singer and Nicolson's fluid mosaic model of membrane structure presumed all of the following statements to be true EXCEPT: a. The phospholipid bilayer is a fluid matrix. b. Proteins can be anchored to the membrane by covalently linked lipid chains. c. Proteins can move laterally across a membrane. d. Membranes should be about 5 nm thick. e. Transverse motion of lipid molecules can occur occasionally.

The cell membrane, often described as the ‘skin’ of the cell, serves as a crucial barrier that separates the interior of the cell from its external environment. It is not simply a static layer, but a dynamic and complex structure that plays a key role in protecting the cell and facilitating communication and transport between the inside of the cell and its surroundings.

According to the fluid mosaic model, the cell membrane is composed of a phospholipid bilayer with various membrane proteins embedded within it. This arrangement allows the membrane to maintain its integrity while being flexible enough to enable the movement of substances in and out of the cell - a process essential for the cell's survival. The membrane's fluidity is a fundamental characteristic that permits the lateral movement of proteins and lipids, which is important for cell signaling and interaction.

The phospholipid bilayer forms the foundational framework of the cell membrane and is responsible for its semi-permeable nature. Each phospholipid molecule consists of a hydrophilic (water-attracting) 'head' and two hydrophobic (water-repellent) 'tails'. In an aqueous environment, these molecules naturally arrange themselves into a bilayer, with the heads facing the water on both the outside and inside of the cell, and the tails tucked away from the water.

This structure creates a selective barrier that allows only certain molecules to easily pass through. It is the continuous movement of the phospholipids that lend the membrane its fluid-like properties, essential for the diverse functions the membrane performs. Crucially, the structure and makeup of the phospholipid bilayer can also affect how the membrane interacts with different substances, influencing the overall functioning of the cell.

Membrane proteins are integral to the functioning of the cell membrane, performing a variety of critical tasks. These proteins come in two main types: integral proteins that span the entire membrane and peripheral proteins that are bound to either the exterior or interior surface of the membrane.

These proteins can serve as channels or pumps controlling the passage of ions and molecules, act as receptors for signal transduction, or provide structural support. Some membrane proteins can move laterally, floating in the fluid lipid bilayer, allowing them to interact with various cellular components and signals. This mobility is vital for the cell's responsiveness to the environment. Additionally, the ability of some proteins to anchor to the membrane with covalently linked lipid chains demonstrates the versatility and dynamic nature of the cell membrane's protein network.