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In the same way as humans have arteries and veins to carry blood, plants have their own ways of transporting substances so that they can survive.
Plants have two main transport systems: xylem and phloem.
And between them there are four types of specialised cells we need to know: the vessels of the xylem, sieve tubes and companion cells of the phloem, and root hair cells in the roots.
Xylem:
Water and minerals are transported by xylem.
They are hollow tubes with continuous walls that are made from specialised dead cells called vessels.
Looking at the xylem in the picture below, these cells are now empty and have no end walls (no ceilings or floors) so they can be joined together smoothly, reducing the water turbulence.
The walls are also strengthened and supported by a chemical called lignin.
This secure structure allows the transport of water and minerals upwards from the roots and through the stem to the rest of the plants in transpiration - so think root to leaves!
Surprisingly, this antigravity direction is what we call a physical process, meaning the xylem doesn’t need energy to push its water contents around.
Phloem:
Phloem transports sugars (from photosynthesis) and amino acids around the plant in a dissolved form.
Because photosynthesis occurs anywhere chloroplasts are, the dissolved sugar and amino acids are transported upwards and downwards to where the plant needs them for growth, energy storage as starch (e.g. in bulbs), making seeds and respiration.
Phloem has its own term for this transport called translocation.
The phloem is actually a combination of two cell types, both of which are alive, depend on each other and are well adapted for their roles (see the diagram above):
Sieve tube cells have no nuclei and are stacked on top of each other, making the tunnels of the phloem. The cytoplasm of the neighbouring cells interacts through the gaps in the ends of each cell.
Translocation needs energy, which companion cells provide, so every sieve tube gets at least one companion cell so that it can function!
When thinking about the plant as a whole, xylem and phloem are arranged differently depending on whether we’re looking at the roots or the stem.
In the roots, xylem offer the most support by being the central portion, with the phloem around the perimeter.
In the stem though, xylem and phloem are arranged in pairs called vascular bundles (imagine an artery and a vein side by side).
Root Hair Cells:
These allow a plant to absorb water from the soil using osmosis.
They are specially adapted to be long and thin so that they can get between pieces of soil and they have a larger surface area for water absorption.
The gradient here is dependent on there being less water in the root hair cytoplasm than in the soil, and so as water comes in, the cell also actively transports mineral salts from their low concentration in the soil to an already high concentration in the cytoplasm, maintaining the influx of water.
And as active transport always needs energy, these cells respire a lot to produce the energy the cell needs to function.
Are you ready to have a go at some questions now?
