Transport of Food and Minerals in Plants

Plants have a sophisticated transport system to move water, minerals, and food between different parts of the plant. This is accomplished through two main vascular tissues: the xylem and the phloem.

Transport of Water and Minerals (Xylem)

The xylem is responsible for the transport of water and dissolved minerals from the roots to the rest of the plant. This movement is primarily unidirectional, from the roots upwards to the stems and leaves.

Key Mechanisms

• Diffusion: The net movement of molecules or atoms from a region of higher concentration to a region of lower concentration. This is a passive process and is important for the movement of substances over short distances.
• Osmosis: The spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration. This is the primary way that plant roots absorb water from the soil.
  • Semipermeable Membrane: A membrane that allows certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion," along with other variations of active transport and passive transport.
  • Root Pressure: The transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.
• Active Transport: The movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy. This is important for the uptake of mineral ions from the soil.
• Transpiration Pull: The main driving force for water movement in the xylem. As water evaporates from the leaves (transpiration), it creates tension, which pulls more water up from the roots through the xylem. Cohesion and adhesion of water molecules help to maintain this continuous column of water.

Pathway

1. Absorption: Plant roots absorb water and minerals from the soil. Root hairs greatly increase the surface area for absorption.
2. Movement to Xylem: Water and minerals move through the root's epidermis, cortex, and endodermis to reach the xylem. They can travel between cells (apoplastic pathway) or through the cells themselves (symplastic pathway).
3. Upward Movement: The water and minerals are then transported upwards to the rest of the plant through the xylem vessels.

Transport of Food (Phloem)

The phloem transports the soluble organic compounds, primarily sugars like sucrose, produced during photosynthesis from the leaves to other parts of the plant where they are needed for growth or storage. This process is called translocation.

Key Features

• Source and Sink: The leaves, where photosynthesis occurs, are the "source" of the food. The "sinks" are the parts of the plant that require energy, such as roots, fruits, and growing tips.
• Mechanism (Pressure-Flow Hypothesis):
  1. Sugars are actively loaded into the phloem at the source, which increases the solute concentration and draws water in from the nearby xylem by osmosis.
  2. This creates high pressure at the source.
  3. At the sink, sugars are unloaded, causing water to move out of the phloem and reducing the pressure.
  4. This pressure gradient drives the bulk flow of sap from source to sink.
• Bidirectional Flow: Unlike the xylem, the flow in the phloem is bidirectional. The direction of transport depends on where the source and sink are located, which can change with the seasons and the plant's developmental stage.
• Composition: Phloem sap is a water-based solution rich in sugars, but it also transports amino acids, proteins, hormones, and other organic molecules.