字幕列表 影片播放 列印英文字幕 Imagine you are taking a walk on a trail through the park. Trees and flowers surround the rocky pathway, birds chirp from nearby branches, clouds float overhead, and a creek babbles nearby. Some of these things are living and some are clearly not living. But where do we draw the line between living and non-living things? How do biologists specifically define life? Living things all share seven distinct characteristics. The grow and develop, they are able to reproduce, they respond to their environment, they use energy they maintain homeostasis, they have cellular organization, and they are able to adapt, evolve, and pass on their genetic information in the form of DNA. So let’s look at these characteristics in terms of the things you encounter on your walk. The trees that surround you are living things. They started out as a single seed, then underwent growth and development to become trees that reproduce again by producing more seeds. They respond to their environment by growing towards light. They make their own sugars, which they then metabolize for energy. They keep their internal environments fairly constant. If you were to zoom in closely on a leaf using a microscope, you would see that it is composed of tiny individual units called cells. DNA resides within the nucleus of each cell. In contrast, let’s consider the creek running alongside the path. While it may grow larger or change size due to increased rainfall or other weather conditions, it is not made up of cells. Water does not contain DNA. It doesn’t reproduce or maintain homeostasis.