GCSE Biology mAbs & plant disease revision

A monoclonal antibody is one antibody — identical in every copy — made in bulk from a single clone of cells, and it binds to just one specific antigen. The trick is combining two useful cell types. A lymphocyte from a mouse immunised against the target antigen makes exactly the antibody you want, but lymphocytes do not divide much. A tumour cell divides endlessly but makes no useful antibody.

1. 11. Immunise and harvest

   A mouse is exposed to the target antigen, and lymphocytes making the matching antibody are taken from its spleen.

2. 22. Fuse

   The lymphocytes are fused with rapidly dividing tumour cells, creating hybridoma cells.

3. 33. Select and grow

   Hybridomas making the desired antibody are selected and cultured in bulk.

4. 44. Collect

   The identical monoclonal antibodies are harvested and purified for use.

Because every hybridoma descends from one fused cell, every antibody molecule is identical — that is the "monoclonal" in the name. The specificity comes from the lymphocyte; the immortality comes from the tumour cell.

The best-known use is the pregnancy test. Early in pregnancy the hormone HCG appears in urine. The test strip carries monoclonal antibodies that bind HCG, along with a colour marker; when urine is applied, HCG in the sample is captured and stuck in a line position — the "two lines" are antibody-bound hormone, a direct readout of the antibody’s specificity.

In diagnosis, monoclonal antibodies are added to blood samples: they bind to the antigens of particular pathogens, revealing an infection, or to proteins released when a blood clot forms, helping locate a clot in a patient suspected of having one. They can also measure levels of particular hormones or chemicals in blood.

In treatment, monoclonal antibodies are attached to a radioactive substance, a toxic drug or other chemical that kills cells. The antibody binds only to its target — for example an antigen found on cancer cells — so the payload is delivered to the tumour while healthy cells are largely spared. That is the theoretical advantage over standard chemotherapy or radiotherapy, which damage dividing healthy cells too.

The evaluation has three established downsides: monoclonal antibodies have caused more side effects than expected, they are expensive to develop and produce, and they have proved suitable for treating fewer types of cancer than scientists first hoped. A balanced answer states the specificity advantage and these three limitations.

Plants show disease in characteristic ways. The GCSE list: stunted growth, spots on leaves, areas of decay, growths, malformed stems or leaves, discolouration, and the presence of visible pests. Any diagnosis starts by matching what you see to these categories.

Identification then moves to evidence. Reference manuals and websites let a grower match symptoms to known diseases; a laboratory can culture and analyse tissue from the plant for a definitive answer; and testing kits containing monoclonal antibodies can confirm the presence of specific plant pathogens — the same specificity that powers pregnancy tests, applied to a leaf sample.

Plants cannot run, so their defences are built in. Physical defences block entry: cellulose cell walls reinforce every cell, a tough waxy cuticle seals the leaf surface, and bark (with layers of dead cells) shields stems and trunks.

Chemical defences fight back: many plants produce antibacterial compounds that kill bacteria, and others make poisons that deter herbivores — the chemistry behind foxglove, deadly nightshade and tobacco.

Mechanical defences deter or misdirect attackers: thorns and hairs make the plant difficult or painful to eat; some leaves droop or curl when touched, knocking insects off or mimicking a dying leaf; and some plants mimic other things — one passion-flower species grows speckles that look like butterfly eggs, so real butterflies avoid laying eggs there, keeping their caterpillars off the plant.

Microorganisms are grown on a culture medium — a carbohydrate for energy plus minerals, vitamins and sometimes nitrogen. In the practical, the medium is set as agar in a Petri dish. The bacteria are transferred with an inoculating loop that is sterilised in a flame before and after use; the lid is taped shut (not fully sealed, so oxygen can still enter) and the plate is incubated upside down so condensation does not drip onto the colonies.

Temperature is the safety decision. School labs incubate at about 25 °C: warm enough for growth, but below body temperature, so any pathogen adapted to humans is unlikely to grow. Industry cultures known microorganisms at higher temperatures — often close to 37 °C — because growth is faster and the safety controls of a lab make that acceptable.

A classic extension tests antibiotics or disinfectants: paper discs soaked in the test substance are placed on a plate spread evenly with bacteria. After incubation, a clear zone (an inhibition zone) appears around any disc whose substance killed the bacteria. The bigger the clear zone, the more effective the substance — and the exam expects you to quantify it with areas.