Tonicity and Osmoregulation - AP Biology

Explain the effect of diuretics on water balance.

Diuretics increase urine production, promoting water excretion. Inhibit sodium reabsorption, leading to water loss.
What is a nonpenetrating solute?

A solute that cannot cross the membrane and therefore affects tonicity. These solutes create permanent osmotic gradients across membranes.
What is tonicity in the context of osmosis across a selectively permeable membrane?

A solution's ability to cause net water movement and cell volume change. Tonicity specifically measures osmotic effect, not just total solute concentration.
Name a condition where blood plasma is hypotonic.

Hyponatremia is a condition where blood plasma is hypotonic. Low sodium levels create hypotonic blood conditions.
What happens to a plant cell placed in a hypertonic solution?

It plasmolyzes as water leaves and the membrane pulls from the wall. Extreme water loss causes membrane separation from rigid wall.
State the relationship between tonicity and osmolarity.

Tonicity depends on the osmolarity and the cell's permeability to solutes. Only nonpenetrating solutes contribute to tonicity effects.
What is the key difference between tonicity and osmolarity?

Tonicity depends on nonpenetrating solutes; osmolarity counts all solutes. Only membrane-impermeable solutes create sustained osmotic pressure.
What type of solution is required for a plant cell to become turgid?

A hypotonic solution causes a plant cell to become turgid. Water influx creates turgor pressure against the cell wall.
What is an osmoregulator?

An organism that maintains internal osmolarity different from its environment. Actively maintains optimal internal conditions regardless of environment.
Which part of the nephron is responsible for reabsorbing water?

The proximal convoluted tubule and loop of Henle reabsorb water. Both regions actively reclaim water from the filtrate.
State the consequence of a cell bursting due to excess water intake.

Lysis occurs when a cell bursts due to excess water intake. Cell membrane ruptures from excessive water influx.
Define osmoregulation.

Osmoregulation is the control of water and solute concentrations in a cell or organism. Maintains proper cell volume and prevents osmotic stress.
What happens to an animal cell placed in a hypertonic solution?

It shrivels (crenates) as water exits. Water loss causes membrane to fold inward at cell edges.
Identify the hormone that regulates water reabsorption in kidneys.

Antidiuretic hormone (ADH) regulates water reabsorption. Increases water permeability of collecting duct cells.
What is an osmoconformer?

An organism whose internal osmolarity matches its external environment. No energy spent maintaining osmotic gradients against environment.
What is a solute in a solution, in the context of tonicity?

A dissolved substance that contributes to total solute concentration. Only nonpenetrating solutes affect tonicity in osmotic calculations.
Which cellular structure allows plant cells to resist lysis in hypotonic solutions?

The rigid cell wall. Rigid walls withstand internal pressure from water uptake.
What does isotonic mean for a cell in a surrounding solution?

No net water movement; cell volume remains stable. Equal effective solute concentrations create osmotic equilibrium.
Which hormone increases water reabsorption in the kidney collecting ducts?

Antidiuretic hormone (ADH, vasopressin). ADH makes collecting ducts more permeable to water reabsorption.
What is plasmolysis?

Plasmolysis is the process where plant cells lose water and the cell membrane detaches from the cell wall. Occurs when plant cells are placed in hypertonic solutions.
Which option best describes negative feedback in osmoregulation?

A deviation triggers responses that restore internal osmolarity toward a set point. Homeostatic mechanism maintains stable internal water balance.
Which structure in plants helps minimize water loss?

The cuticle helps minimize water loss in plants. Waxy layer reduces water loss through transpiration.
Describe the role of contractile vacuoles in osmoregulation.

Contractile vacuoles expel excess water from cells to maintain osmotic balance. Prevent cell bursting in hypotonic environments.
What does hypertonic mean for a cell in a surrounding solution?

Solution has higher effective solute; water leaves the cell. Water moves out to equalize solute concentrations across membrane.
What is turgor pressure?

Pressure of the cell contents against the plant cell wall after water uptake. Created when water enters cell but wall prevents expansion.
Identify the tonicity: Outside and cytosol have equal nonpenetrating solute.

Isotonic. Equal concentrations prevent net water movement across membrane.
What does hypotonic mean for a cell in a surrounding solution?

Solution has lower effective solute; water enters the cell. Water moves in to dilute higher internal solute concentration.
Which type of organism primarily uses contractile vacuoles?

Protists, such as amoebas and paramecia, use contractile vacuoles. Single-celled organisms in freshwater environments need this adaptation.
Which organ is primarily responsible for osmoregulation in mammals?

The kidneys are primarily responsible for osmoregulation in mammals. Filter blood and regulate water and electrolyte balance.
What is osmosis?

Diffusion of water across a selectively permeable membrane. Water moves down its concentration gradient through membrane pores.
What is lysis in the context of osmosis?

Cell rupture caused by excessive water influx. Occurs when osmotic pressure exceeds membrane strength.
How does the body respond to dehydration in terms of ADH?

The body increases ADH secretion to promote water reabsorption. Conserves body water by increasing kidney reabsorption.
Which hormone decreases blood osmolarity by increasing water reabsorption?

Antidiuretic hormone (ADH) decreases blood osmolarity. Promotes water retention to dilute blood solutes.
What happens to a cell in an isotonic solution?

In an isotonic solution, a cell experiences no net water movement. Equal solute concentrations create osmotic equilibrium.
What is the role of the loop of Henle in water reabsorption?

The loop of Henle creates a concentration gradient to facilitate water reabsorption. Countercurrent flow concentrates urine and conserves water.
What is the primary osmotic challenge for freshwater fish?

Freshwater fish must excrete excess water and retain salts. Hypotonic environment requires active water removal.
Identify the tonicity: Outside has higher nonpenetrating solute than cytosol.

Hypertonic. Higher nonpenetrating solute creates net water efflux from cell.
Identify the effect of increased ADH on urine concentration.

Urine becomes more concentrated (greater water reabsorption). More ADH increases water reabsorption, reducing urine volume.
What is the function of the glomerulus in the nephron?

The glomerulus filters blood to form a filtrate in the nephron. Initial step in urine formation within Bowman's capsule.
What is the term for the movement of water across a semipermeable membrane?

Osmosis is the movement of water across a semipermeable membrane. Passive diffusion driven by concentration gradients.
What is a selectively permeable membrane?

A membrane that allows some substances to cross more easily than others. Permeability varies by substance size, charge, and lipid solubility.
What is tonicity?

Tonicity is the ability of a solution to cause a cell to gain or lose water. Based on relative solute concentrations between solution and cell.
What does a hypertonic solution cause a cell to do?

A hypertonic solution causes a cell to lose water and shrink. Higher solute concentration outside draws water out via osmosis.
What is the function of the nephron in osmoregulation?

Nephrons filter blood, reabsorb water, and excrete waste to maintain osmotic balance. The functional unit of the kidney for water and solute regulation.
Which structure prevents animal cells from becoming turgid in hypotonic solutions?

No rigid cell wall is present in animal cells. Animal cell membranes lack rigid walls to resist osmotic pressure.
What happens to an animal cell placed in an isotonic solution?

It maintains normal volume with no net water movement. Equal solute concentrations prevent osmotic water movement.
What is the advantage of producing uric acid instead of urea for some animals?

It conserves water because uric acid can be excreted as a paste. Minimal water loss makes this ideal for arid environments.
What happens to an animal cell placed in a hypotonic solution?

It swells and may lyse. Water influx causes swelling; no cell wall provides structural resistance.
What is crenation?

Shriveling of an animal cell due to water loss in a hypertonic solution. Cell membrane shrinks and creates characteristic spiky appearance.
Identify the primary challenge for terrestrial animals in osmoregulation.

Terrestrial animals must conserve water while excreting wastes. Must balance water retention with waste elimination.
Which cells are responsible for maintaining osmotic balance in the brain?

Astrocytes help maintain osmotic balance in the brain. Glial cells that regulate brain fluid osmolarity.
Identify the direction of water movement in osmosis.

Water moves from regions of low solute concentration to high solute concentration. Water follows its concentration gradient via osmosis.
What is osmolarity?

Osmolarity is the total concentration of solute particles in a solution. Measured in osmoles per liter of solution.
What happens to a plant cell placed in a hypotonic solution?

It becomes turgid due to water uptake and turgor pressure. Cell wall prevents lysis while allowing optimal turgor pressure.
What is the role of the rectal gland in sharks?

The rectal gland excretes excess salts in sharks. Specialized organ for concentrated salt elimination.
What occurs during crenation?

Crenation is the shrinking of animal cells in a hypertonic solution. Water loss causes cell membrane to wrinkle and shrink.
Which group is most commonly osmoconformers: marine invertebrates or freshwater fish?

Marine invertebrates. Their body fluids match seawater osmolarity naturally.
What is osmoregulation?

Regulation of water balance and solute concentration in an organism. Essential for maintaining cellular function and organism survival.
What is the solvent in most biological osmosis problems?

Water. Water is the universal solvent in biological systems.
What role does urea play in osmoregulation?

Urea maintains osmotic balance by being reabsorbed in the kidneys. Acts as an osmolyte to concentrate urine effectively.
What is the main nitrogenous waste in most mammals, relevant to water conservation?

Urea. Less toxic than ammonia but requires some water for excretion.
What adaptation do marine fish have for osmoregulation?

Marine fish drink seawater and excrete salt through gills. Osmoregulation in hypertonic seawater environment.
Identify the tonicity: Outside has lower nonpenetrating solute than cytosol.

Hypotonic. Lower nonpenetrating solute creates net water influx into cell.
What is a stomatal function in osmoregulation?

Stomata regulate gas exchange and water loss in plants. Guard cells control water vapor loss and CO₂ uptake.
Which direction does water move by osmosis relative to solute concentration?

From lower solute concentration to higher solute concentration. Water dilutes concentrated regions to achieve equilibrium.
What is anhydrobiosis?

Anhydrobiosis is a dormant state that allows survival in dry conditions. Extreme dehydration tolerance by suspending metabolism.
What is plasmolysis?

Plant cell membrane detaches from cell wall due to water loss. Water loss shrinks cytoplasm away from inflexible cell wall.
What is a penetrating solute (in tonicity problems)?

A solute that can cross the membrane and may not sustain tonicity effects. These solutes equilibrate across membranes, eliminating osmotic effect.
What is the primary osmoregulatory challenge for marine bony fish?

Losing water and gaining salts due to a hypertonic environment. Hypertonic seawater continuously draws water from fish tissues.
Identify the typical response of marine bony fish: do they drink seawater?

Yes; they drink seawater and excrete excess salts. Drinking replaces lost water; gills pump out excess salt.
What is the role of aquaporins in osmosis?

Aquaporins facilitate the rapid transport of water across cell membranes. These channel proteins increase membrane water permeability.
Which structure is a major site of salt excretion in many marine bony fish?

Gills (specialized ion transport cells). Specialized cells actively transport salt against concentration gradients.
Identify the typical response of freshwater fish to osmosis: do they drink water?

No; they excrete large volumes of dilute urine. They actively excrete excess water through kidneys instead.
What happens to a plant cell placed in an isotonic solution?

It becomes flaccid due to reduced turgor pressure. Reduced turgor makes plant tissue soft and wilted.
Describe the effect of a hypotonic solution on a cell.

A hypotonic solution causes a cell to gain water and swell. Lower external solute concentration drives water into the cell.
What is the effect of aldosterone on the kidneys?

Aldosterone increases sodium reabsorption and potassium excretion in the kidneys. Promotes sodium retention and potassium loss for volume regulation.
Define the term 'osmotic pressure'.

Osmotic pressure is the pressure required to prevent water movement across a semipermeable membrane. Reflects the solution's tendency to draw water across membranes.
What is the primary osmoregulatory challenge for freshwater animals?

Gaining water and losing salts by osmosis and diffusion. Hypotonic environment causes continuous water influx and salt loss.
Identify the process by which salt is actively transported out of saltwater fish.

Salt is actively transported out by chloride cells in the gills. Specialized cells actively pump out excess sodium chloride.
Which part of the brain regulates thirst?

The hypothalamus regulates thirst. Contains osmoreceptors that detect blood osmolarity changes.

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