Main Difference – Apoplast vs Symplast

The root hair cells absorb water from the soil by osmosis. This water is transported to the xylem of the root through the root cortex. The transportation of water also occurs by osmosis. Apoplast and symplast are the two routes by which the water travels from root hair cells to the xylem of the root. In the apoplastic route, water moves through the cell walls and the intracellular spaces of the root cortex. In symplastic route, water moves through the protoplasts of the root cortex. The main difference between apoplast and symplast is that apoplast is a fully permeable route in which the water movement occurs by passive diffusion whereas symplast is a selectively permeable route in which the water movement occurs by osmosis.

Key Areas Covered

1. What is Apoplast
      – Definition, Process, Characteristics
2. What is Symplast
      – Definition, Process, Characteristics
3. What are the Similarities Between Apoplast and Symplast
      – Outline of Common Features
4. What is the Difference Between Apoplast and Symplast
      – Comparison of Key Differences

Key Terms: Apoplast, Protoplast, Cell Wall, Intracellular Spaces, Osmosis, Passive Diffusion, Root Cortex, Root Hair Cells, Symplast, Xylem

What is Apoplast

The apoplast refers to the non-protoplasmic spaces of a plant. It includes the cell walls and the intracellular spaces. The apoplast of the root cortex is used for the movement of water to the xylem, which is absorbed by the root hair cells. This pathway is called the apoplastic pathway. The apoplastic pathway does not cross any cytoplasmic membrane at any time. That means the water moves through the passive diffusion. Therefore, the apoplastic pathway shows the least resistance towards the movement of water. The lignosuberin Casparian strips present in the walls of the endodermic cells may interrupt the water movement through the apoplast. Then the water moves only through the symplastic pathway. The interruption of the water movement by the Casparian strips is shown in figure 1.

What is Symplast

Symplast refers to the protoplasmic components of a plant. The protoplasms of the cells are connected by the cell junctions called plasmodesmata. The symplast of the root cortex is used for the movement of water from the root hair cells to the xylem of the root. This pathway is called the symplastic pathway. The water enters the cytoplasm of the cell through the plasma membrane; hence, the symplastic pathway should cross cell membranes. Since cell membranes are semi-permeable, the movement of the water occurs by osmosis in the symplastic pathway. However, the water that moves through the symplast does not enter the vacuole of the cell. Since the symplastic pathway crosses the cell membrane, it is also called the transmembrane pathway. The movement of water in the symplastic pathway is aided by the cytoplasmic streaming.

Figure 2: Apoplastic and Symplastic Pathway

The mineral nutrients are transported by the active absorption in the symplastic pathway. In the vacuolar symplastic pathway, the water enters to the vacuole of the cell through the tonoplast. Then the water passes to the adjacent vacuoles of other cells. The vacuolar symplastic pathway generates a high resistance towards the movement of water. Both apoplastic and symplastic pathways are shown in figure 2.

Similarities Between Apoplast and Symplast

• Apoplast and symplast are two routes by which the water moves from root hair cells to the xylem.
• Both apoplast and symplast occur in the root cortex.
• Both apoplast and symplast carry water and nutrients towards the xylem.

Difference Between Apoplast and Symplast

Definition

Apoplast: Apoplast refers to the nonprotoplasmic components of a plant, including the cell walls and the intracellular spaces. 

Symplast: Symplast refers to the continuous network of protoplasts of a plant, which are interconnected by plasmodesmata.

Components

Apoplast: The apoplast consists of nonprotoplasmic components such as cell walls and intracellular spaces.

Symplast: The symplast consists of protoplast.

Living/Non-living

Apoplast: The apoplast is composed of nonliving parts of a plant.

Symplast: The symplast is composed of living parts of a plant.

Water Movement

Apoplast: The water movement occurs by passive diffusion.

Symplast:  The water movement occurs by osmosis.

Resistance to Water Movement

Apoplast: The apoplast shows less resistance to the water movement.

Symplast:  The symplast shows some resistance to the water movement.

Speed of the Water Movement

Apoplast: Water movement through the apoplast is rapid.

Symplast:  Water movement through the symplast is slower.

Metabolic State of Roots

Apoplast: The metabolic rate of the cells in the root cortex does not affect the movement of water through the apoplastic route.

Symplast: The metabolic state of the cells in the root cortex highly affect the water movement by the symplastic route.

Significance

Apoplast: With the secondary growth of the root, most of the water moves by the apoplastic route.

Symplast:  Beyond the cortex, water moves through the symplastic route.

Conclusion

Apoplast and symplast are two routes used by plants to transport water from root hair cells to the xylem of the root. Apoplast includes the non-living components of a plant such as cell walls and the intracellular spaces. Symplast includes the living components of a plant such as protoplasms. The water moves by the apoplastic pathway by passive diffusion. In contrast, in the symplastic pathway, water moves by the osmosis since water moves across the cell membranes. The main difference between apoplast and symplast is their mechanism of the water movement.

Reference:

1. “Pathways of Water Movements in Roots (With Diagram).” Biology Discussion, 12 Dec. 2016, Available here. Accessed 23 Aug. 2017.

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