Cytoplasm, Cell Organelles and Endoplasmic Reticulum

1.0Cytoplasm

The fluid content inside the plasma membrane and excluding nucleus is called cytoplasm.

Cytoplasm has cytosol and many cell organelles.

Cytosol forms the aqueous, nearly transparent, structureless ground substance inside the cell.
Cell organelles are sub-cellular structures which have characteristic form, structure and function.

Almost all cell organelles are surrounded by membranes.

Non-living materials in the cytoplasm of a cell such as pigment granules, starch granules, fat droplets, etc. are called cell inclusions.

2.0Cell Organelles

Ribosomes (Engine of cell/Protein construction teams)

Ribosomes are smallest and membrane less cell organelles. They are composed of RNA and proteins. It is also called organelle within an organelle and protein factory of cells.

Ribosomes are the protein builders or the protein synthesizers of the cell. These proteins might be used as enzymes or as support for other cell functions. 

Ribosomes are found in many places around the cell. They are floating in the cytoplasm. Those floating ribosomes make proteins that will be used inside the cell. Other ribosomes are found on the endoplasmic reticulum (rough endoplasmic reticulum).  These attached ribosomes make proteins that will be used inside the cell and for export out of the cell.

The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S. Here ‘S’ (Swedberg’s unit) stands for the sedimentation coefficient; it indirectly is a measure of density and size.

The 60S and 40S model works fine for eukaryotic cells. Prokaryotic cells have ribosomes made of 50S and 30S subunits.

3.0Endoplasmic Reticulum

Structure 

Endoplasmic reticulum is a complex network of membrane bound channels or sheets, tubules and vesicles.  It does not work alone. The ER works closely with the Golgi apparatus and ribosomes.

It creates a network of membranes found through the whole cell. The ER may also look different from cell to cell, depending on the cell's function.

Components of E.R.

(i) Cisternae: These are long flattened and unbranched units arranged in stacks. 

(ii) Vesicles: These are oval membrane bound structures.

(iii) Tubules: These are irregular, often branched tubes bounded by membranes. Tubules may be free or associated with cisternae. 

Types of Endoplasmic Reticulum

Smooth ER/Agranular ER

It has smooth membranes which do not bear ribosomes. It acts as a storage organelle. It is important in the synthesis and storage of steroids. It is mainly made up of vesicles and tubules.

Smooth ERs are the site of lipid synthesis, mainly steroids (a type of ringed organic molecule used for many purposes in an organism).

Rough ER/Granular ER

It has a rough membrane because a number of ribosomes are attached to its outer surface. RER is engaged in protein synthesis. It is mainly made up of cisternae. Tubules are very few.

Function of Endoplasmic Reticulum

(i) ER serves as channels for the transport of materials (especially proteins) between various regions of cytoplasm or between cytoplasm and nucleus.

(ii) It also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

(iii) SER plays an important role in detoxifying many poisons and drugs and is also a site of lipid synthesis.

(iv) RER is engaged in synthesizing proteins.

(v) The proteins and lipids synthesized by RER and SER respectively help in building the cell membrane which is known as membrane biogenesis. Some proteins and lipids formed by RER and SER function as enzymes and hormones.

4.0Also Read