Cells: The Living Units Chapter 3 – Part 1 BACK

Cell Theory Cells are the basic structural and functional units of life. Activity of an organism depends on the activities of its cells. Biochemical activities of a cell are dictated by their organelles. Continuity of life has a cellular basis. Characteristics of Cells Vary greatly in their size, shape, and function. All are composed primarily of CHONS All have the same basic parts and some common functions. A generalized human cell contains the plasma membrane, the cytoplasm, and the nucleus

The Plasma Membrane: Structure Inward-facing and outward-facing surfaces of the plasma membrane differ in the kinds and amounts of lipids they contain. Glycolipids are found only in the outer membrane. Lipid rafts only in outer membrane, assumed to function in cell signaling. Integral proteins are firmly inserted into plasma membrane. Most integral proteins are transmembrane proteins that span the entire width of the membrane and are involved with transport as channels or carriers. Fluid Mosaic Model Plasma membrane is composed of a double layer of phospholipids embedded with small amounts of cholesterol and proteins. Peripheral proteins are not embedded in plasma membrane, but attach to integral proteins or to phospolipids. Peripheral proteins may function as enzymes or mechanical functions of cell. The glycocalyx is the fuzzy, sticky, carbohydrate-rich area surrounding the cell. Phospholipid bilayer is composed of two layers of phospholipids lying tail to tail, with their polar heads exposed to water inside and outside the cell. animation

Specializations of the Plasma Membrane Most body cells are bound together using glycolipids, specialized interlocking regions, or specialized membrane junctions. Gap junctions are a communication junction between cells that allows substances to pass between adjacent cells. Tight junctions are a type of membrane junction in which integral proteins on adjacent cells fuse together to form an impermeable junction in order to prevent molecules from passing through the extracellular space between cells. Desmosomes are mechanical couplings that are scattered along the sides of adjoining cells that prevent their separation and reduce the chance of tearing when a tissue is stressed. Microvilli – increase surface area for absorption (intestines, kidneys) Layers of the skin Cardiac Conduction System Blood Brain Barrier

Plasma Membrane: Functions Separates intracellular fluids from extracellular fluids Membrane Transport – selectively permeable CAM’s Generating & Maintaining a Resting Membrane Potential – nerve and muscle cells Functions of Membrane Proteins Transport Enzymatic activity Receptors for signal transduction Intercellular adhesion Cell-cell recognition Attachment to cytoskeleton and extracellular matrix

Cytoplasm Compostion The cytoplasm is the cellular material between the cell membrane and the nucleus, and is the site of most cellular activity. Contents of Cytoplasm Cytosol – largely water with dissolved protein, salts, sugars, and other solutes Inclusions – chemical substances such as glycosomes, glycogen granules, and pigment Cytoplasmic organelles – metabolic machinery of the cell Membranous - mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus Nonmembranous - cytoskeleton, centrioles, and ribosomes.

Organelles: Mitochondria Double membrane structure with shelflike cristae (internal matrix) Provide most of the cell’s ATP via aerobic cellular respiration Contain their own DNA and RNA Self replicating Protein synthesis of respiratory proteins Passed to offspring by female parent

Organelles: Ribosomes and Endoplamic Reticulum Granules containing protein and rRNA Smooth Endoplasmic Reticulum Tubules arranged in a looping network Catalyzes the following reactions in various organs of the body In the liver – lipid/cholesterol metabolism, detoxification of drugs In the testes – synthesis of steroid-based hormones In the intestinal cells – absorption, synthesis, and transport of fats Rough Endoplasmic Reticulum External surface studded with ribosomes Manufactures all secreted proteins Site of protein synthesis Free ribosomes synthesize soluble proteins Responsible for the synthesis of integral membrane proteins and phospholipids for cell membranes Membrane-bound ribosomes synthesize proteins to be incorporated into membranes Synthesized by the nucleolus in the nucleus In muscle – storage and release of calcium

Golgi Apparatus and Endomembrane System Stacked, flattened membranous sacs Functions in modification, concentration, and packaging of proteins Transport vessels from the ER fuse with the cis face of the Golgi apparatus Proteins then pass through the Golgi apparatus to the trans face Secretory vesicles Golgi stack and move to designated parts of the cell Vesicle Pathways Secretion vesicles New membrane vesicles Lysosome vesicle

Lysosomes Peroxisomes Abundant in liver and kidney cells Spherical membranous bags containing digestive enzymes Digest ingested bacteria, viruses, and toxins Degrade nonfunctional organelles Breakdown glycogen and release thyroid hormone Breakdown bone to release Ca2+ Secretory lysosomes are found in white blood cells, immune cells, & melanocytes Peroxisomes Abundant in liver and kidney cells Membranous sacs containing oxidases and catalases Catalase breaks down H2O2 --->H2O + O2 Detoxify harmful or toxic substances Neutralize dangerous free radicals Free radicals – highly reactive chemicals with unpaired electrons (O2–) Lack of enzyme causes ALD

Cytoskeleton structure and movement Microfilaments – Dynamic strands of protein actin Attached to the cytoplasmic side of plasma membrane Braces and strengthens cell surface Intermediate filaments Scaffolding of the cell Tough, insoluble protein fibers with high tensile strength (keratin) Microtubules -Determines overall shape of cell & distribution of organelles Make up centrioles which function in mitosis Makes up cilia and flagella

Motor Molecules Kinesin Dynein Attach to receptors on organelles and transport organelles Motion accomplished by Motor Molecules and cytoskeleton Protein complexes that function in motility Powered by ATP Cause Cilia and Flagellum Action

Centrioles Small barrel-shaped organelles located in the centrosome near the nucleus Pinwheel array of nine triplets of microtubules Form the bases of cilia and flagella Organize mitotic spindle during mitosis kinetochore

Nucleus Contains the genetic library with blueprints for nearly all cellular proteins Dictates the kinds and amounts of proteins to be synthesized Nuclear Envelope –double membrane Chromatin - Threadlike strands of DNA and histones Arranged in fundamental units called nucleosomes Form condensed, barlike chromosomes when nucleus starts to divide Outer membrane is continuous with the rough ER and is studded with ribosomes Pore complex regulates transport of large molecules into and out of the nucleus Nucleoli - Dark-staining spherical bodies within the nucleus Site of ribosome production

Rafts return Composed of sphingolipids and cholesterol Are concentrating platforms for cell-signaling molecules return