Cytoplasm Definition, Properties, Function, and structure
What is cytoplasm?
The cytoplasm contains all the contents outside the nucleus and is enclosed within the cell membrane of a cell. It is clear in color and has a gel-like appearance. The cytoplasm is mainly composed of water but includes enzymes, salts, organelles, and various organic molecules.
Properties of Cytoplasm
- The cytoplasm is composed of 70% – 80% water and is usually colorless.
- It contains proteins, carbohydrates, salts, sugars, amino acids, and nucleotides.
- The cytoplasm constitutes dissolved nutrients and also dissolves waste products.
- The outer clear and vitreous layer of the cytoplasm is called the ectoplasm or cell cortex and the inner granular mass is called the endoplasm.
- The peripheral area of the cytoplasm is a thick and jelly-like substance, known as a plasmogel. The surrounding area of the atomic region is thin and liquefied in nature and is known as plasmosol.
- The physical nature of the cytoplasm is variable. Sometimes, accelerated proliferation occurs in the cell, causing the cytoplasm to resemble a colloidal solution. At other times, it takes on the properties of a gel-like or glass-like substance.
- It is said to have viscous properties as well as elastic materials – capable of deforming slowly under external force in addition to reclaiming their original shape with minimal loss of energy.
- The cytoskeleton present in the cytoplasm gives the cell its shape.
- The cytoplasm helps the cellular material to move around the cell through a process called cytoplasmic streaming.
- Since the cytoplasm contains many salts, it is a very good conductor of electricity.
- It exhibits differential staining properties, areas stained with basic dyes are basophilic regions of the cytoplasm and for this material are called ectoplasm.
The cytoplasm refers to the fluid filling the cell, including the elements, proteins, ions, and macromolecular structures, as well as organelles suspended in the cytosol.
In eukaryotic cells, cytoplasm refers to the contents of the cell with the exception of the nucleus. Eukaryotes have elaborate mechanisms to maintain a specific nuclear compartment distinct from the cytoplasm. Active transport is involved to form these subcellular structures and to maintain homeostasis along with the cytoplasm. For prokaryotic cells, since they do not have a defined nuclear membrane, the cytoplasm also contains the primary genetic material of the cell. These cells are usually smaller than in eukaryotes and have a simple internal organization of the cytoplasm.
- The cytoplasm serves to support and suspend organelles and cellular molecules.
- The cytoplasm also has many cellular processes, such as protein synthesis, the first phase of cellular respiration (known as glycolysis), mitosis, and meiosis.
- The cytoplasm helps move substances such as hormones around the cell, and also dissolve the cellular waste.
The cytoplasm is the site for most enzymatic reactions and metabolic activity of the cell. Cellular respiration begins with anaerobic respiration or glycolysis in the cytoplasm. This reaction provides intermediates used by mitochondria to generate ATP. In addition, the translation of mRNA into proteins on the ribosome is also mostly in the cytoplasm. Some of it is on the free ribosome in the cytosol while the rest is on the ribosome anchored on the endoplasm reticulum.
The cytoplasm also contains monomers that move to generate the cytoskeleton. In addition to being important for the normal activities of the cell, the cytoskeleton is important for cells that have a particular shape. For example, neurons along their long axes require the presence of intermediate cells, microtubules, and actin filaments to provide a rigid framework for the action potential to be transmitted to the next cell. Additionally, some epithelial cells have small cilia or flagella to move the cell or remove foreign particles through the coordinated activity of cytoplasmic extrusion formed through the cytoskeleton.
The cytoplasm also plays a role in forming sequences with specific locations for different organisms within the cell. For example, the nucleus is usually viewed towards the center of the cell, with a centrosome nearby. The broad endoplasmic reticulum and the Golgi network are also placed in relation to the nucleus, with the vessels passing out towards the plasma membrane.
Movement within the cytoplasm also occurs in bulk through the directed movement of the cytosol around the nucleus or vacuole. This is especially important in large single-celled organisms such as some species of green algae, which may have a length of about 10 cm. Cytoplasmic streaming is also important for positioning the chloroplast close to the plasma membrane to optimize photosynthesis and deliver nutrients through the entire cell. In some cells, such as mouse oocytes, cytoplastic streaming is expected to play a role in the formation of cellular sub-compartments and also in organelle positioning.
Cytoplasmic streaming, or cyclosis, is a process by which substances are circulated within a cell. Cytoplasmic streaming occurs in many cell types, including plant cells, amoebae, protozoa, and fungi. Cytoplasmic movement can be affected by certain factors, including the presence of certain chemicals, hormones, or changes in light or temperature.
Plants employ cyclosis to shuttle chloroplasts to areas that receive the most available sunlight. Chloroplasts are the organs of the plant responsible for photosynthesis and require light for the process. In protists, such as amoebae and sludge molds, cytoplasmic streaming is used for locomotion. Temporal expanses of the cytoplasm known as pseudopodia arise that are valuable for movement and occupation of food.
Cytoplasmic streaming is also required for cell division because the cytoplasm must be distributed between daughter cells produced in mitosis and meiosis.
Structure of Cytoplasm
The cytoplasm is unusual because it is unlike any other fluid found in the physical world. The fluids studied to understand diffusion usually contain some solutes in an aqueous environment. However, the cytoplasm is a complex and crowded system consisting of a wide range of particles – from ions and small molecules to proteins as well as huge multi-protein complexes and organelles. These components are transferred to the cell based on the needs of the cell with a wide cytoskeleton with the help of specialized motor proteins. The motion of such large particles also changes the physical properties of the cytosol.
The physical nature of the cytoplasm is variable. Sometimes, accelerated proliferation occurs in the cell, causing the cytoplasm to resemble a colloidal solution. At other times, it takes on the properties of a gel-like or glass-like substance. It is said to have viscous properties as well as elastic materials – capable of deforming slowly under external force in addition to reclaiming their original shape with minimal loss of energy. Parts of the cytoplasm close to the plasma membrane are also ‘stiffer’, while areas near the interior are similar to free-flowing fluids. These changes in the cytoplasm are dependent on metabolic processes within the cell and play an important role in carrying out specific functions and protecting the cell from stress.
Divisions of Cytoplasm
The cytoplasm can be divided into two primary parts:
The endoplasm (endo -, – plasm) and The ectoplasm. The endoplasm is the central region of the cytoplasm that contains the organelle. The ectoplasm is the more gel-like peripheral part of the cell’s cytoplasm.
Components of Cytoplasm
Prokaryotic cells, such as bacteria and archaeons, do not have a membrane-free nucleus. In these cells, the cytoplasm contains all the contents of the cell inside the plasma membrane. In eukaryotic cells, such as plant cell and animal cells, the cytoplasm has three main components. They are cytosols, organelles, and various particles and corpuscles called cytoplasmic inclusions.
The cytoplasm can be divided into three components:
- Cytoskeleton with its associated motor protein
- Original and other large multi-protein complexes
- Cytoplasmic inclusions and solutes
- Cytosol: Cytosol is the semi-fluid component or liquid medium of the cytoplasm of a cell. It is located outside the nucleus and within the cell membrane.
- Organelles: Organelles are small cellular structures that perform specific functions within a cell. Examples of organelles include mitochondria, ribosomes, nucleus, lysosomes, chloroplasts, endoplasmic reticulum, and Golgi apparatus. The cytoskeleton, also located within the cytoplasm, is a network of fibers that helps the cell maintain its shape and provide support for organelles.
- Cytoplasmic Inclusions: Cytoplasmic inclusions are particles that are temporarily suspended in the cytoplasm. The inclusions include macromolecules and granules. The three types of inclusions found in the cytoplasm are secretory secretions, nutrient inclusions, and pigment corpuscles. Examples of secretory inclusions are proteins, enzymes, and acids. Examples are the inclusion of glycogen (glucose storage molecules) and lipid nutrients. Melanin found in skin cells is an example of the inclusion of a pigmented grain.
Cytoskeleton and Motor Proteins
The basic shape of a cell is provided by its cytoskeleton mainly by three types of polymers –
- Actin filaments,
- Microtubules, and
- Intermediate filaments
Actin filaments or microfilaments are 7 nm in width and are made up of double-stranded polymers of F-actin. These fibers are associated with many other proteins that help with filament assembly and also anchor them closer to the plasma membrane.
This cytoplasmic location helps microfilaments become involved in rapid responses from external molecules to signal molecules and induce cellular responses via signal transduction or chemotaxis. In addition, myosin, an ATP-based motor protein transport cargo and vesicles with microfilament and are also involved in muscle contraction.
Microtubules are polymers of α and, tubulin, which form a hollow tube by the lateral association of 13 protofilaments. Each protofilament is a polymer of alternating α and il tubulin molecules. The internal diameter of a microtubule is 12 nm and its outer diameter is 24 nm.
Microtubules are involved in forming cilia and flagella-like structures for cytoplasmic transport, chromosome isolation, and cellular movement.
Intermediate filaments are larger than microfilaments but smaller than microtubules and are formed by groups of proteins that share structural features. Although they are not involved in cell motility,
They are important for the cells to come together as tissues and remain anchored to the extracellular matrix.
Organelles and Multi-protein Complexes
Most eukaryotic cells have several types of organelles that provide compartments within the cytoplasm for specific microadenomas. For example, lysosomes contain many hydrolases in an acidic environment that is ideal for their enzymatic activity. These hydrolases are actively transported to the lysosome after being synthesized in the cytoplasm. Mitochondria, while keeping their own genome, also require several enzymes synthesized in the cytosol, which then selectively migrate to the organelle. These organisms are placed at specific locations due to the physical gel-like nature of the cytoplasm and anchoring to the cytoskeleton.
In addition, the cytoplasm also hosts multi-protein complexes such as proteasomes and ribosomes. Ribosomes are large complexes of RNA and proteins that are important for the translation of mRNA codes into amino acid sequences of proteins. Proteasomes are massive molecular structures that are 20,000 kilodaltons in mass and 15 nm in diameter. Proteasomes are important for targeted destruction of proteins that are no longer required by the cell.
Cytoplasmic inclusions can include a wide range of biochemicals – from small crystals of protein to dyes, carbohydrates, and fats. All cells, especially in tissue such as fat, contain lipid droplets in the form of triglyceride. These are used to make cellular membranes and are an excellent energy store.
Lipids can produce twice as many ATP molecules per gram as compared to carbohydrates. However, oxygen consumption is intimately the process of releasing this energy from triglycerides, and therefore the cell also contains reserves of glycogen as cytoplasmic inclusions. Glycogen inclusions are particularly important in cells such as skeletal and cardiac muscle cells where a sudden increase in glucose demand may occur. Glycogen can be quickly broken down into individual molecules of glucose and used in cellular respiration before the cell to gain more glucose reserves from the body.
Crystals are another type of cytoplasmic inclusions that are found in many cells and perform a specific function in inner ear cells (maintaining balance). The presence of crystals in testicular cells appears to be associated with morbidity and infertility. Finally, the cytoplasm also contains melanin-like pigments, which give rise to pigmented cells of the skin. These pigments protect the cell and internal body structures from the deadly effects of ultraviolet radiation. Dyes are also prominent in the cells of the iris that surround the pupil of the eye.
Each of these components affects the functioning of the cytoplasm in different ways, making it a dynamic region that plays an overall role in the cell and is affected by the overall metabolic activity of the cell.
The cytoplasm hosts two organelles that have their own genomes – chloroplast and mitochondria. These organelles are directly inherited 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. Cytoplasmic or extranuclear inheritance, therefore, forms an intact genetic line that has not undergone mixing or recombination with the male parent.
The cell membrane or plasma membrane is the structure that maintains the cytoplasm from spreading outside the cell. This membrane is made up of phospholipids, which form a lipid boiler that separates the cell components from the extracellular fluid. The lipid bilayer is semi-permeable, meaning that only a few molecules are able to diffuse across the membrane to enter or exit the cell. Extracellular fluid, proteins, lipids, and other molecules can be added to the cytoplasm of the cell by endocytosis. In this process, molecules and extracellular fluids are internalized, as the membrane constitutes a vesicle.
The vesicle surrounds the fluid and molecules and forms an endosome from the cell membrane. The endosome moves within the cell to transport its contents to their appropriate sites. Substances are removed from the cytoplasm by exocytosis. In this process, vesicles emerging from Golgi bodies fuse with cytoplasm to eject their contents from the cell. The cell membrane also provides structural support for the cell by acting as a stable platform for attachment to the cytoplasm and cell wall (plants).
Cytoplasm Definition, Properties, Function, and Structure