B2.1.11 Relationships between fatty acid composition of lipid bilayers and their fluidity (HL) Notes

Cell Membranes Rely On The Balance of Saturated And Unsaturated Fatty Acids

1. The specific composition of these fatty acids directly impacts the flexibility, permeability, and stability of the membrane.

Saturated vs. Unsaturated Fatty Acids

Saturated Fatty Acids

1. Contain straight hydrocarbon chains that pack tightly together in the phospholipid bilayer.
2. Result in dense packing, which reduces membrane fluidity, flexibility, and permeability.
3. Found in membranes that need to remain more rigid and less permeable.
4. This rigidity can enhance the membrane's strength and stability at high temperatures but may limit its flexibility.

Unsaturated Fatty Acids

1. Feature one or more kinks in their hydrocarbon chains due to double bonds.
2. These kinks prevent tight packing, increasing membrane fluidity, flexibility, and permeability.
3. Found in membranes that require greater adaptability or are exposed to colder temperatures.
4. This fluidity is crucial for maintaining membrane function in colder environments.

Balancing Membrane Properties

1. Cells dynamically regulate the ratio of saturated to unsaturated fatty acids to ensure the membrane has the right balance of properties.
2. Temperature Dependence:
   1. At low temperatures, more unsaturated fatty acids are incorporated to prevent the membrane from becoming too rigid.
   2. At high temperatures, more saturated fatty acids are included to maintain structural integrity and prevent the membrane from becoming too fluid.

Other Components Affecting Membrane Fluidity

1. Cholesterol: Embedded in the hydrophobic core of the membrane, cholesterol plays a dual role:
   1. At low temperatures, it prevents the membrane from becoming too rigid.
   2. At high temperatures, it restricts excessive movement, stabilizing the membrane.
2. Proteins and Carbohydrates:
   1. Peripheral Proteins: Loosely attached to the membrane surface, aiding in signaling and structural support.
   2. Integral Proteins: Span the bilayer, forming channels and facilitating transport.
   3. Glycolipids and Glycoproteins: Project carbohydrates into the extracellular environment, playing roles in cell recognition and adhesion.

Membrane Fluidity in Action

1. Flexibility and Functionality
   1. Membranes must be flexible to allow processes such as vesicle formation and cellular movement.
   2. At the same time, they need to maintain enough strength to avoid perforation or collapse.
2. Selective Permeability
   1. Membranes regulate what enters and exits the cell, balancing permeability to ensure essential molecules pass through while keeping harmful substances out.