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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)"]