The Digestive system

Prokaryotic Cells

Prokaryotic Cells

Prokaryotic cells are smaller than eukaryotic cells and contain no membrane organelles including no nucleus. Their main features are in the table below.

Name Function
Cell wall

Capsule

Flagellum

Cytoplasm

Ribosomes

Nucleoid

DNA

Plasmid

Plasma membrane

This is composed of the glycoprotein murein, as opposed to cellulose in plants.

This is a thick layer outside of the cell wall composed of polysaccharides. It has a number of functions: for reserving food, to stick cell together, and to protect itself against chemicals, desiccation and phagocytosis.

This is used for propulsion. It’s a rigid tail, helical in shape within a motor on the cell membrane which is controlled by a hydrogen ion gradient. It shouldn’t be confused with a eukaryotic flagellum.

As there are no organelles it contains the enzymes required for metabolic reactions.

These are smaller 70S ribosomes. They’re all free in the cytoplasm and are used to make proteins.

This is the region that contains the DNA but it doesn’t consist of a membrane.

The DNA is always circular and isn’t associated with proteins to make chromatin. It’s also known as the bacterial chromosome.

These are small circles of DNA that are separate from the bacterial chromosome. They can be used to exchange DNA between other bacterial cells and have applications in genetic engineering.

Like eukaryotic cells, this is composed of proteins and phospholipids.

Scanning Electron Microscopes

It’s possible to view cells in detail by using scanning electron microscopes (SEM). They use a focused beam of high energy electrons which create a variety of signals at the surface of the solid object being observed. The signals received provide information on the sample’s texture, structure and chemical composition. If data is collected over a number of surfaces then a 3D image can be formed.

However, there are limitations. Not only must the sample be solid but it must also fit in the microscopic chamber on the microscope. Certain elements can’t be detected as most SEMs have inadequate energy resolution to pick them up. Also, an electrically conductive coating needs to be applied for use in conventional SEMs.

A SEM provides a high magnification (of up to about x100,000) and a high resolution.

  • Magnification refers to how much larger an object can be made.
  • Resolution, on the other hand, refers to how well defined the image is: the smallest distance below which two discrete objects will be seen as one.