Before, most biologists believe that life may spontaneously arise from any inanimate matter. Aristotle, a Greek philosopher, was one of the earliest recorded scholars to propose the spontaneous generation theory. This theory persisted in the 17th century, until Francesco Redi, an Italian scientist, disproved it by performing an experiment that refutes the idea that maggots arise spontaneously from meat. It was followed by different scientists who also conducted their own experiments to disprove this theory. Louis Pasteur, a French chemist, disproved this theory conclusively with his famous swan-necked flask experiment. He proposed that life can only come from preexisting life forms.

All living organisms, especially plants and animals, are composed of at least one cell. Most cells are not just visible to our naked eye, thus microscopes are deemed important to study them. Robert Hooke, a British scientist, was the first to use a simple microscope to examine a thin slice of oak tree bark called cork. He observed blocks of tiny packets that make up the cork and called them cells. Today, we studied that the cell is the smallest structure that can perform all activities required to sustain life. It carries out important functions such as metabolism, homeostasis, and reproduction.

Size of the Cell

Cells are generally small. Although they are found at the lower level in the hierarchy of the biological organization, life already exists in them. Most cells are far smaller than 1 mm, and some are even as small as 1 μm (as shown in Fig. 1.1.1). Subcellular structures and macromolecules that are smaller than a micrometer are measured in terms of nanometers. Because of this, the cell can only be viewed under the microscope to magnify its size in the field of view.


Fig. 1.1.1. The diversity of cell sizes ranges from the smallest bacterial cell to the largest avian egg. Note that there are some cells that can be seen with a human's naked eye.

Surface Area to Volume Ratio in Cells

The cell itself is a system. The exchange of nutrients and metabolic wastes happens through its surface. A cell needs a surface area large enough relative to its volume to allow adequate nutrients to enter and sufficient waste to be eliminated. Small cells are likely to have more available surface area for the movement of these molecules. Bigger cells, by contrast, have a larger volume relative to their surface area that it gets difficult for nutrients to diffuse to the center and the wastes to be eliminated. As the size of a cell increases, its volume also increases at a greater rate than its surface area, which then decreases its surface area to volume ratio. On the other hand, if the size of the cell is smaller, its volume decreases at a slower rate than its surface area, which then increases its surface area to volume ratio. The surface area of bigger cells becomes inadequate for the exchange of materials that their volume requires. Surface area to volume ratio is important that it favors a smaller cell size in terms of the efficiency of the movement of molecules. Having a large surface area to volume ratio is important to the functioning of the cells as most processes require molecules obtained from external sources and involve the production of wastes.


Fig. 1.1.2. The small size of cells provides them with a relatively higher ratio of surface area to volume. A living entity may maintain the same total volume, but having small cells will allow it to have a greater surface area available for the movement of molecules.

Shown in Fig. 1.1.2 is a comparison of the surface area to volume ratio of a small box (one-unit dimension) and a big box (five-unit dimension). The larger box is shown to have a smaller surface area to volume ratio compared with the smaller one. Despite its relatively larger volume, the bigger box would be deemed inefficient in terms of the movement of molecules because of the consequent decrease in the surface area. By contrast, the formation of smaller unit boxes from the same large box will maintain volume but significantly increase the total surface area. It should also be emphasized that an organism can still increase its total volume (as shown during growth and development) while maintaining a smaller cell size along the process.

General Functions of the Cell

The cell is a basic feature in any living organism, from the unicellular bacteria, protists, and yeast, to more complex multicellular forms such as plants and animals. It is the smallest unit that exhibits different attributes of life. In multicellular organisms, cells are more specialized—they are committed to performing particular functions (such as in Fig. 1.1.3) that contribute to the overall maintenance of the interacting systems in these organisms.


Fig. 1.1.3. Cells may devote themselves to specialized functions that will contribute to the survival of the organism.

Regulation of the Internal Environment

An organism’s ability to keep a constant internal state is called homeostasis. Homeostasis involves constant adjustments as the internal and external conditions of the cell continuously change. Maintenance of these conditions, usually at a normal or optimal level, is important because most cells of an organism require a specific set of conditions to function normally. If conditions go beyond a particular optimal range, some cells would cease to function properly.

As an example in humans, cells will only function normally at a constant internal temperature of 37 °C. During extremely cold weather, some cells, particularly the fibers of skeletal muscles, may be stimulated to contract involuntarily which results in shivering. This mechanism allows the body to generate heat (or thermogenesis) during cold weather to allow bodily chemical reactions to normally take place. On the other hand, perspiration involves water evaporation through the skin to cool down the body temperature during hot seasons. The sweat glands in the skin release water that covers the body and instantly serve as the cooling system to remove excess heat in the body. Homeostasis is exhibited in the attempt of the body to use its cells to return the temperature to a normal range.

Acquisition and Utilization of Energy

Cells acquire energy from the nutrients in food that organisms consume. This chemical energy is stored in the bonds present in food molecules, and it will be converted by the cells into more usable forms. Energy is needed by cells to drive most of the chemical reactions and other functions in the organism’s body. For example, energy is constantly needed for the heart muscles to continuously pump blood throughout our bodies. Energy is also needed in other bodily functions such as the breakdown of macromolecules during digestion, the contraction of skeletal muscles to initiate motion, and for the cells of the nervous system to conduct information. Cells, too, invest energy to release more energy from the food molecules they metabolize.

Responsiveness to Their Environment

The cell’s environment changes constantly and rapidly. To survive, cells also respond to various signals that indicate any form of change in their environment. These changes may include the shift in the activities of enzymatic molecules, chemicals that pass through the cell membrane, and signals to various membrane-transport processes. Responsiveness is related to homeostasis. A cell must first be able to determine the changes that have taken place before deciding the necessary responses that will ultimately result in the maintenance of normal internal conditions.

One classical example is the pigmented cells in the skin of humans. Whenever these cells are exposed to ultraviolet radiation from the sun, they synthesize and release more pigment to impart protection to the underlying cells especially UV radiation that can damage DNA.


A tanned skin means more pigment is released in that area to impart protection to the underlying cells especially UV radiation that can damage DNA.

Protection and Support

Cells protect and support their internal environment through their cellular membranes. The chemicals outside the cells could affect or influence normal cellular processes. Cells may form linings of organs to serve as the first line of defense from the external environment. In addition, some specialized cells, particularly immune cells in complex multicellular animals, also impart protection against pathogens and other foreign bodies that may enter the general circulation.

History of the Development of Cell Theory

Historically, the cell theory was proposed to disprove the spontaneous generation theory. It is now universally accepted even though it is still a theory. But how does cell theory develop from a mere idea to how it is being widely discussed and accepted today?

Table 1.1.1. Timeline of the history of the development of the cell theory

   


Principles of Cell Theory

During the 19th century, microscopes continued to improve helping scientists to observe the details of the nucleus and other subcellular structures. Matthias Schleiden, a German botanist states that the cell is the fundamental structure of life. Theodor Schwann, a German physician, proposed that all living organisms are made up of cells. They used their observations of many different plant and animal cells to formulate the cell theory which originally had two components. German physiologist, Rudolf Virchow, added the third component, which states that cells come from preexisting cells. Like any scientific theory, cell theory (summarized in Table 1.1.2) is potentially falsifiable, but many studies support each of its components, which makes it one of the most powerful ideas in biology.

Table 1.1.2. The three principles of cell theory


The Cell as a Common Feature among All Organisms

Unicellular organisms consist of only one cell but are already considered living organisms. Examples include bacteria, most protists, and yeasts. These unicellular organisms can function and regulate independently as living organisms. All of the cellular processes needed for their survival take place inside the cell, most of which are biochemical reactions that allow them to process the molecules they directly obtain from their environment to acquire energy.

Living organisms are divided into six kingdoms (but other classification systems can range from five to eight kingdoms). Kingdom Archaea consists of the archaebacteria. These prokaryotic cells thrive in extreme environments such as sulfuric lakes and hydrothermal vents. Bacterial species that usually cause diseases to humans belong to the Kingdom Eubacteria. Although not all bacteria are harmful, the kingdom Eubacteria also consists of good or nonpathogenic bacteria. Kingdom Protista consists of unicellular organisms that are animal like, plant-like, and fungus-like. These organisms do not have the characteristics of true animals, true plants, or true fungi. Kingdom Fungi consists of organisms such as mushrooms, molds, and mildews. Some mushrooms are harmful, but some are edible. Kingdom Plantae consists of plants, and Kingdom Animalia consists of animals, the members of which include complex multicellular organisms.


Organisms, from unicellular forms, such as in paramecium (left), to multicellular forms, such as in plants (right), are made of cells. The images shown above are microscopic views of these representative organisms.

The Cell as the Fundamental Unit of Life

In the hierarchy of biological organization, the cell is the basic level that exhibits all the important attributes of life. These attributes include metabolism, responsiveness, reproduction, energy processing, and homeostasis. It is at the level of the cell that important biochemical reactions take place to keep living organisms alive. Cells process molecules to release energy that can be used to fuel and drive other cellular processes. Cells work together (for multicellular organisms) to maintain balance among the different organ systems and to provide an optimal physical and chemical environment for cellular reactions to proceed. For more complex organisms, cells may be more specialized to perform particular functions.


Plants cells, such as in this microscopic view, perform various functions that contribute to the survival of the entire plant.

The Cell as a Product of Preexisting Cells

Louis Pasteur, who conclusively ended the long-believed theory of spontaneous generation, designed an experiment involving sterile nutrient broth, i.e., he tried to kill all microorganisms in it through heating. He tested whether microbes could arise from preexisting ones or if they would generate spontaneously. He had two setups in his experiment, each of which consisted of nutrient broth. He utilized the curve-necked flasks (or swan-necked flasks) and then boiled the broth to kill any existing microbe. After sterilizing, Pasteur broke off the neck of one of the flasks, which exposed the broth to the air and dust particles, while the other flask remained intact. Over time, the broth of the flask with a broken neck becomes cloudy and teeming with microorganisms. By contrast, the intact flask remained clear. In conclusion, Pasteur’s experiment proved that microorganisms, and living cells in general, cannot arise from nonliving matter such as dust particles.


Reproduction, being one of the major attributes of life, is exhibited at the cellular level through cell division.

Cells can only come from preexisting cells similar to how bacterial and yeast cells produce their daughter cells through binary fission. Another example is the fusion of an egg cell and a sperm cell to form a fertilized cell called zygote which will undergo further division to give rise to complex multicellular organisms such as humans.

Bacterial cells are unicellular organisms that can multiply rapidly through binary fission. During this mode of reproduction, the parent cell divides into two daughter cells. The daughter cells possess DNA identical to that of the parent cell. Some bacterial species are pathogenic, which means they can cause different diseases to other living organisms such as humans, animals, and plants. Because of this property, harmful bacteria can multiply rapidly in a certain environment if proper sanitation and disinfection will not be observed. To avoid diseases from spreading, it is important to practice proper sanitation and disinfection at all times.