A researcher used two equal pieces of leaf tissue from the same plant. A treatment reduced the tensile strength of the cellulose cell walls in piece R. Piece S was not treated. The treatment did not change the waxy cuticle or affect the growth of the fungal pathogen. The same amount of the pathogen was placed inside one living cell in each piece.
| Piece | Treatment | Starting cell infected after 48 hours | Neighbouring cells infected after 48 hours |
|---|---|---|---|
| R | cell-wall tensile strength reduced | yes | 15 |
| S | no treatment | yes | 4 |
Which conclusion is best supported by these results?
Cell walls cannot stop a pathogen already inside a cell, but stronger walls limit its spread to neighbouring cells.
Cell walls release chemicals that kill the pathogen as it passes between cells, so fewer were infected in S.
The waxy cuticle surrounds each leaf cell, so it limits the spread of the pathogen to neighbouring cells.
Weaker cell walls let the pathogen reproduce faster inside the starting cell, so it reached more neighbours.
82 exam-style questions on Edexcel GCSE Biology 5.2 Plant and body defences, covering 5.2.1 Physical plant defences, 5.2.2 Chemical plant defences, 5.2.3 Detecting plant diseases, and 5.2.4 Human physical and chemical defences. Each one has a worked solution and a mark scheme showing where the marks go.