Cytoplasm its composition, structure & functions

The cytoplasm is the semi-viscous and semitransparent substance between the plasma membrane and nuclear envelope. All material enveloped within the cell membrane is protoplasm. The cytoplasm is the main aqueous part of protoplasm rest is the nucleoplasm. Sometimes it is also described as the nonnuclear content of protoplasm.

The term cytoplasm was introduced by Rudolf von Kölliker  first time in 1863, and it was mean the cell substance and organelles outside the nucleus

All the organelles of eukaryotic cells, such as the nucleus (control center of the cell), endoplasmic reticulum (the network of channels in a cell), mitochondria (the powerhouse of the cell), etc., are located in the cytoplasm.

Composition

It contains water in which organic molecules (protein, carbohydrate, lipid, etc) and inorganic salts are dissolved.

A- Soluble sol and semisolid gel-like composition: The cytoplasmic components may make cytoplasm as a sol or gel-like substance. Colloidal solution of various salts and molecules in cytoplasm forms its soluble liquid like composition (Sol) but some times integrated network of components forms a solid mass (Gel), which is jelly-like in composition.

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B-  Solid glass-like cytoplasmic components:  This may be present in dormant cells and are formed by freezing of subcellular structures in place, preventing their damage. This allows the diffusion of very small proteins and metabolites, helping regrowth of cell upon the cell’s revival from dormancy.

Cytoplasmic components which are close to the cell membrane are  ‘stiffer’ while the interior cell regions resemble free-flowing liquids. These changes in the cytoplasm may be due to the metabolic processes within the cell and help in carrying out specific functions.

Structure

The cytoplasmic structure can be divided into three components.

1. The cytoskeleton and its associated motor proteins
2. Organelles with other large multi-protein complexes
3. Dissolved solutes and cytoplasmic inclusions

1- Cytoskeleton and Motor Proteins

Three main types of cytoskeleton fibers are present in the cytoplasm. These include microtubules, actin filaments, and intermediate filaments. These provide not only shape to the cell but also help in cyclosis, cell division, cilia formation, etc.

• Actin filaments or microfilaments: Thes are 7 nm in width and are composed of double-stranded polymers of F-actin. Microfilaments are involved in responding to signal molecules from the extracellular environment and produce cellular responses by chemotaxis.
• Microtubule is a polymer of α and β tubulin, which forms a hollow tube by the association of 13 protofilaments. Each protofilament is formed by alternating α and β tubulin molecules. A microtubule has 12 nm inner diameter and 24 nm outer diameter. Microtubules also take part in cytoplasmic transport, segregation of chromosome and in the formation of structures like cilia and flagella for cellular movement.
• Intermediate filaments are intermediate in size as compared to the other two and are formed by a group of proteins. They help cells to come together to form tissues and anchor cells to the extracellular matrix.

2- Organelles and Protein Complexes

Eukaryotic cells often have a lot of organelles that form compartments within the cytoplasm for specific microenvironments.

Examples of organelles:

• Lysosomes consists of a number of hydrolases enzymes in an acidic environment for their better enzymatic activity.
• Mitochondria (have its own DNA) require many enzymes synthesized in the cytosol. These enzymes are selectively moved into this organelle.
• In addition to these cytoplasms also contains several other organelles such as Endoplasmic reticulum, vacuole, Golgi apparatus, Peroxisomes, Centrioles, Plastids, etc which have specific functioning within the cell.

 Multi-protein complexes:

The cytoplasm also acts as a host to multi-protein complexes like the proteasome and ribosomes.

• The ribosome is a large complex structure of RNA and protein that functions in the translation of mRNA code into specific proteins.
• Proteasomes are giant molecular structures that contain peptidases enzymes present in both cytoplasm and nucleus. These are important for the breakdown of proteins that are useless for the cell.

3- Cytoplasmic Inclusions

These include small crystals of proteins, pigments, carbohydrates, fats, etc present in the cell.

Examples

• The adipose tissue, contain triglyceride lipids in droplets form which are used to create cellular membranes and as an excellent energy store. Lipids can generate twice as many ATP molecules per gram when compared to carbohydrates.
• Glycogen is also present as cytoplasmic inclusions in cells especially the skeletal and cardiac muscle cells where there could be a sudden increase in demand for glucose. Glycogen quickly breaks into glucose molecules which undergo cellular respiration to release energy.
• Crystals are also important cytoplasmic inclusion found in many cells such as in cells of the inner ear ( to maintain balance). In cells of the testis, crystals if present may cause morbidity and infertility.
• Finally, in the cytoplasm, pigments such as melanin, which cause pigmentation in cells of the skin, are also present. These pigments protect the internal body structures and the cells from the harmful effects of UV radiations. Pigments are also present in the iris that around the pupil of the eye.

All these cytoplasmic components make cytoplasm a dynamic region that helps the cell’s in completion of its overall metabolic activity.

Functions of Cytoplasm

• The cytoplasm is the main site for most of the biochemical and metabolic reactions of the cell. For example Glycolysis.Glycolysis in cytoplasm
• The translation of mRNA into polypeptide protein chains on ribosomes occurs mostly in the cytoplasm. Some of it occurs on free ribosomes suspended in the cytosol and the rest occurs on ribosomes joined on the endoplasmic reticulum.
• The cytoplasm contains monomers to generate the cytoskeletal fibers which help cell in maintaining its shape, providing anchorage site to organelles, etc. For example, neurons with their long axons require intermediate filaments, microtubules, and actin filaments to give them shape and a rigid framework for the nerve impulse to be transmitted to the next cell. Besides this, some epithelial cells may contain small cilia or flagella remove foreign particles or to move the cell through the coordinated activity of cytoplasmic protrusions formed by the cytoskeleton.

  

• The cytoplasm provides specific locations for different organelles. For example, the nucleus is usually present in the center of the cell, with a nearby centrosome. The extensive network of endoplasmic reticulum and Golgi network are placed in close relation to the nucleus, for their coordinated working with the nucleus.
• The flow of cytoplasmic components is involved in many cellular functions which involve the permeability of the cytoplasmic membranes.  An example of such a function is cell signalling.
• It provides space for the functioning of cell organelles.