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2.3  Chemicals that Make Cells Work

water, requires assistance to cross this barrier, through protein molecules integrated into

the membrane.

Cells often have a heterogeneous mixture of different phospholipids in their membrane.

Certain combinations of phospholipids can result in a phase transition behavior in which one

type of phospholipid appears to pool together in small microdomains surrounded by a sea

of another phospholipid type. These microdomains are often dynamic with a temperature-​

sensitive structure and have been referred to popularly as lipid rafts, with a range of effective

diameters from tens to several hundred nanometers, and may have a biological relevance as

transient zones of molecular confinement in the cell membrane.

FIGURE 2.3  Fats and nucleic acids. (a) Single phospholipid molecule. (b) Bilayer of phospholipids in

water. (c) Hydrogen-​bonded nucleotide base pairs. (d) B-​DNA double-​helical structure.