Sciences - Co-ordinated (Double) (0654) | Transport in plants

Welcome to Transport in Plants (B8)!

Hello IGCSE scientists! This chapter, Transport in Plants, is all about how plants manage to move essential materials—like water, minerals, and manufactured food—from one part of their body to another. Think of it as the plant's internal delivery system. If this system breaks down, the plant can't survive! Understanding this is key to understanding how plants grow so tall and stay healthy.

Don't worry if some terms seem new; we will break down the plant's transport system into two main highways: one for water and one for food!

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B8.1 The Plant's Transport Highways: Xylem and Phloem

Plants need specialized tubes to move substances efficiently over long distances, similar to the blood vessels in animals. These tubes are clustered together in structures called vascular bundles.

1. The Xylem: Water and Support

The xylem acts like a one-way plumbing pipe, primarily moving things upwards from the roots.

• Function 1: Transport of Water and Mineral Ions
  

  Water absorbed by the roots moves up through the xylem to the leaves where it is used for photosynthesis and lost during transpiration.

• Function 2: Support
  

  Xylem vessels have thick, lignified walls (woody material), which helps provide structural support to the non-woody parts of the plant (like a rigid skeleton).

Analogy: Think of the Xylem as the elevator shaft in a building, only moving supplies up, and also helping the structure stand tall!

2. The Phloem: Food Delivery

The phloem is responsible for distributing the food the plant makes.

• Function: Transport of Sucrose and Amino Acids
  

  Phloem transports soluble sugars (mainly sucrose, the plant's stored food molecule) and amino acids to all parts of the plant that need them for growth or storage.

Did you know? Unlike the xylem, the phloem can transport substances both up and down, depending on where the food is needed!

3. Position of Xylem and Phloem (Vascular Bundles)

It is important to be able to identify where these tissues are located, especially in non-woody dicotyledonous plants (plants with two seed leaves, like beans or roses).

• In Roots: The xylem forms a central 'X' shape, with the phloem located between the arms of the X. This central structure provides resistance against stretching forces as the root pulls through the soil.
• In Stems: The vascular bundles are arranged in a ring just inside the cortex (the outer layer). Xylem is typically towards the inside, and phloem is towards the outside.
• In Leaves: They form the "veins" (vascular bundles), running close together to ensure every cell has quick access to water (via xylem) and quick removal of manufactured food (via phloem).

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B8.2 How Plants Suck Up Water

How does water actually enter the plant from the soil? It all starts with specialized cells in the root.

1. The Role of Root Hair Cells

• Structure and Function: Root hair cells are specialized epidermal cells found near the tips of the roots. They have long, thin extensions (the root hairs).
• Importance of Surface Area: This root hair structure gives the cells a very large surface area. This massive area is crucial because it significantly increases the uptake of water and mineral ions from the soil, making the process much faster.

Analogy: Imagine trying to sweep the floor with a tiny brush versus a wide broom. The broom (large surface area) is much more efficient!

2. The Water Pathway Through the Plant

Water doesn't just jump into the xylem; it follows a clear path dictated by the structure of the root and the forces of osmosis (which you learned about in B3!).

The water pathway is:

Root Hair Cells → Root Cortex Cells → Xylem Vessels (in root/stem) → Mesophyll Cells (in leaf)

1. Water enters the root hair cell from the soil (usually by osmosis).
2. It moves through the inner root cortex cells, travelling from cell to cell.
3. It enters the xylem vessels in the center of the root.
4. The water then travels up the stem through the xylem.
5. Finally, it is delivered to the mesophyll cells in the leaves for use in photosynthesis or evaporation.

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B8.3 Transpiration: The Plant's Breathing and Driving Force

Transpiration is the process that drives water movement throughout the plant. It is defined as the loss of water vapour from the leaves (and stem) of the plant.

1. The Mechanism of Transpiration

Transpiration involves two key steps: evaporation and diffusion.

1. Evaporation: Water moves from the xylem into the surrounding mesophyll cells in the leaf. Here, water evaporates from the wet surfaces of the mesophyll cells into the air spaces inside the leaf.
2. Diffusion: The concentration of water vapour is very high in these air spaces. This water vapour then diffuses out of the leaves through the stomata (small pores, usually on the underside of the leaf) into the drier air outside.

Think of it like sweating: water evaporates from a wet surface (the mesophyll cells) and then escapes into the air (through the stomata).

2. Factors Affecting the Rate of Transpiration (Crucial for Exams!)

The rate at which water is lost changes constantly based on the environment. You need to know the effects of three main factors:

(a) Temperature

Effect: Higher temperature increases the rate of transpiration.

Explanation (Why?): Higher temperature increases the kinetic energy of the water molecules, causing them to evaporate faster from the mesophyll cell surfaces. It also increases the concentration gradient between the inside of the leaf and the outside air.

(b) Wind Speed (Air Movement)

Effect: Increased wind speed increases the rate of transpiration.

Explanation (Why?): When water diffuses out of the stomata, it forms a layer of humid air (a boundary layer) right outside the leaf surface. Wind blows this humid air away, maintaining a steep concentration gradient for water vapour between the leaf and the external air.

(c) Humidity

Effect: Increased humidity (more water vapour in the air) decreases the rate of transpiration.

Explanation (Why?): If the surrounding air is very humid, there is less difference (a shallower concentration gradient) between the concentration of water vapour inside the leaf and outside. Since diffusion depends on a concentration gradient, water diffuses out more slowly.

Memory Aid: Remember the factors using the acronym TWH (Temperature, Wind, Humidity). T and W increase transpiration; H decreases it!

3. Wilting (Extended/Supplement)

If a plant loses water faster than its roots can absorb it, the plant cells lose their turgor pressure (they become flaccid) and the plant starts to droop. This process is called wilting.

How and Why Wilting Occurs:

• How: When water loss is greater than water uptake, the large central vacuole in the plant cells shrinks, and the cytoplasm pulls away from the cell wall (plasmolysis). The cells lose the rigidity provided by turgor pressure.
• Why: Wilting is actually a defense mechanism. As the cells lose water, the leaf droops and the stomata often close. This action reduces the surface area exposed to the sun and stops further water loss, preventing permanent damage or death.

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B8.4 Translocation: Moving the Sugars (Extended/Supplement)

While the xylem handles water, the phloem handles the movement of essential organic nutrients—the food the plant has made.

1. What is Translocation?

Translocation is defined as the movement of sucrose and amino acids in the phloem. This movement is often described as going from sources to sinks.

2. Sources and Sinks

The flow of substances in the phloem is always from a place of high concentration (the source) to a place of low concentration (the sink).

(a) Sources

Sources are the parts of plants that release (or produce) sucrose or amino acids into the phloem.

• Primary Example: Mature, photosynthesising leaves (where food is actively made).
• Other Examples: Storage organs (like tubers or roots) when they are releasing their stored energy for growth.

(b) Sinks

Sinks are the parts of plants that use or store sucrose or amino acids. These parts remove the nutrients from the phloem.

• Examples: Growing points (buds, young leaves, root tips), roots (for storage), and developing fruits or seeds.

Encouragement: Understanding the difference between source and sink is crucial. A potato tuber is a SINK in summer (storing food) but becomes a SOURCE in spring (releasing food for the new shoots).