CIE IGCSE Biology: 复习笔记：8.1.8 Translocation

• The soluble products of photosynthesis are sugars (mainly sucrose) and amino acids
• These are transported around the plant in the phloem tubes which are made of living cells (as opposed to xylem vessels which are made of dead cells)
• The cells are joined end to end and contain holes in the end cell walls (called sieve plates) which allow easy flow of substances from one cell to the next
• The transport of sucrose and amino acids in the phloem, from regions of production to regions of storage or use, is called translocation
• Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year; however dissolved food is always transported from the source (where it’s made) to sink (where it’s stored or used):
• During winter, when many plants have no leaves, the phloem tubes may transport dissolved sucrose and amino acids from the storage organs to other parts of the plant so that respiration can continue
• During a growth period (eg during the spring), the storage organs (eg roots) would be the source and the many growing areas of the plant would be the sinks
• After the plant has grown (usually during the summer), the leaves are photosynthesizing and producing large quantities of sugars; so they become the source and the roots become the sinks – storing sucrose as starch until it is needed again

Translocation through the phloem

Comparison between Xylem and Phloem Tissue Table

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