Plasma Membrane and Cytoplasm

The Plasma Membrane

Like prokaryotes, eukaryotic cells have a plasma membrane (see image below), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. Recall that a phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane.

The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol in it. Image Attribution: OpenStax Biology

The plasma membranes of cells that specialize in absorption fold into fingerlike projections which we refer to as microvilli (singular = microvillus). See the image below. Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function.

People with celiac disease have an immune response to gluten, which is a protein in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.

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. Image Attribution: "micrograph," modification of work by Louisa Howard

The Cytoplasm

The cytoplasm is the entire region of a cell between the plasma membrane and the nuclear envelope (a structure which we will discuss in the next lesson). It consists of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals (see image below).

Can you identify the cytoplasm here? Image Attribution: OpenStax Biology

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 you can find in the cytoplasm. You can find glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol 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.