Sugar, primarily in the form of sucrose, enters plant cells after being produced through photosynthesis in the leaves. It is transported from source cells (like leaf cells) to sink cells (like root cells, developing fruits, and growing stem cells) through the phloem.
Sugar Production and Conversion
- Photosynthesis: Plants create glucose through photosynthesis in chloroplasts, primarily in leaf cells.
- Sucrose Formation: Glucose is then converted into sucrose. This is the main form in which sugar is transported in plants because it is less reactive than glucose.
Phloem Loading: Getting Sugar into the Phloem
- Active Transport: Sucrose enters the phloem cells (specifically sieve tube elements) via active transport. This requires energy because the concentration of sucrose is usually higher in the phloem than in the surrounding source cells.
- Companion Cells: Companion cells, associated with sieve tube elements, play a critical role. They actively load sucrose, often against its concentration gradient, from the source cells into the sieve tube elements.
- Proton Pump and Co-transport: A common mechanism involves a proton pump that uses ATP to pump protons (H+) out of the companion cells. This creates a proton gradient. Sucrose then moves back into the companion cell along with protons through a co-transport protein (symporter). This allows sucrose to move from areas of lower concentration (source cells) to higher concentration (companion cells) and eventually into the sieve tube elements.
Movement Through the Phloem: Pressure Flow
- Pressure Flow Hypothesis: Sucrose entering the phloem at the source creates a high solute concentration. This causes water to move into the phloem from the adjacent xylem via osmosis, increasing pressure in the phloem at the source.
- Movement Towards Sinks: At the sink (e.g., roots, fruits), sucrose is unloaded from the phloem. This reduces the solute concentration in the phloem, and water moves out, reducing the pressure. The pressure difference between the source and the sink drives the flow of phloem sap (containing sucrose) from source to sink.
Phloem Unloading: Getting Sugar out of the Phloem
- Active or Passive Transport: At the sink, sucrose is unloaded from the phloem cells either through active or passive transport, depending on the specific sink tissue and the sugar concentration.
- Conversion back to Glucose: Once unloaded, the sucrose may be converted back into glucose for respiration or used to synthesize other molecules like starch or cellulose for storage and growth.
In summary, sugar enters a plant through photosynthesis, is converted to sucrose, actively loaded into phloem cells (sieve tube elements) often aided by companion cells, and then transported via pressure flow to sink cells where it is unloaded.
