3.1.5Unit 3: Practical Skills in Biology I
Prokaryotic Cells — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 7 of 27 Unit quiz
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.
Part 1 of 5Structure of a Bacterial Cell
Prokaryotes such as bacteria are small cells, usually 0.1–5 μm in diameter.
Cell wall
- Cell wall A rigid wall made of peptidoglycan surrounds the cell, supports its shape and prevents it from bursting.
Cell surface membrane
- Cell surface membrane Lies inside the wall and controls what enters and leaves the cell, as in other cells.
Cytoplasm
- Cytoplasm Contains 70S ribosomes but has no membrane-bound organelles such as mitochondria or a nucleus.
Circular DNA
- Circular DNA A single circular chromosome lies free in the cytoplasm in a region called the nucleoid, and it is not bound to histones.
Plasmids
- Plasmids Small circular loops of extra DNA that often carry genes, such as those for antibiotic resistance.
Extras
- Extras Some bacteria also have a slime capsule for protection and flagella for movement.
Part 2 of 5Comparing Cell Types
Comparing Cell Types
Size
- Size Prokaryotic cells are much smaller than eukaryotic cells, which are about 10–100 μm across.
Ribosomes
- Ribosomes Prokaryotes have smaller 70S ribosomes, but eukaryotes have larger 80S ribosomes in the cytoplasm.
DNA
- DNA Prokaryotes have circular DNA without a nuclear envelope, but eukaryotes have linear DNA inside a nucleus.
Organelles
- Organelles Prokaryotes lack membrane-bound organelles, so reactions occur in the cytoplasm or at the membrane.
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | Absent | Present |
| DNA | Circular, free | Linear, in nucleus |
| Ribosomes | 70S | 80S |
| Cell wall | Peptidoglycan | Cellulose in plants |
| Mitochondria | Absent | Present |
Part 3 of 5Interpreting a Bacterium Image
1
Identify the features visible in the micrograph, such as the cell wall, cytoplasm and any flagella.
2
Measure the length of the cell on the image with a ruler, in millimetres, and note the magnification.
3
Convert the image size to micrometres by multiplying millimetres by 1000.
4
Divide the image size by the magnification to find the real size of the cell.
5
Compare the answer with the typical size range to check that your answer is sensible.
Part 4 of 5Worked Example
Question
- A bacterium appears 24 mm long in a micrograph with a magnification of ×12 000.
- Calculate its real length in micrometres.
- Real size = image size ÷ magnification.
- Image size = 24 mm = 24 × 1000 = 24 000 μm.
- Real size = 24 000 ÷ 12 000 = 2.0 μm.
Answer The bacterium is 2.0 μm long.
Part 5 of 5Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Prokaryote An organism whose cells have no nucleus and no membrane-bound organelles.
- Nucleoid The region of a prokaryotic cell where the circular DNA is found.
- Plasmid A small circular piece of DNA found in bacteria in addition to the main chromosome.
- Peptidoglycan The material that forms bacterial cell walls.
- 70S ribosome The smaller type of ribosome found in prokaryotes.
Common Mistakes
- Writing that prokaryotes have no DNA or that their DNA is in a nucleus.
- Saying that bacterial cell walls are made of cellulose.
- Stating that bacteria have no ribosomes, when they have smaller 70S ribosomes.
- Forgetting that plasmids are extra DNA in addition to the main circular chromosome.
Last updated:
Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.