Question

In a lipid bilayer, the _________ of all the lipid molecules are sandwiched between all of the ______ a. hydrophilic tails; hydrophobic heads b. hydrophilic heads; hydrophilic tails c. hydrophobic tails; hydrophilic heads d. hydrophobic heads; hydrophilic tails

Short answer

c. hydrophobic tails; hydrophilic heads

Step-by-step solution

Understand the Structure of a Lipid Bilayer

A lipid bilayer is composed primarily of phospholipids. Each phospholipid molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repellent) tails. The hydrophilic heads face the aqueous environment on either side of the bilayer, while the hydrophobic tails are tucked away from the water.

Identify the Correct Arrangement

In the bilayer formation, the hydrophobic tails of the molecules face inward, away from the water, while the hydrophilic heads are positioned on the outer surfaces of the bilayer, interfacing with the aqueous environment.

Choose the Correct Option

Given the arrangement where hydrophobic tails are sandwiched between hydrophilic heads, the correct choice is "hydrophobic tails; hydrophilic heads." This matches the description of a lipid bilayer arrangement.

Key concepts

Phospholipids

Phospholipids are essential building blocks of cell membranes, vital for cellular structure and function. Each phospholipid comprises a glycerol backbone, to which two fatty acid chains and a phosphate group are attached. The presence of these components creates a molecule with dual characteristics, lending itself to the structure of membranes.

• Two fatty acid chains make up the hydrophobic tail, which avoids water.
• The phosphate group constitutes the hydrophilic head, which is attracted to water.

This unique structure allows phospholipids to spontaneously arrange themselves into a bilayer, the core of cell membranes, when placed in an aqueous environment.

Hydrophilic Head

The term "hydrophilic" means "water-loving," and for phospholipids, it specifically refers to the polar, phosphate-containing head. This part of the molecule is soluble in water due to its charge, enabling it to interact favorably with water molecules. The interactions stabilize the orientation of phospholipids in an aqueous environment.
The hydrophilic heads of phospholipids orient themselves towards the water, whether that's the intracellular fluid inside the cell or the extracellular fluid outside of it. This alignment forms the outer sides of the lipid bilayer and is crucial for the membrane’s interactions with the cell's environment. By facing outwards, the hydrophilic heads facilitate processes like signaling and transport that are essential to cellular function.

Hydrophobic Tail

The "hydrophobic" tail of a phospholipid, meaning "water-fearing," refers to the two fatty acid chains attached to the glycerol backbone. Due to being non-polar, these tails do not mix well with water and instead prefer to associate with other hydrophobic substances. This aversion to water is paramount in the formation of the lipid bilayer.
In a biological membrane, the hydrophobic tails point inward, shielding themselves from water by creating a water-free zone within the center of the bilayer. The natural inclination of these tails to escape from water not only leads to the bilayer structure but also ensures the membrane's integrity and stability. This organization is essential for forming a semi-permeable barrier that controls substance movement in and out of the cell.

Cell Membrane Structure

The cell membrane, often described as the "fluid mosaic model," is predominantly composed of lipid molecules arranged in a bilayer. The lipid bilayer is the key feature that provides the membrane with its semi-permeable nature, embedding proteins that function in communication, signaling, or transport.

• The hydrophilic heads interact with watery environments on both sides of the membrane.
• The hydrophobic tails form a barrier to most water-soluble substances.

This arrangement of lipids in the membrane not only forms a barrier but also maintains the flexibility and fluidity of the cell membrane. Proteins and lipids within the bilayer can move freely, allowing the membrane to be dynamic, adaptable, and capable of self-repair. These characteristics are crucial for the myriad functions that the cell membrane performs, ranging from nutrient uptake to waste removal, and signaling transduction.