2.4 Eukaryotes and Prokaryotes

Define eukaryote

A eukaryote is an organism whose cells contain a true nucleus enclosed by a nuclear membrane and possess membrane-bound organelles, allowing compartmentalisation of cellular processes.

Define prokaryote

A prokaryote is an organism whose cells lack a true nucleus and membrane-bound organelles, with genetic material present as a loop of DNA in the cytoplasm.

Define endosymbiosis

Endosymbiosis is the theory that certain organelles, such as mitochondria and chloroplasts, originated from free-living prokaryotic cells that were engulfed by larger cells and formed a mutually beneficial relationship.

Describe the structure of a prokaryotic cell

Prokaryotic cells are small (0.5–3.0 µm) and consist of a cell membrane, cytoplasm, ribosomes (70S), a cell wall made of murein, and genetic material as a circular DNA loop in the cytoplasm. Some may also have a capsule, flagella, and plasmids.

Structure and function of the capsule

The capsule is a slimy outer layer that protects bacteria from dehydration and from being engulfed by phagocytes, especially in parasitic species.

Structure and function of the cell wall (prokaryotes)

The cell wall is made of murein (peptidoglycan) and prevents the cell from bursting due to osmotic pressure.

Structure and function of prokaryotic ribosomes

Ribosomes are small (70S, ~20 nm) and are the site of protein synthesis.

Structure and function of prokaryotic DNA

The genetic material consists of a circular loop of DNA (bacterial chromosome) and sometimes plasmids, carrying genetic information; prokaryotes are haploid.

Structure and function of flagellum

The flagellum is a protein fibre that rotates to produce a corkscrew motion, allowing movement of the bacterium.

Structure and function of storage granules

Storage granules contain glycogen, lipids, or phosphate and act as reserves of energy and materials.

Size difference between prokaryotic and eukaryotic cells

• Prokaryotic cells: 0.5–3.0 µm

• Eukaryotic cells: 20–60 µm (much larger)

Differences in DNA between prokaryotes and eukaryotes

• Prokaryotes: circular DNA in cytoplasm, no nucleus

• Eukaryotes: linear DNA in chromosomes within a nucleus

Differences in organelles

• Prokaryotes: no membrane-bound organelles

• Eukaryotes: many membrane-bound organelles (e.g. mitochondria, ER)

Differences in ribosomes

• Prokaryotes: 70S, smaller

• Eukaryotes: 80S, larger

Differences in cell walls

• Prokaryotes: murein (peptidoglycan)

• Plants (eukaryotes): cellulose

• Animals: no cell wall

Structures found in both prokaryotic and eukaryotic cells

• Cell membrane

• Cytoplasm

• Ribosomes

• DNA (genetic material)

Evidence for endosymbiosis

• Mitochondria and chloroplasts have circular DNA

• They contain 70S ribosomes (like bacteria)

• They replicate independently

• They have double membranes

How did mitochondria evolve according to endosymbiosis?

Aerobic bacteria were engulfed by anaerobic cells and formed a symbiotic relationship, eventually evolving into mitochondria.

Aerobic bacteria were engulfed by anaerobic cells and formed a symbiotic relationship, eventually evolving into mitochondria.

Photosynthetic bacteria were engulfed by early eukaryotic cells and evolved into chloroplasts.

How to identify a prokaryotic cell in a microscope

• Very small size

• No nucleus

• No membrane-bound organelles

• Circular DNA in cytoplasm

How to identify a eukaryotic cell in a microscope

• Larger size

• Presence of nucleus

• Presence of organelles (e.g. mitochondria, chloroplasts)

Why are eukaryotic cells more complex than prokaryotic cells?

Eukaryotic cells have membrane-bound organelles that compartmentalise functions, increasing efficiency and allowing specialised processes to occur.

Why are prokaryotic cells simpler but still successful?

Their simple structure allows rapid reproduction, efficient nutrient uptake, and survival in diverse environments.

Structures found only in prokaryotic cells

• Capsule

• Plasmids

• Circular DNA in cytoplasm

• Murein cell wall

Structures found only in eukaryotic cells

• Nucleus

• Mitochondria

• Endoplasmic reticulum

• Golgi apparatus

• Chloroplasts (plants only)