Lecture Notes: Function of Phloem in Plants

Introduction

Phloem is a vascular tissue responsible for transporting substances.
Main function: transport of sucrose and amino acids.
Related to Chapter 2: Sucrose is a disaccharide, non-reducing sugar made from alpha glucose and fructose linked by glycosidic bonds.

Source: Part of the plant that provides sucrose and amino acids (usually through photosynthesis in leaves).
- Mesophyll cells in leaves (spongy or palisade) are typically the source.
Sink: Part of the plant that receives sucrose and amino acids.
- Root cortex cells act as a typical sink due to the absence of chloroplasts.

Transport occurs from leaf cells (source) to root cortex cells (sink) via phloem.
Phloem consists of two cell types:
- Companion cells
- Phloem sieve tube elements

Active Loading of Sucrose:
- Sucrose synthesized by mesophyll cells is actively loaded into the phloem sieve tube element.
- Active loading is a complex process involving transport from source to phloem.
Lowering Water Potential:
- Sucrose in the phloem lowers water potential in the sieve tube element due to increased solute concentration.
- Water enters the sieve tube by osmosis from surrounding cells.
- This increases the volume and pressure inside the sieve tube element.
Pressure Gradient and Mass Flow:
- Pressure gradient created due to higher pressure at the top and lower pressure at the bottom of the sieve tube.
- Mass flow of water and dissolved sucrose occurs down the pressure gradient.
- Sucrose reaches the bottom and is unloaded into the sink (e.g., root cortex).

Transport isn't limited to top-to-bottom; can occur bottom-to-top (e.g., to a fruit above the leaf).
Same principles apply regardless of direction:
- Active loading, lowering water potential, osmosis, pressure gradient, and mass flow.
Key Concepts:
- Movement of substances in phloem can be top-to-bottom or bottom-to-top.
- Important processes: active loading, water potential changes, osmosis, pressure gradients, mass flow.