Cytoplasm - Structure and Function

Cytoplasm - Structure and Function & Cytoplasmic Organelles

All organisms are composed of cells. Some organisms consist of a single cell too small to see with the unaided eye, while others, like us, are composed of many cells. Cells are so much a part of life as we know it that we cannot imagine an organism that is not cellular in nature. In this article, we will take a close look at the internal structure of cells. In the following chapters, we will focus on cells in action—on how they communicate with their environment, grow, and reproduce. The three major features all cells are a plasma membrane, a nucleoid or nucleus, and cytoplasm.

The cytoplasm includes the substance inside and outside the nucleus of the plasma membrane. The cytoplasm is the substance that contains different cellular components. The major difference between eukaryotic and prokaryotic cytoplasm is the complex internal structure of the eukaryotic cytoplasm consisting of excessively small rods (microfilaments and intermediate filaments) and cylinders (microtubules). They are forming the cytoskeleton together. The cytoskeleton provides support and shape and supports the cell's transportation of substances. Cytoplasmic streaming is the movement of eukaryotic cytoplasm from one part of the cell to another, which helps to distribute nutrients and move the cell across a surface. Another difference between prokaryotic and eukaryotic cytoplasm is that many of the important enzymes found in prokaryote cytoplasm are sequestered in eukaryote organelles.

Cytoplasmic Organelles

Cytoplasmic organelles are small cytoplasmic structures that carry out specific functions. Cytoplasmic organelles are classified as membranous or non-membranous organelles, based on whether a membrane surrounds them. The membranous organelles of cytoplasm are endoplasmic reticulum, Gogli complex, mitochondria, plastids (in animal cells) and lysosomes (in plant cells) while non-membranous organelles of cytoplasm include ribosomes, cytoskeleton and centrioles. Let us study the structure and function of each of these cytoplasmic organelles in details.

Endoplasmic reticulum

The endoplasmic reticulum (ER) is an extensive system of folded membranes dividing into compartments and channels the interior of eukaryotic cells. The ER is the largest of the inner membranes. The term endoplasmic means "in the cytoplasm," and for "a small net" the term reticulum is Latin.It weaves via the inside of the cell in sheets, making a series of channels between its folds. The two largest of the many compartments in eukaryotic cells are the inner region of the ER, which is called the cisternal space, and the outer region, the cytosol.

The ER surface areas dedicated to protein synthesis are heavily studied with ribosomes, large molecular protein and ribonucleic acid (RNA) aggregates that translate gene RNA copies into proteins. The ribosome-rich regions of the ER appear pebbly through the electron microscope, like the surface of sandpaper, and are therefore called rough ER. The ER regions are referred to as smooth ER regions with relatively few bound ribosomes. The smooth ER membranes contain many embedded enzymes, most of which are only active when connected to a membrane. Rough ER synthesizes proteins, while smooth ER organizes lipid synthesis and other activities of biosynthesis.

Gogli complex

Ultimately, most of the proteins produced by ribosomes attached to rough ER are transported to other cell regions. The transportation path step is through an organelle called the Golgi complex. It consists of 3 to 20 cisterns that look like a pita bread stack. The cisterns are often curved; giving the Golgi complex a cuplike shape Proteins synthesized by ribosomes on the rough ER are surrounded by a portion of the ER membrane that eventually buds from the surface of the membrane to form a vesicle of transport. The transport vesicle fuses into the cistern with a cistern of the Golgi complex. The proteins are altered and move through vesicles that bud from the edges of the cisternae from one cistern to another. Cisternal enzymes change the proteins into glycoproteins, glycolipids and lipoproteins. A few of the processed proteins leave the cisternae in secretory vesicles that produce the proteins to the plasma membrane detached from the cistern, where they are released by exocytosis. Other processed proteins leave cisterns in vesicles which actually deliver their contents to the membrane of the plasma. Finally, in vesicles called storage vesicles, some processed proteins leave the cisternae.


Lysosomes are formed from complexes in Golgi and actually look like spheres confined to the membrane. Lysosomes have only one membrane and lack internal structure, unlike mitochondria. But they contain as many as 40 powerful digestive enzymes that can break down different molecules. In addition, these enzymes can also digest cell - entry bacteria. Human white blood cells contain large numbers of lysosomes, which use phagocytosis to ingest bacteria.


In the cytoplasm of most eukaryotic cells, spherical or rod-shaped organelles called mitochondria. The number of mitochondria per cell varies widely between different cell types. A mitochondrion is a double membrane similar to the plasma membrane structure. The outer mitochondrial membrane is smooth, but in a series of folds called cristae, the inner mitochondrial membrane is arranged. The mitochondrion's center is a semifluid substance called the matrix. The inner membrane provides an enormous surface area on which chemical reactions can occur due to the nature and arrangement of the cristae. Some proteins that function in cellular respiration are located on the cristae of the inner mitochondrial membrane, including the enzyme that makes ATP, and many of the metabolic steps involved in cellular respiration are concentrated in the matrix. Because of their central role in ATP production, mitochondria are often called the "cell powerhouses".

Mitochondria contains its own 70S ribosomes and some DNA, as well as the machinery needed to replicate, transcribe, and translate the DNA-encoded information. Furthermore, by growing and dividing into two, mitochondria can reproduce more or less on their own. Chloroplasts contain 70S ribosomes, DNA, and protein synthesis enzymes. They can multiply within the cell on their own. The way in which both chloroplasts and mitochondria multiply — growing in size and then dividing into two — remembers bacterial multiplication strikingly.


Ribosomes that are also found free in the cytoplasm are attached to the outer surface of the rough endoplasmic reticulum. Ribosomes are the cell sites of protein synthesis. Eukaryotic endoplasmic reticulum and cytoplasm ribosomes are somewhat larger and denser than prokaryotic cells. These eukaryotic ribosomes are 80S ribosomes, each consisting of a large 60S subunit consisting of three rRNA molecules and a smaller 40S subunit consisting of one rRNA molecule. The subunits are produced separately in the nucleus and exit the nucleus once produced and join in the cytosol together. 

Chloroplasts and mitochondria encompass 70S ribosomes that may indicate prokaryote evolution. Some ribosomes are unattached to any structure in the cytoplasm, called free ribosomes. Free ribosomes primarily synthesize proteins that are used within the cell. Other ribosomes, called membranous ribosomes, are attached to the nuclear membrane and the reticulum endoplasm. These ribosomes synthesize proteins for plasma membrane insertion or cell export. Within mitochondrial ribosomes, mitochondrial proteins are synthesized. 10 to 20 ribosomes sometimes come together in a stringent arrangement called a polyribosome.

The cytoplasmic matrix, microfilaments, intermediate filaments, and microtubules

The cytoplasmic matrix contains the many organs of eucaryotic cells. One of the cell's most important and complex parts is the matrix. It is the organelles ' "environment" and the location of many important processes of biochemistry. 

The cytoskeleton consists of three types of filaments: microfilaments, microtubules and intermediate filaments. Microfilaments are minute protein filaments with a diameter of 4 to 7 nm, which can either be dispersed within the cytoplasm matrix or organized into networks and parallel arrays. Microfilaments consist of an actin protein similar to the muscle tissue's actin - contractile protein. Microfilaments are involved in cell motion and shape changes such as the motion of pigment granules, amoeboid movement, and protoplasmic streaming in slime molds.

Microtubules are shaped like thin cylinders about 25 nm in diameter. The two proteins are the same molecular weight, and their amino acid sequence and tertiary structure differ only slightly. Each tubulin has a diameter of about 4 to 5 nm. These subunits are assembled in a helical arrangement in one turn or circumference to form a cylinder with an average of 13 subunits. 

Microtubules serve at least three purposes: 

(1) Help maintain cell shape,
(2) Involved in microfilaments in cell movements, and
(3) Participate in intracellular transport processes. 

Microtubules are found in long, thin cell structures that require support such as protists ' axopodia. In structures that participate in a cell or organel movements, microtubules are also present— the mitotic spindle, cilia, and flagella. Intermediate filaments are cytoskeleton heterogeneous elements. They are about 10 nm in diameter and can be divided into several classes from a group of proteins. In one or more of these classes of proteins, intermediate filaments with different functions are assembled.