Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 44.1 Osmoregulation Balances the Uptake and Loss of Water and Solutes
Osmoregulation is the process by which animals control water and solute concentrations to maintain internal stability. Depending on the environment, animals use different strategies to regulate water gain and loss, often tightly linked with excretion of metabolic wastes.
1. Osmosis and Osmolarity
- Osmosis is water movement across membranes driven by solute concentration differences
- Osmolarity measures solute concentration (mOsm/L)
- Water flows from hypoosmotic (lower solute) to hyperosmotic (higher solute) areas
- Two isoosmotic solutions exchange water equally across membranes
2. Osmoconformers vs. Osmoregulators
- Osmoconformers (mostly marine invertebrates): internal osmolarity equals environment
- Osmoregulators control internal osmolarity independently from surroundings
- Osmoregulation enables life in freshwater, land, or fluctuating habitats
- Stenohaline animals tolerate narrow osmolarity ranges; euryhaline animals tolerate wide ranges
3. Marine Animals
- Marine invertebrates are typically osmoconformers
- Bony fishes drink seawater, excrete excess salts via gills and kidneys, and produce scanty urine
- Sharks retain urea and TMAO, making their body slightly hyperosmotic to seawater → water enters passively
4. Freshwater Animals
- Freshwater animals are hyperosmotic to their surroundings
- Constant water influx by osmosis; countered by producing large volumes of dilute urine and active salt uptake via gills
- Migratory fish like salmon acclimate when switching between fresh and salt water by altering hormone levels and salt-secreting cell function
5. Terrestrial Animals
- Face risk of desiccation (dehydration) through skin, lungs, and waste excretion
- Conserve water via impermeable body coverings, nocturnal behavior, and metabolic water production
- Desert animals like camels tolerate high body temperature shifts and water loss; desert mice survive on dry seeds alone
6. Anhydrobiosis
- Some animals (e.g. tardigrades) survive extreme dehydration by entering dormant state
- They produce protective sugars like trehalose to stabilize membranes and proteins
- Inspired modern room-temperature storage techniques for biological samples
7. Transport Epithelia
- Specialized epithelial tissues that move solutes directionally for osmoregulation and excretion
- Found in salt glands of marine birds, vertebrate kidneys, and insect excretory systems
- Example: albatrosses use nasal salt glands to excrete excess salt and retain water
8. Energetics of Osmoregulation
- Active transport is energy-intensive; energy cost rises with osmotic gradient and permeability
- Some animals adapt body fluids to match external osmolarity and reduce cost
- Osmoregulation can use up to 30% of resting metabolism in extreme cases (e.g., brine shrimp)
In a Nutshell
Animals regulate water and solute levels through osmosis, active transport, and specialized epithelial systems. Strategies differ based on habitat—marine, freshwater, or terrestrial—but all aim to maintain homeostasis. Some organisms use extreme adaptations like anhydrobiosis or salt glands, while others shift strategies as they migrate across environments.