Biology: Cells to AP Biology · Module 9 of 15
Human Body Systems & Classification
How the body keeps itself stable, an overview of its major systems, and how life is classified.
Homeostasis & Feedback Loops
- Homeostasis is the body's ability to maintain a stable internal environment despite changes outside it; most homeostatic control uses negative feedback, a response that counteracts the change.
- A negative feedback loop has a sensor (detects the change), a control center (processes the signal, often the brain), and an effector (produces the response) — like shivering when body temperature drops.
Blood glucose rises after a meal. How does the body respond, and what kind of feedback is this?
Solution. The pancreas releases insulin, which signals cells to take up glucose from the blood, bringing blood sugar back down toward its normal set point. This is negative feedback — the response counteracts the original change.
Overview of Major Organ Systems
- The circulatory system (heart, blood vessels, blood) transports oxygen, nutrients, and wastes throughout the body; the respiratory system (lungs, airways) exchanges oxygen and carbon dioxide with the environment.
- The nervous system (brain, spinal cord, nerves) sends fast electrical signals to control the body; the digestive system (stomach, intestines) breaks food into nutrients the body can absorb.
After exercise, both your heart rate and breathing rate increase. Why?
Solution. Contracting muscles need more oxygen and glucose and produce more CO2 waste. The circulatory system speeds up to deliver oxygen and nutrients and remove waste faster, while the respiratory system speeds up to take in more oxygen and expel more CO2.
Classification & Taxonomy
- Taxonomy organizes life into a nested hierarchy, broadest to narrowest: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
- Binomial nomenclature names each species with two parts — genus and species (e.g. Homo sapiens) — giving every organism one scientific name recognized worldwide.
Why do biologists use scientific names like Homo sapiens instead of common names like "human"?
Solution. Common names vary by language and region and can refer to different or multiple species. A two-part scientific name is standardized worldwide, so any scientist knows exactly which species is meant.
Domains, Kingdoms & Biodiversity
- The three domains of life are Bacteria, Archaea (both prokaryotic, lacking a nucleus), and Eukarya (organisms with a membrane-bound nucleus, including the kingdoms Protista, Fungi, Plantae, and Animalia).
- Viruses aren't classified in any domain since they aren't made of cells and can't reproduce on their own — they hijack a host cell's machinery to replicate.
Is a virus alive?
Solution. Most biologists say no, by the standard definition: viruses can't grow, respond to stimuli, or reproduce independently — they must hijack a host cell's machinery to make copies of themselves.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is homeostasis?
2. Blood sugar rises after a meal, and insulin is released to bring it back down. What kind of feedback is this?