Which best describes the structure of a biological membrane?
a. Two layers of phospholipids with proteins either embedded in the layers or on the surface of the layers.
b. A mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot.
c. Two layers of phospholipids with each layer covered on the outside with proteins.
d. A fluid structure in which phospholipids and proteins move freely between sides of the membrane.
e. Two layers of phospholipids with proteins embedded between the two layers.
The Correct Answer and Explanation is :
Correct Answer:
a. Two layers of phospholipids with proteins either embedded in the layers or on the surface of the layers.
Explanation:
The structure of a biological membrane is best described by the fluid mosaic model, which was first proposed by Singer and Nicolson in 1972. This model explains how biological membranes function as dynamic and selectively permeable barriers for cells.
A biological membrane consists of two layers of phospholipids (a phospholipid bilayer) arranged with their hydrophobic (water-fearing) tails facing inward and their hydrophilic (water-loving) heads facing outward. This arrangement creates a stable but fluid structure that allows flexibility and movement.
Embedded within this phospholipid bilayer are proteins, which play crucial roles in cell function. These proteins can be:
- Integral proteins: Embedded within the membrane, sometimes spanning the entire bilayer (called transmembrane proteins), which help in transport and signaling.
- Peripheral proteins: Attached to the surface of the membrane (either inner or outer), involved in signaling and structural support.
This model describes the membrane as a fluid structure where phospholipids and proteins move laterally within the layer but rarely flip-flop between layers. The movement of these molecules contributes to the dynamic nature of the membrane, which allows cells to adapt to their environment, communicate, and regulate the passage of molecules.
Other answer choices contain inaccuracies:
- (b) is incorrect because the components are not covalently linked but held together by hydrophobic and hydrophilic interactions.
- (c) is incorrect because proteins are not just on the surface but also embedded within the bilayer.
- (d) is incorrect because while the membrane is fluid, phospholipids and proteins do not move freely between sides.
- (e) is incorrect because proteins are embedded in or on the bilayer, not between two separate layers.
Now, I’ll generate an image to illustrate the structure of a biological membrane.
Here is an illustration of the biological membrane structure based on the fluid mosaic model. It highlights the phospholipid bilayer, integral and peripheral proteins, glycoproteins, and cholesterol molecules, all contributing to membrane fluidity and function. Let me know if you need any modifications or further explanations!