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The left part of this illustration shows an integral membrane protein with a single alpha-helix that spans the membrane. The middle part shows a protein with several alpha-helices spanning the membrane. The right part shows a protein with two beta-sheets spanning the membrane.
Integral membrane proteins may have one or more alpha-helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3). (credit: “Foobar”/Wikimedia Commons)

OpenStax Biology 2e

Proteins comprise the plasma membranes’ second major component. Integral proteins, or integrins, as their name suggests, integrate completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the phospholipid bilayer’s hydrophobic region. Single-pass integral membrane proteins usually have a hydrophobic transmembrane segment that consists of 20–25 amino acids. Some span only part of the membrane—associating with a single layer—while others stretch from one side to the other, and are exposed on either side. Up to 12 single protein segments comprise some complex proteins, which are extensively folded and embedded in the membrane. This protein type has a hydrophilic region or regions, and one or several mildly hydrophobic regions. This arrangement of protein regions orients the protein alongside the phospholipids, with the protein’s hydrophobic region adjacent to the phosopholipids’ tails and the protein’s hydrophilic region or regions protruding from the membrane and in contact with the cytosol or extracellular fluid.

– What is the physical process by which a protein chain acquires its native 3-dimensional structure, a conformation that is usually biologically functional, in an expeditious and reproducible manner?

Peripheral proteins are on the membranes’ exterior and interior surfaces, attached either to integral proteins or to phospholipids. Peripheral proteins, along with integral proteins, may serve as enzymes, as structural attachments for the cytoskeleton’s fibers, or as part of the cell’s recognition sites. Scientists sometimes refer to these as “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens.

– What is a longer, continuous, and unbranched peptide chain of up to fifty amino acids.?

Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called proteases, but may also occur by intra-molecular digestion. Low pH or high temperatures can also cause proteolysis non-enzymatically.

Neuroscientists at MIT’s Picower Institute for Learning and Memory have found that a protein acts like a volume dial for the release of neurotransmitters, the chemicals that neurons release across connections called synapses to stimulate muscles or communicate with other neurons in brain circuits. The findings help explain how synapses work and could better inform understanding of some neurological disorders.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: