Osmosis
The Role of Water as a Solvent
Before understanding osmosis, we must understand why water is so important in living organisms. Water is known as the universal solvent because it dissolves many substances.
This property allows water to perform three critical roles:
- Digestion: Enzymes and food molecules must be dissolved in water within the gut to be broken down and absorbed.
- Transport: Blood plasma (which is mostly water) transports dissolved nutrients, hormones, and waste products around the body.
- Excretion: Waste products like urea and salts are dissolved in water to form urine or sweat, allowing them to leave the body.
Without water acting as a solvent, these chemical reactions and transport systems could not occur.
Building on this concept, for substances to move between cells (like nutrients entering root hair cells or waste leaving kidney cells), they must be in solution. This leads us to the specific movement of water itself across membranes, which is the focus of osmosis.
Osmosis and Water Potential
Core Definition (LO 2 & LO 3):
Water diffuses through partially permeable membranes by osmosis. Specifically, water moves into and out of cells by osmosis through the cell membrane.
Water diffuses through partially permeable membranes by osmosis. Specifically, water moves into and out of cells by osmosis through the cell membrane.
Supplement Definition (LO 7):
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.
Key Terms Explained:
- Net movement: The overall direction of flow. While individual water molecules move randomly in all directions, more molecules move from the high concentration side to the low concentration side than vice versa.
- Partially permeable membrane: A barrier that allows small molecules (like water) to pass through but blocks larger molecules (like sucrose or starch).
- Water Potential (\Psi): A measure of the tendency of water to move out of a solution.
- Higher water potential = Dilute solution (more water molecules per unit volume, fewer solute particles).
- Lower water potential = Concentrated solution (fewer water molecules per unit volume, more solute particles).
Therefore, water always moves from a dilute solution to a concentrated solution.
Effects of Osmosis on Plant Cells
Plant cells have a rigid cell wall outside the cell membrane. This structure is crucial for how they respond to osmosis.
1. In a Dilute Solution (e.g., Distilled Water):
- The external solution has a higher water potential than the cell cytoplasm/vacuole.
- Water moves into the cell by osmosis.
- The vacuole swells and pushes the cytoplasm against the cell wall.
- The cell becomes turgid (swollen and firm).
- Support Mechanism (LO 6): Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. This is called turgor pressure. Turgid cells provide structural support for non-woody parts of the plant.
2. In a Concentrated Solution (e.g., Strong Salt/Sugar Solution):
- The external solution has a lower water potential than the cell cytoplasm/vacuole.
- Water moves out of the cell by osmosis.
- The vacuole shrinks and the cytoplasm pulls away from the cell wall.
- This process is called plasmolysis.
- The cell becomes flaccid (limp/wilted).
| Condition | Water Movement | Cell State | Term Used | Plant Appearance |
|---|---|---|---|---|
| Dilute Solution (High \Psi) | Into cell | Swollen, membrane against wall | Turgid | Firm / Upright |
| Concentrated Solution (Low \Psi) | Out of cell | Membrane pulls away from wall | Plasmolysed | Wilted / Flaccid |
Note on Animal Cells: Animal cells do not have a cell wall. If placed in distilled water, they may burst (lyse). If placed in concentrated solution, they shrivel (crenate).
Investigating Osmosis
LO 4: Using Dialysis Tubing
Dialysis tubing is a synthetic partially permeable membrane. It is often used to model osmosis in the lab.
- Setup: Fill a bag with sucrose solution and tie it into distilled water.
- Result: Water moves into the bag (higher \Psi outside to lower \Psi inside). The bag gains mass/volume.
- Why rinse?: If you dip the bag in water after removing it, you must rinse the outside to remove any sucrose that might have leaked or adhered to the surface. This ensures that when you weigh the bag later, you are measuring only the water that moved through the membrane, not external liquid.
LO 5: Using Plant Tissues (e.g., Potato Cylinders)
- Method: Cut potato cylinders to the same size/length. Weigh them (initial mass). Immerse in sucrose solutions of different concentrations for a set time. Pat dry (to remove surface water) and weigh again (final mass).
- Calculation:
\text{Percentage Change in Mass} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100 - Interpreting Results:
- Positive % change: Water entered the potato. External solution was dilute (higher \Psi) than the potato cells.
- Negative % change: Water left the potato. External solution was concentrated (lower \Psi) than the potato cells.
- Zero % change: No net movement. The external solution has the same water potential as the potato cells (isotonic). This point on a graph represents the internal concentration of the potato tissue.
Biological Importance of Water Potential
LO 9: Uptake and Loss of Water
Understanding water potential explains how organisms manage water balance.
- Root Hair Cells: The cell sap in the vacuole has a lower water potential than the soil water. Therefore, water moves into the root hair cells by osmosis. This is essential for keeping the plant turgid and transporting minerals.
- Wilting: If a plant loses water through transpiration faster than it can absorb it from the soil (e.g., in hot/dry conditions), the external environment becomes effectively more concentrated relative to the cell. Water leaves the cells, they become flaccid, and the plant wilts.
- Red Blood Cells: In a hypotonic (dilute) solution, water enters red blood cells until they burst. In a hypertonic (concentrated) solution, water leaves, and they shrivel. In an isotonic solution (like 0.9% saline), there is no net movement, and the cell maintains its shape.
⚠︎ Common Misconceptions in Osmosis
Mistake 1: Moving Solutes instead of Water
- Incorrect: "Sucrose moves out of the potato into the solution."
- Correct: Only water moves by osmosis. The solute (sucrose/salt) is too large to pass through the partially permeable membrane.
Mistake 2: Ignoring the Cell Wall in Plants
- Incorrect: "The plant cell bursts in distilled water because it takes in too much water."
- Correct: Plant cells do not burst. The rigid cell wall prevents bursting and exerts pressure back on the contents, creating turgor pressure.
Mistake 3: Confusing 'Concentrated' with 'High Water Potential'
- Incorrect: "Water moves to the concentrated solution because it is concentrated."
- Correct: Water moves from a dilute solution (high water potential) to a concentrated solution (low water potential). Remember: High concentration of solute = Low concentration of water.
Examiner Tips for Maximum Marks
Tip 1: Describing Osmosis (LO 7)
- Context: When asked to define or describe osmosis.
- Why examiners accept this: The definition is precise and covers all three necessary conditions: the substance (water), the gradient (high to low water potential), and the barrier (partially permeable membrane).
- Correct Usage: "Osmosis is 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."
Tip 2: Explaining Plant Support (LO 6)
- Context: When asked how plants are supported or why non-woody stems stand upright.
- Why examiners accept this: It directly links the physical pressure to the structural component (cell wall).
- Correct Usage: "Plants are supported by turgor pressure, which is the pressure of water inside the cells pressing outwards on the cell wall."
Tip 3: Interpreting Graphs/Tables (LO 5)
- Context: When asked to find the concentration where there is no change in mass.
- Why examiners accept this: It demonstrates understanding of equilibrium/water potential equality.
- Correct Usage: "The concentration at which the percentage change in mass is zero represents the point where the water potential of the external solution is equal to the water potential of the potato cells."
Practice Questions
Q:
Define osmosis.
A:
Osmosis is 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.
Q:
State two ways in which water acts as a solvent in organisms.
A:
- It dissolves substances for digestion (e.g., in the gut). 2. It transports dissolved materials (e.g., nutrients or waste) around the body.
Q:
Describe the appearance of a plant cell after being placed in a concentrated salt solution.
A:
The cell becomes plasmolysed and flaccid. The cytoplasm/vacuole shrinks and pulls away from the cell wall.
Q:
Explain why potato cylinders gain mass when placed in distilled water.
A:
Distilled water has a higher water potential than the potato cells. Water moves into the cells by osmosis, causing them to swell.
Q:
State how plants are supported by their cells.
A:
Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall (turgor pressure).