Structure of the Cell Membrane (AQA A-Level Biology): Revision Notes

Structure of the Cell Membrane

Basic membrane structure

All cellular membranes share the same fundamental architecture, whether they surround entire cells or form boundaries around internal organelles. These structures are collectively termed plasma membranes. The cell-surface membrane refers specifically to the plasma membrane that encases the cell, creating a selective barrier between the internal cytoplasm and the external environment.

This boundary serves multiple essential roles: it maintains distinct internal conditions, regulates the passage of materials, and enables cells to interact with their surroundings. Understanding the molecular components that create this sophisticated barrier is essential for explaining how cells maintain homeostasis and communicate.

Phospholipids

Phospholipids form the structural foundation of all cell membranes. Each phospholipid molecule contains a hydrophilic (water-attracting) phosphate head group attached to two hydrophobic (water-repelling) fatty acid tails. This dual nature drives their spontaneous arrangement into a phospholipid bilayer.

In this bilayer arrangement, the hydrophilic heads orient towards the aqueous environments on both sides of the membrane, whilst the hydrophobic tails cluster together in the membrane interior, away from water. This creates a continuous double layer that effectively separates the cell's interior from its external environment.

The phospholipid bilayer performs several vital functions:

• Permits lipid-soluble substances to pass through the membrane by dissolving in the fatty acid region
• Blocks water-soluble substances from crossing freely, maintaining distinct internal and external compositions
• Provides membrane flexibility and enables self-sealing properties, allowing the membrane to maintain integrity even when stretched or compressed

Membrane proteins

• Proteins embedded within the phospholipid bilayer dramatically expand the membrane's functional capabilities. These proteins integrate with the bilayer in two distinct patterns.
• Surface proteins associate with one side of the bilayer without spanning its entire width. These proteins typically provide structural support to the membrane or function as receptors for signalling molecules such as hormones and neurotransmitters.
• Transmembrane proteins extend completely across the bilayer from one surface to the other. This group includes protein channels that form water-filled passages, allowing specific water-soluble ions to traverse the membrane. Carrier proteins represent another crucial type - these bind to particular molecules like glucose or amino acids, then undergo conformational changes to transport these substances across the membrane.

Membrane proteins collectively serve six major functions:

1. Provide structural reinforcement to the membrane
2. Create channels for water-soluble substance transport
3. Enable active transport through carrier protein mechanisms
4. Form cell-surface receptors for cell identification and signalling
5. Facilitate cell adhesion by helping adjacent cells bind together
6. Act as hormone receptors for cellular communication

Cholesterol

Cholesterol molecules nestle within the phospholipid bilayer, adding mechanical strength and regulating membrane properties. These highly hydrophobic molecules play a stabilising role by interacting with phospholipid fatty acid chains, which affects membrane fluidity and integrity.

Cholesterol molecules position themselves between phospholipid molecules, where they serve three important functions:

1. Reduce lateral movement of phospholipids and other membrane components, providing structural stability
2. Decrease membrane fluidity at elevated temperatures, preventing excessive molecular motion
3. Prevent water and ion leakage by filling gaps between phospholipid molecules and maintaining membrane integrity

This regulation ensures the membrane maintains optimal fluidity across varying temperature conditions whilst preventing unwanted permeability.

Glycolipids

Glycolipids consist of carbohydrate groups covalently attached to lipid molecules. The carbohydrate portion projects from the phospholipid bilayer into the surrounding aqueous environment, where it serves as a recognition element for specific chemical interactions.

These molecules demonstrate particular importance in biological recognition systems. For example, the human ABO blood group system relies on specific glycolipids present on red blood cell surfaces to determine blood compatibility.

Glycolipids contribute to membrane function through three key roles:

1. Act as recognition sites for specific molecules or cells
2. Help maintain membrane stability under varying conditions
3. Facilitate cell-to-cell attachment and tissue formation

Glycoproteins

Glycoproteins form when carbohydrate chains attach to extrinsic proteins located on the membrane's outer surface. These hybrid molecules function primarily as highly specific cell-surface receptors, particularly for hormone and neurotransmitter recognition.

The carbohydrate portions of glycoproteins extend into the extracellular environment, creating unique molecular signatures that enable precise cellular communication and recognition.

Glycoproteins perform three essential functions:

1. Serve as recognition sites for intercellular communication
2. Enable cell attachment and tissue organisation
3. Allow cellular recognition, such as enabling immune cells to distinguish between the body's own cells and foreign organisms

Membrane permeability

The cell-surface membrane acts as a selectively permeable barrier, controlling which substances can enter or exit the cell. Most molecules cannot freely cross this barrier due to several physical and chemical constraints.

Many substances face barriers because they are:

• Not lipid-soluble, preventing passage through the phospholipid layer
• Too large to fit through existing protein channels
• Electrically charged in ways that prevent passage through available channels
• Polar molecules that cannot traverse the non-polar hydrophobic interior of the phospholipid bilayer

This selective permeability enables cells to maintain distinct internal compositions whilst allowing essential nutrients to enter and waste products to exit through specific transport mechanisms.

Fluid-mosaic model

The fluid-mosaic model describes how membrane components combine to create a dynamic, functional structure. This model explains the membrane's architecture using two key characteristics.

The fluid aspect refers to the ability of individual phospholipid molecules to move relative to each other within their layer. This molecular motion gives the membrane a flexible structure that constantly changes shape whilst maintaining its integrity.

The mosaic characteristic describes how various proteins embed within the phospholipid bilayer in different shapes, sizes, and positions, resembling tiles arranged in a mosaic pattern.

This model successfully explains the membrane's ability to maintain structural integrity whilst accommodating the diverse proteins necessary for cellular functions. The fluid nature allows proteins to move within the membrane when needed, whilst the mosaic arrangement provides the variety of functions required for cellular processes.