15: Water + Electrolyte Balance Pt. 1

• 3 mechanisms of membrane transport:

  Passive transport:

  1. Diffusion
  2. Facilitated diffusion through channels or carriers

  Active transport:

  3. Primary and secondary active transport
• The chemical reactions that make life possible occur in an aqueous solution

  • If the balance of water and solutes in the solution is disturbed, those chemical reactions -and life itself- may stop
• Electrolyte: a compound that dissociates into ions when dissolved in water

  • Because cells require precise concentrations of Na+, Cl-, K+, and Ca2+ to function normally, maintaining electrolyte balance is crucial
• Water and electrolyte balance is associated with excretion

  • Animals produce urine to excrete waste and this leads to water loss
• Animals must maintain water and electrolyte balance in 3 environments

  1. Freshwater
  2. Marine
  3. Terrestrial
• Electrolytes and water move through organisms by diffusion and osmosis respectively
• Small molecules and ions in a solution, called solutes

  • Have thermal energy
  • Are in constant, random motion
  • This spontaneous movement is called diffusion
• Water moves quickly across lipid bilayers

  • This is a special case of diffusion called osmosis
• Concentration Gradient:

  • Created by a difference in solute concentrations
  • When a concentration gradient exists, there is a net movement from high concentration to low concentration
• Diffusion along a concentration gradient

  • Increases entropy
  • Is spontaneous
• Equilibrium occurs when the molecules or ions are randomly distributed

  • Molecules are still moving randomly, but there is no more net movement
• Passive Transport:

  • Occurs when substances diffuse across a membrane in the absence of an outside energy source
• Diffusion across a Selectively Permeable Membrane Establishes an Equilibrium:

  1. Separation of solutes on opposite sides of a lipid bilayer (both molecules diffuse freely across the bilayer
  2. Diffusion (each solute undergoes a net movement along its own concentration gradient)
  3. Equilibrium is established when solutes continue to move back and forth across the membrane, but at equal rates
• Osmosis:

  • Water moving from regions of low solute concentration to regions of high solute concentration
  • This dilutes the higher concentration of solute
  • It equalizes the concentration on both sides of the bilayer
• Hypertonic: an outside solution with a higher concentration than the inside of a cell
• Hypotonic: a solution with a lower concentration of solutes than the cell
• Isotonic: if solute concentrations are equal on the outside and the inside of the cell
• A cell in:

  • A hypertonic solution -> water moves out of cell and cell will shrink
  • A hypotonic solution -> water moves into cell and cell will swell
  • Isotonic solution -> no net water movement and cell will stay the same
• Osmotic stress occurs when the concentration of dissolved substances in a cell or tissue is abnormal

  • Water and solute concentrations are different from their set points
• Osmoregulation: the process by which organisms control the concentration of water and solutes within their bodies
• Seawater nearly matches the electrolyte concentrations found within these animals

  • Such animals are called osmoconformers
• Seawater is isosmotic in comparison to the cells and tissues

  • The solute concentrations inside and outside the animals are equal
• Osmoregulators: animals that actively regulate osmolarity inside their bodies to achieve homeostasis
• Osmoregulation is required in marine vertebrates because seawater is hyperosmotic to their tissues

  • Marine bony fishes keep the osmolarity of their tissues lower than that of seawater
• Passive transport is driven by diffusion along an electrochemical gradient and does not require an expenditure of energy
• Active transport occurs when a source of energy powers the movement of a solute that establishes an electrochemical gradient
• Primary active transport is used to move ions against their gradients

  • Ex. sodium-potassium pump
• Secondary active transport (cotransport) relies on membrane proteins that us an electrochemical gradient established by a pump during primary active transport

  • Symporters move solutes in the same direction
  • Anitporters move solutes in the opposite directions
• No known mechanisms for actively transporting water across membranes
• Cells use pumps to transport ions to set up osmotic gradients

  • Water follows by osmosis, often through aquaporins
• Because water tends to flow by osmosis out of the gill epithelium, marine fishes must replace the water or the cells will shrivel and die

  • These animals face a trade-off between gas exchange and maintaining water and electrolyte balance
• Marine fishes must drink large amounts of water to replace the loss of water, which also brings more electrolytes

  • Ions also diffuse into the gill epithelium down their concentration gradient
• To rid themselves of these excess electrolytes, marine bony fishes actively pump ions out into the seawater

  • Membrane proteins in the gill epithelium carry out this process
  • The fishes also lose electrolytes by excreting small quantities of highly concentrated urine
• Freshwater animals are under osmotic stress because they gain water and lose solutes
• The freshwater is hyposmotic to the fishes' tissues

  • The gain of water across the gill epithelium puts them under osmotic stress
• If a freshwater fish does not get rid of incoming water, its cells will burst and it will die

  • To achieve homeostasis, they excrete large amounts of water in their urine and don't drink
• Electrolytes diffuse out of the gill epithelium into the environment in freshwater fishes

  • The fishes replace electrolytes by eating food or actively transporting them into the body
• Terrestrial animals constantly lose water to the environment, but they lose it by evaporation rather than osmosis
• The epithelial cells in respiratory structures have a moist surface to promote gas exchange in terrestrial animals

  • This allows for a large amount of water loss through evaporation
  • There is a trade off between gas exchange and osmoregulation
• Land animals also lose water when they produce urine and when they sweat or pant