Transport in Plants

Explained

Two tissues, two directions

Plants have two transport tissues running side by side in vascular bundles. They carry different things, in different directions, and questions almost always turn on keeping them apart.

Xylem

Xylem carries water and dissolved mineral ions, upwards only, from the roots to the leaves.

Xylem vessels are made of dead cells stacked end to end with the end walls broken down, forming a continuous hollow tube. The walls are strengthened with lignin, which supports the plant and stops the tube collapsing under the tension of water being pulled up it.

Dead and hollow is the key structural point. There is no cytoplasm in the way, so water moves freely.

Phloem

Phloem carries sucrose and amino acids, in both directions, from where they are made or stored to wherever they are needed. That movement is called translocation.

Phloem is made of living cells, with sieve plates between them that have pores for the contents to pass through, and companion cells alongside supplying the energy the process requires.

Note that the sugar is transported as sucrose, not glucose. Plants convert glucose to sucrose for transport because sucrose is less reactive and is not used up in respiration along the way. Writing glucose is a common answer and it earns nothing.

Comparing them

• Contents: water and mineral ions, or sucrose and amino acids.
• Direction: upwards only, or both ways.
• Cells: dead and hollow, or living with sieve plates.
• Energy: none needed, or energy required.

Four contrasts, and a comparison question wants both halves of each in the same sentence.

Root hair cells

Water enters the root through root hair cells, each of which has a long narrow extension into the soil.

That extension gives a very large surface area for absorption, and it reaches between the soil particles to where the water is. The cell has a thin wall, so the distance water has to cross is short.

Water enters by osmosis, because the soil water is a more dilute solution than the cell contents, so it moves down the water potential gradient into the cell. No energy is required.

Mineral ions are different. They are usually more concentrated inside the cell than in the soil water, so they have to be taken in against the gradient by active transport, using energy from respiration. That is why root hair cells contain many mitochondria, and why a waterlogged plant absorbs fewer minerals: without oxygen there is no aerobic respiration, so no energy for active transport.

How water gets to the top

Water evaporates from the mesophyll cells and diffuses out of the leaf through the stomata, which is transpiration.

That loss pulls water up the xylem from below, because the water molecules stick to each other in a continuous column. The whole column is dragged upwards, which is why the process is called the transpiration stream and why it needs no energy from the plant at all: the sun does the work.