1.1 Transport of Food and Minerals in Plants

Transport of Water and Minerals (Xylem)

• Xylem: Responsible for unidirectional transport of water and dissolved minerals from roots upwards.

Key Mechanisms for Water/Mineral Transport

• Diffusion: Passive movement from high to low concentration (short distances).
• Osmosis: Primary way roots absorb water; movement of water across a semipermeable membrane.
  • Semipermeable Membrane: Allows selective passage of molecules.
  • Root Pressure: Transverse osmotic pressure causing sap to rise.
• Active Transport: Movement against concentration gradient, requires energy (for mineral ions).
• Transpiration Pull: Main driving force; water evaporation from leaves creates tension, pulling water up.

Pathway of Water/Mineral Transport

1. Absorption: Roots absorb water and minerals from soil (root hairs increase surface area).
2. Movement to Xylem: Through root tissues (apoplastic or symplastic pathways).
3. Upward Movement: Transported upwards through xylem vessels to the rest of the plant.

Transport of Food (Phloem)

• Phloem: Transports soluble organic compounds (sugars) from leaves (source) to other parts (sink).
• Process: Called translocation.

Mechanism of Food Transport (Pressure-Flow Hypothesis)

1. Loading at Source: Sugars actively loaded into phloem, drawing water from xylem by osmosis.
2. High Pressure at Source: Water influx creates high pressure.
3. Unloading at Sink: Sugars unloaded at sink, causing water to move out of phloem, reducing pressure.
4. Bulk Flow: Pressure gradient drives sap flow from source to sink.

• Bidirectional Flow: Unlike xylem, phloem flow can be in both directions, depending on source/sink location.
• Composition: Phloem sap is water-based, rich in sugars, amino acids, hormones, etc.