According to the modern concept, cell membrane is

Options

A. solid
B. quasifluid
C. fluid
D. solidified sheath.

Answer

### Step-by-Step Solution:

1. **Understanding the Structure of the Cell Membrane**: The cell membrane is primarily composed of a phospholipid bilayer. This bilayer consists of two layers of phospholipids, where each phospholipid has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) fatty acid tails.

2. **Identifying the Nature of the Phospholipid Bilayer**: In the bilayer, the hydrophilic heads face outward towards the aqueous environment, while the hydrophobic tails face inward, away from water. This arrangement is crucial for the membrane's function.

3. **Role of Proteins in the Membrane**: The cell membrane also contains proteins that can either be intrinsic (spanning the membrane) or extrinsic (located on the surface). These proteins play various roles, including transport, signaling, and structural support.

4. **Describing the Fluid Nature of the Membrane**: The modern concept of the cell membrane is described by the fluid mosaic model. This model suggests that the membrane is not rigid but rather has a quasi-fluid nature, allowing for the movement of proteins within the lipid bilayer.

5. **Conclusion**: Based on the fluid mosaic model, the cell membrane is best described as "quasi-fluid," meaning it has properties of both fluidity and structure, allowing proteins to float within the lipid environment like icebergs in the sea.

### Final Answer:
According to the modern concept, the cell membrane is quasi-fluid.

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