Biological Organisms

What is the meaning of biological organisms?

What are the characteristics shared by biological organisms?

Think of a plant, a fungus, an animal, or a bacteria. Biological organisms, or living beings, are so diverse that sometimes it is difficult to identify which characteristics define them. Do all these entities really share some basic traits? Let's look at the main characteristics biologists use to define a biological organism.

Order

Biological organisms are organized and coordinated structures made up of one or more cells, which are tiny structures we consider as the fundamental unit of life.

Each cell is incredibly complex: at the fundamental level, it is composed of atoms. These atoms make up molecules. These molecules come together to form complex compartmentalized cell structures called organelles.

Then, in multicellular organisms, multiple cells come together to form tissues, which then form structures with specialized functions called organs, which, in turn, work together in organ systems.

Response to stimuli

Organisms can respond by moving toward the stimulus; this is called a positive response. They can also respond by moving away from the stimulus; this is called a negative response.

Reproduction

Organisms can replicate themselves by passing on their genetic information to their offspring. By passing on their genetic information, the offspring will belong to the same species and have similar traits.

Growth and development

Organisms grow and develop, meaning their structures and functions change over time. This change is determined by a combination of the genetic information passed on to the individual organism as well as its environment.

Regulation

Organisms require multiple complex regulatory mechanisms to coordinate their internal processes, such as transporting nutrients and responding to stimuli.

Homeostasis

Organisms need to maintain homeostasis because their internal structures function optimally within a set of internal and external conditions.

Energy processing

Organisms need an energy source to carry out their metabolic processes. Some organisms might produce their own food by capturing energy from the sun and converting it to chemical energy, while other organisms might obtain energy by eating other organisms.

Do all biological organisms need oxygen? What are aerobic and anaerobic biological organisms?

Considering how we often hear that we need oxygen to live, you might think that all biological organisms need oxygen. However, for the first two billion years of the Earth’s existence, the atmosphere contained no free molecular oxygen (O₂).

That means we can trace the production of the world’s first free molecular oxygen to the emergence of these photosynthetic cyanobacteria about 2.6 billion years ago. With this, oxygen slowly accumulated in the atmosphere, enabling the evolution of other more complex life forms, including aerobic organisms (including us humans) that require oxygen to live.

Classification of biological organisms

Biological organisms can be classified into three groups called domains: bacteria, archaea, and eukarya. This classification is illustrated in the phylogenetic tree.

The nodes represent the points in evolutionary history when an ancestor forms two new, distinct species, while the length of each branch corresponds to the amount of time that elapsed since the split.

The organisms that comprise bacteria and archaea are prokaryotic, meaning they are single-celled or colonial organisms that lack membrane-bound organelles. Instead of being enclosed in a nucleus, their DNA is organized into a single circular chromosome. As prokaryotes, they reproduce through fission, a process where an individual cell replicates its chromosome and splits into two distinct cells.

On the other hand, the members of domain eukarya are single-celled or multicellular organisms with eukaryotic cells, which means they have membrane-bound organelles, including a nucleus that separates their DNA from other parts of the cell. Unlike prokaryotes, eukaryotes have multiple linear chromosomes. Unlike prokaryotes, some eukaryotes can reproduce sexually.

What are the three domains of life? What are examples of biological organisms from each domain?

Now that we have cited important similarities and differences among the three domains let us take a closer look at their characteristics and cite some examples.

Domain Bacteria

Bacteria are a highly-diverse group of prokaryotic organisms that we can encounter in our everyday lives. Individual bacteria have three basic shapes:

• Coccus: spherical

• Bacillus: rod-like

• Vibrio, spirillum, or spirochete: curved

Bacteria are so small that the average rod-shaped individual is about 2 micrometers long and half a micrometer wide, while the average spherical bacterium is around 1 micrometer in diameter.

Domain Archaea

Archaea are also prokaryotic organisms but have molecular characteristics that set them apart from bacteria. These include the following characteristics:

• Their membrane lipids are composed of branched hydrocarbon chains attached to glycerol by ether linkages (Fig. 2).

• Their cell walls do not have peptidoglycan, a substance typically found in bacteria cell walls.

• Their ribosomal RNA (a molecule that forms the protein-synthesizing organelle called a ribosome) is different from those of bacteria and eukarya.

Another distinguishing feature of archaea is their ability to live in extreme environments, which can be inhospitable for other living organisms.

Domain Eukarya

As mentioned earlier, organisms under the domain eukarya are different from archaea and bacteria mainly due to the presence of membrane-bound organelles like the nucleus.

You might find references that identify four kingdoms under the domain eukarya, namely:

• Plantae (or Plants)are multicellular organisms that produce their own food by photosynthesis and absorption. Their cells have cell walls and are typically organized into tissues.

  • Plants include mosses, ferns, conifers, and flowering plants.

• Animalia (or Animals) are multicellular organisms that do not carry out photosynthesis and obtain nutrients by eating and digesting other organisms.

  • Examples of animals include sponges, insects, birds, and humans.

• Fungi are unicellular or multicellular organisms with cell walls. Their cells are not organized into tissues. They do not undergo photosynthesis; instead, they absorb nutrients in their dissolved form from the environment.

  • Examples of fungi include yeasts, molds, mildew, and mushrooms.

• Protista (or protists) are mostly unicellular, but some are colonial and multicellular species. They are diverse in terms of their feeding patterns, reproduction, and life cycles.

  • Examples of protists include algae, slime molds, and dinoflagellates.

What is a biological community of interacting organisms?

Organisms interact with each other on various levels. For instance, we usually make the distinction between individuals, populations, and species, which form a biological community. But there are also ecosystems, so, what is the difference between all these biological levels?

Individuals of a species that live together in a specific area are collectively called a population.

When different populations of living organisms inhabit and interact in the same area, they are called a community.

The sum of all living organisms and nonliving components of their physical environment constitute an ecosystem.

The collection of all ecosystems on Earth is called the biosphere. The biosphere represents all the zones of life.