Introduction to Cell Biology

Abstract

The cell membrane confines the contents of the cell. It consists of a continuous bilayer of phospholipid with the polar hydrophilic ends forming the outer and inner layers and the hydrophobic tails forming the central core of the bilayer. The hydrophilic heads on the two sides of the cell membrane are of different composition, those on the outside often being modified by glycosylation: a process that involves the addition of various sugar residues. Embedded in this lipid bilayer are proteins whose function can be classified as follows: Signal transduction. These proteins mediate the action of external ligands such as growth factors and neurotransmitters). These receptor proteins cross the cell membrane and may have intrinsic enzyme functions (such as the tyrosine kinase activity of the receptor for epidermal growth factor) or may be linked to other proteins such as G proteins (e.g., the muscarinic acetylcholine receptor) (Fig. 1). Cell-cell or cell-matrix contact. Specialized junctions between cells and between cells and the intercellular matrix occur, involving transmembrane proteins such as integrins and cadherins that interact on the outside with molecules such as fibronectin, and on the inside with molecules such as catenin, talin, and vinculin that act as intermediate links to the cell skeleton, which is made of actin fibers. These proteins form specialized cellular contacts, such as desmosomes and tight junctions. Carrier proteins. These are involved in the transport of small molecules and ions across the cell membrane (as in glucose, the Na1/K1 pump or the multidrug resistance pump). These proteins may be energy dependent or passive—if they are active, they usually utilize adenosine triphosphate (ATP) as a source of energy. These carrier proteins often transport other molecules at the same time in the opposite direction (Fig. 2). Channel proteins. These are involved in the transport of ions across the cell membrane. These function in a passive way and effectively are hydrophilic pores; they function more efficiently than carrier proteins and can carry ions more than 1000-fold faster. They are selective for certain ions and may be closed or open. The stimuli for opening these channels may be electrical current, ligand binding (as with acetylcholine binding to nicotinic receptors), or mechanical deformation. Intracellular organelles. These are also bounded by membranes and include rough and smooth endoplasmic reticulum (ER), Golgi apparatus, and nuclear membranes. These intracellular membranes compartmentalize the cell into functionally distinct units (Fig. 3) and are extensive, having a surface area 10- to 25-fold greater than the cell membrane itself. Intracellular proteins are synthesized on free cytosolic ribosomes and remain in the cytoplasm, but proteins for export begin their life by being synthesized on ribosomes that have special signaling molecules that dock with receptors on the ER, Golgi apparatus, lysosomes, etc., which mediate transport to their final destination. There are specialized areas of the cell membrane. These include the structure of the synapse or neuromuscular junction, which are dealt with elsewhere in the book. In addition, cells have small vesicles that can be involved in the storage of neurotransmitters. Calveolae are small vesicles formed by budding off the cell membrane. Clathrin-coated pits are other specialized areas of the cell membrane that are responsible for internalization of activated ligand-bound receptors, thereby reducing the intensity of the associated signaling cascade. In such areas of the cell, adapter proteins are found, which may act as signal transduction molecules. Figure 1 A schematic drawing of a G-protein-linked receptor. Receptors that bind protein ligands have a large, extracellular, ligand-binding domain formed by the part of the polypeptide chain shown in light green. Receptors for small ligands such as adrenaline (epinephrine) have small extracellular domains, and the ligand-binding site is usually deep within the plane of the membrane, formed by amino acids from several of the transmembrane segments. The parts of the intracellular domains that are mainly responsible for binding to trimeric G proteins are shown in orange, while those that become phosphorylated during receptor desensitization (discussed later) are shown in red. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429110269/6332914b-1a7a-44bc-933a-4b4cc047c467/content/fig2_1.tif" xmlns:xlink="http://www.w3.org/1999/xlink"/> Figure 2 Three types of carrier-mediated transport. The schematic diagram shows carrier proteins functioning as uniports, symports and antiports. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429110269/6332914b-1a7a-44bc-933a-4b4cc047c467/content/fig2_2.tif" xmlns:xlink="http://www.w3.org/1999/xlink"/> Figure 3 The major intracellular compartments of an animal cell. The cytosol, endoplasmic reticulum, Golgi apparatus, nucleus, mitochondrion, endosome, lysosome, and peroxisome are distinct compartments isolated from the rest of the cell by at least one selectively permeable membrane. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429110269/6332914b-1a7a-44bc-933a-4b4cc047c467/content/fig2_3.tif" xmlns:xlink="http://www.w3.org/1999/xlink"/>