Apoplast - An Overview, Types
The apoplast is the non-living part of the plant tissue that allows the movement of water and solutes through cell walls and intercellular spaces. It plays a key role in root water absorption and transport via xylem. The apoplast pathway is important in CBSE and NEET exams, often compared with symplast and transmembrane pathways.
This Story also Contains
- What is Apoplast?
- Pathways for Water Transport in Plants
- Types of Xylem
- Materials Transported via Apoplast
- Transport of Water and Ions through Apoplast
- Difference Between Apoplast and Symplast Pathway
- Exam Importance
- Tips, Tricks, & Mnemonics
- Apoplast NEET MCQs
- FAQs on Apoplast
Water plays a major role in transpiration, growth and transport in plants. All cells contain water as the solvent in which biochemical reactions take place and in which cell structures are maintained. Water molecules are in constant random motion. Diffusion occurs when molecules migrate as a result of this motion. Molecules will move down a concentration gradient.
In most of the plants, water uptake occurs predominantly through root hairs, fine extensions of single epidermal cells. They provide a very large surface area for absorption. There are different pathways for water transport. Water can flow either through the cell wall spaces, the apoplast or through the cell contents, the symplast. Movement of a water molecule across the root involving both pathways is termed transcellular transport.
What is Apoplast?
The apoplast is the continuous system of cell walls and intercellular spaces in plant tissues that allows for the movement of water and solutes outside of the cell membranes. It forms a crucial pathway for water and nutrient transport in plants because it provides a low-resistance pathway for water movement from roots to leaves. It allows for rapid, long-distance water transport without crossing cell membranes, which is essential for maintaining plant hydration and supporting transpiration.
Pathways for Water Transport in Plants
Plants require water to survive just as animals and mammals need. Plants require water to complete their daily basis of biological processes. Water plays a very important role in processes such as seed germination, and photosynthesis. Three major water means of transport are discussed below:
Apoplast Pathway
Apoplast is defined as the pathway in which the movement of water occurs through the cell wall and other intercellular spaces that are present in the cell. The apoplast, also known as the cell wall of the plant, is located on the outer side of the cell.
Symplast Pathway
Symplast is defined as the transfer of water from one cell to another cell with the help of some connections that are called plasmodesmata. The inner side of the plasma membrane consists of symplast.
Transmembrane Pathway
As the name suggests, transmembrane is defined as a combination of both the Apoplast and Symplast pathways and forms a transmembrane pathway. This is the pathway that helps in the movement of water across cells and cell walls.
Types of Xylem
Different types of xylem are discussed below in the table:
Types of Xylem | Explanation |
Xylem Vessels |
|
Xylem Tracheids |
|
Xylem Fibres |
|
Materials Transported via Apoplast
Materials required during the Apoplast Pathway are discussed below in the table:
Materials Needed | Explanation |
Oxygen |
|
Carbon dioxide |
|
Organic Nutrients |
|
Inorganic ions and water |
|
Transport of Water and Ions through Apoplast
The basic transportation of water and ions through the apoplast path is discussed below in the table:
Xylem Tissue Functions | Explanation |
Support |
|
Transport of water |
|
Difference Between Apoplast and Symplast Pathway
Apoplast Pathway | Symplast pathway |
Apoplast pathway is the movement of water through adjacent cell walls. | Symplast pathway, water moves from one cell to another through plasmodesmata. |
Movement through apoplast does not involve crossing the cell membrane. | During symplast movement, the water travels through cells. |
Movement is faster. | It is relatively slower. |
It does not provide any barrier in water movement. | Water molecules are unable to penetrate casparian strips. |
It is not aided by the streaming movement of cytoplasm. | It is aided by the streaming movement of cytoplasm. |
The metabolic rate of the cells in the root cortex does not affect the movement of water. | The metabolic rate of the cells in the root cortex highly affects the movement of water. |
Exam Importance
During exam preparation, different types of questions about the Apoplast Pathway are asked. The table below contains the different patterns of questions asked in different exams.
Exam Type | Types of Questions Asked | Weightage |
| 4% | |
| 5-4% | |
Paramedical |
| 5% |
Tips, Tricks, & Mnemonics
It takes effort to remember everything in a single go. We made the entire problem easy. Some of the tricks regarding Apoplast Pathway are given below which you can use to memorise the important points.
Apoplast Pathway Overview
"WINS: Water, Ions, Nutrients, Support"
W: Water - Primarily absorbed by root hairs and transported through the apoplast pathway to various plant parts via the xylem.
I: Ions - Inorganic ions (e.g., potassium, calcium) are absorbed from the soil and travel through the apoplast to support cellular functions.
N: Nutrients - Organic nutrients transported via the phloem; essential for plant growth and development.
S: Support - Xylem provides structural support while facilitating the movement of water.
Key Components of the Apoplast Pathway
"XLA: Xylem, Lignin, Apoplast"
X: Xylem
L: Lignin
A: Apoplast
Functions of Xylem in Apoplast Pathway
"STW: Support, Transport, Water"
S: Support
T: Transport
W: Water Storage
Comparison with Symplast Pathway
"CSD: Cell, Symplast, Diffusion"
C: Cell Membranes - Unlike the apoplast, the symplast pathway involves movement through cell membranes and cytoplasm.
S: Symplastic Transport - Refers to the movement of water and solutes through living plant cells, requiring membrane transport.
D: Diffusion - The apoplast pathway relies on passive diffusion, allowing for rapid transport.
Apoplast NEET MCQs
Q1. The apoplast pathway involves movement of water through:
Cytoplasm
Vacuoles
Cell walls and intercellular spaces
Plasmodesmata
Correct answer: 3) Cell walls and intercellular spaces
Explanation:
The apoplast pathway refers to the movement of water through the cell walls and intercellular spaces without crossing the plasma membrane. It is a non-living continuum outside the protoplast. Since it does not involve the cytoplasm, vacuoles, or plasmodesmata (which are cytoplasmic connections used in the symplast pathway), the correct answer is cell walls and intercellular spaces. This makes apoplast transport rapid and passive.
Hence, the correct answer is option 3) Cell walls and intercellular spaces.
Q2. Which of the following is absent in apoplast transport?
Cell membrane crossing
Passive movement
Intercellular spaces
Diffusion
Correct answer: 1) Cell membrane crossing
Explanation:
In apoplast transport, water and solutes move outside the cell membranes through cell walls and intercellular spaces, so they do not cross the plasma membrane until they reach the endodermis (Casparian strip). Passive movement, intercellular spaces, and diffusion are all part of apoplast transport. Therefore, the correct answer is cell membrane crossing, which is absent until the symplast pathway takes over at the endodermis.
Hence, the correct answer is option 1) Cell membrane crossing.
Q3. Lignin in xylem vessels mainly provides:
Photosynthesis
Structural support and water transport
Respiration
Nutrient storage
Correct answer: 2) Structural support and water transport
Explanation:
Lignin is a complex organic polymer deposited in the cell walls of xylem vessels. It provides mechanical strength to withstand the negative pressure generated during transpiration pull. Additionally, lignin makes the walls impermeable to water leakage, creating an efficient, continuous channel for long-distance water transport. It does not play a role in photosynthesis, respiration, or direct nutrient storage, making structural support and water transport the correct answer.
Hence, the correct answer is option 2) Structural support and water transport.
Also Read:
FAQs on Apoplast
What is apoplast in plants?
The apoplast is the non-living continuum in plants formed by the cell walls, intercellular spaces, and xylem vessels. It excludes the protoplasm (cytoplasm and plasma membrane). Apoplast provides a network through which water and solutes can move freely without crossing cell membranes. Since it is made of dead tissues like xylem vessels and porous cell walls, movement through apoplast is passive and faster than symplastic transport.
What is the apoplast pathway?
The apoplast pathway is the movement of water and dissolved minerals through the apoplast—that is, along the cell walls and intercellular spaces without entering the cytoplasm. This pathway allows rapid, bulk movement of water from root hairs towards the inner layers of the root. However, at the endodermis, the Casparian strip (suberin layer) blocks apoplastic flow, forcing water to enter the symplast pathway for selective absorption into the stele.
What is the difference between apoplast and symplast pathways?
The apoplast pathway involves movement of water through cell walls and intercellular spaces, without crossing cell membranes. It is faster but not selective. In contrast, the symplast pathway involves transport through the cytoplasm of cells interconnected by plasmodesmata, making it slower but highly controlled. The apoplast is non-living, while the symplast is living.
What is the function of apoplast in water transport?
The apoplast provides a low resistance pathway for the movement of large volumes of water and solutes in plants. It helps in rapid distribution of water from the soil into root tissues up to the endodermis. In leaves, apoplast movement also facilitates guttation and apoplastic phloem loading. By allowing free flow outside the plasma membrane, the apoplast plays a major role in long-distance water transport via xylem vessels.
Frequently Asked Questions (FAQs)
The apoplast is crucial in determining grafting compatibility. Successful grafting requires the formation of new connections between the apoplastic pathways of the scion and rootstock. Differences in apoplastic composition or structure can affect the success of this process.
Apoplastic ROS play important roles in plant physiology and stress responses. They can act as signaling molecules, contribute to cell wall loosening during growth, and play a role in plant defense against pathogens. The balance of ROS in the apoplast is carefully regulated by plants.
The apoplast plays a crucial role in plant responses to wounding. It can serve as a conduit for wound signals, such as electrical signals or chemical messengers. Changes in apoplastic composition can also occur as part of the wound healing process.
During seed germination, the apoplast is involved in water uptake and the movement of stored nutrients. Changes in cell wall properties and apoplastic pH can affect the weakening of seed coats and the expansion of embryonic tissues.
In some salt-tolerant plants (halophytes), the apoplast plays a role in salt secretion. Excess salt can be transported through the apoplast to specialized glands or salt hairs on the leaf surface, where it is secreted, helping the plant maintain ion balance.
The apoplast can serve as a pathway for the movement of plant hormones. Some hormones, like auxin, can be transported through the apoplast. Additionally, the apoplast can contain enzymes that modify hormones, affecting their activity and distribution within plant tissues.
Apoplastic enzymes play various roles in plant physiology, including cell wall modification, defense against pathogens, and nutrient mobilization. These enzymes can affect the properties of the apoplast itself, influencing transport processes and plant-environment interactions.
Cell wall extensibility, which affects plant growth, is closely related to apoplastic transport. Changes in cell wall properties can affect the movement of water and solutes through the apoplast, influencing both growth processes and the plant's ability to respond to environmental changes.
In waterlogged conditions, the apoplast can play a role in plant adaptation by facilitating the formation of aerenchyma (air spaces in roots). The apoplast may also be involved in the transport of oxygen from aerial parts to submerged tissues.
During heat stress, the apoplast can act as a heat sink, helping to buffer temperature changes. Changes in apoplastic composition, such as the accumulation of heat shock proteins, can also occur as part of the plant's stress response.