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Eukaryotic cells are the fundamental building blocks of all plants, animals, fungi and protoctists. Understanding their structure and the function of each organelle is essential for Edexcel A-Level Biology (9BI0) Topic 2. In this lesson, we examine the ultrastructure of eukaryotic cells as revealed by electron microscopy, and explore how each organelle contributes to the overall functioning of the cell.
Eukaryotic cells are cells that possess a true nucleus — a membrane-bound compartment containing the cell's genetic material (DNA). The term "eukaryote" derives from the Greek eu (true) and karyon (kernel/nucleus). All eukaryotic cells share a common set of features, including membrane-bound organelles, a cytoskeleton, and linear chromosomes packaged with histone proteins.
Eukaryotic organisms include:
Exam Tip: The Edexcel specification (9BI0) requires you to compare and contrast eukaryotic and prokaryotic cells. Make sure you can list the key structural differences in a table — this is a very common exam question.
The nucleus is the largest organelle in most eukaryotic cells, typically 5–10 μm in diameter. It is the control centre of the cell, containing the genetic information encoded in DNA.
| Feature | Description |
|---|---|
| Nuclear envelope | A double membrane with nuclear pores; the outer membrane is continuous with the rough endoplasmic reticulum (RER) |
| Nuclear pores | Allow the passage of large molecules such as mRNA and ribosomes between the nucleus and cytoplasm |
| Nucleoplasm | The gel-like matrix inside the nucleus |
| Chromatin | DNA associated with histone proteins; condenses into visible chromosomes during cell division |
| Nucleolus | A dense region within the nucleus responsible for ribosomal RNA (rRNA) synthesis and ribosome assembly |
Exam Tip: When describing the nuclear envelope, always state it is a double membrane with nuclear pores. Many students lose marks by simply calling it a "membrane" without specifying "double".
The endoplasmic reticulum is an extensive network of membrane-bound flattened sacs (cisternae) and tubules that extends throughout the cytoplasm. There are two types:
| Feature | Rough ER | Smooth ER |
|---|---|---|
| Ribosomes | Present | Absent |
| Main function | Protein synthesis and transport | Lipid synthesis, detoxification |
| Appearance | Flattened cisternae | More tubular |
| Abundant in | Cells that secrete proteins (e.g. pancreatic cells) | Cells that synthesise lipids (e.g. liver cells) |
The Golgi apparatus (also called the Golgi complex or Golgi body) consists of a stack of flattened, membrane-bound sacs called cisternae, typically 4–8 in a stack. It has a distinct polarity:
The Golgi apparatus is particularly well-developed in cells that secrete large amounts of substances, such as goblet cells (which secrete mucus) and B lymphocytes (which secrete antibodies).
Exam Tip: Remember the pathway: RER → transport vesicle → cis face of Golgi → modification through cisternae → trans face of Golgi → secretory vesicle → cell surface membrane. This is the secretory pathway and is frequently tested.
Mitochondria (singular: mitochondrion) are double-membrane organelles responsible for aerobic respiration — the production of ATP (adenosine triphosphate), the cell's energy currency.
| Feature | Description |
|---|---|
| Outer membrane | Smooth, contains porin proteins that allow small molecules to pass through |
| Inner membrane | Highly folded into cristae; contains the electron transport chain (ETC) and ATP synthase enzymes |
| Intermembrane space | The narrow space between the outer and inner membranes; important for the proton gradient (chemiosmosis) |
| Matrix | The gel-like interior; contains enzymes for the Krebs cycle, mitochondrial DNA (mtDNA), 70S ribosomes and various metabolites |
The folding of the inner membrane into cristae greatly increases the surface area available for the electron transport chain and oxidative phosphorylation. Cells with high energy demands — such as muscle cells, sperm cells and active transport cells in the kidney — contain large numbers of mitochondria with well-developed cristae.
Mitochondria possess their own circular DNA and 70S ribosomes, similar to those found in prokaryotes. This provides strong evidence for the endosymbiotic theory, which proposes that mitochondria evolved from free-living aerobic bacteria that were engulfed by an ancestral eukaryotic cell.
Ribosomes are the sites of protein synthesis (translation). They are not membrane-bound and are found either free in the cytoplasm or attached to the rough endoplasmic reticulum.
| Feature | Eukaryotic ribosomes | Prokaryotic ribosomes |
|---|---|---|
| Size | 80S | 70S |
| Subunits | 60S + 40S | 50S + 30S |
| Location | Free in cytoplasm or attached to RER | Free in cytoplasm |
Exam Tip: The "S" in 70S and 80S stands for Svedberg units, a measure of sedimentation rate during centrifugation — not a simple addition of subunit values. This is a common point of confusion.
Lysosomes are single-membrane-bound vesicles containing powerful hydrolytic (digestive) enzymes (hydrolases). These enzymes function optimally at an acidic pH of around 4.5–5.0, maintained by proton pumps in the lysosomal membrane.
Lysosomes are abundant in phagocytic white blood cells (neutrophils and macrophages), which engulf and destroy pathogens.
The cytoskeleton is a dynamic network of protein filaments within the cytoplasm that provides:
The three main components are:
| Component | Diameter | Composition | Function |
|---|---|---|---|
| Microfilaments | ~7 nm | Actin | Cell movement, cytokinesis, muscle contraction |
| Intermediate filaments | ~10 nm | Various proteins (e.g. keratin) | Mechanical strength, anchoring organelles |
| Microtubules | ~25 nm | Tubulin | Intracellular transport, spindle formation, cilia/flagella |
Plant cells are eukaryotic but possess additional structures not found in animal cells:
| Feature | Animal Cell | Plant Cell |
|---|---|---|
| Cell wall | Absent | Present (cellulose) |
| Chloroplasts | Absent | Present (in green parts) |
| Large central vacuole | Absent (small temporary vacuoles) | Present |
| Centrioles | Present | Usually absent |
| Shape | Irregular / round | Fixed / rectangular |
The detailed internal structure of cells — the ultrastructure — can only be observed using electron microscopes, which use a beam of electrons instead of light.
| Feature | Light Microscope | Transmission Electron Microscope (TEM) | Scanning Electron Microscope (SEM) |
|---|---|---|---|
| Maximum resolution | ~200 nm | ~0.5 nm | ~3–10 nm |
| Maximum magnification | ~x1,500 | ~x500,000 | ~x100,000 |
| Specimen | Living or dead, thin sections or whole mounts | Dead, ultra-thin sections | Dead, surface-coated |
| Image type | 2D, colour | 2D, black and white | 3D, black and white |
| Advantages | Cheap, portable, can view living specimens | Very high resolution, can see internal ultrastructure | 3D images of surface detail |
Magnification=Actual sizeImage size
This can be rearranged:
Actual size=MagnificationImage size
Exam Tip: Magnification calculations are commonly tested. Always ensure your units are consistent — convert mm to μm by multiplying by 1,000, and μm to nm by multiplying by 1,000. Show your working clearly.
Eukaryotic cells are complex, highly organised structures containing membrane-bound organelles that compartmentalise cellular activities. Each organelle has a specific function that contributes to the survival of the cell and the organism. Understanding the relationship between structure and function at the ultrastructural level is a central theme of A-Level Biology.
Key points to remember:
The following diagram shows how key organelles work together in the secretory pathway:
graph TD
A["Nucleus<br/>(DNA → mRNA)"] --> B["Rough ER<br/>(protein synthesis)"]
B --> C["Golgi Apparatus<br/>(modification & packaging)"]
C --> D["Vesicles<br/>(transport)"]
D --> E["Cell Membrane<br/>(secretion)"]
C --> F["Lysosomes<br/>(digestion)"]
The Edexcel 9BI0 specification places eukaryotic ultrastructure within Topic 3: Voice of the Genome, with substantial overlap into Topic 2: Membranes, Proteins, DNA and Gene Expression and Topic 4: Biodiversity and Natural Resources. The relevant statements concern: identifying organelles in animal and plant cells from electron-micrograph images (nucleus, nucleolus, ribosomes, rough and smooth endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes, centrioles, plasma membrane, cell wall, chloroplasts, vacuole and tonoplast); describing the function of each organelle; and explaining how organelles cooperate in the protein-secretion pathway. The specification also requires candidates to calculate magnification and actual size from scale bars, to convert between mm, μm and nm, and to compare the resolution and magnification of light versus electron microscopes. Synoptic threads run into Topic 4 (the role of 70S ribosomes in selective antibiotic toxicity) and Topic 7 (mitochondrial structure as the site of oxidative phosphorylation). (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).
Question (8 marks):
A transmission electron micrograph of a pancreatic acinar cell shows a structure with a double membrane, the outer of which is studded with ribosomes and is continuous with a network of flattened sacs occupying much of the cytoplasm. A scale bar on the image reads 2μm and measures 40mm long when printed.
(a) Identify the structure described and the network with which it is continuous, justifying your answer with reference to the ultrastructural features given. (3)
(b) Calculate the magnification of the printed image, expressing your answer in standard form. (2)
(c) The nucleus of the same cell measures 60mm across on the print. Calculate its actual diameter in μm. (3)
Solution with mark scheme:
(a) Step 1 — interpret the ultrastructural clues. A double membrane that is outer-studded with ribosomes and continuous with a network of flattened sacs identifies the nuclear envelope, with the ribosome-bearing outer membrane continuous with the rough endoplasmic reticulum (RER).
M1 (AO2.1) — correct identification of the nuclear envelope from the double-membrane clue. Common error: candidates write "mitochondrion" because they remember "double membrane" but ignore the ribosomes on the outer surface and the continuity with a sheet network.
M1 (AO2.1) — correct identification of the RER from the ribosome-studded sacs.
A1 (AO1.2) — explicit justification linking each structural feature to its identification (e.g. "ribosomes on the outer nuclear membrane mark the start of co-translational protein synthesis into the RER lumen"). A bare "nuclear envelope and RER" with no justification scores M1 M1 only.
(b) Step 1 — convert units consistently. Scale bar represents 2μm=2×10−6m=2×10−3mm. Printed length =40mm.
M1 (AO2.1) — correct conversion of μm to mm (or mm to μm); units must match before dividing.
Step 2 — apply magnification=actual sizeimage size.
M=2×10−3mm40mm=2×104
A1 (AO1.1b) — magnification =2×104 (or ×20000). Common error: leaving the answer as 20000 when the question demands standard form, or writing ×104 without the leading coefficient.
(c) Step 1 — rearrange. Actual size =magnificationimage size=2×10460mm.
M1 (AO1.1a) — correct rearrangement of the magnification equation.
Step 2 — evaluate. 2×10460=3×10−3mm.
M1 (AO2.1) — correct numerical division with consistent units.
Step 3 — convert to μm. 3×10−3mm=3μm.
A1 (AO1.1b) — final answer 3μm. Common error: candidates leave the answer as 0.003mm when the unit asked for is μm, losing the A1 for failing to honour the units of the question stem.
Total: 8 marks.
Question (6 marks): A scientist examines an unknown eukaryotic cell using both a light microscope and a transmission electron microscope. Explain how the electron micrograph would allow her to distinguish a plant cell from an animal cell, and how it would allow her to determine whether the cell is metabolically active in protein secretion.
Mark scheme decomposition by AO:
| Marking point | AO | Credit-worthy content |
|---|---|---|
| 1 | AO1.1 | States that a plant cell has a cell wall, large central vacuole and chloroplasts whereas an animal cell does not. |
| 2 | AO1.2 | Explains that centrioles are typically present in animal cells but absent from higher-plant cells. |
| 3 | AO2.1 | Applies to the micrograph: the cell wall appears as a thick external boundary outside the plasma membrane; the vacuole appears as a single large electron-lucent region. |
| 4 | AO2.1 | Applies to the question's second clause: extensive RER, prominent Golgi stacks and abundant secretory vesicles indicate active protein secretion. |
| 5 | AO3.1a | Interprets relative organelle abundance (e.g. high mitochondrial density implies high ATP demand for synthesis and exocytosis). |
| 6 | AO3.2a | Concludes by linking the visible structure to the metabolic function — e.g. "this combination of features is consistent with a secretory plant cell such as a glandular trichome". |
Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. This is a classic Section A "explain how" question — Edexcel rewards candidates who use the micrograph (AO2/AO3) rather than merely listing organelles (AO1).
| AO | Typical share on this topic | Earned by |
|---|---|---|
| AO1 (knowledge) | 40–50% | Naming organelles, stating their function, recalling magnification/resolution definitions, distinguishing 70S from 80S ribosomes |
| AO2 (application) | 35–45% | Identifying organelles from electron micrographs, performing magnification/actual-size calculations, applying structure–function reasoning to unfamiliar cells |
| AO3 (analysis / evaluation) | 10–20% | Evaluating which microscopy technique is appropriate; interpreting abnormal organelle abundance in pathological cells; linking ultrastructure to whole-cell metabolic phenotype |
Examiner-rewarded phrasing: "the cristae of the inner mitochondrial membrane provide an increased surface area for the electron-transport chain"; "the rough endoplasmic reticulum is studded with 80S ribosomes engaged in co-translational synthesis of proteins destined for secretion or membrane insertion"; "lysosomes contain hydrolytic enzymes that function optimally at the acidic pH maintained by membrane-bound proton pumps".
Phrases that lose marks: "the mitochondrion makes energy" (energy is transferred, not made; ATP is the credited product); "ribosomes are organelles" (they are sub-cellular structures lacking a membrane, not membrane-bound organelles); "the cell wall protects the cell" (vague — credit requires "prevents lysis by withstanding turgor pressure" or "provides mechanical support"); answers that conflate the smooth ER (lipid synthesis, detoxification, calcium storage) with the rough ER despite naming the structural difference.
Question: Describe two structural differences between the rough endoplasmic reticulum (RER) and the Golgi apparatus, and state one functional consequence of these differences. (3)
Grade C response (~195 words):
The RER has ribosomes on its surface but the Golgi apparatus does not have ribosomes. The RER is also a network of long tubes whereas the Golgi is a stack of flattened sacs. A functional consequence is that the RER makes proteins because of the ribosomes, while the Golgi modifies them.
Examiner commentary: Awarded 2/3. The candidate identifies two valid structural differences (presence/absence of ribosomes; tubular network versus stacked cisternae) and gives a functional consequence. The answer scores M1 for the ribosome difference, M1 for the morphology difference, but loses the third mark because "modifies them" is too vague — examiners need a specific modification (glycosylation, addition of carbohydrate side chains, sorting/packaging into vesicles). A common pitfall is to use everyday verbs like "deals with" or "handles" instead of biology-specific terminology. The candidate also misses the opportunity to link the absence of ribosomes on the Golgi to its post-translational rather than translational role, which would secure the AO2 credit.
Grade A response (~225 words):*
The rough endoplasmic reticulum (RER) is a network of interconnected flattened sacs and tubules whose cytoplasmic face is studded with 80S ribosomes; the Golgi apparatus is a discrete stack of flattened, curved cisternae bearing no ribosomes. A second structural difference is the polarity of the Golgi: it has a distinct cis (receiving) face and trans (shipping) face, whereas the RER is morphologically uniform along its length. A direct functional consequence of the ribosome distribution is that the RER is the site of co-translational synthesis and initial folding of proteins destined for secretion, lysosomes or the plasma membrane, while the Golgi performs post-translational modifications — notably the trimming and elaboration of N-linked glycan chains and the addition of mannose-6-phosphate tags that direct lysosomal hydrolases to their target compartment.
Examiner commentary: Full marks (3/3). The candidate uses precise vocabulary (cisternae, cis and trans faces, co-translational, N-linked glycosylation, mannose-6-phosphate) and links each structural feature to a specific molecular event. The mannose-6-phosphate detail is beyond the strict specification but examiners credit any biologically correct extension, and it signals the depth that distinguishes A* from A.
Question: Pancreatic acinar cells secrete digestive enzymes such as trypsinogen. Explain how the ultrastructure of an acinar cell is adapted for this function. (6)
Grade B response (~265 words):
Pancreatic acinar cells make and secrete trypsinogen, so they are adapted to do this efficiently. They have lots of rough endoplasmic reticulum which is where the protein is made on the ribosomes. The Golgi apparatus then modifies and packages the protein into vesicles. The vesicles travel to the cell membrane and are released by exocytosis. Acinar cells also have many mitochondria because exocytosis requires ATP. The nucleus is large because it contains the DNA that codes for trypsinogen.
Examiner commentary: Awarded 4/6. The candidate covers the secretory pathway (RER → Golgi → vesicles → exocytosis) and references mitochondrial abundance for ATP. However, the answer is structurally adequate but mechanistically thin: the candidate does not state that ribosomes on the outer face of the RER allow co-translational threading into the lumen, nor that the Golgi performs specific modifications such as glycosylation, nor that trypsinogen is stored as an inactive zymogen in condensing secretory vesicles to prevent premature autodigestion. The mitochondrial point is correct but undeveloped — A* candidates would specify that exocytosis requires ATP for vesicle docking and SNARE-mediated membrane fusion. The nucleus comment adds nothing because every cell has a nucleus; the examiner expects a feature uniquely relevant to high-rate protein secretion.
Grade A response (~290 words):*
Pancreatic acinar cells are specialised for high-rate synthesis, processing and regulated secretion of trypsinogen, and their ultrastructure reflects each stage of this pathway. The basal cytoplasm is densely packed with rough endoplasmic reticulum (RER); ribosomes on its cytoplasmic face thread the nascent polypeptide co-translationally into the RER lumen, where signal-peptide cleavage and initial folding occur. A prominent supranuclear Golgi apparatus receives transport vesicles at its cis face, performs glycosylation and proteolytic maturation, and concentrates the inactive zymogen at its trans face into condensing vacuoles that mature into secretory granules. These granules cluster apically and undergo regulated exocytosis at the apical membrane in response to cholecystokinin signalling. The cytoplasm contains numerous mitochondria positioned near the RER and apical membrane, supplying the ATP required for vesicle trafficking, SNARE-mediated membrane fusion and the maintenance of ion gradients that drive secondary active transport. The nucleus is unusually euchromatic, indicating active transcription of digestive-enzyme genes, with a prominent nucleolus reflecting high rRNA synthesis to sustain ribosome biogenesis. Storage of trypsin as an inactive zymogen, segregated within membrane-bound granules, prevents autodigestion of the acinar cell itself.
Examiner commentary: Full marks (6/6). The candidate links every named ultrastructural feature to a specific stage of the secretory pathway, uses correct molecular terminology (signal peptide, glycosylation, condensing vacuole, SNARE), and reaches AO3 by reasoning about why zymogen storage prevents autodigestion. The euchromatic-nucleus and nucleolar-prominence details show that the candidate understands organelle ultrastructure as a predictive indicator of cellular activity, not just a memorised list.
Question: Compare and contrast the ultrastructure of a typical eukaryotic plant cell, a typical eukaryotic animal cell, and a typical prokaryotic bacterial cell. Discuss how these structural differences relate to differences in cellular function and to the evolutionary endosymbiotic origin of mitochondria and chloroplasts. (9)
Grade A response (~395 words):*
All three cell types are bounded by a phospholipid bilayer plasma membrane, but their internal organisation differs sharply. The eukaryotic plant cell possesses a true nucleus, an extensive endomembrane system (RER, SER, Golgi, lysosome-equivalent vacuoles), 80S cytoplasmic ribosomes, mitochondria, a tonoplast-bound large central vacuole, chloroplasts and a cellulose cell wall reinforced with pectin in the middle lamella. The eukaryotic animal cell shares the nucleus, endomembrane system, 80S ribosomes and mitochondria, but lacks chloroplasts, a cellulose cell wall and a large permanent vacuole; it possesses centrioles organising the spindle during mitosis. The prokaryotic bacterial cell has no membrane-bound nucleus — its circular DNA lies free in a nucleoid region — and no membrane-bound organelles. Its ribosomes are 70S (smaller subunits and different rRNA than 80S), and its cell wall contains peptidoglycan rather than cellulose. Plasmids carry accessory genes; pili and flagella may project from the surface.
These structural differences map onto functional capacities. Compartmentalisation in eukaryotes allows incompatible reactions to proceed simultaneously — for example, lysosomal hydrolysis at pH 4.5 cannot coexist with cytoplasmic glycolysis at pH 7.4 without a membrane boundary. The plant cell's chloroplasts permit autotrophic carbon fixation, and the large vacuole sustains turgor for mechanical support and growth by cell expansion. The animal cell's lack of cell wall permits the dynamic shape changes required for movement, phagocytosis and tissue morphogenesis. The bacterial cell's small size and lack of compartmentalisation reflect a high surface-area-to-volume ratio that supports rapid diffusion-based metabolism and division.
The endosymbiotic theory accounts for several of these features. Mitochondria and chloroplasts each retain a double membrane, circular DNA, 70S ribosomes and divide by binary fission — features inherited from free-living prokaryotic ancestors engulfed by an early eukaryote. The 70S ribosomes of mitochondria and chloroplasts explain why antibiotics targeting prokaryotic translation (e.g. chloramphenicol) can have eukaryotic mitochondrial side-effects, a synoptic link to selective antibiotic toxicity.
Examiner commentary: Full marks (9/9). The candidate organises the answer in three clear stages — structural comparison, functional consequence, evolutionary explanation — matching the question's three demands. Specific molecular details (peptidoglycan, 70S/80S, double membrane, circular DNA) earn AO1 marks; the surface-area-to-volume reasoning and the chloramphenicol synoptic link earn AO3. The integration of endosymbiotic theory with antibiotic toxicity is the kind of cross-topic synthesis Edexcel rewards at the very top.
Oxbridge-style interview prompt: "Mitochondria and chloroplasts both have double membranes, circular DNA and 70S ribosomes. How would you design an experiment to test whether a newly described eukaryotic organelle is also of endosymbiotic origin?"
This topic links to Edexcel 9BI0 Core Practical 8 (preparation of stained microscope sections — typically a root-tip squash of garlic, onion or broad bean stained with acetic orcein or by the Feulgen reaction). Strictly, CP8 probes mitotic dividing cells and the morphology of condensed chromosomes, not the ultrastructure of resting eukaryotic cells; the technique is therefore an indirect but instructive partner to this topic. The shared craft is the same: fixation, hydrolysis, staining and squashing to render organelles or chromatin visible at light-microscope resolution. The Feulgen reaction is mechanistically informative — mild HCl hydrolysis cleaves purines from deoxyribose, exposing aldehyde groups on the sugar that the Schiff reagent stains magenta. This is DNA-specific (RNA lacks deoxyribose), and the resulting contrast is what allows nuclei and mitotic chromosomes to be unambiguously identified — a tangible demonstration of the principle that stain chemistry, not just magnification, is what makes ultrastructure interpretable under the microscope.
This content is aligned with the Pearson Edexcel GCE A Level Biology B (9BI0) specification, Paper 1 — Lifestyle, Transport, Genes and Health, Topic 2: Cells, Viruses and Reproduction. For the most accurate and up-to-date information, please refer to the official Pearson Edexcel specification document.
graph TD
A["Nucleus<br/>(DNA, nucleolus)"] --> B["RER<br/>(co-translational<br/>protein synthesis)"]
B --> C["Transport vesicles"]
C --> D["Golgi cis face<br/>(receive)"]
D --> E["Golgi medial<br/>(glycosylation)"]
E --> F["Golgi trans face<br/>(sort and tag)"]
F --> G["Secretory vesicles<br/>(exocytosis)"]
F --> H["Lysosomes<br/>(M6P-tagged hydrolases)"]
I["Mitochondria<br/>(ATP for trafficking)"] --> C
I --> G
style A fill:#27ae60,color:#fff
style G fill:#3498db,color:#fff
style H fill:#e67e22,color:#fff