules connected to external openings. o The openings are capped by a flame bulb with a tuft of cilia that draws water and solutes from the interstitial fluid, through the flame bulb, and into the tubule system.  Protonephridia are found in rotifers, some annelids, larval molluscs, and lancelets.  In freshwater flatworms, the major function of the flame-bulb system is osmoregulation, whereas most metabolic wastes diffuse across the body surface or are excreted into the gastrovascular cavity.  Metanephridia, a tubular excretory system found in most annelids, consist of internal openings that collect body fluids from the coelom through a ciliated funnel. o Beating of the cilia draws fluid into a coiled collecting tubule, which includes a storage bladder that opens to the outside.  An earthworm’s metanephridia have both excretory and osmoregulatory functions. Protonephridia and Metanephridia

 Insects and other terrestrial arthropods have organs called Malpighian tubules that remove nitrogenous wastes and also function in osmoregulation. o The tubules open into the digestive system and dead-end at tips that are immersed in the hemolymph. o The filtration step common to other excretory systems is absent. Malpighian tubules of an insect

 The transport epithelium lining the tubules secretes certain solutes, including nitrogenous wastes, from the hemolymph into the lumen of the tubule. o Water follows the solutes into the tubule by osmosis, and the fluid then passes into the rectum, where most of the solutes are pumped back into the hemolymph. o Water again follows the solutes, and the nitrogenous wastes, primarily insoluble uric acid, are eliminated along with the feces.  This system is highly effective in conserving water and is one of several key adaptations contributing to the tremendous success of insects on land.  Some terrestrial insects have an additional adaptation for water balance: the rectal end of their gut enables water uptake from the air.  The kidneys of vertebrates usually function in both osmoregulation and excretion.  Like the excretory organs of most animal phyla, kidneys are built of tubules.  The vertebrate excretory system includes a dense network of capillaries intimately associated with the tubules, along with ducts and other structures that carry urine out of the tubules and kidney and eventually out of the body.

A closer look at the mammalian excretory system.  Mammals have a pair of bean-shaped kidneys.  Each kidney is supplied with blood by a renal artery and drained by a renal vein. o In humans, the kidneys account for less than 1% of body weight, but they receive about 25% of the blood exiting the heart.  Urine exits each 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, which empties to the outside near the vagina in females or through the penis in males. o Sphincter muscles near the junction of the urethra and the bladder control urination.  The mammalian kidney has two distinct regions: an outer renal cortex and an inner renal medulla.  Both regions are packed with microscopic excretory tubules, nephrons, and their associated blood vessels.  The mammalian excretory system

 Each nephron consists of a single long tubule and a ball of capillaries, called the glomerulus.  The blind end of the tubule forms a cup-shaped swelling, called Bowman’s capsule, that surrounds the glomerulus.  Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule. o The porous capillaries, along with specialized capsule cells, are permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins. o The filtrate in Bowman’s capsule contains salt, glucose, amino acids, vitamins, nitrogenous wastes such as urea, and other small molecules. o Because filtration of small

molecules is nonselective, the mixture mirrors the relative concentrations of solutes in blood plasma.

 From Bowman’s capsule, the filtrate passes through three regions of the nephron: the proximal tubule; the loop of Henle, a hairpin turn with a descending limb and an ascending limb; and the distal tubule.  The distal tubule empties into a collecting duct, which receives processed filtrate from many nephrons.  The many collecting ducts empty into the renal pelvis, which is drained by the ureter.  In the human kidney, about 85% of the nephrons, the cortical nephrons, have reduced loops of Henle and are almost entirely confined to the renal cortex.  The other 15%, the juxtamedullary nephrons, have well-developed loops that extend deeply into the renal medulla. o It is the juxtamedullary nephrons that enable mammals to produce urine that is hyperosmotic to body fluids, conserving water.  The nephron and the collecting duct are lined by a transport epithelium that processes the filtrate to form the urine. Their most important task is to reabsorb solutes and water. o This reabsorption reduces 180 L of initial filtrate to about 1.5 L of urine to be voided.  Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus.  The capillaries converge as they leave the glomerulus, forming an efferent arteriole. o This vessel subdivides again into the peritubular capillaries, which surround the proximal and distal tubules. o Additional capillaries extend downward to form the vasa recta, a loop of capillaries that serves the loop of Henle. 28

 The tubules and capillaries are immersed in interstitial fluid, through which substances diffuse.  The tubules and capillaries are immersed in interstitial fluid, through which various materials diffuse between the plasma in the capillaries and the filtrate within the nephron tubule.  The vasa recta and the loop of Henle function together as part of a countercurrent system that enhances nephron efficiency.

44.4 The nephron is organized for stepwise processing of blood filtrate.  The porous capillaries and specialized cells of Bowman’s capsule are permeable to water and small solutes, but not blood cells or large molecules, such as plasma proteins. o Thus, the filtrate produced in the capsule contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules. o Because such molecules pass freely between glomerular capillaries and Bowman’s capsule, the concentrations of these substances in filtrate mirror those in blood plasma.  As the filtrate moves through the nephron and collecting duct, each region contributes to the stepwise processing of filtrate into urine.  The nephron and collecting duct: regional functions of transport

1. One of the most important functions of the proximal tubule is the reabsorption of ions, water, and valuable nutrients from the initial filtrate volume.  NaCl (salt) in the filtrate diffuses into the cells of the transport epithelium.  The epithelial cells actively transport Na+ into the interstitial fluid.

o This transfer of positive charge drives the passive transport of Cl− out of the tubule, as well as the movement of more Na+ from the lumen into the tubule by facilitated diffusion and cotransport mechanisms.  As salt moves from the filtrate to the interstitial fluid, water follows by osmosis.  Salt and water diffuse into the peritubular capillaries.  Glucose, amino acids, potassium ions, and other essential molecules are also actively or passively transported from the filtrate to the interstitial fluid, and then move into the peritubular capillaries.  Processing of filtrate in the proximal tubule helps maintain a relatively constant pH in body fluids. + + o Cells of the transport epithelium secrete H in the form of ammonium ions (NH4 ). o The more acidic the filtrate, the more ammonia the cells secrete. - o The proximal tubules also reabsorb about 90% of the buffer bicarbonate (HCO3 ) from the filtrate.  As the filtrate passes through the proximal tubule, materials to be excreted become concentrated. o Waste products such as urea remain in the filtrate, while water and salts leave.  Some toxic materials are actively secreted into the filtrate from surrounding tissues. o Drugs that have been processed in the liver pass from the peritubular capillaries and into the interstitial fluid, where they are actively secreted into the lumen of the proximal tubule.

2. Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle.  The transport epithelium in the descending limb is freely permeable to water with water channels formed by the protein aquaporin, but it is not very permeable to salt and other small solutes. o For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate.  Because the osmolarity of the interstitial fluid becomes progressively greater from the outer cortex to the inner medulla, the filtrate moving within the descending loop of Henle continues to lose water.

3. In contrast to the descending limb, the transport epithelium of the ascending limb of the loop of Henle is permeable to salt, not water.  Rare in biological membranes, the membrane in the ascending limb is impermeable to water. o Unlike the descending limb, the ascending limb has a transport epithelium studded with ion channels, but not water channels.  The ascending limb has two specialized regions: a thin segment near the loop tip and a thick segment adjacent to the distal tubule. o As filtrate ascends the thin segment of the ascending limb, NaCl diffuses out of the permeable tubule into the interstitial fluid, increasing the osmolarity of the medulla. o The active transport of salt from the filtrate into the interstitial fluid takes place in the thick segment of the ascending limb.  By losing salt without giving up water, the filtrate becomes progressively more dilute as it moves up to the cortex in the ascending limb of the loop.

4. The distal tubule plays a key role in regulating the K+ and NaCl concentrations of body fluids.  This regulation involves variation in the amount of K+ that is secreted into the filtrate and the amount of NaCl that is reabsorbed from the filtrate. o Like the proximal tubule, the distal tubule also contributes to pH regulation by the controlled + − secretion of H and the reabsorption of bicarbonate (HCO3 ).

5. The collecting duct carries the filtrate through the medulla to the renal pelvis.  The transport epithelium of the nephron and collecting duct processes the filtrate, forming the urine.  One of this epithelium’s most important tasks is reabsorption of solutes and water. o Under normal conditions, approximately 1,600 L of blood flows through a pair of human kidneys each day, a volume about 300 times the total volume of blood in the body. 30

o From this blood, the nephrons and collecting ducts process about 180 L of initial filtrate. o Of this, about 99% of the water and nearly all of the sugars, amino acids, vitamins, and other organic nutrients are reabsorbed into the blood, leaving only about 1.5 L of urine to be transported to the bladder.  As filtrate passes along the transport epithelium of the collecting duct, hormonal control of permeability and transport determines the extent to which the urine becomes concentrated.  When the kidneys are conserving water, aquaporin channels in the collecting duct allow water molecules to cross the epithelium, while the epithelium remains impermeable to salt and, in the renal cortex, to urea.  As the collecting duct traverses the gradient of osmolarity in the kidney, the filtrate becomes increasingly concentrated as it loses more and more water by osmosis to the hyperosmotic interstitial fluid.  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. o Along with NaCl, this urea contributes to the high osmolarity of the interstitial fluid in the medulla.  The net result is urine that is hyperosmotic to the general body fluids.  To produce dilute rather than concentrated urine, the kidney actively reabsorbs salts without allowing water to follow by osmosis. o Aquaporin molecules are removed from the epithelium, and NaCl is actively transported out of the filtrate.  The collecting duct epithelium is controlled by hormones that maintain homeostasis for osmolarity, blood pressure, and blood volume.

The mammalian kidney’s ability to conserve water is a key terrestrial adaptation.  The osmolarity of human blood is about 300 mOsm/L, but the kidney can excrete urine up to four times as concentrated—about 1,200 mOsm/L.  In a mammalian kidney, the maintenance of osmotic differences and the production of hyperosmotic urine are possible only because considerable energy is expended by the active transport of solutes against concentration gradients. o The nephrons—especially the loops of Henle—can be thought of tiny energy-consuming machines whose function is to produce a region of high osmolarity in the kidney, which can then extract water from the urine in the collecting duct.  The two primary solutes affecting osmolarity are NaCl, which is deposited in the renal medulla by the loop of Henle, and urea, which passes across the epithelium of the collecting duct in the inner medulla.  The juxtamedullary nephrons, which maintain an osmotic gradient in the kidney and use that gradient to excrete hyperosmotic urine, are the key to understanding the physiology of the mammalian kidney as a water-conserving organ.  As the filtrate flows from the cortex to the medulla in the descending limb of the loop of Henle, water leaves the tubule by osmosis.  NaCl diffusing from the ascending limb helps maintain a high osmolarity in the interstitial fluid of the renal medulla.  Thus, the two limbs of the loop of Henle cooperate in generating the gradient of osmolarity in the interstitial fluid of the kidney.  The loop of Henle has several qualities common to a countercurrent system. o The countercurrent system involving the loop of Henle expends energy to actively transport NaCl from the filtrate in the upper part of the ascending limb of the loop. o Such countercurrent systems, which expend energy to create concentration gradients, are called countercurrent multiplier systems.

o The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the interior of the kidney, enabling the kidney to form concentrated urine.  The vasa recta is also a countercurrent system, with descending and ascending vessels carrying blood in opposite directions through the kidney’s osmolarity gradient.  The countercurrent-like characteristics of the loop of Henle and the vasa recta maintain the steep osmotic gradient between the medulla and the cortex.  This active transport and other active transport systems in the kidney consume considerable ATP, requiring the kidney to have one of the highest relative metabolic rates of any organ.  By the time the filtrate reaches the distal tubule, it is actually hypoosmotic to body fluids because of active transport of NaCl out of the thick segment of the ascending limb.  As the filtrate descends again toward the medulla in the collecting duct, water is extracted by osmosis into the hyperosmotic interstitial fluids, but salts cannot diffuse in because the epithelium is impermeable to salt.  This process concentrates salt, urea, and other solutes in the filtrate. o Some urea leaks out of the lower portion of the collecting duct, contributing to the high interstitial osmolarity of the inner medulla.  Before leaving the kidney, the urine may reach the osmolarity of the interstitial fluid in the inner medulla, which can be as high as 1,200 mOsm/L. o Although isoosmotic to the inner medulla’s interstitial fluid, the urine is hyperosmotic to blood and interstitial fluid elsewhere in the body. How the kidney concentrates urine

Diverse adaptations of the vertebrate kidney have evolved in different environments.  The juxtamedullary nephron is a key adaptation to terrestrial life, enabling mammals to get rid of salts and nitrogenous wastes without squandering water.  The remarkable ability of the mammalian kidney to produce hyperosmotic urine is completely dependent on the precise arrangement of the tubules and collecting ducts in the renal cortex and medulla.  Variations in nephron structure and function equip the kidneys of different vertebrates for osmoregulation in their various habitats. o Mammals that excrete the most hyperosmotic urine, such as hopping mice and other desert mammals, have exceptionally long loops of Henle that extend deep into the medulla. Long loops maintain steep osmotic gradients, resulting in very concentrated urine. o In contrast, aquatic mammals like beavers, which rarely face problems of dehydration, have nephrons with short loops, resulting in much less concentrated urine.  The vampire bat’s ability to alternate rapidly between producing large amounts of dilute urine and small amounts of very hyperosmotic urine is an essential part of its adaptation to an unusual food source.  Birds, like mammals, have kidneys with juxtamedullary nephrons that specialize in conserving water. o However, the nephrons of birds have much shorter loops of Henle than do mammalian nephrons, and bird kidneys cannot concentrate urine to the osmolarities achieved by mammalian kidneys. o The main water conservation adaptation of birds is the use of uric acid as the nitrogen excretion molecule.  The kidneys of other reptiles, which have only cortical nephrons, produce urine that is, at most, isoosmotic to body fluids. o However, the epithelium of the cloaca helps conserve fluid by reabsorbing some of the water present in urine and feces. o Also, like birds, most other terrestrial reptiles excrete nitrogenous wastes as uric acid.  In contrast to mammals and birds, a freshwater fish must excrete excess water because it is hyperosmotic to its surroundings.  Amphibian kidneys function much like those of freshwater fishes. o When a frog is in fresh water, its skin accumulates certain salts from the water by active transport, and the kidneys excrete dilute urine. o On land, where dehydration is the most pressing problem, frogs conserve body fluid by reabsorbing water across the epithelium of the urinary bladder.  Marine bony fishes, being hypoosmotic to their surroundings, have the opposite problem of their freshwater relatives.

44.5 Hormonal circuits link kidney function, water balance, and blood pressure.  One important aspect of the mammalian kidney is its ability to adjust both the volume and the osmolarity of urine, depending on the animal’s water and salt balance and the rate of urea production. o With high salt intake and low water availability, a mammal can excrete urea and salt with minimal water loss in small volumes of hyperosmotic urine. o If salt is scarce and fluid intake is high, the kidney can get rid of excess water with little salt loss by producing large volumes of hypoosmotic urine

A combination of nervous and hormonal controls manages the osmoregulatory function of the mammalian kidney.  Antidiuretic hormone (ADH), also called vasopressin, is important in regulation of water balance.  ADH is produced in the hypothalamus of the brain and stored in and released from the pituitary gland, which lies just below the hypothalamus.  Osmoreceptor cells in the hypothalamus monitor the osmolarity of the blood and regulate the release of ADH.

o When blood osmolarity rises above a set point of 300 mOsm/L, more ADH is released into the bloodstream and reaches the kidney. o ADH induces the epithelium of the distal tubules and collecting ducts to become more permeable to water, amplifying water reabsorption. o This reabsorption concentrates urine, reduces urine volume, and lowers blood osmolarity back toward the set point.

Control of collecting duct permeability by ADH

1) ADH binds to membrane receptors

2) Receptor triggers signal transduction

3) Vesicles with aquaporin water channels are inserted into membrane lining lumen of the collecting duct.

4) Aquaporin channels reabsorption of water from collecting duct into interstitial fluid.

 By negative feedback, the subsiding osmolarity of the blood reduces the activity of osmoreceptor cells in the hypothalamus, and less ADH is secreted. o ADH alone prevents further movements away from the set point, but only intake of additional water in food and drink can bring osmolarity back down to 300 mOsm/L.  Conversely, if a large intake of water has reduced blood osmolarity below the set point, very little ADH is released.

Regulation of fluid retention in the kidney

 A low level of ADH decreases the permeability of the distal tubules and collecting ducts, so that water reabsorption is reduced, resulting in an increased discharge of dilute urine. o Diuresis refers to increased urination, and ADH is called antidiuretic hormone because it opposes this state.  ADH influences water uptake in the kidney through regulation of the water-selective channels formed by aquaporin proteins. o Binding of ADH to its receptor leads to a transient increase in the number of aquaporin molecules in the membranes of collecting duct cells, increasing water uptake and reducing urine volume.  Mutations that prevent ADH production or that inactivate the human ADH receptor block the increase in the number of channels and thus the ADH response. o The resulting disorder, diabetes insipidus, causes severe dehydration and solute imbalance due to production of urine that is abnormally large in volume and dilute.  Even in the absence of genetic changes, certain substances can alter the regulation of osmolarity. o For example, alcohol can disturb the water balance by inhibiting the release of ADH, leading to excessive urinary water loss and dehydration (which may cause some of the symptoms of a hangover).  Normally, blood osmolarity, ADH release, and water reabsorption in the kidney are all linked in a feedback loop that contributes to homeostasis.