hormones of the steroid family. Indeed, any foreign chemical that is not strongly hydrophilic will readily pass into and out of the cell. In contrast, simple ions and charged molecules are strongly hydrophilic. They cannot dissolve in the hydrophobic interior of the membrane and therefore cannot cross membranes by simple diffusion (Figure 2.2). In this case, specialized proteins are present in our cells to facilitate their entry and exit. We will cover these in detail in Chapter 9.
The nucleus, mitochondrion, and chloroplast (in plants) are enclosed within an envelope consisting of two parallel membranes. These major cell organelles all contain the genetic material, DNA.
The Nucleus
The nucleus is often the most prominent cell organelle. It contains the genome, the cell's database, which is encoded in molecules of the nucleic acid, DNA. The nucleus is bounded by a nuclear envelope composed of two membranes separated by an intermembrane space (Figure 2.3). The inner membrane of the nuclear envelope is lined by the nuclear lamina, a meshwork of lamin proteins that provide rigidity to the nucleus and anchorage for the DNA. A two‐way traffic of proteins and nucleic acids between the nucleus and the cytoplasm passes through holes in the nuclear envelope called nuclear pores. The nucleus of a cell that is synthesizing proteins at a low level will have few nuclear pores. In cells that are undergoing active protein synthesis, however, virtually the whole nuclear surface is perforated.
IN DEPTH 2.1 WATER, WATER (AND AQUAPORINS) EVERYWHERE
Our bodies are ~70% water. Water can readily diffuse into and out of cells through the process of osmosis. It is thus essential that we maintain correct water balance otherwise our cells would distort or even lyse. Osmosis can be defined as the movement of water across a membrane down its concentration gradient from a solution of low osmolarity to a solution of high osmolarity. Osmolarity is calculated by summing the molar concentration of all the solutes in a particular solution. The more concentrated a solution is, the lower its water concentration and the higher its osmolarity.
For most mammalian fluids, the osmolarity is approximately 300 mOsm/l. Hypertonic solutions have an osmolarity that is higher, whereas hypotonic solutions have an osmolarity that is lower. If cells were placed in a hypertonic solution, water would move out of the cell from the cytosol where the osmolarity is lower than the bathing medium. Cells shrink under these conditions. Conversely, water would move into the cell if placed in hypotonic solutions because the osmolarity of the cytosol is now higher relative to the bathing medium. This can cause lysis. Changes in osmolarity can occur in pathological situations.
Water is a hydrophilic molecule, obviously! Yet it can cross the hydrophobic membrane relatively quickly. Why? The answer is that membranes have water channels known as aquaporins. They facilitate the diffusion of water across the membrane. Aquaporins are expressed in all cells but are particularly abundant in red blood cells and kidney tubules. Consequently, the plasma membranes of these cells are highly permeable to water.
Within the nucleus, it is usually possible to recognize discrete areas. Much of it is occupied by chromatin, a complex of DNA and certain DNA‐binding proteins such as histones (page 39). In most cells, it is possible to discern two types of chromatin. A central region of lightly staining euchromatin is that portion of the cell's DNA database that is being actively read out by being transcribed into RNA, another nucleic acid (Chapter 5). In contrast the peripheral, darkly staining heterochromatin is the inactive portion of the genome where no RNA synthesis is occurring. The DNA in heterochromatin is densely packed, leading to its dark appearance.
Example 2.1 DNA Destruction in the Cytosol
An animal cell's own DNA should remain in the nucleus, except for the tiny amount that is within mitochondria. DNA in the cytosol will likely belong to a pathogen such as an invading virus. Cells therefore contain active DNAses in the cytosol that rapidly destroy DNA, while leaving RNA intact. It is to evade this defense mechanism that many viruses use RNA as their genetic material, even though RNA is a much less stable molecule than is DNA.
Unlike DNA, RNA is also found in the cytoplasm associated with particles called ribosomes whose function is to make proteins. Ribosomes are made in the nucleus, in specialized regions called nucleoli that form at specific nucleolar organizer region sites on the DNA. These contain blocks of genes that code for the ribosomal RNA. Nuclear pores allow ribosomal subunits to exit the nucleus.
It should be stressed that the appearance of the nucleus we have described thus far relates to the cell in interphase, the period between successive rounds of cell division. As the cell enters mitosis (Chapter 14) the organization of the nucleus changes dramatically. The DNA becomes more and more tightly packed and is revealed as a number of separate rods called chromosomes, of which there are usually 46 in human cells. The nucleolus disperses, and the nuclear envelope fragments. Upon completion of mitosis, these structural rearrangements are reversed and the nucleus resumes its typical interphase organization.
Mitochondria
Like nuclei, mitochondria are encapsulated by an outer and inner membrane (Figure 2.4). Perhaps the most distinctive feature of mitochondria is that the inner membrane is markedly elaborated and folded to increase its surface area. These shelf‐like projections, named cristae, make mitochondria among the most easily recognizable organelles (e.g. Figure 1.4 on page 8). The number of cristae, like the number of mitochondria themselves, depends upon the energy budget