What is the difference between a 'dilute' solution and a 'concentrated' solution in terms of water potential?

A dilute solution has a high concentration of water molecules and a low concentration of solutes, resulting in a high (less negative) water potential. A concentrated solution has a high solute concentration, which lowers the free energy of water, resulting in a low (more negative) water potential.

How does the response of a plant cell in a hypotonic solution differ from that of an animal cell?

A plant cell becomes turgid because its rigid cellulose cell wall withstands the internal pressure and prevents bursting. An animal cell, lacking a cell wall, will continue to expand as water enters until the cell membrane ruptures, leading to cytolysis.

What is the distinction between 'plasmolysis' and 'flaccidity' in plant cells?

Flaccidity occurs when the water potential is equal inside and outside the cell (isotonic), resulting in no net movement and a loss of turgor. Plasmolysis is a more extreme state occurring in hypertonic solutions where the protoplast actually shrinks away from the cell wall due to significant water loss.

Why is it incorrect to say water moves from a 'low water potential' to a 'high water potential'?

Water moves down a potential gradient, from an area of higher free energy (high water potential) to lower free energy (low water potential). Confusing this direction would violate the laws of thermodynamics regarding passive transport.

What is a common mistake when comparing water potential values like $-500$ kPa and $-200$ kPa?

Students often mistake the larger absolute number for a 'higher' potential. In reality, $-500$ kPa is a lower water potential than $-200$ kPa because it is further from the maximum value of $0$ kPa (pure water).

Why is the term 'semi-permeable' often considered less accurate than 'partially permeable'?

'Semi-permeable' implies that exactly half of the substances can pass through, which is rarely true. 'Partially permeable' correctly describes a membrane that is selective, allowing only specific molecules (like water) to pass based on size or charge.

Define 'Water Potential' ($\\Psi$).

Water potential is a measure of the tendency of water molecules to move from one area to another due to osmosis, gravity, or mechanical pressure. It is typically measured in kilopascals (kPa), with pure water at $0$ kPa.

What is a 'Hypotonic' solution?

A hypotonic solution is one that has a lower solute concentration (and therefore a higher water potential) compared to the cytoplasm of a cell. This causes a net movement of water into the cell.

Define 'Turgor Pressure'.

Turgor pressure is the hydrostatic pressure exerted by the fluid inside the central vacuole of a plant cell against the cell wall. It provides structural strength to the plant.

Cytolysis is the bursting of an animal cell caused by the excessive intake of water via osmosis when the cell is placed in a hypotonic environment.

Osmosis | AQA AS-Level Biology

Osmosis

Summary

Osmosis is a specialized form of passive transport involving the net movement of water molecules across a partially permeable membrane. Driven by differences in water potential, it is a critical process for maintaining cellular integrity and physiological balance in both plant and animal organisms.

1. Definition & Core Concepts

Osmosis is defined as the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
A partially permeable membrane (such as the cell surface membrane) allows small molecules like water to pass through while blocking larger solute molecules like sucrose or proteins.
The process is passive, meaning it does not require metabolic energy (ATP) and occurs spontaneously down a concentration gradient of water molecules.

U-tube diagram showing osmosis. Water moves from the left side (high water potential, low solute) to the right side (low water potential, high solute) across a central partially permeable membrane.

2. Underlying Principles: Water Potential

3. Osmosis in Animal Cells

Animal cells lack a rigid cell wall, making them highly sensitive to changes in the surrounding water potential.
In a hypotonic solution (higher water potential outside), water enters the cell, causing it to swell and eventually burst, a process known as cytolysis.
In a hypertonic solution (lower water potential outside), water leaves the cell, causing it to shrink and shrivel, often referred to as crenation.
To prevent damage, animal bodies use osmoregulation to maintain an isotonic environment where the net movement of water is zero.

4. Osmosis in Plant Cells

Plant cells possess a strong cellulose cell wall that provides structural support and prevents the cell from bursting when water enters.
When placed in a hypotonic solution, water enters the vacuole, increasing the internal pressure against the cell wall; this state is called turgid and is essential for plant support.
In a hypertonic solution, water leaves the cell, causing the protoplast to shrink away from the cell wall, a phenomenon known as plasmolysis.
If the water potential is equal inside and outside, the cell is flaccid, and the plant may begin to wilt as it loses its structural rigidity.

5. Key Distinctions

6. Exam Strategy & Tips