How does the ultrastructure of prokaryotic and eukaryotic cells relate to their functions, and how do we distinguish the two?
Describe the ultrastructure of prokaryotic and eukaryotic cells and relate the structure of organelles to their functions
A focused answer to the H2 Biology Cell Biology outcome on cell ultrastructure. The organelles of eukaryotic cells and how their structure fits their function, the simpler organisation of prokaryotes, and the key differences between the two cell types.
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What this dot point is asking
SEAB wants you to describe the detailed (ultra) structure of cells as revealed by the electron microscope, name the organelles of a eukaryotic cell, relate each organelle's structure to its function, and contrast the eukaryotic cell with the simpler prokaryotic cell. This is foundational: almost every later topic refers back to an organelle or to the difference between the two cell types.
The answer
The eukaryotic cell and its organelles
A eukaryotic cell is defined by a true nucleus enclosed in a double membrane and by a set of membrane-bound organelles that compartmentalise its chemistry.
- Nucleus. Enclosed by a double membrane (the nuclear envelope) perforated by nuclear pores. It holds the linear DNA wound around histone proteins. The nucleolus inside it makes ribosomal RNA.
- Rough endoplasmic reticulum (RER). Flattened membrane sacs studded with ribosomes; synthesises and folds proteins for secretion or for membranes.
- Smooth endoplasmic reticulum (SER). Tubular membranes without ribosomes; synthesises lipids and steroids and stores calcium ions.
- Golgi apparatus. A stack of flattened sacs that modifies, sorts and packages proteins and lipids into vesicles.
- Mitochondrion. A double membrane organelle; the inner membrane is folded into cristae carrying the electron transport chain. It is the site of aerobic respiration and ATP synthesis.
- Ribosomes. Sites of translation. Free in the cytoplasm or bound to the RER; the eukaryotic cytoplasmic ribosome is 80S.
- Lysosome. A membrane sac of hydrolytic enzymes that digests worn-out organelles and engulfed material.
Plant cells additionally have a cellulose cell wall, a large central vacuole, and chloroplasts (the site of photosynthesis, with their own double membrane and internal thylakoid stacks).
The prokaryotic cell
A prokaryotic cell (a bacterium) is much smaller and simpler. It has no nucleus and no membrane-bound organelles. Its single circular DNA molecule lies free in the cytoplasm in the nucleoid region, often with small extra rings called plasmids. It has a cell surface membrane, a peptidoglycan cell wall, smaller 70S ribosomes, and frequently a flagellum or a protective capsule.
Relating structure to function
The recurring theme is that structure fits function. The cristae of the mitochondrion give a large surface area for the electron transport chain. The ribosome-studded RER places protein synthesis next to the lumen where folding occurs. The nuclear envelope separates transcription from translation so that mRNA can be processed before it is read.
Examples in context
Example 1. Antibiotic selectivity. Many antibiotics bind the bacterial 70S ribosome and block translation without affecting the host's 80S cytoplasmic ribosomes. The structural difference between prokaryotic and eukaryotic ribosomes is what makes this selective toxicity possible.
Example 2. Endosymbiotic theory. Mitochondria and chloroplasts have their own circular DNA and 70S ribosomes and divide by binary fission, just as free-living prokaryotes do. This is the central evidence that these organelles arose when a prokaryote was engulfed by an ancestral eukaryotic cell.
Try this
Q1. State two structures found in a prokaryotic cell but not in an animal cell. [2 marks]
- Cue. Cell wall (peptidoglycan) and a plasmid or capsule or flagellum; circular DNA in a nucleoid is also acceptable.
Q2. Explain why a cell that makes and exports lipid-based hormones would contain abundant smooth endoplasmic reticulum. [2 marks]
- Cue. SER is the site of lipid and steroid synthesis, so a steroid-secreting cell needs a large amount of it.
Q3. A mitochondrion has folded inner membranes called cristae. Explain how this folding supports the organelle's function. [2 marks]
- Cue. Cristae increase the surface area of the inner membrane, providing more space for the electron transport chain and ATP synthase, raising the rate of ATP production.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksDescribe three structural features that distinguish a prokaryotic cell from a eukaryotic cell, and explain how each relates to a functional difference between the two cell types.Show worked answer →
Examiners want a feature, then the functional consequence, for each of three points.
First, prokaryotes have no membrane-bound nucleus; their circular DNA lies free in the cytoplasm in a region called the nucleoid, whereas eukaryotic DNA is linear, associated with histones and enclosed by a double nuclear envelope. The consequence is that in eukaryotes transcription and translation are physically separated, allowing post-transcriptional processing of mRNA, while in prokaryotes translation can begin on an mRNA still being transcribed.
Second, prokaryotes lack membrane-bound organelles such as mitochondria and the endoplasmic reticulum. Aerobic respiration in prokaryotes is associated with the cell surface membrane rather than a dedicated organelle, so they cannot compartmentalise incompatible reactions to the same degree.
Third, prokaryotic ribosomes are smaller (70S) than eukaryotic cytoplasmic ribosomes (80S). This difference is the basis for selective antibiotics that bind the 70S ribosome without harming the host.
Markers reward three correctly paired feature-and-function statements, accurate use of terms such as nucleoid and 70S, and a clear distinction rather than a list of features alone.
Original4 marksA secretory cell exports large amounts of protein. Name two organelles you would expect to be especially abundant and explain the role of each in the secretory pathway.Show worked answer →
The answer should follow the route a secreted protein takes through the cell.
Rough endoplasmic reticulum would be abundant. Its membranes are studded with ribosomes that synthesise proteins destined for secretion; the newly made polypeptides enter the ER lumen where they fold and undergo initial modification, then bud off in transport vesicles.
The Golgi apparatus would also be abundant. It receives vesicles from the rough ER, modifies the proteins further (for example by adding carbohydrate to form glycoproteins), sorts them, and packages them into secretory vesicles that travel to the cell surface membrane for release by exocytosis.
A strong answer may also note abundant mitochondria to supply the ATP for synthesis and vesicle transport. Markers reward correct naming, the link of structure to role, and a sense of the ordered pathway from rough ER to Golgi to vesicle.
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