2.17 Root hair cells, xylem and phloem

Transport in plants

Plants need to move two things around:

• water and mineral ions, taken up from the soil by the roots and carried to every part of the plant in the xylem
• dissolved sugars, made in the leaves by photosynthesis and carried to where they are used or stored in the phloem.

Root hair cells

• The long root hair gives a very large surface area for absorbing water and mineral ions. Millions of root hairs make the total area enormous.
• Water moves in by osmosis, because the cytoplasm of the root hair is more concentrated than the soil water.
• Mineral ions, such as nitrate and magnesium, are usually at a lower concentration in the soil than in the cell, so they must be taken in by active transport. This needs energy from respiration, so root hair cells have many mitochondria.
• Root hair cells have no chloroplasts: they are underground, where there is no light.

Xylem

• Xylem is made of dead cells joined end to end.
• The end walls have broken down, so the cells form a long hollow tube that water flows through easily.
• The walls are strengthened with rings or spirals of lignin, so the tubes do not collapse and they help support the plant.
• Water and mineral ions only travel upwards, from the roots to the leaves. This is the transpiration stream.

Phloem

• Phloem is made of living cells called sieve tube elements, joined end to end.
• Their end walls have pores, forming sieve plates, so dissolved sugars can flow from one cell to the next.
• The sieve tube cells have very little cytoplasm and no nucleus. Companion cells next to them have mitochondria and supply the energy they need.
• Sugars move both up and down the plant: from the leaves to growing tips, roots and storage organs. This is translocation.