We learned that cells want to maintain a surrounding that is not too hot, too cold, too wet, or too dry even though the environment is constantly changing. This maintenance of a constant internal state in a changing environment is called homeostasis. Homeostasis ensures that cells can carry out tasks such as obtaining energy, making new cells, exchanging materials, and eliminating waste. Organisms maintain homeostasis by the cells working together. When we are cold muscle cells will contract to generate heat by shivering. Some trees lose leaves in the winter to reduce the amount of water loss. The green pigment, chlorophyll used for photosynthesis breaks down changing the color of the leaves before they fall.