The Human Organism

Organ Systems, Cells, Organelles, and Our Microbiota

Cells are the basic building blocks for all living organisms and provide support and optimal conditions for biochemical processes needed for cellular functions. This chapter mainly provides overviews of properties of living cells, cellular structures, and intracellular organelles in eukaryotic cells. Also, different types of cells and microbiota in human health and diseases are discussed.

Keywords

cell structures; cell functions; intracellular organelles; stem cells; microbiota

Key Points

Organ Systems: Integrated Function at the Highest Level

The highest level of integrated functionality in humans is the organ system; these systems have been structurally recognized for thousands of years but recognized for their functional importance for only a few hundred. The 11 organ systems of humans are as follows:

Organs are composed of cells that compartmentalize the lower-level functions that are the basis for organ function in living organisms. These are, in turn, composed of organelles that further compartmentalize the biochemical reactions and processes that are highly evolved and specialized to produce the chemical substances needed to create these structures and to extract energy to drive the chemical reactions and mechanical actions which the various organ systems provide. Normal health and disease diagnosis are related to the organ systems of humans, and thus the primary sections and chapters of this biochemical text are similarly organized.

Cells: Structures and Functions

The unifying principle of biology is that all living organisms from the smallest and least complex (bacteria) to the largest (whales) and most complex (humans) are composed of cells. The precise location of cells in the multicellular organisms and the location of intracellular organelles within cells are vital in normal development and function. During injury, wound repair, or morphogenesis, the precise location and migratory patterns of cells in multicellular organisms involve several strategies, which include establishment of gradient of small molecules, regulatory networks, and genetic diversity [1]. The membrane trafficking and metabolites to correct intracellular locations are precisely regulated. Defects in the membrane trafficking lead to pathological consequences [2].

In the simplest forms of life, such as bacteria, cellular organization and biochemical functions are relatively uncomplicated and are primarily devoted to growth and reproduction. As a consequence, bacteria have evolved to survive and thrive in the widest range of environments imaginable—soil, rivers and oceans, hot springs, and frozen land, as well as in most areas of the human body. The only regions of the body that are normally sterile are the respiratory tract below the vocal chords; the sinus and middle ear; the liver and gall bladder; the urinary tract above the urethra; bones; joints; muscles; blood; the linings around the lungs; and cerebrospinal fluid. Intestinal colon contains numerous microorganisms, and they are collectively known as microbiota. Humans and microorganisms have a symbiotic relationship. A population of normal healthy microbiota is essential for the maintenance of optimal host physiology by providing nutrients and energy balance. In germ-free mice, changes in the microbial flora have ameliorated obesity. Fecal flora from healthy persons has been used to re-establish normal microbiota in a recurrent Clostridium difficile enteric infection. Diet, the quality of the gut microbiota, and genetic makeup of the host all may play a role in producing a proatherosclerotic metabolite known as trimethylamine-N-oxide (TMAO) leading to cardiovascular disease. Recent studies have revealed that dietary sources of carnitine and phosphatidylcholine (lecithin) are converted to trimethylamine by gut microbes, which in turn enter the enterohepatic blood circulation followed by its conversion to TMAO by hepatic flavin-containing monooxygenases. Colon epithelial barrier damage can provoke infection and inflammation and may lead to a spectrum of diseases. (These aspects, along with required reading references, are discussed in Chapter 11).

All bacteria belong to the super kingdom called prokaryotes. Yeasts, molds, and protozoa are also single-celled organisms, but their cellular structures and functions are more complex than those of bacteria. These organisms belong to the other super kingdom called eukaryotes, along with all higher plants and all multicellular animals. A prokaryote cell has no true nucleus or specialized organelles in the cytoplasm. Bacteria reproduce asexually by cell division (fission). Because mitochondria (discussed later) have many properties in common with bacteria, it suggests that bacteria-like organisms were assimilated into eukaryotic cells early in their evolution.

All eukaryotic cells have a well-defined nucleus surrounded by a nuclear membrane and cytoplasm containing organelles that perform specialized functions. All eukaryotic somatic cells reproduce by the complex mechanisms of mitosis and cytokinesis. Germinal cells (sperm and ova) are formed by a slightly different mechanism called meiosis.

Although the size and complexity of eukaryotic organisms differ...