Phloem transports assimilates such as sucrose and amino acids. Unlike xylem, it is made mainly of living cells. It contains sieve tube elements, companion cells, fibres and parenchyma.Sieve tube elements are joined end to end to form long sieve tubes.1. Their end walls are perforated to form sieve plates, which let the phloem sap flow from one element to the next.
2. As they mature, they lose their nucleus and most of their organelles. Only a thin layer of cytoplasm lines the walls, leaving more space for the sap to flow.
3. Their walls are made of cellulose and are not lignified.Because a sieve tube element has no nucleus and few organelles, it cannot survive on its own.
Companion cells lie alongside the sieve tube elements. Each one is linked to its sieve tube element by many plasmodesmata, strands of cytoplasm passing through the cell walls.1. A companion cell has a nucleus and dense cytoplasm, and it carries out the metabolic reactions needed to keep its sieve tube element alive.
2. It contains many mitochondria, which supply the ATP needed for the active loading of sucrose into the phloem.Phloem fibres provide support, and phloem parenchyma cells store substances.
2. As they mature, they lose their nucleus and most of their organelles. Only a thin layer of cytoplasm lines the walls, leaving more space for the sap to flow.
3. Their walls are made of cellulose and are not lignified.Because a sieve tube element has no nucleus and few organelles, it cannot survive on its own.
Companion cells lie alongside the sieve tube elements. Each one is linked to its sieve tube element by many plasmodesmata, strands of cytoplasm passing through the cell walls.1. A companion cell has a nucleus and dense cytoplasm, and it carries out the metabolic reactions needed to keep its sieve tube element alive.2. It contains many mitochondria, which supply the ATP needed for the active loading of sucrose into the phloem.Phloem fibres provide support, and phloem parenchyma cells store substances.