MCAT Biology Review
Chapter 1: The Cell
The human body contains approximately 37 trillion cells. These cells create tissues from which organs form. Each cell serves a purpose, communicating and carrying out the reactions that make life possible. Interestingly, bacteria outnumber the eukaryotic cells in our bodies about ten to one. But the sheer number of cells from which the human body is created is not nearly as impressive as the numerous functions these cells can perform, from conduction of impulses through the nervous system, allowing for memory and learning, to the simultaneous contraction of cardiac myocytes to allow for the pumping of blood through the entire human body. In order to understand the human organism as a whole, and how the human body reacts to various pathogens, a thorough understanding of cell biology is required. It is not enough to simply memorize each part of the cell; the MCAT requires an understanding of how each cell structure carries out its functions and affects the entire organism.
1.1 Cell Theory
Prior to the 1600s, organisms were perceived as being complete and inseparable into smaller parts. This was due in part to the inability to see smaller structures through optical instruments like the microscope. In 1665, Robert Hooke assembled a crude compound microscope and tested its properties on a piece of cork. He noticed a honeycomb-like structure and compared the spaces within the cork to the small rooms of a monastery, known as cells. Because cork consists of desiccated nonliving cells, Hooke was not able to see nuclei, organelles, or cell membranes. In 1674, Anton van Leeuwenhoek was the first to view a living cell under a microscope. Later researchers noted that cells could be separated, and that each cell was a distinct structure. Further research indicated that tissues were made of cells, and the function of a tissue was dependent upon the function of the cells from which it is formed. Two centuries later, in 1850, Rudolph Virchow demonstrated that diseased cells could arise from normal cells in normal tissues.
Robert Hooke, who invented the first crude microscopes to look at cork, is also known for his characterization of springs. Hooke’s Law, F = –kx, describes the relationship between elastic force, the spring constant, and the displacement of a spring from equilibrium. While Hooke’s law does not appear on the official MCAT content lists, the related topic of elastic potential energy, , is testable content. This equation, as well as other forms of energy, is discussed in Chapter 2 of MCAT Physics and Math Review.
The original form of the cell theory consisted of three basic tenets:
· All living things are composed of cells.
· The cell is the basic functional unit of life.
· Cells arise only from preexisting cells.
Through advances of molecular biology, a fourth tenet has been added to the theory:
· Cells carry genetic information in the form of deoxyribonucleic acid (DNA). This genetic material is passed on from parent to daughter cell.
Cell theory has created an interesting dilemma with respect to viruses. Viruses are small structures that contain genetic material, but are unable to reproduce on their own. This violates the third and fourth tenets of the cell theory because virions can only replicate by invading other organisms and because they may contain ribonucleic acid (RNA) as their genetic information. Therefore, viruses, discussed later in this chapter, are not considered living organisms.
MCAT Concept Check 1.1:
Before you move on, assess your understanding of the material with this question.
1. What are the four fundamental tenets of the cell theory?