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A cell is a form of the miniature factory that encloses a distinct chemical system that is involved in structural maintenance, homeostasis, energy generation and reproduction. Every organism is made of these structural living units. The cell divides and produces more cells and assists in the growth of the organism. There are two types of cells, eukaryotic and prokaryotic. Eukaryotic cells are characterised by a membrane-bound nucleus enclosing the genetic material. The Eukaryotic cells can be divided into three main parts, plasma membrane, cytoplasm, and nucleus (Cooper & Hausman, 2000).
Plasma Membrane
The plasma membrane separates the intracellular fluid from the extracellular fluid. It encloses all the cellular organelles and act as a limiting membrane for the cell. It consists of the double layer made up of lipids and phospholipids, and proteins are embedded in this lipid bilayer (Lodish et al., 2000). Plasma membrane is also termed as cell membranes that also surround the various organelles where it plays significant roles in living processes. The membranous organelles are mitochondria, endoplasmic reticulum, Golgi apparatus, plastids and lysosomes as well as chloroplast of plant cells. The non-membranous organelles include ribosomes, centrioles, and cytoskeleton components as well as nucleoli. The phospholipid cell membranes embed several proteins that are enzymes, hormone receptors, and cell recognition proteins or antigens. Cell membrane plays a significant role in cell to cell recognition. Membrane proteins serve as a carrier and facilitates the transportation of many nutrients and other proteins inter and intracellularly across the membrane. This transportation occurs either via active or passive diffusion through various channels such as the sodium-potassium pump. Selective diffusion of material develops a membrane potential which enforces further diffusion processes. Cell membranes are important for maintaining the homeostatic balance inside and outside the cells and organelles. It helps in contact signaling, and communication and chemical signaling through G-receptors (Marieb & Hoehn, 2007).
Cytoplasm
Cytoplasm is a cellular gel that consists of three main components, cytosol, organelles, and inclusions. Cytosol is a semi-transparent and viscous fluid with colloidal properties. Various macro nutrients like sugar protein are dissolved in this liquid. The organelles embedded in the cytosol are metabolic machinery and each of it that performs specific functions. The inclusions are the chemical substances that may be present in few cells, such as glycogen granules in liver, and lipid droplets.
Cytoplasmic Organelles
The cytoplasmic organelles are specialized structures without a membrane. Below are the list of cytoplasmic organelles.
Mitochondria: Mitochondria are called the powerhouse of the cell. The extent of mitochondria in a cell determines the energy needs of the cell. Mitochondria is a two membranous structure with a smooth outer layer and folded inner layers that form shelf like crusts into the mitochondrial matrix. The main function of mitochondria is to produce energy and generation of ATP. It is a site of aerobic cellular respiration.
Ribosomes: Ribosomes are small granules made of proteins and multiple RNA types or rRNA. Two globular subunits combine themselves to make a single ribosomal unit. It is a site of protein synthesis. Some ribosomes are free in the cytoplasm while few of them attach themselves to the membrane forming a rough endoplasmic reticulum.
Chloroplast: Chloroplasts are found only in eukaryotic plant cells that are also known as food producers of the cell. These are double layered organelles containing chloroplast substance. Its main function is to perform photosynthesis in which the light energy of the Sun is converted into the glucose which is consumed and stored in the form of food by the plants.
Endoplasmic Reticulum (ER): ER is a system of interconnected tubes and parallel membranes that enclose the fluid-filled cavities or cistern. It can be categorised into two types of ER, rough and smooth. On the rough ER, the ribosomes are studded, and the proteins are enclosed in the ER vesicles that are transported to Golgi bodies. Smooth ER is the tubules and sacs containing system without ribosomes. The external site of rough ER is involved in phospholipid synthesis, while the smooth ER is the site of steroid and lipid synthesis and metabolism as well as involved in drug detoxification (Marieb & Hoehn, 2007).
Golgi apparatus: Golgi apparatus contains stacked flattened membranes that are associated with small vesicles. It acts as a traffic operator for various cellular proteins. Its vesicles are located near the nuclear wall. Inside Golgi apparatus, the proteins are modified that include trimming and the addition of some groups (phosphate) to the protein. The Golgi apparatus acts as a package for proteins and digestive enzymes to specific sites.
Peroxisomes: Peroxisomes are round shaped membranous sacs. It contains the powerful enzymes such as catalases and oxidases. The main function of peroxisomes is the neutralization of free radicals that can harm the biological environment. In liver and kidney peroxisomes are present in a greater number due to the high demand of detoxification. They are also involved in energy metabolism.
Lysosomes: Lysosomes are round shaped membranous organelles like peroxisomes. They contain inactive acid hydrolases and activated digestive enzymes. These are found in phagocytes abundantly; that is a significant part of immune system. Lysosomal enzymes can digest all types of biological molecules. It can break down the bone and release the calcium ions into the blood.
Cytoskeleton
Cytoskeleton refers to the rod-like structures that move through cytosol and accessory proteins. These are like the bones and ligaments of the cell that help in cell movements and support the cellular structures. These are of three types.
Microfilaments: These are the thinnest semi-flexible strands of the cytoskeleton that are made of protein actin. These form a specific terminal web in each cell, connected to the cytoplasmic site of the plasma membrane. These are also involved in cell movement and help in maintaining the cell shape.
Intermediate Filaments: These filaments are complex fibres that are composed of insoluble fibril tetramer protein units. These possess an average size of diameter and present the most stable form of the cytoskeleton. Their main function is to resist the mechanical forces exerted on the cell wall.
Microtubules: These have the largest diameter with a hollow structure. These tubes are made of tubulins protein subunits. Microtubules mostly extend from an area near the nucleus that is also called centrosome. The main function of microtubules is to support the cell structure; it is also involved in cellular and intracellular movements such as in cilia, flagella, and centrioles
Centrosoms: Centrosome anchors the microtubules and act as a microtubule organizing centre. They are involved in forming the mitotic spindle during cell division (Marieb & Hoehn, 2007).
Nucleus
The nucleus is the largest organelle in the cell which is surrounded by the nuclear membrane. It contains fluid nucleoplasm, nucleoli, and chromatin.
Nuclear Envelope: It is a double membrane structure which is separated by fluid-filled spaces; its outer layer is attached to rough ER, and inner face is lined by a nuclear lamina. It is made of rod-shaped lamins protein that plays a significant role in DNA organization inside the nucleus. It encloses a gel-like nucleo plasm fluid in which other nuclear components are suspended such as proteins, and RNA.
Nucleoli: The nucleolus is the dense non-membranous round shaped body embedded in the nucleoplasm. It is made up of proteins and ribosomal RNA. Mostly there are one or two nucleoli in each nucleus, but there may be numerous in one cell. Nucleolus is connected to the specific nucleolar region that encloses DNA. The nucleolus is the specific manufacturing site of ribosomal subunits.
Chromatin: Chromatin is a system of woven threads through the nucleoplasm. It is made up of three major components with 30% genetic material i.e. DNA, 60% of histone proteins and 10% RNA. The nucleosomes are the basic unit of chromatin that is made up of eight disc-shaped clusters of histone proteins that are connected by a DNA molecule. DNA constitutes the genes. DNA is attached to each nucleosome twice with a ribbon like pattern, and two such shapes are connected via linker DNA segments. Histone proteins facilitate the long packaging of DNA molecules as well as gene regulation. When a cell undergoes a division, the chromatin threads become condensed and form short coloured bodies called chromosomes (Marieb & Hoehn, 2007).
References
Cooper, G. M., & Hausman, R. E. (2000). The cell (pp. 725-730). Sunderland: Sinauer
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Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., & Darnell, J. (2000). Cell-
Marieb, E. N., & Hoehn, K. (2007). Human anatomy & physiology. Pearson Education
