Cell structure for A-Level Biology
All living organisms are made of cells, but the level of internal organisation differs sharply between groups. Eukaryotic cells (animal, plant, fungal and protist) contain membrane-bound organelles including a nucleus. Prokaryotic cells (bacteria and archaea) have no membrane-bound organelles and a single circular DNA molecule in the cytoplasm. This division is the most important comparison in AQA A-Level Biology section 3.2.1.
This guide covers the structure and function of every organelle on the specification, the differences between animal, plant and prokaryotic cells, and the microscopy techniques (light, TEM, SEM) that allow you to see them. You will also see where students lose marks on the resolution and magnification calculations.
Eukaryotic vs prokaryotic
Eukaryotic cells have membrane-bound organelles and linear DNA in a nucleus. Prokaryotic cells have no nucleus and a single circular DNA molecule.
Three microscope types
Light microscopes see whole cells. TEM shows internal organelles in 2D. SEM shows surface detail in 3D, both at much higher resolution.
Magnification = image / actual
Magnification is how many times larger an image appears. Resolution is the smallest distance two points can be apart and still appear separate.
Eukaryotic cell organelles
For A-Level Biology you need to know the structure and function of every organelle in a eukaryotic cell, and the easiest way in is to group them by membrane structure (single, double or none). Learn function alongside structure, because exam questions almost always pair the two.
The nucleus, mitochondria and chloroplasts are double-membraned. The Golgi apparatus, lysosomes, vesicles, endoplasmic reticulum and vacuoles are single-membraned. Ribosomes and the cytoskeleton have no membrane at all.
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Double membrane (nuclear envelope) with nuclear pores; contains chromatin and a nucleolus | Stores DNA, controls protein synthesis, makes ribosomes (in nucleolus) |
| Mitochondrion | Double membrane; inner folded into cristae; matrix contains enzymes and a small loop of DNA | Site of aerobic respiration and ATP synthesis |
| Chloroplast (plant only) | Double membrane; thylakoids stacked in grana; stroma contains enzymes | Site of photosynthesis |
| Rough ER | Network of flattened membrane sacs studded with ribosomes | Synthesises and transports proteins |
| Smooth ER | Network of tubular membranes, no ribosomes | Synthesises lipids and steroids |
| Golgi apparatus | Stack of flattened membrane sacs (cisternae) | Modifies, packages and dispatches proteins in vesicles |
| Lysosome | Membrane-bound vesicle containing hydrolytic enzymes | Breaks down old organelles, pathogens, cellular debris |
| Ribosome | Two subunits (large and small), made of rRNA and protein | Site of protein synthesis (translation) |
| Cell wall (plant, fungal) | Rigid layer of cellulose (plants) or chitin (fungi) | Provides shape and prevents osmotic lysis |
Differences between animal and plant cells
Animal and plant cells share most organelles. The differences are easy to recall: Plant cells additionally have chloroplasts, a permanent vacuole and a cellulose cell wall. Animal cells have centrioles (involved in spindle formation during mitosis) and lysosomes that are typically more prominent.
In exam answers, do not just list what is present. State the function of each unique structure. For example, the permanent vacuole maintains turgor pressure, which keeps the plant cell rigid. Without that detail, only the identification mark is awarded.
| Feature | Animal cell | Plant cell |
|---|---|---|
| Cell wall | Absent | Present (cellulose) |
| Chloroplasts | Absent | Present |
| Permanent vacuole | Small or absent | Large, central, tonoplast-bound |
| Centrioles | Present | Absent in flowering plants |
| Shape | Variable, rounded | Regular, often rectangular due to cell wall |
Prokaryotic cell structure
Prokaryotic cells are smaller (typically 1 to 5 micrometres) and structurally simpler than eukaryotic cells. They have no nucleus, no membrane-bound organelles, and no linear chromosomes. The genome is a single circular DNA molecule in the cytoplasm, often supplemented by smaller circular plasmids that can be exchanged between cells.
The cell wall of bacteria is made of peptidoglycan (also called murein), not cellulose. Some bacteria have an outer slime capsule for protection, and many have flagella for movement or pili for attaching to surfaces. All prokaryotes have 70S ribosomes, which are smaller than the 80S ribosomes found in eukaryotic cytoplasm.
| Feature | Eukaryotic cell | Prokaryotic cell |
|---|---|---|
| Size | 10 to 100 micrometres | 1 to 5 micrometres |
| Nucleus | Present (membrane-bound) | Absent |
| DNA | Linear, in chromosomes, with histone proteins | Single circular molecule, plus plasmids, no histones |
| Membrane-bound organelles | Yes (mitochondria, ER, Golgi, etc.) | None |
| Ribosomes | 80S in cytoplasm, 70S in mitochondria and chloroplasts | 70S only |
| Cell wall | Cellulose (plants), chitin (fungi), absent (animals) | Peptidoglycan / murein |
Endosymbiotic origin of mitochondria and chloroplasts Mitochondria and chloroplasts both have 70S ribosomes, double membranes and their own small loops of DNA. This evidence supports the endosymbiotic theory: They were once free-living prokaryotes that became permanent residents inside larger ancestral cells. The theory is not directly examined, but it is a useful synoptic link.
Microscopy: Light, TEM and SEM
You need to be able to compare three microscopy techniques. Light microscopes use visible light and glass lenses, can see live specimens, and have a resolution of about 200 nm (limited by the wavelength of light). Transmission electron microscopes (TEM) fire electrons through a thin specimen, producing a high-resolution 2D image of internal structures with resolution down to about 0.2 nm.
Scanning electron microscopes (SEM) bounce electrons off the surface of a specimen and produce a 3D-looking image with resolution around 5 to 20 nm. Both electron microscopes can only image dead, dehydrated specimens in a vacuum, which is why they cannot show live cellular processes.
| Microscope | Resolution | Strengths | Limitations |
|---|---|---|---|
| Light | About 200 nm | Cheap, can view live specimens in colour | Cannot resolve small organelles like ribosomes |
| TEM | About 0.2 nm | Very high resolution, 2D internal detail | Specimens must be dead, very thin, in a vacuum |
| SEM | About 5–20 nm | 3D surface images, larger specimens | Lower resolution than TEM, surface only |
Magnification and resolution calculations
Magnification is calculated using the equation: Magnification = image size / actual size. Rearrange to find actual size when given image size and magnification, or to find image size when given actual size and magnification. The trick is keeping all measurements in the same units, usually micrometres (μm) or nanometres (nm).
Resolution is the smallest distance two separate points can be apart and still appear as two points in the image. It is set by the wavelength of light or electrons used. Increasing magnification beyond the resolution limit just produces a bigger but blurrier image, not more detail. Mark schemes reward students who can explain why resolution matters more than magnification.
Worked example for magnification A mitochondrion appears 8 mm long under a TEM at magnification 20,000 times. Convert 8 mm to micrometres: 8 mm = 8000 μm. Actual size = image / magnification = 8000 / 20,000 = 0.4 μm. Mitochondria vary in size (typically around 0.5 to 10 μm long, 0.5 to 1 μm wide), so 0.4 μm sits just below the typical range, which is plausible for a small or end-on mitochondrion.
Cell fractionation and ultracentrifugation
To study organelles separately, biologists isolate them using cell fractionation. The tissue is first homogenised in a cold, isotonic, buffered solution. Cold slows enzyme activity to prevent organelle damage. Isotonic prevents osmotic lysis. Buffered keeps pH stable so enzymes do not denature.
The homogenate is then filtered to remove debris and centrifuged at increasing speeds (ultracentrifugation). The densest organelles (nuclei) settle out first at the lowest speed, then mitochondria and chloroplasts at intermediate speeds, then the endoplasmic reticulum, and finally ribosomes at the highest speed. Each pellet is collected and the supernatant is re-centrifuged at the next speed.
Where students lose marks
AQA examiner reports flag the same handful of slips on the cell structure topic every year. Most are about precision rather than missing knowledge.
Common mistakes that cost easy marks Confusing 70S and 80S ribosomes (70S is prokaryotic, 80S is eukaryotic). Saying a vacuole is bound by a cell membrane when it is bound by the tonoplast. Calling peptidoglycan cellulose. Writing that electron microscopes can see live cells. Forgetting to convert units in magnification calculations. Describing the cell wall as semi-permeable when it is fully permeable.
Key facts to memorise for the exam
- Eukaryotic cells have membrane-bound organelles and a nucleus
- Prokaryotic cells have no nucleus and a single circular DNA molecule
- Mitochondria and chloroplasts have a double membrane and 70S ribosomes
- 80S ribosomes are eukaryotic cytoplasmic; 70S are prokaryotic and organellar
- Plant cell wall is cellulose; fungal is chitin; bacterial is peptidoglycan
- Resolution is set by wavelength: Light ~200 nm, TEM ~0.2 nm, SEM ~5–20 nm
- Magnification = image size / actual size, with consistent units
- Cell fractionation: Cold, isotonic, buffered solution; then differential centrifugation
