Prokaryotic and Eukaryotic Cells
All living organisms are composed of cells, the fundamental unit of life. At A-Level, you must understand the key structural differences between prokaryotic and eukaryotic cells, including the nature of their organelles, genetic material, and modes of reproduction. This lesson covers the ultrastructure of both cell types in the detail required by the AQA specification (section 3.2.1).
Key Definition: A prokaryotic cell is a cell that lacks a true nucleus and membrane-bound organelles. Prokaryotes include bacteria and archaea. A eukaryotic cell possesses a true membrane-bound nucleus and a range of membrane-bound organelles.
General Features of Eukaryotic Cells
Eukaryotic cells are found in animals, plants, fungi, and protoctists. They share the following features:
- Nucleus — enclosed by a double membrane (nuclear envelope) with nuclear pores. Contains chromosomes made of linear DNA associated with histone proteins, forming chromatin. The nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis.
- Mitochondria — double-membrane organelles; the inner membrane is folded into cristae to increase surface area for oxidative phosphorylation. The interior matrix contains enzymes for the Krebs cycle, as well as the organelle's own 70S ribosomes and small circular DNA.
- Endoplasmic reticulum — an extensive network of membranes. Rough endoplasmic reticulum (RER) is studded with 80S ribosomes and synthesises proteins for secretion or for membranes. Smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in lipid and steroid synthesis.
- Golgi apparatus — a stack of flattened membrane-bound cisternae that modifies, packages, and sorts proteins and lipids into vesicles for transport to their destination.
- Lysosomes — single-membrane vesicles containing hydrolytic (digestive) enzymes. They break down worn-out organelles, engulfed pathogens, and cellular debris via intracellular digestion.
- Ribosomes — 80S ribosomes (made of a 60S large subunit and a 40S small subunit) are the site of translation (protein synthesis). Found free in the cytoplasm or bound to RER.
- Centrioles — cylindrical structures composed of nine triplets of microtubules arranged in a ring. Found in animal cells and some lower plant cells, they organise the spindle fibres during cell division.
- Cell surface membrane — a phospholipid bilayer with embedded proteins; controls entry and exit of substances.
Additional Features of Plant Cells
- Cellulose cell wall — lies outside the cell surface membrane; provides structural support and prevents the cell from bursting when turgid.
- Chloroplasts — double-membrane organelles containing thylakoid membranes arranged into grana (stacks) and stroma (fluid matrix). The site of photosynthesis. Like mitochondria, chloroplasts possess their own 70S ribosomes and circular DNA.
- Large permanent vacuole — surrounded by a tonoplast membrane; contains cell sap and helps maintain turgor pressure.
- Plasmodesmata — cytoplasmic channels through the cell wall that connect adjacent cells, allowing transport of substances.
Exam Tip: When asked to compare animal and plant cells, list the features they share (nucleus, mitochondria, ribosomes, ER, Golgi, cell surface membrane) and then the features unique to plant cells (cell wall, chloroplasts, large permanent vacuole, plasmodesmata). Do not state that animal cells have no vacuoles — they can have small, temporary vacuoles.
General Features of Prokaryotic Cells
Prokaryotic cells are considerably smaller than eukaryotic cells, typically 1–5 µm in diameter compared with 10–100 µm for eukaryotic cells. Key features include:
- No true nucleus — DNA is found in the nucleoid region, a less well-defined area of the cytoplasm. The DNA is a single, circular molecule that is not associated with histone proteins (it is described as 'naked' DNA).
- Plasmids — small, additional circular loops of DNA that can replicate independently. Plasmids often carry genes for antibiotic resistance and can be exchanged between bacteria via conjugation. They are widely used in genetic engineering as vectors.
- 70S ribosomes — smaller than the 80S ribosomes of eukaryotes. The 70S ribosome consists of a 50S large subunit and a 30S small subunit. This difference is exploited by antibiotics such as tetracycline and erythromycin, which target 70S ribosomes specifically without affecting human 80S ribosomes.
- Cell wall — made of peptidoglycan (also called murein), a polymer of sugars cross-linked by short peptide chains. This is chemically distinct from the cellulose cell wall of plants or the chitin cell wall of fungi.
- Capsule (slime layer) — some bacteria possess a polysaccharide capsule outside the cell wall. This provides protection from phagocytes and desiccation, and aids adhesion to surfaces (e.g., biofilm formation).
- Flagella — some prokaryotes have one or more flagella for locomotion. Prokaryotic flagella have a simpler structure than eukaryotic flagella, composed of the protein flagellin and rotating like a propeller driven by a proton gradient.
- Pili — short, hair-like projections on the surface used for attachment to host cells or other bacteria during conjugation.
- No membrane-bound organelles — prokaryotes lack mitochondria, ER, Golgi apparatus, lysosomes, and chloroplasts. Some photosynthetic prokaryotes (e.g., cyanobacteria) carry out photosynthesis on infoldings of the cell surface membrane called thylakoid membranes or mesosomes.
- Cell surface membrane — similar in structure to that of eukaryotic cells (phospholipid bilayer), but prokaryotes do not typically contain cholesterol.
Comparison Table: Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|
| Typical size | 1–5 µm | 10–100 µm |
| Nucleus | No true nucleus; nucleoid region | True membrane-bound nucleus |
| DNA | Circular, naked (no histones) | Linear, associated with histones |
| Plasmids | Often present | Absent (except in yeast, rarely) |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) in cytoplasm; 70S in mitochondria/chloroplasts |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, Golgi, etc.) |
| Cell wall | Peptidoglycan (murein) | Cellulose (plants), chitin (fungi), absent (animals) |
| Reproduction | Binary fission | Mitosis (and meiosis) |
| Flagella | Simple (flagellin) | Complex (9+2 microtubule arrangement) |
| Cytoskeleton | Rudimentary or absent | Well-developed (microtubules, microfilaments, intermediate filaments) |
Binary Fission
Prokaryotes reproduce asexually by binary fission. This is not the same as mitosis; there is no spindle formation and no condensation of chromosomes.
Steps of Binary Fission
- The circular DNA molecule replicates. Both copies attach to the cell surface membrane.
- The cell elongates, and the two DNA copies are pulled apart as the membrane grows between the attachment points.
- A new cross-wall (septum) of peptidoglycan forms across the middle of the cell, dividing the cytoplasm.
- The cell splits into two genetically identical daughter cells, each with a copy of the circular DNA and a roughly equal share of ribosomes and cytoplasm.
- Plasmids replicate independently and are distributed between the daughter cells (not necessarily equally).
Binary fission can occur very rapidly — Escherichia coli can divide every 20 minutes under optimal conditions. This rapid reproduction rate means that beneficial mutations (or acquired plasmid genes) can spread quickly through a population, which has implications for the evolution of antibiotic resistance.
Exam Tip: If asked to compare binary fission with mitosis, emphasise that binary fission involves no spindle, no condensation of chromosomes, and no nuclear envelope breakdown. Both processes produce genetically identical daughter cells (barring mutations), but mitosis is part of the eukaryotic cell cycle and involves distinct phases (prophase, metaphase, anaphase, telophase).
The Endosymbiont Theory
The endosymbiont theory (proposed by Lynn Margulis) explains the origin of mitochondria and chloroplasts. According to this theory:
- An ancestral prokaryote was engulfed by a larger cell through endocytosis but was not digested.
- The engulfed prokaryote became an endosymbiont, eventually evolving into mitochondria (from an aerobic bacterium) or chloroplasts (from a photosynthetic cyanobacterium).
Evidence supporting the endosymbiont theory:
- Mitochondria and chloroplasts have their own circular DNA (similar to prokaryotic DNA).
- They possess 70S ribosomes, the same size as prokaryotic ribosomes.
- They have a double membrane — the inner membrane may represent the original prokaryotic membrane, and the outer membrane may be derived from the host cell's engulfing vesicle.
- They replicate independently by binary fission within the cell.
- Their DNA sequences are more similar to bacterial DNA than to the nuclear DNA of the host cell.
Gram Staining and Bacterial Cell Walls
Although detailed bacteriology is not part of the core AQA A-Level specification, understanding Gram staining provides context for cell wall structure:
- Gram-positive bacteria have a thick peptidoglycan wall that retains the crystal violet stain (appearing purple).
- Gram-negative bacteria have a thin peptidoglycan wall with an outer lipopolysaccharide membrane; they lose the crystal violet and take up the counterstain safranin (appearing pink/red).
- This distinction is clinically important because Gram-negative bacteria are generally more resistant to antibiotics due to their outer membrane acting as an additional barrier.
Summary
- Prokaryotic cells lack a true nucleus and membrane-bound organelles; eukaryotic cells have both.
- Prokaryotic DNA is circular and naked; eukaryotic DNA is linear and wrapped around histones.
- Prokaryotic ribosomes are 70S; eukaryotic cytoplasmic ribosomes are 80S.
- Prokaryotic cell walls are made of peptidoglycan; plant cell walls of cellulose; fungal cell walls of chitin.
- Binary fission is the asexual reproduction method of prokaryotes — it does not involve a spindle or chromosome condensation.
- The endosymbiont theory explains the prokaryotic features of mitochondria and chloroplasts.
Key Exam Command Words: Compare — identify similarities and differences. Describe — give an account of features. Explain — give reasons for, linking structure to function.