Prokaryotes
Applied Science: BTEC Level 3 National Extended Diploma, topic 1.3.2
Prokaryotic Cells
Prokaryotic cells have some features that are also found in eukaryotic cells. Prokaryotic cells may also contain plasmids, a capsule and one or more flagella.
Prokaryotes vs eukaryotes
- Features common to these types of cells are:
- A plasma membrane.
- Cytoplasm.
- DNA.
- Ribosomes (ribosomes are 70S in prokaryotes).
- So, the main difference in terms of organelles is that prokaryotes do NOT have any membrane-bound organelles (e.g. nucleus, mitochondria, Golgi apparatus etc.).
DNA in prokaryotes
- Prokaryotic DNA is found as a circular molecule in the cytoplasm.
- Some prokaryotes have smaller loops of DNA called plasmids that are not part of the main circular DNA molecule.
- Bacteria can exchange plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA.
- Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange.
Flagella, pili and fimbriae
- Some prokaryotes have flagella, pili, or fimbriae:
- Flagella are tail-like and used for locomotion.
- Some prokaryotes have none and others have more than one.
- Pili are used to exchange genetic material during a type of reproduction called conjugation.
- Fimbriae are used by bacteria to attach to a host cell.
Capsule & cell wall
- The capsule enables prokaryotic cells to attach to surfaces in its environment.
- Most prokaryotes have a peptidoglycan cell wall.
- The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration.
Gram staining
- Gram-positive organisms have a thick peptidoglycan layer fortified with teichoic acids.
- Gram-negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and lipoproteins.
- The name of these bacteria comes from a lab technique called Gram staining. Gram staining dyes the thick peptidoglycan layer of Gram-positive bacteria purple.
- This purple colour can be seen when stained Gram-positive bacteria are visualised under a light microscope.
- Gram-negative bacteria appear pink/red.
Antibiotics
- Cell walls are an important target for antibiotics.
- Penicillin inhibits the crosslinking of peptidoglycan. This weakens cell walls and causes lysis.
- As both types of bacteria contain peptidoglycan in their cell walls, penicillin is effective against both.
- Some antibiotics only target one type of bacteria.
- Vancomycin is an antibiotic that inhibits the formation of the backbone of Gram-positive cell walls.
- Gram-negative bacteria make their cell walls using a different mechanism, so vancomycin is ineffective.
Test yourself on Prokaryotes
Which four features do all cells contain?
What enables a prokaryotic cell to attach to surfaces in its environment?
What colour do Gram-positive bacteria appear under a microscope?