Transport in plants — xylem, phloem and transpiration (KCSE Biology Form 2)

TL;DR

Plants have special tissues, xylem and phloem, to move water and food around. Xylem transports water and minerals from roots to leaves, while phloem moves sugars from leaves to other parts. Transpiration, the loss of water vapour from leaves, is the main force pulling water up the xylem.

1. The Mental Model

Think of a plant like a building with a plumbing system. Xylem is like the water pipes bringing water up from the ground, and phloem is like the food delivery system moving energy around. Transpiration is the "pump" that makes the water move upwards.

2. The Core Material

2.1 Why Plants Need Transport

Just like you need to move nutrients and waste around your body, plants need to move water, minerals, and food (sugars) to all their cells. Water and minerals are absorbed by the roots, and food is made in the leaves during photosynthesis. These need to reach every part of the plant for growth and survival.

2.2 Xylem: The Water Highway

Xylem is the tissue responsible for transporting water and dissolved mineral salts from the roots, up the stem, to the leaves and other aerial parts of the plant. It's made of dead cells that form continuous tubes.

• Xylem vessels: These are wide, continuous tubes formed from dead cells joined end-to-end, with their end walls broken down. They provide a low-resistance pathway for water flow.
• Tracheids: These are narrower, elongated cells with tapered ends. Water moves between tracheids through pits (small pores) in their walls.
• Xylem parenchyma: Living cells that store food and help in lateral transport.
• Xylem fibres: Provide mechanical support to the plant.

Water movement in xylem is mostly one-way: from roots to leaves.

2.3 Phloem: The Food Delivery Service

Phloem is the tissue that transports manufactured food (sugars, mainly sucrose) from the leaves, where it's made during photosynthesis, to all other parts of the plant, including roots, fruits, and growing tips. This process is called translocation.

• Sieve tubes: These are living cells arranged end-to-end, forming continuous tubes. Their end walls, called sieve plates, have pores that allow sap to flow through.
• Companion cells: These are living cells located next to sieve tubes. They have a nucleus and many mitochondria, providing energy and controlling the activities of the sieve tube cells, which lack a nucleus.
• Phloem parenchyma: Living cells for storage.
• Phloem fibres: Provide support.

Food movement in phloem can be multi-directional, from "source" (where food is made or stored) to "sink" (where food is used or stored).

2.4 Transpiration: The Driving Force

Transpiration is the process by which plants lose water vapour to the atmosphere, mainly through tiny pores called stomata on the surface of their leaves.

2.4.1 How Transpiration Works

1. Water evaporates from the moist cell surfaces inside the leaf into the air spaces.
2. This water vapour then diffuses out of the leaf through the stomata into the drier atmosphere.

2.4.2 The Transpiration Stream

This loss of water from the leaves creates a "pull" or tension, like sucking on a straw. This pull is transmitted down the continuous column of water in the xylem vessels, all the way to the roots. This is known as the transpiration pull or cohesion-tension theory.

• Cohesion: Water molecules stick to each other due to hydrogen bonds.
• Adhesion: Water molecules stick to the walls of the xylem vessels.

These forces, along with root pressure (a minor upward push from the roots), help maintain the continuous water column and pull water up against gravity.

2.4.3 Factors Affecting Transpiration Rate

Several environmental factors influence how fast a plant transpires:

• Temperature: Higher temperature increases the rate of evaporation from the leaf and the rate of diffusion of water vapour out of the stomata. Higher temperature = higher transpiration.
• Humidity: High humidity means the air already has a lot of water vapour, reducing the water potential gradient between the leaf and the air. Higher humidity = lower transpiration.
• Wind: Wind blows away the humid air around the leaf, maintaining a steep water potential gradient. More wind = higher transpiration.
• Light Intensity: Light causes stomata to open to allow CO2 for photosynthesis. Open stomata mean more water loss. Higher light intensity = higher transpiration.
• Water Availability: If the plant doesn't have enough water, stomata will close to conserve water, reducing transpiration. Less water = lower transpiration.

Here's a diagram showing the transpiration stream:

graph TD
    A[Sunlight & Warmth] --> B(Water evaporates from leaf cells)
    B --> C{Water vapour diffuses out through Stomata}
    C --> D[Atmosphere]
    C -- "Creates pull" --> E[Water column in Xylem]
    E -- "Cohesion & Adhesion" --> F[Water moves up Stem]
    F --> G[Water absorbed by Root Hairs]
    G --> H[Soil Water]

3. Worked Example

Imagine a plant in a hot, windy, and dry environment. How would its transpiration rate compare to a plant in a cool, still, and humid environment, assuming both have enough water?

Step 1: Analyze the first environment (hot, windy, dry).
* Hot: High temperature increases evaporation.
* Windy: Wind removes humid air, maintaining a steep gradient.
* Dry: Low humidity means a large water potential gradient.

Step 2: Analyze the second environment (cool, still, humid).
* Cool: Low temperature decreases evaporation.
* Still: No wind, so humid air builds up around the leaf.
* Humid: High humidity means a small water potential gradient.

Step 3: Compare the effects.
All factors in the first environment (hot, windy, dry) will increase the rate of transpiration. All factors in the second environment (cool, still, humid) will decrease the rate of transpiration.

Conclusion: The plant in the hot, windy, and dry environment will have a significantly higher transpiration rate than the plant in the cool, still, and humid environment.

4. Key Takeaways

• Xylem transports water and minerals upwards from roots to leaves.
• Phloem transports sugars (food) from leaves to all parts of the plant.
• Transpiration is the loss of water vapour from leaves, mainly through stomata.
• The transpiration pull, driven by evaporation from leaves, is the main force moving water up the xylem.
• Factors like temperature, humidity, wind, and light intensity affect the rate of transpiration.

Common mistakes to avoid:
* Confusing the direction of transport for xylem (up only) and phloem (up or down).
* Thinking transpiration is just "breathing" for plants; it's specifically water loss.
* Forgetting that cohesion and adhesion are crucial for the continuous water column in xylem.
* Mixing up the factors that increase vs. decrease transpiration rate.
* Believing phloem transports water; it primarily transports sugars.

5. Now Try It

Design a simple experiment to investigate how one environmental factor (e.g., wind or humidity) affects the rate of transpiration in a potted plant. Describe the materials you would need, the steps you would follow, and what you would measure to determine the rate of transpiration. What would success look like?

Success would be describing an experiment that clearly isolates one variable, has a control, and uses a measurable outcome (like water loss from a pot or a potometer reading) to show the effect of that variable on transpiration.