Life Science: Living Systems, Energy Flow, and Inheritance
What to know
Living things share a set of observable characteristics: they are made of cells, obtain and use energy, grow and develop, respond to stimuli, reproduce, and maintain a stable internal environment. Viruses are a classic edge case - they reproduce only inside a host cell and are usually described as non-living outside one. Cells are the basic unit of structure and function; plant cells add a cell wall and chloroplasts, but plant and animal cells both carry out cellular respiration.
Organisms are classified by shared characteristics, from broad groups down to species. In ecosystems, producers capture energy (usually from sunlight), consumers eat other organisms, and decomposers break down dead material and return nutrients to the soil. Arrows in a food chain point in the direction energy flows - from the organism being eaten to the eater. Matter cycles through an ecosystem, but energy flows one way and is gradually lost as heat.
Adaptations are inherited traits that help survival in a particular environment; an individual adjusting within its lifetime (a tan, a thicker winter coat) is not an adaptation. Traits pass from parents to offspring through genes, and both inherited traits and environmental factors shape how an organism looks and behaves.
Traps to watch
Plants are often assumed not to respire - they do, day and night. "Energy cycles" is wrong; matter cycles. Watch for questions that confuse a population with a community, or a habitat with a niche.
Worked example
If producers hold 10,000 units of energy and roughly 10% passes to each next level, primary consumers get 1,000, secondary consumers 100, and a third-level consumer about 10. Rule of thumb: at each step up a food chain, expect only about one-tenth of the energy to move up.