Problem 2 - Campbell Biology

Question

According to the fluid mosaic model of membrane structure, proteins of the membrane are mostly (A) spread in a continuous layer over the inner and outer surfaces of the membrane. (B) confined to the hydrophobic interior of the membrane. (C) embedded in a lipid bilayer. (D) randomly oriented in the membrane, with no fixed insideoutside polarity.

Step-by-Step Solution

Answer

C: embedded in a lipid bilayer.

Step 1: Understand the fluid mosaic model

The fluid mosaic model describes the structure of cell membranes. According to this model, the membrane is a fluid combination of lipids, proteins, and carbohydrates.

Step 2: Analyze Option A

Option A suggests that proteins are spread in a continuous layer over the inner and outer surfaces of the membrane. This does not align with the fluid mosaic model as proteins are not uniformly spread in layers.

Step 3: Analyze Option B

Option B states that proteins are confined to the hydrophobic interior of the membrane. Proteins in the fluid mosaic model are not only confined to the interior but also span the membrane or are on the surface.

Step 4: Analyze Option C

Option C indicates that proteins are embedded in a lipid bilayer. This is consistent with the fluid mosaic model, where proteins are embedded within the lipid bilayer and can move laterally.

Step 5: Analyze Option D

Option D suggests that proteins are randomly oriented with no fixed polarity. This does not match the fluid mosaic model, which states that integral proteins have specific orientations and functions.

Step 6: Choose the Correct Answer

Based on the analysis, Option C is correct because the proteins are indeed embedded in the lipid bilayer as per the fluid mosaic model.

Key Concepts

Cell Membrane Structure

The cell membrane, also known as the plasma membrane, acts as a barrier and gatekeeper for the cell. It separates the interior of the cell from the outside environment. The membrane is composed of a double layer of lipids, often called the lipid bilayer, along with various proteins and carbohydrates. The fluid mosaic model explains that this membrane is not static but rather dynamic and flexible. Lipid molecules can move sideways within the layer, making the membrane fluid. Embedded proteins also float in this sea of lipids like boats on water, contributing to the 'mosaic' aspect of the model. This structure allows the cell membrane to be selectively permeable, meaning it can control what enters and exits the cell.

Lipid Bilayer

The lipid bilayer forms the fundamental structure of the cell membrane. It consists mainly of phospholipids, which have a hydrophilic (water-attracting) 'head' and two hydrophobic (water-repelling) 'tails'. These phospholipids arrange themselves in a bilayer, with the heads facing outwards towards the water inside and outside the cell, and the tails facing inwards, away from the water. This arrangement creates a hydrophobic interior, which prevents water-soluble substances from easily passing through. Besides phospholipids, other lipids like cholesterol are also present, which add to the fluidity and stability of the membrane. The lipid bilayer acts as a fluid foundation that allows proteins and other molecules to move within it.

Membrane Proteins

Membrane proteins play critical roles in the function of the cell membrane. They can be classified into two main types: integral proteins and peripheral proteins.
Integral proteins penetrate the lipid bilayer and can span the entire membrane. These are often called transmembrane proteins. They help in various functions like transporting molecules across the membrane, acting as receptors for signaling molecules, or serving as enzymes.
Peripheral proteins are not embedded in the lipid bilayer. Instead, they attach loosely to the exterior or interior surfaces of the membrane, often connected to integral proteins or the lipid heads.
Both types of proteins contribute to the dynamic nature of the cell membrane, allowing it to adapt and respond to various internal and external conditions. They play essential roles in maintaining the cell's homeostasis and enabling communication with its environment.