Biology (0610) | Osmosis

🧬 Movement into and out of Cells: OSMOSIS

👋 Introduction: Why Water Movement Matters!

Welcome to the fascinating world of Osmosis! This topic explains exactly how water moves in and out of every cell in every living thing, from tiny bacteria to massive redwood trees.

Understanding osmosis is crucial because the movement of water controls things like:

• Why plants stand upright (or wilt!).
• How your cells stay healthy and don't burst.
• The process of absorbing nutrients and getting rid of waste.

Let’s dive into how this essential process works!

Part 1: The Core Concepts of Osmosis (Core Content)

1.1 Water: The Essential Solvent (Core 3.2.1)

In biology, water is not just something we drink; it plays a critical role as a solvent.

• A solvent is a liquid that dissolves other substances (solutes). In living organisms, water dissolves vital substances like glucose, ions, and waste products (like urea).
• This ability is essential for processes like: digestion, transport (moving substances around the body via blood/xylem), and excretion (removing soluble waste).

1.2 The Partially Permeable Membrane (Core 3.2.2 & 3.2.3)

Osmosis only happens when there is a barrier involved: the cell membrane.

• The cell membrane (which surrounds all cells) is a partially permeable membrane (PPM).
• "Partially permeable" means it allows some particles to pass through easily (like tiny water molecules), but blocks others (like larger solute molecules, such as sugar or protein).

Think of it like a fine sieve: water molecules are small enough to pass through the holes, but large dissolved sugar molecules get stuck.

1.3 Defining Osmosis (Core Definition)

Osmosis is essentially the diffusion of water.

Osmosis is the net movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution), through a partially permeable membrane.

Quick Review: What does "Net Movement" Mean?

Water molecules are always moving randomly. In osmosis, water moves in both directions across the PPM. However, when we say net movement, we mean the overall direction in which more molecules are moving.
Example: If 100 water molecules move left, but only 50 move right, the net movement is 50 molecules to the left.

Part 2: The Extended Definition: Water Potential (Supplement Content)

For extended (Supplement) students, you must use the term Water Potential. Don't worry, it's just a more precise way of talking about water concentration!

2.1 Understanding Water Potential (Supplement 3.2.7)

Water potential (\(\Psi\)) measures the tendency of water molecules to move freely.

• Pure water has the highest water potential (defined as zero).
• Adding solutes (like salt or sugar) lowers the potential, making the value negative.
• Therefore, a dilute solution has a high water potential (closer to zero).
• A concentrated solution has a low water potential (more negative).

The Advanced Definition (Supplement 3.2.7):
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.

Part 3: The Effects of Osmosis on Cells

3.1 Effects on Animal Cells

Animal cells (like red blood cells) have a cell membrane but no cell wall. This means they are vulnerable to changes in water pressure.

A. Animal cell in Pure Water (High Water Potential Solution)

• Water potential outside the cell is much higher than inside.
• Water moves into the cell by osmosis.
• The cell swells up until the membrane bursts. This is called lysis (or haemolysis for red blood cells).

The animal cell has no cell wall to protect it from bursting.

B. Animal cell in Highly Concentrated Salt Solution (Low Water Potential Solution)

• Water potential outside the cell is much lower than inside.
• Water moves out of the cell by osmosis.
• The cell shrinks and the membrane wrinkles. This is called crenation.

3.2 Effects on Plant Cells (Core 3.2.5 & Supplement 3.2.8)

Plant cells have a strong, rigid cell wall outside the cell membrane. This wall provides crucial protection.

A. Plant Cell in Pure Water (High Water Potential)

Water moves into the cell by osmosis.

• The cell membrane pushes outwards against the rigid cell wall.
• The cell wall prevents the cell from bursting.
• The pressure exerted on the cell wall is called turgor pressure.
• The cell is described as turgid (swollen and firm).

Importance (Core 3.2.6): Turgid cells pressing outwards on the cell wall provide support to the plant, keeping leaves and stems upright.

Analogy: A turgid cell is like an overfilled balloon inside a rigid box. The balloon pushes hard on the box walls.

B. Plant Cell in Highly Concentrated Solution (Low Water Potential)

Water moves out of the cell by osmosis.

• The cell loses water and the volume of the cytoplasm shrinks.
• The cell membrane pulls away from the rigid cell wall.
• The cell is described as plasmolysed, and the process is called plasmolysis.
• If the cell loses some water but the membrane has not yet pulled away from the wall, the cell is flaccid (limp).

When an entire plant wilts, its cells have become flaccid or plasmolysed due to water loss.

C. Plant Cell in Isotonic Solution (Equal Water Potential)

• There is no net movement of water.
• The cell is usually flaccid or just slightly supported.

Part 4: Investigations and Importance

4.1 Investigating Osmosis (Core 3.2.4 & 3.2.5)

You may be asked to describe or interpret experiments on osmosis.

1. Using Dialysis Tubing (Visking Tubing):
A bag of dialysis tubing (a PPM) filled with a concentrated sugar solution is placed in a beaker of pure water.

• Observation: The bag swells and increases in mass.
• Conclusion: Water moves from the beaker (high water potential) into the bag (low water potential) by osmosis. The sugar molecules cannot pass out.

2. Using Plant Tissues (Potato/Carrot Cylinders):
Identical pieces of plant tissue are weighed and placed into solutions of different concentrations (e.g., pure water, dilute sugar, concentrated sugar) for a set period.

• Observation:
  • Tissue in pure water increases in mass and becomes firm (turgid).
  • Tissue in concentrated solution decreases in mass and becomes soft (flaccid/plasmolysed).
• Calculation: You calculate the percentage change in mass to measure the extent of water movement.

4.2 Importance in Organisms (Supplement 3.2.9)

Osmosis is vital for life processes, primarily controlling water balance.

• Plant Water Uptake: Water enters the root hair cells of plants by osmosis. Soil water usually has a higher water potential than the cytoplasm of the root hair cell, driving water in.
• Maintaining Cell Health: In animals, tissues are kept in a fluid environment (like blood plasma) that maintains an isotonic balance (equal water potential), ensuring cells neither swell nor shrink permanently.
• Excretion: In kidney function, osmosis helps reabsorb necessary water back into the blood, preventing dehydration.