← All notesBiology · Characteristics & classification⬇ Download PDF
IGCSE Biology 0610 · Chapter 1 notes · ~12 min read

Characteristics & classification

The seven things every living organism does, how scientists name and group organisms, how to use a dichotomous key — and the features of each kingdom, vertebrate and arthropod group.

MRS GRENSpecies & binomial namesDichotomous keysKingdomsVertebrates & arthropodsViruses
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • All living organisms show seven characteristics: movement, respiration, sensitivity, growth, reproduction, excretion, nutrition (MRS GREN).
  • A species is a group of organisms that can reproduce to produce fertile offspring. Every species has a two-part binomial name, e.g. Homo sapiens.
  • Organisms are sorted into groups by shared features. A dichotomous key uses yes/no questions to identify an organism.
  • EXT Classification reflects evolutionary relationships, and similar DNA base sequences mean a more recent common ancestor.

By the end you can

  • State and define the seven characteristics of living organisms.
  • Define species and write a binomial name correctly.
  • Use and construct a dichotomous key.
  • Describe the main features of animals, plants, the vertebrate classes and the arthropod classes.
  • Describe the five kingdoms, ferns, flowering plants and viruses. EXT

1What makes something alive?

The seven life processes

MMovementmoving the bodyor a part of itto a new placeRRespirationreactions in cellsthat releaseenergy from foodSSensitivitydetect stimuliand respondto themGGrowthpermanent risein size anddry massRReproductionmake more ofthe same kindof organismEExcretionremove toxicwaste and excesssubstancesNNutritiontake in materialsfor energy, growthand developmentEvery living organism does all seven: “MRS GREN”
The seven characteristics of living organisms. If a thing does not do all seven, it is not a living organism.

Movement — an action by an organism or part of an organism causing a change of position or place.

Respiration — the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Sensitivity — the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses.

Growth — a permanent increase in size and dry mass by an increase in cell number or cell size or both.

Reproduction — the processes that make more of the same kind of organism.

Excretion — removal from organisms of toxic materials and substances in excess of requirements (waste from metabolism).

Nutrition — taking in of materials for energy, growth and development. Plants need light, carbon dioxide, water and ions. Animals need organic compounds, ions and usually water.

Memory trickM R S G R E N — Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition. Two Rs: respiration and reproduction.
Exam tipAlways define each process using the key phrase: movement = change of position, growth = permanent increase, excretion = toxic or excess, respiration = release energy in cells.
Common mistake"Respiration is breathing." Breathing is moving air in and out of the lungs. Respiration is a chemical reaction in cells that releases energy.
Common mistake"Plants do not move." Plants move parts of themselves (leaves turn towards light, flowers open). Movement means a change of position of the organism or part of it.
Common mistake"Excretion is getting rid of faeces." Faeces are egested — undigested food that was never part of the body's cells. Excretion removes waste made by the cells (CO₂, urea).

A quick test: is it alive?

  • A flame seems to grow, move and use oxygen — but it does not reproduce, respond to stimuli, or make waste from cells. It is not alive.
  • A seed shows few signs of life, but it is alive: it respires slowly and will grow when conditions are right.
Picture itTreat the seven processes as a checklist. If you cannot tick every box, the thing is not a living organism.

2Species and naming

What is a species?

Species — a group of organisms that can reproduce to produce fertile offspring.

  • A horse and a donkey can breed to make a mule — but a mule is sterile (infertile), so horses and donkeys are different species.
  • Fertile matters. Two organisms that can only produce infertile offspring do not belong to the same species.

Binomial names

Binomial system — a naming system that gives every species a two-word Latin name: the genus (capital letter) then the species (small letter).

OrganismBinomial nameGenusSpecies
humanHomo sapiensHomosapiens
lionPanthera leoPantheraleo
tigerPanthera tigrisPantheratigris
maizeZea maysZeamays
  • Typed names are in italics. In handwriting, underline each word separately.
  • Lions and tigers share the genus Panthera, so they are closely related but are different species.
  • Why use Latin names? One name is used by scientists all over the world. Common names vary: "robin" means a different bird in Britain and in America.
Exam tipWrite it correctly: Genus with a capital, species in lower case, e.g. Homo sapiens, not homo Sapiens and not Homo Sapiens.

The classification hierarchy

Groups get smaller going downExample: humanKingdomAnimaliaPhylumChordataClassMammaliaOrderPrimatesFamilyHominidaeGenusHomoSpeciessapiensMnemonic: King Philip Came Over For Good SoupSpecies = organisms that can breed to produce fertile offspring
Organisms are put into groups within groups. Each step down contains fewer organisms that are more alike.
  • The groups, from biggest to smallest: kingdom → phylum → class → order → family → genus → species.
  • Biologists group organisms using shared features — what they look like, how their bodies work and what their DNA says.
Memory trickKing Philip Came Over For Good Soup = Kingdom, Phylum, Class, Order, Family, Genus, Species.

How classification worksEXTENDED

  • Classification should reflect evolutionary relationships: organisms that share a recent common ancestor are grouped together.
  • Early classification used appearance. Today, scientists also compare DNA base sequences: the more similar the sequences, the more closely related the organisms and the more recently they shared a common ancestor.
Worked example

EXT The DNA base sequence of the same gene in three species differs as follows: species P and Q differ in 2 bases out of 100, species P and R differ in 14 bases out of 100. Which two are more closely related, and why?

  1. P and Q differ least
  2. so their DNA is most similar
  3. so P and Q shared a common ancestor more recently than P and R did, which makes P and Q more closely related

3The main groups of organisms

Animals and plants (Core)

FeatureAnimalsPlants
cellsmany cells, no cell wallsmany cells with cellulose cell walls
chloroplastsnonepresent: they photosynthesise
nutritionfeed on organic substances made by other organismsmake their own food from CO₂ and water
carbohydrate storeglycogenstarch
movementmost can move from place to placefixed in place (parts do move)
coordinationnervous coordination, quick responsesslow responses

The five kingdomsEXTENDED

ANIMALSmany cellsno cell wallno chloroplastsfeed on other organismse.g. humans, insectsPLANTSmany cellscellulose cell wallchloroplasts: make foodby photosynthesise.g. ferns, maizeFUNGImostly threads (hyphae)chitin cell wallno chloroplastsfeed on dead mattere.g. mould, yeastPROTOCTISTSmostly single-celledhave a nucleussome have chloroplastse.g. Amoeba, ChlorellaPROKARYOTESsingle-celledNO nucleusDNA free in cytoplasmcell wall (not cellulose)e.g. bacteriaVIRUSESnot made of cellsnot in any kingdomprotein coat + DNA/RNAmultiply only insidehost cellsCore: animals and plants · Extended: all five kingdoms + viruses
The five kingdoms, plus viruses, which belong to no kingdom.
  • Animals and plants — as in the table above.
  • Fungi — a cell wall made of chitin, no chloroplasts. They feed by releasing digestive enzymes onto food and absorbing the products (saprotrophs, living on dead matter). Examples: Mucor (made of threads called hyphae), and yeast (single-celled).
  • Protoctists — mostly single-celled with a nucleus. Some have chloroplasts (Chlorella), some feed like animals (Amoeba). Plasmodium, which causes malaria, is a protoctist.
  • Prokaryotes — single-celled with no nucleus; DNA is a circular loop in the cytoplasm. Examples: Lactobacillus (makes yoghurt) and Pneumococcus (causes pneumonia).

Ferns and flowering plantsEXTENDED

FeatureFernsFlowering plants
leavesfronds (often divided into many leaflets)many shapes
reproductionspores, no flowers or seedsflowers, then seeds inside fruits
roots and veinsyesyes

Vertebrates

FishsalmonAmphibiansfrogReptileslizardBirdspigeonMammalshumanSkinBreathingEggsyoungBodytemp.wet scalesmoist, thinno scalesdry scalesfeathershair or furgillslungs andskinlungslungslungssoft eggsin waterjelly eggsin waterleathery eggson landhard-shelledeggs on landlive youngfed on milkvaries withsurroundingsvaries withsurroundingsvaries withsurroundingsconstant(warm)constant(warm)Warm-blooded = constant body temperature (birds, mammals)
The five classes of vertebrate (animals with a backbone).
  • All vertebrates have a backbone. Invertebrates (insects, snails, worms…) do not.
  • Warm-blooded (birds, mammals) means the body keeps a constant temperature whatever the surroundings. The others change temperature with their surroundings.
  • Mammals have hair or fur, mammary glands (they feed milk to their young) and give birth to live young.
  • Birds have feathers, wings, a beak and lay hard-shelled eggs.
Exam tipRemember one feature for each class: fish = gills, amphibians = moist skin, reptiles = dry scales, birds = feathers, mammals = hair.

Arthropods

INSECTS3 body parts6 legsusually wings1 pair of antennaeARACHNIDS2 body parts8 legsno antennaeno wingsCRUSTACEANS2 body parts5+ pairs of legs2 pairs of antennaeMYRIAPODSlong body, manysegmentsmany legs (1–2 pairsper segment)All arthropods: jointed legs · hard outer skeleton (exoskeleton) · segmented body
Arthropods have jointed legs and a hard exoskeleton. Count the legs and body parts to name the group.
ClassBody partsLegsOther features
insects361 pair of antennae, usually wings
arachnids28no antennae, no wings
crustaceans25 or more pairs2 pairs of antennae
myriapodsmany segmentsmany pairs1 pair of antennae (centipedes and millipedes)
Common mistake"Spiders are insects." Spiders have 8 legs and 2 body parts: they are arachnids.

VirusesEXTENDED

protein coatgenetic material(DNA or RNA)surface proteins(attach to host cell)● NOT made of cells: no nucleus, cytoplasm or membrane● Reproduce ONLY inside a living host cell● Much smaller than bacteria (about 100 nm across)● Antibiotics do not work on viruses● Examples: influenza, HIV, tobacco mosaic virus
A virus is not a cell. It is a piece of genetic material inside a protein coat.
  • Viruses have no cell structure: no nucleus, cytoplasm or cell membrane. They are far smaller than bacteria.
  • They reproduce only inside living host cells, so they are not placed in any kingdom.
  • Their genetic material is DNA or RNA. Examples: influenza, HIV, tobacco mosaic virus.
  • Antibiotics do not kill viruses (see Diseases, immunity & drugs).

4Dichotomous keys

Using a key

Does it have legs?Does it have a shell?How many legs?noyessnailearthwormantspideryesnosixeightThe same key written as numbered pairs of statements:1aLegs present → go to 21bNo legs → go to 32aSix legs → ant2bEight legs → spider3aShell present → snail3bNo shell → earthworm
A dichotomous key is a series of two-choice questions. Follow the answers until you reach one named organism.

Dichotomous key — a way to identify organisms using a series of paired statements (yes/no questions), each leading to another question or to a name.

  • Start at question 1. Choose the statement that fits. Follow the instruction (go to 2, or the name).
  • Use features you can observe (number of legs, wings, shell) — not size or colour, which vary within a species.

Building your own key

  • 1 List the features of each organism.
  • 2 Pick a feature that splits the group into two sets. Write it as a yes/no question.
  • 3 Repeat on each set until every organism is alone at the end of a branch.
  • 4 Check each organism can be reached by only one route. Test the key on a friend.
Worked example

Use the key on this page: a small animal has no legs and no shell. What is it?

  1. Question 1: does it have legs? No
  2. Question 2: does it have a shell? No
  3. the animal is an earthworm

5Test yourself

Test yourself — cover the answers first

Quick check

  1. Name the seven characteristics of living organisms.
  2. Define excretion.
  3. What is a species?
  4. Write the binomial name for humans correctly.
  5. How many legs and body parts does an insect have?
  6. Name the vertebrate class that has feathers.
  7. What is the difference between respiration and breathing?
  8. EXT Name the five kingdoms.
  9. EXT Why are viruses not placed in any kingdom?

Exam-style

  1. Define the term growth. [2]
  2. A mule is the offspring of a horse and a donkey. Explain why horses and donkeys are classed as different species. [2]
  3. Give three differences between a fungus and a plant. [3]
  4. EXT Explain how DNA sequences help scientists to classify organisms. [3]
Show answers
  1. movement, respiration, sensitivity, growth, reproduction, excretion, nutrition
  2. removal of toxic materials and substances in excess of requirements from organisms
  3. a group of organisms that can reproduce to produce fertile offspring
  4. Homo sapiens (genus capital, species lower case)
  5. 6 legs, 3 body parts
  6. birds
  7. respiration is a chemical reaction in cells releasing energy; breathing is moving air in and out of the lungs
  8. animals, plants, fungi, protoctists, prokaryotes
  9. they are not made of cells and only reproduce inside host cells
  10. a permanent increase
    in size and dry mass (by an increase in cell number, cell size or both)
  11. a mule is sterile
    so they cannot produce fertile offspring
  12. fungi have chitin cell walls, plants have cellulose
    fungi have no chloroplasts, plants do
    fungi feed by absorbing digested food, plants make their own by photosynthesis
  13. compare the base sequences of the same gene
    the more similar the sequences, the more closely related
    they share a more recent common ancestor
Practise this chapter
Characteristics of living organismsClassificationFeatures of organisms
Next chapterCells & organisation →
Stuck on a question?

Send it to us on WhatsApp. We read every message.

Want regular help? Send this to your parent →
Share with a friendAsk on WhatsApp