Question

In plasma membrane, the lipids have their polar heads facing (a) Outer side (b) Inner side (c) In the middle (d) Stable facing nowhere

Short answer

The lipids' polar heads in the plasma membrane are facing the outer side and the inner side.

Step-by-step solution

Understanding Plasma Membrane Configuration

The plasma membrane is a lipid bilayer, it consists of two layers of lipids where their hydrophilic 'head' regions are facing the aqueous 'outside' and 'inside' environments, and their hydrophobic 'tail' regions are facing each other, inside the plasma membrane.

Identifying Orientation of Lipids

From the membrane configuration, it can be concluded that the lipids' polar heads are not only facing one side, instead, they are facing towards both the outer and the inner aqueous environments.

Key concepts

Lipid Bilayer

The lipid bilayer forms the fundamental structure of the plasma membrane, playing a critical role in protecting and enclosing the cell's internal environment. Imagine it as a sandwich, where the 'bread' comprises hydrophilic (water-attracting) heads facing the watery surroundings of the cell, and the 'filling' consists of hydrophobic (water-repelling) tails that hide from water, facing each other.

In terms of its arrangement, the lipid bilayer is a double-layered sheet that creates a stable barrier between two aqueous compartments. Due to the contrasting properties of hydrophilic heads and hydrophobic tails, the tails gather together, avoiding water, while the heads seek it out. This organization is fundamental as it creates a selectively permeable membrane, only allowing specific substances to pass in and out of the cell, thus maintaining a distinct internal environment, or homeostasis.

Hydrophilic and Hydrophobic

Understanding the terms 'hydrophilic' and 'hydrophobic' is key to grasping how biomolecules interact with water.
'Hydrophilic' describes molecules or portions of molecules that are attracted to water and can form hydrogen bonds, leading to easy mixing or dissolution. This is characteristic of the polar heads of the lipids in the plasma membrane, which face the aqueous solutions inside and outside of the cell.

Contrastingly, 'hydrophobic' refers to the part of molecules that repel and do not mix well with water. The nonpolar lipid tails in the plasma membrane represent this as they tend to cluster together away from water, creating a barrier that certain substances cannot easily pass through. The interplay between these hydrophilic and hydrophobic properties is a fundamental concept in biology, especially when considering the formation of cell membranes and the movement of substances across them.

Cell Membrane Structure

The cell membrane has a complex structure that is pivotal for its function. In addition to the lipid bilayer, the membrane includes proteins, cholesterol, and carbohydrate molecules, which contribute to its fluid nature and functionality.

Integral and Peripheral Proteins

Integral proteins span the entire membrane and can act as channels, receptors or enzymes, while peripheral proteins are attached only to the surface, aiding in communication and connection to the cytoskeleton.

Cholesterol

Exists within the lipid bilayer, modulating membrane fluidity and stability across different temperatures.

Carbohydrates

Often bound to proteins or lipids on the outer surface, they form glycoproteins and glycolipids essential for cell recognition, signalling, and adhesion.

Together, these components ensure the cell membrane is not just a static barrier but a dynamic interface for various cellular processes, such as nutrient uptake, waste expulsion, and signal transduction.