The Study of Life

Science is not a set of truths written in stone. It is an approach to discovering truths about the world that applies to problems encountered in everyday life. New ideas, new experiments, new tools, and new ways of gathering data are constantly adding to discovery, so scientific ideas are always subject to change. Science includes many fields of study attempting to comprehend the nature of the universe.

Scientific method is a process for investigating natural phenomena to accurately explain how the natural world works. A hypothesis is a suggested explanation for an observation based on previous research. Theories are established explanations for a phenomenon or observations that are broader in scope than a hypothesis. Theories are also testable and well supported by a tremendous amount of research, like gravitational theory.

Scientific method begins with an observation, which leads to one question being generated that can be used to form a testable hypothesis. The hypothesis is a falsifiable suggested explanation for a particular phenomenon or set of observations. An experiment will be constructed with results that will either support or fail to support the hypothesis. Lab experiments require a control group, which is not exposed to the variable, and a variable group, which is exposed to the variable. Experiments can have only one variable. Two variables would keep anyone from knowing which variable caused the change.

Following the experiment, the results will be analyzed and reported. The next step is to share those results. Scientists share findings to expand and build upon their discoveries. Collaboration with other scientists—when planning, conducting, and analyzing results—are all important for scientific research. Scientists communicate with peers and share results. Most scientists present their results in peer-reviewed papers that are published in scientific journals. Peer-reviewed papers are reviewed by qualified colleagues. These colleagues are often experts in the same research area, who judge whether or not the paper is suitable for publication. Scientists publish their work so other scientists can reproduce their experiments under similar or different conditions to expand on the findings. When additional experimental results are consistent with the paper, the original paper is validated.

Exploring the Properties of Life

Order: Organisms are highly organized structures with one or more cells. Even very simple organisms are complex. Inside each cell, atoms make up molecules that make up organelles. In multicellular organisms, similar cells form tissues. Tissues build organs. Organs work together to form organ systems. Smallest to largest in humans follows this order atoms, molecules, organelles, cells, tissues, organs, and organ systems to form each organism.

Sensitivity or Response to Stimuli: Organisms respond to a variety of stimuli. Plants bend toward a source of light, climb on fences and walls. Even bacteria can move toward or away from chemicals or light.

Reproduction: Single-celled organisms reproduce by first duplicating their DNA and dividing it equally into two new cells. Multicellular organisms often produce specialized reproductive cells that form new individuals. Reproductive cells pass DNA, containing genes, to offspring. Genes ensure offspring belong to the same species and have similar characteristics.

Growth and Development: Organisms grow and develop following specific instructions coded for by their genes, based on the principles of the Central Dogma of Biology. DNA codes for RNA codes for proteins. The genes composed of DNA provide instructions to direct cellular growth and development, ensuring offspring will grow up to exhibit many of the same characteristics as its parents.

Regulation: Even the smallest organisms are complex and require multiple mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.

Homeostasis: In order to function properly, cells need proper temperature, pH, and an appropriate concentration of diverse molecules. These conditions can change from one moment to the next. Organisms maintain internal conditions almost constantly through homeostasis. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear, have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, blubber, and fat. In hot climates, organisms have other methods to regulate body heat.

Energy Processing: All organisms use a source of energy for their metabolic activities. Some organisms are producers that capture energy from the sun and convert it into chemical carbohydrate energy. Consumers are organisms that use the chemical energy from producers as food.

Some organisms consist of a single cell, and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei. In contrast, the cells of eukaryotes have membrane-bound organelles and a membrane-bound nucleus.

Above the level of organism, there are higher levels of organization. Organisms form populations, communities, ecosystems, and the biosphere. These are in order from smallest to largest.

This phylogenetic tree was constructed by microbiologist Carl Woese using data obtained from sequencing ribosomal RNA genes. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes, single-celled organisms lacking intracellular organelles. (credit: Eric Gaba; NASA Astrobiology Institute)

Living things can be divided into three domains: Bacteria, Archaea, and Eukarya. Both bacteria and archaea are prokaryotic domains. The eukaryotic domain contains both single-celled organisms and complex organisms. In addition, organisms are either producers or consumers. Producers extract energy from nonliving environmental resources. Consumers extract energy from producers, either directly or indirectly.