Welcome to Osmosis and Plant Transport!
Have you ever wondered how tall trees manage to pull gallons of water from the soil all the way up to their highest leaves without a mechanical pump? Or why a crisp piece of celery goes limp if left out on the counter, but crisps right back up when you put it in a bowl of cold water?
In this chapter, part of Unit 2: Body Systems, Genetics, Microorganisms and Health, we will explore the science behind these everyday events. Don't worry if biology terms sometimes feel overwhelming; we will break down each concept step by step so you can master this topic for your CCEA GCSE exams!
Quick Summary of What We Will Cover:
1. What osmosis is and how selectively permeable membranes work.
2. What happens to plant and animal cells in different solutions.
3. How plants take up and transport water and nutrients (Xylem vs. Phloem).
4. The process of transpiration and what changes its rate.
5. Key practical experiments (potato cylinders, Visking tubing, and potometers).
1. Understanding Osmosis: The Basics
To understand how plants move water, we must first master the concept of osmosis.
The Official Definition of Osmosis
Osmosis is the 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 selectively permeable membrane.
Let's break down the key parts of this definition:
• Dilute Solution (High Water Concentration): A solution that contains a lot of water molecules and very few dissolved solute particles (like sugar or salt). Think of pure water or very weak squash.
• Concentrated Solution (Low Water Concentration): A solution that contains fewer water molecules relative to a large amount of dissolved solute particles. Think of thick, syrupy squash.
• Selectively Permeable Membrane: A microscopic barrier (like a cell membrane) that has tiny holes in it. It allows small molecules, such as water, to pass through freely, but blocks larger dissolved molecules, such as sucrose or proteins.
Everyday Analogy: Imagine a crowd of people trying to get through a doorway with a tennis net across it. Small ping-pong balls (water molecules) can easily bounce through the gaps in the net, but large basketballs (sugar molecules) are blocked!
Examiner Warning: Common Mistake to Avoid!
Never just write that osmosis is the movement of particles "from high to low concentration." You must state that it is the movement of water molecules from a higher water concentration to a lower water concentration (or dilute to concentrated solution) across a selectively permeable membrane to get full marks.
Key Takeaway: Osmosis is simply the diffusion of water molecules down their concentration gradient across a selectively permeable membrane.
2. Effects of Osmosis on Cells
What happens when you place living cells into pure water or concentrated salt/sugar solutions? The result depends on whether the cell is a plant cell or an animal cell.
A. Plant Cells in Different Solutions
Plant cells have two key features: a cell membrane on the inside and a strong, rigid cell wall made of cellulose on the outside.
1. Dilute Solution / Pure Water (Hypotonic Environment):
• Water enters the plant cell by osmosis.
• The vacuole swells and pushes the cytoplasm hard against the cell wall.
• The strong cell wall prevents the cell from bursting.
• The cell becomes firm and swollen, which we call turgid.
• This creates turgor pressure, which gives non-woody plant stems and leaves mechanical support to stay upright.
2. Equal Concentration (Isotonic Environment):
• Water enters and leaves the cell at the exact same rate (there is no net movement of water).
• The cell stays the same size and volume.
3. Concentrated Sugar/Salt Solution (Hypertonic Environment):
• Water leaves the plant cell by osmosis.
• As water leaves, the vacuole shrinks and the cell loses its firmness, becoming soft or flaccid.
• In severe water loss, so much water leaves that the cytoplasm pulls completely away from the cell wall. This state is called plasmolysis, and the cell is described as plasmolysed.
Memory Trick: Turgid = Tight and firm. Plasmolysed = Pulled away from the wall.
B. Animal Cells vs. Plant Cells
Unlike plant cells, animal cells (such as red blood cells) do not have a cell wall. This leads to very different results:
• In pure water / dilute solutions: Animal cells take in water by osmosis, swell up, and burst. This bursting of animal cells is called lysis.
• In concentrated solutions: Animal cells lose water by osmosis, causing them to shrink, wrinkle, and shrivel. This is known as crenation.
Quick Comparison Box:
• Plant cell in dilute water = Turgid (cell wall prevents bursting).
• Animal cell in dilute water = Lysis (bursts because it has no cell wall).
• Plant cell in concentrated solution = Flaccid / Plasmolysed.
• Animal cell in concentrated solution = Crenation (shrivels).
Key Takeaway: The rigid cell wall protects plant cells from bursting in dilute water (making them turgid) and keeps the plant upright via turgor pressure.
3. Plant Transport Tissues: Xylem and Phloem
Plants have a specialized transport system made of two distinct pipeline tissues: xylem and phloem.
1. Xylem Tissue
• Structure: Made of dead, hollow cells joined end-to-end to form continuous, unbroken tubes. Their walls are strengthened with a tough substance called lignin.
• Function: Transports water and dissolved mineral ions.
• Direction of Flow: One-way only — upwards from the roots, up the stem, and to the leaves.
2. Phloem Tissue
• Structure: Made of living cells comprising sieve tubes and supporting companion cells.
• Function: Transports dissolved sugars (sucrose) and amino acids.
• Direction of Flow: Two-way — moves both up and down the plant to wherever food is needed or stored.
• Name of Process: The movement of dissolved food through phloem is called translocation.
Memory Aid:
• Xylem = Water & Minerals (X goes up like a ladder).
• Phloem = Food (transports sugars/sucrose both ways).
3. Water Uptake in the Roots
Water enters the plant through microscopic specialized cells called root hair cells.
• Adaptation: They have long, thin extensions that provide a very large surface area to absorb water quickly.
• Mechanism: Soil water is dilute compared to the cell contents inside the root. Therefore, water enters the root hair cells by osmosis down a water concentration gradient.
Key Takeaway: Xylem moves water and minerals upwards in dead, hollow tubes; phloem moves sugars and amino acids in living cells via translocation.
4. Transpiration and the Transpiration Stream
The Exact Definition of Transpiration
Examiners look for a very specific two-part description for this process:
Transpiration is the evaporation of water from spongy mesophyll cells followed by the diffusion of water vapour through leaf intercellular air spaces and out through the stomata.
The Transpiration Stream
The transpiration stream is the continuous, uninterrupted column of water moving through the plant: entering the roots by osmosis, travelling upwards through the stem inside xylem vessels, and evaporating out through the leaves.
Why is the Transpiration Stream Important? (3 Key Functions)
1. Supplying Water: Provides water for photosynthesis and maintains cell turgidity (keeping the plant firm).
2. Transporting Minerals: Carries dissolved essential mineral nutrients from the soil to the leaves.
3. Cooling: Cools the leaf surfaces as water evaporates (evaporative cooling), preventing leaves from overheating in the sun.
5. Environmental Factors Affecting Transpiration Rate
Four main weather and environmental factors change how fast a plant loses water:
1. Temperature (Increase = Increases Transpiration Rate)
• Warmer temperatures give water molecules more kinetic energy.
• This speeds up both the rate of evaporation from spongy mesophyll cells and the rate of diffusion out through the stomata.
2. Light Intensity (Increase = Increases Transpiration Rate)
• In brighter light, stomata open wider to let in carbon dioxide for photosynthesis.
• Wider stomata allow more water vapour to diffuse out of the leaf.
3. Wind / Air Movement (Increase = Increases Transpiration Rate)
• Moving air blows away the moist water vapour that builds up directly around the outside of the leaf.
• This keeps a steep concentration gradient between the inside of the leaf and the surrounding air, speeding up diffusion.
4. Humidity (Increase = Decreases Transpiration Rate)
• High humidity means the air outside the leaf already has a lot of water vapour.
• This decreases (flattens) the concentration gradient between the leaf interior and the air, which slows down the diffusion of water vapour.
Key Takeaway: Transpiration is fastest when it is warm, bright, windy, and dry.
6. Prescribed Practical Investigations
Prescribed Practical 2.1: Investigating Osmosis
A. Model Cells Using Visking Tubing:
• Visking tubing acts as a model for a selectively permeable cell membrane.
• It is filled with a concentrated sucrose solution, tied tightly at both ends, dried, weighed, and placed into a beaker of pure water.
• Over time, water enters the tubing by osmosis down the concentration gradient, causing the tubing to increase in mass and volume.
B. Potato Cylinder Experiment:
• Potato cylinders of identical length and diameter are cut.
• They are weighed and placed into test tubes containing different concentrations of sugar or salt solutions.
• After a set time, the cylinders are removed, blotted dry with a paper towel (to remove excess surface liquid which causes errors), and re-weighed.
We calculate the percentage change in mass using this formula:
\(\text{Percentage Change} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100\)
Exam Tip on Calculations:
• If the potato gains mass, the percentage change is positive (\(+\)).
• If the potato loses mass, remember to include the minus sign (\(-\)).
• Always divide by the Initial Mass, never the Final Mass!
Finding the Isotonic Point:
When you plot a graph of Solution Concentration (\(x\)-axis) against Percentage Change in Mass (\(y\)-axis), the point where your line of best fit crosses the horizontal axis (at \(0\%\) change) tells you the exact internal concentration of the potato cells.
Prescribed Practical 2.2: Investigating Water Uptake and Transpiration
1. The Bubble Potometer:
• A cut leafy shoot is attached to a capillary tube filled with water.
• An air bubble is introduced into the capillary tube.
• As the plant loses water from its leaves, it pulls water up the stem, causing the air bubble to move along a ruler.
• By timing how far the bubble moves in a given time, we measure the rate of water uptake.
Examiner Warning: A bubble potometer strictly measures the rate of water uptake, not directly the exact rate of transpiration (because a small percentage of water taken up is used inside the plant for photosynthesis and keeping cells turgid).
2. The Weight Potometer:
• A potted plant has its pot and soil sealed inside a plastic bag so water cannot evaporate from the soil.
• The whole setup is weighed on a balance, left for a set period, and re-weighed.
• The mass lost is due to water evaporating directly from the leaves.
3. The Washing Line Method:
• Several leaves are hung from a string line.
• Different leaf surfaces are sealed with petroleum jelly (Vaseline): one with the upper surface greased, one with the lower surface greased, one with both greased, and an ungreased control.
• By comparing the loss of mass, this experiment demonstrates that the vast majority of water vapour is lost through the stomata on the lower surface of the leaf.
Quick Review: Check Your Understanding!
Before moving on to exam practice questions, make sure you can confidently answer these:
1. Can you recite the 3-part definition of osmosis without looking?
2. What is the difference between a flaccid plant cell and a plasmolysed plant cell?
3. Why does an animal cell burst in pure water while a plant cell becomes turgid?
4. What are the two main transport tissues in plants, and what does each carry?
5. How does high humidity affect the rate of transpiration, and why?
6. Why must you blot potato cylinders dry before weighing them?