16: Membrane Structure

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16.1: Overview
The plasma membrane has the same phospholipid bilayer construction as all intracellular membranes. All membranes are a fluid mosaic of proteins attached to or embedded in the phospholipid bilayer. The different proteins and to some extent, different phospholipids, structurally and functionally differentiate one kind of cellular membrane from another. Integral (trans-membrane) proteins span the phospholipid lipid bilayer, with one hydrophobic domain and two hydrophilic domains.
16.2: Plasma Membrane Structure
In eukaryotic cells, the plasma membrane surrounds a cytoplasm filled with ribosomes and organelles. Organelles are structures that are themselves encased in membranes. Some organelles (nuclei, mitochondria, chloroplasts) are even surrounded by double membranes. All cellular membranes are composed of two layers of phospholipids embedded with proteins. All are selectively permeable (semi-permeable), allowing only certain substances to cross the membrane.
16.3: Membrane Proteins
Clearly, membrane proteins themselves have domains that keep membranes in or attached to the membrane, provide catalytic surfaces and allow interactions inside, across and outside of cells and organelles. Membranes anchor proteins in several ways. As noted, membrane proteins, like phospholipids, are amphipathic, with hydrophobic domains that non-covalently interact strongly with the fatty acid interior of membranes. Some integral membrane proteins span the entire membrane, with hydrophilic domai
16.4: How Membrane Proteins are Held in Membranes
The hydrophobic domain of integral membrane proteins consists of one or more alphahelical regions that interact with the hydrophobic interior of the membranes. Hydrophilic domains tend to have more tertiary structure with hydrophilic surfaces, and so face the aqueous cytosol and cell exterior. Two trans-membrane proteins are cartooned below.
16.5: A Diversity of Membrane Protein Functions
Transmembrane proteins perform most of the functions illustrated here. However, peripheral membrane proteins also play vital roles in membrane function. Remember that Cytochrome c in the electron transport system on the mitochondrial cristal membrane is a peripheral protein. Other peripheral membrane proteins may serve to regulate the transport or signaling activities of transmembrane protein complexes or may mediate connections between the membrane and cytoskeletal elements.
16.6: Glycoproteins
Membrane proteins are often covalently linked to oligosaccharides, which are branched glycoside-linked sugars (averaging around 15 sugar residues). As glycans, they are the sugars linked to glycoproteins. Glycoproteins are rare in the cytosol, but common on secreted and membrane proteins. Oligosaccharides are typically linked to proteins via the hydroxyl group on serine or threonine. Occasional linkages are to modified amino acids like hydroxylysine or hydroxyproline (O-glycosylation), and to th
16.7: Glycolipids
Glycolipids are phospholipids attached to oligosaccharides, and as noted, are part of the glycocalyx. Both are only found on the extracellular surface. Glycolipids are synthesized in much the same way as glycoproteins. Specific enzymes catalyze initial glycosylation of either phospholipids or polypeptides, followed by the addition of more sugars. Along with glycoproteins, glycolipids play roles in cell-cell recognition and the formation of tissues. The glycans on the surfaces of one cell will re
16.8: Glycoproteins and Human Health
We’ll close this chapter with a few examples of glycoproteins that play crucial roles in human physiology. Let’s look first at the major human blood groups. The major A, B, AB, O and Rh blood groups result from the presence or absence of glycoprotein antigens embedded in red blood cell membranes and the presence or absence in the blood, of antibodies against the antigens.
16.9: Key Words and Terms

Thumbnail: The cell membrane, also called the plasma membrane or plasmalemma, is a semipermeable lipid bilayer common to all living cells. It contains a variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes. It also serves as the attachment point for both the intracellular cytoskeleton and, if present, the cell wall. (Public Domain; LadyofHats via Wikipedia)