Welcome to Osmosis and Plant Transport!
Have you ever wondered how giant oak trees pull hundreds of litres of water all the way up from the soil to leaves dozens of metres in the air without a single mechanical pump? Or why a limp stick of celery crunches up again after being placed in a glass of water?
In this chapter for CCEA GCSE Double Award Science (Unit B2), we will explore the wonderful world of osmosis, discover how water and nutrients travel through plants, and look at the essential experiments you need to know for your exam. Don't worry if this topic feels a bit tricky at first—we will break down every single idea into simple, bite-sized steps!
1. What is Osmosis?
To understand plant transport, we first need to master one vital biological process: osmosis.
The Official Definition
Osmosis is the net movement of water molecules from a region of high water concentration (a dilute solution or higher water potential) to a region of lower water concentration (a concentrated solution or lower water potential) through a selectively permeable membrane.
Breaking Down the Key Terms
Let's look at the three critical parts of this definition that examiners look for:
• Water Molecules Only: Osmosis is a special type of diffusion that deals exclusively with water. Solute particles (like sugar or salt) do not move by osmosis.
• Concentration Gradient: A dilute solution has lots of water molecules and very few solute particles (high water concentration). A concentrated solution has lots of solute particles crowding the space, meaning fewer free water molecules (low water concentration). Water naturally flows down its own concentration gradient from where there is more water to where there is less water.
• Selectively Permeable Membrane: This is a microscopic barrier with tiny pores. It allows small molecules like water to pass through freely, but blocks larger solute molecules like sucrose or starch from getting through.
Analogy Corner: Imagine a crowded dance floor separated by a small turnstile. If one room is packed with people holding balloons (solute) and water bottles (water), while the other room has only water bottles, the water bottles can easily slip through the turnstile back and forth until they balance out, but the big balloons are too large to fit!
Key Takeaway for Section 1
Always state three things in your exam answers: (1) movement of water molecules, (2) from high water concentration to low water concentration, and (3) across a selectively permeable membrane.
2. Osmosis in Plant and Animal Cells
What happens when living cells are placed in different solutions? Because plant cells have a strong cell wall and animal cells do not, they respond in very different ways.
A. Plant Cells in Different Solutions
1. In Pure Water or Dilute Solution (Hypotonic):
Water enters the plant cell by osmosis because the water concentration outside is higher than inside the cell. As water enters, the vacuole expands and the cytoplasm pushes firmly against the rigid cellulose cell wall. The strong cell wall pushes back, preventing the cell from bursting. When the cell is full of water and firm, it is described as turgid.
Why is this important? Turgidity provides vital mechanical support to keep non-woody plant stems, leaves, and seedlings standing upright!
2. In a Concentrated Sugar/Salt Solution (Hypertonic):
Water leaves the plant cell by osmosis because the water concentration inside the cell is higher than outside. As water exits, the vacuole shrinks and the cytoplasm pulls away from the cell wall. When the cell becomes soft and limp, it is flaccid. When the cell membrane completely pulls away from the cell wall, the cell is plasmolysed (this process is called plasmolysis).
B. Animal Cells (e.g., Red Blood Cells)
Animal cells do not have a rigid cell wall. They only have a flexible cell surface membrane:
• In Pure Water / Dilute Solution: Water rushes into the cell by osmosis. Without a tough cell wall to resist the pressure, the animal cell swells and bursts. In red blood cells, this bursting is called lysis (or haemolysis).
• In Concentrated Solution: Water leaves the cell rapidly by osmosis. The red blood cell shrinks, shrivels, and crinkles. This is called crenation.
Memory Trick:
• Plant cells: Turgid (Terrifically firm) vs. Plasmolysed (Pulled away from wall).
• Animal cells: Lysis (Loses its shape and bursts) vs. Crenation (Crinkles and shrinks).
Key Takeaway for Section 2
Plant cells never burst in pure water because of their strong cellulose cell wall—they become turgid. Animal cells burst (lysis) because they lack a cell wall.
3. Plant Transport: Xylem, Phloem, and Root Hair Cells
Plants have specialised transport systems to move water, minerals, and sugars to every cell.
A. Root Hair Cells
Roots absorb water and dissolved mineral ions from the soil:
• Structure: They have long, finger-like microscopic extensions.
• Function: This shape provides a very large surface area for the rapid absorption of water by osmosis and mineral ions by active transport.
B. Xylem Tissue
Xylem vessels act like the plant's plumbing pipes for water:
• Structure: Made of hollow, dead, elongated cells joined end-to-end with no end walls. Their walls are thickened and waterproofed with a tough substance called lignin.
• Function: Transports water and dissolved mineral ions in a one-way direction (upwards only) from the roots, through the stem, to the leaves.
• Extra Role: Lignified xylem walls provide strong physical support to the plant stem.
C. Phloem Tissue
Phloem vessels distribute food throughout the plant:
• Structure: Made of living cells arranged end-to-end with perforated end walls called sieve plates, supported by companion cells.
• Function: Transports dissolved sugars (sucrose) and amino acids in both directions (up and down) to growing regions and storage organs. This movement of food is called translocation.
Quick Comparison: Xylem vs. Phloem
• Xylem: Dead hollow tubes, contains lignin, transports water and mineral ions, flows upwards only.
• Phloem: Living cells, has sieve plates, transports sucrose and amino acids (translocation), flows bidirectionally (up and down).
4. Transpiration and the Transpiration Stream
What is Transpiration?
Transpiration is the evaporation and diffusion of water vapour from the surface of spongy mesophyll cells into the leaf air spaces, followed by diffusion out through the stomata into the atmosphere.
The Transpiration Stream
Water moves in an unbroken, continuous column through the plant. This journey is called the transpiration stream:
Soil \(\rightarrow\) Root Hair Cells \(\rightarrow\) Root Cortex \(\rightarrow\) Xylem Vessels \(\rightarrow\) Stem \(\rightarrow\) Leaf Mesophyll Cells \(\rightarrow\) Stomata \(\rightarrow\) Atmosphere
Why is Transpiration Essential for Plants?
1. Cooling: Evaporation of water cools leaf tissues in hot weather.
2. Photosynthesis: Supplies necessary water directly to leaf cells.
3. Mineral Transport: Carries dissolved mineral ions up from the soil.
4. Support: Keeps cells fully hydrated and turgid to support the plant.
Factors Affecting the Rate of Transpiration
Examiners love asking how environmental conditions change transpiration rates:
1. Temperature:
• Effect: Higher temperature \(\rightarrow\) increases transpiration rate.
• Why: Heat gives water molecules more kinetic energy, causing faster evaporation and diffusion.
2. Wind Speed (Air Movement):
• Effect: Higher wind speed \(\rightarrow\) increases transpiration rate.
• Why: Moving air blows away the moist water vapour gathered outside the stomata. This maintains a steep concentration gradient between the inside of the leaf and the outside air.
3. Humidity:
• Effect: High humidity \(\rightarrow\) decreases transpiration rate.
• Why: Humid air is already full of water vapour. This reduces the concentration gradient, so water diffuses out much more slowly.
4. Light Intensity:
• Effect: Brighter light \(\rightarrow\) increases transpiration rate.
• Why: Stomata open wider in bright light to let in carbon dioxide for photosynthesis, allowing more water vapour to escape.
5. Required Practical Investigations & Calculations
A. Investigating Osmosis: The Potato Cylinder Experiment
In this classic experiment, potato cylinders are placed into test tubes containing different concentrations of sucrose or salt solution.
Step-by-Step Method:
1. Use a cork borer to cut cylinders of uniform diameter from a single potato.
2. Trim the cylinders to identical lengths with a scalpel.
3. Gently blot the cylinders dry with a paper towel to remove excess surface water before weighing.
4. Record the initial mass of each cylinder using an accurate electronic balance.
5. Place cylinders into different sucrose concentrations (e.g., \(0.0\text{ M}\), \(0.2\text{ M}\), \(0.4\text{ M}\), \(0.6\text{ M}\), \(0.8\text{ M}\), \(1.0\text{ M}\)) for a set period.
6. Remove, blot gently again, and measure the final mass.
Calculating Percentage Change in Mass
Because potato cylinders do not always start with the exact same initial mass, calculating the raw change in mass is unfair. Calculating percentage change allows a fair and valid comparison:
\(\text{Percentage Change in Mass} = \left( \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \right) \times 100\)
• If the cylinder gains mass, the value is positive (\(+\)). Water entered by osmosis (the surrounding solution was more dilute than the potato cell contents).
• If the cylinder loses mass, the value is negative (\(-\)). Water left by osmosis (the surrounding solution was more concentrated than the potato cell contents).
Interpreting the Osmosis Graph
When you plot Sucrose Concentration (x-axis) against Percentage Change in Mass (y-axis):
• The line/curve starts above zero in dilute solutions (mass gained).
• The line crosses the 0% line (the x-intercept).
• Key Exam Fact: The concentration where the curve crosses the 0% line is the isotonic point. At this point, there is no net movement of water because the water concentration of the solution matches the internal water concentration of the potato cells!
B. Measuring Transpiration: The Potometer
A bubble potometer is used to investigate how environmental factors affect water loss.
• Important Principle: A potometer measures the rate of water uptake by the cut leafy shoot (assumed to be directly proportional to the transpiration rate, even though a tiny fraction of water is retained for photosynthesis).
• Formula:
\(\text{Rate of Water Uptake} = \frac{\text{Distance moved by bubble (mm)}}{\text{Time taken (min or s)}}\)
• Crucial Setup Steps:
1. Cut the leafy shoot underwater to prevent air bubbles entering the xylem vessels.
2. Seal all rubber tubing joints with petroleum jelly (Vaseline) to ensure the apparatus is completely airtight.
C. The Washing-Line Leaf Experiment
To investigate where water is lost from leaves:
• Leaves are hung from a line with different surfaces covered in petroleum jelly (e.g., upper surface covered, lower surface covered, both covered, neither covered).
• Cobalt chloride paper can be used to detect water loss (it turns from blue to pink when wet).
• Result: Leaves lose significantly more water from their lower surface because that is where the density of stomata is highest.
6. Common Exam Pitfalls to Avoid
Make sure you do not lose easy marks by remembering these examiner warnings:
• Don't forget the membrane: Writing "osmosis is the movement of water from high to low concentration" gets zero marks if you omit "through a selectively permeable membrane".
• Specify "water" concentration: Never write "water moves from a high concentration to a low concentration" without including the word water. Saying simply "high concentration" could mean solute concentration!
• Plant vs. Animal Terms: Never say a plant cell has "burst" (the cell wall prevents this) and never call an animal cell "turgid" or "plasmolysed". Use lysis (bursting) and crenation (shrivelling) for animal cells.
• Potometer Precision: If an exam question asks what a potometer directly measures, write rate of water uptake, not "transpiration rate".
• Why use percentage change? If asked why raw mass is not used, state: "To account for differences in the initial starting masses of the potato cylinders."
Quick Chapter Summary
• Osmosis: Net movement of water molecules from high water concentration to low water concentration across a selectively permeable membrane.
• Turgor: Water entering plant cells pushes cytoplasm against the cell wall, making cells turgid and supporting the plant.
• Plasmolysis: In concentrated solutions, plant cells lose water and the cytoplasm pulls away from the cell wall.
• Xylem: Dead, lignified tubes transporting water and minerals upwards.
• Phloem: Living cells with sieve plates transporting sugars/amino acids up and down (translocation).
• Transpiration Stream: Pull of water from soil \(\rightarrow\) roots \(\rightarrow\) xylem \(\rightarrow\) leaf mesophyll \(\rightarrow\) stomata.
• Factors Increasing Transpiration: Higher temperature, higher wind speed, higher light intensity, and lower humidity.