Eukaryotic Cell Architecture
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You should understand the basic structure of prokaryotic cells, including the function of the plasma membrane and the role of ribosomes in protein synthesis.
Why this matters
Eukaryotic cells solve the problem of genetic complexity by compartmentalising functions.
While prokaryotes operate as a single open compartment, eukaryotes evolved internal membranes to create distinct environments for specific biochemical processes. This architectural shift allows for the coexistence of incompatible reactions, such as the acidic environment of lysosomes and the neutral cytosol, and enables the cell to grow to much larger sizes while maintaining efficient transport and communication. Understanding this architecture is fundamental to grasping how complex life forms, from yeast to humans, function at the cellular level.
The Membrane System
The defining feature of a eukaryotic cell is the presence of a nucleus and a system of internal membranes. Unlike prokaryotes, where DNA floats freely in the cytoplasm, eukaryotic DNA is enclosed within a double-membrane structure called the nuclear envelope. This separation allows for distinct stages of gene expression: transcription occurs in the nucleus, while translation occurs in the cytoplasm. The internal membranes also form the endomembrane system, which includes the endoplasmic reticulum (ER), the Golgi apparatus, and vesicles. These organelles work together to modify, package, and transport proteins and lipids. The ER serves as the site for protein folding and lipid synthesis, while the Golgi apparatus acts as a sorting and distribution centre. This compartmentalisation is not just about separation; it is about creating specific chemical environments that optimise enzymatic reactions and protect the cell from potentially harmful intermediates.
Nuclear Structure and Function
The nucleus is the control centre of the eukaryotic cell. It is bounded by the nuclear envelope, a double membrane continuous with the rough endoplasmic reticulum. The envelope is perforated by nuclear pores, which are complex protein structures that regulate the transport of molecules between the nucleus and the cytoplasm. Small molecules and ions can diffuse freely through these pores, but larger molecules, such as mRNA and ribosomal subunits, require active transport mediated by nuclear transport receptors. Inside the nucleus, the chromatin is organised into distinct regions. The nucleolus is a dense region where ribosomal RNA is transcribed and ribosomal subunits are assembled. The nuclear matrix, a network of proteins, provides structural support and organises the chromatin. The nuclear lamina, a meshwork of intermediate filaments lining the inner nuclear membrane, helps maintain nuclear shape and anchors the chromatin.
The Endomembrane System
The endomembrane system is a network of membranes that are physically, chemically, and functionally related. It begins with the endoplasmic reticulum (ER), which exists in two forms: rough ER (studded with ribosomes) and smooth ER (lacking ribosomes). The rough ER is the primary site for the synthesis of secretory proteins, membrane proteins, and proteins destined for lysosomes. As proteins enter the ER lumen, they undergo folding and post-translational modifications, such as the addition of oligosaccharides (glycosylation). The smooth ER is involved in lipid synthesis, carbohydrate metabolism, and detoxification of drugs and poisons. From the ER, proteins are transported to the Golgi apparatus via transport vesicles. The Golgi apparatus consists of a series of flattened membrane-bound sacs called cisternae, arranged in a stack. It modifies, sorts, and packages proteins for delivery to their final destinations, such as the plasma membrane, lysosomes, or secretion outside the cell. The Golgi has a cis face (receiving side) and a trans face (shipping side), ensuring directional flow of materials.
Mitochondria and Chloroplasts
Mitochondria and chloroplasts are unique organelles that possess their own DNA and ribosomes, suggesting an endosymbiotic origin. Mitochondria are the sites of cellular respiration, where organic molecules are oxidised to produce ATP. They have a double membrane: the outer membrane is permeable to small molecules, while the inner membrane is highly folded into cristae, increasing the surface area for the electron transport chain and ATP synthase. The matrix contains the enzymes for the citric acid cycle. Chloroplasts, found in plant cells and algae, are the sites of photosynthesis. They also have a double membrane, but the inner membrane is not folded. Instead, it encloses a system of flattened sacs called thylakoids, which are stacked into grana. The thylakoid membranes contain chlorophyll and the photosystems, while the stroma (the fluid surrounding the thylakoids) contains the enzymes for the Calvin cycle. Both organelles replicate independently of the cell cycle, supporting the endosymbiotic theory that they were once free-living prokaryotes engulfed by an ancestral eukaryotic cell.
The concept
Prokaryotic and eukaryotic cells
Two fundamentally different cellular architectures.
A common misconception is that the nuclear envelope is a static barrier. In reality, it is highly dynamic; during mitosis, the nuclear envelope breaks down to allow spindle fibres to access the chromosomes, and it reforms around the daughter nuclei in telophase. (Many fungi keep the envelope intact through a "closed" mitosis.) This breakdown and reassembly are tightly regulated and essential for accurate chromosome segregation. C
Worked example
Problem. A eukaryotic cell has a nuclear diameter of \( 10\ \mu\mathrm{m} \). The central channel through which small molecules diffuse freely across a nuclear pore complex is about \( 9\ \mathrm{nm} \) wide (the whole complex is roughly 100 nm across). Calculate the ratio of the nuclear diameter to the pore diameter. Express your answer as a dimensionless number.
- Convert the nuclear diameter to nanometres: \( 10\ \mu\mathrm{m} = 10 \times 1000\ \mathrm{nm} = 10000\ \mathrm{nm} \).
- Identify the pore diameter: \( 9\ \mathrm{nm} \).
- Calculate the ratio: \( \frac{10000\ \mathrm{nm}}{9\ \mathrm{nm}} \).
- Perform the division: \( 10000 / 9 \approx 1111.11 \).
Units check. The units of nanometres cancel out, leaving a dimensionless ratio, which is correct for a comparison of lengths.
Three quick tests
- What is the primary function of the rough endoplasmic reticulum?
Answer
Synthesis of secretory proteins, membrane proteins, and lysosomal proteins. - How do mitochondria and chloroplasts support the endosymbiotic theory?
Answer
They have their own circular DNA and ribosomes, similar to prokaryotes, and replicate independently. - What structure regulates the transport of molecules between the nucleus and the cytoplasm?
Answer
Nuclear pore complexes.
Back to BU-101 →
Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.