Which one of the following is a feature of Membrane?
Bilayer of phospholipid molecule in which proteins and cholesterol are embedded
Understanding Biological Membrane Features
Biological membranes are essential components of cells, separating the internal environment from the external one and also compartmentalizing organelles within eukaryotic cells. Their structure is crucial for their function.
Key Features of a Biological Membrane
The widely accepted model for the structure of biological membranes is the Fluid Mosaic Model, proposed by Singer and Nicolson. According to this model, a biological membrane has several key features:
- Phospholipid Bilayer: The fundamental structure is a double layer (bilayer) of phospholipid molecules. These molecules have a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. The hydrophilic heads face outwards towards the aqueous environment (both inside and outside the cell), while the hydrophobic tails face inwards, away from water.
- Proteins: Various proteins are embedded within or associated with the phospholipid bilayer. These proteins perform diverse functions, including transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment to the cytoskeleton or extracellular matrix. Proteins can be integral (spanning the membrane), peripheral (loosely attached), or lipid-anchored.
- Cholesterol: In animal cells, cholesterol molecules are interspersed among the phospholipid molecules in the bilayer. Cholesterol helps regulate membrane fluidity, making the membrane less fluid at high temperatures and more fluid at low temperatures.
- Carbohydrates: Carbohydrate chains are often attached to proteins (forming glycoproteins) or lipids (forming glycolipids) on the outer surface of the plasma membrane. These play roles in cell recognition and cell adhesion.
Analyzing the Options
Let's examine each option based on the known features of biological membranes:
-
Bilayer of phospholipid molecule in which proteins and cholesterol are embedded
This option accurately describes the core structure of a biological membrane according to the Fluid Mosaic Model. It correctly identifies the phospholipid bilayer as the foundation and mentions the presence of embedded proteins and cholesterol.
-
Bilayer of proteins molecule in which lipid and cholesterol are embedded
This option is incorrect. The primary structural basis of the membrane is a lipid bilayer (specifically phospholipid), not a protein bilayer.
-
Bilayer of neutral lipid in which proteins and cholesterol are embedded
This option is incorrect. While membranes contain lipids, the structural bilayer is predominantly formed by phospholipids, which are a type of polar lipid, not neutral lipids like triglycerides.
-
Bilayer of neutral lipid lacking cholesterol and proteins
This option is incorrect for multiple reasons. The bilayer is primarily phospholipids, not neutral lipids, and biological membranes characteristically contain proteins and often cholesterol (in animal cells) which are essential for their function.
Based on this analysis, the feature that correctly describes a biological membrane is a bilayer of phospholipid molecules with embedded proteins and cholesterol.
| Component | Description | Primary Location/Role |
|---|---|---|
| Phospholipids | Amphipathic molecules with hydrophilic head and hydrophobic tail | Forms the structural bilayer foundation |
| Proteins | Integral, peripheral, or lipid-anchored | Transport, enzymes, receptors, structural support |
| Cholesterol | Steroid lipid (in animal cells) | Modulates membrane fluidity |
| Carbohydrates | Often attached to lipids or proteins (glycolipids/glycoproteins) | Cell recognition, adhesion |
Revision Table: Key Membrane Features
| Feature | Description |
|---|---|
| Basic Structure | Phospholipid Bilayer |
| Embedded Molecules | Proteins, Cholesterol (in animal cells) |
| Model | Fluid Mosaic Model |
Additional Information: Fluid Mosaic Model
The "Fluid" part of the Fluid Mosaic Model refers to the ability of membrane components, particularly phospholipids and many proteins, to move laterally within the membrane layer. The "Mosaic" part refers to the "patchwork" of proteins scattered throughout the phospholipid bilayer.
The fluidity of the membrane is important for various cellular processes such as:
- Cell movement
- Cell growth
- Cell division
- Formation of vesicles
- Protein function and transport across the membrane
Factors affecting membrane fluidity include temperature (higher temperature increases fluidity), the type of fatty acid tails in phospholipids (unsaturated tails increase fluidity due to kinks), and the presence of cholesterol.
The plasma membrane is selectively permeable, meaning it regulates the passage of substances into and out of the cell. This property is largely due to the lipid bilayer and the transport proteins embedded within it.
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