1.1: The Science of Life (2023)

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    Biology is the science that studies life. What exactly is life? This may seem like a silly question with an obvious answer, but it's not easy to define life. For example, a branch of biology called virology studies viruses, which have some of the characteristics of living things but lack others. It turns out that while viruses can attack living organisms, cause disease, and even reproduce, they don't meet the criteria biologists use to define life.

    Since its inception, biology has wrestled with four questions: What are the shared properties that make something "alive"? How do these various living things work? When faced with the remarkable diversity of life, how do we organize the different types of organisms in order to better understand them? And finally, what biologists ultimately seek to understand, how did this diversity come about and how does it continue? As new organisms are discovered every day, biologists continue to search for answers to these and other questions.

    properties of life

    All groups of living organisms share several key characteristics or functions: order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. When viewed together, these eight characteristics serve to define life.


    Organisms are highly organized structures consisting of one or more cells. Even very simple single-celled organisms are remarkably complex. Inside each cell, atoms form molecules. These in turn form cellular components or organelles. Multicellular organisms, which can be made up of millions of individual cells, have the advantage over unicellular organisms in that their cells can specialize to perform specific functions and even sacrifice themselves in certain situations for the good of the organism as a whole. Later we will analyze how these specialized cells unite to form organs such as the heart, lungs or skin in organisms such as the toad shown in Figure \(\PageIndex{1}\).

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    Sensitivity or response to stimuli

    Organisms respond to various stimuli. For example, plants can lean toward a light source or respond to touch (Figure \(\PageIndex{2}\)). Even the smallest bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Moving towards a stimulus is considered a positive response, while moving away from a stimulus is considered a negative response.

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    See thisvideoto see how the sensitive plant responds to a tactile stimulus.


    Single-celled organisms reproduce by first duplicating their DNA, which is the genetic material, and then dividing it equally as the cell prepares to divide to form two new cells. Many multicellular organisms (those made up of more than one cell) produce specialized reproductive cells that will form new individuals. When reproduction occurs, DNA containing genes is passed on to an organism's offspring. These genes are the reason why the offspring will belong to the same species and will have characteristics similar to those of the parent, such as coat color and blood type.


    All living organisms exhibit an "adjustment" to their environment. Biologists refer to this adjustment as adaptation, and it is a consequence of evolution by natural selection, which operates in every lineage of reproductive organisms. Examples of adaptations are diverse and unique, from heat-resistant Archaea that live in boiling hot springs to the length of a nectar-eating moth's tongue matching the size of the flower it feeds on. All adaptations enhance the reproductive potential of the individual exhibiting them, including their ability to survive to reproduce. The adaptations are not constant. As the environment changes, natural selection causes the characteristics of individuals in a population to follow those changes.

    Growth and development

    Organisms grow and develop according to specific instructions encoded by their genes. These genes provide instructions that will direct cell growth and development, ensuring that the offspring of a species (Figure \(\PageIndex{3}\)) will grow and exhibit many of the same characteristics as their parents.

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    Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, such as nutrient transport, response to stimuli, and coping with environmental stress. For example, organ systems such as the digestive or circulatory systems perform specific functions such as transporting oxygen throughout the body, removing waste, delivering nutrients to each cell, and cooling the body.


    To function properly, cells require appropriate conditions, such as temperature, pH, and concentrations of various appropriate chemicals. These conditions, however, may change from time to time. Organisms are capable of maintaining internal conditions within a narrow range almost constantly, despite environmental changes, through a process called homeostasis or "steady state": the ability of an organism to maintain constant internal conditions. For example, many organisms regulate their body temperature in a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure \(\PageIndex{4}\)), have body structures that help them withstand low temperatures and conserve body heat. In hot climates, organisms have methods (such as sweating in humans or panting in dogs) that help them get rid of excess body heat.

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    energy processing

    All organisms (such as the California condor shown in Figure \(\PageIndex{5}\)) use an energy source for their metabolic activities. Some organisms capture energy from the Sun and convert it into chemical energy in food; others use chemical energy from the molecules they absorb.

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    Levels of organization of living beings

    Living things are highly organized and structured, following a hierarchy on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms linked by a chemical bond. Many molecules that are biologically important are macromolecules, large molecules that are usually made by combining smaller units called monomers. An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure \(\PageIndex{6}\)), which contains the instructions for the functioning of the organism that contains it.

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    To see an animation of this DNA molecule, clickhere.

    Some cells contain aggregates of membrane-enclosed macromolecules; these are called organelles. Organelles are small structures that exist within cells and perform specialized functions. All living things are made of cells; hecell itselfIt is the smallest fundamental unit of structure and function in living organisms. (This requirement is the reason viruses are not considered living: they are not made of cells. To create new viruses, they have to invade and hijack a living cell; only then can they obtain the materials they need to reproduce.) Some organisms They are made up of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled organisms that lack membrane-enclosed organelles and do not have nuclei enclosed by nuclear membranes; On the contrary, the cells ofeukaryotesThey have organelles and nuclei surrounded by membranes.

    In most multicellular organisms, cells combine to form tissues, which are groups of similar cells that perform the same function. Organs are groups of tissues grouped according to a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization consisting of functionally related organs. For example, vertebrate animals have many organ systems, such as the circulatory system that carries blood throughout the body and to and from the lungs; includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Unicellular prokaryotes and unicellular eukaryotes are also considered to be organisms and are commonly referred to as microorganisms.


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    Which of the following statements is false?

    1. Tissues exist within organs that exist within organ systems.
    2. Communities exist within populations that exist within ecosystems.
    3. Organelles exist within cells that exist within tissues.
    4. Communities exist within ecosystems that exist in the biosphere.


    All individuals of a species living within a specific area are collectively called a population. For example, a forest may include many white pines. All these pines represent the white pine population of this forest. Different populations may live in the same specific area. For example, the pine forest includes populations of flowering plants as well as populations of insects and microbes. A community is the set of populations that inhabit a certain area. For example, all the trees, flowers, insects, and other populations in a forest make up the forest community. The forest itself is an ecosystem. An ecosystem is made up of all living things in a given area together with theabioticNon-living or non-living parts of that environment, such as soil nitrogen or rainwater. At the highest level of organization (Figura \(\PageIndex{7}\)), the biosphere is the set of all ecosystems and represents the life zones on Earth. Includes land, water, and parts of the atmosphere.

    The diversity of life

    The science of biology is very broad in scope because there is an enormous diversity of life on Earth. The source of this diversity is evolution, the process of gradual change during which new species arise from older species. Evolutionary biologists study the evolution of living things in all areas, from the microscopic world to ecosystems.

    In the 18th century, a scientist named Carl Linnaeus first proposed organizing the known species of organisms into a hierarchical taxonomy. In this system, the species that are most similar to one another are grouped together within a group known as a genus. In addition, within a family similar genera are grouped together (the plural of genus). This grouping continues until all agencies are brought together in groups at the highest level. The current taxonomic system now has eight levels in its hierarchy, from lowest to highest, they are: species, genus, family, order, class, phylum, kingdom, domain. Thus, species are grouped within genera, genera are grouped within families, families are grouped within orders, etc. (Figure \(\PageIndex{8}\)).

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    The highest level, the domain, is a relatively new addition to the system since the 1990s. Scientists now recognize three domains of life: Eukarya, Archaea, and Bacteria. The Eukarya domain contains organisms that have cells with a nucleus. It includes the kingdoms of fungi, plants, animals, and various kingdoms of protists. Archaea are single-celled organisms without a nucleus and include many extremophiles that live in harsh environments such as hot springs. Bacteria are quite a different group of single-celled organisms without a nucleus (Figure \(\PageIndex{9}\)). Both Archaea and Bacteria are prokaryotes, an informal name for cells without a nucleus. The recognition in the 1990s that certain "bacteria," now known as Archaea, were genetically and biochemically as different from other bacterial cells as they were from eukaryotes, prompted the recommendation to divide life into three domains. This dramatic change in our knowledge of the tree of life demonstrates that the classifications are not permanent and will change as new information becomes available.

    In addition to the hierarchical taxonomic system, Linnaeus was the first to name organisms using two unique names, now called the binomial naming system. Before Linnaeus, the use of common names to refer to organisms caused confusion because there were regional differences in these common names. Binomial names consist of the genus name (which is capitalized) and the species name (all lowercase). Both names appear in italics when printed. Each species is assigned a unique pairing that is recognized worldwide, so a scientist anywhere can tell which organism is being referred to. For example, the North American blue jay is known only asCyanocitta cristata. Our own kind isA wise man.

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    EVOLUTION IN ACTION: Carl Woese and the phylogenetic tree

    The evolutionary relationships of various life forms on Earth can be summarized in a phylogenetic tree. A phylogenetic tree is a diagram that shows the evolutionary relationships between biological species based on similarities and differences in genetic or physical traits or both. A phylogenetic tree is made up of branch points or nodes and branches. Internal nodes represent ancestors and are points in evolution where, based on scientific evidence, one ancestor is believed to have diverged to form two new species. The length of each branch can be considered as estimates of relative time.

    In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and bacteria. However, pioneering work by American microbiologist Carl Woese in the early 1970s has shown that life on Earth has evolved along three lineages, now called domains: Bacteria, Archaea, and Eukarya. Woese proposed the domain as a new taxonomic level and Archaea as a new domain, to reflect the new phylogenetic tree (Figure \(\PageIndex{10}\)). Many organisms belonging to the Archaea domain live in extreme conditions and are called extremophiles. To build his tree, Woese used genetic relationships rather than similarities based on morphology (shape). Several genes were used in phylogenetic studies. Woese's tree was constructed from comparative sequencing of genes that are universally distributed, found in some slightly altered form in each organism, conserved (meaning that these genes have remained only slightly modified throughout evolution). ) and of a suitable length.

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    Branches of biological study

    The scope of biology is wide and therefore contains many branches and sub-disciplines. Biologists may pursue one of these subdisciplines and work in a more specific field. For example, molecular biology studies biological processes at the molecular level, including the interactions between molecules such as DNA, RNA, and proteins, as well as how they are regulated. Microbiology is the study of the structure and function of microorganisms. This is quite a broad branch in itself, and depending on the subject of study, there are also microbial physiologists, ecologists, and geneticists, among others.

    Another biological field of study, neurobiology, studies the biology of the nervous system, and while it is considered a branch of biology, it is also recognized as an interdisciplinary field of study known as neuroscience. Due to its interdisciplinary nature, this subdiscipline studies different functions of the nervous system using molecular, cellular, developmental, medical, and computational approaches.

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    Paleontology, another branch of biology, uses fossils to study the history of life (Figure \(\PageIndex{11}\)). Zoology and botany are the study of animals and plants, respectively. Biologists may also specialize as biotechnologists, ecologists, or physiologists, to name just a few areas. Biotechnologists apply knowledge of biology to create useful products. Ecologists study the interactions of organisms in their environment. Physiologists study the functioning of cells, tissues, and organs. This is just a small sample of the many fields that biologists can develop. From our own bodies to the world we live in, discoveries in biology can affect us in very direct and important ways. We depend on these discoveries for our health, our food sources, and the benefits our ecosystem provides. Because of this, knowledge of biology can benefit us when making decisions in our day to day.

    The development of technology in the 20th century that continues today, particularly the technology for describing and manipulating the genetic material, DNA, has transformed biology. This transformation will allow biologists to continue to understand the history of life in greater detail, how the human body works, our human origins, and how humans can survive as a species on this planet despite the stress caused by our increasing numbers. Biologists continue to unravel enormous mysteries about life, suggesting that we have only just begun to understand life on the planet, its history, and our relationship to it. For this and other reasons, the knowledge of biology gained through this textbook and other print and electronic media should be a benefit in any field you enter.

    CAREERS IN ACTION: Forensic Scientist

    Forensic science is the application of science to answer questions related to the law. Both biologists and chemists and biochemists can be forensic scientists. Forensic scientists provide scientific evidence for use in court, and their job is to examine trace material associated with crimes. Interest in forensic science has increased in recent years, possibly due to popular television shows featuring forensic scientists at work. Additionally, the development of molecular techniques and the establishment of DNA databases have updated the types of work that forensic scientists can perform. His work activities are mainly related to crimes against people such as murder, rape and assault. His job involves analyzing samples such as hair, blood, and other bodily fluids, and also processing DNA (Figure \(\PageIndex{12}\)) found in many different environments and materials. Forensic scientists also analyze other biological evidence left at a crime scene, such as insect parts or pollen grains. Students who want to pursue careers in forensic science will likely need to take chemistry and biology courses, as well as some intensive math courses.

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    Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. Living things are highly organized following a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped into populations, communities, ecosystems, and the biosphere. Evolution is the source of the tremendous biological diversity that exists on Earth today. A diagram called a phylogenetic tree can be used to show the evolutionary relationships between organisms. Biology is very broad and includes many branches and sub-disciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others.


    A basic unit of matter that cannot be broken down by normal chemical reactions.
    the study of living organisms and their interactions with each other and with their environment
    a collection of all ecosystems on Earth
    she was whole
    The smallest fundamental unit of structure and function in living things.
    a set of populations that inhabit a particular area
    all living things in a particular area along with the abiotic and non-living parts of that environment
    Organism with cells that have nuclei and organelles surrounded by membranes.
    the process of gradual change in a population that can also lead to new species arising from older species
    the ability of an organism to maintain constant internal conditions
    A large molecule typically formed by joining together smaller molecules.
    Chemical structure consisting of at least two atoms joined by a chemical bond.
    Structure made up of tissues that work together to perform a common function.
    organ system
    The highest level of organization consisting of functionally related organs.
    a membrane-enclosed compartment or sac within a cell
    a single living entity
    phylogenetic tree
    a diagram showing the evolutionary relationships between biological species based on similarities and differences in genetic or physical traits or both
    all individuals of a species living in a specific area
    A single-celled organism that lacks a nucleus or any other membrane-enclosed organelle.
    a group of similar cells that perform the same function

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