Plasma Membrane

Learning Goals

By the end of this lesson, you will be able to:

• Describe the role of the cell (plasma) membrane in a eukaryotic cell
• Describe the parts that make up a cell (plasma) membrane
• Explain the functions of the different parts of the cell (plasma) membrane

Key Vocabulary

• Plasma Membrane – (also known as the cell membrane) a thin, semi-permeable membrane that surrounds and encloses the contents of a cell, acting as a barrier to the outside environment.

• Phospholipid – a type of lipid molecule that is an essential component of cell membranes. They are composed of a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.

• Integral Proteins – proteins anchored within the plasma membrane 

• Channel proteins – Channel proteins are a type of membrane protein that form channels or pores in the cell membrane, allowing the passive diffusion of specific molecules or ions across the membrane

• Carrier proteins – a type of membrane protein that facilitates the movement of molecules across the cell membrane

Plasma Membrane

The cell membrane, also known as the plasma membrane, is a thin, flexible barrier that surrounds and encloses the contents of a cell. One of the primary functions of the plasma membrane is to control the passage of substances into and out of the cell. The membrane is selectively permeable, meaning it allows certain molecules, such as oxygen, carbon dioxide, and small hydrophobic molecules, to freely diffuse across it while restricting the passage of larger, polar, or charged molecules.

This selective permeability is due to the structure of the membrane, which is made of a bilayer of phospholipid molecules. Phospholipids are amphipathic, meaning they have both hydrophobic (water-repelling) and hydrophilic (water-loving) regions, which allows them to form a stable bilayer in an aqueous environment. The hydrophobic tails of the phospholipids face inward, while the hydrophilic heads face outward, creating a barrier that separates the inside of the cell from the outside environment.

Another important function of the cell membrane is to facilitate cell-to-cell communication and signaling. The membrane contains various receptors and proteins that allow the cell to detect and respond to external stimuli (hormones, neurotransmitters, pathogens, etc). These signals can trigger a series of molecular events within the cell that can result in changes in gene expression, cell growth, differentiation, or apoptosis. In addition, the membrane can form junctions with adjacent cells, which allow cells to adhere to each other and coordinate their activities, as in the case of tissues and organs.

The plasma membrane also plays a role in maintaining the structure of the cell. The membrane is anchored to the cytoskeleton, a network of protein filaments that provides support and shape to the cell. The membrane also contains various lipids and proteins that allow it to withstand mechanical stress and changes in temperature or pressure.

The plasma membrane can also actively transport molecules across the membrane, using energy from ATP or ion gradients, which is critical for various cellular processes such as nutrient uptake, waste removal, or signaling. Overall, the plasma membrane is a highly dynamic and complex structure that performs numerous essential functions in maintaining the proper functioning of the cell.

Quiz/CFU