General Biology Chapter 2 Structure of the Cell Mohammed Al-Gayyar - 10 - Autumn 2012 General Biology Introduction to the cells The word cell comes from the Latin cellula, meaning "a small room". The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some notes about cells should be kept in mind: § Nothing less than cell can be called living: The vital functions of an organism occur within cells. All cells come from preexisting. Like ourselves, the individual cells that form our bodies can grow, reproduce, process information, respond to stimuli and carry out an amazing array of chemical reactions. These abilities define life. Even simple unicellular organisms exhibit all the hallmark properties of life, indicating that the cell is the fundamental unit of life. § The Diversity: Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). Cells come in an amazing variety of sizes and shapes. Some move rapidly and have fast-changing structures. Others are largely stationary and structurally stable. Oxygen kills some cells but is an absolute requirement for others. § Similar basic chemistry: Despite the extraordinary diversity of plants and animals, all living things are fundamentally similar inside. Cells resemble one another to an astonishing degree in the details of their chemistry and sharing the same machinery for the most basic functions. All cells are composed of the same sorts of molecules that participate in the same types of chemical reactions. § Invention of the light microscope led to the discovery of cells: The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he in 1665 when he compared the cork cells he saw through his microscope to the small rooms. The name cell stuck even though the structures Hooke described were only the cell walls that remained after the living plant cells inside them had died. Types of cells: The biological universe consists of two types of cells: Mohammed Al-Gayyar - 11 - Autumn 2012 General Biology § Prokaryotic cells consist of a single closed compartment that is surrounded by the plasma membrane, lacks a defined nucleus, and has a relatively simple internal organization. Examples: Bacteria and Blue-green algae. § Eukaryotic cells: unlike prokaryotic cells, contain a defined membrane-bound nucleus and extensive internal membranes that enclose other compartments. The region of the cell lying between the plasma membrane and the nucleus is the cytoplasm, comprising the cytosol (aqueous phase) and the organelles. Eukaryotes comprise all members of the plant and animal kingdoms, including the fungi, which exist in both multicellular forms (molds) and unicellular forms (yeasts). Eukaryotes Prokaryotes Complex in structure, with nuclei and more complex in structure, with nuclei and membrane-bound organelles membrane-bound organelles Large (100 - 1000 µm) Small (1-10 µm) DNA in nucleus, bounded by membrane DNA circular, unbounded Genome consists of several chromosomes Genome consists of single chromosome Sexual reproduction common, by mitosis and Asexual reproduction common, not by mitosis meiosis or meiosis Mitochondria and other organelles present No general organelles Most forms are multicellular Most forms are singular Aerobic Anaerobic Mohammed Al-Gayyar - 12 - Autumn 2012 General Biology Biological MembraneMembranessss Membranes are the outer boundary of individual cells and of certain organelles. Plasma membranes are the selectively permeable outermost structures of cells that separate the interior of the cell from the environment. Certain molecules are permitted to enter and exit the cell through transport across the plasma membrane. Components of biological membranes: All cell membranes are composed of the same materials: 1. Lipids Lipids are the most abundant type of macromolecule present. Plasma and organelle membranes contain between 40% and 80% lipid. There are three types of lipids are found: § Phospholipids: The most abundant of the membrane lipids are the phospholipids. They are polar, ionic compounds that are amphipathic (have both hydrophilic and hydrophobic components). The hydrophilic or polar portion is in the “head group”. Within the head group is the phosphate and an alcohol that is attached to it. The hydrophobic portion of the phospholipid is a long, hydrocarbon (structure of carbons and hydrogens) fatty acid tail. While the polar head groups of the outer leaflet extend outward toward the environment, the fatty acid tails extend inward. § Cholesterol: Another major component of cell membranes is cholesterol. An amphipathic molecule, cholesterol contains a polar hydroxyl group as well as a hydrophobic steroid ring and attached hydrocarbon. Cholesterol is dispersed throughout cell membranes, intercalating between phospholipids. Mohammed Al-Gayyar - 13 - Autumn 2012 General Biology Its polarhydroxyl group is near the polar head groups of the phospholipidswhile the steroid ring and hydrocarbon tails of cholesterol areoriented parallel to those of the phospholipids. Cholesterolfits into the spaces created by the kinks of the unsaturatedfatty acid tails, decreasing the ability of the fatty acids to undergomotion and therefore causing stiffening and strengthening of themembrane. § Glycolipids: Lipids with attached carbohydrate (sugars), glycolipidsare found in cell membranes in lower concentration than phospholipids and cholesterol. The carbohydrate portion is always oriented toward the outside of the cell, projecting into the environment. Glycolipids help to form the carbohydrate coat observed on cells and are involved in cell- to-cell interactions. 2. Proteins While lipids form the main structure of the membrane, proteins arelargely responsible for many biological functions of the membrane. The types of proteins within a plasma membrane vary depending on the cell type. However, all membrane proteins are associated with membrane in one of three main ways: § Transmembrane proteins: They are embedded within the lipid bilayer of the membrane with structures that extend from the environment into the cytosol. All trans membrane proteins contain both hydrophilic and hydrophobic components. These proteins are oriented with their hydrophilic portions in contact with the aqueous exterior environment and with the cytosol and their hydrophobic portions in contact with the fatty acid tails of the phospholipids. § Lipid-anchored proteins: They are attached covalently to a portion of a lipid without entering the core portion of the bilayer of the membrane. Both trans membrane and lipid- anchored proteins are integral membrane proteins since they can only be removed from a membrane by disrupting the entire membrane structure. § Peripheral membrane proteins: These proteins are located on the cytosolic side of the membrane and are only indirectly attached to the lipid of the membrane; they bind to other proteins that are attached to the lipids. Mohammed Al-Gayyar - 14 - Autumn 2012 General Biology Structure of biological membranes: The proteins and lipids of a cellular membrane are arranged in a certain way to form a stable outer structure of the cell. The membrane components, including lipids and proteins, are not fixed rigidly into a particular location. Both can exhibit several types of motions. Membrane proteins can also move laterally and can rotate. Despite its fluidity, the membrane structure is very stable and supportive for the cell. The arrangement of the phospholipids provides the basic structure which is then augmented by cholesterol, with functional roles played by proteins. The biological membranes have the following characters: § Bilayer arrangement: Membrane phospholipids are oriented with their hydrophobic fatty acid tails facing away from the polar, aqueous fluids of both the cytosol and the environment. The hydrophilic portions of the phospholipids are oriented toward the polar environment. Two layers of phospholipids are required to achieve this structure. § Asymmetry: The fatty acid tails of all the phospholipids are structurally very similar to each other. Some phospholipids are found on the outer leaflet while others are more commonly seen on the inner leaflet. In addition, glycolipids are differentially arranged as well and are always on the outer leaflet with their attached carbohydrate projecting away from the cell. § Fluid mosaic model: The membrane is described as a fluid, owing to the ability of lipids to diffuse laterally. The overall structure is likened to a flowing sea. Membrane proteins are dispersed throughout the membrane. Many of the membrane proteins retain the ability to undergo lateral motion and are likened to icebergs floating within the sea of lipids. Mohammed Al-Gayyar - 15 - Autumn 2012 General Biology Organelles Organelles are complex intracellular locations where processes necessary for eukaryotic cellular life occur. Most organelles are membrane-enclosed structures. Their membranes are composed of the same components asplasma membranes that form the outer boundaries of cells. Together with the cytosol (liquid portion of the cytoskeleton), the organelles help to form the cytoplasm, composed of all materials contained within the boundaries of the plasma membrane. Organelles do not float freely within the cytosol but are interconnected and joined by the framework established