Week 1, Session 1: What Makes Life Alive?

Estimated time: 70 minutes
NGSS connection: HS-LS1-2; Developing and Using Models

Start with a puzzle

A seed can sit in a dry envelope for years without moving, eating, or visibly growing. A flame moves, uses fuel, gives off waste, and can spread. Yet biologists call the seed alive and the flame nonliving. Clearly, one dramatic behavior is not enough to define life. We need a pattern of evidence.

By the end, you can

• describe the shared characteristics of living systems;
• arrange biological organization from molecules to the biosphere; and
• explain why an emergent property appears at one level but not at a lower level.

Learn the idea

Living things show an organized set of characteristics. They are made of one or more cells, use energy and matter in metabolism, maintain internal stability through homeostasis, grow and develop, respond to their environment, reproduce, store hereditary information in DNA, and belong to populations that evolve over generations. Not every organism displays each characteristic every moment. A hibernating bear has a slow metabolism, and a sterile worker ant cannot reproduce by itself, but both belong to living systems that show the full pattern.

Biology also works across levels of organization. Atoms combine into molecules. Large biological molecules build organelles, and organelles work inside cells. Similar cells may form tissues; tissues cooperate in organs; organs join organ systems; and organ systems sustain an organism. Organisms of one species in one area form a population. Interacting populations form a community. A community plus its nonliving environment forms an ecosystem. All ecosystems together make up the biosphere.

New abilities can appear when simpler parts interact. A molecule of cardiac muscle protein cannot pump blood. Millions of organized cardiac cells form tissue that contracts, and several tissues form a heart that moves blood through the body. Pumping is therefore an emergent property. It depends on the parts, but it belongs to their organized system.

Life is both unified and diverse. A bacterium, an oak tree, and a human use DNA, cell membranes, ribosomes, and many of the same chemical pathways. At the same time, evolution has produced strikingly different structures and ways of living. Biologists look for both the shared rules and the useful variations.

Study the visual

Alt text: A wetland ecosystem narrows through magnified views of an organism, a heart, cardiac tissue, an animal cell, and DNA.

Read the image from left to right. Name each level you recognize. Then reverse direction and explain how a change in DNA might eventually affect the wetland. One reasonable chain is: a DNA change alters a protein; the protein changes a cell; affected cells change tissue or organ function; the organism's survival or behavior changes; repeated across a population, that change can influence relationships in the ecosystem.

Try it yourself

Choose a familiar living thing. Create a seven-step "zoom map" that connects one molecule in that organism to its ecosystem. At every arrow, write one sentence explaining what becomes possible at the next level.

Check yourself

1. A robot senses obstacles, uses electricity, and moves. Why is that not enough to classify it as alive?
2. Put these in order: organ, cell, organism, tissue, organ system.
3. Is sweating a response, homeostasis, or both? Explain.
4. Why is reproduction not a requirement that every individual organism must personally meet?

Answer and feedback

1. Several life-like behaviors can be engineered. The robot does not show the complete cellular, metabolic, hereditary, and evolutionary organization of life.
2. Cell → tissue → organ → organ system → organism.
3. Both. The body responds to increased temperature, and evaporation helps restore a stable internal temperature.
4. Some living individuals are sterile or not yet mature. Reproduction is a property of the continuing biological system or population.

Key vocabulary: cell, metabolism, homeostasis, evolution, emergent property, population, community, ecosystem

Exit reflection: Which characteristic of life seems easiest to observe? Which requires the most evidence?