Most of the lipids in the plasma membrane are of a specific type known as phospholipids. A phospholipid molecule has a head region at one end that is hydrophilic—it can mix with water. At the other end are two long tails that are hydrophobic—they do not mix well with water. In the plasma membrane’s bilayer construction, phospholipid molecules are arranged so that their hydrophilic heads point outward on either side of the membrane, and their hydrophobic tails point toward each other in the middle of the membrane. This orientation keeps the hydrophobic tails away from the watery fluids that both fill and surround living cells. In fact, the plasma membrane stays intact precisely because the phospholipid molecules strongly resist any change in configuration that would expose their hydrophobic tails to the watery environment.
While the phospholipids are held in a bilayer, scientists believe the plasma membrane as a whole is a fluid structure because phospholipid molecules and some proteins can move sideways within the membrane. In one second, a single phospholipid molecule can travel the length of a large bacterial cell. Proteins drift more slowly through the membrane. With protein molecules scattered among the phospholipid molecules, the plasma membrane appears to be a mosaic of phospholipids and proteins. Some of the proteins are found on the inner or outer surface of the plasma membrane, while others span the membrane and protrude on either end. Scientists refer to this concept of the plasma membrane’s structure as the fluid mosaic model.
The plasma membrane forms an extremely effective seal around the cell. Only a very few molecules can pass directly through the lipid bilayer to get from one side of the membrane to the other. Many substances that a cell needs in order to survive cannot cross the lipid bilayer on their own, including glucose (a sugar that cells burn for energy), amino acids (the building blocks of proteins), and ions, such as sodium and potassium. A cell uses two methods to move such substances from one side of the plasma membrane to another, known as passive transport and active transport. Both of these processes involve proteins in the plasma membrane.
The most extensive organelle in the cell is the cytoskeleton, a web of protein filaments that branches extensively throughout the cytoplasm and gives the cell its shape. The cytoskeleton proteins, as well as other proteins in the cell, are made by tiny spherical organelles known as ribosomes. Several other important organelles are found in the cells. Among them are the lysosomes, membranous sacs storing enzymes that digest and recycle worn out cell parts; and the mitochondria, sacs where the cell's energy is generated. The endoplasmic reticulum, another organelle, is an extensive network of membrane folds and tubes that serves in part as the cell's factory floor where large molecules, such as lipids, are manufactured. These large molecules are sent to another organelle, the Golgi apparatus, which consists of layers of membranes where the molecules are modified, sorted, and packaged for transport.
The largest and most conspicuous organelle is the nucleus. The nucleus encloses and protects the cell’s genetic material, deoxyribonucleic acid (DNA), so that it is not damaged by biochemical reactions in the cell. Within the eukaryotic nucleus, DNA is wrapped around specialized proteins called histones, like a thread wound around a series of spools. Each DNA strand and its histones fold back and forth several times to form a compact, stick-shaped structure called a chromosome. Depending on the organism, the nucleus contains from one to over a thousand chromosomes. Surrounding the nucleus is the nuclear envelope, a membrane with numerous pores. The pores, ringed by special protein, regulate the flow of substances into and out of the nucleus.