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By the end of this section, you will be able to:
  • Identify and describe the properties of life
  • Describe the levels of organization among living things
  • Recognize and interpret a phylogenetic tree
  • List examples of different sub disciplines in biology

Biology is the science that studies life, but what exactly is life? This may sound like a silly question with an obvious response, but it is not always easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. It turns out that although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life. Consequently, virologists are not biologists, strictly speaking. Similarly, 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.

From its earliest beginnings, biology has wrestled with three questions: What are the shared properties that make something “alive”? And once we know something is alive, how do we find meaningful levels of organization in its structure? And, finally, when faced with the remarkable diversity of life, how do we organize the different kinds of organisms so that we can better understand them? As new organisms are discovered every day, biologists continue to seek answers to these and other questions.

Properties of life

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

Order

A photo shows a light-colored toad covered in bright green spots.
A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems. (credit: “Ivengo”/Wikimedia Commons)

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 ( [link] ), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.

Sensitivity or response to stimuli

A photograph of the Mimosa pudica shows a plant with many tiny leaves connected to a central stem. Four of these stems connect together.
The leaves of this sensitive plant ( Mimosa pudica ) will instantly droop and fold when touched. After a few minutes, the plant returns to normal. (credit: Alex Lomas)

Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch ( [link] ). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis ) or light ( phototaxis ). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.

Questions & Answers

who proposed Cell doctrine
ANJOUS-YINKA Reply
biology introduction
Khab Reply
describe how the gurd cells regulate the movement of gases in and out of a leaf.
Tony
state the main functions of the leaf
kalisto Reply
what is the similarity between meiosis and mitosis
kalisto
Regulation of blood sugar level
Kyara Reply
Describes larmack theory
Mayiik Reply
Mention five modes of nutrition in ecology
Mayiik
Saprophytic
Evans
what is biology
Imisi Reply
Biology is the study of life
Aisha
Yea
Mayiik
Biology is the study of biography
Musoke
what is insulin and where is it found in the body
Cecilia Reply
An illustration of the negative feedback mechanism in homeostasis
Cecilia
How we know
Nhial
That is not the meaning
Mayiik
what are chromosomes
Ibrahim
factor that affect the diffusion of substance
Nwafor Reply
nature of substances size increase in temperature
Quadri
Explain the specialized functions of the organs involved in processing food in the body
Brah Reply
none
Emmanuel
Processing food involves ingestion eating, digestion mechanical and enzymatic breakdown of large molecules, absorption cellular uptake of nutrients, and elimination removal of undigested waste as feces. Many organs work together to digest food and absorb nutrients.
Timileyin
biology is the study of life
Evelyn Reply
introduction of biology
Alison Reply
what is biology
Alison
biology is the scientific study of life
Gloria
Biology is the study of science. living and non living things in our environment. that's biotics and abiotic components of life.
Oluwadare
what are limit factors of photosynthesis
Mpolokeng Reply
what are limit factors of photosynthesis
Friday
Light intensity Carbon dioxide Temperature
Timileyin
what is virus
Chidimmma
virus are said to be microscopic organisms that can't be seen with naked eyes. but With the aid of electron microscope.
Oluwadare
are they unicellular or multicellular
Samkelo
sunlight.. water Carbon dioxide glucose
Oluwadare
what is a cell
Veronica Reply
are building blocks of the body
Mercy
cell is a smalles unit of life
Joeflexy
Cell is the basic ,structural and functional unit of life
ANGELINA
cell is the microscopic unit makes up an organ
Lance
Cell is the basic, structural and functional unit of life.
Esther
A cell is a basic fundamental unit which all organisms obtain
Musoke
A cell is a basic fundamental unit of an organism
Musoke
why do phosphate group attaches to the protein?
Abel Reply
what are enzymes?
Reiah Reply
enzymes are chemical substances that speed up the rate of chemical reactions
Favour
yes it is still available
hashim
This are chemical compound that speed up the digestion of food
Mayiik
these are chemical substances that speed up the rate of reactions and remain the same at the end of the reaction
kalisto

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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