Components of Cell: Cytoplasm, Organelles, and cellular components

 

Components of Cell: Cytoplasm, Organelles, and cellular components

Components of Cell: Cytoplasm, Organelles, and cellular components

There are three main Components of Cell: Cytoplasm, Organelles, and cellular components. The cytoplasm is a liquid inside the cell that surrounds the organelles of the cell. The cell is surrounded by a cell membrane and controls the movement of substances entering or leaving the cell. 

Many of the cell functions that are performed in the cytoplasmic compartment result from the activity of specific structures called organelles. This article summarizes the structure and function of these organelles.

• Cytoplasm

The cytoplasm of a cell has two distinct parts:

1.       The cytomembrane system consists of well defined structures, such as the endoplasmic reticulum, Golgi apparatus, vacuoles, and vesicles.

2.       The fluid cytosol suspends the structures of the cytomembrane system and contains various dissolved molecules.

• Ribosomes: Protein Workbenches

Ribosomes are non-membrane-bound structures that are the sites for protein synthesis. They contain almost equal amounts of protein and a special kind of ribonucleic acid called ribosomal RNA. Some ribosomes attach to the endoplasmic reticulum, and some float freely in the cytoplasm. Whether ribosomes are free or attached, they usually cluster in groups connected by a strand of another kind of ribonucleic acid called messenger RNA. These clusters are called polyribosomes or polysomes.

• Endoplasmic Reticulum: Production and storage

The endoplasmic reticulum (ER) is a complex, membrane-bound labyrinth of flattened sheets, sacs, and tubules that branch and spread throughout the cytoplasm. The endoplasmic reticulum is continuous from the nuclear envelope to the plasma membrane and is a series of channels that helps various materials circulate throughout the cytoplasm. It is also a storage unit for enzymes and other proteins and a point of attachment for ribosomes.

The endoplasmic reticulum with attached ribosomes is rough endoplasmic reticulum, and the endoplasmic reticulum without attached ribosomes is called the smooth endoplasmic reticulum.

Smooth endoplasmic reticulum is the site for lipid production, detoxification of a wide variety of organic molecules, and storage of calcium ions in muscle cells. Most cells contain both types of endoplasmic reticulum, although the relative proportion varies among cells.

• Golgi Apparatus: Packaging, sorting, and export

 The Golgi apparatus or complex (named for Camillo Golgi, who discovered it in 1898) is a collection of membranes associated physically and functionally with their endoplasmic reticulum in the cytoplasm. It is composed of flattened stacks of membrane-bound cisternae (sing. cisterna; closed spaces serving as fluid reservoirs). The Golgi apparatus sorts, packages, and secretes proteins and lipids.

Proteins that ribosomes synthesize are sealed off in little packets called transfer vesicles. Transfer vesicles pass from the endoplasmic reticulum to the Golgi apparatus and fuse with it. In the Golgi apparatus, the proteins are concentrated and chemically modified. One function of this chemical modification seems to mark and sort the proteins into different batches for different destinations.

Eventually, the proteins are packaged into secretory vesicles, which are released into the cytoplasm close to the plasma membrane. When the vesicles reach the plasma membrane, they fuse with it and release their contents to the outside of the cell by exocytosis. Golgi apparatus are most abundant in cells that secrete chemical substances (e.g., pancreatic cells secreting digestive enzymes and nerve cells secreting neurotransmitters). As noted in the next section, the Golgi apparatus also produces lysosomes.

• Lysosomes: Digestion and degradation

Lysosomes are membrane-bound spherical organelles that contain enzymes called acid hydrolases, which are capable of digesting organic molecules (lipids, proteins, nucleic acids, and polysaccharides) under acidic conditions. The enzymes are synthesized in the endoplasmic reticulum, transported to the Golgi apparatus in the form of lysosomes or as vesicles that fuse with the lysosomes, Lysosomes fuse with phagocytic vesicles, thus exposing the vesicle's contents to lysosomal enzymes.

Cells can also selectively digest portions of their own cytoplasm or organelles and then reuse the digested materials when they are returned to the cytoplasm.

• Mitochondria: Power generators

Mitochondria are double membrane-bound organelles that are spherical to elongate in shape. A small space separates the outer membrane from the inner membrane. The inner membrane folds and doubles in on itself to form incomplete partitions called cristae. The cristae increase the surface area available for the chemical reactions that trap usable energy for the cell. The space between the cristae is the matrix. The matrix contains ribosomes, circular DNA and other material. Because they convert energy to a usable form, mitochondria are frequently called the "power generators" of the cell. Mitochondria usually multiply when a cell needs to produce energy.

• Cytoskeleton: Microtubules, intermediate filaments, and microfilaments

In most cells, the microtubules, intermediate filaments, and microfilaments for the flexible cellular framework called the cytoskeleton. This latticed framework extends throughout the cytoplasm, connecting the various organelles and cellular components.

1. Microtubules are hollow, slender, cylindrical structures in animal's cells. Each microtubule is made of spiraling subunits of globular proteins called tubulin subunits. Microtubules function in the movement of organelles, such as secretory vesicles, and in chromosome movement during division of the cell nucleus. They are also part of a transport system within the cell.

For example, in nerve cells, they help move materials through the long nerve processes. Microtubules are an important part of the cytoskeleton n the cytoplasm, and they are involved in the overall shape changes that cells undergo during periods of specialization. 

2. Intermediate filaments are a chemically heterogenous group of protein fibers, the specific proteins of which can vary with cell type.. These filaments help to maintain cell shape and the spatial organization of organelles, as well as promote mechanical activities within the cytoplasm. 
3. Microfilaments are solid strings of protein (actin) molecules. Actin microfilaments are most highly developed in muscle cells as myofibrils, which help muscle cells to shorten or contract. Actin microfilaments in non muscle cells provide mechanical support for various cellular structures and help form contractile systems responsible for some cellular movements (e.g., amoeboid movement in some protozoa).

• Cilia and Flagella: Movement

Cilia and flagella are elongated appendages on the surface of some cells by which the cells, including many unicellular organisms, propel themselves. In stationary cells, cilia or flagella move material over the cell's surface.

Although flagella are 5 to 20 times as long as cilia move somewhat differently, cilia, and flagella move material over the cell's surface. Both are membrane-bound cylinders that enclose a matrix. In this matrix is an axoneme or axial filament, which consists of nine pairs of microtubules arranged in a circle around two central tubules. This is called a 9 + 2 pattern of microtubules. Each microtubule pair (a doublet) also has pairs of dynein (protein) arms projecting towards a neighboring doublet and spokes extending towards the central pair of microtubules double sliding one another.

In the cytoplasm at the base of each cilium or flagellum lies a short, cylindrical basal body, also made up of microtubules and structurally identical to the centriole. The basal body controls the growth of microtubules in cilia or flagella. The microtubules in the basal body form a 9 + 0 pattern: nine sets of three with none in middle.

• Centrioles and microtubule: Organizing centers

The specialized membranous regions of the cytoplasm near the nucleus are microtubule-organizing centers. These centers of dense material give rise to a large number of microtubules with different functions in the cytoskeleton. For example, one type of center gives rise to the centrioles that lie at right angles to each other. Each centriole is composed of nine triplet micro-tubules that radiate from the center like the spokes of a wheel. The centrioles are duplicated preceding cell division, are involved with chromosome movement, and help organize the cytoskeleton.

• Vacuole: Cell maintenance

Vacuoles are membranous sacs that are part of the cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. For example, some single-celled organisms (e.g., protozoa) and sponges have contractile vacuoles that collect water and pump it ti the outside to maintain the organism's internal environment. Other protozoa and sponges have vacuoles for storing food.

• The Nucleus: Information center

The nucleus contains the DNA and is the control and information center of the eukaryotic cell. It has two major functions. The nucleus directs chemical reactions in cells by transcribing genetic information from DNA to RNA, which translates this specific information into proteins (e.g., enzymes) that determine the cell's specific activities. The nucleus also stores genetic information and transfers it during cell division from one cell to next, and from one generation to the next.
Nuclear envelope: Gateway to the Nucleus
The Nuclear envelope is a membrane that separates the nucleus from the cytoplasm that is continuous with the endoplasmic reticulum at a number of points. Over three thousand nuclear pores penetrate the surface of the nuclear envelope. These pores allow materials to enter and leave the nucleus, and give the nucleus direct contact with the endoplasmic reticulum. Nuclear pores are not simply holes in the nuclear envelope; each is composed of an ordered array of globular and filamentous granules, probably proteins. These granules form the nuclear pore complex, which governs the transport of molecules into and out of the nucleus. The size of the pores prevents DNA from leaving the nucleus but permits RNA to be moved out.
Chromosomes: genetic containers
The nucleoplasm is the inner mass of the nucleus. In a nondividing cell, it contains genetic material called chromatin. Chromatin consists of a combination of DNA and protein and is the uncoiled, tangled mass of chromosomes ("colored bodies") containing the hereditary information in segments of DNA called genes. During cell division, each chromosome coils tightly, which makes the chromosome visible when viewed through a light microscope.
Nucleolus: Preassembly Point for Ribosomes
The nucleolus is a non-membrane-bound structure in the nucleoplasm that is present in non-dividing cells. Two or three nucleoli for in most cells, but some cells (e.g., amphibian eggs) have thousands. Nucleoli are preassembly points for ribosomes and usually contain proteins and RNA in many stages of synthesis and assembly. The assembly of ribosomes is completed after they leave the nucleus through the pores of the nuclear envelope.

This is all about Components of Cell: Cytoplasm, Organelles, and cellular components. Have a good day!