IGCSE Biology / Transport in plants
The two transport tissues
Edexcel International GCSE Biology (4BI1), chapter 11: Transport in plants, specification point 2.53, 2.54. Free lesson.
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Xylem and phloem are the two transport tissues in plants. Their different structures suit what they carry, the direction they transport it, and whether transport needs energy.
How xylem is built
A xylem vessel is a long, hollow tube that carries substances from roots towards leaves. Its structure leaves a clear path for water and helps the vessel resist collapse as water is pulled upwards. Xylem is made from dead cells stacked end to end. Their end walls break down, forming one continuous open tube. There is no cytoplasm or nucleus inside, so the open space, called the lumen, is clear for water to flow through. The walls are thickened with lignin, a hard, waterproof substance. Lignin is arranged in rings or spirals, which help stop the vessel collapsing.
What xylem carries and where it goes
Xylem transports water and dissolved mineral ions from the roots towards the leaves. This movement is upwards only. It uses no energy from the plant, because evaporation at the leaves drives water upwards through the vessel. Xylem carries water and mineral ions. Transport is upwards only, from root to leaf. A dead, hollow tube cannot load sugars into itself, so xylem does not carry sugars.
How phloem is built
Phloem is a column of living cells joined end to end. Their end walls have holes, forming sieve plates that let sap pass between cells. Sieve tube cells have cytoplasm but no nucleus. Companion cells beside them have a nucleus and many mitochondria. The sieve plates are perforated end walls, allowing sap to pass from one sieve tube cell to the next. Each sieve tube cell is paired with a companion cell. The companion cell provides energy for loading sugars into the sieve tube and helps keep it alive.
What phloem carries and why it is alive
Phloem transports dissolved food substances around the plant. Loading these substances into the sieve tube needs energy, so phloem cells must be alive. Unlike xylem, phloem can move substances in either direction. Phloem carries sucrose and amino acids. Transport can be upwards or downwards, as the plant needs. Phloem walls are ordinary cellulose walls. The sieve plates remain between the cells.
Compare the two transport tissues
The key differences follow from what each tissue does. Xylem is a dead, open pathway for water and minerals, while living phloem cells use energy to load and transport food substances. Xylem: dead cells, hollow continuous tube, lignified walls, no end walls, water and mineral ions, upwards only, no energy from the plant needed. Phloem: living cells, sieve plates with holes, ordinary cellulose walls, sucrose and amino acids, movement in both directions, energy needed.