Phloem vessels consist of living cells. They have walls made of cellulose. At the end of each cell is a cross wall of cellulose with holes called a sieve plate, which link cells together, forming a sieve tube. The tubes transport products of photosynthesis from leaves to other parts of the plant. Sucrose, amino acids are carried to young leaves and other growing points. Despite being living cells, the phloem sieve tubes have no nucleus and are controlled by the companion cells. Foods flow in both directions (up and down).
Xylem vessels are made of dead cells. There is a hollow space down the middle called lumen, where water passes. The walls contain a woody material called lignin. Unlike the phloem, xylem vessels contain no cytoplasm. Their role is to carry water and minerals from the roots up the shoot to the leaves in the transpiration stream.
Water Uptake – The Roots
Roots of a plant are covered with root hair cells, which increase the surface area of the root epidermis. Each hair is a single, specialized cell of the root epidermis. The long thin outer projection penetrates between soil particles.
In I/GCSE Biology, water uptake is done by osmosis. The soil has a higher water potential than inside the cell, which causes water to move into the very first of the root hair cells. This increases the water potential in the first cells. However, the cells behind the first ones have a lower water potential, which causes water to move from the first cells to the second ones. Continuing in this way, a water potential gradient is set up across the root cortex, kept going by water being taken up the xylem (or stele, in the picture) in the middle of the root.
That's the end of the topic!
Drafted by Joey (Biology)