The Three Pathways of Water Movement in Tree Roots

A picture of a sprouting tree next to the words: Apoplast, Symplastic and Transmembrane Pathways

Tree roots absorb water, but have you ever wondered how the water is absorbed? There are three pathways the water can take. These are the apoplast, symplastic, or transmembrane pathways.

At a basic level, water travels into a tree's fine roots. The water must then travel inside the root to the xylem. There are three different paths water can take to get to the xylem. This article will cover each pathway in depth.

Before reading this article, you may want to brush up on your root anatomy knowledge. This article will cover each piece of anatomy in-depth. And this article is an extensive guide to roots as a whole, with a brief look at their anatomy.

Osmosis refers to the diffusion of water through a semipermeable membrane NOT the diffusion of information through impermeable skulls

Apoplast Pathway

When using the apoplast pathway, water will travel between cells. There will be tiny gaps between each cell. Water will move between these gaps.

Water will need to navigate these gaps as they move deeper inside the tree root. Eventually, the water will reach the endodermis. The endodermis is the inner skin of the tree root.

The endodermis controls how much water and nutrients move further inside the cell. The Casparian Strip helps with this process.

The Casparian Strip is a corky substance that prevents foreign material from getting deeper into the root. Something can only pass through the strip if that something is inside a cell.

So, to get past the Casparian Strip, water and nutrients need to enter a cell. Substances can enter a cell by moving through the cell's plasma membrane.

After passing through the endodermis, the water can make their way to the tree's vascular system.

A diagram showing the apoplast pathway
Water (green line) travels along the cell walls (blue and black) to get to the endodermis (red). Water cannot pass through the endodermis without being inside a cell. So, the water must then enter an adjacent cortical cell (black) and cross the endodermis via the plasmodesmata (gaps connecting cells)

Symplastic Pathway

Between plant cells, there is something called the plasmodesmata. The plasmodesmata is like a tiny bridge that connects cells. This bridge connects the cytoplasm of one cell to the cytoplasm of neighboring cells.

The cytoplasm is the gel-like liquid inside a cell.

Plasma membrane covers all cells and the plasmodesmata is an extension of this membrane. So, one big, continuous plasma membrane surrounds all the connected cells.

From the Principles of Soil and Plant Water Relations, 2005:

The symplast or symplasm is the continuum of communication cytoplasm, which is created by the intracellular connections.

So, the symplast is the connected plasma membrane.

Water must cross the plasma membrane to travel the symplastic pathway. Once the molecules are inside the membrane, they can travel through the cell's cytoplasm.

The water molecules will reach the end of the cell. From here, the molecules can use the plasmodesmata (bridge) to cross to the next cell.

Water molecules will continue this process until they enter the xylem. Remember, water needs to be inside a cell to cross the endodermis. When using the symplastic pathway, the molecules are already inside a cell.

A diagram showing the symplastic pathway
Water (green line) travels into the epidermis cells (blue), then crosses into the cortical cells (black) via the plasmodesmata (bridge connecting cells). The water makes its way through the cortical cells, all the way to the endodermis (red). The water pass through the endodermis, and continue deeper into the root.

Transmembrane Pathway

The transmembrane pathway is a kind of mix between the two previous paths.

When molecules travel along the transmembrane pathway, the molecules enter and exit each cell by crossing the plasma membranes.

Water molecules will travel into each cell. But, rather than use the plasmodesmata to enter the next cell, the molecules will hop through the cells walls.

So, water will travel through the symplast by moving through interconnected cytoplasms. But, the water will also travel through the apoplast by moving through cell walls and gaps between cell walls.

Thus, the transmembrane pathway combines both apoplast and symplastic paths.

Water (green line) will travel through cells (blue and black) until it passes through the endodermis (red). The water enter and exit cells via the cell wall. The water doesn't use the plasmodesmata (bridges between cells) to enter and exit each cell.

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