Characteristics and classification of living organisms

• If your reason for putting an organism in a group would still be true of an organism in a different group, it is not a classification feature. "Lives in water" is true of a shark, a whale, a crab and a duck.
• The initials spell MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition. The mnemonic gets you the seven words. It gets you none of the seven definitions, and the definitions are where the marks are.
• Groups of organisms which share a more recent ancestor — that is, groups which are more closely related — have base sequences in DNA that are more similar than those of groups that share only a distant ancestor.
• If you can reproduce the first three grids from memory, you can answer any Core recall question in Topic 1. Everything else in this chapter exists to make the grids make sense — and to teach the reasoning the grids alone cannot.
• One question carries most of the Core marks: is there a cell wall, and what is it made of? No wall at all means animal; a cellulose wall with chloroplasts means plant. Extended candidates add a second question, is there a nucleus?, which separates prokaryotes from everything else.
• Two things are worth over-learning, because a single missing word changes whether the definition is right and because you will need both of them again in later topics: a species is a group of organisms that can reproduce to produce fertile offspring, and growth is a permanent increase in size and dry mass.

• Describe all seven characteristics of living organisms, using the syllabus definition of each one. (1.1.1)
• Separate respiration from breathing, excretion from egestion, and growth from a temporary gain in mass. (1.1.1)
• State that organisms can be classified into groups by the features they share. (1.2.1)
• Describe a species using the fertile-offspring test, and explain why "they look alike" and "they can breed" are both incomplete. (1.2.2)
• Describe the binomial system as an internationally agreed two-part name, write any scientific name correctly, and correct one written wrongly. (1.2.3)
• Use a dichotomous key to identify an unfamiliar organism, and write down the route you took. (1.2.4)
• Construct a dichotomous key from a set of specimens, using observable, mutually exclusive statements, and explain why a weak pair fails. (1.2.4)
• State the main features used to place animals and plants into the appropriate kingdoms. (1.3.1)
• State the main features that place a vertebrate into the mammal, bird, reptile, amphibian or fish group. (1.3.2a)
• State the features shared by all arthropods, and the features that separate myriapods, insects, arachnids and crustaceans. (1.3.2b)
• Classify an unfamiliar organism from a description using those features, and say which pieces of evidence you used. (1.3.3)
• Explain that classification systems aim to reflect evolutionary relationships. (1.2.5)
• Explain that the sequences of bases in DNA are used as a means of classification. (1.2.6)
• Explain that organisms sharing a more recent ancestor have more similar DNA base sequences than those sharing only a distant ancestor. (1.2.7)
• State the main features used to place all organisms into one of the five kingdoms: animal, plant, fungus, prokaryote and protoctist. (1.3.4)
• State the main features used to place organisms into groups within the plant kingdom, limited to ferns and flowering plants (monocotyledons and dicotyledons). (1.3.5)
• Classify organisms using the five-kingdom and plant-group features. (1.3.6)
• State the features of viruses: a protein coat and genetic material. (1.3.7)

Why ferns come first. The fern is the group that shows you what a flowering plant's features actually mean. A fern has proper roots, stems and vascular tissue — so those are not what makes a plant a flowering plant. What a fern lacks is flowers and seeds: it reproduces by spores instead. That single contrast is the whole of the fern question.

• 1. Defining growth as "getting bigger" Fix Growth is a permanent increase in size and dry mass. A plant that takes up water swells but has not grown: the water is temporary and adds no dry mass.
• 2. Writing "respiration is breathing" Fix Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. Breathing moves air; it is not one of the seven characteristics.
• 3. Confusing excretion with getting rid of faeces Fix Excretion is the removal of the waste products of metabolism and substances in excess of requirements. Faeces are undigested food that was never taken into the cells, so removing them is egestion, not excretion.
• 4. Defining a species without the word "fertile" Fix A species is a group of organisms that can reproduce to produce fertile offspring. A horse and a donkey produce a mule, which is not fertile — so they are not one species.
• 5. Capitalising the species part of a binomial Fix Homo sapiens, never Homo Sapiens. Genus capital, species lower case, both italic when typed.
• 6. Using habitat as a classification feature Fix Habitat is not a structural feature. Sharks, whales, crabs and ducks all live in water and sit in four different groups.
• 7. "A spider is an insect" Fix A spider is an arachnid: four pairs of legs, two body regions, no antennae. An insect has three pairs of legs, three body regions and one pair of antennae.
• 8. "A whale is a fish" Fix A whale is a mammal: it has hair, lungs and mammary glands, and it is endothermic. Fish have scales, fins and gills.
• 9. Writing overlapping key statements Fix "Has wings / is small" lets one organism satisfy both. Use one feature, present or absent: "wings present / wings absent".
• 10. Turning "usually" into "always" Fix Mammals usually give birth to live young; monocotyledon flower parts are commonly in threes. Both are typical features, not laws without exceptions, so keep the qualifier and classify on the whole set of evidence rather than on one feature.
• 11. Supplement. "Classification shows which organisms look alike" Fix A classification system aims to reflect evolutionary relationships. Organisms are grouped by shared features because shared features usually mean shared ancestry — resemblance is the evidence, ancestry is the thing being reflected.
• 12. Supplement. "More similar DNA means the organisms are more alike" Fix Say what the syllabus says: organisms sharing a more recent ancestor have DNA base sequences that are more similar than those of organisms sharing only a distant ancestor. The comparison is about ancestry, and it is always relative — "more similar than", never a fixed percentage.
• 13. Supplement. Adding replication to the features of a virus Fix Statement 1.3.7 is limited to two features: a protein coat and genetic material. Anything you add about host cells is background, not one of the features being asked for.
• "Respiration is breathing." Repair Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. Breathing moves air; it is not one of the seven characteristics at all. A tree and a yeast cell both respire and neither breathes.
• "Growth is when something gets bigger." Repair Growth is a permanent increase in size and dry mass. Both missing words are doing work: a wilted plant watered at noon is bigger by the afternoon, but the gain is water, so it is neither permanent nor an increase in dry mass.
• "Excretion is getting rid of faeces." Repair Excretion removes the waste products of metabolism and substances in excess of requirements. Faeces are undigested food that never entered the cells, so removing them is egestion. Ask one question: did the organism's own chemistry make it?
• "Movement means moving from place to place, so plants do not show it." Repair Movement is an action by an organism or part of an organism causing a change of position or place. A shoot bending towards light and a flower opening are both movement. The syllabus wrote "or part of an organism" precisely so that plants are included.
• "Members of a species look identical." Repair They can vary. The defining test is that they can reproduce to produce fertile offspring, not that they resemble each other. Two dogs of different breeds are one species; two identical-looking beetles that cannot produce fertile offspring are two species.
• Writing homo Sapiens. Repair Homo sapiens. The genus takes the capital, the species stays lower case, both are italicised when typed and underlined separately when handwritten.
• "Both statements in a key can be true — you just pick the best one." Repair They cannot. The two statements at a stage must be mutually exclusive, so that exactly one is true of any specimen. If both can be true, the pair is wrong and must be rewritten — "has wings / is small" becomes "wings present / wings absent".
• "Every plant cell has chloroplasts." Repair Chloroplasts are present in the photosynthetic cells of a plant. Root cells have none. Write the kingdom feature that way and a question about roots cannot catch you.
• Calling a spider an insect. Repair Spiders are arachnids: four pairs of legs, two body regions and no antennae. Insects have three pairs of legs, three body regions and one pair of antennae.
• Using "lives in water" to identify a fish. Repair Use scales, fins and gills. Whales are aquatic mammals: they have hair, produce milk and breathe with lungs. Habitat is not a classification feature.
• "The organism is a bird because it flies." Repair Bats and many insects fly, and penguins do not. Classify on structure: feathers, a beak and hard-shelled eggs make a bird.
• "All mammals give birth to live young." Repair Keep the qualifier. Mammals usually give birth to live young. Dropping "usually" turns a correct feature into a false statement. The features that carry no qualifier — hair or fur, and females producing milk — are the safer ones to quote.
• "A classification system shows which organisms look alike." Repair A classification system aims to reflect evolutionary relationships. Shared features are the evidence used to build it, because features are inherited; shared ancestry is what the system is trying to show. That distinction is the whole of statement 1.2.5.
• "Species A evolved from species B because their DNA is similar." Repair Two living species do not evolve from each other — they share a common ancestor. Similar base sequences mean that ancestor was more recent, not that one species is the parent of the other.
• "They share 90% of their DNA, so they split 10 million years ago." Repair Statement 1.2.7 is a comparison, nothing more: groups sharing a more recent ancestor have base sequences that are more similar than groups sharing only a distant ancestor. Nothing in this syllabus lets you convert a similarity into a date.
• "A fungus is a plant that has lost its chlorophyll." Repair A fungus is its own kingdom. Its cell walls are not made of cellulose, it has no chloroplasts, and it feeds saprotrophically by releasing enzymes onto its food and absorbing the products. Nothing about it is a modified plant.
• "It is a prokaryote because it is one cell." Repair Many protoctists are unicellular too. The dividing feature is the nucleus: prokaryotes have none and carry circular DNA free in the cytoplasm; protoctist cells have a nucleus.
• "The features of a virus are a protein coat, genetic material, and that it replicates inside living cells." Repair Statement 1.3.7 is limited to two features: a protein coat and genetic material. The third item is background, not a feature, and a question asking for the features of a virus is not asking for it. Separately, do not place a virus in a kingdom: a prokaryote is a complete cell with cytoplasm and a cell wall, and a virus has neither.
• "All dicotyledons have a tap root." Repair A main tap root is often present in dicotyledons, and monocotyledon flower parts are commonly in threes. Both are typical features rather than laws without exceptions, so keep the qualifier and classify on more than one feature.

• Two habits that protect the marks. First, read both statements at a stage before choosing — the contrast tells you how strictly the first is meant. Second, write down the route as you go (1b, 2b, 3b). If you land on the wrong organism you can see exactly which stage you misread instead of starting again.
• Notice the design. Stage 1 uses the biggest available split — backbone present or absent — so half the groups are removed in one move. Every later stage then uses one feature in the present-or-absent form. That is exactly the pattern you are expected to reproduce when you construct a key yourself.
• Count antennae first. It splits all four groups faster than anything else: none means arachnid, two pairs means crustacean, and one pair leaves insect or myriapod, which the number of legs then separates immediately.
• The habit worth building. Underline every observation in the question before you answer, then cross out the ones that are habitat, behaviour or size without a boundary. What remains is your evidence. In specimen A, striking out "spends its whole life at sea" is the whole question.
• Two levels, two answers. When an Extended question gives you a plant, read how far it is asking you to go. "Name the kingdom" wants plant. "Name the group" wants dicotyledon. Giving the wrong level of answer is one of the commonest ways to lose a mark you plainly knew.
• Always write the route. When a question asks you to show how you used the key, the middle column is the working, and it is what lets you see where an identification went wrong. A bare answer of "arachnid" shows none of that; "1b, 6b, 7a" shows every choice you made, so a misread at one stage is visible instead of invisible.
• Working backwards is also allowed. If a question gives you the group and asks for features, read the map from right to left: take the chip and write the evidence row next to it. It is the same information, and it is already in the form a "state the features" answer needs: named observable features, nothing else.
• What is not in this check, and why. There is no question here about virus replication or host cells. That is deliberate: statement 1.3.7 is limited to a protein coat and genetic material, so replication is background rather than examinable content, and a retrieval check that tested it would be training you for a question the syllabus does not ask.
• Self-marking honestly. Count a point only if your answer contains the idea, not if you feel you "knew that". Copy the points you missed into your notes in the syllabus's own wording — for the definitions in this topic, that wording is the whole of the answer.

• Topic 1 is short-answer and definition territory. Most of its statements begin with state or describe, so it rewards precision far more than length. The syllabus writes several of its definitions out word for word — "fertile" offspring, "permanent increase in size and dry mass" — and an answer that drops one of those words is no longer the definition the syllabus gives, so write them in full.
• Forty four-choice items, so a stem of this shape is easy to set: "Which organism has an exoskeleton and four pairs of legs?" Because the groups in 1.3.2 differ by countable features, work from the count — legs, body regions, antennae — rather than from which option sounds right. Paper 2 can draw the same style of question from Supplement content as well.
• "Describe a species." "State the main features used to place an animal in the mammal group." Marks come from named features and exact wording, not from sentences about them. Write the feature list, not a paragraph.
• You are given an unfamiliar key and several drawings. You follow the key and write the letter or name. The biology is not the hard part here — reading each pair of statements carefully is, so slow down and read both halves before choosing.
• You are given specimens and asked to complete or construct a dichotomous key. Marks are awarded for observable, mutually exclusive, paired statements that actually separate the specimens.
• "Explain how DNA base sequences are used to decide how closely two species are related." This is the one part of Topic 1 that asks for a chain of reasoning rather than a list, and it is Extended only.