Is it alive? — answer key

1. LQ1 · Practice

Options as numbered on the pupil sheet: 1 — An action causing a change of position or place; 2 — Chemical reactions in cells that break down nutrient molecules and release energy; 3 — Detecting and responding to changes in the internal or external environment; 4 — A permanent increase in size and dry mass; 5 — Processes that make more of the same kind of organism; 6 — Removal of waste products of metabolism and substances in excess; 7 — Taking in materials for energy, growth and development.

Model answer. Movement is an action that changes position or place. Respiration is the chemical reactions in cells that break down nutrient molecules and release energy. Sensitivity is detecting and responding to changes inside or outside the organism. Growth is a permanent increase in size and dry mass. Reproduction makes more of the same kind of organism. Excretion removes the waste products of metabolism and substances in excess of requirements. Nutrition is taking in materials for energy, growth and development.

A full answer includes

• Movement: an action causing a change of position or place.
• Respiration: chemical reactions in cells that release energy from nutrient molecules.
• Sensitivity: detecting and responding to changes in the environment.
• Growth: a permanent increase in size and dry mass.
• Reproduction: processes that make more of the same kind of organism.
• Excretion: removal of waste products of metabolism and substances in excess.
• Nutrition: taking in materials for energy, growth and development.

Also accept

• movement: change of position; respiration: reactions in cells releasing energy; sensitivity: detect and respond; growth: permanent increase in size and dry mass; reproduction: more of the same kind; excretion: removing waste of metabolism and excess; nutrition: taking in materials

Watch for

• “Respiration is breathing.” — Match respiration to the reactions in cells: breathing only moves air in and out of the lungs.
• “Growth is any increase in size.” — Match growth to the description with permanent and dry mass: swelling with water is not growth.

2. LQ2 · Practice

Options as numbered on the pupil sheet: 1 — Movement; 2 — Respiration; 3 — Sensitivity; 4 — Growth; 5 — Reproduction; 6 — Excretion; 7 — Nutrition.

Model answer. The mouse cells show respiration: reactions in cells that release energy. The bean plant shows nutrition: it takes in materials to make its food. The child shows growth: a permanent increase in dry mass. The kidneys show excretion: urea is a waste product of metabolism. The budding yeast shows reproduction. The flytrap shows sensitivity: it detects the touch and responds. The snail shows movement.

A full answer includes

• Cells of a mouse break down glucose and release energy: respiration.
• A bean plant takes in carbon dioxide, water and mineral ions: nutrition.
• A child has more dry mass than a year ago: growth.
• A cat's kidneys remove urea in urine: excretion.
• A yeast cell buds off a new yeast cell: reproduction.
• A Venus flytrap snaps shut when an insect touches its hairs: sensitivity.
• A snail glides across a leaf: movement.

Also accept

• mouse cells: respiration; bean plant: nutrition; child: growth; kidneys and urea: excretion; yeast bud: reproduction; flytrap: sensitivity; snail: movement

Watch for

• “The flytrap snapping shut is movement, not sensitivity.” — It does move, but the point of the example is that it detects the touch and responds: match it to sensitivity.
• “Urine is egestion.” — Urea is made by reactions in cells, so removing it is excretion; egestion is passing out undigested food as faeces.

3. LQ3 · Practice

Model answer. c. Burning uses oxygen and makes carbon dioxide, but it is not respiration: the flame has no cells. It does not grow, reproduce, sense or take in nutrients. Living organisms show all seven characteristics; the flame shows one or two at most.

A full answer includes

• c: burning is not respiration, because a flame has no cells; it shows only one or two of the seven characteristics, while living organisms show all seven.

Also accept

• c

Watch for

• “The flame is alive because it uses oxygen and grows.” — A flame shows a few of the characteristics, and none by reactions in cells; living organisms show all seven, and they are made of cells.

4. LQ4 · Practice

Model answer. b and d. A seedling and a puppy build new cells, so their dry mass increases for good. A dry bean swelling in water (a) only takes in water, which can dry out again. A salt crystal (c) and a balloon (e) are not organisms at all.

A full answer includes

• b and d: growth is a permanent increase in size and dry mass, made by building new cells; a swelling bean has only taken in water, and a crystal or a balloon is not an organism.

Also accept

• b, d

Watch for

• “Growth is any increase in size.” — Growth needs a permanent increase in dry mass: a swelling bean has only taken in water.
• “The crystal grows, so it must be alive.” — The crystal gets bigger as salt comes out of the solution onto it; it takes nothing in and is not an organism.

5. LQ5 · Practice

Model answer. b. Carbon dioxide is made by respiration in the fish's cells, so getting rid of it is excretion. Faeces (a) are mostly food that was never digested and never entered the cells: removing them is egestion. Taking in water (c) and eating (d) are nutrition.

A full answer includes

• b: excretion removes waste made by reactions in cells, such as carbon dioxide from respiration; faeces were never part of the cells, so passing them out is egestion.

Also accept

• b

Watch for

• “Faeces are excretion.” — Faeces were never part of the cells' reactions: passing them out is egestion, not excretion.

6. LQ6 · Practice

Model answer. The pupil has mixed up respiration and breathing. Breathing is moving air in and out of the lungs. Respiration is the chemical reactions in cells that break down nutrient molecules and release energy. Every living cell respires, so the living cells of a tree respire all the time, even though a tree has no lungs and does not breathe.

A full answer includes

• Respiration is chemical reactions in cells that release energy from nutrient molecules.
• Breathing (moving air in and out of lungs) is a different thing; every living cell respires, including the living cells of a tree.

Also accept

• respiration = reactions in cells releasing energy; breathing = moving air in and out of lungs; all living cells, including the living cells of a tree, respire

Watch for

• “Respiration is breathing.” — Breathing moves air; respiration is reactions in every living cell. A tree respires but does not breathe.

7. LQ7 · Practice

Model answer. The crystal only gets bigger, and only because salt settles on its surface. (1) It has no cells, so it cannot respire. (2) It does not take in materials and use them (no nutrition): it gets bigger only because salt comes out of the solution onto its surface. Other good reasons: it cannot reproduce, it does not sense or respond, and it does not excrete.

A full answer includes

• Any two, each linked to a characteristic: it has no cells, so it does not respire; its getting bigger is salt coming out of solution onto its surface, not growth by taking in and using materials (no nutrition); it cannot reproduce (make new crystals of its own); it does not sense or respond; it does not excrete.

Also accept

• two of: no respiration (no cells); no nutrition (salt just deposits on the surface); no reproduction; no sensitivity; no excretion

Watch for

• “The crystal grows, so it must be alive.” — Getting bigger is not enough: living organisms show all seven characteristics, and the crystal shows none of them by reactions in cells.

8. LQ8 · Practice

Model answer. The seven characteristics are movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. Their first letters spell MRS GREN.

A full answer includes

• Movement, respiration, sensitivity, growth, reproduction, excretion, nutrition (any order).

Also accept

• movement, respiration, sensitivity, growth, reproduction, excretion, nutrition

Watch for

• “Breathing is one of the seven.” — The characteristic is respiration, the reactions in cells; breathing is only how some animals get oxygen in.

9. LT1 · Apply

Model answer. (a) Carbon dioxide could come from respiration: cells breaking down the nutrients and releasing energy, then excreting the carbon dioxide. (b) It could also come from a chemical reaction in the soil, with no cells at all, just as a candle flame releases carbon dioxide without being alive. Living organisms show all seven characteristics, so one clue cannot decide. (c) The probe could look for another characteristic: for example growth (the dry mass of the soil organisms increasing), reproduction (more and more cells in the sample, so more carbon dioxide each day), or cells under a microscope.

A full answer includes

• (a) Respiration (cells using the nutrients and releasing carbon dioxide), or excretion of the carbon dioxide.
• (b) A chemical reaction in the soil without cells could also release carbon dioxide, like the candle; one characteristic is not enough: living organisms show all seven.
• (c) Any other characteristic, e.g. growth (an increase in dry mass over time), reproduction (more cells, more carbon dioxide each day), or cells seen under a microscope.

Also accept

• (a) respiration (or excretion of CO₂); (b) a non-living chemical reaction can also release CO₂; one characteristic is not enough; (c) another characteristic, e.g. growth, reproduction, cells

Watch for

• “Anything that releases carbon dioxide must be alive.” — A flame releases carbon dioxide too. One characteristic cannot decide whether something is alive.

10. LT2 · Apply

Model answer. (a) No. A dry seed seems to do nothing, but if it is alive its cells are still respiring very slowly. Looking cannot tell a living seed from a dead one. (b) Put the seeds on damp paper in a warm place, with air. A living bean seed usually germinates within a week or two: a root grows down and a shoot grows up (growth and movement, showing sensitivity to gravity and light), and the seedling then makes its own food and gains dry mass (nutrition and growth). (c) No. Germination shows that a seed is alive, but no germination does not prove that it is dead: some living seeds stay dormant for a long time, and others need a different temperature. A seed that only swells, then goes soft and rots, is probably dead.

A full answer includes

• (a) No: a seed can be alive even though it seems to do nothing; its cells respire very slowly.
• (b) Give the seeds water, air and warmth; a seed that germinates is alive: a root and shoot appear (movement, growth), and later the seedling gains dry mass and takes in materials (growth, nutrition).
• (c) No: germination shows that a seed is alive, but a seed that does not germinate may be dead, or it may be dormant or kept in the wrong conditions.

Also accept

• (a) no; living seeds respire very slowly; (b) water, air and warmth; a seed that germinates is alive: root and shoot, growth (dry mass), movement, sensitivity; (c) no: it may be dormant or the conditions wrong

Watch for

• “The seeds are dead, because they show no characteristics.” — A dry seed can be alive, respiring very slowly. Give it water, air and warmth: if it germinates, the other characteristics appear.
• “A seed that does not germinate must be dead.” — Some living seeds stay dormant even with water and warmth; germination proves life, but no germination does not prove death.
• “A seed that swells in water has grown.” — Swelling is only water taken in; growth is a permanent increase in dry mass.