كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

A2.3Theme A — Unity and diversity

Viruses — IB Biology HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 9 of 14 2 quizzes

📚 IB Biology HL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

0%read

Lesson summary

A2.3 Viruses — IB BIOLOGY HL notes: Describe virus structure, the lytic and lysogenic cycles, theories of origin and why viruses evolve rapidly.Status — viruses are acellular infectious particles, not living organisms, so they sit outside the three-domain system.Size — 20–300 nm,…

Fixed size
Nucleic acid core
Missing features
Lipid envelope
Parasitism
Host specificity

Section 1 of 11Structure common to all viruses

Structure common to all viruses

Status

Status viruses are acellular infectious particles, not living organisms, so they sit outside the three-domain system.

Size

20–300 nm, much smaller than prokaryotic cells, so only an electron microscope shows them.

Fixed size

Fixed size viruses contain few molecules and do not grow.

Nucleic acid core

Nucleic acid core the genome is DNA or RNA, single or double stranded, linear or circular.

Capsid

Capsid a protein coat, with attachment proteins on its outer surface for binding and entering host cells.

Missing features

Missing features no cytoplasm, no metabolism, no respiration and very few or no enzymes.

Lipid envelope

Lipid envelope some viruses have an outer layer, usually from the membrane phospholipids of the cell they were made in,
- which can be involved in cell recognition.

Parasitism

Parasitism all viruses reproduce only inside living cells using the host's ribosomes and energy.

Section 2 of 11Diversity of virus structure

Diversity of virus structure

Variation

Variation genetic material may be DNA or RNA, enveloped or not, in threadlike, polyhedral or spherical shapes.

Host specificity

Host specificity attachment proteins decide which cell a virus infects, for example HIV infects white blood cells and hepatitis infects liver cells.

Virus	Genome	Distinguishing structure
Bacteriophage lambda	Double-stranded DNA in the capsid head	Tail and fibrils attach to E. coli; the tail contracts and injects DNA through the cell wall
Coronavirus	Single-stranded RNA	Spherical, enveloped, with glycoproteins projecting to form a corona
HIV	Two RNA strands	Reverse transcriptase, protein capsid, lipid envelope with glycoprotein attachment proteins

Section 3 of 11Coronaviruses and HIV

Coronaviruses

Coronaviruses they cause respiratory disease in mammals and birds and spread by respiratory fluids.
Examples are SARS-CoV-2 (COVID-19), MERS and SARS.

HIV

HIV it is a retrovirus because reverse transcriptase makes DNA from viral RNA.
The envelope's lipid bilayer comes from the membrane of the host helper T cell it left.

HIV transmission needs direct exchange of body fluids.

Sexual intercourse.
Blood donation.
Sharing needles by intravenous drug users.
Mother to child across the placenta, by mixing of blood at birth or through breast milk.

Section 4 of 11Lytic cycle

1Basis

Basis viruses cannot divide, so they replicate only by infecting host cells.

2Requirements

Requirements attach to a specific site, inject nucleic acid, use host protein synthesis machinery, assemble new particles and release them.

Steps

The virus attaches to the host cell membrane using attachment proteins.

It injects its nucleic acid into the cytoplasm.

The host's proteins and enzymes make new viral parts in biosynthesis.

Particles are assembled and mature into virions.

The host cell lyses, released by the virus-coded lysozyme enzyme, and virions infect further cells.

Section 5 of 11Lysogenic cycle

1Difference from lytic

Difference from lytic new particles are not released immediately and disease does not follow at once.

2Integration

Integration the viral nucleic acid combines with the host DNA.

3Latency

Latency a viral gene for a repressor protein stops the viral DNA being transcribed and translated, and this period is lysogeny.

4Persistence

Persistence the host cell divides normally, so later cells carry the viral nucleic acid in their genome.

5Trigger

Trigger dormant viral DNA enters the lytic pathway when conditions change, such as exposure to UV rays or certain chemicals.

Section 6 of 11Origin of viruses

Age and coevolution

Age and coevolution viruses infect nearly all species and may predate humans by 3.5 billion years.

Endogenous retroviruses

Endogenous retroviruses fragments of ancient viral DNA that make up about 8% of the human genome.

Evidence problem

Evidence problem viruses do not fossilise, so evidence is limited.

Theory	Proposal
Escape	Genetic elements gained the ability to move between cells and became surrounded by a boundary
Regressive (reduction)	Viruses were small cells that became parasites and lost unneeded structures
Virus-first	Viruses predate their cellular hosts because simple forms come before complex ones

Diversity
- viruses may have had different origins, and shared features suggest convergent evolution.
- All have a protein capsid and no cytoplasm.
- All have DNA or RNA using the standard genetic code.
- All are parasitic and need a host cell.

Section 7 of 11Rapid evolution of viruses

Rapid evolution

Rapid evolution influenza viruses and HIV evolve quickly.
They have high mutation rates, mostly because RNA is copied into DNA with errors during replication.
They have large populations.
They have short generation times.

Result

Result they evolve quickly to evade the host immune system.

Section 8 of 11Antigenic drift and shift

Feature	Antigenic drift	Antigenic shift
Change	Small changes accumulate in viral genes over time	Major change in a short time
Cause	Mutations slowly vary the surface proteins	Two or more virus types infect one cell and combine genetic material
Outcome	The immune system eventually fails to recognise the virus	A new virus appears that is not recognised
Example	HIV	Influenza

Section 9 of 11Treating rapidly evolving viruses

Vaccines for drift

Vaccines for drift they must be updated yearly, which works when changes are small.

HIV

HIV drift is unusually rapid, so no vaccine has yet been successful.

Vaccines for shift

Vaccines for shift they are less successful because changes are rapid and unpredictable.

Isolation

Isolation infected individuals may need to be isolated to stop the spread of fast-evolving viruses.

Section 10 of 11Key terms

Term	Meaning
Capsid	the protein coat of a virus.
Virion	a complete, fully functional virus particle.
Lytic cycle	replication ending with host cell lysis and release of virions.
Lysogeny	a latent phase where viral nucleic acid is integrated in the host genome.
Retrovirus	an RNA virus that uses reverse transcriptase to make DNA.
Endogenous retrovirus	viral DNA left in a host genome from ancient infections.
Antigenic drift	gradual accumulation of small changes in viral surface proteins.
Antigenic shift	sudden major change from the combination of genomes of different viruses.

Section 11 of 11Exam technique

Viruses are living cells — they are acellular, cannot respire and cannot replicate without a host.

Viruses divide like cells — they are assembled from host machinery and released by lysis.

Lysogeny means the virus is gone — the viral DNA is integrated and dormant until a trigger.

Drift and shift are the same — drift is gradual mutation and shift is a sudden combination of genomes.

Last updated: 18 October 2026