Characteristics of Life: Videos & Practice Problems

Living organisms share eight fundamental characteristics that distinguish them from nonliving entities. Understanding these characteristics is essential for recognizing what defines life. The first characteristic is that all living things are composed of cells, which are the basic units of life. This cellular structure is crucial for all biological processes.

The second characteristic is order, indicating that living organisms exhibit a structured organization. They take simpler molecules and systematically assemble them into more complex structures, which is vital for their survival and function.

Living things also respond to stimuli, the third characteristic, meaning they can react to environmental triggers. This responsiveness is essential for adapting to changes in their surroundings.

The fourth characteristic is homeostasis, the ability of organisms to maintain stable internal conditions despite external fluctuations. This regulation is critical for sustaining life processes.

Reproduction is the fifth characteristic, allowing living organisms to produce offspring either sexually or asexually, thereby ensuring the continuation of their species.

The sixth characteristic is dynamic metabolism, which encompasses all chemical reactions within an organism. Metabolism is flexible and adapts to varying conditions, enabling organisms to extract and transform energy from their environment for survival.

Heredity, the seventh characteristic, refers to the transmission of genetic information, primarily through DNA, from one generation to the next. This genetic continuity is fundamental for the evolution of species.

Finally, the eighth characteristic is evolution, which involves changes in DNA over time, leading to variations that can enhance an organism's fitness in its environment. These adaptations are crucial for the survival and diversification of life forms.

While these characteristics define living organisms, it is important to note that viruses do not exhibit all of these traits, which is why they are classified as nonliving entities. In future discussions, we will explore which specific characteristics viruses lack.

Viruses are fascinating entities that challenge our understanding of life due to their unique characteristics. Unlike living organisms, viruses are not cellular; they consist of a protein shell called a capsid that encases nucleic acid, which can be either DNA or RNA. Some viruses also possess a lipid envelope surrounding the capsid, making them significantly smaller than cellular components.

While viruses exhibit ordered structures, as they are assemblies of macromolecules, they lack several key characteristics of life. For instance, there is insufficient evidence to suggest that viruses can respond to stimuli, and they do not maintain homeostasis through complex mechanisms like living organisms. Although the capsid and envelope may help stabilize internal conditions, this does not equate to true homeostasis.

Reproduction is another area where viruses diverge from living organisms. They cannot reproduce independently, either sexually or asexually; instead, they must hijack the reproductive machinery of host cells to replicate. Additionally, viruses do not possess a dynamic metabolism, which is a hallmark of living organisms.

Despite these limitations, viruses do have the ability to evolve over time, indicating a form of heredity. This blend of characteristics has led some scientists to reconsider the definitions of life, yet as it stands, viruses are classified as non-living entities. Understanding these distinctions is crucial as we explore the biological levels of organization in living organisms.

Living organisms exhibit a structured biological hierarchy of organization, which can be visualized as a spectrum from larger, more complex entities to smaller, simpler ones. At the far left of this hierarchy are multicellular organisms, such as humans, which consist of various organ systems. Each organ system is a collection of individual organs that collaborate to perform specific functions. These organs are further composed of tissues, which are groups of similar cells working together.

Delving deeper, cells are made up of organelles, which are specialized structures that carry out distinct tasks within the cell. Organelles themselves are constructed from macromolecules, which are large molecules essential for life. The term "macro" indicates their size, distinguishing them from smaller molecules. As we progress in biochemistry, we will explore these macromolecules and organelles in greater detail.

Macromolecules are formed from smaller molecules, which in turn consist of individual atoms. These atoms, the fundamental building blocks of matter, are represented on the periodic table. The biological hierarchy illustrates that lower levels of organization serve as the foundational components for the higher levels. This concept is encapsulated in the idea of biochemical unity of life, which posits that all living organisms share a fundamental resemblance at the atomic and molecular levels.

This resemblance supports the theory of a common universal ancestor, suggesting that all life on Earth has evolved from a single ancestral organism. Despite the vast diversity of life forms, the similarities observed at the biochemical and molecular levels highlight the interconnectedness of all living beings, making the study of these relationships both intriguing and essential.