EXTENDED LECTURE OUTLINE 3.1 What is a Cell? Organisms are composed of cells which are quite small and can be seen only under a microscope. The Cell Theory According to the cell theory, the cell is the basic unit of life, all living things are composed of cells, and new cells arise only from preexisting ones. Cell Size Cells are small to maintain an optimal surface areato-volume ratio. Microscopy Micrographs are photographs of objects obtained by using a microscope. The compound light microscope passes light rays through specimens and uses glass lenses to view objects. The transmission electron microscope passes electrons through a specimen and projects images onto a fluorescent screen or photographic film. The scanning electron microscope provides a three-dimensional view of the surface of an object. 3.2 How Cells are Organized Biologists classify cells into two broad categories-the prokaryotes and the eukaryotes. Both types of cells have a plasma membrane, which is selectively permeable, and cytoplasm. Internal Structure of Eukaryotic Cells The most prominent organelle within the eukaryotic cell is a nucleus. Prokaryotic cells lack a nucleus. Evolutionary History of the Animal Cell The first cells to arise were prokaryotic cells, lacking a membrane-enclosed nucleus. Prokaryotic cells today are represented by the bacteria and archaea. The eukaryotic cell, which does have a nucleus, is believed to have evolved from the archaea. During evolution, the plasma membrane may have invaginated to form a nucleus. Other organelles in eukaryotic cells may have arisen by engulfing prokaryotic cells. 3.3 The Plasma Membrane and How Substances Cross It The plasma membrane is a bilayer of phospholipids with embedded or attached proteins. The fluidmosaic model is a working description of membrane structure. Glycoproteins and glycolipids serve as identification markers. Embedded proteins may serve as hormone receptors, transport channels, and enzymes in metabolic reactions. Plasma Membrane Functions The plasma membrane keeps a cell intact. It is selectively permeable, and therefore regulates the entrance and exit of molecules. Diffusion Diffusion is the random movement of molecules from an area of higher concentration to one of lower concentration. Osmosis Osmosis is the diffusion of water across a plasma membrane. Solutions that cause cells to swell due to the intake of water are considered hypotonic. Solutions that cause cells to shrink are considered hypertonic. In an isotonic solution, there is the same concentration of solutes and water on both sides of the membrane and the cells maintain their size and shape. Facilitated Transport A protein carrier helps a molecule across the membrane with the concentration gradient without using energy. Active Transport A protein carrier transports a molecule across the concentration gradient and requires energy. Endocytosis and Exocytosis Endocytosis occurs as the plasma membrane forms a vesicle around a particle as it enters the cell. Phagocytosis and Pinocytosis are forms of endocytosis. Exocytosis is the reverse process. 3.4 The Nucleus and Endomembrane System The Nucleus The nucleus stores genetic information in the form of chromatin, a combination of DNA molecules and proteins. The nucleus also contains nucleoli where ribosomal RNA is produced. The nucleus is separated from the cytoplasm by a double membrane known as the nuclear envelope which is continuous with the endoplasmic reticulum. Ribosomes Ribosomes are a mix of rRNA and proteins. They are the sites of protein synthesis in the cytoplasm. The Endomembrane System The endomembrane system consists of the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vesicles. The Endoplasmic Reticulum The endoplasmic reticulum (ER) is a series of membranes that form tubular channels within the cytoplasm. Rough ER (with ribosomes) specializes in protein synthesis. Smooth ER produces different compounds in different cells. The Golgi Apparatus The Golgi apparatus, a stack of flattened vacuoles with vesicles near the edges, packages, stores, and distributes the proteins the ER produces. Lysosomes The Golgi apparatus produces lysosomes, which contain hydrolytic enzymes that digest unwanted materials inside the cell, including worn-out cell parts. 3.5 The Cytoskeleton, Cell Movement, and Cell Junctions Microtubules, actin filaments, and intermediate filaments form a cytoskeleton that serves as a framework for the cell's interior. Cilia and Flagella Cilia and flagella are hair-like projections of cells responsible for locomotion. Each cilium and flagellum shares the same organization of microtubules within a plasma membrane. Motor molecules help them move. Junctions Between Cells Adhesion junctions serve to mechanically attach adjacent cells. In tight junctions, connections between the plasma membrane proteins of neighboring cells produce a zipperlike barrier. Gap junctions serve as communication portals between cells. 3.6 Mitochondria and Cellular Metabolism Mitochondria are organelles bounded by a double membrane. The inner membrane is folded into cristae. The gel-like material between the cristae is matrix. Mitochondria convert the energy stored in glucose into ATP molecules in a process called cellular respiration. Cellular Respiration and Metabolism Cellular respiration is a very important component of metabolism which includes all the chemical reactions that occur in a cell. Chemical reactions in a cell are organized into metabolic pathways where every reaction has its own enzyme. Enzymes Enzymes are named for their substrates and have an active site specific for the substrate. Many enzymes have nonprotein molecules, called coenzymes, to assist them. Some coenzymes, like NAD+, remove hydrogen from substrates. Cellular Respiration The breakdown of glucose into carbon dioxide and water requires three subpathways: glycolysis, the citric acid cycle, and the electron transport system. Glycolysis needs no oxygen and takes place in the cytoplasm of the cell. During glycolysis, glucose breaks down into two molecules of pyruvate. If oxygen is present, pyruvate can enter the mitochondrion and the citric acid cycle. If no oxygen is present, fermentation occurs. The citric acid cycle is a cyclical series of enzymatic reactions that occurs in the matrix of the mitochondria and completes the breakdown of glucose. Carbon dioxide is released and hydrogen atoms are carried away by NADH. NADH molecules from glycolysis and the citric acid cycle deliver electrons to the electron transport chain, a group of carrier proteins embedded in the cristae of a mitochondrion. Oxygen serves as the final acceptor of the electrons at the end of the chain. Fermentation When no oxygen is present for pyruvate to enter the citric acid cycle, pyruvate is converted to lactic acid through fermentation, producing few ATP.