Living things and keys — final check key

1. LK1 · Final check

Model answer. (a) Growth is a permanent increase in size and dry mass. (b) Excretion is the removal of the waste products of metabolism (the reactions in cells), such as carbon dioxide and urea, and of substances in excess of requirements. (c) Sensitivity is the ability to detect and respond to changes in the internal or external environment.

A full answer includes

• (a) Growth: a permanent increase in size and dry mass.
• (b) Excretion: removal of the waste products of metabolism and substances in excess of requirements.
• (c) Sensitivity: the ability to detect and respond to changes in the internal or external environment.

Also accept

• (a) permanent increase in size and dry mass; (b) removal of waste of metabolism and excess; (c) detect and respond to changes in the environment

Watch for

• “Growth is any increase in size.” — Add permanent and dry mass: swelling with water is not growth.
• “Faeces are excretion.” — Excretion removes waste made in cells; faeces are egestion.

2. LK2 · Final check

Model answer. The pupil is wrong. The toy shows movement and something like a response, but living organisms show all seven characteristics. The toy has no cells, so it does not respire; it does not grow, reproduce, excrete or take in nutrients. It stops when its spring runs down.

A full answer includes

• The pupil is wrong.
• The toy shows only movement (and a mechanical response), not all seven characteristics.
• It has no cells, so it does not respire; it does not grow, reproduce, excrete or take in nutrients.

Also accept

• wrong; only movement (and a response), not all seven; no cells, no respiration, growth, reproduction, excretion or nutrition

Watch for

• “Anything that moves and responds is alive.” — A machine can move and respond. Living organisms show all seven characteristics, including respiration in their cells, growth and reproduction.

3. KK1 · Final check

Model answer. (a) For example: 1. Has legs, go to 2 / Has no legs: earthworm. 2. Has six legs: ladybird / Has more than six legs, go to 3. 3. Has eight legs: spider / Has ten legs: crab. (b) Crab: pair 1 has legs, pair 2 more than six legs, pair 3 ten legs: crab. Other keys work too, if each pair is two opposite statements about a clear feature.

A full answer includes

• (a) Three pairs of opposite statements about identifiable features (for example legs / no legs; six legs / more than six; eight legs / ten legs), each animal on its own at the end.
• (b) The route through the pupil's own key that reaches crab (for example 1 → 2 → 3 → crab).

Also accept

• (a) three pairs of opposite statements about identifiable features, each animal alone at the end; (b) a correct path to crab in that key

Watch for

• “A key can have three choices at a step.” — Use exactly two statements at each step.

4. KK2 · Final check

Model answer. Each pair of statements splits one group into two. Starting with one group of 20, it takes 19 splits to leave 20 groups of one, so the key needs 19 pairs of statements.

Checked automatically

• Number of pairs of statements: 19 (exact value)
  • Common wrong answer 20 : One pair fewer than the number of kinds: each pair splits one group into two.
  • Common wrong answer 10 : Halving once is only one step; keep splitting until each seaweed is on its own.
  • Common wrong answer 38 : That is the number of statements (two in each of 19 pairs); the question asks for pairs.
  • Common wrong answer 40 : That is two statements for each kind. Each pair splits one group into two, so 20 kinds need 19 pairs.

A full answer includes

• 19 (one pair fewer than the number of kinds).

Also accept

• 19

Watch for

• “A key needs one pair for each organism.” — Each pair splits one group in two: n organisms need n − 1 pairs.

5. SK1 · Final check

Model answer. The binomial system is an internationally agreed system in which the scientific name of an organism has two parts: the first shows the genus (Homo) and the second shows the species (sapiens). The genus starts with a capital letter and the species part with a small letter, and the name is printed in italics or underlined by hand. The name is the same in every country.

A full answer includes

• An internationally agreed system.
• The scientific name has two parts: the genus (Homo) and the species (sapiens).
• The genus has a capital letter, the species part a small letter; the name is in italics or underlined.

Also accept

• internationally agreed; two parts: genus (Homo) and species (sapiens); capital for genus, small letter for species; italics or underlined

Watch for

• “Both words of a scientific name start with a capital letter.” — Only the genus has a capital letter.

6. SK2 · Final check

Model answer. Zebras and horses are different species. A species is a group of organisms that can reproduce to produce fertile offspring. A zebra and a horse can produce young, but the zorse is infertile, so it cannot reproduce.

A full answer includes

• They are different species.
• A species can reproduce to produce fertile offspring; the zorse is infertile.

Also accept

• different species; species produce fertile offspring; zorse is infertile

Watch for

• “Two animals that can mate and have young must be the same species.” — The offspring must be fertile; a zorse is not.

7. SK3 · Final check

Model answer. Line up the base sequences of the same piece of DNA from each species and count the places where they differ for each pair. Base sequences change slowly over many generations, so species that share a more recent common ancestor have had less time to change: the pair with the most similar sequences (fewest differences) is the most closely related. Classification aims to reflect these evolutionary relationships.

A full answer includes

• Line up the base sequences and count the places where the bases are different.
• Base sequences change slowly over many generations, so the pair with the fewest differences (most similar sequences) shares the most recent common ancestor.
• Classification aims to reflect these evolutionary relationships.

Also accept

• compare base by base and count differences; fewest differences = most similar = most recent common ancestor; classification reflects evolutionary relationships

Watch for

• “The more differences, the more closely related.” — More differences means more time since the common ancestor: a more distant relationship.