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Biology LibreTexts


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  • What is Life?


    In this chapter we will learn how living organisms change as they become better adapted to their environment. Over millions of years, these changes have produced a large number of different kinds of organisms. It has been estimated that there may be 10 to 30 million or more different species living on earth.

    Living organisms are comprised of the same chemical elements that make up nonliving things and both obey the same laws of physics and chemistry. We can better understand what distinguishes living from nonliving by examining characteristics that all living organisms have in common. Some of these characteristics are discussed below. The student is encouraged to add to or modify these criteria so that they can develop a better understanding of life.

    Characteristics Common to All Living Organisms

    Living things are composed of cells.

    Small organisms such as bacteria and many protists are composed of a single cell. Larger organisms are composed of many cells; they are multicellular.

    Living things are organized.

    The list below shows increasing levels of biological organization.

    • atoms
    • molecules
    • cells
    • tissues
    • organs
    • organ systems
    • individual organism
    • population
    • community
    • ecosystem
    • biosphere

    The first three items on this list (atoms and molecules) will be discussed further in the chapter on chemistry.

    Cells are considered to be the smallest structure that is alive. They are often too small to see without the aid of a microscope. All living organisms are composed of cells. The smallest organisms are composed of a single cell; larger organisms are composed of more than one cell.

    Similar kinds of cells may be arranged together to form a tissue. Tissues have specific properties and functions. For example muscle tissue is composed of muscle cells. It functions to move body components. 

    Two or more tissues that form a structure with a specific function is an organ. For example, the heart is an organ formed from muscle tissue, nervous tissue, connective tissue, and epithelial tissue. It functions to pump blood.

    An organ system consists of two or more organs which perform a specific task. Some organ systems are: the integumentary, nervous, sensory, endocrine, skeletal, muscular, circulatory, immune, lymphatic, digestive, respiratory, excretory, and reproductive systems.

    A population is an interbreeding group of organisms (the same species) that occupies a particular area.

    Two or more populations form a community.

    The word community refers to the organisms. The word ecosystem refers to the organisms of a community and also the nonliving environment.

    All of the ecosystems on earth form the biosphere.

    Living things require nutrients and energy

    Organisms need nutrients and energy for their activities, growth, reproduction, and maintenance.

    Chemical reactions are needed to store and release energy and to synthesize compounds needed by the organism. The word metabolism refers to the chemical reactions that occur within a cell.

    Energy cannot be created or destroyed, but it can be transformed from one form to another. For example, photosynthetic organisms such as plants are able to transform radiant (solar) energy to chemical energy. 

    Plants, some algae, and some bacteria obtain their energy from light. The light energy is used to bond molecules of carbon dioxide together to form sugar (glucose). The energy is stored in glucose. This process is called photosynthesis. When a cell needs energy, chemical reactions within the cell are able to release this stored energy for it's needs. The energy stored in glucose can be used to form other chemicals. The new chemicals now contain some of the energy. Whenever energy is transferred from one chemical to another, a little is lost as heat. Animals that eat plants obtain their energy from the chemicals in the plants. As with plants, chemical reactions within the animal cells release the energy stored in their food and make it available for the cell.


    Two processes occur in ecosystems- energy flows and is eventually lost, nutrients cycle and are not lost.

    1) Energy flows through ecosystems and is eventually lost as heat. Green organisms such as plants (called producers) capture solar energy. Some of the energy is used for maintenance, growth, reproduction or other needs. Some is stored in chemical compounds and some is lost as heat. Organisms that feed on other organisms are able to use the energy that has been stored by those organisms. These consumers also use some of the energy, lose some as heat, and store some. Eventually, all of the available energy has been lost as heat.

    2) Nutrients cycle. They may exist in soil, rocks, water, the atmosphere, or any other part of the nonliving environment. They may be taken up by organisms and passed from one organism to another in a food chain. Eventually, the nutrient makes its way back to the nonliving environment where it may remain in one form or another until it is again taken up by living organisms.

    Living things respond to their environment

    Organisms must sense, interact with, and respond to their environment because they need nutrients and energy from the environment.

    Organisms need to protect themselves to prevent other organisms from taking their energy (by eating them).

    The internal environment of an organism fluctuates less than the external environment. For example the temperature of some organisms remains fairly constant even though the outside temperature fluctuates. The maintenance of constant internal conditions is called homeostasis.

    Living things contain DNA

    The genetic instructions of all living organisms is contained in molecules of deoxyribonucleic acid (DNA).

    DNA contains instructions that are used by cells to produce proteins. The vast array of different chemical reactions that build and maintain cells are controlled by proteins.

    The instructions for making proteins are found in the genes; different genes contain instructions for different proteins or parts of proteins.

    Before protein is synthesized, the information in DNA must first be copied. The copy is composed of a substance similar to DNA called mRNA (for messenger RNA). It is mRNA that is used in the manufacture of protein. The diagram below illustrates that information in DNA is used to create mRNA and that information in mRNA is used to synthesize protein. The circular arrow in the diagram indicates that DNA has the ability to replicate itself.

    Living things reproduce

    Within a multicellular organism, cells reproduce to enable growth and tissue repair.

    Individual organisms can also reproduce.

    Asexual Reproduction

    The advantage of asexual reproduction is that it can produce large numbers of offspring very rapidly and it does not require a mate.

    Asexual reproduction, however, produces offspring which are identical to the parent. Populations in which all of the individuals are identical are more likely to go extinct if the environment fluctuates. Moreover, these populations are less likely to change over time in response to environmental change.

    Sexual Reproduction

    Sexual reproduction requires two parents and thus promotes genetic variation. Populations which show variability are more likely to survive environmental fluctuations because there is an increased likelihood that at least some individuals are going to be able to survive due to their being better adapted. Imagine a situation where the environment fluctuates and it becomes too extreme for one individual. If there is little genetic variability in the population so that all of the individuals are nearly identical then if the environment is too extreme for one individual to survive, then it is too extreme for all of them. The population will go extinct.

    Populations of living things evolve

    Evolution refers to changes in the genetic composition of a population. Genetic changes may result in changes in the physical or behavioral characteristics of the individuals. 

    A mutation is a change in the genetic instructions (DNA) of an individual.

    The change is usually harmful but occasionally it is beneficial.

    Any beneficial mutations that occur are likely to spread within a population because individuals that possess the mutations will have higher reproductive output and they will reproduce the mutation. Beneficial mutations are therefore likely to result in evolutionary change. The increase in beneficial genes due to their allowing the individual to have higher reproductive success is called natural selection.

    Evolutionary change has led to diversity among organisms. To date, approximately 1.8 million different species of organisms have been identified. Biologists estimate that there are between 10 and 200 million species on earth.