An Overview of Cell

A quick tour of cell structure shows both how much cells have in common and how greatly they differ. The familiar examples from earlier observation — an onion peel and human cheek cells — make a good starting point.

The outer boundary and the nucleus

• The onion cell is a typical plant cell. Its outermost boundary is a distinct cell wall, and lying just inside the wall is the cell membrane.
• The human cheek cell has an outer membrane as its delimiting structure, but no cell wall.
• Inside each cell sits a dense, membrane-bound structure, the nucleus. The nucleus holds the chromosomes, which in turn carry the genetic material, DNA.

This single feature — whether or not the nucleus is enclosed in a membrane — divides all cells into two great groups:

• Eukaryotic cells have a membrane-bound nucleus.
• Prokaryotic cells lack a membrane-bound nucleus.

The cytoplasm

In both prokaryotic and eukaryotic cells, a semi-fluid matrix called the cytoplasm fills the volume of the cell. The cytoplasm is the main arena of cellular activity in plant and animal cells alike — it is where the many chemical reactions that keep the cell in the 'living state' take place.

Organelles

Besides the nucleus, eukaryotic cells contain other membrane-bound structures called organelles, including:

• the endoplasmic reticulum (ER)
• the golgi complex
• lysosomes
• mitochondria
• microbodies
• vacuoles

Prokaryotic cells lack these membrane-bound organelles.

Ribosomes are a special case: they are non-membrane-bound organelles present in all cells, both eukaryotic and prokaryotic. Within a cell, ribosomes occur not only in the cytoplasm but also inside two organelles — chloroplasts (in plants) and mitochondria — and on the rough ER. Animal cells also contain another non-membrane-bound organelle, the centrosome, which assists in cell division.

Variation in size, shape and number

Cells differ enormously in size, shape and activity. …