Cytoplasm: Definition, Structure, Functions, Diagram

The cytoplasm is the semi-viscous ground substance of the cell. All the volume of such substances outside the nucleus and inside the plasma membrane is cytoplasm.

• It is sometimes described as the non-nuclear content of the protoplasm.
• All the cellular contents in prokaryotes are contained within the cell’s cytoplasm.
• In eukaryote organisms, the nucleus of the cell is separated from the cytoplasm.
• The cytoplasm is the substance of life, it serves as a molecular soup and it is in the cytoplasm where all the cellular organelles are suspended and are bound together by a lipid bilayer membrane.
• The cytoplasm was discovered in the year 1835 by Robert Brown and other scientists.

Structure/ Components of the Cytoplasm

The main components of the cytoplasm are:

1. Cytosol– a gel-like substance
2. Organelles – the cell’s internal sub-structures, and
3. Various cytoplasmic inclusions.

The Cytosol 

The cytosol is the part of the cytoplasm that is not occupied by any organelle. It is a gelatinous fluid, where other components of the cytoplasm remain suspended. It mainly consists of cytoskeleton filaments, organic molecules, salt, and water.

Organelles 

Organelles mean “little organs”, that are membrane-bound. They are present inside the cell and perform specific functions that are necessary for the survival of the cell. Some of the constituents of the cell that are suspended in the cytosol are cellular organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles, lysosomes, and chloroplasts in plant cells. 

Cytoplasmic Inclusions

The cytoplasmic inclusions consist of different types of insoluble particles or molecules that remain suspended in the cytosol. Cytoplasmic inclusions are not surrounded by any membrane. They are basically granules of starch and glycogen, and they can store energy. A vast range of inclusions are present in different cell types. The inclusions range from calcium oxalate crystals or silicon dioxide crystals in plants to storage granules of materials like starch, glycogen, etc. Lipid droplets are a widespread example of inclusions, these are spherical droplets, they are made of lipids and proteins and are present in both prokaryotes and eukaryotes as a medium to store lipids like fatty acids and sterols.

Properties of Cytoplasm

• The cytoplasm is made of 70% – 80% water and is usually colorless.
• It contains proteins, carbohydrates, salts, sugars, amino acids, and nucleotides.
• The cytoplasm constitutes of dissolved nutrients and also dissolved waste products.
• The outer clear and glassy layer of the cytoplasm is called the ectoplasm or the cell cortex and the inner granular mass is called the endoplasm.
• The peripheral zone of cytoplasm is a thick and jelly-like substance, known as the plasmogel. The surrounding area of the nuclear zone is thin and liquefied in nature and is known as the plasmosol. 
• The physical nature of the cytoplasm is variable. Sometimes, there is quick diffusion across the cell, making the cytoplasm resemble a colloidal solution. At other times, it appears to take on the properties of a gel-like or glass-like substance.
• It is said to have the properties of viscous as well as elastic materials – capable of deforming slowly under external force in addition to regaining its original shape with minimal loss of energy. 
• The cytoskeleton present in the cytoplasm gives the cell its shape.
• Cytoplasm helps the movement of the cellular materials around the cell through a process called cytoplasmic streaming.
• Since the cytoplasm constitutes numerous salts, it is a very good conductor of electricity.  
• It shows differential staining properties, the areas stained with the basic dyes are the basophilic areas of the cytoplasm and is termed as ergatoplasm for this material. 

Functions of Cytoplasm

1. The cytoplasm is the site for most of the enzymatic reactions and metabolic activity of the cell.
2. The cytoplasm is the place where the cell expands and the growth of the cell takes place. 
3. The cytoplasm provides a medium for the organelles to remain suspended.
4. The cytoplasm acts as a buffer and protects the genetic material of the cell and also the cellular organelles from damage caused due to movement and collision with other cells. 
5. Cellular respiration begins in the cytoplasm with glycolysis. This reaction provides the intermediates that are used by the mitochondria to generate ATP.
6. The translation of mRNA into proteins on ribosomes also occurs mostly in the cytoplasm.
7. The cytoplasm also contains the monomers that go on to generate the cytoskeleton. The cytoskeleton, in addition to being important for the normal activities of the cell, is crucial for cells that have a specialized shape. 
8. The cytoplasm also plays a role in creating order within the cell with specific locations for different organelles. For instance, the nucleus is usually seen towards the center of the cell, with a centrosome nearby.
9. Cytoplasmic streaming is important for positioning chloroplasts close to the plasma membrane to optimize photosynthesis and for distributing nutrients through the entire cell. In some cells, such as mouse oocytes, cytoplasmic streaming is expected to have a role in the formation of cellular sub-compartments and in organelle positioning as well.
10. Cytoplasmic Inheritance: The cytoplasm plays hosts to two organelles that contain their own genomes – the chloroplast and mitochondria. These organelles are inherited directly from the mother through the oocyte and therefore constitute genes that are inherited outside the nucleus. These organelles replicate independently of the nucleus and respond to the needs of the cell.

References

1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
2. Stephen R. Bolsover, Elizabeth A. Shephard, Hugh A. White, Jeremy S. Hyams (2011). Cell Biology: A short Course (3 ed.).Hoboken,NJ: John Wiley and Sons.
3. Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.
4. https://biology.tutorvista.com/animal-and-plant-cells/cytoplasm.html
5. https://biologywise.com/structure-functions-of-cytoplasm
6. https://biologydictionary.net/cytoplasm/
7. http://www.softschools.com/science/biology/function_of_cytoplasm/