Prokaryotic Cell - Cell: The Unit of Life | Class 11 Biology

Prokaryotic Cells

This lesson introduces prokaryotic cells, describes their main structures and how they differ from eukaryotic cells, and explains how bacteria can be classified and examined.

Prokaryotic and Eukaryotic Cells

• Cells are broadly divided into prokaryotic and eukaryotic types.
• Prokaryotic cells lack a membrane-bound nucleus. Their genetic material lies in the cell’s interior rather than inside a nuclear envelope.
• They are generally small and structurally simpler than eukaryotic cells, and can multiply rapidly.
• Bacteria, including cyanobacteria, and mycoplasmas are examples of prokaryotes.
• Like other cells, prokaryotes have cytoplasm, where many cellular reactions take place.

The Prokaryotic Cell Envelope

The lesson describes the layers surrounding a bacterial cell:

• Glycocalyx: The outer covering, which may be a loose slime layer or a more organized capsule. It can help protect the cell and aid its attachment to surfaces or host tissues.
• Cell wall: Provides support, helps maintain the cell’s shape, and protects it. Its main structural material is peptidoglycan, also called murein.
• Plasma membrane: A selectively permeable boundary that regulates what enters and leaves the cell. It is composed mainly of lipids and proteins, with carbohydrates also present.

Gram-Positive and Gram-Negative Bacteria

The lecture compares the two groups by their cell-envelope structure:

• Gram-positive bacteria have a relatively thick peptidoglycan layer.
• Gram-negative bacteria have a thinner peptidoglycan layer and an additional outer membrane containing lipopolysaccharides (LPS). The space between the inner and outer membranes is called the periplasmic space.

Gram staining is used to distinguish these groups:

1. Apply crystal violet.
2. Add iodine, which helps retain the dye.
3. Wash with alcohol or another decolorizing agent.
4. Apply safranin as a counterstain.
5. Examine the sample under a microscope:
   • Gram-positive bacteria retain crystal violet and appear purple.
   • Gram-negative bacteria lose the violet stain during decolorization and take up safranin, appearing pink.

Plasma-Membrane Modifications

The lesson describes inward folds of the prokaryotic plasma membrane as mesosomes. In the account presented, these folds increase membrane surface area and are associated with cellular activities such as respiration and DNA-related processes.

It also describes chromatophores in photosynthetic prokaryotes as membrane structures associated with photosynthetic pigments.

Genetic Material and Plasmids

• The main genetic material is located in a non-membrane-bound region of the cytoplasm.
• Prokaryotes may also contain plasmids: small, extra-chromosomal DNA molecules that replicate separately from the main chromosome.
• Plasmids can carry genes and can be transferred between bacteria. The lesson connects them to gene transfer and genetic engineering.

Ribosomes and Protein Production

• Prokaryotic ribosomes are 70S, made of 50S and 30S subunits.
• Ribosomes carry out protein synthesis. Some are free in the cytoplasm, while others may be associated with the plasma membrane.

Storage Inclusions

• Prokaryotic cells generally lack membrane-bound storage organelles.
• Instead, they can store materials in inclusion bodies, including substances such as glycogen, phosphate, or sulfur compounds. The lesson also mentions gas-filled structures in some bacteria.

Cell-Surface Appendages

• Fimbriae and pili help bacteria attach to surfaces or host cells. Some pili are involved in transferring genetic material between bacterial cells.
• Flagella are associated with movement. The lecture describes three parts: the filament, hook, and basal body.

Bacterial Shapes

The teacher briefly refers to classification by shape, including spherical, rod-shaped, comma-shaped, and spiral forms. The auto-generated subtitles make some of the corresponding terminology difficult to distinguish.

Speakers or Sources Featured

• Shipra Tiwari, the teacher and presenter from Magnet Brains.
• No other speaker or source is clearly identifiable in the subtitles.