Osmosis, Compartmentalization, & Origins

What is osmosis?

Net movement of water across a selectively permeable membrane from higher water potential toward lower water potential.

What is a solute, and what is a solvent?

The solute is a dissolved substance; the solvent is the substance doing the dissolving, usually water in cells.

What does hypertonic mean relative to a cell?

The surrounding solution has a higher concentration of nonpenetrating solutes and tends to draw water out of the cell.

What does hypotonic mean relative to a cell?

The surrounding solution has a lower concentration of nonpenetrating solutes and tends to drive water into the cell.

What does isotonic mean relative to a cell?

The solution has the same effective concentration of nonpenetrating solutes, so there is no net change in cell water content.

How does an animal cell respond to hypertonic versus hypotonic surroundings?

It shrinks in a hypertonic solution and may swell or lyse in a strongly hypotonic solution.

How does a plant cell respond to hypotonic surroundings?

Water enters, the central vacuole swells, and the cell becomes turgid as the wall resists further expansion.

What is plasmolysis?

In a hypertonic environment a plant cell loses water and its plasma membrane pulls away from the cell wall.

Why can a plant cell become flaccid without undergoing plasmolysis?

With little net water entry its turgor drops; severe water loss is needed for the membrane to pull away from the wall.

How can a freshwater paramecium avoid bursting?

A contractile vacuole collects and expels excess water entering by osmosis.

In an isotonic system, do water molecules stop crossing the membrane?

No. They move both ways at equal rates, producing no net water movement.

In a simple comparison of solutions, which way does water usually move?

From lower effective solute concentration toward higher effective solute concentration; pressure can modify the direction.

What is water potential (Ψ)?

A measure of water's tendency to move; water moves from higher Ψ to lower Ψ.

What is the water-potential equation in this packet?

Ψ = Ψs + Ψp, where Ψs is solute potential and Ψp is pressure potential.

What is the water potential of pure water in an open system?

0; adding solute lowers water potential below zero.

What is the solute-potential equation?

Ψs = −iCRT; i is ionization factor, C molar concentration, R = 0.0831 L·bar/(mol·K), and T is temperature in Kelvin.

How do you convert Celsius to Kelvin for the packet's water-potential problems?

T(K) = temperature in °C + 273 (approximately 273.15 for greater precision).

What is i for sucrose compared with NaCl in idealized AP Biology problems?

Sucrose does not dissociate, so i = 1; NaCl dissociates into two ions, so i is approximately 2.

At 21°C, what is Ψ for 0.5 M sucrose in an open system?

Ψp = 0; Ψ = −(1)(0.5)(0.0831)(294) ≈ −12.22 bars.

What can cause water to move toward a solution with lower solute concentration?

A sufficient pressure-potential difference can outweigh its solute-potential difference; compare total Ψ, not solute alone.

Why do organelle membranes help maintain different local conditions?

They restrict exchange and allow each compartment to keep conditions suited to its reactions, such as an acidic lysosome interior.

Why can damage to a lysosome's membrane harm a cell?

Its digestive enzymes and acidic contents may leak or the organelle may lose the conditions needed for normal breakdown and recycling.

What is an advantage of folded internal membranes in mitochondria and chloroplasts?

They increase area for membrane proteins and reactions while maintaining distinct compartments and gradients.

What is the nucleoid?

The region containing most of a prokaryote's DNA; it is not enclosed by a nuclear membrane.

Which organelles likely originated through endosymbiosis?

Mitochondria and chloroplasts arose from formerly free-living bacteria incorporated into ancestral cells.

What evidence beyond DNA supports endosymbiosis?

Bacterial-like ribosomes, division resembling binary fission, double membranes, and genetic similarity to bacteria.

What origin is commonly proposed for the nuclear envelope and ER?

Infolding of an ancestral plasma membrane; this is a model for parts of the endomembrane system.

When should a line graph be preferred to a bar chart?

Use a line graph for measured change across ordered continuous values such as time; use a bar chart to compare categories.

What belongs on the axes of a graph of mass change in cells versus external solute concentration?

X axis: external solute concentration with units; Y axis: mass change (or percent mass change) with units; include a clear title and appropriate scale.