Podcast
Questions and Answers
What is the primary role of aquaporins in the kidneys?
What is the primary role of aquaporins in the kidneys?
Which of the following best describes an amphipathic compound?
Which of the following best describes an amphipathic compound?
Which term describes a solution with a lower solute concentration compared to another solution?
Which term describes a solution with a lower solute concentration compared to another solution?
What process is primarily responsible for the movement of water across a semipermeable membrane in response to solute concentrations?
What process is primarily responsible for the movement of water across a semipermeable membrane in response to solute concentrations?
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What distinguishes tonicity from osmolarity?
What distinguishes tonicity from osmolarity?
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In a hypertonic solution, what effect does this have on a cell?
In a hypertonic solution, what effect does this have on a cell?
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What is osmotic pressure?
What is osmotic pressure?
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What is the consequence of antidiuretic hormone (ADH) on cell volume?
What is the consequence of antidiuretic hormone (ADH) on cell volume?
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Why are macromolecules not stored in their monomeric form?
Why are macromolecules not stored in their monomeric form?
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Which of the following best describes the hydrophobic effect?
Which of the following best describes the hydrophobic effect?
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What is the primary role of aquaporins in cell membranes?
What is the primary role of aquaporins in cell membranes?
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Which statement accurately describes the interstitial fluid?
Which statement accurately describes the interstitial fluid?
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How does osmosis occur?
How does osmosis occur?
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What is defined as the pressure required to stop the net flow of water molecules during osmosis?
What is defined as the pressure required to stop the net flow of water molecules during osmosis?
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Which fluid compartment contains the majority of the body's total-body water?
Which fluid compartment contains the majority of the body's total-body water?
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How does the body primarily regulate fluid balance?
How does the body primarily regulate fluid balance?
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What primarily determines tonicity of a solution?
What primarily determines tonicity of a solution?
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What characterizes a hypertonic solution in relation to cells?
What characterizes a hypertonic solution in relation to cells?
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What is the typical total body water percentage in a young adult man?
What is the typical total body water percentage in a young adult man?
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Which of the following best describes the function of sodium-potassium ATPase pumps?
Which of the following best describes the function of sodium-potassium ATPase pumps?
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Study Notes
Water Functions
- Chemical reactions
- Nutrient catabolism
- Body temperature regulation
- Water has a high specific heat, does not readily change temperature
- Sweat facilitates heat removal
- Lubrication and protection
- Secretions facilitate body processes as well as cushion and protect tissues
- Solvent and transport medium
- Water in body fluids contains numerous dissolved substances (solutes)
- Maintenance of blood volume
- Water helps to maintain blood pressure and sustain cardiovascular system function
- Acid-base (pH) balance
- Water is needed for reactions involving buffers
Hydrogen Bond and H₂O Properties
- Hydrogen atom shares an electron pair with oxygen
- Oxygen attracts electrons more strongly
- Hydrogen partial positive
- Oxygen partial negative
- Attraction between oxygen of one water molecule and hydrogen of another
- Hydrogen bond
- Cohesive forces
H₂O Forms H-Bonds with Polar Solutes
- Hydrogen bonds are not unique to water
- Readily form between an electronegative atom and a hydrogen atom covalently bonded to another electronegative atom
- H-C do not participate in hydrogen bonding
- Butane (CH3(CH2)2CH3) has a boiling point of only -0.5°C
- Butanol (CH3(CH2)2CH2OH) has a boiling point of 117°C
H₂O Forms H-Bonds with Polar Solutes
- Uncharged but polar biomolecules dissolve readily in water
- Hydrogen bond between hydroxyl (OH) group or carbonyl oxygen and H₂O
- Alcohols, aldehydes, ketones and compounds containing N—H form hydrogen bonds with H₂O
H₂O Interacts with Charged Solutes
- Water plays an important role in the solubility of biomolecules
- Water is a polar solvent
- Hydrophilic dissolves in water
- "Water loving"
- Greek, hydros means "water" and philia means "friendship"
- Nonpolar solvents easily dissolve hydrophobic biomolecules
- "Water fearing"
- Greek, phobos means "fear”
H₂O as a Solvent
- Water dissolves salts such as NaCl by hydrating and stabilizing the Na⁺ and Cl⁻ ions
- Readily dissolves charged biomolecules
- Compounds with functional groups such as ionized carboxylic acids (-COO⁻), protonated amines (-NH₃⁺)
- Water replaces solute-solute H-bonds with solute-water H-bonds
Nonpolar Gases Poorly Soluble in H₂O
- Biologically important gases are nonpolar
- O₂ and N₂, electrons are shared equally by both atoms
- CO₂, C=O bond is polar, but the two dipoles cancel each other out
- Water soluble "carrier proteins"
- Hemoglobin and myoglobin
- CO₂ + H₂O → H₂CO₃
Nonpolar Force Unfavorable Changes
- Biomolecules with polar groups readily dissolve in water
- Glucose can form hydrogen bonds with multiple water molecules
- Uncharged nonpolar molecules disrupt hydrogen bonds between water molecules without forming new H-bonds
- Water molecules form a water shell – motion is restricted
Amphipathic
- Amphipathic compounds contain both polar and nonpolar regions
- Nonpolar regions cluster together, polar regions maximize interactions with each other and the solvent
- Hydrophobic effect
- Micelles - stable structures of amphipathic compounds
- Biological membranes
Hydrophobic Effect
- Contribute to the structure and function of biomolecules
- Hydrophobic portions cluster together away from water
- Leucine and isoleucine often found in the interior of folded protein
- Hydrophobic amino acids minimize exposure to water
Hydrophobic Effect
- Hydrophobic effects between nonpolar amino acids in proteins play a major role in protein folding
- Nonpolar collapse into the core of the protein
- Polar amino acids located on the surface to increase solubility
- Stabilize the overall three-dimensional structure
Water and Sodium Balance
- Water movement among the various compartments is strongly affected by sodium in the ECF
- Osmotic pressure regulates the water movement between interstitial and intracellular space
- Hydrostatic pressure contributes to movement across capillary walls
Osmosis
- Water spontaneously moves across a semipermeable membrane from low solute to higher solute concentration
- Osmosis or osmotic flow
- Water molecules will diffuse by osmosis from higher water concentration to lower water concentration
- Net effect is solute concentration becomes equal in two solutions
Osmotic Pressure
- Consequence of osmosis
- Pressure required to stop the net flow of water molecules
- Depends on the number of solute molecules
- Solutions with the same osmotic pressure are isosmotic
- Hyperosmotic if a solution has a higher osmotic pressure
- Hyposmotic if a solution has a lower osmotic pressure
Osmolarity/Osmolality Regulate Volume
- The unit of measurement of osmotic pressure is osmole (Osm)
- Osmolarity is a measure of a solute's ability to generate osmotic pressure
- Number of osmoles of solute per liter of solution (Osm/L)
- Osmolality uses water mass in place of volume
- Number of osmoles per kilogram of solvent (Osmol/kg H₂O)
- Glucose does not dissociate in solution – 180 mg = 1 mmol can potentially generate 1 mOsm of osmotic pressure
- NaCl → Na⁺ and Cl⁻ 1 mmol NaCl will generate ~2 mOsm of osmotic pressure
Tonicity
- Osmolarity/osmolality is determined by total concentration of all solutes present
- In contrast, tonicity is determined by the concentration of only those solutes that do not enter the cell
- Extracellular osmolarity = cytosol are isotonic relative to that cell
- Hypertonic: ↑ osmolarity
- Hypotonic: ↓ osmolarity
Glucose Storage
- Effect of solutes on osmolarity depends on the number of dissolved particles, not their mass
- Macromolecules have less effect on osmolarity than equal mass of their monomeric components
- Storing fuel as a polysaccharide (glycogen) rather than glucose avoids an increase in osmotic pressure
Nature and Distribution of Solutes
- Osmolarity of the ECF is regulated to reduce deleterious osmotic movement of water
- Cell membranes allow passage of water in and out of cells
- Osmotic pressure associated with solute concentrations influences water movement, which occurs by osmosis
- Solutes maintain osmolarity and water distribution
- Within cells (intracellular), potassium represents the major cation and phosphate as the major anion
- In contrast to the intracellular fluid, the ECF contains sodium as the major cation and chloride as the major anion
- Sodium-potassium ATPase pumps maintain the intracellular K⁺ and extracellular Na⁺
Aquaporins Increase H₂O Permeability
- Water molecules can move across cell membranes by simple diffusion
- Plasma membranes of many cell types contain aquaporins (AQPs)
- Family of membrane channel proteins
- Allow water and a few other small uncharged molecules
- Greatly increase water permeability
- Transport water in either direction depending on osmotic gradient
- Tetramer of identical 28-kDa subunits
- Each subunit contains six membrane-spanning a-helices that form a central pore
- Several water molecules can move simultaneously—hydrophilic amino acids extend into the middle of the channel
Water Reabsorption
- Aquaporin 2 is found in the kidney epithelial cells
- Absorb water from the urine
- When water intake exceeds homeostatic needs, dilute urine passes through the collecting ducts to the bladder
- If there is a need to conserve water, fluid can be recovered
- Recovery of water and final urine concentration is governed by the presence of AQPs and regulated by antidiuretic hormone (ADH; also known as arginine vasopressin or vasopressin)
- When cells are in their resting state and water is being excreted to form urine, aquaporin 2 is sequestered inside of the cell
- When ADH/vasopressin binds to the cell-surface receptor → aquaporin 2-containing vesicles fuse with the plasma membrane
- Increasing the rate of water uptake and return to circulation
- When water intake increases and circulating ADH/vasopressin levels fall, AQPs are removed from the membrane
Effects of Antidiuretic Hormone (ADH)
- In humans, water input is controlled through thirst, water output is adjusted by the kidneys
- Water deficiency, kidneys diminish excretion
- Water excess, kidneys ↑ water excretion and urine flow
- Renal excretion of water is controlled by ADH
- Urine volume is linked to solute amount and ADH levels
Control of ADH Secretion
- ADH decreases urine output → more concentrated urine
- Increase in plasma osmolality → release of ADH
- Osmoreceptor cells shrink → ↑ Plasma ADH
- ↑ Water reabsorption → Osmotically concentrated urine
- ADH and thirst protect against hypernatremia
- Decrease in plasma osmolality → Osmoreceptor cells swell
- ↓ ADH release → ↓ Water reabsorption
- Dilute urine is excreted
- Plasma osmolality restored by elimination of excess water
- Urine osmolality is a linear function of plasma ADH
- Blood volume controls ADH release
- ↑ Blood volume inhibits ADH release
- ↓ Blood volume (hypovolemia) stimulates ADH release
- Excess volume, ↓ plasma ADH would promote excretion
- With hypovolemia, ↑ plasma ADH would promote reabsorption or conservation of water
- Receptors for blood volume include stretch receptors in the right atrium of the heart and in the pulmonary veins
- More stretch results in more impulses to the brain, which inhibit ADH release
- Producing large volume of dilute urine when lying down in bed at night, exposed to cold weather, or when immersed in water
- Blood into more central vessels
- Increase central blood volume
Diabetes Insipidus (DI)
- Inactivating mutations in either vasopressin receptor or aquaporin 2 gene
- Central (CDI): lack of ADH release
- Nephrogenic (NDI): resistant to ADH
- Fail to significantly decrease urine volume or increase urine osmolality as plasma osmolality increases
- CDI can result from damage to hypothalamus or pituitary gland
- Surgery is the most common
- Head injury is also common
- Hereditary NDI
- Most result from mutations in the AVPR2 gene encodes the vasopressin V2 receptor
- Smaller percentage caused by mutations in AQP2 gene encodes aquaporin 2 protein
Diabetes Mellitus vs. Diabetes Insipidus
- Diabetes (German for "siphon" or "to pass through") mellitus
- Elevated blood glucose due to relative (Type II) or absolute (Type I) deficiency in insulin
- Type 1 – formerly called insulin-dependent diabetes mellitus
- Absolute deficiency of insulin – autoimmune attack on B cells of pancreas
- Type 2 – formerly called non-insulin-dependent diabetes mellitus
- Combination of insulin resistance and dysfunctional B cells
- Diabetes insipidus
- Diminished water reabsorption due to lack of or resistance to ADH
- Urinate large volumes of dilute urine
- Do NOT have elevated blood glucose
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