A cell is considered as a mass of cytoplasm where it is bounded externally by a cell membrane. Usually, it is microscopic in size, the cells are the smallest structural units of the living matter and they compose all the living things inside it. Most of the cells have one or more nuclei and other organelles that carry out a variety of tasks. The study of cell structure and function is called cell biology. Some of the organisms have only one cell, while others are organized in the form of cooperative groups with a huge number of cells. On the whole, cell biology focuses on the structure and function of a particular cell, from the most general functions and characteristics of cell properties are shared by all cells, to the unique, highly intricate functions particular to specialized cells.
There are several main subfields within cell biology. One is the study of the energy of the cell and the biochemical mechanisms that support cell metabolism. Another subfield in cell biology concerns the genetics of the cell and its interconnection with the proteins that are present for controlling the release of genetic information from the nucleus to the cytoplasm of the cell. Another subfield that focuses on the structure of the cell components, these are known as subcellular compartments. And finally, there is the subfield that is primarily concerned with the cell cycle. The rotation of the phases of the beginning and ending with the process of cell division is focused on different periods of growth and DNA replication.
Like the cell membrane, membranes of some of the organelles contain the permeases or transport proteins, that allow the chemical communication found in between the organelles. Communication between the various organelles is also achieved by the membrane budding processes of endocytosis and exocytosis, which are essentially the same as in the cell membrane. On the other hand, the biosynthetic and degradative processes that take place in the different organelles may require some of the conditions that greatly differ from those of the other organelles or of the cytosol. Internal membranes can maintain these different conditions by isolating them from each other.
Another major function of organelles is to prevent the cell undergoing competing enzymatic reactions from interfering with one another. For instance, the essential proteins are known to be synthesized on the rough endoplasmic reticulum and in the cytosol. Whereas the unwanted proteins are broken down in the form of lysosomes and to some extent, these are broken down in the form of the cytosol. Similarly, the fatty acids that are made in the cytosol and then either they are broken down in the mitochondria for the synthesis of ATP or are degraded in the peroxisomes with concomitant generation of heat. These processes must be kept isolated. Organelle membranes are also involved in the prevention of potentially lethal by-products or enzymes. It prevents the attack on the sensitive molecules in some other regions of the cell by sequestering such degradative activities in their respective membrane-bounded compartments.
The internal membranes present in the eukaryotic cells can differ both structurally and chemically from the outer cell membrane. The outer membrane is made up of a phospholipid bilayer into which these are embedded, or bound by specific membrane proteins. The three major lipids that form the outer membrane are phospholipids, cholesterol, and glycolipids; they are also found in the internal membranes, but in different concentrations. The phospholipid is the primary lipid involved in the formation of all cellular membranes. Cholesterol contributes to the fluidity and stability of all the membranes that are found in internal membranes at 25 percent of the concentration in the outer membrane. Glycolipids are a type of lipids that are found only as a trace of components of internal membranes, whereas they constitute approximately five percent of the outer membrane lipid.
The cellular components are also known as cell organelles. These cell organelles include both membrane and non-membrane bound organelles that are present within the cells and are distinct in their structures and functions. They can coordinate and function efficiently for the normal functioning of the particular cell. A few of these cells can function by providing the shape and support, whereas some of them are involved in the locomotion and reproduction of a cell. There are various organelles that are present within the cell and these are classified into three categories depending on the presence or absence of the membrane.
Organelles without Membrane: Cell wall, Ribosomes, and Cytoskeleton. These are found in both prokaryotic and eukaryotic cells.
Single Membrane-bound Organelles: Vacuole, Golgi Apparatus, Lysosome, Endoplasmic Reticulum. These are found in the eukaryotic cells.
Double Membrane-bound Organelles: Nucleus, mitochondria, and chloroplast. These are found in the eukaryotic cells.
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Vacuole: Most of the plant cells contain one or more membrane-bound vesicles which are called vacuoles. Within the vacuole the cell sap is present, it is a water solution that is made up of salts and sugars that are kept at a high concentration by the active transport of ions through permeases present in the vacuole membrane. Proton pumps are present in order to maintain the high concentrations of protons in the vacuole interior. These high concentrations cause the entry into the vacuole through the process of osmosis, which in turn expands the vacuole and generates hydrostatic pressure. This pressure is called turgor pressure, which compresses the cell membrane against the cell wall. Turgor pressure is the cause of rigidity found in living plant tissue.
Lysosome: Lysosomes are bound by a single phospholipid bilayer membrane. These vary in size and are formed by the fusion of the vesicles that are derived from Golgi with the endosomes that are derived from the cell surface. Enzymes that are known to be present within the lysosomes include hydrolases that can degrade proteins, lipids, glycolipids, nucleic acids, and glycoproteins. Hydrolases are the most active enzymes in the acidity maintained in the lysosomes. After the material is broken down, the lipids and the amino acids are transported across the lysosomal membrane by the permeases for use in the process of biosynthesis.
Microbodies: Microbodies are roughly spherical in shape, these are bound by a single membrane, and they are usually about one micrometre in diameter. There are several types, the most common of which is the peroxisome. Peroxisomes derive their name from the molecule hydrogen peroxide. It is a reactive intermediate that is found present in the process of a molecular breakdown which usually occurs in the microbody. Peroxisomes contain type II oxidases, these are the enzymes that use molecular oxygen in the reactions in order to oxidize the organic molecules.
Endoplasmic Reticulum: The Endoplasmic Reticulum is a cell organelle that consists of a network of membranous canals that are filled with fluid. They are the transport system of the cells that are involved in transporting materials throughout the cell.
There are two different types of Endoplasmic Reticulum:
Rough Endoplasmic Reticulum: These are composed of cisternae, tubules, and vesicles. These are found throughout the cell and thus these are involved in the manufacture of protein.
Smooth Endoplasmic Reticulum: These are the storage organelles that are associated with the production of the lipids and steroids and also these are responsible for detoxifying the cell.
Mitochondria: Mitochondria is called the powerhouse of the cell as they can produce energy-rich molecules for the cell. It is a double membrane-bound organelle that is sausage-shaped and is found in almost all eukaryotic cells.
Plastids: Plastids are large, membrane-bound organelles that contain pigments. Based on the type of pigments, plastids can be divided into three types:
Chloroplasts: Chloroplasts are double membrane-bound organelles that usually vary in their shape from a disc shape to spherical or discoid or oval or ribbon. They are present in the mesophyll cells of leaves and these are the pigments that are responsible for trapping the light energy for photosynthesis.
Chromoplasts: The chromoplasts that include fat-soluble and carotenoid pigments like xanthophylls, carotene. These can provide the plants with their characteristic colour such as yellow, orange, red, etc.
Leucoplasts: Leucoplasts are colourless plastids that can store nutrients. Amyloplasts help to store carbohydrates, aleuroplasts can store proteins, and elaioplasts can store oils and fats.
Ribosomes: Ribosomes are the non-membrane-bound organelles and these are the important cytoplasmic organelles that are found in close association with the endoplasmic reticulum. Ribosomes are found in the form of tiny particles in a large number of cells and are mainly composed of RNA and protein. These are named the 70s and 80s. Ribosomes are either present within the endoplasmic reticulum or are freely traced in the cytoplasm of the cell. The primary function of the ribosomes is involved in the synthesis of proteins in all the living cells that can ensure the survival of the cell.
Golgi Apparatus: Golgi Apparatus is also termed as Golgi Complex. It is a membrane-bound organelle that is mainly composed of a series of flattened and stacked pouches called cisternae. This organelle is primarily responsible for the transportation modification, and packaging of proteins and lipids to targeted destinations. Golgi Apparatus is present within the cytoplasm of a cell and is present in both plant and animal cells.
Nucleus: The nucleus is a double-membraned organelle that is found in all the eukaryotic cells. It is the largest organelle that functions as the control centre of cellular activities and acts as the storehouse of DNA. The structure of the nucleus is dark and round in nature and it is surrounded by a nuclear membrane. It is a porous membrane and it forms a wall between the cytoplasm and the nucleus. Within the nucleus, there are certain tiny spherical bodies called the nucleolus. It also carries another essential structure called chromosomes.
Chromosomes: Chromosomes are thin and thread-like structures that carry another important structure called a gene. Genes are considered as a hereditary unit in organisms where it helps in the inheritance of traits from one generation to another. Hence, the nucleus can control the characters and functions of cells in our body. The primary function of the nucleus is to monitor cellular activities such as metabolism and growth by using genetic information from DNA. Nucleoli present in the nucleus are responsible for the synthesis of protein and RNA.
Endocytosis and Exocytosis
Endocytosis is the process of capturing a substance or a particle by engulfing it along with the cell membrane. At this point, a vesicle or a membrane-bound sac pinches off and pushes the substance into the cytosol. There are two kinds of endocytosis:
The phagocytosis process or cellular eating process occurs when the dissolved materials enter the cell. The plasma membrane can engulf the solid material and forms a phagocytic vesicle.
Pinocytosis or the process of cellular drinking occurs when the plasma membrane folds inward to form a channel to allow the dissolved substances to enter the cell. When this channel is closed the liquid is encircled within a pinocytic vesicle.
Exocytosis describes the process of fusion of vesicles with the plasma membrane and releases the contents to the outside of the cell. Exocytosis can occur when a cell produces the substances for the purpose of export of the protein, or when the cell can get rid of a waste product or a toxin. The newly formed membrane proteins and membrane lipids are moved on to the top of the plasma membrane by the process of exocytosis.
Cell biology is associated with both prokaryotic and eukaryotic cells and it can be divided into many sub-topics that include the study of the cell cycle, cell metabolism, biochemistry, cell communication, and cell composition. The study of cells can be performed using several techniques such as cell culture and cell fractionation. Both endocytosis and exocytosis processes are active transport processes. The active transport process is the energy-requiring process that involves the pumping of molecules and ions across membranes against a concentration gradient.