Comparing Prokaryotes and Eukaryotes
~7 min read · Tier toggle changes depth (T1 plain · T2 standard · T3 rigour)
You should be familiar with the basic components of a cell, including membranes, ribosomes, and the concept of genetic material, as covered in the previous lecture on Eukaryotic Cell Architecture.
Why this matters
Understanding the fundamental differences between prokaryotes and eukaryotes is essential for grasping the evolutionary history of life and the basis of cellular function.
The distinction between prokaryotic and eukaryotic cells represents one of the most significant divisions in biology. Prokaryotes, which include bacteria and archaea, are generally smaller and simpler in structure, lacking a defined nucleus and membrane-bound organelles. In contrast, eukaryotes, which encompass plants, animals, fungi, and protists, possess a complex internal architecture characterised by a nucleus and specialised organelles. This structural dichotomy dictates differences in gene expression, metabolism, and reproduction, influencing everything from antibiotic efficacy to the development of multicellular organisms. Recognizing these differences provides a framework for understanding cellular diversity and the mechanisms that drive biological complexity.
The Fundamental Dichotomy
At the most basic level, the primary distinction between prokaryotes and eukaryotes lies in the organisation of their genetic material and cellular compartmentalisation. Prokaryotic cells do not have a membrane-bound nucleus; instead, their DNA is located in a region called the nucleoid, which is not separated from the rest of the cytoplasm by a membrane. This means that transcription and translation can occur simultaneously in prokaryotes, as there is no physical barrier preventing ribosomes from accessing mRNA as it is being synthesised. Eukaryotic cells, however, enclose their DNA within a double-membrane-bound nucleus. This separation necessitates a more complex process of gene expression, where transcription occurs in the nucleus and translation occurs in the cytoplasm, often requiring the processing and export of mRNA before it can be translated.
Furthermore, prokaryotes lack membrane-bound organelles such as mitochondria, the endoplasmic reticulum, and the Golgi apparatus. While they may have internal structures, these are not enclosed by membranes in the same way as eukaryotic organelles. Eukaryotes, on the other hand, use these organelles to compartmentalise specific metabolic processes, increasing efficiency and allowing for greater cellular complexity. This compartmentalisation is a key feature that enables eukaryotic cells to perform a wider range of functions and to grow to larger sizes than prokaryotic cells.
Structural and Size Differences
Prokaryotic cells are typically much smaller than eukaryotic cells. The average diameter of a prokaryotic cell is approximately \( 1\ \mu\mathrm{m} \), whereas eukaryotic cells are generally around \( 10\ \mu\mathrm{m} \) in diameter. This difference in size is directly related to the surface area-to-volume ratio, which limits the rate of diffusion of materials across the cell membrane. Prokaryotes, being smaller, have a higher surface area-to-volume ratio, allowing for efficient exchange of nutrients and waste products without the need for complex internal transport systems. Eukaryotes, being larger, require specialised structures to manage transport and maintain homeostasis.
The cell wall composition also differs significantly. Most prokaryotes possess a rigid cell wall made of peptidoglycan (in bacteria) or pseudopeptidoglycan (in some archaea). In contrast, plant eukaryotes have cell walls composed of cellulose, while animal eukaryotes lack cell walls entirely. The presence and composition of the cell wall have implications for cell shape, protection, and interaction with the environment. Additionally, prokaryotes often possess external structures such as flagella for motility and pili for attachment, which are structurally distinct from the flagella and cilia found in eukaryotes.
Genetic Organization and Ribosomes
The genetic material in prokaryotes is usually a single, circular chromosome located in the nucleoid region. Prokaryotes may also contain small, circular DNA molecules called plasmids, which can carry additional genes, such as those for antibiotic resistance. In contrast, eukaryotes have multiple linear chromosomes housed within the nucleus. Eukaryotic chromosomes are associated with histone proteins, forming chromatin, which helps in packaging the DNA and regulating gene expression. Prokaryotic DNA is generally not associated with histones, although some archaea do use histone-like proteins.
Ribosomes, the sites of protein synthesis, also differ between the two groups. Prokaryotes possess 70S ribosomes, which consist of a 50S large subunit and a 30S small subunit. Eukaryotic cytoplasmic ribosomes are 80S, composed of a 60S large subunit and a 40S small subunit. It is important to note that the "S" (Svedberg) unit is a measure of sedimentation rate and is not additive; hence, 50S + 30S does not equal 80S. The difference in ribosome structure is the basis for the action of many antibiotics, which target prokaryotic ribosomes without affecting eukaryotic ribosomes, allowing for selective toxicity in treating bacterial infections.
Metabolism and Reproduction
Prokaryotes exhibit a wide variety of metabolic strategies, including aerobic respiration, anaerobic respiration, and fermentation. Some prokaryotes are capable of photosynthesis, using different pigments and mechanisms than those found in eukaryotic plants. The metabolic versatility of prokaryotes allows them to inhabit a vast array of environments, from extreme heat and acidity to deep-sea vents. Eukaryotes, while also metabolically diverse, generally rely on mitochondria for aerobic respiration and chloroplasts for photosynthesis (in plants and algae). These organelles are believed to have originated from endosymbiotic events, where ancestral prokaryotes were engulfed by a host cell and evolved into specialised organelles.
Reproduction in prokaryotes is primarily asexual, occurring through binary fission. In this process, the cell replicates its DNA and divides into two genetically identical daughter cells. This method of reproduction allows for rapid population growth under favourable conditions. Eukaryotes, on the other hand, can reproduce both asexually (through mitosis) and sexually (through meiosis and fertilization). Sexual reproduction introduces genetic variation through recombination and independent assortment, which is a key driver of evolution. The ability to undergo meiosis and produce gametes is a defining feature of the eukaryotic life cycle, enabling the formation of multicellular organisms with complex developmental programs.
The concept
Prokaryotic and eukaryotic cells
Two fundamentally different cellular architectures.
A common misconception is that prokaryotes are "primitive" or "less evolved" than eukaryotes. In reality, prokaryotes are highly adapted to their environments and have survived for billions of years. The term "prokaryote" refers to a structural characteristic (lack of a nucleus) rather than an evolutionary hierarchy. Archaea, a group of prokaryotes, are actually more closely related to eukaryotes than to bacteria in terms of genetic and biochemical features, highlighting the complexity of evolutionary relationships. C
Worked example
Problem. A bacterial cell has a diameter of \( 1\ \mu\mathrm{m} \) and a eukaryotic cell has a diameter of \( 10\ \mu\mathrm{m} \). Assuming both cells are spherical, calculate the ratio of the surface area-to-volume ratio of the bacterial cell to that of the eukaryotic cell.
- Recall the formula for the surface area (\( A \)) and volume (\( V \)) of a sphere: \( A = 4\pi r^2 \) and \( V = \frac{4}{3}\pi r^3 \).
- The surface area-to-volume ratio is \( \frac{A}{V} = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r} \).
- For the bacterial cell, \( r_{\text{bact}} = 0.5\ \mu\mathrm{m} \). Thus, \( \left(\frac{A}{V}\right)_{\text{bact}} = \frac{3}{0.5} = 6\ \mu\mathrm{m}^{-1} \).
- For the eukaryotic cell, \( r_{\text{euk}} = 5\ \mu\mathrm{m} \). Thus, \( \left(\frac{A}{V}\right)_{\text{euk}} = \frac{3}{5} = 0.6\ \mu\mathrm{m}^{-1} \).
- Calculate the ratio: \( \frac{\left(\frac{A}{V}\right)_{\text{bact}}}{\left(\frac{A}{V}\right)_{\text{euk}}} = \frac{6}{0.6} = 10 \).
Units check. The ratio is dimensionless, as the units of \(\mu\mathrm{m}^{-1}\) cancel out.
Three quick tests
- What is the primary structural difference between prokaryotic and eukaryotic cells?
Answer
Prokaryotes lack a membrane-bound nucleus and membrane-bound organelles, while eukaryotes possess both. - Why are 70S ribosomes significant in the context of antibiotic treatment?
Answer
Many antibiotics target 70S ribosomes specifically, allowing them to inhibit protein synthesis in bacteria without affecting the 80S ribosomes of human cells. - How does the surface area-to-volume ratio influence the size of prokaryotic cells?
Answer
Prokaryotes are small to maintain a high surface area-to-volume ratio, which ensures efficient diffusion of nutrients and waste products across the cell membrane.
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.