Phloem Transport | Source to Sink Movement | Phloem Loading скачать в хорошем качестве

Phloem Transport | Source to Sink Movement | Phloem Loading

Phloem Transport: Source to Sink Movement Phloem transport is the process by which plants move food (mainly sucrose) from the source (where it is produced) to the sink (where it is used or stored). This movement is essential for plant growth, storage, and survival. 🌱 1. Source The source is any organ that produces or releases sugars. Most commonly: mesophyll cells of leaves (where photosynthesis occurs). Here, sucrose is synthesized and then transported towards the phloem. 🌱 2. Pathway of Loading (Leaf side) Mesophyll cell – produces sucrose by photosynthesis. Phloem parenchyma cell – helps in transfer and temporary storage. Companion cell – actively loads sucrose into the sieve tube (via plasmodesmata or active transport). Sieve tube element – becomes filled with sucrose solution, creating high osmotic pressure. 🌱 3. Translocation in Sieve Tubes Water enters from xylem into sucrose-rich sieve tubes by osmosis. This generates turgor pressure (mass flow/pressure flow). The pressure pushes the sucrose solution through sieve tubes towards the sink. 🌱 4. Sink A sink is any organ that consumes or stores sugars. Examples: roots, fruits, seeds, tubers, young leaves. At the sink: Sucrose unloading occurs into phloem parenchyma and then into sink cells. Water diffuses back into xylem, maintaining circulation. 🌱 5. Pathway of Unloading (Root side) Sieve tube element – delivers sucrose solution. Companion cell – regulates unloading. Phloem parenchyma – receives and distributes sugars. Root cell (sink cell) – uses sucrose for respiration or stores it as starch. 🌱 6. Key Points Phloem transport is bidirectional (can move upward or downward depending on source–sink relation). Driven by the Pressure Flow Hypothesis (Münch’s hypothesis). Energy is required mainly for loading/unloading, but bulk movement is passive (pressure-driven). ✅ In summary: Leaves (source) → Mesophyll → Phloem Parenchyma → Companion Cell → Sieve Tube → Pressure Flow → Companion Cell → Phloem Parenchyma → Roots/Other sinks