Cytology - Cell Structure & Function PDF

## The Cell ### Chapter 2 - The body is composed of three different elements: cells, extracellular substance, and body fluids. - The cell is the basic structural and functional unit of the body. - It is not a static entity in life, but its organelles, chemical constitution, and morphology are in c...

## The Cell ### Chapter 2 - The body is composed of three different elements: cells, extracellular substance, and body fluids. - The cell is the basic structural and functional unit of the body. - It is not a static entity in life, but its organelles, chemical constitution, and morphology are in continuous changes. - The cell consists of the nucleus and the cytoplasm (fig. 2.1). ### The cytoplasm - The cell is partially isolated from the extracellular environment by the cell membrane. - The cell membrane surrounds a viscous fluid called the cytoplasm, composed of water, salts, organic molecules and many enzymes that catalyze reactions. - Numerous structures: organelles, inclusions, and usually one nucleus, are suspended in the cytoplasm. - The cytoplasm can be cytogel (gel-like) or cytosol (liquid) depending on the activity of the cell. - Living components of the cell are called organelles while non-living components are called inclusions. ### Cell Membrane - All cells are enveloped by 7.5 nm thick cell membrane, visible only with the EM. - The cell membrane and membranous organelles are composed mainly of proteins, lipids (phospholipids and cholesterol) and small amounts of carbohydrates. - By EM, the cell membrane shows the so-called unit membrane, a structure of two dense lines (each about 2.5 nm thick) separated by light intermediate line (fig. 2.2). ### Fluid Mosaic Model - According to the fluid mosaic model of Singer and Nicholson (fig. 2.3); the light line of the unit membrane consists of lipid molecules organized into two layers. - Each lipid molecule has a head (hydrophilic) and tail (hydrophobic). In the two layers, the heads of the molecules are directed outward and the tails inward. - The two dense lines represent the protein molecules aggregated on the outer and inner surfaces of the lipid. - Although looking trilaminar in routine EM, it behaves as if it comprises a double layer of lipid molecules, in which proteins are distributed asymmetrically in a mobile mosaic pattern. ### Transport by changes in cell membrane - Some proteins span the width of the membrane, and may control rates of transport by changes in their conformation. - Others, as enzymes, receptors, or adhesion molecules, etc. have active domains (parts) at the surface held in correct position by intramembranous domains embedded in the lipid layer, and intracellular domains to engage in events inside the cell. ### Integral and peripheral proteins - The proteins are divided into two groups: integral proteins, which are passing across the lipid bilayer, and peripheral proteins on both surfaces. - The carbohydrate portions of glycoproteins and glycolipids project from the external surface, forming the cell coat or glycocalyx. They are important components of specific molecules called receptors. ### Plasma Membrane - The plasma membrane plays an important role in the way a cell interacts with its environment. It acts as a selective barrier that regulates the passage of certain materials into and out of the cell. Membranes also carry out a number of specific recognition and regulatory functions. ### Endocytosis - Endocytosis is a process in which the cell takes material from the extracellular fluid across the cell membrane. It involves folding and fusion of the membrane to form cytoplasmic membranous structures (vesicles or vacuoles) containing the material. - Endocytosis includes three processes (fig. 2.4): 1. **Pinocytosis:** - It is the uptake of fluids by invagination of the cell membrane to form a pit. - The pit pinches off inside the cell when the cell membrane fuses and forms a pinocytotic vesicle containing the fluid. - Transcytosis is when soluble substances pass through the cell from one pole to another, enter by endocytosis and exit by exocytosis. 2. **Phagocytosis:** - It involves the extension from the cell of large folds called pseudopodia which engulf particles, for example bacteria, and then internalize this material into a cytoplasmic vacuole or phagosome. - Phagosomes formed by endocytosis, are called heterophagosomes, while vacuoles containing damaged cell organelles are called autophagosomes. 3. **Receptor-mediated endocytosis:** - It is the uptake of specific protein molecules (ligands) from the extracellular fluid. - It depends upon the presence of a receptor, for each ligand, in the plasma membrane. - Molecules of the receptor, bound by their ligands, move within the membrane to form a shallow depression (or pit) of the cell surface. - Other protein molecules, called clathrin, bound themselves to the cytoplasmic surface of the pit (now called coated pit). - The coated pits invaginate and pinch off from the cell membrane to become coated vesicles. - The coated vesicles rapidly lose their clathrin coat and fuse together to form a larger vacuole called early endosome, which fuses later with a primary lysosome. - Acidity of the endosome separates the ligands from their receptors, which return to the plasma membrane by vesicles that bud off the endosome. ### Exocytosis - Cytoplasmic vesicles fuse with the plasma membrane, resulting in the release of their contents into the extracellular space without compromising the integrity of the plasma membrane (fig. 2.4). - Membrane trafficking: is a continuous process of separation of portions of the cell membrane caused by endocytosis and their return by recytosis (fig. 2.4). ### Endosomes - The vesicles originating from endocytosis fuse to form early endosomes under the cell membrane. - Composition of early endosome change progressively as more vesicles fuse and others separate from them. - As the early endosome moves deeper near the Golgi apparatus and nucleus, it develops into late endosome (fig. 2.4). - The membrane of endosomes contains ATP-driven H+ pumps that acidify their interior; early endosomes are slightly acidic (pH 6.5) than the neutral (pH 7.2) cytoplasm, while late endosomes are more acidic (pH 5.5). - Proteins are sorted in early endosomes into two ways: - Some proteins (e.g. receptors) are recycled to the cell membrane by vesicles bud off from the early endosomes. - Others (ligands) remain in the late endosomes that fuse with lysosomes for degradation of their contents. ### Endoplasmic Reticulum (ER) - The endoplasmic reticulum is a system of membranous channels within the cytoplasm. These channels take various forms; including cisternae (flattened sacs), tubules, and vesicles (fig. 2.5). - There are two types of channel systems; those with attached ribosomes are called the rough endoplasmic reticulum (rER), others without ribosomes are called the smooth endoplasmic reticulum (SER). ### Rough Endoplasmic Reticulum (rER) - It synthesizes proteins, by its attached ribosomes. Some carbohydrates are also synthesized by the rER. - Transport vesicles, carrying the product, bud from the rER and fuse with Golgi apparatus. ### Smooth Endoplasmic Reticulum (SER) - This associates with carbohydrate metabolism and many other metabolic processes, including detoxification and synthesis of lipids, cholesterol, and other steroids. - Highly specialized sER is present in striated muscle cells. It stores calcium ions necessary to initiate muscle contraction. ### Ribosomes - Ribosomes are granules about 15 nm in diameter, composed of protein and rRNA. Each ribosome is made of two, large and small, subunits. - Ribosomes are found as single monosomes or polysomes linked by mRNA. - The function of ribosomes is the synthesis of proteins by translating mRNA into a polypeptide chains. - Ribosomes may be attached to membranes (rER and nuclear envelope) or lie free in the cytoplasm (fig. 2.6): - Attached ribosomes synthesize proteins that are transported from the rER by transport vesicles to the Golgi apparatus. Then, secretory vesicles bud from the Golgi apparatus; either released from the cell by exocytosis, or form lysosomes. - Free ribosomes synthesize proteins that remain in the cytoplasm to be used inside the cell. ### Golgi complex - The Golgi complex is a membranous organelle has a cup shape consisting of a stack of flattened cisternae with vesicles and vacuoles. - There is usually one Golgi apparatus in the cell, but in cells with high secretory activities, e.g. fibroblasts, there are two or more groups of Golgi stacks, and in liver cells up to 50. - It consists of; a convex side near the nucleus called the cis-face, several medial cisternae, and a concave side called the trans-face (fig. 2.7). - Proteins synthesized in the rER are carried by transporting vesicles fuse with the cis-face and then by vesicles that bud from one cisterna to another until they separate from the trans-face. - Within the Golgi stack, proteins undergo a series of chemical modifications; glycosylation, sulphation, and phosphorylation.