How is a plant disease identified by observation in the field?

The plant is examined for characteristic symptoms such as discoloured or mottled leaves, spots, galls or stunted growth, and these are matched against known descriptions.

How does a portable microscope help a grower diagnose disease in the field?

It shows fungal threads, spores and damaged cells on the leaf surface that are far too small to see with the eye alone.

Why is testing for a pathogen's DNA a reliable way of identifying it?

The base sequence is unique to that species, so the result is certain even when very few pathogens are present and no symptoms have appeared.

OCR GCSE Biology A J247

Cell Level Systems

Topic overview Microscopy, and sub-cellular structures DNA structure: polymer, double helix and nucleotides Protein synthesis and the triplet code Enzyme action, and investigating enzymes Cellular respiration as an exothermic process Aerobic and anaerobic respiration compared Synthesis and breakdown of carbohydrates, proteins and lipids Photosynthesis: producers and the process Photosynthesis as endothermic, and investigating it Limiting factors in photosynthesis

Community Level Systems

Topic overview Cycling materials, and the role of microorganisms The carbon and water cycles, and decomposition Levels of organisation, and abiotic and biotic factors Interdependence, competition and trophic levels Pyramids of biomass and the efficiency of transfer

Genes, Inheritance and Selection

Topic overview Genetic terms, and the genome Genome, environment and mutations How genetic variants influence phenotype Asexual and sexual reproduction compared Haploid, diploid and meiosis Single gene inheritance and Punnett squares Sex determination, multiple genes and Mendel's work Variation within a population, and classification Natural selection and evolution Evidence for evolution, and the work of Darwin and Wallace

Global Challenges

Topic overview Field investigations, and human impact on ecosystems Maintaining biodiversity, and evidence for environmental change Food security, agricultural and biotechnological solutions Selective breeding and genetic engineering Health, disease, and types of disease Interactions between diseases, and how they spread Reducing the spread of disease, and named examples Physical and chemical plant defences Detecting plant disease; white blood cells and platelets Non-specific defences and the immune system Monoclonal antibodies and their uses Vaccines, medicines and aseptic technique Developing new medicines, and non-communicable disease Treating cardiovascular disease, and lifestyle factors Cancer, and the use of stem cells in medicine Gene technology in medicine, and the human genome

Organism Level Systems

Topic overview The nervous system and coordinated responses The reflex arc The eye and its common defects The structure and function of the brain Investigating and treating the brain Hormonal coordination, thyroxine and adrenaline Hormones in reproduction and the menstrual cycle Contraception and treating infertility Plant hormones and their commercial uses Homeostasis, and the skin in temperature control Insulin and glucagon in blood sugar control Type 1 and type 2 diabetes Osmotic changes in body fluids, and the kidneys The kidney tubule and the effect of ADH Responding to temperature and osmotic challenges

Scaling Up

Topic overview Diffusion, osmosis and active transport Mitosis, the cell cycle and cell differentiation Stem cells in animals and meristems in plants Surface area to volume ratio and exchange surfaces The circulatory system, the heart and blood vessels Red blood cells and plasma Root hair cells, transpiration and translocation Xylem and phloem Factors affecting water uptake, and the potometer

Global Challenges

Detecting plant disease; white blood cells and platelets

3 things to know
Paper 4

Overview

Diagnosing plant disease from symptoms in the field and with a portable microscope, why symptoms can mislead, identifying pathogens in the laboratory by DNA and antigen tests, the advantages of each approach, how phagocytes engulf pathogens and are adapted to it, antibodies and antitoxins made by lymphocytes, why antibodies are specific, and how platelets and blood clotting keep pathogens out.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why can identification from symptoms alone give the wrong answer?

Key term

Describe how DNA is used in a laboratory to identify the organism causing a plant disease.

Key term

Describe how antigens are used in a laboratory to identify the cause of a plant disease.

Questions from this subtopic

Each one with the model answer underneath.

1
How is a plant disease identified by observation in the field?
Model answer
The plant is examined for characteristic symptoms such as discoloured or mottled leaves, spots, galls or stunted growth, and these are matched against known descriptions.
2
How does a portable microscope help a grower diagnose disease in the field?
Model answer
It shows fungal threads, spores and damaged cells on the leaf surface that are far too small to see with the eye alone.
3
Why can identification from symptoms alone give the wrong answer?
Model answer
Different pathogens, and mineral deficiencies as well, can produce very similar symptoms in a plant.
4
Describe how DNA is used in a laboratory to identify the organism causing a plant disease.
Model answer
DNA is extracted from the infected tissue and tested for base sequences that are found only in that pathogen.
5
Why is testing for a pathogen's DNA a reliable way of identifying it?
Model answer
The base sequence is unique to that species, so the result is certain even when very few pathogens are present and no symptoms have appeared.
6
Describe how antigens are used in a laboratory to identify the cause of a plant disease.
Model answer
The sample is tested with antibodies that bind only to the antigens of one pathogen, and a colour change shows that pathogen is present.

See the rest and revise

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.

1Closed book

2Open book

3Closed book

See a huge jump in your memorisation

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Cell Level Systems

Community Level Systems

Genes, Inheritance and Selection

Global Challenges

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

Algae and .

phytoplanktonfungianimals

Blank Blitz consolidates the facts

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

AThe blood becomes too dilute.

BLess ADH is released.

CThe collecting ducts become less permeable.

Build It trains answer structure

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

Egestion means passing out of undigested food as faeces.

TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Every question in Global Challenges, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Start revising, £5 a month See how the method works

Or £46 a year, about £3.83 a month. Cancel anytime.