What is Biology?

Biology is a branch of science concerned with the study of living things. At its core, it seeks to unravel the fundamental principles that govern living organisms.

With this in mind, it would be appropriate to understand what specifically defines life, what it means to be alive, and how we define an organism.

What is Life?

Most people have an intuitive understanding of what it means for something to be alive, however when it comes down to specifics, it becomes hard to define - especially when examples of exceptions (that "live" within grey areas) are so prevalent.

Despite much disagreement on the many differing key characteristics of life, a mainstream checklist containing the most common defining properties of all life might go as follows:

• Homeostasis: regulating the internal environment in order to maintain a constant state; for example, sweating to reduce temperature when overheating.

• Organisation: being structurally composed of one or more cells – the basic units of life.

• Metabolism: transformation of energy, used to convert chemicals into cellular components (anabolism) and to decompose organic matter (catabolism). Living things require energy for homeostasis and other activities.

• Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size and structure.

• Adaptation: the evolutionary process whereby an organism becomes better able to live in its habitat.

• Response to stimuli: such as the contraction of a unicellular organism away from external chemicals, the complex reactions involving all the senses of multicellular organisms, or the motion of the leaves of a plant turning toward the sun (phototropism), and chemotaxis.

• Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.

(Wikipedia Contributors, 2019)

Cells and Organisms: Understanding the Basic Units of Life

Cells are the smallest units of life, and all living things are made up of cells. They can be broadly classified into two types: prokaryotic and eukaryotic. [Much more of this will be covered later]

What is an organism?

Organisms are individual living entities that independently carry out all basic life processes, such as metabolism, growth, reproduction, and response to stimuli. They are usually composed of multiple cells organised into tissues, organs, and systems working together to sustain life - however this generalisation has an exception in the case of prokaryotes.

• Prokaryotes: Unicellular organisms like bacteria and archaea are composed of a single cell that performs all life functions independently. These are considered complete organisms.

• Eukaryotes: Multicellular organisms such as, plants, animals, and fungi, have specialised cells that form tissues and organs. Each individual cell in a multicellular organism is not considered an independent organism by itself despite being alive and performing basic functions.

In summary, whether a single cell is considered an organism depends on the context. In multicellular eukaryotes, cells are fundamental units of life but not organisms. In unicellular prokaryotes, a single cell constitutes a complete organism.

Differentiating living and non-living entities

Some non-living entities may show several, but not all, properties of life - many exist in grey areas being difficult to define whether they should be considered living. There are also countless phenomena that carry attributes of life despite being unanimously considered non-living.

We have already mentioned how the properties listed above don't give us a perfect grasp in determining whether or not something is alive (see question 1-2), however generally most organisms under typical conditions should fit into all 7 defining properties. Living things like humans, dogs, apes, flowers, trees and even grass meet all seven criteria of life: they are made of cells, metabolise, maintain homeostasis, grow, respond, reproduce, and their populations evolve/adapt.

Arguably the most relevant example of a grey area in defining life is with viruses, for instance. Despite exhibiting many properties they can't reproduce without a host, they have no cells, and don't maintain homeostasis or metabolise independently. Thus, they are generally not considered alive, though this is debated.

Life criteria we've discussed apply to life on Earth. Extraterrestrial life might have different characteristics. NASA’s definition—“life is a self-sustaining system capable of Darwinian evolution”—is broader but harder to apply quickly (Khan Academy, n.d.).

Isn't something missing?

Some properties that you'd might expect aren't listed because defining life is complex; some living organisms don't fit neatly into these criteria. Different sources may include characteristics like movement, carrying genetic information in DNA, or being carbon-based (Khan Academy, n.d.).

Are these properties definitive?

Not really. For example, reproduction is a key property of life, yet a lone mammal couldn't ever reproduce without a mate.
Would this animal then be considered non-living? Obviously not.

Nonetheless, the list provides a reasonable set of properties to distinguish between living and non-living things.

Examples of phenomena displaying aspects of life?

Fire and Snow crystals are examples of a phenomena that display aspects of life; fire exhibit the ability to reproduce, grow and respond to stimuli whilst snow crystals exhibit organisation and growth also, however both lack other crucial criteria.

Fri 2nd Aug 2024 (02/08/2024), N. Heath

Byju's, 2017