Functions of Human Life

Core Concepts

Organization

All life depends on organized structure — the six-level hierarchy from Topic 2. Organization also means upkeep: molecules are replaced, damaged cells are repaired or removed, worn structures are rebuilt. A living body actively maintains its order, which is one thing that separates living from non-living systems.

Metabolism

Metabolism is the sum of all chemical reactions in the body, running in two directions at once. Catabolism breaks larger molecules into smaller ones and releases energy — cells break down glucose and capture the energy in its bonds. Anabolism builds larger molecules from smaller ones and requires energy — assembling amino acids into proteins, building bone. Released energy is captured in ATP (adenosine triphosphate), the molecule often called the body's "energy currency," which cells spend to power contraction, synthesis, and transport. The process that makes most ATP, cellular respiration, requires oxygen — which is why oxygen is a requirement for human life (Topic 4). Enzymes, protein catalysts studied in Chapter 2, speed all of these reactions.

Responsiveness (excitability)

Responsiveness is the ability to sense changes in the environment — stimuli — and react. Examples: pulling your hand off a hot surface, pupils constricting in bright light, shivering in the cold, white blood cells streaming toward an infection. Nervous and muscle tissue are the most excitable tissues, which makes sense: nerves must detect and transmit changes, and muscles act on them. Note the two halves of the process — sensing and responding — and be ready to say which half is failing.

Movement

Movement happens at every scale. At the cellular level, white blood cells crawl through tissues, sperm swim using flagella, and muscle cells shorten when they contract; inside cells, motor proteins haul cargo along internal tracks. At the organ and whole-body level, movement includes walking, the heart pumping, the diaphragm rising and falling with each breath, and food being churned along the digestive tract. Most of it is powered by the ATP produced during metabolism.

Development

Development is the sum of changes over a lifetime, with two components students often mix up. Growth is an increase in body size — more cells (via cell division), larger cells, and added material between cells. Differentiation is unspecialized cells becoming specialized: an early embryonic cell can become a muscle cell, a nerve cell, or a red blood cell depending on the signals it receives. Differentiation is most dramatic in the embryo (Topic 1's embryology), but it continues throughout life — stem cells in bone marrow mature into the various blood cell types daily.

Reproduction

Reproduction has two distinct meanings. Cellular reproduction is cell division: replacing worn-out cells, repairing injuries, and supporting growth — how the body renews itself. Organismal reproduction is producing offspring, the function of the reproductive system. When a question says "reproduction," check the context: in a chapter about cells it usually means division; in a chapter about the reproductive system, a new individual.

Homeostasis: the point of it all

Each process helps maintain a stable internal environment: organization keeps the machinery coherent, metabolism supplies energy and building blocks, responsiveness lets the body react, movement carries out the reactions, and development and reproduction renew the body and species. When they succeed, the body stays in balance; when they fail, the result is homeostatic imbalance — Topic 5, and the physiological definition of disease.

Common Confusions