Living things are made of cells, carry out metabolism, respond to stimuli while keeping their internal conditions steady, develop and grow, pass genetic information to offspring, and adapt and evolve over generations.
Non-living things can mimic some of these six features, so deciding whether something is alive means weighing all six together.
Metabolism is anabolism, building large molecules from small ones, plus catabolism, breaking them down for energy; viruses cannot do it on their own, which puts them on the border of life.
In the diagram you can pick a tree, a virus, a candle flame or a salt crystal and see how each of the six features is judged, and why.
What It Means to Be Alive
Whether something is alive cannot be decided from one feature. If growth were enough, a salt crystal would count; if using energy were enough, a candle flame would, since it draws in oxygen and gives off heat and light. So biology weighs six features together: being made of cells, metabolism, response to stimuli and homeostasis, development and growth, reproduction and heredity, and adaptation and evolution. Non-living things can mimic a few of these, but only living things have all six. Looking at them one by one is what lets us describe cases like viruses, which have only some.
🌱 What Being Alive Means
①Unlike a stone, a tree is made of cells, makes its own food, grows and leaves offspring
②Living things have all six shared features, while non-living things only mimic some of them in appearance
③A virus has genetic material and evolves, but it has no cell structure and no metabolism of its own, so it sits on the border between living and non-living
Is It Alive?
tree
seegongsik.com
A table judging whether a tree, a virus, a candle flame and a salt crystal have the six features of life
The title names the chosen object and how many of the six features it fully has. Each row below is one feature: a green circle means present, a red circle absent, and a yellow triangle present only under a condition, with the reason on the right. A tree is green in every row, while the candle flame and the salt crystal are red in every row, though one gives off heat and the other grows. A virus is red for cells and metabolism, yellow for reproduction because it multiplies only inside a host cell, and green for evolution because it changes through mutation.
🔬 What to Look for in the Table
①A tree has all six features, so it is alive
②A candle flame uses oxygen to give off heat and a salt crystal keeps growing, but neither has cells or metabolism, so every feature is absent
③A virus has no cells or metabolism, but it multiplies inside host cells and evolves through variation, so it has only some of the features
④Resembling life in one or two ways is not enough; all six must be weighed together
The Six Features of Life
Moving is not the same as being alive. A car burns fuel and moves, yet it has no cells, builds no body of its own and leaves no offspring. A rooted tree never walks, yet it bends toward light, opens and closes its stomata to control its water, and spreads seeds that carry its DNA. Another common mistake is to treat adaptation and evolution as changes in one individual's lifetime. Evolution changes a population's genetic makeup over many generations, which is why viruses, changing from generation to generation, match living things in the evolution row.
Characteristics of Life
Features Shared by Living Things
Anabolism and Catabolism
Metabolism Comparison
Anabolism and Catabolism
Item
Anabolism
Catabolism
Direction
small → large molecules
large → small molecules
Energy
absorbed (endothermic)
released (exothermic)
Examples
photosynthesis, protein synthesis
cellular respiration, digestion
ATP
uses ATP
makes ATP with the energy released
The Heart of Metabolism
metabolism = anabolism + catabolism
every metabolic reaction is a chemical reaction involving enzymes, with energy going in or out
Worked Examples
Example 1
Which of the following are examples of anabolism? (a) photosynthesis (b) cellular respiration (c) protein synthesis (d) digestion
1
Anabolism builds large molecules from small ones and absorbs energy.
2
Photosynthesis (carbon dioxide + water → glucose) and protein synthesis (amino acids → protein) are anabolism; cellular respiration and digestion break large molecules down, so they are catabolism.
▸ (a) and (c)
If molecules get bigger and energy is absorbed, it is anabolism; if they get smaller and energy is released, it is catabolism. The energy released by cellular respiration is stored in ATP, and anabolism spends that ATP.
Example 2
Explain, in terms of cells and metabolism, why viruses show features of both living and non-living things.
1
Living features: they have genetic material (DNA or RNA), multiply inside host cells, and evolve as mutations create variation.
2
Non-living features: they have no cell structure and cannot carry out metabolism on their own, so outside a host they exist much like protein crystals.
▸ genetic material, multiplication and evolution inside a host (living features) vs no cell structure and no metabolism of their own (non-living features)
Choose the virus in the table and the cell and metabolism rows turn red while the evolution row is green. Lacking some of the six features, viruses sit on the border between living and non-living.
Summary
The Six Features of Life
made of cells · metabolism · response to stimuli and homeostasis · development and growth · reproduction and heredity · adaptation and evolution
living things have all six; non-living things only resemble some of them on the surface
CSAT-style problem, adapted
Which pairing of a characteristic of life with its example is INCORRECT?
①Metabolism — ATP produced by cellular respiration
②Stimulus and response — a plant bending toward light
③Homeostasis — keeping body temperature constant
④Reproduction and heredity — parental traits passed to offspring
⑤Metabolism — a rock worn down by wind and rain
▸ ⑤ Metabolism — a rock worn down by wind and rain
1
Metabolism is the building up and breaking down of substances inside the cells of living things, with enzymes involved and energy going in or out.
2
A stone worn away by wind and rain is weathering, a physical and chemical change rather than a feature of life, so the pairing in ⑤ is incorrect.
🎯 Exam Points
①The cell is the basic unit of structure and function in living things
②Metabolism consists of anabolism (building, energy absorbed) and catabolism (breaking down, energy released), and enzymes are involved
③Many questions ask you to tell response to stimuli and homeostasis apart from development and growth by example
④A virus has no cell structure and no metabolism of its own, but it has genetic material, multiplies inside a host and evolves
⑤Adaptation and evolution happen in populations over many generations, not in individuals