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- Phloem loading is the process of loading carbon into the phloem for transport to different '' in a plant. Sinks include metabolism, growth, storage, and other processes or organs that need carbon solutes to persist. It can be a passive process, relying on a pressure gradient to generate diffusion of solutes through the symplast, or an active process, requiring energy to create membrane-bound transporter proteins that move solutes through the apoplast against a gradient. Passive phloem loading transports solutes freely through plasmodesma in the symplast of the minor veins of leaves. Active transport occurs apoplastically and does not use plasmodesmata. An intermediate type of loading exists that uses symplastic transport but utilizes a size-exclusion mechanism to ensure diffusion is a one-way process between the mesophyll and phloem cells. This process is referred to as polymer-trapping, in which simple solutes such as sucrose are synthesized into larger molecules such as stachyose or raffinose in intermediary cells. The larger molecules cannot diffuse back to the mesophyll but can move into the phloem's sieve cells. Therefore, the synthesis of larger compounds uses energy and is thus 'active' but this strategy does not require specialized proteins and can still move symplastically. (en)
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- Phloem loading is the process of loading carbon into the phloem for transport to different '' in a plant. Sinks include metabolism, growth, storage, and other processes or organs that need carbon solutes to persist. It can be a passive process, relying on a pressure gradient to generate diffusion of solutes through the symplast, or an active process, requiring energy to create membrane-bound transporter proteins that move solutes through the apoplast against a gradient. Passive phloem loading transports solutes freely through plasmodesma in the symplast of the minor veins of leaves. Active transport occurs apoplastically and does not use plasmodesmata. An intermediate type of loading exists that uses symplastic transport but utilizes a size-exclusion mechanism to ensure diffusion is a one- (en)
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