Apoplast and symplast pathway

Introduction

Water is essential for plants to carry out different life processes like photosynthesis, germination and transportation of nutrients. The plant body loses water through leaves by transpiration. This exerts pressure along the length of the plant body allowing roots to absorb soil water with minerals.

Water and minerals enter the root hair cells by osmosis and are transported across the root cortex to reach the xylem vessels. Xylem vessels occur in the vascular stele and aid the longitudinal transport of water. The xylem and phloem are the vascular components in plants. Xylem transports water while phloem chiefly transports nutrients. Across the root cortex, water molecules are transported through apoplastic, symplastic and transmembrane pathways

Apoplast and Symplast- Definition

Plant cells have a rigid cell wall around the cell membrane that encloses cytosol with membrane-bound organelles. All space lying outside the cell membrane is regarded as apoplast. While symplast is the living component inside the cell membrane of a plant cell. It lies on the cytosolic side

Apoplast

The apoplast is the space between the plant cell wall and the cell membrane. It is the non-living part and includes all non-protoplasmic components of plant cells. Generally, the cell wall and intercellular space are regarded as apoplast

Symplast

It is the protoplasmic content and is the living part involved in water transportation across root cortex via plasmodesmata. Plasmodesmata are the small channels connecting the cytoplasm of neighbouring plant cells. Plasmodesmata lie across the cell walls and have microscopic pores or channels that act as bridges for the movement of molecules across the cells.

Differences and similarities between apoplast and symplast

Both apoplast and symplast are two separate compartments aiding the transportation of water and solutes. Both can occur simultaneously or separately with varying rates. They possess some similarities as well as some dissimilarities which need to be understood

Similarities between apoplast and symplast

• Both apoplast and symplast are transport pathways

• The main aim is to carry water and solute molecules

• Transport is directed towards the root xylem from the outer root hair cells through the root cortex

Apoplast	Symplast
It includes non-protoplasmic components like cell walls and intercellular spaces.	It includes the protoplasmic content
Transport is through non-living parts	Transport is through living parts.
It is Passive diffusion from high to low concentrations.	It is Osmosis across cell membranes.
Not affected by Cellular metabolism.	Cellular metabolism highly impacts the symplast rate.
It is a relatively faster process.	It is relatively slow.
Movement is free of resistance.	Movement experiences resistance to some extent.
Also called cell wall pathway.	Also called a cytoplasmic pathway.

What is apoplast movement in plants?

• Apoplast movement is the transport of water along channels made of cell walls. The transport through cell walls and intercellular spaces without taking entry into the cellular interior is considered an apoplastic pathway.

• This movement generally takes place from the root hairs to the xylem passing through the epidermis and cortex. However, beyond the cortex, there is endodermis having Casparian strips. They are waterproof bands (composed of suberin) on the side walls of the root endodermal cells. Suberin is impermeable and corky substance which opposes apoplastic transport to some extent. Hence, the water and solutes take a symplastic pathway beyond the cortex for reaching the xylem.

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• Secondary roots are the slender branches arising by the side of the main roots. They emerge from the pericycle (a layer adjoining endodermis towards the vascular bundles when seen on a root transsection). Secondary roots are devoid of Casparian strips and the apoplastic pathway is continuous without any shift to the symplast pathway

• This transport bypasses the protoplasm and there is no need to cross the lipid bilayer. Hence there is no selectivity over solutes

• Since the apoplast transport involves non-protoplasmic content, the cellular metabolism which takes place inside of cell membrane has no impact on the rate of transport. Also, the apoplast transport does not experience any resistance from the cellular contents as there are barriers separating the cellular contents from the apoplast

• Symplast pathway

• Symplast pathway is the transport pathway involving the living components. Water along with solutes enters the cytoplasm of cells and moves through plasmodesmata to maintain continuity

• Plasmodesmata act like bridges between cells allowing the intercellular movement of substances including photosynthetic products, proteins etc. They are referred to as cell junctions in plant cells

• In the symplast pathway, plasmodesma (Plural plasmodesmata) forms the transport channels in between the cells

• This movement takes place only after water enters the cytoplasm by crossing the selectively permeable lipid bilayer by osmosis. However, there is no entry of water into the vacuole. Symplast only involves water movement into the cytoplasm and is influenced by cytoplasmic streaming.

• Selective permeability of the plasma membrane allows only some substances to enter the cytoplasm. Which has an obvious impact on the components transported through symplast

• Since the cellular environment is directly involved in symplast transport, there is a direct effect of metabolism on symplast transport. Apart from this the symplast pathway also experiences resistance due to cytoplasmic components to some extent. This makes the symplast a relatively slow process when compared with the apoplast pathway.

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Importance of Casparian strips

• Casparian bands are the cork-like, suberin bands which are impermeable to water. They are present on the root endodermal cell side walls. As the absorbed water moves through the root epidermis and cortex (through the apoplastic pathway), it reaches the endodermis. Water takes a symplastic pathway to enter into pericycle in order to reach xylem bundles. Casparian strips deny water transport through apoplast and it has to adopt a symplastic pathway.

• Casparian strips avoid water reentry back into the cortex. Since the water flow is from high potential to low potential, the salts secreted by endodermal cells into vascular bundles create a low potential allowing movement into the vascular tissues only.

• Casparian strips regulate the water and solutes flowing directly into vascular bundles. They are forced to enter the cytoplasm through a semipermeable membrane to protect the inner vascular tissues from soil water

Conclusion

Plants absorb water and minerals from the root hair zone and transport them to all other parts through vascular tissues called xylem. The water and minerals have to cross different cell layers to reach the xylem. Apoplast and symplast pathways are the transport pathways adopted by water molecules. Although both are transport pathways, they differ from one another in various aspects. In the apoplast pathway, water moves through the cell walls and intercellular spaces without entering the cellular interior. However, the water molecules experience impermeability in the endodermis because of the Casparian strips on the side walls. Water molecules undertake the symplastic pathway which is a cellular route. In the symplastic pathway, the water along with solutes pass through the cell membrane into the cytoplasm. Both symplast and apoplast can occur simultaneously or separately varying with the necessity based on the physiological state the plant is experiencing

FAQs

Q1. What factors influence the speed of the symplast pathway?

Ans. Water molecules moving through symplast have to cross through the selectively permeable barrier for entering the cytoplasm. After the cellular entry, the molecules encounter cytoplasmic streaming both of which slow down the speed.

Q2. Does plasmodesmata allow the transfer of biotic components apart from abiotic water?

Ans. Plasmodesmata allow cell-to-cell communication by allowing the transfer of macromolecular entities like proteins, and hormones apart from water and solutes. Viruses are also noted to spread infections throughout the plant body via plasmodesmata

Q3. State the permeability range of apoplast

Ans. The apoplast pathway is a completely permeable pathway, unlike the symplast pathway which is semipermeable.

Q4. Does apoplast and symplast transport need energy?

Ans. Apoplast and symplast pathways are passive diffusion and osmosis respectively. They do not need energy and molecules move from high concentration to low concentration

Q5. Why is apoplast transport faster?

Ans. Apoplast transport is free-flowing and does not experience any resistance due to cytoplasm and hence is a faster transport pathway