Source: Lecture notes / textbook readings
Difficulty: Introductory | Prerequisites: None
This topic is the foundation of every biology course. Before you study cells, genetics, ecology or evolution in detail, you need a shared vocabulary for what counts as "alive" and why. The characteristics of life are the framework biologists use to classify organisms, and understanding where viruses fall (or do not fall) within that framework is a perennial exam topic. If you have missed earlier lectures, start here: everything else in the course builds on these definitions.
You should already be comfortable with basic chemistry concepts (atoms, molecules) and have a general sense of what DNA is, though both will be covered more fully later.
All living things share a set of properties: they are made of cells, contain genetic material, metabolise energy, maintain homeostasis, grow, reproduce, respond to stimuli and evolve. Viruses have some of these traits (genetic material, evolution) but lack others (independent metabolism, reproduction), so most biologists classify them as non-living biological entities that sit in a grey area.
Cell
The basic structural and functional unit of all living organisms. Think of it as the smallest package that can carry out all the activities we associate with life.
Metabolism
The sum of all chemical reactions occurring within an organism to maintain life. In simple terms, this means everything your body does to convert food into energy and raw materials.
Catabolism
The subset of metabolism that breaks down molecules to release energy. Think of it as the "tear down" side of metabolism.
Anabolism
The subset of metabolism that uses energy to build complex molecules (proteins, membranes, DNA). The "build up" side.
Homeostasis
The ability of an organism to regulate and maintain stable internal conditions (temperature, pH, osmotic balance) despite changes in the external environment. In simple terms, your body's thermostat and all the systems like it.
DNA (deoxyribonucleic acid)
The molecule that carries hereditary information in most organisms. It encodes the instructions for building proteins.
RNA (ribonucleic acid)
A nucleic acid involved in protein synthesis and, in some viruses, serves as the primary genetic material instead of DNA.
Adaptation
A heritable trait that enhances an organism's survival and reproductive success in a specific environment. In simple terms, a feature shaped by natural selection to fit the organism's surroundings.
Natural selection
The mechanism of evolution in which individuals with advantageous heritable traits survive and reproduce at higher rates, causing those traits to become more common over generations.
Capsid
The protein coat that encloses a virus's genetic material. Think of it as the virus's protective shell.
Obligate parasite
An organism (or entity, in the case of viruses) that cannot complete its life cycle without a host. Viruses are the textbook example: outside a host cell they are inert.
Asexual reproduction
Reproduction involving a single parent that produces genetically identical offspring (clones).
Sexual reproduction
Reproduction involving two parents whose genetic material recombines, producing offspring with genetic variation.
Biologists identify eight properties that all living organisms share. An entity must exhibit all of them to be classified as alive.
Cellular organisation. All life is made of one or more cells. Single-celled organisms (bacteria, many protists) carry out every life function in one cell. Multicellular organisms (plants, animals, fungi) divide labour among specialised cells.
Genetic material. Every organism contains DNA or RNA that encodes hereditary information. Genes carry instructions for building proteins, and these instructions pass from parent to offspring.
Order and organisation. Living things display highly ordered structures, from the molecular level (protein folding) up to organ systems.
Metabolism. Organisms carry out chemical reactions to obtain and use energy. Catabolism breaks molecules down to release energy; anabolism uses energy to build complex molecules.
Growth and development. Organisms increase in size and complexity following genetic instructions.
Homeostasis. Internal conditions (temperature, pH, water balance) are actively regulated. Examples: sweating cools the body; shivering generates heat; hormones adjust blood sugar.
Reproduction. Species persist by producing new individuals, either asexually (one parent, genetically identical offspring) or sexually (two parents, genetic recombination introduces variation).
Response to stimuli. Organisms detect and react to environmental changes, from a bacterium swimming toward nutrients to a human pulling a hand away from a hot surface.
A ninth property, evolution through adaptation, is sometimes listed separately: populations change over generations through natural selection, with advantageous traits becoming more common.
Cells handle energy production, waste removal and information processing. They range from simple prokaryotic cells (no membrane-bound nucleus) to complex eukaryotic cells (nucleus and organelles). Regardless of type, the cell is the smallest unit that can independently carry out all life functions.
DNA is the primary hereditary molecule in all cellular life. Some viruses use RNA instead. Genes, which are segments of DNA (or RNA), encode proteins that carry out cellular work. The transmission of genetic information from parent to offspring ensures continuity of traits and allows populations to evolve.
Living organisms need a continuous supply of energy. That energy comes from sunlight (photosynthesis) or from chemical sources (chemosynthesis, or breaking down food). Catabolism releases energy by breaking bonds; anabolism consumes energy to form new bonds. Together they sustain movement, growth and reproduction.
Homeostasis is not a fixed state but a dynamic equilibrium. The body continuously adjusts. A few examples worth remembering:
Temperature regulation: vasodilation (blood vessels widen to release heat) vs vasoconstriction (vessels narrow to conserve heat)
Blood glucose: insulin lowers it, glucagon raises it
Osmotic balance: kidneys adjust water and solute reabsorption
Asexual reproduction is faster and requires only one parent, but produces clones with no genetic variation. Sexual reproduction is slower and requires two parents, but the recombination of genetic material generates variation, which is the raw material for natural selection.
Viruses occupy a grey area between living and non-living. They possess some life-like properties but fail others.
What viruses have:
Genetic material (DNA or RNA)
The ability to evolve through mutation and natural selection
Structural organisation (genetic material enclosed in a protein capsid, sometimes with a lipid envelope)
What viruses lack:
Cells (they are not made of cells)
Independent metabolism (they cannot generate or use energy on their own)
Independent reproduction (they must hijack a host cell's machinery to replicate)
Growth and development
Homeostasis
Because viruses cannot metabolise or reproduce without a host, most biologists classify them as biological entities rather than living organisms. They are obligate parasites: without a host cell, a virus particle (virion) is inert.
Structure of a virus:
Genetic core: DNA or RNA (single- or double-stranded)
Capsid: the protein shell surrounding the genetic material
Envelope (some viruses): a lipid membrane derived from the host cell, which helps the virus attach to and enter new hosts
The scientific consensus is that viruses are best described as occupying a grey area, possessing some properties of life but not fulfilling all the criteria.
The characteristics-of-life framework is how microbiologists decide whether a newly discovered entity (e.g. giant viruses such as Mimivirus) should be classified as living. It also underpins medical virology: because viruses are not alive in the conventional sense, antibiotics (which target living bacterial cells) do not work against them, which is why viral infections require antivirals or vaccines instead.
Students often think viruses are alive because they can evolve. Evolution alone is not sufficient; an entity must also metabolise and reproduce independently.
Students sometimes confuse "metabolism" with "eating." Metabolism includes all chemical reactions in a cell, not just digestion.
Homeostasis does not mean conditions never change. It means the organism actively works to keep internal conditions within a viable range.
Asexual reproduction does not mean an organism reproduces "by itself" in the colloquial sense. It means there is only one genetic parent and the offspring are clones.
⚠️ Be able to list all eight characteristics of life and give a one-sentence example of each.
⚠️ Expect a question asking you to explain why viruses are not considered alive, referencing specific characteristics they lack.
⚠️ Know the difference between catabolism and anabolism, with an example of each.
⚠️ Understand that homeostasis is active regulation, not a static state.
⚠️ The virus "grey area" question appears frequently. Be ready to name the properties viruses have (genetic material, evolution) and the ones they lack (cells, independent metabolism, independent reproduction).
True or false: Viruses can carry out metabolism independently. (False. They require a host cell's machinery.)
Fill in the blank: The protein coat surrounding a virus's genetic material is called a __________. (Capsid.)
True or false: Homeostasis means an organism's internal conditions never change. (False. It means conditions are actively regulated to stay within a viable range.)
Fill in the blank: Breaking down nutrients to release energy is called __________. (Catabolism.)
True or false: All living organisms contain DNA or RNA. (True.)
Q: List four characteristics that all living organisms share and briefly explain each.
A: (1) Cells – all life is composed of one or more cells. (2) Metabolism – organisms carry out chemical reactions to obtain and use energy. (3) Homeostasis – organisms regulate internal conditions such as temperature and pH. (4) Reproduction – organisms produce new individuals, either sexually or asexually.
Q: Why are viruses not considered living organisms?
A: Viruses lack several key characteristics of life. They have no cells, cannot carry out metabolism on their own, and cannot reproduce without hijacking a host cell's machinery. They are classified as biological entities in a grey area between living and non-living.
Q: A student argues that fire is alive because it grows, reproduces (spreads), and responds to stimuli (wind). Explain why fire does not meet the biological definition of life.
A: Fire is not composed of cells, does not contain genetic material, does not maintain homeostasis, and does not evolve through natural selection. While it superficially resembles some life characteristics, it fails the majority of the criteria.
Q: Distinguish between catabolism and anabolism. Give one example of each.
A: Catabolism is the breakdown of complex molecules to release energy (e.g. cellular respiration breaking down glucose). Anabolism is the synthesis of complex molecules using energy (e.g. building proteins from amino acids).
Q: Explain how sexual reproduction contributes to evolution.
A: Sexual reproduction introduces genetic variation through recombination of parental DNA. This variation provides the raw material for natural selection: individuals with advantageous trait combinations are more likely to survive and reproduce, shifting allele frequencies in the population over time.
This material connects directly to the evolution and natural selection unit: the characteristics of life (especially reproduction with variation and response to environmental pressures) are the preconditions for evolution to occur. It also connects to the cell biology unit, where you will study how cells actually carry out metabolism, maintain homeostasis and process genetic information. Understanding why viruses are not alive will come back when you study infectious disease and immunology.
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