5.2 notes
Osmoregulation Introduction
Osmoregulation is a fundamental type of homeostasis in animals.
It's the process by which an organism controls the concentrations of water and dissolved molecules in its body.
Cells require constant concentrations of water and electrolytes to function properly.
Electrolytes are substances that separate into ions when dissolved in water.
Disturbances in water and electrolyte concentrations disrupt enzymes and chemical reactions, potentially leading to cell death.
Animals face a constant threat of imbalances due to diffusion and osmosis because the external environment often differs in water and electrolyte concentrations from the internal environment.
Diffusion and Osmosis Recap
Diffusion:
If a concentration gradient exists for an atom or molecule across a membrane.
AND the membrane is permeable to that atom or molecule.
THEN it will diffuse across the membrane until the concentration is equal on both sides.
Osmosis:
Wherever there is a concentration gradient, osmosis will also occur.
Osmosis is strongest when there is a concentration gradient of an atom or molecule that cannot diffuse through the membrane.
Water will diffuse through the membrane until the concentration of the atom or molecule is the same on both sides.
Water flows from the side with low solute concentration to the side with high solute concentration (water follows salt).
Difference between Osmosis and Diffusion:
Diffusion occurs in response to gradients of individual molecules or ions (e.g., sodium diffuses if there is a gradient of sodium ions).
Osmosis occurs in response to the total concentration of ALL dissolved solutes (osmolarity).
Red Blood Cell Examples:
Hypertonic Environment:
The concentration of solutes is higher outside the cell.
High osmosis.
Water flows out of the cell, causing it to shrivel up (crenation), which can disrupt metabolic processes and kill the cell.
Hypotonic Environment:
The concentration of solutes is lower outside the cell.
Water flows into the cell.
Increases water pressure inside the cell, causing it to swell; extreme swelling can cause the plasma membrane to burst open, killing the cell.
Isotonic Environment:
The concentration of solutes is the same on both sides.
There is no net flow of water, so the cell remains unchanged.
Osmoregulation
Combined action of diffusion and osmosis:
A cell in a hypertonic environment will lose water by osmosis and gain excess electrolytes by diffusion.
A cell in a hypotonic environment will gain excess water by osmosis and lose electrolytes by diffusion.
Hypertonic and hypotonic environments create osmotic stress.
Osmotic stress: Water or electrolyte imbalance disrupts the metabolic processes of the cell.
Prolonged or extreme osmotic stress can be damaging or fatal to cells.
Animals have evolved ways to maintain homeostasis of water and salt concentrations.
Osmoconformers vs. Osmoregulators:
Marine invertebrates and marine fish with cartilage-based skeletons are osmoconformers.
Their body cells have the same osmolarity as their surrounding environment (isotonic).
All other animals are osmoregulators.
Their body cells have a different osmolarity than their surrounding environment.
Osmoregulation in Marine Fish
Marine fish live in seawater (hypertonic environment).
They lose water by osmosis.
They gain excess electrolytes by diffusion.
How they osmoregulate:
Replace water by drinking seawater and producing very little urine.
Excrete excess electrolytes through the gills via active transport.
Gill cells have a protein that moves salt by active transport.
Osmoregulation in Freshwater Fish
Freshwater fish live in a hypotonic environment.
They gain excess water by osmosis.
They lose electrolytes by diffusion.
How they osmoregulate:
Get rid of excess water by producing large amounts of urine and drinking very little.
Absorb lost electrolytes back into the body through the gills.
Gill cells have a protein that moves electrolytes by active transport.
Osmoregulation in Fish inhabiting both Marine and Freshwater
These fish can swap the location of their active transporter.
In marine environments, the transporter is located on the inside of the gill cells to pump excess electrolytes out of the body.
In freshwater environments, the transporter is located on the outside of the gill cells to pump lost electrolytes back into the body.
Osmoregulation in Terrestrial Animals
More complex due to:
Multiple sources of water loss: evaporation from respiratory surfaces (e.g., lungs), evaporation from body surface, water lost in feces and urine.
Multiple sources of electrolyte loss: freshwater sources are hypotonic, electrolytes lost in urine.
Limited sources of electrolyte gain: primarily from food.
Limited sources of water gain: eating, drinking, and metabolic water (hydrolysis and dehydration synthesis reactions releasing water).
Osmoregulation by Retention
Retention: absorbing a substance back into the body before it is excreted.
Retention strategies depend on filtration and reabsorption.
Filtration: separating the water-based fluid of the body from cells and large molecules.
Reabsorption: selectively absorbing the parts of the filtered material that we want to retain.
Retention strategies in insects
Minimize water loss
Thick exoskeleton coated in hydrophobic wax
A thick layer of protein and a material called chitin minimize diffusion
Wax layer blocks evaporation of water from the body
Their respiratory system can be closed
Respiratory openings called spiracles can be closed by small muscles
This minimizes loss of water from the respiratory system
Retention Strategies in Insects
Insects have Malphigian tubules, which are a set of primitive kidneys that work to regulate the internal environment.
The tubules are in direct contact with the insect’s version of blood, called hemolymph.
The Malphigian tubule removes only electrolytes, water, and waste products from the hemolymph by acting as a filter.
The Malphigian tubule empties its filtered material directly into the gut of the insect.
The hindgut will reabsorb the amount of electrolytes and water that the insect must retain.
The Malphigian tubule is the filter and the hindgut is the reabsorber
All waste products and any excess water or electrolytes are removed from the body in the feces.