Answer
True.
Plasma membrane proteins play important roles in characterizing the functional capabilities of a cell:
Proteins help determine the selective permeability of the membrane for certain chemical species such as ions and large uncharged polar particles. That is, some of the integral membrane proteins serve as ion channels and as transporters or carriers. The membrane lipid bilayer allows some chemical species such as oxygen, carbon dioxide and steroids to diffuse through freely by simple diffusion, but it retards the passage of ions and large uncharged polar particles such as potassium, calcium, chlorine (ions) and glucose, fructose, galactose (molecules). Diffusion of these latter moities is facilitated by ion channel proteins, and transporter/carrier proteins, respectively; these proteins that assist these solutes across membranes through the passive processes of facilitated diffusion.
Membrane proteins can also function as receptors, enzymes, as tags (self identifiers) Receptor proteins integral to cell membranes can bind with hormones ( e.g. ADH) to alter the activity of cells e.g. kidney tubles. In the GI tract epithelial cells of the small intestines secrete several digestive hormones which aid in the hydrolysis of nutrient substances. A case in point is the secretion of lactase by small intestine epilthelium. Lactase splits lactose into glucose and galactose, making glucose available for ATP generation. Certain proteins eg. glycoproteins are important in the expression of "self" in cells. These markers distinguish the cells of one individual from those of others, except a twin. Thus major histocompatibility complex proteins (MHC) are very important in cell and tissue transplant medicine and in autoimmune diseases.
With regard to determination of membrane selective permeability to chemical moities proteins do play vital roles. but the role of the lipid bilayer in this context is also very significant. While small hydrophobic molecules can easily diffuse through the lipid bilayer, ions and large polar molecules cannot. go through without help; The aid of integral membrane proteins is required to help the latter chemical species across the membrane. This aid takes two forms. Some membrane proteins act as gated channels which may or may not create channels or pores to allow certain substances across, for example, water(H2O) and hydrophilic molecules. In carrier-mediated facilitated transport, hydrophilic molecules bind to carrier(transport) proteins; the latter picks up molecules on one side of the membrane, carries them through, and releases them on the other side. In active transport, ATP is required to maintain the Na/K+ ATPAse membrane pump that maintains the electrochemical gradient involved in active cross-membrane transport.
Work Step by Step
Membrane proteins are important factors in controlling cell functions and capacities. With respect to plasma membrane permeability, however, the hydrophobic nature of the membrane lipid bilayer is probably the most important determinant of selectivity. Because of this hydrophobic nonpolar interior, nonpolar hydrophobic chemical species pass through easily by simple diffusion. But large, charged particles need assistance to get through the plasma membrane. This is provided by proteins as gated channels, transporter/carriers in facilitated diffusion and as membrane pumps in active transport.
