Question

A phospholipid has a head and two tails. The tails are found a. at the surfaces of the membrane. b. in the interior of the membrane. c. spanning the membrane. d. where the environment is hydrophobic. e. Both b and d are correct.

Short answer

The correct answer is option e (Both b and d are correct) because the tails of a phospholipid are found in the interior of the membrane and in a hydrophobic environment.

Step-by-step solution

Understand phospholipid structure

A phospholipid has a head and two tails. The head is hydrophilic, meaning it is attracted to water molecules, while the tails are hydrophobic, meaning they are repelled by water molecules. In the phospholipid bilayer, the head interacts with the aqueous environment, and the tails interact with each other and form the interior of the bilayer.

Evaluate each option

a. at the surfaces of the membrane: This statement refers to the head of the phospholipid, not the tails. b. in the interior of the membrane: This statement seems to be correct, as the tails form the interior of the bilayer due to their hydrophobic nature. c. spanning the membrane: This statement is inaccurate because it is the entire phospholipid molecule that spans the membrane, not just the tails. d. where the environment is hydrophobic: This statement appears to be accurate because the tails of the phospholipid are hydrophobic and will be found in an environment that is hydrophobic. e. Both b and d are correct: As both statements b and d seem to be accurate, this option might be the correct answer.

Choose the correct answer

Based on the evaluation of all the options, it is concluded that option e (Both b and d are correct) is the correct answer, because the tails of the phospholipid are found in the interior of the membrane and in a hydrophobic environment.

Key concepts

Cell Membrane Structure

The cell membrane, often referred to as the plasma membrane, serves as a vital border that separates the internal components of a cell from its external environment. It plays a crucial role in protecting the cell, facilitating the transport of materials in and out, and maintaining homeostasis within the cellular environment.

Structurally, the membrane is primarily composed of a phospholipid bilayer which is essentially a double layer of phospholipids. Each phospholipid is made up of a glycerol backbone, two fatty acid tails, which are hydrophobic (repelled by water), and a phosphate group head, which is hydrophilic (attracted to water). The bilayer forms with the hydrophobic tails facing inward, shielded from water, while the hydrophilic heads face outward, interacting with the aqueous environment on both sides of the membrane.

Embedded within this membranous structure are various proteins, cholesterol, and carbohydrates that support a plenitude of functions, such as signal transduction, cell recognition, and structural support. These additional elements contribute to the selective permeability of the cell membrane, allowing it to regulate the passage of substances and respond to various stimuli.

Hydrophilic and Hydrophobic

Understanding the terms 'hydrophilic' and 'hydrophobic' is essential for comprehending how biological membranes function. 'Hydrophilic' refers to molecules or portions of molecules that are attracted to water, typically due to the presence of polar or ionic groups. These groups can form hydrogen bonds with water, making them soluble in aqueous environments.

On the other hand, 'hydrophobic' pertains to molecules or portions of molecules that repel water. These are generally nonpolar and cannot form hydrogen bonds with water, leading to their insolubility in aqueous environments. The fatty acid tails of phospholipids are a classic example of hydrophobic structures. They tend to aggregate and form structures such as micelles or bilayers to minimize their exposure to water.

The hydrophilic and hydrophobic properties are crucial for the formation and function of the cell membrane as they drive the self-assembly of phospholipids into a bilayer structure, thus creating a barrier that protects the cell while allowing for selective permeability.

Lipid Bilayer Interaction

The lipid bilayer interaction refers to how the individual phospholipids in the membrane arrange themselves and interact with each other and their environment. The most crucial aspect of this interaction is the hydrophobic effect, which encourages the hydrophobic tails to avoid water and consequently drives the formation of the bilayer with hydrophobic tails inward.

Within the bilayer, the phospholipids are not static; they are able to move laterally, allowing the membrane to be fluid and flexible, which is essential for the functioning of the cell. This fluidity is influenced by the types of fatty acids in the phospholipids and the presence of cholesterol. Saturated fatty acids, which have no double bonds, result in a more rigid membrane, while unsaturated fatty acids, containing one or more double bonds, lead to increased fluidity.

The bilayer's structure provides a matrix for proteins and other molecules to reside in or pass through, enabling the interactions necessary for cell communication, signaling, and transport. These mechanisms are closely regulated and vital for the survival of the cell.