1.2What are Organisms?
Organisms have the following characteristics:
- They are made of units known as cells, each of which is surrounded by a phospholipid bilayer.
- They self-replicate through DNA, a genetic material.
- They respond to stimuli from the surrounding environment.
- They synthesize adenosine triphosphate (ATP), an energetic material, and use the energy produced for life and growth.
Let’s look at these characteristics more closely.
Organisms and Cells
The fact that all organisms are made of cells, the minimum unit of life, is of enormous importance. It is believed that the first organisms to emerge on earth some 3.8 billion years ago were anaerobic unicellular organisms, which depended on organic compounds in the ocean rather than oxygen. Then came photosynthetic bacteria and cyanobacteria, which were capable of synthesizing organic matter from carbon dioxide, thus gradually increasing the amount of oxygen in the air. This oxygen was gradually transformed into ozone in the stratosphere 10 to 50 km above the ground by ultraviolet rays. The ozone layer blocked these harmful ultraviolet rays, thus allowing the emergence of organisms on the planet. The first organisms found on the ground surface were plants, which began to grow approximately 400 million years ago. This marked the start of an explosive diversification of life on earth.
Take a moment to look around you. What you see are very complex and diverse biotic communities. An adult human is made of some 60 trillion cells. All organisms consist not of uniform cells but of specialized versions with many different functions. In other words, cell differentiation is needed for the creation of complex organisms (see Chapter 10). For groups of differentiated cells to be highly organized in a spatial sense and become an organism, another step known as cell-to-cell interaction is required (see Chapter 11). This mechanism is called hierarchization, and is essential for the creation of higher organisms.
Another characteristic of organisms is that they produce offspring that are visually similar to them. Unicellular cells procreate by asexual reproduction such as division (e.g., in paramecia) and budding (e.g., in baker’s yeast) (see Chapter 12), and the offspring cells have the same traits as the parent cells provided no mutation occurs in their DNA. On the other hand, multicellular organisms procreate by sexual reproduction, and the offspring inherit half their genes from each parent. Although different species do not result from this method of reproduction, any DNA mutation that occurs during the process of self-replication can be reflected in the traits of the offspring. This is called evolution. Mutation occurs in DNA bases at a certain frequency, meaning that evolution may be observed in the characteristics of reproduced organisms.
Response to Stimuli
One of the key characteristics of organisms is that they respond to stimuli from the surrounding environment (i.e., the outside world). The cell membrane has receptors - proteins that receive stimuli. When these receptors are stimulated by some external influence, a cascade of chemical reactions is triggered in the cytoplasm, leading to the synthesis of new proteins via DNA transcription. This chain reaction mechanism is called the signal transduction pathway (see Chapter 9). Various types of receptor are coded into the genes of all organisms, from E. coli to humans, and many such receptors are common to all organisms. This indicates that organisms on earth have evolved from the same protobionts.
ATP - Currency of Energy Transfer
The last characteristic of organisms discussed here is their capacity for metabolic activities (i.e., synthesis and degradation of substances) within their cells. During these processes, ATP (see Chapter 2) - an energetic material - is synthesized, the resulting energy is used to produce heat, and metabolic activities are retained (see Chapters 7 & Chapters 8).