Characteristics and Classification of Living Organisms
Characteristics of Living Organisms
All living organisms are defined by seven fundamental characteristics that distinguish them from non-living matter. Movement is an action made by an organism that causes a change of position or place. Respiration involves the chemical reactions that take place in cells to break down nutrient molecules and release energy for metabolism. Sensitivity is defined as the ability to sense stimuli in the internal or external environment and make appropriate responses. Growth is a permanent increase in size and dry mass, which occurs through an increase in cell number, cell size, or both. Reproduction is the biological process that makes more of the same kind of organism. Excretion is the removal from organisms of waste products of metabolism, toxic materials, and excess substances. Finally, nutrition is the process of taking in materials for growth, energy, and development.
Common Cellular Features and Modern Classification
All living things share certain features in common, including the presence of cytoplasm, a cell membrane, and DNA as genetic material. They also contain ribosomes in the cytoplasm, which may be floating freely or attached to membranes known as the rough endoplasmic reticulum. These ribosomes are responsible for making the proteins and enzymes involved in respiration.
Classification is the process of grouping organisms. A species is defined as a group of organisms that can reproduce to produce fertile offspring. Scientists classify organisms for several reasons, including the identification of those at risk of extinction and to understand evolutionary relationships. Traditionally, classification was done by studying morphology, which is the study of form or outward appearance, and anatomy, which is the study of internal structures through dissection. However, modern classification utilizes more accurate methods, such as sequences of DNA and amino acids in proteins. Each species possesses a unique number of chromosomes and a specific sequence of bases in its DNA. For instance, humans have chromosomes. Organisms with recent ancestors have DNA sequences that are more similar than those that are distantly related.
The Binomial Naming System and Taxonomic Hierarchy
The Binomial Naming system is used to make classification standardized globally. Every organism is given a scientific name consisting of its Genus and species. The Genus is always capitalized, while the species remains in lower case. In print, these names are italicized, and when written by hand, they are underlined. An example is the scientific name for humans, which is Homo sapiens. Organisms are classified under a hierarchy that starts from general levels and moves to specific ones: Kingdom, Phylum, Class, Order, Family, Genus, and Species. Following the Linnatus's System of Classification for humans, the sequence is: Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, and Species sapiens.
Whittaker's Five Kingdom Scheme: Animalia and Plantae
Whittaker’s scheme classifies all living things into five kingdoms: Animal, Plant, Fungus, Prokaryote, and Protoctist. The Animal kingdom consists of multicellular organisms that do not have cell walls or chloroplasts. This kingdom is further divided into vertebrates and arthropods. The Plant kingdom consists of multicellular organisms with cell walls made of cellulose. They contain chloroplasts with chlorophyll, allowing them to make their own food. The Plant kingdom is divided into ferns and flowering plants.
Ferns are land plants and vascular plants, meaning they possess xylem and phloem for transport. They undergo sexual reproduction and produce gametes (sex cells) without seeds; thus, the zygote directly becomes the fern plant. Fern plants also have structures called sporangia, which make spores for reproduction. Flowering plants are divided into two main categories: Monocotyledon and Dicotyledon. Monocotyledons typically have long and narrow leaves with parallel leaf veins, a single cotyledon, and flower parts arranged in groupings of . Dicotyledons have broad leaves with branching veins, two cotyledons, and flower parts arranged in groupings of .
Kingdoms Fungus, Prokaryote, and Protoctist
The Fungus kingdom includes organisms like yeast and mushrooms. These are made up of thread-like structures called hyphae rather than traditional cells. Many nuclei are distributed throughout the cytoplasm within these hyphae. A basic fungal cell contains a cell wall made of chitin rather than cellulose, a cell membrane, ribosomes, mitochondria, and cytoplasm. Fungi also feature a mycelium and a fruiting body.
Kingdom Prokaryote includes unicellular organisms such as bacteria and certain algae. In these organisms, chromosomes are not organized into a nucleus. A typical bacterial cell contains a single chromosome consisting of a circular loop of DNA strands, a cell wall made from peptidoglycan, a cell membrane, cytoplasm, ribosomes, and plasmids.
Kingdom Protoctist also contains unicellular organisms, but unlike prokaryotes, their chromosomes are enclosed in a nuclear membrane to form a distinct nucleus. A common example of a protoctist is the Amoeba.
Viruses and Identification Tools
Viruses are distinct from the five kingdoms because virus particles are not cells. They consist of a central core of DNA or RNA surrounded by a protein coat, sometimes further encased in an envelope. Viruses do not feed, excrete, respire, or grow. While they do reproduce, they can only do so inside the cells of a living organism by using materials from the host cell.
To identify organisms, scientists use dichotomous keys. The term dichotomous means " branches," and each key is made of contrasting features. Users follow a series of numbered choices based on observable characteristics to reach the correct identification. For example, a key for leaves might ask if a leaf has several small leaflets or one large leaf blade, then further distinguish if the edge is smooth or toothed, guiding the user step-by-step through specific biological traits.