2. Cell Membrane (Plasma Membrane)

Structure

The cell membrane is just under the cell wall.
It’s like a thin, flexible skin that controls what goes in and out of the cell.

  • In bacteria without a cell wall (like Mycoplasmas and Sarcoplasmas), the cell membrane is the outermost layer.

Chemical Composition

  • Unlike animal cells, bacterial membranes do not contain sterols (like cholesterol).

Special Structures: Mesosomes

In some places, the cell membrane folds inward, creating structures like vesicles, tubules, or lamellae. These folds are called mesosomes.

Functions of Mesosomes

Mesosomes play important roles:

  • DNA replication (copying genetic material before cell division).
  • Cell division (helping the cell to split into two).
  • Act as respiratory centers — in bacteria, respiration (energy production) happens at the cell membrane, so mesosomes help in producing energy.

3. Cytoplasm and Genetic Material

Let’s now explore what’s inside the cell.

Cytoplasm

The cytoplasm is a jelly-like fluid filling the inside of the cell. It holds all the internal parts and dissolved substances.

It Contains:

  • Nucleoid (area with genetic material)
  • Ribosomes (for making proteins)
  • Mesosomes

It Lacks:

  • Cytoskeleton (no internal framework like in animal cells)
  • Membrane-bound organelles (e.g., no mitochondria, no endoplasmic reticulum)

Ribosomes

Ribosomes are small structures that help the cell make proteins, which are essential for cell function and growth.

  • They are either floating freely in the cytoplasm or attached loosely to the cell membrane.
  • Bacterial ribosomes are smaller than those in human or animal cells.
  • They are called 70S ribosomes, made of:
    • A large subunit (50S)
    • A small subunit (30S)

Genetic Material

Bacteria have two types of DNA:

1. Nucleoid (Main DNA)

  • It’s the central area in the cytoplasm where the main DNA is located.
  • Not surrounded by any membrane.
  • Contains a single, circular, double-stranded DNA molecule.
  • This DNA controls the cell’s functions and passes traits to the next generation.
  • It does not have histone proteins, which are usually found in human DNA.

2. Plasmids (Extra DNA)

  • Some bacteria also carry plasmids — small, circular, double-stranded pieces of DNA.
  • These are not part of the main chromosome.
  • They can replicate on their own, even before or after the cell divides.
  • Plasmids often carry useful genes, such as those that give the bacteria resistance to antibiotics or help them survive in tough conditions.

Conclusion

So to sum it up:

  • Bacteria are simple, one-celled organisms with no nucleus.
  • They have a strong cell wall (made of peptidoglycan) and a flexible cell membrane.
  • Inside the cell, you’ll find the cytoplasm, ribosomes, mesosomes, and the genetic material.
  • Their DNA is free-floating, not in a nucleus.
  • Some bacteria also have plasmids and capsules to help them survive better.
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