Biological membranes

Membrane functions

  1. Barrier that creates compartments

    1. Plasma membrane

    2. Organelle membranes: creates subcellular compartments where cell functions can be isolated 

  2. Selective Transport 

    1. Solutes must travel between compartments and with the exterior of the cell. 

  3. Communication

    1. Between cells (signaling and cell-cell interaction) and between organelles to allow cells to respond to changes in a coordinated way

  4. Scaffold for biochemical activities (energy transduction) 

Discuss how biological membranes can self-seal, be fluid, dynamic, semipermeable and adaptable and how these characteristics are essential to understand their biological role


“Biological membranes are PERFECT barriers: 

  1. Self-seal: can close the barrier back after being disturbed or perturbed 

  2. Fluid (viscosity) and deformable: they are NON-rigid structures

    1. TEMP: lipids move around more at higher temp

    2. Membrane composition

      1. Length of fatty acids: Longer fatty acids are less fluid

      2. Number of unsaturations in a fatty acid: higher number = more fluid 

      3. Presence of sterols - can't predict

  3. Adaptable: cells can change membrane composition using the same basic components. 

  4. Dynamic: their basic components can move inside 

  5. Semipermeable: they allow the transport of certain substances and not others

And all these emerge from the basic components that they are made of 

Membrane components

  1. Lipids 

    1. Phosphoglycerides

      1. Derivative of glycerol 3-phosphate

      2. Two long fatty acid chains (varies by amount of carbons and degrees of saturation) and a polar head group 

    2. Sphingolipids 

      1. Ceramide: sphingosine + fatty acid + amide bond

      2. Two main classes:

        1. Sphingomyelins: ceramide + phosphate polar head group

        2. Glycosphingolipids: ceramide + sugar

    3. Sterols 

      1. have a four-ring isoprenoid-based hydrocarbon with a hydroxyl group. This hydroxyl makes these lipids amphipathic as well but their polar region is much smaller

All are amphipathic

  1. Proteins 

    1. Transmembrane

    2. Anchored to membrane 

    3. Peripheral 

  2. Saccharides

Found covalently bound to proteins

Membrane Proteins

Transmembrane Protein:

  • Can cross from one side of the membrane to the other 

    • Single pass 

    • Multipass 

    • Beta Barrel (in bacteria or from bacterial origin)


What are the requirements for an alpha helix to cross completely the membrane? 

  • Will be in Non polar/hydophobic amino acids

  • The minimal amount of non polar amino acids is 20. Allows the protein to go through 4-5nm bilayer


Anchored Protein

  • Protein is lipid modified and this libid can anchor the protein to the membrane

  • Fatty acid, Cholesterol or GPI (glycosylphosphatidylinositol) is covalently bound to a Cys or to the C terminal group of the Protein. 



Peripheral protein: 

  • Associated with a transmembrane protein. NOT mediated by a covalent bound. Instead → Van der Waals, hydrogen bounds or electrostatic interactions are involved!  

    • Will be released from the membrane using high salt! 

Hydrophobicity plots 

  1. Hydrophobicity plots are used to display the distribution of polar (hydrophobic) and apolar (hydrophilic) residues along a protein sequence. The plots identify possible transmembrane regions in the primary sequence of a protein

  2. Anything above the line is hydrophobic, anything below the line is hydrophilic (one way to remember:think phallic being below the waist lmao)

  3. Any region that is 20+ AAs long is long enough to cross the whole membrane`


Fluid Mosaic model 

  1. Describe the fluid-mosaic model

    1. It is a model based on thermodynamics, microscopy, and biochemistry data obtained for many years by different scientists.

    2. Hydrophilic exterior and hydrophobic interior.

    3. Mosaic: it is composed of different elements all mixed with each other

    4. Fluid: the lipid bilayer can reseal, is dynamic, fluid, and semipermeable.

  1. Discuss how membrane composition differs between different organisms, different subcellular membranes, opposite leaflets or domains.

  1. Domains: region of a membrane that has different properties from the membrane around it.

    1. Membrane composition differs between concrete areas of the same leaflet, called domains. 

  2. Leaflet: the lipid bilayer consists of two leaflets. 

    1. Membrane composition differs between two leaflets of the same membrane. 

  3. Membrane composition differs between different organisms and organelles by a number of factors such as the number of fatty acids and amount of kinks or unsaturations present. 

    1. Eg: presence of Beta barrel proteins in Bacterial membrane