Cell Structure and Ultrastructure

Understanding the intricate architecture of cells is fundamental to modern biology, moving beyond simple diagrams to the detailed reality revealed by advanced microscopy. By comparing prokaryotic and eukaryotic ultrastructure—the fine details visible only with powerful electron microscopes—you gain insight into the evolutionary relationships between life forms and the functional compartmentalization that defines complex organisms. This knowledge is not only core to A-Level Biology but also forms the basis for advancements in medicine, genetics, and biotechnology.

The Limits of Sight: Resolution and Magnification

To study cell ultrastructure, you must first understand the tools that make it visible. Resolution is the minimum distance between two points that can be distinguished as separate. The human eye has a resolution of about 0.1 mm, while a light microscope improves this to roughly 200 nm. This is insufficient to see most organelles. Transmission Electron Microscopes (TEMs) and Scanning Electron Microscopes (SEMs) use a beam of electrons instead of light, achieving resolutions down to 0.1 nm, allowing us to see the detailed internal structure or 3D surface of cells.

Magnification is simply how much larger an image appears compared to the object's real size. It is calculated using the formula: $Magnification=\frac{ImageSize}{ActualSize}$. You must be able to manipulate this formula. For example, if a mitochondrion in an electron micrograph measures 20 mm and its actual length is 2 μm, first convert measurements to the same units (2 μm = 0.002 mm). The magnification is then $\frac{20mm}{0.002mm}=\times 10,000$. Always show your working and double-check unit conversions—a common source of error.

Prokaryotic vs. Eukaryotic Ultrastructure

The most fundamental division in cellular life is between prokaryotic and eukaryotic cells, a distinction starkly clear in electron micrographs.

Prokaryotic cells, like bacteria, are simpler and smaller (typically 0.5-5 μm). Their key ultrastructural features, visible under a TEM, include:

• No membrane-bound nucleus: Genetic material is a single, circular DNA molecule that lies free in the cytoplasm, in a region called the nucleoid.
• No membrane-bound organelles. Their cytoplasm contains 70S ribosomes (smaller than eukaryotic ones) and often stored glycogen or lipid droplets.
• A cell wall composed of peptidoglycan (murein) for protection and shape.
• Possession of plasmids (small circular DNA molecules) and sometimes a capsule for further protection.
• Flagella for movement, which have a fundamentally different, simpler structure than eukaryotic flagella.

In contrast, eukaryotic cells (animal, plant, fungal, protist) are larger (typically 10-100 μm) and defined by compartmentalization. They possess a true, membrane-bound nucleus and a variety of specialized, membrane-bound organelles. This compartmentalization allows for separate, efficient, and often incompatible metabolic processes to occur simultaneously within the cell.

Organelles of Genetic Control and Protein Synthesis

The command center and protein factories of the eukaryotic cell are its most prominent features.

The nucleus is enclosed by a double membrane called the nuclear envelope, which is studded with nuclear pores that control the movement of molecules like mRNA and ribosomes. Inside, chromatin (DNA wrapped around histone proteins) is visible as dark-staining material. A denser region within, the nucleolus, is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Ribosomes are the sites of protein synthesis. They appear as small, dense granules in TEM images. They can be free in the cytoplasm, producing proteins for use within the cell, or attached to the rough endoplasmic reticulum (RER), producing proteins for export or for membranes. The RER is a network of flattened, membrane-bound sacs (cisternae) with a "rough" appearance due to attached ribosomes. Its primary function is to synthesize, fold, and modify proteins.

The Endomembrane System: Modification, Transport, and Breakdown

Proteins and lipids manufactured in the cell are processed, sorted, and transported via a coordinated system of organelles.

The smooth endoplasmic reticulum (SER) is a system of tubular membranes continuous with the RER but lacking ribosomes. It synthesizes lipids (including steroids and phospholipids), metabolizes carbohydrates, and detoxifies drugs and poisons. In liver and muscle cells, the SER is particularly abundant.

The Golgi apparatus (Golgi body) appears in micrographs as a stack of flattened membrane sacs (cisternae). It receives transport vesicles from the ER. Here, proteins and lipids are further modified (e.g., by adding carbohydrate chains to form glycoproteins), sorted, and packaged into new vesicles. These vesicles are then dispatched to their destinations: the plasma membrane for secretion, other organelles, or to form lysosomes.

Lysosomes are membrane-bound vesicles containing a cocktail of powerful hydrolytic (digestive) enzymes. They function as the cell's stomach, digesting material ingested by phagocytosis, breaking down worn-out organelles (autophagy), and, in some cases, digesting the entire cell (autolysis). Their intact membrane is crucial to prevent these enzymes from destroying the cell itself.

Energy-Converting Organelles and Endosymbiosis

Two organelles are responsible for transforming energy, and their structure provides key evidence for their evolutionary origin.

Mitochondria (singular: mitochondrion) are the sites of aerobic respiration and ATP production. In TEMs, they appear as oval-shaped organelles with a double membrane. The inner membrane is highly folded into cristae, which greatly increase the surface area for the electron transport chain. The interior fluid is the matrix, containing enzymes for the Krebs cycle, mitochondrial DNA, and 70S ribosomes.

Chloroplasts, found in plant and algal cells, are the sites of photosynthesis. They also have a double membrane and contain an internal system of thylakoid membranes stacked into grana. The fluid-filled space surrounding the thylakoids is the stroma, which contains the enzymes for the Calvin cycle, chloroplast DNA, and 70S ribosomes.

The presence of their own DNA and 70S ribosomes (characteristic of prokaryotes) in mitochondria and chloroplasts is central to the endosymbiotic theory. This theory proposes that these organelles were once free-living prokaryotic organisms (an aerobic bacterium and a photosynthetic cyanobacterium, respectively) that were engulfed by a larger ancestral eukaryotic host cell. Rather than being digested, they formed a mutually beneficial (symbiotic) relationship, eventually becoming integrated as essential organelles. The double membrane is seen as evidence of the engulfment process.

Common Pitfalls

1. Confusing Magnification and Resolution: A common exam mistake is to state that electron microscopes simply provide "higher magnification." The critical advantage is their superior resolution. You can magnify a light micrograph indefinitely, but the image will just become a larger blur—you cannot see details finer than the microscope's resolution limit.
2. Misidentifying Organelles in Micrographs: Vesicles, lysosomes, and small mitochondria can look similar. Look for key identifiers: lysosomes are often very dense and dark; mitochondria have cristae; vesicles are simple, single-membrane spheres. The Golgi apparatus is always a stack of flattened sacs, not a single tube like the SER.
3. Overgeneralizing Organelle Presence: Remember that not all eukaryotic cells contain all organelles. For instance, mature mammalian red blood cells lack a nucleus and mitochondria, and chloroplasts are only in plant and algal cells. Always consider the cell's specific function.
4. Misstating the Evidence for Endosymbiosis: Be precise. The evidence includes: their own circular DNA (like prokaryotes), their own 70S ribosomes, the ability to replicate independently of the cell cycle via binary fission, and a double membrane (where the inner one would be the original prokaryote's plasma membrane).

Summary

• Ultrastructure refers to the fine cellular details visible only with electron microscopes, which have a far higher resolution than light microscopes.
• Prokaryotic cells lack a membrane-bound nucleus and organelles, while eukaryotic cells are compartmentalized, containing specialized organelles within a cytoplasm.
• Key organelles form functional systems: the nucleus and ribosomes for genetic control; the RER, SER, Golgi apparatus, and lysosomes for synthesis, modification, and breakdown; and mitochondria and chloroplasts for energy conversion.
• The endosymbiotic theory, supported by the presence of DNA and 70S ribosomes in mitochondria and chloroplasts, explains their evolutionary origin from engulfed prokaryotes.
• Interpreting electron micrographs requires careful attention to distinguishing features like membranes, cristae, and grana, and skill in calculating magnification using the formula $Magnification=\frac{ImageSize}{ActualSize}$.