Prokaryotic vs Eukaryotic Cell
PROKARYOTIC CELLS
Assessment statements
• Draw and label a diagram of the ultrastructure of Escherichia coli as an example of a prokaryote.
• Annotate the diagram with the functions of each named structure.
• Identify named structures in an electron micrograph of E. coli.
• State that prokaryotic cells divide by binary fission.
Cells are divided into two types according to their structure. Cells in the
fi rst group, the prokaryotic cells, are usually much smaller than those in
the second group, the eukaryotic cells. They have a much simpler structure
and are thought to be the fi rst cells to have evolved. Bacteria are all
prokaryotic cells.
Prokaryotic cells are so called because they have no nucleus
(‘prokaryote’ comes from the Greek, meaning ‘before the nucleus’).
They also have no organelles (internal structures), so there is little
compartmentalisation of function within them. From the mid-20th
century, when the electron microscope was developed, it became possible
to study the internal detail of cells. Figures 2.6 and 2.7 show the main
features of a typical prokaryotic cell.
• The cell wall surrounds the cell. It protects the cell from bursting and
is composed of peptidoglycan, which is a mixture of carbohydrate and
amino acids.
• The plasma membrane controls the movement of materials into and
out of the cell. Some substances are pumped in and out using active
transport.
• Cytoplasm inside the membrane contains all the enzymes for the
chemical reactions of the cell. It also contains the genetic material.
• The chromosome is found in a region of the cytoplasm called the
nucleoid. The DNA is not contained in a nuclear envelope and also it is
‘naked’ – that is, not associated with any proteins. Bacteria also contain additional small circles of DNA called plasmids. Plasmids replicate
independently and may be passed from one cell to another.
• Ribosomes are found in all prokaryotic cells, where they synthesise
proteins. They can be seen in very large numbers in cells that are
actively producing protein. • A agellum is present in some prokaryotic cells. A fl agellum, which
projects from the cell wall, enables a cell to move.
• Some bacteria have pili (singular pilus). These structures, found on the
cell wall, can connect to other bacterial cells, drawing them together so
that genetic material can be exchanged between them.
Prokaryotic cells are usually much smaller in volume than more complex
cells because they have no nucleus. Their means of division is also simple.
As they grow, their DNA replicates and separates into two diff erent areas of
the cytoplasm, which then divides into two. This is called binary fission. It
differs slightly from mitosis in eukaryotic cells.
EUKARYOTIC CELLS
Assessment statements
• Draw and label a diagram of the ultrastructure of a liver cell as an example of an animal cell.
• Annotate the diagram with the functions of each named structure.
• Identify named structures in an electron micrograph of liver cells.
• Compare prokaryotic and eukaryotic cells.
• State three differences between plant and animal cells.
• Outline two roles of extracellular components.
Eukaryotic organisms have cells that contain a nucleus. Animals, plants,
fungi and protoctista all have eukaryotic cells.
The complexity of a eukaryotic cell cannot be fully appreciated using a
compound light microscope. In images made using an electron microscope,
however, the fi ne details of many diff erent organelles are visible. Figure 2.8
shows what can be seen of animal and plant cells using a light microscope
– compare these images with the electron micrographs and interpretive
drawings in Figures 2.9 to 2.12 (pages 24–25).
Eukaryotic cells contain structures called organelles, each of which
has its own specifi c function. Organelles enable a cell to carry out various
chemical reactions or processes in separate parts of the cell. Diff erent types
of cell have diff erent organelles in diff erent proportions, depending on the
role of the cell.
The largest and most obvious structure in a eukaryotic cell is the nucleus,
which contains the cell’s chromosomes. Chromosomes are composed of
DNA combined with protein, to form a material known as chromatin. The
nucleus is surrounded by a double-layered membrane, the nuclear envelope.
Small gaps in the envelope, called nuclear pores, are visible and it is through
these that material passes between the nucleus and the rest of the cell. A
distinctive feature of the nucleus is the darkly staining nucleolus. This is the
site of production of ribosomes.
Continuous with the nuclear envelope is a series of membranes known
as the endoplasmic reticulum (ER). Ribosomes attach to this network
to form rough endoplasmic reticulum (rER), the site of protein synthesis. As proteins are produced, they collect in the space between the
membranes, known as the cisternae. From here they can be transported
in vesicles to other parts of the cell such as the Golgi apparatus. ER
that has no ribosomes attached is known as smooth endoplasmic
reticulum (sER). The membranes of smooth ER have many enzymes on
their surfaces. Smooth ER has diff erent roles in diff erent types of cell – in
liver cells, it is where toxins are broken down; in the ovaries, it is the site
of oestrogen production. Smooth ER also produces phospholipids for the
construction of membranes and lipids for use in the cell.
The Golgi apparatus is similar in appearance to the sER, composed
of stacks of fl attened, folded membranes. It processes proteins made in the
rER, collecting, packaging and modifying them, and then releasing them
in vesicles for transport to various parts of the cell or for secretion from
the cell. The pancreas contains many secretory cells, which have large areas
of Golgi apparatus.
Eukaryotic cells also contain mitochondria (singular
mitochondrion). These are elongated structures surrounded by a double
membrane that are found throughout the cytoplasm. Mitochondria are
known as the cell’s ‘powerhouses’ because they are the site of aerobic
respiration. The inner membrane is folded to form cristae, which greatly increase the surface area for the production of ATP in the cell. Cells that
respire rapidly, such as muscle cells, have numerous mitochondria.
Lysosomes are spherical organelles with little internal structure which
are made by the Golgi apparatus. They contain hydrolytic enzymes for
breaking down components of cells. They are important in cell death, in
breaking down old organelles and, in white blood cells, digesting bacteria
that have been engulfed by phagocytosis. Plant cells do not normally
contain lysosomes.
Ribosomes are the site of protein synthesis in cells. They may be free
in the cytoplasm or attached to the rER. They are made of RNA and
protein but they do not have a membrane around them.
Plant cells have three additional structures. All plant cells have an outer
cellulose cell wall and most have a large central vacuole. Some plant cells
contain chloroplasts.
The chloroplasts are found in cells exposed to the light, as they are the
sites of photosynthesis. Chloroplasts have a double membrane and are about
the same size as bacteria. Both chloroplasts and mitochondria have their own
DNA and ribosomes and are able to reproduce independently of the cell.
The large central vacuole contains water and salts. The membrane
that surrounds it is under pressure from within and exerts a force on the
cytoplasm, which in turn exerts a force on the cell wall, making the cell
turgid and fi rm. The outer cell wall is composed of cellulose and other
carbohydrates such as lignin and pectin, giving plant cells further support
and a more rigid structure than animal cells. The cell walls and turgidity of
plant cells give strength and support to tissues like leaves, holding them in
the optimum position to catch the energy from sunlight for photosynthesis.
As in prokaryotic cells, the plasma membrane controls the movement
of materials into and out of the cell, and the gel-like cytoplasm, which
fi lls much of the volume of the cell, provides a medium for many
metabolic reactions.
DNA present, not associated with proteins, circular plasmids may also be present
cell wall always present in plant cells, made of cellulose,never present in animal cells always present, made of peptidoglycan cilia and flagella sometimes present some have flagella, but these have a different structure from those in eukaryotic cells
Table 2.2 Differences between prokaryotic and eukaryotic cells.
Here’s the Prokaryotic vs Eukaryotic Cell Diagram with Anatomy — it clearly shows the structural differences between the two cell types.
On the left, the prokaryotic cell (like a bacterium) is simpler: no nucleus, no membrane-bound organelles, and features like a nucleoid, plasmid, ribosomes, cell wall, and flagellum.
On the right, the eukaryotic cell (like an animal cell) is more complex: it has a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles — all enclosed within membranes.
This visual comparison makes it easy to understand how these two cell types differ in structure and complexity.
Prokaryotic cells are simpler, smaller, and lack a nucleus or membrane-bound organelles, found in organisms like bacteria and archaea; eukaryotic cells are larger, more complex, and found in animals, plants, fungi, and protists, with a true nucleus and organelles like mitochondria and Golgi apparatus. These differences are foundational in biology and help classify all life forms into two major domains.
🧬 Key Differences Between Prokaryotic and Eukaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (DNA in nucleoid region) | Present (DNA enclosed in nuclear membrane) |
| Membrane-bound organelles | Absent | Present (e.g., mitochondria, ER, Golgi) |
| Cell size | Small (1–10 µm) | Larger (10–100 µm) |
| Cell wall | Present (usually peptidoglycan) | Present in plants (cellulose), fungi (chitin); absent in animals |
| DNA structure | Circular DNA | Linear DNA with histones |
| Reproduction | Asexual (binary fission) | Mostly sexual (mitosis/meiosis) |
| Examples | Escherichia coli, Streptococcus, Halobacterium | Human cells, Yeast, Amoeba, Mango tree cells |
🦠 Examples of Prokaryotic Cells
- Bacteria: E. coli, Staphylococcus aureus, Lactobacillus
- Archaea: Methanogens, Halophiles, Thermophiles
- Found in soil, water, human gut, extreme environments.
🌿 Examples of Eukaryotic Cells
- Animal cells: Muscle cells, nerve cells, blood cells
- Plant cells: Leaf cells, root cells, xylem/phloem
- Fungi: Yeast (unicellular), mushrooms (multicellular)
- Protists: Amoeba, Paramecium, Euglena
🔍 Why This Matters
- Medical relevance: Antibiotics target prokaryotic features (like cell walls or ribosomes) without harming eukaryotic cells.
- Evolutionary insight: Eukaryotes likely evolved from prokaryotes via endosymbiosis (e.g., mitochondria originated from bacteria).
- Biotech applications: Bacteria are used in genetic engineering, fermentation, and bioremediation.
📌 Takeaway: Prokaryotic cells are structurally simpler and found in bacteria and archaea, while eukaryotic cells are complex and form the basis of all multicellular life. Understanding their differences is essential for biology, medicine, and biotechnology.
The ultrastructure of Escherichia coli (E. coli) shows the typical features of a prokaryotic cell: no true nucleus, circular DNA in a nucleoid region, plasmids, ribosomes, and external structures like pili and flagella. It is a model organism widely studied in microbiology and biotechnology.
🔬 Ultrastructure of E. coli
External Structures
- Capsule: Protective outer layer that helps evade host immune responses.
- Cell wall (peptidoglycan): Provides shape and prevents bursting due to osmotic pressure.
- Plasma membrane: Semi-permeable barrier controlling transport of substances.
- Pili (fimbriae): Short hair-like projections for attachment and conjugation (DNA transfer).
- Flagella: Long whip-like structures for motility.
Internal Structures
- Cytoplasm: Contains enzymes for metabolism and all cellular components.
- Nucleoid (DNA): Region with circular double-stranded DNA, not enclosed by a membrane.
- Plasmids: Small, circular DNA molecules carrying extra genes (e.g., antibiotic resistance).
- Ribosomes (70S): Sites of protein synthesis, smaller than eukaryotic ribosomes.
- Storage granules: Reserve materials like glycogen or polyphosphate.
- Mesosomes (controversial): Invaginations of the plasma membrane, once thought to aid in respiration and division (now considered artifacts in electron microscopy).
📊 Quick Comparison: E. coli vs Eukaryotic Cell
| Feature | E. coli (Prokaryote) | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (nucleoid only) | Present |
| DNA | Circular, no histones | Linear, with histones |
| Ribosomes | 70S | 80S |
| Organelles | None (no mitochondria, ER, Golgi) | Present |
| Size | 1–2 µm | 10–100 µm |
🌍 Importance of E. coli
- Model organism: Used in genetics, molecular biology, and biotechnology.
- Medical relevance: Some strains are harmless (gut flora), while pathogenic strains cause food poisoning and urinary tract infections.
- Industrial use: Engineered for insulin production, recombinant proteins, and bioengineering.
📌 Takeaway: The ultrastructure of E. coli highlights the simplicity of prokaryotic cells—yet this simplicity makes it an ideal model for research and biotechnology.
Would you like me to also prepare a labeled diagram comparing E. coli ultrastructure with a typical eukaryotic cell so you can visually see the differences side by side?
microscopy of escherichia coli
- Light Microscopy (Gram Stain):
- Appearance: E. coli appears as small, pink/red rods.
- Reasoning: Because it is Gram-negative, it has a thin peptidoglycan layer that does not retain the purple crystal violet stain but instead picks up the pink safranin counterstain.
- Resolution: At 1000x magnification, you can clearly distinguish its "straight rod" shape.
- Scanning Electron Microscopy (SEM):
- Appearance: Provides a 3D view of the cell's external surface.
- Features: It highlights the flagella (long whip-like structures for movement) and pili/fimbriae (hair-like projections for attachment).
- Transmission Electron Microscopy (TEM):
- Appearance: Provides a 2D cross-section of the internal ultrastructure.
- Features: Shows the complex cell envelope (outer membrane, periplasmic space, and inner membrane), the granular ribosomes, and the nucleoid region containing DNA.
- Cell Envelope (Three-layered):
- Outer Membrane: A unique lipid bilayer containing lipopolysaccharides (LPS) that acts as a selective barrier.
- Periplasmic Space: The region between the outer and inner membranes containing a thin layer of peptidoglycan (cell wall).
- Plasma Membrane (Inner Membrane): A phospholipid bilayer that regulates the transport of molecules and is the site of metabolic processes.
- Cytoplasm:
- Nucleoid: The irregularly shaped region containing the single, circular bacterial chromosome (DNA).
- Ribosomes (70S): Small granular structures responsible for protein synthesis, scattered throughout the cytoplasm.
- Plasmids: Small, circular, extrachromosomal DNA molecules that often carry genes for antibiotic resistance.
- External Structures:
- Flagella: Long, whip-like appendages used for motility through a rotating mechanism.
- Pili (Fimbriae): Short, hair-like projections used for attachment to surfaces or other cells during conjugation.
- Capsule: A polysaccharide layer outside the cell wall that provides protection (present in some strains).
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