Transfer of Molecules Across the Plasma Membrane

Foodstuffs, signaling molecules, and waste products pass through the plasma membrane. Some of these enter and exit the cell via transport proteins. Others are taken in through a process called endocytosis. Exocytosis, which is the reverse of endocy-tosis, can be used to expel material.

Transport Proteins

The transport proteins of eukaryotic cells may function as either a channel or a carrier. Channels are pores in the membrane. These are so small that only specific ions can diffuse through. They allow ions to move with the concentration gradient; they do not create such a gradient. To control ion passage, the channel has a gate, which can be either opened or closed, depending on environmental conditions. Carriers are analogous to proteins in prokary-otic cells that mediate facilitated diffusion and active transport.

Cells of multicellular organisms can often take up nutrients by facilitated diffusion, because the nutrient concentration of surrounding environments can be controlled. For example, glucose levels in the blood are maintained at a concentration higher than in most tissues. Consequently, animal cells generally do not need to expend energy transporting glucose.

The active transport mechanisms of eukaryotic cells are structurally analogous to the major facilitator superfamily and ABC transporter systems found in prokaryotic cells. Some of these are of medical interest because they can eject drugs from the cell. For example, some strains of Plasmodium falciparum, the eukaryotic parasite that causes a deadly form of malaria, use

72 Chapter 3 Microscopy and Cell Structure

Table 3.7 Comparison of Prokaryotic and Eukaryotic Cell Structures/Functions

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