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How do polar molecules pass through cell membranes?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

Because the cell membrane consists of a nonpolar lipid bilayer, it acts as a barrier that repels many polar, water-soluble molecules and charged ions. To cross this barrier, polar molecules utilize specialized transport systems facilitated by membrane proteins.123

These transport mechanisms include:

Channel Proteins: These form water-filled pathways that allow specific small polar molecules and ions to diffuse directly through the membrane.123
Facilitated Diffusion: Carrier proteins bind to specific polar molecules and help them diffuse across the membrane down their concentration gradient without the use of cellular energy.23
Active Transport: When molecules must move against a concentration gradient, specialized protein "pumps" use energy—typically in the form of ATP—to force them through the membrane.23
Vesicular Transport: For very large polar molecules or particles, the cell membrane can engulf the substance through processes like endocytosis or pinocytosis, bringing them into the cell within membrane-bound vesicles.23