Living things are distinguished from non-living things by a set of shared characteristics. These features, often called the characteristics of life, include:
1. Organization: Living organisms are highly organized, with complex structures at various levels:
- Cellular level: All living things are composed of at least one cell.
- Tissue level: Similar cells work together to form tissues.
- Organ level: Different tissues cooperate to form organs.
- Organ system level: Organs work together to form organ systems.
- Organism level: All organ systems function together to form a complete organism.
2. Metabolism: Living things carry out chemical reactions to maintain life. These reactions include:
- Anabolism: Building complex molecules from simpler ones (e.g., photosynthesis).
- Catabolism: Breaking down complex molecules into simpler ones (e.g., cellular respiration).
3. Growth and Development: Living organisms increase in size and complexity over time. Growth involves an increase in the number of cells or the size of existing cells. Development refers to changes in an organism's form and function.
4. Responsiveness: Living things can react to stimuli (changes in their environment). These responses can be immediate (e.g., pulling your hand away from a hot stove) or gradual (e.g., a plant turning towards the sun).
5. Reproduction: Living organisms produce offspring to ensure the continuation of their species. This can be sexual (involving two parents) or asexual (involving only one parent).
6. Adaptation: Living things evolve over time, adapting to changes in their environment to ensure their survival. Adaptations can be physical (e.g., camouflage) or behavioral (e.g., migration).
7. Homeostasis: Living things maintain a stable internal environment, even when the external environment changes. This internal stability is essential for life and includes factors like temperature, pH, and nutrient levels.
These features, while generally applicable, are not always absolute. For example, viruses are not cellular and can't reproduce on their own, but they exhibit other features of life. This raises the question of what defines life, a topic that continues to be debated.
