2.8 Investigating water potential

Define osmosis

Osmosis is the diffusion of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

Define water potential gradient

A water potential gradient is the difference in water potential between two regions, which causes water to move by osmosis from higher to lower water potential.

Define turgid

A cell is turgid when it is swollen with water, with the cytoplasm pressing against the cell wall due to water entering by osmosis.

Define turgor pressure

Turgor pressure is the pressure exerted by the cell contents against the cell wall when water enters a plant cell by osmosis.

Define pressure potential

Pressure potential is the opposing force exerted by the cell wall against the expanding cell contents, balancing turgor pressure.

Define flaccid

A flaccid cell is one that has lost water and is soft, with reduced pressure against the cell wall.

Define plasmolysis

Plasmolysis is the process in which the cell membrane pulls away from the cell wall due to loss of water by osmosis in a solution with lower water potential.

Why is osmosis a special type of diffusion?

Because it specifically involves the movement of water molecules across a partially permeable membrane down a water potential gradient.

Direction of water movement

From higher water potential → lower water potential.

What happens when there is no net movement of water?

Water molecules still move in both directions, but at equal rates, so there is no net diffusion of water.

Explain why eggplant tissue becomes firmer in distilled water

Water enters the cells by osmosis because the external solution has higher water potential, increasing turgor pressure and making the tissue firm.

Explain why tissue becomes softer in concentrated salt solution

Water leaves the cells by osmosis due to lower external water potential, reducing turgor pressure and making cells flaccid.

Why are no cells seen in distilled water?

Red blood cells burst (haemolysis) as water enters by osmosis, causing them to swell and rupture.

Why do red blood cells shrink in concentrated salt solution?

Water leaves the cells by osmosis due to lower water potential outside, causing them to shrink (crenation).

Why do animal cells not undergo plasmolysis?

Because they lack a cell wall, so the membrane cannot pull away from a rigid structure.

What is the independent variable in the potato experiment?

Concentration of sucrose solution.

What is the dependent variable?

Change in mass of the potato tissue.

What is the derived variable?

Percentage change in mass.

Formula for percentage change in mass

Percentage change = (change in mass ÷ original mass) × 100

Percentage change = (change in mass ÷ original mass) × 100

Because initial masses differ, so percentage change allows fair comparison.

Explain why potato gains mass in dilute solutions

Water enters the cells by osmosis due to higher external water potential.

Explain why potato loses mass in concentrated solutions

Water leaves the cells by osmosis due to lower external water potential.

What happens to cells in distilled water?

Cells become turgid as water enters, vacuole expands, and cytoplasm presses against cell wall.

What happens in concentrated solution?

Cells become plasmolysed as water leaves, vacuole shrinks, and membrane pulls away from wall.

What happens at equilibrium concentration?

Cells are flaccid with no net movement of water and no pressure against the cell wall.

Why must root cells have lower water potential than soil?

To maintain a water potential gradient so water enters the roots by osmosis.

How do plants in salty soils survive?

They maintain very low internal water potential to continue absorbing water from the environment.

Explain Investigation 1 using water potential

In dilute solutions, water enters cells (higher WP outside → lower inside), making tissue firm; in concentrated solutions, water leaves cells (lower WP outside), making tissue soft.

Explain Investigation 2 using water potential

In distilled water (high WP), water enters RBCs causing bursting; in concentrated solutions (low WP), water leaves cells causing shrinkage.

Why do cells not gain mass indefinitely in water?

Once fully turgid, pressure potential balances water potential, preventing further net water entry.

Why should you use “water potential” instead of “hypotonic/hypertonic”?

Because water potential explains the movement of water scientifically, whereas hypotonic/hypertonic only describe the outcome.

Describe the effects of immersing plant and animal cells in solutions of different water potential

• In high water potential solutions, water enters cells by osmosis → plant cells become turgid, animal cells may burst

• In low water potential solutions, water leaves cells → plant cells become flaccid or plasmolysed, animal cells shrink

• At equal water potential, there is no net movement of water