Cell Structure — A-Level Biology Study Guide

For: A-Level Biology candidates sitting A-Level Biology.

Covers: microscopy types and magnification calculations, eukaryotic organelle structure and function, prokaryotic cell features, the fluid mosaic model of cell membranes, and cell wall composition across plant, fungal, and bacterial groups.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the A-Level Biology style for educational use. They are not reproductions of past Cambridge International examination papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official Cambridge mark schemes for grading conventions.

1. What Is Cell Structure?

Cell structure is the study of the microscopic components, organisation, and functional roles of biological cells, the fundamental unit of all life on Earth. It is a core foundational topic across all A-Level Biology papers (1, 2, 3 and 4), with 8-12% of total qualification marks allocated to questions on this content. Common synonyms include cellular ultrastructure and cell anatomy, and questions range from multiple-choice identification of organelles to extended response questions comparing cell types and model explanations.

2. Microscopy — light vs electron, units and magnification

All cell structure observations rely on microscopy, so you first need to master the standard units of measurement, magnification calculations, and the key differences between light and electron microscopes, all of which are regularly tested.

First, the standard metric units for cellular measurements, with conversion factors you must memorize: $$1\text{metre (m)}=10^3\text{millimetres (mm)}=10^6\text{micrometres (μm)}=10^9\text{nanometres (nm)}$$ Examiners frequently test unit conversion, so always convert all values to the same unit before completing calculations. The core magnification formula is: $$magnification=\frac{imagesize}{actualsize}$$ This can be rearranged to calculate actual size: $actualsize=\frac{imagesize}{magnification}$

Key microscope comparisons

Note: Resolution is the minimum distance between two points that can be distinguished as separate. A common exam trap is confusing magnification (how large the image is) with resolution (how clear the image is).

Worked Example: A student views a red blood cell under a TEM at x15,000 magnification. The image of the cell is 105mm wide. Calculate the actual diameter of the red blood cell in μm.

1. Convert image size to μm: $105\text{mm}=105\times 1000=105,000\text{μm}$
2. Apply the formula: $actualsize=\frac{105000}{15000}=7\text{μm}$

3. Eukaryotic cell organelles — function and structure

Eukaryotic cells (plants, animals, fungi, protoctists) have membrane-bound organelles with specialised functions, outlined below for the A-Level Biology syllabus:

Worked Example: Explain why cells that secrete steroid hormones (e.g. cells in the testes) have a high density of smooth endoplasmic reticulum. Steroid hormones are lipid-based molecules, and the smooth endoplasmic reticulum is the organelle responsible for lipid synthesis. A high density of SER supports the high rate of steroid hormone production required for normal function of these cells.

4. Prokaryotic cells — bacteria and archaea

Prokaryotic cells (bacteria and archaea) are smaller than eukaryotic cells (0.1-5μm diameter, compared to 10-100μm for eukaryotes) and have no membrane-bound organelles. Key structural features of all prokaryotes include:

• No nucleus: genetic material is a single circular DNA molecule found in the nucleoid region, not associated with histone proteins
• Small 70S ribosomes
• Cell wall (present in almost all prokaryotes)
• Small circular DNA fragments called plasmids, which carry non-essential genes (e.g. antibiotic resistance)

Bacteria are the most common prokaryotes, with additional optional features: a protective polysaccharide capsule outside the cell wall, flagella for locomotion, and pili for attachment to surfaces or exchange of genetic material during conjugation. Archaea are structurally similar to bacteria but have key molecular differences: their cell walls lack peptidoglycan, their ribosomal structure is closer to eukaryotes than bacteria, and many are adapted to survive extreme environments (e.g. hot springs, high salt lakes).

Exam Tip: Examiners frequently ask for 3 structural differences between prokaryotic and eukaryotic cells, so memorize the core differences outlined above for quick recall.

Worked Example: State three features found in prokaryotic cells that are not present in animal eukaryotic cells.

1. 70S ribosomes (animal eukaryotes have 80S ribosomes)
2. Circular DNA not associated with histone proteins (animal eukaryotes have linear DNA with histones)
3. Cell wall (animal cells have no cell wall)

5. Cell membrane — fluid mosaic model

The cell surface membrane (plasma membrane) controls movement of substances into and out of the cell, and is described by the fluid mosaic model, first proposed by Singer and Nicholson in 1972. It is named for two key properties:

1. Fluid: Phospholipids and embedded proteins can move laterally within the membrane layer, so the membrane is not static
2. Mosaic: Proteins are scattered throughout the phospholipid bilayer like tiles in a mosaic

Key membrane components:

1. Phospholipid bilayer: The core structure, made of phospholipid molecules with hydrophilic phosphate heads facing the aqueous cytoplasm and extracellular fluid, and hydrophobic fatty acid tails facing inwards, forming a non-polar core that repels charged and polar molecules.
2. Integral proteins: Span the full width of the bilayer, including channel and carrier proteins for transport of polar/charged molecules, and receptor proteins for cell signalling.
3. Peripheral proteins: Attached to the inner or outer surface of the bilayer, including enzymes and cell adhesion molecules.
4. Cholesterol: Only present in animal cell membranes; regulates membrane fluidity: reduces fluidity at high temperatures, and prevents the membrane from freezing at low temperatures.
5. Glycolipids/glycoproteins: Short carbohydrate chains attached to lipids or proteins on the extracellular surface of the membrane; function in cell recognition, cell signalling, and immune response.

Worked Example: Explain why small non-polar molecules (e.g. oxygen, carbon dioxide) can diffuse freely across the cell membrane, while charged ions (e.g. sodium, potassium) cannot. The inner core of the phospholipid bilayer is hydrophobic, so non-polar molecules can dissolve in this core and diffuse across. Charged ions are repelled by the hydrophobic core, so they cannot cross the membrane without the help of integral channel or carrier proteins.

6. Cell wall composition — plants, fungi, bacteria

Cell walls are rigid outer layers found outside the cell membrane in plant, fungal, and prokaryotic cells, but not in animal cells. Their composition is specific to each group, a frequently tested fact in A-Level Biology exams:

1. Plant cell walls: Made of cellulose, a polysaccharide of β-glucose monomers joined by 1,4 glycosidic bonds, arranged into strong microfibrils. The cell wall is freely permeable to all solutes, and provides tensile strength to prevent the cell from bursting when it takes up water by osmosis, and maintains cell shape.
2. Fungal cell walls: Made of chitin, a nitrogen-containing polysaccharide of N-acetylglucosamine monomers. Chitin is stronger than cellulose, and provides structural support for fungal hyphae.
3. Bacterial cell walls: Made of peptidoglycan (murein), a polymer of alternating N-acetylglucosamine and N-acetylmuramic acid residues cross-linked by short peptide chains. It provides shape and protects the cell from osmotic lysis. Antibiotics like penicillin work by inhibiting peptidoglycan synthesis, weakening the cell wall and killing the bacteria.

Worked Example: A researcher treats three unknown cell samples with three different enzymes: cellulase, chitinase, and lysozyme (which breaks down peptidoglycan). Sample 1 breaks down when treated with cellulase, Sample 2 breaks down when treated with chitinase, and Sample 3 breaks down when treated with lysozyme. Identify each sample.

• Sample 1: Plant cell (cell wall made of cellulose, digested by cellulase)
• Sample 2: Fungal cell (cell wall made of chitin, digested by chitinase)
• Sample 3: Bacterial cell (cell wall made of peptidoglycan, digested by lysozyme)

7. Common Pitfalls (and how to avoid them)

• Wrong move: Confusing magnification and resolution, stating that higher magnification always lets you see smaller structures. Why students do it: They mix up the two definitions. Correct move: Remember magnification is how much larger an image is than the actual specimen, while resolution is the ability to tell two separate points apart. You can magnify an image infinitely but it will be blurry if resolution is too low.
• Wrong move: Stating that all prokaryotes have cell walls, or that all eukaryotes do not. Why: They forget that animal eukaryotes have no cell walls, but plant and fungal eukaryotes do, and a small number of prokaryotes (e.g. Mycoplasma bacteria) have no cell wall. Correct move: Always specify the organism group when talking about cell walls.
• Wrong move: Saying ribosomes are membrane-bound organelles, or that prokaryotes have 80S ribosomes. Why: Students assume all organelles have membranes, and mix up ribosome sizes. Correct move: Ribosomes have no membrane; eukaryotic cytoplasmic ribosomes are 80S, prokaryotic ribosomes are 70S (the same as ribosomes in mitochondria and chloroplasts, a key piece of evidence for the endosymbiont theory).
• Wrong move: Describing the phospholipid bilayer as static, or saying that cholesterol is present in all cell membranes. Why: They learn the diagram but not the fluid property, and forget cholesterol is only in animal cell membranes, not plant, fungal or prokaryotic membranes. Correct move: Explicitly state the bilayer is fluid (components move laterally), and note that cholesterol is exclusive to animal cell membranes.
• Wrong move: Mixing up TEM and SEM function, stating SEM is used to view internal cell structure. Why: They don't remember how each electron microscope works. Correct move: TEM transmits electrons through thin specimens, so shows internal ultrastructure; SEM scans the surface of specimens, so produces 3D external images.

8. Practice Questions (A-Level Biology Style)

Question 1 (Paper 1 Multiple Choice)

Which of the following is a feature of scanning electron microscopes but not transmission electron microscopes? A) Requires a vacuum to operate B) Produces 3D images of specimen surfaces C) Has a maximum resolution of 0.5nm D) Uses visible light to produce images

Worked Solution: Correct answer B. A is incorrect: both types of electron microscope require a vacuum. C is incorrect: this is the resolution of TEM, SEM has a resolution of 1nm. D is incorrect: both use electron beams, light microscopes use visible light. B is the only correct feature specific to SEM.

Question 2 (Paper 2 Structured)

(a) State two functions of proteins found in the cell surface membrane. [2 marks] (b) Explain why the cell membrane is described as a 'fluid mosaic'. [2 marks]

Worked Solution: (a) Any two of the following, 1 mark each:

• Transport of polar/charged molecules across the membrane (channel/carrier proteins)
• Cell signalling (receptor proteins)
• Enzyme activity for membrane-associated reactions
• Cell adhesion to adjacent cells
• Cell recognition (glycoproteins)

(b) 1 mark for 'fluid': Phospholipids and proteins can move laterally within the membrane, so it is not a static structure. 1 mark for 'mosaic': Proteins are scattered throughout the phospholipid bilayer in a pattern resembling a mosaic of tiles.

Question 3 (Paper 3 Calculation)

A student measures the diameter of a mitochondrion on an electron micrograph as 24mm. The actual diameter of the mitochondrion is 1.2μm. Calculate the magnification of the micrograph. Show your working. [2 marks]

Worked Solution: Step 1: Convert the image size to the same unit as actual size (μm): $24\text{mm}=24\times 1000=24,000\text{μm}$ (1 mark for correct unit conversion) Step 2: Apply magnification formula: $magnification=\frac{24000}{1.2}=20,000$ (1 mark for correct final answer, must include 'x' prefix) Final answer: x20,000

9. Quick Reference Cheatsheet

Key Formulas & Conversions

$$magnification=\frac{imagesize}{actualsize}$$ $$1\text{mm}=1000\text{μm},1\text{μm}=1000\text{nm}$$

Microscope Key Features

Cell Wall Composition

Prokaryote vs Eukaryote Key Differences

Fluid Mosaic Model Components

• Phospholipid bilayer: hydrophobic core, barrier to polar/charged molecules
• Integral/peripheral proteins: transport, signalling, enzymes
• Cholesterol (animal only): regulates membrane fluidity
• Glycoproteins/glycolipids: cell recognition, immune response

10. What's Next

This cell structure topic is the foundation for almost all subsequent content in A-Level Biology. Your understanding of membrane structure will directly support your study of cell membrane transport (diffusion, osmosis, active transport, bulk transport) in the next unit, while knowledge of prokaryotic cell structure will be essential when you learn about infectious diseases, antibiotic action, and genetic engineering later in the syllabus. Your grasp of organelle function will also help you understand complex metabolic pathways such as photosynthesis (chloroplasts) and aerobic respiration (mitochondria) in the A2 content, so it is critical to memorize this content thoroughly.

If you have any questions about specific subtopics, need additional practice questions, or want to test your knowledge with interactive quizzes, you can ask Ollie, our AI tutor, at any time on the homepage. Make sure you also review official A-Level past paper questions on cell structure to get familiar with common mark scheme requirements and exam question phrasing, as this will help you maximise your marks on test day.

Aligned with the Cambridge International AS & A Level Biology 9700 syllabus. OwlsAi is not affiliated with Cambridge Assessment International Education.