AP Biology Investigation 4: Diffusion, Osmosis & Water Potential

What is the primary function of cell membranes?

To move water, nutrients, and other materials across membranes to maintain homeostasis.

What does selectively permeable mean in the context of cell membranes?

It means the membrane allows certain substances to pass through while restricting others.

What is the phospholipid bilayer?

It forms the membrane and limits the passage of certain substances.

What are aquaporins?

Special protein channels that allow water to move quickly across the membrane.

What role do protein channels play in cell membranes?

They help substances such as ions cross the membrane.

What are transport proteins?

Proteins that help move other substances, including larger molecules, across the membrane.

What are protein pumps?

They move substances against their concentration gradients using ATP.

Define diffusion.

The movement of particles from an area of higher concentration to lower concentration.

Define osmosis.

The diffusion of water across a selectively permeable membrane.

What is active transport?

The movement of substances against their concentration gradients using energy from ATP.

What is a hypotonic solution?

A solution with lower solute concentration outside the cell, causing water to move into the cell.

What happens to animal cells in a hypertonic solution?

Water moves out of the cell, causing the cells to shrink.

What is an isotonic solution?

A solution with equal effective water potential on both sides, resulting in no net water movement.

What does water potential indicate?

It tells us which direction water will move.

What is the formula for calculating water potential?

ψ = ψP + ψS (Water Potential = Pressure Potential + Solute Potential)

What does pressure potential (ψP) refer to?

The pressure exerted on a solution; in plant cells, positive turgor pressure contributes to this.

What is solute potential (ψS)?

The effect of dissolved solutes on water potential.

What is turgor pressure?

The pressure inside a plant cell caused by water pushing the cell membrane against the cell wall.

What occurs during plasmolysis?

The plasma membrane pulls away from the cell wall due to water loss.

How does cell size affect surface area-to-volume ratio?

As cell size increases, the surface area-to-volume ratio decreases, making larger cells less efficient at exchanging materials.

What is the significance of the surface area-to-volume ratio in cells?

Smaller cells have a higher surface area relative to their volume, allowing for more efficient material exchange.

What happens to plant cells when they lose turgor pressure?

They become limp or wilted.

What is the ionization constant (i) for sucrose?

1

What is the ionization constant (i) for NaCl?

2

What is the effect of adding solutes to water potential?

It makes water potential more negative.

What is the purpose of Procedure 1 in the lab experiments?

To determine how cell size and surface area-to-volume ratio influence diffusion.

What materials are used in Procedure 1?

Agar blocks, NaOH, HCl, and phenolphthalein.

What color does phenolphthalein turn in a basic solution?

Pink.

What happens to the agar blocks when acid diffuses into them?

The pink color disappears.

What is predicted about smaller agar blocks compared to larger ones?

Smaller blocks should become completely penetrated by the acid faster due to a higher surface area-to-volume ratio.

What is the purpose of Procedure 2?

To investigate how differences in solute concentration affect water movement.

What does dialysis tubing represent in Procedure 2?

A model cell membrane.

What is the formula for calculating percent mass change?

\[%\text{ Mass Change}=\frac{\text{Final Mass}-\text{Initial Mass}}{\text{Initial Mass}}\times100\]

What does a positive percent change indicate in Procedure 2?

Net water movement into the bag.

What does a negative percent change indicate?

Net water movement out of the bag.

What is the control setup in Procedure 2?

A dialysis bag containing water placed in a cup containing water.

What is the purpose of Procedure 3?

To observe how living plant cells respond to different solute concentrations.

What plant cells are suggested for observation in Procedure 3?

Elodea or moss cells.

What happens to plant cells in a hypotonic environment?

They gain water and become more turgid.

What happens to plant cells in a hypertonic environment?

They lose water and may plasmolyze.

What does water-potential equilibrium mean?

There is no net water movement.

What does the water potential equation represent?

ψ = ψP + ψS, where ψ is total water potential.

What does isotonic mean in the context of this lab?

Water potentials are equal on both sides.

What is diffusion?

Particles moving from high to low concentration without ATP.

What is osmosis?

Water moves from higher water potential to lower water potential.

What is tonicity?

Hypertonic causes net water loss; hypotonic causes net water gain; isotonic means no net water movement.

Why are small cells generally efficient at diffusion?

They have a higher surface area-to-volume ratio.

What is plasmolysis?

The cell membrane pulling away from its wall due to water loss.

How do you calculate the total water potential of a plant cell?

By adding solute potential (ψS) and pressure potential (ψP).