* Photosynthesis: Plants use sunlight, water, and carbon dioxide to produce sugars (mainly sucrose) during photosynthesis in their leaves.
* Loading: The sugars are loaded into the phloem from the source cells (usually in the leaves). This loading requires energy, and the movement of sugar molecules is often facilitated by active transport.
* Pressure Flow: Once loaded into the phloem, the sugars create a high concentration of solutes, drawing water from surrounding tissues. This creates pressure within the phloem tubes, driving the sugar solution from areas of high pressure (source) to areas of low pressure (sink).
* Unloading: At the sink (e.g., roots, fruits, growing stems), the sugar is unloaded from the phloem for use in growth, storage, or respiration.
The phloem is a living tissue composed of specialized cells called sieve tubes and companion cells. Sieve tubes are long, connected cells with porous end walls, allowing the free flow of sugar solution. Companion cells provide the sieve tubes with energy and support.
Key points to remember:
* The movement of sugars in the phloem is bi-directional. This means sugars can move from leaves to roots, roots to stems, or from one leaf to another.
* The direction of flow is determined by the relative concentration of sugars at different points in the plant.
* The phloem is a critical component of the plant's vascular system, allowing for the transport of essential nutrients and sugars throughout the organism.
