Roots: Mineral Uptake
Roots are where the plant takes in mineral ions from the soil. The rood structure is made to have a very large surface area by the branching of roots and additionally the growth of rood hairs.
Some of the main mineral ions absorbed are potassium, nitrate, phosphate and many others.
Due to the fact that the soil is often less rich in minerals than the root itself, active transport is required to absorb them.
There are various ways in which the ions can be absorbed into the roots. This includes diffusion of mineral ions, mass flow of water that carries the ions as the water drains through the soil and also with the help of fungal hyphae, which grow around the roots of plants and help with the uptake of minerals in a mutualistic relationship.
Stems
The leaves and flowers of a plant are connected to the roots by the stem. The stem transports minerals to the places where they are required using the xylem and phloem.
Stems also play a major role in the support of plants. The cells in the stem absorb water to develop a high pressure, causing the vacuole of the cell to press against the cell wall, causing the cell to become turgid. Certain cells also develop a cellulose wall. The xylem also helps with support, the cell walls become thickened and lignified.
In the centre of the stem is the pith, surrounded by vascular bundles, which are made up of the xylem, cambium and phloem. Surrounding the vascular bundles is the cortex, then the epidermis.
Xylem
The xylem is actually dead when mature. In mature xylem vessels, there are no plasma membranes present. This allows water to move freely through it. The cytoplasm and nuclei of the original cells in the xylem break down. Because of this the lumen of the xylem vessel is filled with sap. Within the lignin are ring-shaped structures, which are thickenings of the cellulose in the cell wall. These rings help with support. The outer cellulose cell wall of the xylem has many pores, which allow the water to pass through to other cell walls of leaf cells.
Water Transport
Water passes through the xylem in an unbroken column of water. Transpiration causes the water to be moved up the xylem from the roots to the leaves. This is called the transpiration stream. The movement of water through the xylem is passive. Heat causes water to evaporate from the leaves. The water lost from the leaves is replaced by water from the xylem vessels. A slight suction is created when the water is pulled out of the leaf, this is the cause of the transpiration pull. This pull causes a sort of chain reaction right down to the roots. The water is pulled upwards in a continuous stream. This stream is usually unbroken due to the cohesion of the water molecules because of hydrogen bonds, meaning they stick together.
Phloem
The phloem is where sugars and amino acids are transported. Unlike the xylem, the phloem is still alive and uses energy to transport substances. This is called active translocation. The phloem transports sugars and amino acids from sources to sinks, where they are unloaded. An example of a source would be the leaves, where photosynthesis occurs, or storage units such as tubers, roots or fruits.
