ABSTRACT
The plasma membrane is a selectively permeable barrier. Some molecules (water, gases, urea) can pass directly through the bilayer unaided, whereas other molecules (sugars, amino acids, ions) require the presence of integral membrane transport proteins. A pure phospholipid bilayer is permeable to water, gases and permeability small uncharged polar molecules, but is impermeable to large uncharged polar molecules, ions and charged polar molecules. The passive transport of molecules across a membrane does not require an input of metabolic energy. Only relatively small uncharged or hydrophobic molecules cross the lipid bilayer by simple diffusion. An example of facilitated diffusion is the uptake of glucose into erythrocytes by the glucose transporter. The active transport of molecules requires an input of metabolic energy. Active transport of a molecule across a membrane against its concentration gradient requires an input of metabolic energy. D-Mannose and D-galactose, which differ from D-glucose in the configuration at one carbon atom, are transported at intermediate rates.
