OxfordAQA International AS and A-level Biology: HIV and plant virus transmission by aphids (9610) – Revision Notes

These revision notes condense HIV (section 3.2.4) and the spread of plant viruses by aphids (section 3.2.9) from the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, Unit 2, together with the principle that viruses are acellular and non-living. For full explanations and worked examples, read the study guide first.

Then test yourself with the practice questions. Other links: the causes of disease revision notes, the course hub, the printable checklist and the free diagnostics.

Key definitions

Term	Meaning
Acellular	Not made of cells; no cytoplasm, ribosomes or metabolism of its own
Non-living	Cannot respire, grow or reproduce outside a host cell
Capsid	Protein coat enclosing the genetic material (and, in HIV, three enzymes)
Envelope	Outer layer taken from the host cell plasma membrane; carries glycoproteins
CD4 receptor	Protein on helper T-cells to which HIV binds
Vector	A living carrier that moves a pathogen between hosts
Stylet	Needle-like sucking mouthpart of an aphid
Phloem sap	Solution of organic substances, under high pressure, carried in phloem vessels

3.2.4.1 HIV structure at a glance

From the outside in: envelope (host membrane lipid) with glycoproteins, then protein capsid, then inside the capsid two copies of RNA plus reverse transcriptase, integrase and protease.

Part	Job to quote
Glycoproteins	Bind to CD4 receptors on helper T-cells
Reverse transcriptase	Makes DNA from the viral RNA template
Integrase	Inserts viral DNA into a host chromosome
Protease	Cuts long viral proteins into functional lengths
Capsid	Protects and holds RNA and enzymes

Magnification reminder: magnification = image size / actual size, using the same units. Worked reminder: an 18 mm image of a particle at ×150 000 means actual size = 18 000 000 nm / 150 000 = 120 nm.

3.2.4.2 Replication cycle: method in steps

1. Glycoproteins bind to CD4 receptors on a helper T-cell.
2. The capsid contents (RNA and enzymes) are released into the cell.
3. Reverse transcriptase makes double-stranded DNA from the RNA.
4. Integrase inserts the DNA into a host chromosome.
5. The viral DNA may be transcribed to give new viral RNA; proteins are made on host ribosomes and protease trims them.
6. RNA, proteins and enzymes are packaged and the particles are released from the cell, completing the cycle.

Helper T-cells initiate the body’s response to infections. Losing them leaves the patient unable to respond effectively, so infections that are normally controlled become serious.

Model answer pattern: HIV and the immune system

Question style: “Explain why a person with untreated HIV becomes unable to fight infections.” A full answer links four ideas in order.

1. HIV glycoproteins bind to CD4 receptors on helper T-cells.
2. The virus uses the cell to make new particles, which are released from the cell, and infected cells are lost.
3. Helper T-cells initiate the body’s response to infections, so fewer of them means a weaker response.
4. Infections that are normally kept in check can then cause serious illness.

Keep to the cycle you have learned. Do not add detail about antibodies or memory cells unless the question supplies it.

Explaining drug effects from information

Always follow: what is blocked, which step that is, what is missing afterwards, effect on new particles.

Observation in treated cells	Step blocked	Explanation
No viral DNA in the cell	Binding, entry or reverse transcriptase	No DNA copy of the RNA, so nothing to integrate or transcribe
Viral DNA present in cytoplasm but not in chromosome	Integrase	DNA cannot be inserted, so no new viral RNA is made from it
Viral RNA and proteins made; uncut proteins; particles not infectious	Protease	Proteins not cut to size, so particles cannot work

Worked reminder (percentage inhibition). An enzyme assay shows reverse transcriptase activity of 480 units without the drug and 60 units with it. Inhibition = (480 − 60) / 480 × 100 = 87.5%.

Worked reminder (treatment data). CD4 count rises from 220 to 330 cells per mm³: (330 − 220) / 220 × 100 = +50%. Viral load falls from 90 000 to 900 copies: 90 000 / 900 = 100-fold, which is a fall of 2 in log₁₀ units.

Must-know distinctions

Compare	Point to make
Virus and bacterium	Virus has no cell structure or metabolism; bacterium is a living cell. Antibiotics act on bacterial structures and do not affect viruses
Reverse transcriptase and integrase	One makes DNA from RNA; the other inserts that DNA into a chromosome
Capsid and envelope	Capsid is viral protein; envelope is host membrane with viral glycoproteins
Transcription and reverse transcription	Transcription makes RNA from DNA; reverse transcription makes DNA from RNA
Winged and wingless aphids	Winged females migrate to new host plants; wingless ones stay on the plant

3.2.9 Plant viruses and aphids

• Many important plant diseases are caused by viruses, and they cause losses in crop production and quality of produce in all parts of the world.
• Symptoms depend on the disease: yellowing of leaves, leaf distortion, or abnormalities of other parts of the plant.
• Plants are immobile, so transmission usually involves vectors such as aphids.
• Aphids have sucking mouthparts called stylets, which they insert into phloem vessels.
• Phloem sap is under high pressure, so once a vessel is punctured, the sap is forced into the aphid’s gut.
• Some species produce winged females that migrate to start colonies on a new host plant.

Aphid feeding in three lines: stylet reaches phloem vessel; pressure forces sap into the gut; virus taken up from an infected plant can be passed to the next plant fed on.

Why winged females matter for disease spread: they travel further than wingless aphids, reach plants far from the source and start new colonies, so virus can reach distant fields.

Model answer pattern for transmission. “Describe how an aphid spreads a plant virus” can be answered in four linked points:

1. The aphid pushes its stylets into a phloem vessel of an infected plant.
2. Sap under high pressure is forced into its gut, and virus particles in the sap are taken up.
3. The aphid moves to a healthy plant and inserts its stylets into that plant’s phloem.
4. The virus is passed into the healthy plant. Winged females can do this over long distances.

Symptom-to-effect links worth learning:

Symptom	Likely consequence
Yellowing of leaves	Less chlorophyll, so less light absorbed and slower photosynthesis
Leaf distortion	Smaller or misshapen leaf surface, so less light intercepted
Abnormal fruit or stunted growth	Reduced yield and poorer quality of produce

Worked reminder (honeydew rate). An aphid excretes honeydew (sugary liquid) as 10 drops of 0.5 µL in 25 minutes. Volume = 10 × 0.5 = 5 µL. Rate = 5 / 25 = 0.2 µL per minute = 12 µL per hour.

Worked reminder (colony growth). 5 aphids doubling every 2 days for 10 days: 10 / 2 = 5 doublings, 5 × 2⁵ = 160.

Quick self-test

1. Give two reasons why a virus is described as non-living.
2. Where does the HIV envelope come from?
3. Name the three enzymes in the HIV capsid.
4. What do HIV glycoproteins bind to?
5. Which enzyme makes DNA from viral RNA?
6. A drug stops viral DNA joining a host chromosome. Which enzyme does it target?
7. Calculate the actual diameter of a virus drawn 24 mm across at ×200 000, in nm.
8. CD4 count rises from 150 to 270 cells per mm³. Calculate the percentage increase.
9. State two symptoms of a plant virus disease.
10. Why are aphid stylets able to take up sap without a strong sucking action?
11. What is the role of winged females?

Answers

1. It has no cells, cytoplasm or ribosomes; it cannot carry out metabolism or reproduce outside a host cell.
2. The host cell’s plasma membrane.
3. Reverse transcriptase, integrase and protease.
4. CD4 receptors on helper T-cells.
5. Reverse transcriptase.
6. Integrase.
7. 24 mm = 24 000 000 nm; 24 000 000 / 200 000 = 120 nm.
8. (270 − 150) / 150 × 100 = 80%.
9. Any two of: yellowing of leaves, leaf distortion, abnormalities of other parts of the plant.
10. Phloem sap is under high pressure, so it is forced into the gut once the vessel is punctured.
11. They migrate to start colonies on new host plants, spreading the virus.

Where marks are usually lost

• Describing HIV as a “cell” or saying it “divides”.
• Forgetting that the envelope comes from the host membrane.
• Writing “reverse transcriptase makes RNA”.
• Saying the virus “enters the nucleus” without saying DNA is integrated into a chromosome.
• Naming the immune cells as “white blood cells” instead of CD4 helper T-cells.
• Giving a drug effect without saying which step is missing downstream.
• Mixing units in magnification, for example mm with nm.
• Stating that aphids “bite” or “chew” leaves.
• Treating a correlation in trap data as proof of causation.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, published by OxfordAQA: sections 3.2.4 and 3.2.9 of Unit 2.