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Cells
Living organisms are composed of one or more cells. A cell is a collection of living matter, which is encased within a protective barrier that shields it from its surroundings. Entities like bacteria are unicellular -- consisting of only one cell -- while organisms like animals and human beings are multicellular.
Reproduction
Living organisms can create new organisms like themselves. This reproduction can be asexual or sexual in nature. In asexual reproduction, the organism reproduces on its own without involving another organism. Examples of asexual reproduction include microscopic organisms, bacteria and some plants. In sexual reproduction, organisms require the unity of cells from different individual organisms in order to produce a new organism. There must be the union of a male sperm cell with a female egg cell.
Energy
Living organisms take in and use raw materials from their surroundings as energy for growth and tissue repair. Humans and animals do this through eating food. Human beings and animals eat food. Plants have to take in carbon dioxide and water to make their food.
Waste Excretion
Living organisms possess the ability and need to remove waste materials and substances from within themselves. Within all living organisms is a chemical reaction, which produces substances that have to be gotten rid of. Sometimes these substances are toxic; other times these substances are not toxic or harmful, but exist in excess of what the organism needs. The casting off of these waste materials is referred to as excretion.
Reactions
Living organisms can react to the environment around them. They can detect and sense surrounding changes and respond to them. These changes include shifts in light and increases or drops in temperature. Very often, living organisms use this response to surroundings as a means of self-preservation: a move away from danger and toward survival. An example of this is the way plants shift the position of their leaves to increase their access to needed sunlight.