Edexcel A Level (IAL) Biology -3.5 Prokaryotic Cells- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -3.5 Prokaryotic Cells- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -3.5 Prokaryotic Cells- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 3.5 (i) know the ultrastructure of prokaryotic cells, including cell wall, capsule, plasmid, flagellum, pili, ribosomes and circular DNA
(ii) knderstand the function of the structures listed in (i)
Ultrastructure & Functions of Prokaryotic Cell Components
🌱 Introduction
Prokaryotic cells (like bacteria) are simpler than eukaryotic cells. They lack a true nucleus and membrane-bound organelles but have specialized structures that help them survive, reproduce, and interact with their environment.
1. Cell Wall
- Ultrastructure: Rigid outer layer outside the plasma membrane. Made of peptidoglycan (in bacteria) or other polysaccharides.
- Function: Maintains shape of the cell. Provides mechanical strength and protection against osmotic lysis.
2. Capsule
- Ultrastructure: Gel-like layer outside the cell wall. Made of polysaccharides or polypeptides.
- Function: Protects bacteria from desiccation and phagocytosis. Helps in adhesion to surfaces and host tissues.
3. Plasmid
- Ultrastructure: Small, circular, extra-chromosomal DNA. Can replicate independently of the main circular DNA.
- Function: Carries additional genes (e.g., antibiotic resistance, metabolic enzymes). Can be transferred between bacteria (conjugation).
4. Flagellum
- Ultrastructure: Long, whip-like filament made of flagellin protein. Anchored in the cell membrane/cell wall by a basal body.
- Function: Provides motility. Allows movement toward nutrients (chemotaxis).
5. Pili (Fimbriae)
- Ultrastructure: Short, hair-like projections on the cell surface.
- Function: Attachment to surfaces and other cells. In sex pili, enables DNA transfer during conjugation.
6. Ribosomes
- Ultrastructure: Small, 70S ribosomes (smaller than eukaryotic 80S). Found free in the cytoplasm.
- Function: Site of protein synthesis.
7. Circular DNA (Nucleoid)
- Ultrastructure: Single, circular, double-stranded DNA molecule. Not enclosed by a membrane.
- Function: Carries essential genetic information for cell growth, metabolism, and reproduction.
📊 Summary Table
| Structure | Ultrastructure | Function |
|---|---|---|
| Cell wall | Rigid layer of peptidoglycan | Shape, protection against osmotic lysis |
| Capsule | Gel-like polysaccharide/polypeptide | Protection, adhesion |
| Plasmid | Small circular DNA | Extra genes, antibiotic resistance, conjugation |
| Flagellum | Long filament (flagellin) | Motility, chemotaxis |
| Pili | Short hair-like projections | Attachment, DNA transfer |
| Ribosomes | 70S, free in cytoplasm | Protein synthesis |
| Circular DNA | Single circular DNA molecule | Stores genetic information |
⚡ Quick Recap
Cell wall → shape & protection.
Capsule → adhesion & defense.
Plasmid → extra genes, transferable.
Flagellum → movement.
Pili → attachment & conjugation.
Ribosomes → protein synthesis.
Circular DNA → main genetic material.