Apoplast - Definition and Its Function

How do plants absorb water for their growth? Well, we all know the roots absorb water for the plant's growth. The absorption of water by the roots takes place in two pathways- Symplast and Apoplast pathways. The following article explores Apoplast in detail.

What is Apoplast?

The Apoplast is an intercellular space outside the plasma membrane that connects from one cell to another and to the external environment. The water is absorbed by roots in two ways:

Active absorption: The water enters the cell sap and passes from one cell to another. In this absorption, protoplasm is involved. This movement is called Symplast.
Passive absorption: The water enters through the apoplast of the root and passes from one cell to another. In this absorption, cell walls and intercellular spaces are involved.

Apoplast Pathway

Apoplast is the space around the plasma membrane where materials can diffuse easily. The apoplast pathway has Casparian strips between the plant cells and the plant cuticle. It also includes non-living spaces between the cells and the cell membranes.

Wikipedia

Source: Wikipedia

The water is transported to the xylem from root hair through the cell wall of the cells. In Apoplast, the water movement is blocked beyond the cortex by the Casparian strip of endodermal cells. Thus, the water is delivered beyond the cortex by the Symplast pathway. The apoplast pathway is made up of non-living components, so it is least affected by the metabolic state of the root.

Difference between Apoplast and Symplast

Though Apoplast and Symplast function for a similar cause, they have some major differences given below:

Criteria	Apoplast	Symplast
Parts involved	It consists of intercellular spaces and cell walls.	It consists of Protoplasm.
Nature of the parts	The parts are non-living in nature.	The parts are living in nature.
Speed of the pathways	The water transports fast in the Apoplastic pathway.	The water transports comparatively slower in the Symplastic pathway.
Metabolic State	There is no effect of the metabolic state on the water movement	There is interference by metabolic state in the flow of water.
Transportation	In this pathway, ions and water are transported in the cortex.	In this pathway, ions and water are transported beyond the cortex.

Apoplast - Practice Problems

Q1. Which of the following refers to passive absorption?

A. Active pathway
B. Apoplast pathway
C. Symplast pathway
D. Passive pathway

Answer: B. Apoplast pathway

Explanation: In passive absorption, the water enters through the apoplast of the root and passes from one cell to another.

Q2. Which of the following blocks the water movement beyond the cortex?

A. Cell wall
B. Cytoplasm
C. Apoplast
D. Casparian strip

Answer: D. Casparian strip

Explanation: In Apoplast, the water movement is blocked beyond the cortex by the Casparian strip of endodermal cells.

Q3. Apoplast is made up of______?

A. Casparian strips
B. Living components
C. Non-living components
D. None of the above

Answer: C. Non-living components

Explanation: The Apoplast pathway is made up of non-living components, so it is least affected by the metabolic state of the root.

Frequently Asked Questions

Q1. What is the significance of the Casparian strip?
Answer: The Casparian strip is a waterproof tissue present on the endodermis of roots. It blocks the water movement beyond the cortex to prevent water from entering the pericycle, which is vital for inducing root pressure.

Q2. Where does the Apoplast movement of water occur?
Answer: Apoplast movement of water occurs through the cell wall and the intercellular spaces.

Q3. According to what gradient the pathway occurs?
Answer: Both in Apoplast and Symplast pathway occurs according to the water potential gradient. The movement of water in the root layers is from higher water potential to lower water potential.