Read the given passage carefully and answer the following question.

The human body is made up of millions of cells - little building subunits (1) of life. Each cell contains many functional blocks (2) (organelles) that enable its proper functioning and is protected from the external environment by a cell membrane. This article is dedicated to providing a short introduction to the basic functions and anatomy of a cell membrane.
 
Cell membranes protect and organize cells. Most importantly they serve as barriers, discriminating the cell’s interior from the outer milieu. Because cells always exist in an aqueous energy (3) their membranes should be structured in such a way that they do not dissolve in water. This function is ideally carried by special chemical molecules - phospholipids. These molecules are constructed from two parts: tails made up of two molecules of fat that ‘avoid’ water and heads that have an affinity for water. For this specific behaviour, the phospholipid’s tails are called hydrophobic (‘hydro’ means water and ‘phobia’ means fear) and their heads are called hydrophilic (‘philos’ means love). When phospholipids are added to water, they self-assemble into double-layered structures, shielding their hydrophobic portions from water and exposing their hydrophilic portions to the environment. This phospholipid bilayer may resemble a sandwich, where phospholipid heads are bread rolls and tails are the sandwich filling.
 
In addition to lipids, membranes are loaded with proteins. They usually go through the lipid bilayer and are exposed to both the aqueous environment and the cell's interior. In fact, proteins account for roughly half the mass of most cellular membranes. They make the membrane semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly. Small molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly. On the other hand, cell membranes restrict the diffusion of charged molecules, such as ions, and large molecules, such as sugars and amino acids. The passage of these molecules relies on specific transport proteins embedded in the membrane.

Membrane transport proteins are specific and selective for the molecules they move, and they often use energy to enhance passage. Also, these proteins transport some nutrients against the concentration gradient, which requires additional materials (4). The ability to maintain concentration gradients and sometimes move the environment (5) against them is vital to cell health and maintenance. Thanks to membrane barriers and transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment and, conversely, dispose of waste products. Other membrane-embedded proteins perform communication-related jobs. Large molecules from the extracellular environment, such as hormones or immune mediators, bind to the receptor proteins on the cell membrane. Such binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules. Like transport proteins, receptor proteins are specific and selective for the molecules they bind.