17 The Plasma Membrane and the Cytoplasm

At this point, it should be clear that eukaryotic cells have a more complex structure than do prokaryotic cells. Organelles allow for various functions to occur in the cell at the same time. Before discussing the functions of organelles within a eukaryotic cell, let us first examine two important components of all cells (prokaryotic and eukaryotic): the plasma membrane and the cytoplasm.

a green hot-dog shaped bacteria. On the outside short squiggly lines pointing outward represent pili and a longer squiggly green line represents the flagellum. A thick light green layer labeled capsule is on on the outside of the cell under the pili. A thinner green area inside the capsule is labeled cell wall. A very thin green layer inside the cell wall is labeled cell membrane. Filling the inside of the cell is light blue region. Inside the light blue is a squiggly brown area labeled nucleoid region and chromosome (DNA). Small brown dots floating in the light blue area are labeled ribosomes.
Diagram of an animal cell.
Figure 2 This figure shows a typical animal cell
Diagram of a typical plant cell
Figure 3 This figure shows a typical plant cell.

The Plasma Membrane

Like prokaryotes, eukaryotic cells have a plasma membrane (Find it in Figures 1-3, then look at the detailed structure in Figure 4) made up of a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule composed of two fatty acid chains, a glycerol backbone, and a phosphate group. The plasma membrane regulates the passage of some substances, such as organic molecules, ions, and water, preventing the passage of some to maintain internal conditions, while actively bringing in or removing others. Other compounds move passively across the membrane.

A representation of the cell membrane. A phospholipid bilayer is illustrated as brown circles each connected to two yellow lines. two layers of these are facing each other. proteins are shown as blue blobs going partway or all the way through the membrane. some have channels through them, some are solid. branched chains of green polygons represent carbohydrates connected to lipids to form glycoplipids and connected to proteins to form glycoproteins.
Figure  4 The plasma membrane is a phospholipid bilayer with embedded proteins. There are other components, such as cholesterol and carbohydrates, which can be found in the membrane in addition to phospholipids and protein.

The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli(singular = microvillus). This folding increases the surface area of the plasma membrane. Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food (Figure 5). This is an excellent example of form matching the function of a structure.

electron micrograph and cartoon of microvilli. The micrograph is black and white and shows tendrils reaching upward. The diagram shows cells with purple nuclei at the bottom. At the top, tendrils are labeled microvilli and "side of cell facing inside of small intestine".
Figure 5 Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit “micrograph”: modification of work by Louisa Howard)

The Cytoplasm

The cytoplasm comprises the contents of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). It is made up of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals (Find it in Figures 1-3). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm. Take note that the cytoplasm is not “empty” or “filler” – it is a vitally important component of cells that allows chemical reactions to take place!


Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10


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