Problem 3 - Campbell Biology Concepts & Connections

Question

Which best describes the structure of a cell membrane? a. proteins between two bilayers of phospholipids b. proteins embedded in a bilayer of phospholipids c. a bilayer of protein coating a layer of phospholipids d. cholesterol embedded in a bilayer of phospholipids

Step-by-Step Solution

Answer

Option b: proteins embedded in a bilayer of phospholipids.

Step 1: Identify the structure of a cell membrane

The cell membrane is primarily composed of a phospholipid bilayer with embedded proteins. This arrangement is often described by the fluid mosaic model.

Step 2: Analyze option a

Option a suggests proteins are between two bilayers of phospholipids, which does not align with the widely accepted model of cell membrane structure.

Step 3: Analyze option b

Option b correctly states that proteins are embedded in a bilayer of phospholipids, which is consistent with the fluid mosaic model.

Step 4: Analyze option c

Option c describes a bilayer of protein coating a layer of phospholipids, which is not accurate according to the established understanding of cell membrane structure.

Step 5: Analyze option d

Option d mentions cholesterol embedded in a bilayer of phospholipids. While cholesterol is present in the membrane, this option does not fully describe the primary structure involving embedded proteins.

Step 6: Conclusion

Given the above analysis, the best description of the cell membrane structure is provided by option b: 'proteins embedded in a bilayer of phospholipids.'

Key Concepts

phospholipid bilayer

The cell membrane's foundational structure is the phospholipid bilayer.
Phospholipids are unique molecules with a hydrophilic (water-attracting) 'head' and two hydrophobic (water-repelling) 'tails'.
In water, phospholipids arrange themselves into a bilayer because the hydrophilic heads face outward, toward the water, while the hydrophobic tails face inward, away from the water.
This dual-layer structure forms a flexible and semi-permeable boundary around the cell.
It allows selective passage of substances, maintaining the internal environment of the cell while interacting with the external environment.

fluid mosaic model

The fluid mosaic model is a way of describing the structure and behavior of cell membranes.
According to this model, the phospholipid bilayer behaves like a fluid, allowing proteins to move sideways within the layer.
This fluidity is crucial for the functioning of the cell membrane, enabling flexibility, self-healing, and the movement of materials in and out of the cell.
The 'mosaic' part of the model refers to the variety of proteins that are embedded within the bilayer.
These proteins float among the phospholipids, like boats on a sea, each performing specific functions to maintain cell life and communication.
This dynamic and adaptable structure contributes to the cell's ability to respond and adapt to its environment.

embedded proteins

Embedded proteins are integral components of the cell membrane, contributing to its diverse functions.
These proteins can either span across the entire membrane or be attached to the surface of the bilayer.
They perform various roles, such as:

• Transport proteins: Help move substances across the membrane.
• Receptor proteins: Allow cells to receive signals from their environment, facilitating communication.
• Enzymatic proteins: Catalyze reactions essential for cell metabolism and function.

Embedded proteins ensure the cell membrane is not just a passive barrier but an active and functional interface between the cell and its surroundings.