3.02:_Cell_Membranes

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<p class="lt-bio-3966">One universal feature of all cells is an outer limiting membrane called the <strong>plasma membrane</strong>. In addition, all <strong>eukaryotic cells</strong> contain elaborate systems of <strong>internal membranes</strong> which set up various membrane-enclosed compartments within the cell. Cell membranes are built from <strong>lipids</strong> and <strong>proteins</strong>.</p>

<span id="The_Plasma_Membrane"></span><span id="The_Plasma_Membrane-2"></span><h2 class="lt-bio-3966">The Plasma Membrane</h2>

<p class="lt-bio-3966">The plasma membrane serves as the interface between the machinery in the interior of the cell and the extracellular fluid (ECF) that bathes all cells. The lipids in the plasma membrane are chiefly phospholipids like phosphatidyl ethanolamine. Phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophobic; its polar head hydrophilic. As the plasma membrane faces watery solutions on both sides, its phospholipids accommodate this by forming a <strong>phospholipid bilayer</strong> with the hydrophobic tails facing each other. Substantial amounts of cholesterol are tucked within the hydrocarbon tails (not shown).</p>

<figure><img height="250" width="606" class="internal" alt="Labeled diagram of the plasma membrane showing the phospholipid bilayer, embedded proteins, cholesterol, carbohydrates, and cytoskeleton." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5326/877px-Cell_membrane_detailed_diagram_en.svg.png?revision=1" aria-describedby="remedy-description-978b17da829e8402" /><span id="remedy-description-978b17da829e8402" data-remedy-image-description="978b17da829e8402" style="display: block; white-space: pre-line;">A labeled cross-section diagram of the plasma membrane. The extracellular fluid lies above the membrane and the cytoplasm below it. The core structure is the phospholipid bilayer: two layers of phospholipid molecules with round hydrophilic heads facing the watery fluids on both sides and hydrophobic tails pointing toward each other in the membrane's interior; one phospholipid molecule is highlighted. Embedded in the bilayer are several blue proteins: a protein channel (transport protein) spanning the full membrane, an integral protein (globular protein), and an alpha-helix protein (integral protein) drawn as a coiled strand weaving back and forth through the bilayer. A surface protein sits on the cytoplasmic face, and a peripheral protein attaches beneath it. Yellow cholesterol molecules are tucked among the hydrophobic tails. Carbohydrate chains project from the outer surface into the extracellular fluid, attached either to proteins (glycoproteins) or to lipids (glycolipids, shown in green). Pale filaments of the cytoskeleton lie under the membrane in the cytoplasm, contacting the inner-face proteins. Labels with pointer lines identify each component.</span><figcaption>Figure 3.2.1: A detailed diagram of the cell membrane </figcaption></figure>

<span id="Integral_Membrane_Proteins"></span><span id="Integral_Membrane_Proteins-2"></span><h3 class="lt-bio-3966">Integral Membrane Proteins</h3>

<p class="lt-bio-3966">Many of the proteins associated with the plasma membrane are tightly bound to it. Some are attached to lipids in the bilayer. In others &mdash; the <strong>transmembrane proteins</strong> &mdash; the polypeptide chain actually traverses the lipid bilayer (Figure 3.2.2). The figure shows a transmembrane protein that passes just once through the bilayer and another that passes through it 7 times. All G-protein-coupled receptors (e.g., receptors of peptide hormones, and odors) each span the plasma membrane 7 times.</p>

<figure><img height="180" width="375" class="internal" alt="Schematic of transmembrane proteins: single-pass and three-pass alpha helices (A) and a beta-barrel (B) crossing a lipid bilayer." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5324/1280px-IntegralMP.png?revision=1" aria-describedby="remedy-description-fe1c72e4431c29d8" /><span id="remedy-description-fe1c72e4431c29d8" data-remedy-image-description="fe1c72e4431c29d8" style="display: block; white-space: pre-line;">Figure 3.2.2, schematic representation of transmembrane proteins, shown in two panels against a horizontal yellow band representing the lipid bilayer. Panel A (labeled 'A' at top left) shows alpha-helical membrane proteins drawn as vertical pink cylinders. On the left is a bitopic protein with a single pink helix spanning the bilayer once; a gray squiggly polypeptide chain extends from its top (one side of the membrane) and a longer coiled gray chain extends from its bottom (other side). To its right is a polytopic protein with three pink helices spanning the bilayer, joined by gray loop segments; short gray chain ends protrude above the top helix and below the bottom helix, with a loop connecting the first and second helices on the upper side and a loop connecting the second and third on the lower side. Panel B (labeled 'B' at top) shows a polytopic beta-sheet (beta-barrel) protein: tilted, elongated strands in alternating dark blue and pale green cross the yellow bilayer at an angle, arranged side by side to form a barrel. Thin gray polypeptide loops connect adjacent strands and project from both the top and bottom surfaces of the membrane. The figure illustrates that membrane-spanning regions are either alpha helices (A) or beta sheets (B).</span><figcaption>Figure 3.2.2: Schematic representation of transmembrane proteins: 1. a single transmembrane &alpha;-helix (bitopic membrane protein) 2. a polytopic transmembrane &alpha;-helical protein 3. a polytopic transmembrane &beta;-sheet protein </figcaption></figure>

<p class="lt-bio-3966">In all these cases, the portion within the lipid bilayer consists primarily of hydrophobic amino acids. These are usually arranged in an alpha helix so that the polar -C=O and -NH groups at the peptide bonds can interact with each other rather than with their hydrophobic surroundings. Those portions of the polypeptide that project out from the bilayer tend to have a high percentage of hydrophilic amino acids. Furthermore, those that project into the aqueous surroundings of the cell are usually glycoproteins, with many hydrophilic sugar residues attached to the part of the polypeptide exposed at the surface of the cell. Some transmembrane proteins that span the bilayer several times form a hydrophilic channel through which certain ions and molecules can enter (or leave) the cell.</p>

<span id="Peripheral_Membrane_Proteins"></span><span id="Peripheral_Membrane_Proteins-2"></span><h3 class="lt-bio-3966">Peripheral Membrane Proteins</h3>

<p class="lt-bio-3966">These are more loosely associated with the membrane. They are usually attached noncovalently to the protruding portions of integral membrane proteins (Figure 3.2.3). Membrane proteins are often restricted in their movements. A lipid bilayer is really a film of oil. Thus we might expect that structures immersed in it would be relatively free to float about. For some membrane proteins, this is the case. For others, however, their mobility is limited:</p>

<ul>
    <li class="lt-bio-3966">Some of the proteins exposed at the interior face of the plasma membrane are tethered to cytoskeletal elements like actin microfilaments.</li>
    <li class="lt-bio-3966">Some proteins are the exterior face of the plasma membrane are anchored to components of the extracellular matrix like collagen.</li>
    <li class="lt-bio-3966">Integral membrane proteins cannot pass through the tight junctions found between some kinds of cells (e.g., epithelial cells).</li>
</ul>

<figure><img height="191" width="437" class="internal" alt="Four ways proteins associate with one surface of a lipid bilayer." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5325/1280px-PeriferalMP.png?revision=1" aria-describedby="remedy-description-f22b3ea9365c1226" /><span id="remedy-description-f22b3ea9365c1226" data-remedy-image-description="f22b3ea9365c1226" style="display: block; white-space: pre-line;">The yellow horizontal band represents the membrane. A: an orange segment lies parallel to its upper surface, with the rest of the chain above it. B: red loops dip into the upper portion of the membrane; two gray structural segments remain above the surface. C: a green lipid-like anchor extends into the membrane from a protein chain below. D: a protein chain remains below the membrane, contacting its lower surface at four red points. None of the illustrated chains crosses the entire yellow band.</span><figcaption>Figure 3.2.3: Schematic representation of peripheral membrane proteins </figcaption></figure>

<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/03:_The_Cellular_Basis_of_Life/3.02:_Cell_Membranes">3.2: Cell Membranes</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0 (creativecommons.org)</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball (biology-pages.info)</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content (biology-pages.info)</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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