Transport of assimilates
Transport of assimilates, also known as translocation, is the process by which organic substances (solutes) like sugars (primarily sucrose) and amino acids are moved to where they are needed within a plant. Unlike water transport in xylem, translocation is an energy-requiring (active) process that occurs in the phloem tissue.
1. Substances Transported and Their Form
The main substances transported as assimilates are sucrose and amino acids, which are dissolved in water within the phloem sap. Sugars are primarily transported as sucrose because it is highly soluble in water, allowing large quantities to be moved in a relatively small volume of phloem sap. Additionally, sucrose is metabolically inert, meaning it is less likely to be used along the transport route from source to sink. Amino acids are also transported to sites where protein synthesis occurs, such as buds, young leaves, young roots, and developing fruits.
2. Source and Sink
Translocation moves solutes from 'sources' to 'sinks'.
Source: This is where assimilates are produced (e.g., from photosynthesis) or where they are released from storage, leading to a high concentration of assimilates. Examples include mature leaves (where sucrose is produced), and storage organs (like tubers or tap roots) that are unloading their stored substances at the start of a growth period, or germinating seeds.
Sink: This is where assimilates are used up or stored, resulting in a lower concentration of assimilates. Examples include roots (where sucrose is stored as starch), growing points (e.g., new leaves, buds, flowers), developing fruits, or storage organs. The contents of phloem sap may vary, and it can flow in either direction depending on the location of the source and sink. For instance, a leaf can be a source or a sink at different times.
3. Mechanism: The Mass Flow Hypothesis
The primary mechanism for solute transport in the phloem is the mass flow hypothesis (also known as the pressure flow hypothesis). This theory is the best supported, though scientists are still not entirely certain how the movement works. The hypothesis explains the movement through a hydrostatic pressure gradient:
Loading at the Source: At the source (e.g., leaves), solutes like sucrose are actively loaded into the sieve tubes of the phloem, typically from companion cells. This loading process involves specialized co-transporter proteins and requires ATP from cell respiration. For example, hydrogen ions are pumped out of companion cells into their cell walls, creating a concentration gradient. These hydrogen ions then diffuse back into the companion cell via co-transporter proteins, carrying sucrose molecules with them against sucrose's concentration gradient. This process lowers the water potential inside the sieve tubes, causing water to enter the tubes by osmosis from the xylem and companion cells. This creates a high hydrostatic pressure inside the sieve tubes at the source end.
Flow: The resulting pressure gradient pushes the solutes along the sieve tubes towards the sink.
Unloading at the Sink: At the sink end, solutes are removed from the phloem to be used or stored (e.g., as starch). This removal increases the water potential inside the sieve tubes, so water leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes at the sink. Unloading is also an energy-requiring process, often involving similar active transport methods as loading, and enzymes converting sucrose into other molecules to maintain the concentration gradient.
4. Structure of Phloem
Phloem tissue is composed of living cells, primarily sieve tube elements and companion cells.
Sieve tube elements are long, tube-like structures with thin layers of cytoplasm and perforated end walls called sieve plates. They lack a nucleus and most organelles, reducing resistance to flow.
Companion cells are closely associated with sieve tube elements. They contain many mitochondria, which provide the ATP necessary for active loading of solutes into the phloem at the source. They also have plasmodesmata that connect them to sieve tube elements, allowing diffusion of sucrose. Some companion cells have many cell wall in-growths, forming transfer cells that greatly increase the surface area for active transport.
5. Factors Affecting Translocation Rate
The rate of translocation can be affected by factors such as:
Sucrose Concentration at the Source: A higher concentration of sucrose at the source leads to a higher rate of translocation.
Temperature: Translocation requires living cells and is an active process that depends on respiration. High temperatures can slow or stop translocation, while lower temperatures reduce the rate of metabolic activity, affecting the rate of translocation.
6. Evidence for Mass Flow Hypothesis
Evidence supporting the mass flow hypothesis includes:
Ringing Experiments: If a ring of bark (containing phloem) is removed from a woody stem, a bulge forms above the ring. Analysis of the fluid in this bulge shows a higher concentration of sugars than the fluid below the ring, indicating a downward flow of sugars that is interrupted by the removal of the phloem.
Radioactive Tracer Experiments: Using radioactive carbon dioxide (14CO2) in photosynthesis, it can be shown that the organic substances produced are transported from the leaves (source) towards the roots (sink). The radioactivity travels from areas of high pressure in the leaves to areas of lower pressure towards the root end of the stem, supporting the concept of a pressure gradient.
Presence of Sieve Plates: Sieve plates allow for uninterrupted flow through the sieve tubes.
7. Objections to Mass Flow Hypothesis
Some objections to the mass flow hypothesis exist:
Sugar can travel to many different sinks, not just the one with the highest concentration.
Sieve plates act as a barrier to mass flow, which might not be expected if mass flow was the sole mechanism.
Mass flow does not fully explain how different solutes can move at different speeds or even in different directions within the phloem. Cytoplasmic streaming is suggested as an additional process.
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