Question

Bulk flow of water occurs through (a) Living cells (b) Cell vacuoles (c) Apparent free spaces (d) Plasmodesmata

Short answer

The correct option is (c) Apparent free spaces.

Step-by-step solution

Understanding the terms

Living cells refer to any cell in the plant that is actively functioning. Cell vacuoles are compartments within plant cells that store water and nutrients. Apparent free spaces are intercellular spaces or air spaces within a plant, typically found in spongy mesophyll tissue of leaf. Plasmodesmata are microscopic channels traversing the cell walls of plant cells enabling transport and communication between them.

Identify the process

Bulk flow is associated with long-distance movement of sap in xylem and phloem cells, not in individual cells or their organelles.

Match the options with the process

Given that neither living cells, cell vacuoles, nor plasmodesmata are involved in bulk flow of water, and knowing that apparent free spaces are part of long-distance water transport system, the correct answer would be 'apparent free spaces'.

Key concepts

Living Cells

In the context of plants, living cells are the fundamental units that make up all plant tissues. These cells are alive, meaning they actively metabolize, divide, and contribute to the plant's growth and survival. Living plant cells include various types, such as:

• Parenchyma cells: These are versatile and often found in leaves, roots, and fruits.
• Collenchyma cells: Known for providing support while still allowing flexibility, these cells usually reside in young stems and leaves.
• Sclerenchyma cells: They offer rigidity and strength due to their thickened cell walls.

Living cells perform vital processes, including photosynthesis, respiration, and transport of nutrients and water. They work collectively but remain individual units that are not directly involved in the bulk flow of water, which is largely dependent on non-living structures like xylem vessels.

Cell Vacuoles

Cell vacuoles are significant organelles found within plant cells, primarily serving as storage compartments. Each vacuole is a membrane-bound space that can hold:

• Water
• Enzymes
• Waste products
• Nutrients

The role of vacuoles extends beyond simple storage. They help in maintaining cell rigidity, which is crucial for standing upright and reaching sunlight—a process known as turgor pressure. Vacuoles also manage the cellular pH and play a role in the detoxification of harmful substances. Although water content in vacuoles is important for the cell's turgor pressure, they are not primary pathways for the large-scale movement of water, which involves major cell components like the xylem.

Apparent Free Spaces

Apparent free spaces are gaps or intercellular spaces within the tissue of plants, often seen in the spongy mesophyll layer of leaves. These spaces are not void but filled with air or, in some cases, water vapor. This structure allows for:

• Efficient gas exchange
• Movement of water and nutrients over longer distances within the plant

In the context of bulk flow, apparent free spaces are vital as they form part of the extensive network that facilitates the movement of water and solutes across different parts of the plant. These spaces, in combination with xylem, aid in rapid water transport, leveraging differences in pressure to move substances swiftly through the plant.

Plasmodesmata

Plasmodesmata are tiny, pore-like structures that create direct channels between the cell walls of adjacent plant cells. These channels serve crucial roles by enabling:

• Exchange of small molecules and ions
• Direct communication between cells
• Transport of nutrients and hormones

The structure of plasmodesmata allows them to bridge the symplast, which is the continuous network of cytoplasm among plant cells. This connectivity contributes to the circulation of substances like sugars but does not act in the bulk flow of water. Instead, they facilitate localized cell-to-cell transport, providing a pathway for coordinated cellular activities across the plant.