Chapter 5: Membranes

Membrane Structure and Function

  • Membrane Composition: Composed of phospholipids, proteins, and carbohydrates.

  • Fluid Mosaic Model: Supported by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM).

Phospholipids: The Foundation of Membranes

  • Components of Phospholipids:

    • Glycerol Phospholipids

    • Single lipids

    • Sterols (e.g., Cholesterol)

  • Spontaneous Formation: Membranes can form spontaneously due to the amphipathic nature of phospholipids, arranging themselves into a bilayer.

  • Diversity: Membrane compositions differ in various organisms and cell types.

Experimental Evidence of Membrane Fluidity

  • Scientific Hypothesis: The plasma membrane is fluid and non-rigid.

  • Prediction: Membrane proteins will diffuse laterally if the membrane is fluid.

  • Experiment:

    • Fuse mouse and human cells and label specific proteins to observe diffusion over time.

    • Result: Hybrid cells show increased mixing of proteins, suggesting lateral diffusion of membrane proteins.

    • Conclusion: At least some membrane proteins can diffuse laterally, indicating fluidity.

Domain Structure within Membranes

  • Substructure of Cellular Membranes: Initially thought to be uniform, evidence indicates the presence of domains with distinct lipid and protein compositions.

    • Example: Epithelial cells have different apical and basal membrane compositions.

  • Lipid Rafts: Microdomains enriched in cholesterol and sphingolipids that interact to create ordered structures.

  • Membrane Fluidity:

    • Similar to triglycerides, membrane fluidity can varying with lipid composition.

    • Saturated or monounsaturated glycerol phospholipids decrease fluidity, making membranes less fluid-like.

Implications and Future Research

  • Further Experiments: Consider alternative explanations such as the insertion of newly synthesized proteins during the experiment. Test ideas to rule out other explanations to reinforce findings.