![]() There is also commonality in the components of the membranes, though the relative amount of constituents varies. Though some cells do not have cell walls (animal cells) and others do (bacteria, fungi, and plants), there is commonality among cells in that they all possess plasma membranes. Figure 3.1: Lipid bilayer closeup Image by Aleia Kim Cell walls will be covered near the end of this chapter. Plasma membranes differ from cell walls both in the materials comprising them and in their flexibility. Before we discuss movement of materials across membranes, it is appropriate we discuss the composition of cellular membranes. This is referred to as facilitated diffusion. In other cases, no external energy is required and they move by diffusion through specific cellular channels. In some cases, energy is required to move the substances (active transport). The orderly movement of these compounds is critical for the cell to be able to 1) get food for energy 2) export materials 3) maintain osmotic balance 4) create gradients for secondary transport 5) provide electromotive force for nerve signaling and 6) store energy in electrochemical gradients for ATP production (oxidative phosphorylation or photosynthesis). With the notable exceptions of water, carbon dioxide, carbon monoxide, and oxygen, most polar/ionic require transport proteins to help them to efficiently navigate across the bilayer. ![]() The last two of these will, when mixed vigorously with water, spontaneously form what is called a lipid bilayer (Figure 3.1), which serves as a protective boundary for the cell that is largely impermeable to the movement of most materials across it. The protective membrane around cells contains many components, including cholesterol, proteins, glycolipids, glycerophospholipids, and sphingolipids.
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