Structure of prokaryotic cells and of viruses
AQA
A-Level
Prokaryotes are typically 0.5-5 micrometres across against 10-100 micrometres for a eukaryotic cell, and the differences are not just of scale. Prokaryotic cytoplasm contains no membrane-bound organelles at all: no nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus or chloroplasts, so respiratory enzymes are located on the cell-surface membrane instead. Its ribosomes are smaller, 70S rather than the 80S of eukaryotes. Its genetic material is one circular DNA molecule lying free in the cytoplasm as a nucleoid, not wound around histone proteins and not enclosed by a nuclear envelope, whereas a eukaryote has several linear chromosomes associated with histones inside a nucleus.
12
Objectives
8
Exam Tips
14
Pitfalls
14
Key Terms
15
Mark Points
Subtopics in this area
Prokaryotic cells are much smaller than eukaryotic cells. They also differ from eukaryotic cells in having: cytoplasm that lacks membrane-bound organelles smaller ribosomes no nucleus; instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins a cell wall that contains murein, a glycoprotein.
In addition, many prokaryotic cells have: one or more plasmids a capsule surrounding the cell one or more flagella.
Viruses are acellular and non-living. The structure of virus particles to include genetic material, capsid and attachment protein.
Structure of prokaryotic cells and of viruses Revision Guide
Learning Objectives
What you need to know and understand
- Give two features shared by every prokaryotic cell that no eukaryotic cell has, avoiding plasmids, capsules and flagella.
- Describe how the genetic material of a bacterium differs from that of a human cell in shape, in location and in whether it is bound to protein.
- Explain why a drug that blocks murein synthesis damages bacteria but leaves human cells and viruses unaffected.
- Compare the ribosomes of a bacterium and a human cell and use the difference to account for the selective action of some antibiotics.
- Name the three structures found in many prokaryotes and state a function for each.
- Explain why a plasmid, unlike the circular DNA molecule, cannot be quoted as a universal difference between prokaryotes and eukaryotes.
- Suggest how the presence of a capsule makes a bacterium more likely to survive an encounter with a phagocyte.
- Describe how plasmid transfer between bacteria spreads antibiotic resistance through a population.
- Give the three structural features of every virus particle and state the function of each.
- Explain separately why a virus is described as acellular and why it is described as non-living.
- Explain why the attachment protein determines which host cells a particular virus can infect.
- Justify why antibiotics cannot be used to treat a viral infection, using named absent structures.
Marking Points
Key points examiners look for in your answers
- one mark for giving features true of all prokaryotes, for example no membrane-bound organelles and murein in the cell wall
- one mark for a single circular DNA molecule free in the cytoplasm, with nucleoid accepted, and not associated with proteins or histones
- one mark for smaller ribosomes, with 70S accepted as the alternative wording
- one mark for a cell wall containing murein, with peptidoglycan accepted
- one mark for the contrast with eukaryotes, whose linear DNA is enclosed in a nucleus and associated with histones
- one mark for naming a plasmid as a small circular DNA molecule separate from the main circular DNA, replicated independently of it
- one mark for plasmids carrying additional genes such as those for antibiotic resistance, which can be transferred between bacteria
- one mark for the capsule lying outside the cell wall and protecting the cell, for example against phagocytosis or drying out, or helping it adhere to surfaces
- one mark for the flagellum producing movement of the whole cell
- one mark for recognising that these structures are present in many but not all prokaryotes
- Acellular means lacking a cell-surface membrane, cytoplasm and organelles.
- Non-living means lacking independent metabolism and being unable to replicate without a host cell.
- Core viral structure includes genetic material (DNA or RNA), a protein capsid and attachment proteins.
- Attachment proteins bind to complementary receptor molecules on specific host cells.
- Antibiotics are ineffective against viruses because viruses lack bacterial structures, such as a murein cell wall, and metabolic pathways.
Examiner Tips
Expert advice for maximising your marks
- 💡If the question says features of all prokaryotic cells, cross out anything only some of them have before you write: plasmid, capsule and flagellum are incorrect here.
- 💡Ensure you explicitly state 'membrane-bound' when discussing the lack of organelles in prokaryotes.
- 💡Use the specification's own words, murein and 70S ribosomes, rather than paraphrases like tough wall or small ribosomes.
- 💡Read whether the stem says all or many; the same three structures are creditworthy in one wording and incorrect in the other.
- 💡When asked for a function, give the benefit to the cell, not just the location: sticks to surfaces and resists phagocytosis, not simply surrounds the cell.
- 💡Keep plasmid answers ready for the genetic technology topic; the same structure returns as a vector in 3.8.4.1.
- 💡When explaining why viruses are acellular and non-living, provide distinct reasons for each term rather than grouping them together.
- 💡If asked for the function of a viral structure, explicitly name the structure before stating its function to ensure clarity.
Common Mistakes
Pitfalls to avoid in your exam answers
- offering a plasmid, a capsule or a flagellum as a feature of all prokaryotic cells, which is incorrect because only some prokaryotes have them
- writing nucleosome instead of nucleoid, or calling the circular DNA a eukaryotic chromosome
- saying prokaryotes have no ribosomes rather than smaller (70S) ribosomes
- describing the prokaryotic cell wall as cellulose or chitin instead of murein
- writing no organelles unqualified, when the credited point is no membrane-bound organelles
- listing a plasmid, capsule or flagellum when the question asks for features of all prokaryotic cells
- confusing the plasmid with the single circular DNA molecule, or calling either a eukaryotic chromosome
- describing a bacterial flagellum as beating like a eukaryotic flagellum instead of rotating
- saying the capsule is the cell wall, or that it is made of murein
- claiming plasmids carry the genes the bacterium needs to survive
- Stating viruses are cells with no organelles. Correction: Viruses are acellular, meaning they are not cells and lack a cell-surface membrane and cytoplasm.
- Calling the viral surface protein a receptor. Correction: The virus has attachment proteins, which bind to complementary receptors on the host cell.
- Stating genetic material is only DNA. Correction: Viral genetic material can be either DNA or RNA.
- Using capsomere as a synonym for capsid. Correction: Capsomeres are the individual protein subunits that assemble to form the capsid structure.