Big Ideas
Characteristics of Life

There are a few aspects of living things that are common to all living things. These characteristics of living things include: Are made up of one or more cells, can reproduce, grow and develop, obtain and use energy, and respond to their environments. Knowing these characteristics and being able to identify them in an organism can determine whether or not something is living.

Living things are made up of small self-contained units called cells. Each cell is a collection of living matter enclosed by a barrier that separates the cell from its surroundings. Most cells can perform all the functions we associate with life. Organisms consisting of only a single cell are called unicellular. Most of the organisms you are familiar with, however, are multicellular, meaning they are made up of many cells.

Living things can reproduce, or produce new organisms of the same type. There are two basic types of reproduction; sexual reproduction and asexual reproduction. Sexual reproduction requires the two cells from different individuals unite to produce the first cell of a new organism. In asexual reproduction a single organism can reproduce without the aid of another.

All living things at one stage or another are capable of growth. During growth, most living things go through a cycle of change called development. Living things obtain energy from their environment and use that energy to grow, develop, and reproduce. All organisms require energy to build the substances that make up their cells. Any process in a living thing that involves putting together or synthesizing, complex substances from simpler substances is called anabolism (Photosynthesis). The breaking down of complex substances into simpler ones, resulting in the release of energy is called catabolism. The total sum of all chemical reactions in the body – the balance of anabolism and catabolism – is called metabolism.

Living things respond to their environments. Anything in the environment that causes a change is called a stimulus. Organisms react to many stimuli, including light, temperature, odor, sound, gravity, heat, water, and pressure. The ability of living things to react to stimuli is known as irritability (this does not mean grouchy). The process by which organisms respond to stimuli in ways that keep conditions in their body suitable for life is called homeostasis.

Experiences Patterns Explanations

Sewer lice

Things that can be observed about living things:
Are made up of one or more cells
can reproduce
grow and develop
obtain and use energy
respond to their environments

Things that are living appear to fulfill all characteristic of living things

Things that are not living can fulfill some but not all characteristics

Living things must fulfill all characteristics of living things to be considered living


L2.p1A Distinguish between living and nonliving systems. (prerequisite)

L2.p1C Describe growth and development in terms of increase in cell number, cell size, and/or cell products.

L2.p1D Explain how the systems in a multicellular organism work together to support the organism. (prerequisite)

L2.p1E Compare and contrast how different organisms accomplish similar functions (e.g., obtain oxygen for
respiration, and excrete waste). (prerequisite)