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Complete Info on the Structure and Function of a Casparian Strip

Structure and Function of a Casparian Strip
The Casparian strip was named after German botanist Robert Caspary, who discovered it. BiologyWise tells you what a Casparian strip actually is, with its definition, structure, function, along with a systematic diagram for better understanding.
Akshay Chavan
Last Updated: Jul 14, 2017
Did You Know?
Despite being discovered way back in the 19th century, the Casparian strip's exact chemical structure is still under debate.
Green plants are autotrophs, which means, they prepare their own food using sunlight and carbon dioxide from the air, and substances like water and minerals from the soil. Apart from anchoring them to the ground, their roots play a major role in absorbing required materials from the soil. However, if water and salts are not absorbed in sufficient amounts, it can be dangerous for the plant, as can the absorption of excess amounts. Moreover, since the roots are nothing but a part of the plant, this places them in direct contact with the soil, which may contain harmful substances like chemicals or bacteria. This raises the question as to how does the plant control what substances it absorbs from the soil, and defends itself from harmful ones? The answer lies in a structure called the Casparian strip, whose function, or purpose, is explained below.
The Casparian strip is a ring-like thickening of certain parts of the endodermal cell walls in plant roots, which forces water and dissolved minerals to pass through the semipermeable plasma membrane of these cells, rather than their cell walls.
Casparian Strip Structure
Plant roots are complex structures, and are made up of different types of layers and cells. Going inwards from the root surface, the various layers in order are - the 'outer epidermis' made up of a single layer of cells, the 'middle cortex' made of several layers, a single layer called the 'endodermis', and finally, the 'vascular cylinder'. Of all these layers, the location of the Casparian strip is only in the endodermis.
Casparian Strip Diagram
In the single-celled layer, called the endodermis, all cells show a peculiar thickening in their radial and transverse cell walls. This means that the thickening is present on all walls of the endodermis, except those facing the central axis of the plant, and those facing the root surface. This ring-like corky band, called the Casparian strip, is composed of impermeable substances like lignin and suberin, while ordinary cell walls are made of lignin alone. This strip is closely attached to the endodermal plasma membrane to ensure that it does not come apart when the cell shrinks or expands due to osmosis (flow of water in and out of the cell).
Casparian Strip Functions
Plants need water and minerals to perform photosynthesis and prepare food for their survival. These substances are absorbed by the plant's roots, and then travel inwards from the root surface to the vascular cylinder, from where they travel upwards via the xylem (a type of transportation tissue). These substances travel from the root surface to the vascular cylinder in two ways - apoplastic and symplastic pathways.
An apoplastic pathway is when water and the substances dissolved in it travels from the spaces in the cell wall of one cell to the cell wall of another, without ever entering any cell. The symplastic pathway is when these substances travel from the cytoplasm (material inside cells) of one cell to the cytoplasm of another, through its plasma membrane.
When substances travel inwards from the root epidermis through the cortex, and finally arrive at the endodermis, they encounter the Casparian strip. Since it is an impermeable thickening in the cell wall, it stops materials from traveling through it, thus terminating the apoplastic pathway. Thus, it forces all substances to pass through the cytoplasm by the symplastic pathway. This is very important for the plant, as the cell wall cannot control the type and amount of substances passing through it, but the plasma membrane can. So, by forcing substances to shift from the apoplastic to the symplastic pathway, the Casparian strip allows the plant to control how much water and minerals it absorbs from the soil.

There is also another method, called 'active transport', by which minerals are transported from the soil into the vascular cylinder, in opposition to their concentration gradient. This means that, contrary to the normal process of diffusion by which salts move from an area of higher concentration (soil) to an area of lower concentration (root), this pathway brings salts into the root irrespective of where their concentration is higher. So, in conjunction with the Casparian strip, this pathway causes a buildup of salts inside the vascular cylinder, since once they are inside the cylinder, the impermeable strip prevents them from flowing back out. This high salt content of the vascular cylinder encourages the flow of more water from the soil into it, by osmosis. Thus, the Casparian strip maximizes the absorption of water from the soil.
The Casparian strip creates a barrier to harmful chemicals, like herbicides, which cannot pass through it. Without this strip, such chemicals would have spread throughout the plant via the xylem, thus killing it. It also prevents harmful microbes from gaining entry into the plant and causing infections.
To conclude, it can be said that the Casparian strip is a thickening in parts of the endodermal cell wall, which helps regulate the absorption of water and nutrients from the soil, and also plays an active role in the defense of the plant. However, there are a lot of things about its structure and function that we do not know as yet.