5.2.1 Physical plant defences

Plant Surfaces Block Pathogen Entry

1. The cellulose cell wall surrounds each plant cell and resists penetration by pathogens.
2. Bark contains layers of dead cells that form a tough barrier around older stems.
3. Cellulose microfibrils give cell walls tensile strength, resisting penetration and limiting the spread of pathogens between cells.
4. Bark is formed from layers of dead cells and contains waterproof substances, creating a thick barrier that is difficult for pathogens to cross.
5. The waxy cuticle forms a waterproof physical barrier over the epidermis, making it difficult for pathogens to enter leaf cells.

Damage Weakens Physical Defences

1. Cuts, insect feeding and damaged roots create openings through which pathogens can enter.
2. A plant may seal damaged tissue by depositing extra material around the wound.
3. The waxy cuticle is continuous over most of the leaf surface, so a pathogen must pass through a stomatal opening or damaged tissue to reach living cells.
4. If a pest breaks a barrier while feeding, the wound exposes living tissue and provides moisture and nutrients that support pathogen growth.

Match Structure to Function

1. The cuticle covers the leaf surface, the cell wall surrounds each cell, and bark protects older stems.
2. Each structure reduces contact between pathogens and living cell contents.

Separate Barriers From Chemicals

1. A physical barrier blocks entry through its structure, while a chemical defence kills pathogens or deters pests.
2. The two types can act together, but they should not be confused in an answer.

5.2.2 Chemical plant defences

Plants Produce Defensive Chemicals

1. Some plant chemicals kill microorganisms or slow their growth, reducing damage from pathogens.
2. Other chemicals are toxic, bitter or unpleasant to herbivores, so less plant tissue is eaten.
3. Poisons and bitter-tasting compounds reduce herbivore feeding, so less photosynthetic tissue is lost and fewer wounds are created.

Antimicrobial Chemicals Limit Pathogens

1. Antibacterial chemicals interfere with bacterial growth or cell processes.
2. Antifungal chemicals inhibit spore germination or fungal growth through plant tissue.
3. Plants may produce more defensive chemicals after tissue is damaged or infected.
4. Antimicrobial plant chemicals may disrupt microbial membranes, inhibit enzymes or prevent spores germinating.

Plant Chemicals Can Become Medicines

1. Scientists can isolate a plant chemical, test its biological effect and develop it into a medicine.
2. Aspirin was developed from compounds associated with willow, and digitalis medicines were developed from foxglove chemicals.
3. Medicines require controlled doses because a useful plant chemical may be toxic at a higher concentration.
4. A clear zone in a microbial culture shows inhibition, but disc size, extract volume, concentration, incubation time and microorganism must be controlled.

Evaluate Evidence for a Treatment

1. A laboratory result showing antimicrobial action is preliminary evidence, not proof that a substance is safe and effective in patients.
2. Further tests compare doses, side effects and effectiveness against a control treatment.
3. A potential medicine must pass toxicity, dose and effectiveness testing because activity in a culture does not show that it will be safe inside the body.

Use Precise Biological Language

1. A chemical that inhibits a pathogen reduces its growth or reproduction.
2. A chemical that deters a pest reduces feeding rather than necessarily killing it.

5.2.3 Detecting plant diseases

Plant Disease Diagnosis Combines Evidence

1. Visible symptoms, environmental measurements, the distribution of affected plants and diagnostic tests are combined to identify a cause.
2. One symptom is not conclusive because infection and environmental stress can produce similar effects.
3. A random pattern affecting one species may suggest infection, while similar symptoms across several species may suggest an environmental cause.

Observe Symptoms in the Field

1. Possible symptoms include spots, lesions, abnormal growth, decay, discolouration, wilting and leaf loss.
2. Chlorosis is yellowing caused by reduced chlorophyll and may result from infection or mineral deficiency.
3. Record photographs, dates, plant species and comparisons with healthy plants.
4. Nitrate deficiency causes stunted growth because nitrate ions are needed to make amino acids and proteins.
5. Magnesium deficiency causes chlorosis because magnesium ions are needed to make chlorophyll.

Eliminate Environmental Causes

1. Measure soil moisture, pH and mineral-ion availability before concluding that a pathogen caused the symptoms.
2. Uniform symptoms across one area may suggest waterlogging, drought or mineral deficiency.

Map the Distribution

1. Map affected plants and compare the pattern with wind direction, water flow, insect activity or newly introduced plants.
2. A spreading cluster supports a transmissible cause but does not identify the pathogen.

Confirm the Pathogen

1. Laboratory identification may use microscopy, culturing, antibody tests or analysis of pathogen genetic material.
2. A positive specific test provides stronger evidence than symptoms alone.
3. Microscopy may reveal fungal hyphae or spores, culture can grow a pathogen, antibody tests detect specific antigens and genetic tests identify pathogen DNA or RNA.
4. Healthy controls show the normal result, while repeated samples reduce the effect of individual variation and sampling error.

Reach a Supported Conclusion

1. The conclusion must fit all the evidence and acknowledge anomalous or conflicting results.
2. Correlation between a symptom and a site condition does not prove causation.

5.2.4 Human physical and chemical defences

Body Surfaces Prevent Pathogen Entry

1. Intact skin forms a tough physical barrier that prevents pathogens reaching living tissues.
2. Blood clotting seals cuts, and the clot dries to form a scab that restores the barrier.
3. Keratinised cells at the skin surface are tightly packed and continually shed, removing microorganisms attached to them.
4. Platelets trigger clotting at a cut, fibrin forms a mesh and trapped blood cells form a clot that dries into a scab.

Mucus and Cilia Clear the Airways

1. Goblet cells produce sticky mucus that traps pathogens and particles in the nose, trachea and bronchi.
2. Cilia beat towards the throat, moving mucus away from the lungs.
3. The mucus is swallowed, allowing stomach acid to kill many trapped pathogens.
4. Mucus contains water and glycoproteins that make it sticky enough to trap inhaled pathogens before they reach alveoli.
5. Coordinated ciliary beating moves mucus to the throat, where swallowing transfers trapped pathogens to the acidic stomach.

Chemical Defences Kill Pathogens

1. Lysozyme is an enzyme in tears, saliva and mucus that breaks down bacterial cell walls.
2. Hydrochloric acid in the stomach creates a very low pH that kills many swallowed pathogens and denatures their enzymes.
3. Lysozyme hydrolyses components of bacterial cell walls, while stomach acid denatures proteins and creates conditions in which many pathogens cannot survive.

Link Each Defence to Its Action

1. Skin blocks entry, mucus traps, cilia move, lysozyme breaks bacterial walls and stomach acid kills.
2. A complete explanation names the defence and gives its biological action.

Defences Work in Sequence

1. Pathogens that pass one barrier may meet another defence deeper in the body.
2. If pathogens enter tissues, white blood cells provide the next line of defence.