CH 03 CELLS: THE LIVING UNITS This chapter provides a review of critical information regarding cells the basic units of structure and function of all living things. CELL THEORY The cell theory resulted from the work of many researchers over many years. The theory summarizes the known research of the time in four statements: The cell is basic unit of structure and function of all life forms The activity of a life form is dependent upon the activities of its cell(s), Subcellular (internal) structures provide the mechanism for each cell s particular activities The continuity of life has a cellular basis (living things come from other living things and do not arise spontaneously from non-life forms) Research leading to the cell theory includes: Robert Hooke Schleiden and Schwann Rudolf Virchow INFORMATION REGARDING EUKARYOTIC CELLS They have a very complex and well-coordinated structure. There are approximately 200 types within the human body. Cell length varies from 200 µm to more than one m. There is great variation in their shape, form and function. All cells, however, do possess common structures and perform similar operations. Two basic cell designs exist: Prokaryote Eukaryote 19
A GENERALIZED EUKARYOTIC ANIMAL CELL MODEL These cells contain protoplasm, which includes membrane-associated/bound structures termed organelles. Protoplasm Nucleoplasm Cytoplasm Organelles NUCLEUS Nuclear Envelope Nucleoplasm Centrosome Chromatin Chromosomes Histones Nucleoli Sing: nucleolus CYTOPLASMIC ORGANELLES Mitochondria Golgi Apparatus Lysosomes Proteasomes Peroxisomes Ribosomes ER Smooth ER 20
Rough ER Cytoskeleton Microtubules Microfilaments Tubulin protein Actin protein CELL MEMBRANE (PLASMALEMMA, PLASMA MEMBRANE) The cell membrane is a selectively permeable regulatory boundary. It is a dynamic, fluid entity. Its design is described as fluid mosaic (Singer and Nicholson, 1972). The membrane is composed of these materials: a double layer of phospholipids with hydrophilic polar heads and hydrophobic non-polar tails. It contains cholesterol, which is a steroid lipid ring structure that helps stabilize the phospholipid bilayer. It has various proteins (integral and peripheral). Some of the functions of the cell membrane include: Providing a protective barrier between cell and its environment. Maintenance of a population of special proteins that allow cells to identify one another and recognize invaders. Provision of channels and other mechanisms for communication and/or attachment with other cells. Selective permeability, which allows it to regulate everything that enters and/or leaves the cell. Membrane proteins can be classified based upon their location or their function: Location Peripheral Integral Function Anchoring Proteins Receptor Proteins Aquaporin Proteins Recognition Proteins Enzymes 21
Carrier Proteins Channel Proteins Leak Channels Gated Channels Specializations of the plasma membrane include mechanisms for attachment, communication and interactions between the cell and the environment. These specializations include: Glycocalyx Microvilli Sing: microvillus Tight Junctions Gap Junctions Desmosomes CAMS Cell Adhesion Molecules Contact Signaling Receptors Chemical Electrical MEMBRANE TRANSPORT SYSTEMS The cell membrane is selectively permeable due to its construction and can therefore control passage of substances in as well as out. There are two major categories of transport: Passive Transport Diffusion Facilitated Diffusion Filtration Osmosis Cellular Environments 22
Isotonic Hypertonic Hypotonic Active Transport Exocytosis Endocytosis Phagocytosis Pinocytosis Receptor-Mediated 23