Function of the Phloem

Overview

• The phloem is a vascular tissue responsible for transporting substances in plants.
• Its primary function is to transport sucrose and amino acids.

Key Concepts

Sucrose

• Sucrose is a disaccharide, a non-reducing sugar composed of alpha glucose and fructose linked by glycosidic bonds.
• It is synthesized by mesophyll cells in the leaves.

Source and Sink

• Source: Part of the plant providing sucrose and amino acids (e.g., mesophyll cells in leaves).
• Sink: Part of the plant receiving sucrose and amino acids (e.g., root cortex cells).
• The source can typically perform photosynthesis, providing nutrients.
• The sink lacks chloroplasts, must obtain nutrients from the source.

Transport Process

Active Loading

• Sucrose is transported from the source into the phloem sieve tube element via active loading.
• Active loading involves transporting sucrose against a concentration gradient, not simple diffusion.

Water Potential and Osmosis

• Sucrose in the sieve tube lowers water potential.
• Water from surrounding cells moves into the phloem sieve tube by osmosis to balance water potential.
• This influx of water increases the volume and pressure in the sieve tube.

Pressure Gradient and Mass Flow

• A pressure gradient is established due to the differential pressures in the sieve tubes.
• Mass flow: Movement of water and dissolved sucrose along the pressure gradient.
• Sucrose moves from areas of high pressure (near source) to low pressure (near sink).

Unloading

• Sucrose is unloaded at the sink (e.g., root cortex), providing energy where needed.

Considerations

• Transport can occur from top to bottom or vice versa, depending on the plant's needs.
• The source is not always at the top, nor the sink always at the bottom (e.g., fruit above leaves).

Important Terms

• Active Loading: Process of sucrose entering sieve tubes.
• Water Potential: The potential energy of water in a system; affects osmosis.
• Pressure Gradient: Difference in pressure that drives mass flow.
• Mass Flow: Movement driven by pressure differences.

Exam Tips

• Be familiar with key processes: active loading, water potential changes, pressure gradients, and mass flow.
• Understand that transport can be directional and context-dependent.
• Active loading specifics will be covered in a future lecture.