Answers · Living things and keys
Living things and keys — return questions key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. LR1 · Return
Model answer. a. Growth is a permanent increase in size and dry mass. Taking in water (c) can be lost again, and making more of the same kind of organism (d) is reproduction.
A full answer includes
- a: growth is a permanent increase in size and dry mass; water taken in can be lost again.
Also accept
- a
Watch for
- “Growth is any increase in size.” — The increase must be permanent and in dry mass, not just water.
2. LR2 · Return
Model answer. Respiration is the chemical reactions in cells that break down nutrient molecules and release energy. Excretion is the removal of the waste products of metabolism. Passing out faeces, which are mostly undigested food, is called egestion.
A full answer includes
- g1: cells.
- g2: energy.
- g3: metabolism (the reactions in cells).
- g4: egestion.
Also accept
- cells; energy; metabolism; egestion
Watch for
- “Respiration happens in the lungs.” — Respiration happens in every living cell; the lungs are for breathing.
- “Faeces are excretion.” — Faeces are mostly undigested food that never entered the cells: removing them is egestion.
3. LR3 · Return
Model answer. (a) Movement is an action by an organism or part of an organism that changes its position or place: a snail glides, and a daisy closes its petals at night. (b) Reproduction is the processes that make more of the same kind of organism, such as a yeast cell budding off a new cell. (c) Nutrition is the taking in of materials for energy, growth and development: a bean plant takes in carbon dioxide, water and mineral ions to make its food.
A full answer includes
- (a) Movement: an action by an organism or part of an organism causing a change of position or place.
- (b) Reproduction: the processes that make more of the same kind of organism.
- (c) Nutrition: the taking in of materials for energy, growth and development.
Also accept
- (a) action causing a change of position or place (of the organism or a part); (b) processes making more of the same kind of organism; (c) taking in materials for energy, growth and development
Watch for
- “Movement means going from place to place.” — A plant moves parts of itself: its leaves turn towards the light. Movement can be of the whole organism or of a part.
- “Nutrition means eating.” — A plant does not eat: it takes in carbon dioxide, water and mineral ions and makes its own food. Nutrition is taking in materials.
4. KR1 · Return
Model answer. b. Dichotomous means "cutting in two": every pair has two statements, and only one of them fits.
A full answer includes
- b: dichotomous means cutting in two: every pair has two opposite statements, and only one of them fits the organism.
Also accept
- b
Watch for
- “A key can have three choices at a step.” — A dichotomous key always has exactly two statements in each pair.
5. KR2 · Return
Model answer. For example: "Sharp spines on the edge of the leaf: holly / No spines on the edge: beech". The two statements are opposites about one feature that can be seen, so only one fits each leaf.
A full answer includes
- A pair of opposites about one identifiable feature, e.g. "Sharp spines on the edge (holly) / No spines on the edge (beech)".
Also accept
- spines on the edge / no spines on the edge (or another pair of opposites about an identifiable feature)
Watch for
- “"Is it big?" makes a good step in a key.” — Use a feature that is either there or not, such as spines.
6. KR3 · Return
Model answer. c. Pair 1: the leaf is divided into leaflets, so go to 2. Pair 2: the leaflets are in pairs along the stalk, with one at the tip, so it is an ash. A route that goes to pair 3 would need a leaf that is one blade.
A full answer includes
- 1 → 2: ash. Pair 1: divided into leaflets, go to 2; pair 2: leaflets in pairs along the stalk with one at the tip, ash.
Also accept
- c
Watch for
- “You can jump straight to the pair that looks right.” — Always start at pair 1 and follow the numbers.
7. SR1 · Return
Model answer. a. The first part shows the genus, a group of closely similar species; the second part shows the species.
A full answer includes
- a: the first part shows the genus, a group of closely similar species; the second part shows the species within it.
Also accept
- a
Watch for
- “The second word alone is the name of the species.” — The first word shows the genus; both words together name the species.
8. SR2 · Return
Model answer. b. Species that share a recent common ancestor have had little time for their base sequences to change, so the sequences are still very similar.
A full answer includes
- b: base sequences change slowly over many generations, so very similar sequences mean a recent common ancestor.
Also accept
- b
Watch for
- “Organisms that look alike are always closely related.” — Similar DNA base sequences, not looks alone, show a recent common ancestor.
9. SR3 · Return
Model answer. b. Organisms are classified into groups by the features they share. The lion, tiger and leopard share the same build of skull and teeth; the two maples share their lobed leaves, leaves in opposite pairs and paired winged fruits. Where they live (a), their size (c) and their common names (d) are not used.
A full answer includes
- b: organisms are classified into groups by the features that they share, such as the lobed leaves and paired winged fruits of the two maples.
Also accept
- b
Watch for
- “Organisms are grouped by where they live.” — A lion and a leopard can live in the same place as a zebra; classification uses the features organisms share.
10. SR4 · Return
Model answer. c. A Chihuahua and a Great Dane look very different, but their puppies are fertile, so they are one species. The horse and the donkey (a) and the zebra and the horse (b) have infertile young. A cat and a dog (d) cannot have young together at all.
A full answer includes
- c: a species is a group of organisms that can reproduce to produce fertile offspring; only the two dog breeds have fertile young.
Also accept
- c
Watch for
- “Organisms that look very different must be different species.” — Fertile offspring, not looks, makes one species.