Plant diseases and plant defences – GCSE Biology exam questions with mark scheme

1Grade 1–3FoundationTriple only4 marks

A student checks the plants in a school garden for signs of disease.

(a) One rose plant has black spots on its leaves.

Give two other signs that could show a plant has a disease. [2]

(b) The student sees lots of very small green insects feeding on the stems of a bean plant.

Name these insects. [1]

Give one other cause of stunted growth in a plant. [1]

(a) any two from: stunted growth / plant smaller than normal; areas of decay / rot; growths (on the plant); malformed / misshapen / curled stems or leaves; discolouration / yellowing of leaves (allow mosaic pattern); visible pests (on the plant) — first mark(1)
(a) second correct sign from the list (ignore spots on leaves — given in the question; ignore 'plant looks unhealthy'; ignore wilting unless linked to rot)(1)
(b) aphids (allow greenfly; ignore 'pests' / 'bugs' alone)(1)
(c) (lack of) nitrate / nitrate (ion) deficiency (allow lack of minerals / mineral ions / nutrients / fertiliser; allow lack of light / water; do not accept lack of magnesium alone)(1)
Answer: (a) Any two of: stunted growth, decay/rot, growths, malformed stems/leaves, discolouration, visible pests. (b) Aphids. (c) Nitrate ion deficiency.
Examiner tip: In (a) don't repeat 'spots' — the question has already given it, so it scores nothing.

2Grade 1–3FoundationTriple only3 marks

Plants have defences against pathogens and animals that eat them.

Draw one line from each defence to the type of defence it is. Each type may be used more than once. [3]

Defence		Type of defence
Tough waxy cuticle on leaves		Physical
Thorns on stems		Physical
Poisons in leaves		Chemical
Bark made of layers of dead cells		Chemical
Leaves that droop when touched		Mechanical
Antibacterial chemicals		Mechanical

waxy cuticle → physical and bark → physical; thorns → mechanical and drooping leaves → mechanical; poisons → chemical and antibacterial chemicals → chemical. Marking: 6 correct lines = 3 marks(1)
4 or 5 correct lines = 2 marks(1)
2 or 3 correct lines = 1 mark (0 or 1 correct = 0 marks; if more than one line is drawn from a defence, that defence scores no credit)(1)
Answer: Physical: waxy cuticle, bark. Chemical: poisons, antibacterial chemicals. Mechanical: thorns, leaves that droop when touched.
Examiner tip: Thorns are not 'physical' in AQA's scheme — they are mechanical (like hairs, drooping leaves and mimicry).

3Grade 4–5Triple only6 marks

A student grew three groups of seedlings in different mineral solutions. Each group was given the same volume of solution, light and temperature.

Group 1: complete solution (all mineral ions)
Group 2: complete solution with no nitrate ions
Group 3: complete solution with no magnesium ions

The bar chart shows the mean height of each group after 4 weeks.

(a) Calculate the percentage decrease in mean height for the seedlings grown with no nitrate compared with the seedlings grown in the complete solution. [2]

(b) Explain why the seedlings grown with no nitrate were shorter. [2]

Name this condition and explain why it happens. [2]

(a) 24 − 9 = 15 (cm) (allow 15/24 or 9/24 seen)(1)
(a) 62.5 (%) (allow 62 or 63; correct answer with no working = 2 marks; 37.5 scores 1 mark only if 9/24 working shown)(1)
(b) nitrate is needed to make amino acids / proteins (allow for protein synthesis; ignore 'for growth' alone)(1)
(b) (proteins are) needed for (cell) growth / to make new cells / tissues (allow so less protein made so less growth)(1)
(c) chlorosis (allow phonetic spelling)(1)
(c) magnesium is needed to make chlorophyll (so less chlorophyll is made) (do not accept magnesium is chlorophyll / makes chloroplasts)(1)
Answer: (a) 62.5% (b) Nitrate is needed to make proteins (amino acids), which are needed for growth / new cells. (c) Chlorosis — magnesium is needed to make chlorophyll.
Examiner tip: In (b) 'nitrate is needed for growth' just repeats the result — you must name protein (or amino acids).

4Grade 6–7HigherTriple only5 marks

A farmer grows potatoes. Some plants have curled, yellow-spotted leaves. The farmer thinks the plants may be infected with potato virus Y.

(a) Give one method, other than a testing kit, that could be used to identify the disease. [1]

(b) The farmer uses a testing kit. A leaf sample is crushed and placed on a test strip. The strip contains monoclonal antibodies attached to a coloured dye.

Explain how the testing kit shows whether the plant is infected with potato virus Y. [3]

(c) Suggest one advantage to the farmer of using a testing kit rather than sending samples away to a laboratory. [1]

(a) compare the symptoms with a gardening manual / (gardening) website (allow reference book / photographs of diseases) or laboratory testing / send a sample to a laboratory (ignore 'look at the plant' alone)(1)
(b) the monoclonal antibodies are specific to / complementary to / only bind to an antigen on potato virus Y (allow protein on the surface of the virus; do not accept antibodies bind to the virus' antibodies)(1)
(b) (if virus present) the antibodies bind / attach to the virus (antigens) (allow form antibody–antigen complex)(1)
(b) so the dye collects / is held in one place and a coloured line / colour change shows the virus is present (allow no colour (line) means the virus is not present)(1)
(c) results are quick / immediate / can be used in the field so infected plants can be removed (or treated) sooner (allow so the disease spreads less; allow cheaper; ignore 'easier' or 'more accurate' alone)(1)
Answer: (a) Gardening manual / website or laboratory testing. (b) Antibodies are specific to an antigen on the virus; they bind to it; the attached dye then gives a coloured line, so a colour shows the virus is present. (c) Quick results on site, so infected plants can be removed before the virus spreads.
Examiner tip: The key word is 'specific': the antibodies bind only to the antigen on this one virus — saying they 'kill' or 'fight' the virus scores nothing.

5Grade 4–5Triple only4 marks

After a hailstorm, many leaves on the plants in an orchard were torn or bruised. A week later, many of the damaged leaves had areas of fungal rot. Very few undamaged leaves had rot.

(a) Explain why the damaged leaves were more likely to become infected by the fungus. [2]

(b) The trunks of the trees in the orchard are covered in bark.

Describe how bark helps to protect a tree from pathogens. [2]

(a) the (waxy) cuticle / epidermis is broken / damaged (allow leaf surface is broken; allow the leaf has wounds / cuts)(1)
(a) so the fungus / pathogen / spores can enter the leaf (tissue / cells) (allow the cuticle normally acts as a barrier to pathogens)(1)
(b) bark is (made of) layers of dead cells (allow tough / thick outer layer)(1)
(b) forms a (physical) barrier that stops pathogens getting in / reaching living tissue (allow outer layers fall off, taking pathogens with them; ignore 'protects the tree' alone)(1)
Answer: (a) The waxy cuticle was broken, so fungal spores could get into the leaf tissue. (b) Bark is layers of dead cells forming a barrier that stops pathogens entering (the outer layers fall off, taking pathogens with them).
Examiner tip: Say what the barrier is (waxy cuticle / dead cells) AND what it does (stops pathogens getting in) — each idea is a separate mark.

6Grade 6–7Triple only5 marks

(a) Thorns, hairs on leaves and mimicry are all examples of one type of plant defence.

Name this type of defence. [1]

(b) The leaves of the sensitive plant, Mimosa pudica, fold up and droop within a few seconds of being touched.

Suggest how this protects the plant from animals. [2]

(c) Some passion flower plants grow small yellow bumps on their leaves. The bumps look like the eggs of a butterfly. Butterflies avoid laying their eggs on leaves that already have eggs on them. The caterpillars that hatch from the eggs eat passion flower leaves.

Explain how the yellow bumps help the passion flower plant to survive. [2]

(a) mechanical(1)
(b) the movement may knock off / dislodge insects (allow startles / scares animals)(1)
(b) the plant looks wilted / smaller / less attractive (so animals eat less of it / move on to other plants) (allow so fewer leaves are eaten)(1)
(c) butterflies do not lay (as many) eggs on the plant (allow the bumps are mimicry / trick butterflies into thinking eggs are already there)(1)
(c) so fewer caterpillars (hatch to) eat the leaves, so more leaf area remains for photosynthesis (allow so fewer leaves are eaten; ignore 'protects the plant' alone)(1)
Answer: (a) Mechanical. (b) Sudden movement knocks off insects / startles animals, and drooping leaves look wilted and less appealing, so less is eaten. (c) Mimicry: butterflies are tricked into not laying eggs, so fewer caterpillars eat the leaves.
Examiner tip: In (c) finish the chain: fewer eggs → fewer caterpillars → fewer leaves eaten. Stopping at 'fewer eggs' loses a mark.

7Grade 6–7Triple only6 marks

Aphids are insects that feed on plant sap. A scientist compared two varieties of a crop plant. Variety H has many hairs on its leaves. Variety S has smooth leaves.

The scientist counted the number of aphids on five leaves of each variety. The table shows the results.

Leaf	1	2	3	4	5	Mean
Number of aphids on variety H	4	7	3	6	5	5
Number of aphids on variety S	18	22	15	20	25	X

(a) Calculate value X. [1]

(b) How many times greater is the mean number of aphids on variety S than on variety H? [1]

(d) Crops with large numbers of aphids produce a lower yield.

Explain why. [2]

(a) 20(1)
(b) 4 (times) (allow ecf from (a): their X ÷ 5)(1)
(c) hairs make it difficult for aphids to move (over the leaf) / to land / to reach the leaf surface (allow hairs may trap aphids)(1)
(c) so aphids find it harder to feed / pierce the leaf (so they move to other plants) (allow hairs are a mechanical defence that deters aphids; ignore 'hairs are sharp')(1)
(d) aphids remove sap / sugars (from the phloem) (allow aphids damage leaves / stems)(1)
(d) so less sugar / glucose is available for (respiration for) growth / to make new tissue or crop (allow aphids carry / spread pathogens / viruses that cause disease)(1)
Answer: (a) X = 20 (b) 4 times (c) Hairs make it hard for aphids to move and reach the leaf surface, so they cannot feed easily. (d) Aphids take sugars from the sap, leaving less for growth (and they can spread viruses).
Examiner tip: In (d) 'aphids eat the plant' is too vague — say they take sap (sugars) that the plant needs for growth.

8Grade 8–9HigherTriple only6 marks

A grower has a large greenhouse of lettuce plants. Some of the plants are smaller than normal and have yellow leaves.

The grower is not sure whether the plants have a disease caused by a pathogen, or whether they are short of a mineral ion.

Describe how the grower could find out the cause of the problem. [6]

look for visible signs of pathogens or pests, e.g. aphids / other insects, spots, growths, rot or fungal growth on the leaves(1)
look at the pattern: a disease usually spreads outwards from one group of plants over time; a mineral shortage usually affects plants evenly (all plants on the same compost / nutrient supply)(1)
compare the symptoms with pictures / descriptions in a gardening manual or website(1)
send samples of leaves / plants to a laboratory to be tested for pathogens(1)
use a testing kit containing monoclonal antibodies (specific to the antigen of a suspected pathogen); a positive result identifies that pathogen(1)
yellow leaves (chlorosis) suggest a lack of magnesium ions, because magnesium is needed to make chlorophyll(1)
small / stunted plants suggest a lack of nitrate ions, because nitrate is needed to make proteins for growth(1)
test the compost / nutrient solution for nitrate / magnesium ions(1)
add fertiliser / magnesium or nitrate ions to some plants but not others (control group) and see whether the new leaves become green / growth improves(1)
a pathogen would not be cured by adding mineral ions; a deficiency would not spread to plants given the ion(1)

Level	Marks	What it looks like
Level 3	5–6	A clear, logical plan that both checks for a pathogen (signs, pests, identification methods) and tests for a mineral-ion deficiency, with reasons linking symptoms to causes (e.g. yellow leaves → magnesium → chlorophyll). Includes a way of confirming the cause, such as a monoclonal antibody kit or adding the ion to some plants with a comparison group.
Level 2	3–4	Several relevant methods are described, with some linked reasoning. The answer may cover only one possible cause in detail, or may name methods without explaining how they show the cause.
Level 1	1–2	One or two simple, relevant statements, e.g. 'look for insects' or 'add fertiliser', with little or no linking to the cause.
No relevant content	0	Nothing worthy of credit.

Answer: A full answer combines checking for pests/visible pathogen signs and the pattern of spread, identifying pathogens (manual/website, lab test, monoclonal antibody kit), and testing for ion deficiency (chlorosis → magnesium, stunting → nitrate; soil test or add the ion to some plants with controls).
Examiner tip: Top-band answers do both jobs: rule a pathogen in or out AND test for a specific ion — and explain WHY yellowing points to magnesium.

9Grade 4–5Triple only4 marks

Foxglove plants make poisons in their leaves. Mint plants make antibacterial chemicals.

(a) Explain how making poisons in its leaves helps a foxglove plant to survive. [2]

(b) Explain how making antibacterial chemicals helps a mint plant. [1]

Suggest why. [1]

(a) poisons deter / put off herbivores / animals from eating the plant (allow animals that eat it become ill or die; ignore 'kills pathogens')(1)
(a) so more leaves remain for photosynthesis / to make glucose (allow so the plant is not destroyed / can grow and reproduce)(1)
(b) kills bacteria / stops bacteria growing, so the plant is less likely to get a bacterial disease / infection (ignore 'kills germs'; do not accept kills viruses)(1)
(c) the chemicals could be used to make (new) medicines / drugs / antibiotics (allow e.g. aspirin from willow, digitalis from foxgloves)(1)
Answer: (a) Poisons deter herbivores from eating the leaves, so more leaf is left for photosynthesis. (b) The chemicals kill bacteria, preventing bacterial infection. (c) They may be useful as new medicines (e.g. digitalis comes from foxgloves).
Examiner tip: Poisons are aimed at animals, antibacterial chemicals at bacteria — mixing these two up is the most common error.

10Grade 6–7Triple only5 marks

A plant virus is carried from plant to plant by aphids. Aphids pick up the virus when they feed on an infected plant.

A farmer recorded the percentage of plants infected with the virus in two fields over 8 weeks. Field B was sprayed with an insecticide at week 0. Field A was not sprayed.

The graph shows the results.

(a) Calculate the mean rate of increase in the percentage of infected plants in field A between week 2 and week 6.

Give the unit. [2]

(b) Explain why the percentage of infected plants increased more slowly in field B. [2]

(a) (40 − 10) ÷ 4 = 7.5(1)
(a) unit: % per week (allow %/week or percentage points per week; unit mark independent)(1)
(b) insecticide kills (most of the) aphids, so there are fewer aphids / vectors (in field B)
do not award for only restating that aphids carry the virus(1)
(b) so fewer aphids carry / transfer the virus (from infected plants) to healthy plants(1)
(c) not all aphids were killed / some aphids were resistant (to the insecticide) (allow aphids flew in from other fields / insecticide wore off; allow virus spread by contact between plants / on tools or hands; ignore 'plants were already infected' — graph shows 0% at week 0)(1)
Answer: (a) 7.5 % per week (b) Insecticide killed aphids, the vectors, so fewer aphids carried the virus from infected to healthy plants. (c) e.g. some aphids survived / were resistant, or new aphids arrived from other fields.
Examiner tip: A rate always needs a 'per time' unit — 7.5 on its own only gets one of the two marks.

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