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Chapter 5

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Chapter 5

Membrane Structure, Synthesis and Transport

Key Concepts:

• Membrane Structure

• Fluidity of Membranes

• Synthesis of Membrane Components in Eukaryotic Cells

• Overview of Membrane Transport

• Transport Proteins

• Exocytosis and Endocytosis



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Membrane Structure

• The framework of the membrane is the phospholipid bilayer

• Phospholipids are amphipathic molecules Hydrophobic (water-fearing) region faces in

Hydrophilic (water-loving) region faces out

• Membranes also contain proteins and carbohydrates

• The two leaflets (halves of bilayer) are asymmetrical, with different amounts of each component



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Fluid-mosaic model

• Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules

• Membrane resembles a fluid because lipids and proteins can move relative to each other within the membrane



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Figure 5.1



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Proteins bound to membranes

Integral or intrinsic membrane proteins

Transmembrane proteins

• Region(s) are physically embedded in the hydrophobic portion of the phospholipid bilayer

Lipid-anchored proteins

• An amino acid of the protein is covalently attached to a lipid

Peripheral or extrinsic membrane proteins

Noncovalently bound either to integral membrane proteins that project out from the membrane, or to polar head groups of phospholipids



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Figure 5.2



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Approximately 20 to 30% of All Genes Encode Transmembrane Proteins

• Membranes are important medically as well as biologically

• Computer programs can be used to predict the number of transmembrane proteins

• Estimated percentage of membrane proteins is substantial: 20 to 30% of all genes may encode transmembrane proteins

• This trend is found throughout all domains of life including archaea, bacteria, and eukaryotes

• Function of many genes is unknown – study may provide better understanding and better treatments for disease



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Table 5.2

Table 5.2 Estimated Percentage of Genes That Encode Transmembrane Proteins*

Organism Percentage of protein-encoding genes that encode transmembrane proteins


Archaeoglobus fulgidus 24.2

Methanococcus jannaschii 20.4

Pyrococcus horikoshii 29.9

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