Characteristics of life

• Biology is the science that studies life, but what exactly is life?
• Virology, a branch of biology, studies viruses, which exhibit some of the characteristics of living entities but lack others.
• Virologists are not biologists, strictly speaking.
• Some biologists study the early molecular evolution that gave rise to life; since the events that preceded life are not biological events, these scientists are also excluded from biology in the strict sense of the term.
• All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, growth and development, regulation, homeostasis, and energy processing.
• When viewed together, these characteristics serve to define life.
• Organisms are highly organized, coordinated structures that consist of one or more cells.
• Even very simple, single-celled organisms are remarkably complex: inside each cell, atoms make up molecules; these in turn make up cell organelles and other cellular inclusions.
• In multicellular organisms, similar cells form tissues.
• Tissues, in turn, collaborate to create organs.
• The biosphere is the collection of all ecosystems, and it represents the zones of life on earth, including land, water, and even the atmosphere to a certain extent.
• Organisms are individual living entities, such as each tree in a forest, single-celled prokaryotes and single-celled eukaryotes, and are typically referred to as microorganisms.
• An ecosystem consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water.
• Mammals have many organ systems, such as the circulatory system which transports blood through the body and to and from the lungs, and includes organs such as the heart and blood vessels.
• Organs are present not only in animals but also in plants.
• A community is the sum of populations inhabiting a particular area.
• All the individuals of a species living within a specific area are collectively called a population.
• Living organisms are parts of a highly structured hierarchy, from a single organelle to the entire biosphere.
• An organ system is a higher level of organization that consists of functionally related organs.
• Organs work together to form organ systems.
• Organisms respond to diverse stimuli.
• Plants can bend toward a source of light, climb on fences and walls, or respond to touch.
• Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis).
• Organisms grow and develop following specific instructions coded for by their genes.
• These genes provide instructions that will direct cellular growth and development, ensuring that a species’ young will grow up to exhibit many of the same characteristics as its parents.
• Organisms require multiple regulatory 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.
• In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions.
• Polar bears maintain their body temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin.
• All organisms use a source of energy for their metabolic activities.
• Organelles are small structures that exist within cells, such as mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars.
• Organisms that live in cold climates have body structures that help them withstand low temperatures and conserve body heat.
• Organisms in hot climates have methods to shed excess body heat.
• Organs perform specific functions such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.
• Some organisms capture energy from the sun and convert it into chemical energy in food (photosynthesis); others use chemical energy in molecules they take in as food (cellular respiration).
• Organisms need to regulate body temperature through a process known as thermoregulation.
• Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization.
• Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles.
• Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei or organelles; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus.
• Cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals.