Living things are made of cells and share metabolism, response to stimuli, homeostasis, reproduction and heredity, and adaptation and evolution.
Metabolism balances anabolism, building large molecules, with catabolism, breaking them down for energy.
Cells stay alive by keeping that balance, while viruses sit on the edge of life without a cell structure or independent metabolism.
Slide the energy input and watch anabolism and catabolism change.
What is a Living Organism?
A change that only looks like matter shifting and a change that spends energy inside a cell to build and break molecules must not share one label. Wind carving a rock has no cell and no ATP, so it is not metabolism, and that is why the page sets several names side by side: being made of cells, metabolism, response to stimuli, homeostasis, reproduction and heredity, and adaptation and evolution. Declare life from the look of one scene and the definition wobbles; a virus stays on the boundary because it may carry genetic material yet cannot run synthesis and breakdown on its own. The list comes first so names are fixed before you pair them with cases.
🌱 What Does it Mean to Be Alive?
①What is the difference between a stone and a tree? — A tree grows, responds, and uses energy
②Living organisms share common properties
③Understanding these reveals "what life is"
④a virus sits on the living/non-living boundary: no cell, no independent metabolism
Visualizing Cells and Metabolism
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Balance of anabolism and catabolism in a cell — change with energy input
An oval cell is drawn with a nucleus in the middle, and on the left an orange arrow writes the energy input as a number. The blue bar above is anabolism (building) and its length is 0.6 times the input; the red bar below is catabolism (breakdown) and its length is 0.4 times the input plus 2, so even when you raise the input from 1 to 10 the breakdown side keeps a floor-like length. A yellow curve on the right is labeled homeostasis. There is only the energy slider, no picker for the six trait names, so those names are read separately in the classify box underneath. What changes is the length of the bars; the cell does not shift its place.
🔬 Key Observations
①Higher energy → anabolism (synthesis) becomes more active
②Catabolism (breakdown) stays at a baseline — survival energy
③Cells maintain homeostasis through balance of anabolism and catabolism
Six Characteristics of Life
Rolling wheels are not enough to call an object alive. A car changes place without cells, without running synthesis and breakdown on its own, and without leaving offspring. A rooted plant, on the other hand, does not walk and still bends toward light, keeps its internal conditions, and passes on DNA. Declare life from genetic material alone and you cannot explain a case that sits still like a crystal outside a host, and adaptation and evolution drop out of the same list if you treat them as one individual's change instead of a population's genes across generations. A verdict holds only when you check the six cells together, not one or two scenes.
Characteristics of Life
Common Properties of Living Things
Anabolism vs Catabolism
Metabolism Comparison
Anabolism vs Catabolism
Item
Anabolism
Catabolism
Direction
small → large molecules
large → small molecules
Energy
absorbed (endothermic)
released (exothermic)
Examples
photosynthesis, protein synthesis
cellular respiration, digestion
ATP
ATP consumed
ATP produced
Key of Metabolism
metabolism = anabolism + catabolism
Foundation of life — balance of synthesis and breakdown
Worked Examples
Example 1
Which of the following are anabolic processes? (a) photosynthesis (b) cellular respiration (c) protein synthesis (d) digestion
1
Anabolism builds large molecules from small ones and absorbs energy.
2
Photosynthesis (CO₂+H₂O→glucose) and protein synthesis (amino acids→protein) are anabolic. Respiration and digestion are breakdown (catabolic).
▸ (a), (c)
Anabolism = building / absorbs energy; catabolism = breaking down / releases energy.
Example 2
Explain why a virus shows both living and non-living characteristics, in terms of cells and metabolism.
1
Living traits: it carries genetic material (DNA/RNA) and reproduces inside a host cell, where heredity and mutation occur.
2
Non-living traits: it has no cell structure of its own and cannot perform independent metabolism, existing as a protein crystal outside a host.
▸ Genetic material/reproduction (living) vs no cell structure/no independent metabolism (non-living)
It fails the "cell" and "independent metabolism" criteria of life, so it sits at the living/non-living boundary.