Biology is the science of life and living organisms (Greek “bios” — life, “logos” — study). It studies the structure, life processes, development, origin and relationships with the environment of every living thing — from microscopic bacteria to giant trees and whales. The first exam questions often come from this section: the branches and methods of biology, the characteristics of life, the levels of organisation of living matter, and viruses. This topic acts as a “map” for all the others: each new chapter is easier when you know which level and which characteristic of life it relates to.
§1. Biology and its branches
Biology is a system of many sciences. Main branches by object of study:
botany — plants; zoology — animals; microbiology — microorganisms; mycology — fungi; virology — viruses; human anatomy and physiology — the human body.
By level of organisation and property of living matter:
molecular biology and biochemistry — the molecular basis of life;
cytology — the cell; histology — tissues; anatomy — structure; physiology — life processes (function);
genetics — heredity and variation; embryology — individual development;
ecology — relationships between organisms and their environment; evolutionary theory — the historical development of life; palaeontology — fossil organisms; systematics — classification of organisms.
New fields arise where sciences meet: biophysics, bioinformatics, biotechnology (using living organisms in production).
§2. Methods and history of biology
Main research methods:
observation — describing events in natural conditions;
experiment — changing conditions and comparing the result with a control group;
comparison — finding similarities and differences between organisms (systematics, comparative anatomy);
the historical method — studying how organisms arose and developed;
modelling — studying a process with a model (for example, a computer model);
microscopy, chemical and physical methods, statistical analysis.
From the history of biology: Aristotle was among the first to classify animals; Ibn Sina (Avicenna) described the human body and diseases in “The Canon of Medicine”; A. van Leeuwenhoek saw microorganisms; C. Linnaeus founded systematics; C. Darwin created the theory of evolution; G. Mendel discovered the laws of heredity; J. Watson and F. Crick worked out the structure of DNA (1953).
§3. Characteristics of life
The set of features that distinguish a living organism from a non-living object:
Unity of chemical composition — all living things are made of the same elements and contain proteins, nucleic acids, carbohydrates and lipids.
Cellular structure — all organisms except viruses are made of cells.
Metabolism and energy exchange — an organism is an open system: it takes in matter and energy and gives out wastes.
Irritability (response to stimuli) — plants show tropisms (growth towards light), animals show reflexes, protozoa show taxes.
Growth and development — increase in size and changes in form (from egg to butterfly).
Reproduction — producing offspring like oneself; it rests on DNA copying itself.
Heredity and variation — passing traits to offspring and the appearance of new traits.
Adaptation and evolution — fitting the environment and historical development.
Discreteness and integrity — an organism is made of separate parts (cells, organs) but works as one system.
No single characteristic proves life: a crystal also “grows”, a fire also “exchanges” matter and “spreads”. Only the whole set of characteristics defines a living thing.
Molecular level — DNA, RNA, proteins, ATP: storing and passing on hereditary information, metabolic reactions.
Cellular level — the cell is the smallest structural and functional unit of life.
Tissue and organ levels — in multicellular organisms cells specialise and form tissues and organs.
Organism level — an individual; in unicellular organisms the cellular and organism levels coincide.
Population–species level — a group of individuals of one species; the elementary unit of evolution is the population.
Ecosystem (biogeocoenosis) level — populations of different species together with their non-living environment; the cycling of matter, energy flow and food chains appear at this level.
Biosphere level — all life on Earth and its environment; global cycling of matter.
Each higher level includes the lower ones and has new properties that they lack: a population has a gene pool and changing numbers, an ecosystem has food chains, the biosphere has global cycles.
§5. Viruses
Viruses are a non-cellular form of life standing on the border between living and non-living.
Structure: a nucleic acid (DNA or RNA — only one) and a protein coat (capsid) around it; some have an outer lipid envelope (influenza, HIV).
No metabolism of their own — they multiply only inside a living cell, using its ribosomes and energy (obligate intracellular parasites). Outside cells some can even be kept as crystals.
Characteristics of life: they carry hereditary information, multiply, mutate (vary) and evolve.
Discovered by D. I. Ivanovsky (1892, tobacco mosaic virus).
Bacteriophages are viruses of bacteria: they attach by the tail and inject their DNA; when new phages have been assembled, the bacterium bursts and dies. Phages are used to treat some bacterial infections.
Diseases: influenza, measles, hepatitis, polio, rabies, HIV infection/AIDS (HIV is an RNA retrovirus: it makes DNA from its RNA by reverse transcription), COVID-19; in plants — tobacco mosaic.
§6. The practical importance of biology
Medicine — finding the causes of disease, vaccines, antibiotics, genetic diagnosis.
Agriculture — breeding new plant varieties and animal breeds, biological pest control.
Biotechnology — producing insulin, vitamins and enzymes with microorganisms; genetic engineering.
Nature conservation — protecting rare species, solving environmental problems, the Red Book.
Worked example
At which level of organisation does the idea of a “food chain” first appear?
At the organism level only one individual is considered.
A population consists of one species, but a food chain links different species.
Different species together with their non-living environment form a biogeocoenosis.
Answer: at the ecosystem (biogeocoenosis) level.
Worked example
Why do antibiotics not work against viral diseases?
Antibiotics act on bacterial structures such as the cell wall and ribosomes.
A virus has no cell wall or ribosomes of its own; it uses the host cell’s machinery.
Answer: the antibiotic has nothing in the virus to act on; vaccines and special antiviral drugs are used instead.
Worked example
A salt crystal “grows” in solution. Why is the crystal not alive?
A crystal grows by matter sticking to its surface from outside; there is no internal metabolism.
A crystal has no cells, no hereditary information (DNA or RNA), no response to stimuli, no homeostasis.
One characteristic (growth) does not prove life — the whole set is needed.
Answer: the crystal is non-living because it lacks the set of characteristics of life.
Check yourself
What is homeostasis?
Which features bring viruses close to living things?
Which levels coincide in a unicellular organism?
What does histology study?
Who discovered the tobacco mosaic virus?
What is the elementary unit of evolution?
A plant growing towards light is an example of which characteristic of life?
What is the protein coat of a virus called?
Common mistakes
Thinking a virus has both DNA and RNA — it has only one.
Believing one characteristic (such as growth) proves something is alive.
Mixing up the population–species and ecosystem levels — food chains appear at the ecosystem level.
Calling a bacteriophage a bacterium — it is a virus.