Cytoplasm- Definition, Structure, Functions and Diagram

The cytoplasm is the semi-fluid substance that fills the entire space of a cell from the outer layer of the double membrane-bound nucleus to the inner layer of the cell membrane. It is composed of water, salts, and various organic molecules. Some intracellular structures, such as the nucleus and mitochondria, are enclosed by membranes that separate them from the cytoplasm. The cytoplasm is sometimes described as the non-nuclear content of protoplasm, which is the living material of the cell.

The cytoplasm was discovered in the year 1835 by Robert Brown and other scientists . Brown was the first to observe the nucleus in plant cells and coined the term "cell nucleus". He also noticed a granular substance surrounding the nucleus, which he called "areolar tissue" or "cell substance". Later, other scientists such as Felix Dujardin, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow studied this substance and gave it different names, such as "sarcode", "cytoblastema", "protoplasm", and "cytoplasm" .

The term "cytoplasm" was first used by Carl Nägeli and C. Cramer in 1855 to refer to the fluid inside the cell membrane. However, it was not until 1896 that Albrecht von Kölliker defined cytoplasm as the part of protoplasm that excludes the nucleus. Since then, cytoplasm has been recognized as a complex and dynamic system that contains various components and performs various functions in the cell.

The cytoplasm is the part of the cell that lies between the plasma membrane and the nuclear envelope. It consists of a fluid matrix called the cytosol, in which various organelles and other structures are suspended. The cytosol is mainly composed of water, dissolved ions, small molecules, and macromolecules such as proteins and nucleic acids. The cytoplasm also contains the cytoskeleton, a network of protein filaments that provide shape, support, and movement to the cell.

The structure of the cytoplasm varies depending on the type of cell. In eukaryotic cells, which have a nucleus and membrane-bound organelles, the cytoplasm contains several specialized compartments that perform different functions. Some of these organelles are:

• Mitochondria: These are the powerhouses of the cell, where cellular respiration takes place and ATP (energy) is produced. Mitochondria have a double membrane, with an inner membrane folded into cristae that contain the enzymes for oxidative phosphorylation. Mitochondria also have their own DNA and ribosomes, and can replicate independently of the nucleus.
• Endoplasmic reticulum (ER): This is a network of membranous tubules and sacs that extends throughout the cytoplasm. The ER has two regions: the smooth ER and the rough ER. The smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage. The rough ER has ribosomes attached to its surface and is involved in protein synthesis and modification.
• Golgi apparatus: This is a stack of flattened membranous sacs that receives proteins from the rough ER and modifies them further by adding sugars, lipids, or other groups. The Golgi apparatus also sorts and packages the proteins into vesicles that are transported to other parts of the cell or secreted outside.
• Lysosomes: These are spherical vesicles that contain hydrolytic enzymes that break down macromolecules, worn-out organelles, pathogens, or debris. Lysosomes fuse with other vesicles or vacuoles that contain the material to be digested, forming a larger structure called a phagosome.
• Peroxisomes: These are small vesicles that contain enzymes that catalyze various oxidative reactions, such as breaking down fatty acids or detoxifying alcohol. Peroxisomes produce hydrogen peroxide as a by-product, which they then convert into water using catalase.
• Vacuoles: These are large vesicles that store water, ions, nutrients, waste products, or pigments. Vacuoles are especially prominent in plant cells, where they occupy most of the cytoplasm and help maintain turgor pressure and cell shape.

In addition to these organelles, eukaryotic cells may also have other specialized structures in their cytoplasm depending on their function. For example, plant cells have chloroplasts, which are organelles that perform photosynthesis and have their own DNA and ribosomes. Animal cells have centrioles, which are cylindrical structures that organize microtubules during cell division.

In prokaryotic cells, which lack a nucleus and membrane-bound organelles, the cytoplasm is simpler but still contains some structures that perform vital functions. Some of these structures are:

• Nucleoid: This is a region of the cytoplasm where the circular DNA molecule of the prokaryote is located. The nucleoid is not enclosed by a membrane but is associated with proteins that help compact and regulate the DNA.
• Plasmids: These are small circular DNA molecules that can replicate independently of the nucleoid. Plasmids often carry genes that confer advantages to the prokaryote, such as antibiotic resistance or metabolic capabilities.
• Ribosomes: These are complexes of RNA and protein that synthesize proteins using the information encoded in the DNA. Prokaryotic ribosomes are smaller than eukaryotic ribosomes and have a different structure and composition.
• Inclusions: These are granules or crystals of various substances that are stored in the cytoplasm for later use or disposal. Some examples of inclusions are glycogen granules (energy storage), polyphosphate granules (phosphate storage), sulfur globules (sulfur oxidation), gas vesicles (buoyancy), magnetosomes (magnetic orientation), or carboxysomes (carbon fixation).

The diagram below shows a simplified representation of the structure of the cytoplasm in a typical eukaryotic cell (left) and a typical prokaryotic cell (right). Note that not all cells have all the structures shown here.

The cytoplasm is the semi-fluid substance that fills the interior of a cell. It consists of three main components: cytosol, organelles, and cytoplasmic inclusions.

Cytosol

The cytosol is the part of the cytoplasm that is not occupied by any organelle. It is a gel-like fluid that is mostly water and contains dissolved ions, small molecules, and proteins. The cytosol is the site for many biochemical reactions, such as glycolysis, protein synthesis, and signal transduction. The cytosol also contains the cytoskeleton, a network of protein filaments that provide structural support and movement for the cell.

Organelles

Organelles are membrane-bound structures that perform specific functions within the cell. They are suspended in the cytosol and have different shapes and sizes. Some of the major organelles found in eukaryotic cells are:

• Nucleus: The largest organelle that contains the genetic material (DNA) and controls the gene expression and cell division.
• Mitochondria: The powerhouse of the cell that produces energy (ATP) through cellular respiration.
• Endoplasmic reticulum (ER): A system of membranous tubules and sacs that synthesize and modify proteins and lipids.
• Golgi apparatus: A stack of flattened membranes that sort and package proteins and lipids for transport to other destinations.
• Lysosomes: Spherical vesicles that contain digestive enzymes and break down waste materials and foreign substances.
• Vacuoles: Large sacs that store water, nutrients, waste products, or other substances.
• Chloroplasts: The site of photosynthesis in plant cells that convert light energy into chemical energy.
• Ribosomes: Small particles composed of RNA and protein that translate mRNA into proteins.

Cytoplasmic Inclusions

Cytoplasmic inclusions are various types of insoluble particles or molecules that remain suspended in the cytosol. They are not surrounded by any membrane and are not considered as true organelles. They serve as storage sites for energy or other materials. Some examples of cytoplasmic inclusions are:

• Glycogen granules: Clusters of glucose molecules that store energy in animal cells.
• Starch granules: Clusters of glucose molecules that store energy in plant cells.
• Lipid droplets: Spherical droplets of lipids (fatty acids and sterols) that store energy in both prokaryotic and eukaryotic cells.
• Protein granules: Aggregates of proteins that store amino acids or perform specific functions, such as hemoglobin in red blood cells or crystallins in lens cells.
• Pigment granules: Particles of colored substances that give color to certain cells, such as melanin in skin cells or carotenoids in plant cells.

The cytoplasm is the semi-viscous ground substance of the cell that occupies the space between the nucleus and the plasma membrane. It consists of three main components: cytosol, organelles and cytoplasmic inclusions.

The cytosol is the fluid part of the cytoplasm that is not occupied by any organelle. It is mainly composed of water (about 70% to 80%) and dissolved substances such as proteins, carbohydrates, salts, sugars, amino acids and nucleotides. The cytosol contains various enzymes that catalyze metabolic reactions and also serves as a medium for transport of molecules and ions within the cell.

The organelles are membrane-bound structures that perform specific functions for the cell. Some of the major organelles in the cytoplasm are mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles and chloroplasts (in plant cells). Each organelle has its own characteristic shape, size and composition.

The cytoplasmic inclusions are insoluble particles or molecules that are suspended in the cytosol. They are not surrounded by any membrane and they do not have any metabolic activity. They mainly serve as storage sites for energy or materials. Some examples of cytoplasmic inclusions are granules of starch, glycogen, lipid droplets, crystals and pigments.

The physical nature of the cytoplasm is variable and depends on the state of the cell and its environment. Sometimes, the cytoplasm behaves like a colloidal solution, where the solutes are evenly distributed and can diffuse freely. At other times, it behaves like a gel or a glass, where the solutes are aggregated and have limited mobility.

The factors that influence the physical nature of the cytoplasm include temperature, pH, osmotic pressure, electric charge, concentration of solutes and presence of cytoskeleton. The cytoskeleton is a network of protein filaments that provides shape, support and movement to the cell and its organelles. The cytoskeleton can also regulate the viscosity and elasticity of the cytoplasm by forming cross-links or breaking them.

The physical nature of the cytoplasm affects its functions such as enzymatic reactions, metabolic activity, cellular respiration, protein translation and cytoskeleton generation. For example, when the cytoplasm is more fluid-like, it allows faster diffusion of molecules and ions across the cell. When it is more solid-like, it provides more stability and protection to the cell and its organelles.

The cytoplasm is the site for most of the enzymatic reactions and metabolic activity of the cell. It provides a medium for the organelles to remain suspended and carry out their specific functions. Some of the important functions of the cytoplasm are:

• Enzymatic Reactions: The cytoplasm contains various enzymes that catalyze biochemical reactions in the cell. For example, glycolysis, the first stage of cellular respiration, occurs in the cytosol of the cytoplasm. Glycolysis breaks down glucose into pyruvate and generates ATP, the energy currency of the cell. Other metabolic pathways, such as fatty acid synthesis and degradation, also take place in the cytosol.
• Metabolic Activity: The cytoplasm is involved in various metabolic activities that are essential for the cell`s growth, maintenance and survival. For instance, the cytoplasm transports nutrients, hormones and other molecules across the cell through diffusion or active transport. The cytoplasm also dissolves cellular waste products and helps in their elimination.
• Cellular Respiration: The cytoplasm plays a crucial role in cellular respiration, which is the process of converting organic molecules into energy. As mentioned earlier, glycolysis occurs in the cytosol of the cytoplasm. The products of glycolysis are then transported to the mitochondria, which are organelles in the cytoplasm that produce ATP through oxidative phosphorylation. The mitochondria also use oxygen and release carbon dioxide as a by-product of cellular respiration. The cytoplasm facilitates the exchange of these gases between the cell and its environment.
• Protein Translation: The cytoplasm is also the site where protein translation occurs. Protein translation is the process of synthesizing proteins from mRNA molecules. The mRNA molecules are transcribed from DNA in the nucleus and then exported to the cytoplasm. In the cytoplasm, they bind to ribosomes, which are organelles that read the mRNA sequence and assemble amino acids into polypeptide chains. The ribosomes can be either free-floating in the cytosol or attached to the endoplasmic reticulum, another organelle in the cytoplasm that modifies and transports proteins.
• Cytoskeleton Generation: The cytoplasm also contains the monomers that go on to generate the cytoskeleton. The cytoskeleton is a network of protein filaments that give shape and support to the cell. It also enables cell movement, division, and communication. The main components of the cytoskeleton are microtubules, microfilaments, and intermediate filaments. These components are assembled from tubulin, actin, and keratin proteins respectively, which are synthesized in the cytosol of the cytoplasm.

These are some of the major functions of the cytoplasm that illustrate its importance for cellular life. The cytoplasm is not just a passive filler of space, but a dynamic and active component of every cell.

The cytoplasm is not only a medium for chemical reactions and organelle suspension, but also a vital factor for cell expansion, growth and protection. Cell expansion refers to the increase in cell size and volume, which is essential for many biological processes such as development, differentiation and adaptation. Cell growth refers to the increase in cell mass and number, which is crucial for tissue formation and repair. Cell protection refers to the defense mechanisms that the cell employs to prevent or cope with damage from various sources such as pathogens, toxins and stress.

How does the cytoplasm contribute to these roles? Here are some examples:

• The cytoplasm contains various enzymes and metabolites that are involved in cellular metabolism, such as glycolysis, cellular respiration, protein synthesis and cytoskeleton formation. These processes provide the energy and materials needed for cell expansion and growth.
• The cytoplasm also contains various signaling molecules and pathways that regulate cell cycle progression, gene expression, differentiation and apoptosis. These processes control the timing and extent of cell expansion and growth.
• The cytoplasm is the site of cytoplasmic streaming, a phenomenon where the cytosol and organelles move around the cell in a coordinated manner. This movement facilitates the distribution of nutrients, waste products, hormones and signals throughout the cell. It also helps to position organelles in optimal locations for their functions. For example, chloroplasts can be moved closer to the plasma membrane to optimize photosynthesis, or away from it to avoid excess light damage. Cytoplasmic streaming also enables cells to change their shape and polarity, which are important for cell expansion and growth.
• The cytoplasm acts as a buffer and protects the cell from mechanical stress and injury. It absorbs shock and distributes pressure evenly across the cell. It also prevents the cell from bursting or collapsing due to osmotic changes. The cytoplasm also contains various antioxidants and detoxifying agents that protect the cell from oxidative stress and toxic substances.
• The cytoplasm hosts two organelles that have their own genomes: the mitochondria and the chloroplasts. These organelles are inherited directly from the mother through the oocyte and therefore constitute genes that are inherited outside the nucleus. These genes encode for proteins that are essential for cellular respiration and photosynthesis, respectively. They also play roles in signaling, apoptosis and immunity.

As we can see, the cytoplasm is a dynamic and multifunctional component of the cell that plays a significant role in cell expansion, growth and protection.

Importance of Cytoplasm in Protein Translation and Cytoskeleton Generation

Protein translation is the process of synthesizing proteins from the information encoded in messenger RNA (mRNA) molecules. Protein translation takes place on ribosomes, which are complex molecular machines composed of ribosomal RNA (rRNA) and proteins. Ribosomes are located in the cytoplasm, either free-floating or attached to the endoplasmic reticulum (ER). The cytoplasm provides the environment and the components for protein translation to occur.

The main components of protein translation are:

• mRNA: The mRNA molecule carries the genetic code for the protein to be synthesized. It is transcribed from DNA in the nucleus and exported to the cytoplasm.
• tRNA: The transfer RNA (tRNA) molecules are small RNA molecules that carry specific amino acids to the ribosome. Each tRNA has an anticodon that matches a codon on the mRNA. There are 20 different types of tRNA, one for each amino acid.
• Amino acids: The amino acids are the building blocks of proteins. They are linked together by peptide bonds to form polypeptide chains. There are 20 different amino acids, each with a unique chemical structure and properties.
• Ribosomes: The ribosomes are the sites of protein synthesis. They consist of two subunits, a large one and a small one, that join together around the mRNA. The ribosomes have three binding sites for tRNA: the A site, where the incoming aminoacyl-tRNA binds; the P site, where the growing polypeptide chain is attached; and the E site, where the empty tRNA exits.
• Initiation factors: The initiation factors are proteins that help to assemble the ribosome-mRNA complex and start the translation process. They include factors that recognize the start codon (AUG) on the mRNA, recruit the initiator tRNA (tRNA^Met^), and join the ribosomal subunits together.
• Elongation factors: The elongation factors are proteins that facilitate the elongation of the polypeptide chain by adding new amino acids. They include factors that help to move the ribosome along the mRNA, deliver new aminoacyl-tRNAs to the A site, and catalyze the formation of peptide bonds between amino acids.
• Termination factors: The termination factors are proteins that recognize the stop codons (UAA, UAG, or UGA) on the mRNA and terminate the translation process. They include factors that release the polypeptide chain from the ribosome and dissociate the ribosome-mRNA complex.

The cytoplasm plays an important role in protein translation by providing:

• The necessary components for translation, such as amino acids, tRNAs, rRNAs, and various factors.
• The optimal conditions for translation, such as temperature, pH, salt concentration, and energy supply.
• The regulation of translation, such as by controlling the availability and stability of mRNAs, tRNAs, rRNAs, and factors.

The cytoplasm also plays an important role in cytoskeleton generation by providing:

• The monomers for cytoskeleton formation, such as tubulin for microtubules, actin for microfilaments, and intermediate filament proteins.
• The accessory proteins for cytoskeleton assembly and disassembly, such as motor proteins, cross-linking proteins, severing proteins, and capping proteins.
• The signals for cytoskeleton organization and dynamics, such as by responding to external stimuli or internal cues.

The cytoskeleton is a network of protein filaments that provides structural support, shape, movement, and organization to cells. It consists of three main types of filaments:

• Microtubules: Microtubules are hollow tubes made of tubulin dimers. They are involved in cell division, intracellular transport, cell motility, and cell polarity.
• Microfilaments: Microfilaments are thin strands made of actin monomers. They are involved in muscle contraction, cell crawling, cytokinesis, endocytosis, and cell shape changes.
• Intermediate filaments: Intermediate filaments are rope-like fibers made of various proteins. They are involved in mechanical strength, nuclear stability, cell adhesion, and stress response.

The cytoskeleton is dynamic and constantly changing according to the needs of the cell. It interacts with other cellular components such as membranes, organelles, signaling molecules, and extracellular matrix.

References:

Cytoplasmic streaming, also known as protoplasmic streaming or cyclosis, is the flow of the cytoplasm inside the cell, driven by forces from the cytoskeleton. It is likely that its function is, at least in part, to speed up the transport of molecules and organelles around the cell . It is usually observed in large plant and animal cells, greater than approximately 0.1 mm. In smaller cells, the diffusion of molecules is more rapid, but diffusion slows as the size of the cell increases, so larger cells may need cytoplasmic streaming for efficient function.

Cytoplasmic streaming is strongly dependent upon intracellular pH and temperature. It has been observed that the effect of temperature on cytoplasmic streaming created linear variance and dependence at different high temperatures in comparison to low temperatures. This process is complicated, with temperature alterations in the system increasing its efficiency, with other factors such as the transport of ions across the membrane being simultaneously affected. This is due to cells homeostasis depending upon active transport which may be affected at some critical temperatures.

In plant cells, chloroplasts may be moved around with the stream, possibly to a position of optimum light absorption for photosynthesis. The rate of motion is usually affected by light exposure, temperature, and pH levels. The optimal pH at which cytoplasmic streaming is highest, is achieved at neutral pH and decreases at both low and high pH. The flow of cytoplasm may be stopped by adding Lugol`s iodine solution or Cytochalasin D (dissolved in dimethyl sulfoxide).

The mechanism for cytoplasmic flow involves motor proteins that use adenosine triphosphate (ATP) to move along microfilaments or microtubules in the cytoskeleton . These motor proteins are usually composed of two proteins that can change their shape and attachment to each other or to other molecules. One of the proteins remains fixed on a substrate, such as a microfilament or a microtubule, while the other protein moves along it, dragging the organelles or molecules with it . The most common motor proteins are myosin and actin, which form long protein fibers aligned in rows parallel to the streaming just inside the cell membrane. Myosin molecules attached to cellular organelles move along the actin fibers, towing the organelles and sweeping other cytoplasmic contents in the same direction . Cytoplasmic flow rates can range between 1 and 100 micron/sec.

Cytoplasmic streaming is important for positioning organelles within the cell according to their functions and needs. For instance, the nucleus is usually seen towards the center of the cell, with a centrosome nearby. Chloroplasts are often distributed near the plasma membrane to optimize photosynthesis. Mitochondria are often located near sites of high energy demand, such as muscle fibers or nerve terminals. Endoplasmic reticulum and Golgi apparatus are often arranged around the nucleus to facilitate protein synthesis and transport. Vacuoles are often positioned near the cell wall to maintain turgor pressure and osmotic balance.

Cytoplasmic streaming also plays a role in creating order within the cell with specific locations for different organelles. For example, in some algae and fungi, cytoplasmic streaming helps to distribute nuclei evenly throughout the multinucleated cells. In some plant cells, such as Chara corallina, cytoplasmic streaming exhibits a cyclic pattern around a large central vacuole. The vacuole occupies around 80% of the cell`s diameter and is used for storage . The cytoplasmic flow helps to circulate nutrients and waste products around the vacuole and also influences the orientation of microtubules and cellulose deposition on the cell wall.

Cytoplasmic streaming is therefore a vital process for maintaining cellular function and organization. It enables cells to transport materials faster than diffusion alone and to adjust their internal structure according to environmental cues or cellular needs.

Cytoplasmic inheritance is the transmission of genes that occur outside the nucleus, in the cytoplasmic organelles such as mitochondria and chloroplasts. These organelles have their own DNA, which is distinct from the nuclear DNA and can encode some essential functions for the cell. Cytoplasmic inheritance is also known as extranuclear inheritance or maternal inheritance, because in most cases, the cytoplasmic genes are inherited only from the mother through the egg cell. However, there are some exceptions where cytoplasmic genes can be inherited from the father, both parents, or neither parent.

Chloroplast Inheritance

Chloroplasts are the organelles that perform photosynthesis in plant cells and some algae. They have circular DNA molecules that contain genes for photosynthetic proteins, ribosomal RNA, and transfer RNA. Chloroplast DNA is usually inherited maternally in plants, meaning that the offspring inherits the chloroplasts from the mother plant only. This can be observed in some plants that have variegated leaves, where different sectors of the leaf have different chloroplast genotypes. For example, in Mirabilis jalapa (four o`clock plant), the leaf color is determined by the type of chloroplasts present in the cells. Green chloroplasts produce chlorophyll, while white chloroplasts do not. If a plant has both green and white chloroplasts in its cells, it will have variegated leaves. However, if a plant has only one type of chloroplasts, it will have either green or white leaves. When a variegated plant is crossed with a green or white plant, the offspring will have either green or white leaves, depending on the maternal parent.

However, chloroplast inheritance is not always strictly maternal. In some plants and algae, chloroplast DNA can be inherited paternally (from the father), biparentally (from both parents), or doubly uniparentally (from one parent but different organelles). For example, in Chlamydomonas reinhardtii (a green alga), chloroplast DNA is inherited paternally during sexual reproduction. The sperm cells carry chloroplasts that enter the egg cell and replace its original chloroplasts. In Pinus (pine), chloroplast DNA is inherited biparentally during sexual reproduction. The pollen grains carry chloroplasts that enter the ovule and fuse with its chloroplasts. In Pelargonium (geranium), chloroplast DNA is inherited doubly uniparentally during sexual reproduction. The egg cell carries two types of chloroplasts: one from its mother and one from its grandmother. The sperm cell does not contribute any chloroplasts.

Mitochondrial Inheritance

Mitochondria are the organelles that produce energy for the cell through cellular respiration. They have circular DNA molecules that contain genes for respiratory proteins, ribosomal RNA, and transfer RNA. Mitochondrial DNA is usually inherited maternally in animals, meaning that the offspring inherits the mitochondria from the mother only. This can be observed in some diseases that are caused by mutations in mitochondrial genes, such as Leber`s hereditary optic neuropathy (LHON) or mitochondrial myopathy. These diseases affect only individuals who inherit the mutated mitochondria from their mothers.

However, mitochondrial inheritance is not always strictly maternal. In some animals and plants, mitochondrial DNA can be inherited paternally (from the father), biparentally (from both parents), or doubly uniparentally (from one parent but different organelles). For example, in Drosophila melanogaster (fruit fly), mitochondrial DNA is inherited paternally during sexual reproduction. The sperm cells carry mitochondria that enter the egg cell and replace its original mitochondria. In Brassica (cabbage), mitochondrial DNA is inherited biparentally during sexual reproduction. The pollen grains carry mitochondria that enter the ovule and fuse with its mitochondria. In Beta vulgaris (beet), mitochondrial DNA is inherited doubly uniparentally during sexual reproduction. The egg cell carries two types of mitochondria: one from its mother and one from its grandmother. The sperm cell does not contribute any mitochondria.

Conclusion

Cytoplasmic inheritance is a fascinating phenomenon that shows how genetic information can be transmitted through different pathways than nuclear genes. Cytoplasmic inheritance can affect various traits and functions of organisms, such as photosynthesis, respiration, disease susceptibility, and evolution. Cytoplasmic inheritance can also reveal interesting patterns of genealogy and ancestry through molecular analysis of cytoplasmic genomes. Therefore, cytoplasmic inheritance is an important aspect of biology that deserves more attention and research.

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