Health, disease and the development of medicines Revision Notes

Chemical Defences Against Pests

Plants produce chemicals to deter or harm pests. Examples include nicotine, which is toxic to many insects, and tannins, which make plant tissues unpalatable. Some chemicals disrupt insect nervous systems or inhibit their growth and development. These compounds can act as direct toxins or antifeedants.

Chemical Defences Against Pathogens

Plants also produce chemicals with antimicrobial properties to combat pathogens like bacteria, fungi, and viruses. These chemicals can inhibit pathogen growth or kill them directly. Some plant compounds are involved in strengthening cell walls to prevent pathogen entry or spread after infection.

Human Applications of Plant Chemicals

Numerous plant-derived chemicals have medicinal value for humans. For instance, quinine from cinchona trees treats malaria. Salicylic acid from willow bark is a precursor to aspirin, used for pain. Other plant chemicals can have analgesic, anti-inflammatory, or antimicrobial effects, demonstrating their pharmaceutical potential.

Sterilisation of Equipment and Media

To prevent contamination, all equipment and growth media must be sterile before use. An autoclave uses high-pressure steam at temperatures around 121°C for about 15-20 minutes, effectively killing bacteria, spores, and viruses. This method is crucial for sterilising things like nutrient agar, broth, and empty Petri dishes. Without proper sterilisation, unwanted microorganisms would grow alongside the desired culture, rendering experimental results unreliable and potentially creating biohazards. Other methods include dry heat for glassware or filtration for heat-sensitive liquids.

Transferring Microorganisms Aseptically

When transferring microorganisms, sterile inoculating loops are essential. These loops, made of wire, are typically sterilised by passing them through a Bunsen flame until red hot, immediately before and after each transfer. This process incinerates any microbes present on the loop. The cooled, sterile loop is then used to pick up and transfer microorganisms to new growth media. It's crucial to cool the loop to avoid killing the culture or melting the agar. Minimising exposure of cultures and media to the air is also vital; this often means working quickly and close to a flame to utilise the sterile updraft it creates.

Maintaining Culture Purity

Keeping Petri dishes and culture vials covered at all times, except for the brief moment of inoculation or sampling, is a fundamental aseptic practice. The lids act as physical barriers against airborne contaminants like dust and spores. When opening a culture vessel or dish, it should be done quickly and close to a sterile zone, such as near a Bunsen flame, to minimise the time the sterile contents are exposed to non-sterile air. The base should be lifted off the lid, rather than the lid off the base, to reduce the chance of airborne particles falling in.

Leaf Cuticle as a Barrier

The leaf cuticle is a protective, waxy layer covering the epidermis of leaves and stems. Its primary functions include reducing water loss through transpiration and forming a physical barrier. This barrier deters pests from feeding directly on the leaf tissue and prevents the penetration of pathogens, such as bacteria and fungi, into the plant's internal cells.

Cell Wall as a Barrier

Every plant cell is surrounded by a rigid cell wall, located external to the cell membrane. Composed mainly of cellulose, the cell wall provides structural support and maintains cell turgor. Crucially, it acts as a strong physical barrier, making it difficult for pathogens to enter the cell and infect the plant. Some pathogens produce enzymes to break down cell walls, but the cell wall's robustness is an initial line of defense.

Layers of Protection

The combination of the waxy cuticle on the outer surface and the cell wall around individual cells provides a multi-layered defense system. The cuticle is the outermost defense, while the cell wall is the ultimate physical barrier that pathogens must breach to reach the cytoplasm. This double-barrier system significantly contributes to plant survival against environmental threats.

Field Investigation Steps

Start with careful observation for disease symptoms such as discoloured leaves or wilting. Systematically check for and rule out environmental causes including drought, nutrient deficiency, and physical injury. Analyse how symptoms are distributed – uniform across all plants suggests an abiotic cause, while patchy or clustered symptoms point to a potential infectious pathogen.

Laboratory Confirmation

After field analysis, collect samples from affected plants. In the lab, techniques like microscopy are used to detect structures like fungal spores or bacteria. Culturing allows growth and identification of specific microorganisms, while DNA-based tests help rapidly pinpoint the pathogen. These lab approaches provide the evidence to identify the disease agent.