Plant Physiology Transport Systems for the ESAT

The Structure and Adaptation of Vascular Tissues

Plants have two main types of transport tissue: xylem and phloem. These tissues are adapted to move specific substances across the plant body.

Xylem Tissue Xylem is responsible for the transport of water and mineral ions from the roots, through the stem, and to the leaves. Its structure is highly specialised for this function:

1. Lignified Dead Cells: Xylem vessels are made of dead cells joined end-to-end to form a continuous, hollow tube. The absence of cytoplasm and end walls allows water to flow with minimal resistance.
2. Lignin: The cell walls are thickened with a tough, waterproof substance called lignin. This provides mechanical strength to the stem and prevents the vessels from collapsing under the negative pressure created by the transpiration pull.

Phloem Tissue Phloem transports dissolved sugars (primarily sucrose) from the leaves (where they are produced by photosynthesis) to the rest of the plant for immediate use in respiration or for storage as starch. Unlike xylem, phloem consists of living cells:

1. Sieve Tube Elements: These cells are arranged in columns. Their end walls, known as sieve plates, are perforated to allow the passage of dissolved sugars.
2. Companion Cells: Because sieve tube elements have very little cytoplasm and no nucleus, they are supported by companion cells which contain many mitochondria to provide the energy (ATP) required for the active loading of sugars.

Water and Mineral Uptake by Root Hair Cells

Plants absorb water and mineral ions from the soil through root hair cells. These cells are found near the tips of growing roots and are uniquely adapted for absorption.

As shown in the diagram, root hair cells have a long, hair like extension that significantly increases the surface area available for absorption.

1. Water Uptake: Water enters the root hair cell from the soil by osmosis. This occurs because the soil water has a higher water potential than the cell cytoplasm.
2. Mineral Uptake: Mineral ions are often in lower concentration in the soil than inside the cell. Therefore, they are taken up by active transport, a process that requires energy from respiration.

Transpiration and Translocation

Transpiration Transpiration is the loss of water vapour from the surface of a plant, mainly through the leaves. Water evaporates from the surfaces of the mesophyll cells into the air spaces within the leaf and then diffuses out through the stomata.

Stomata are small pores usually found on the underside of leaves. Each stoma is surrounded by two guard cells. When the plant has plenty of water, the guard cells become turgid and curve, opening the stoma to allow gas exchange for photosynthesis. When water is scarce, the guard cells become flaccid and the stoma closes to reduce water loss.

Translocation Translocation is the movement of dissolved sugars from the leaves to other parts of the plant. This process is bidirectional, moving sugars to 'sinks' such as growing shoot tips, roots, or storage organs (like tubers).

Environmental Factors Affecting Transpiration

The rate of water uptake (and thus the rate of transpiration) is influenced by four main environmental factors:

1. Light Intensity: Increased light causes stomata to open wider to allow more carbon dioxide in for photosynthesis, which also allows more water vapour to escape.
2. Air Movement (Wind): Increased air movement removes the water vapour accumulating near the leaf surface. This maintains a steep concentration gradient between the inside and outside of the leaf, increasing the rate of diffusion.
3. Humidity: High humidity means there is a high concentration of water vapour in the air surrounding the leaf. This reduces the concentration gradient, slowing down the rate of transpiration.
4. Temperature: Higher temperatures give water molecules more kinetic energy, leading to faster evaporation from the mesophyll cells and faster diffusion through the stomata.

Calculating the Rate of Transpiration

The rate of transpiration can be measured using a potometer, which tracks the volume of water taken up by a plant over time. The formula used is:

$\text{rate of transpiration} = \frac{\text{volume of water}}{\text{time taken}}$

Worked Example A student uses a potometer to measure the water uptake of a leafy shoot. If the shoot absorbs $15\text{ cm}^3$ of water over a period of $60\text{ minutes}$, calculate the rate of transpiration in $\text{cm}^3\text{ min}^{-1}$.

1. Identify the variables: $\text{Volume} = 15\text{ cm}^3$; $\text{Time} = 60\text{ minutes}$.
2. Apply the formula: $\text{Rate} = \frac{15}{60}$.
3. Calculate the result: $\text{Rate} = 0.25\text{ cm}^3\text{ min}^{-1}$.