When water molecules enter a plant through the roots, they move across the cortex and into the xylem tissue, where they are transported to other parts of the plant. There are two pathways that water can follow as it moves from the root to the xylem tissue. Complete Table 5.1 with information about the two pathways.
Fig. 5.1 shows a photomicrograph of a transverse section through the stem of a flowering plant. On Fig. 5.1, draw a label line and add the letter T to show xylem tissue.
Scientists have investigated the cell death process that takes place as cells develop into mature xylem vessel elements. During this development, the tonoplast ruptures (bursts) and hydrolytic enzymes stored in the vacuole are released into the cytoplasm. Name an organelle in animal cells that has a similar function to the vacuole in developing xylem vessel elements.
The rupture of the tonoplast during development of xylem vessel elements is caused by changes in the permeability of the tonoplast. Suggest how the permeability of the tonoplast changes and explain how this change could lead to rupture of the tonoplast.
During development of the xylem vessel elements the pH of the cytoplasm decreases. This change in pH activates enzymes in the cytoplasm that cause organelles to swell. Suggest how a change in pH of the cytoplasm can activate enzymes.
When an insect feeds on xylem fluid it can act as a vector of plant diseases such as Pierce’s disease. This bacterial disease affects many fruit trees, causing the leaves to turn brown and fall from the plant, which results in much less fruit being produced. Explain why Pierce’s disease can be described as an infectious disease.
Malaria is another infectious disease that also uses an insect vector. State the type of organism that causes malaria and name one organism that causes malaria in humans.