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Transcript
Chapter 7: A tour of the cell How we study cells Microscopes provide windows to the world of the cell Brightfield (unstained and stained) Phase contrast and Nomarski optics Fluorescence Confocal TEM and SEM Cell biologists can isolate organelles to study their functions Homogenization Differential centrifugation Low speed centrifuge Ultracentrifuge A panoramic view of the cell Prokaryotic and eukaryotic cells differ in size and complexity All cells have Plasma membrane Cytosol which makes up the cytoplasm Chromosomes Ribosomes Cells differ in Size (mycoplasma are the smallest at .1 to 1 micron; bacteria at 110 micron; eukaryotes at 10-100 micron) Geometric relationship between surface area and volume explains why most cells are microscopic Presence or absence of a nucleus and membrane defined organelles Internal membranes compartmentalize the functions of the eukaryotic cell Familiarize yourself with the diagrams of a representative animal cell (page 114) and a representative plant cell (page 115) The nucleus and ribosomes The nucleus contains a eukaryotic cells genetic library Nucleus Contains MOST of the genes (excludes mitochondrial and chloroplastic DNA) Nuclear membrane aka nuclear envelope Envelope is a double (2-lipid bilayers) membrane Perforated by nuclear pores Pores are lined with proteins, pore-complex proteins Nuclear side of envelope is line with a mesh of protein filaments aka nuclear lamina A nuclear matrix (like a cytoskeleton for the nucleus) of protein filaments maintains the shape of the nucleus Chromatin DNA is organized as chromatin (condenses prior to cell division as chromosomes) Each eukaryotic species has a characteristic # of chromosomes Nucleolus Assembles ribosome subunits Pass out to cytoplasm through the nuclear pores Ribosomes build a cell’s proteins Made of ribosomal RNA and protein Two pieces: large subunit and small subunit Two structurally and functionally identical types: free (located in the cytoplasm) bound (attached to rough ER) Free type makes proteins to be used in the cytoplasm (glycolysis enzymes) Bound type makes proteins to be inserted into membrane; or moved to and used in organelles like lysosomes; or secreted from the cell The endomembrane system Consists of: nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various vacuoles, and plasma membrane They are NOT identical in structure or function The ER manufactures membranes and performs other biosynthetic functions The inside of the ER is called the cisternal space and is continuous with the space between the two nuclear envelope layers Smooth ER No ribosomes attached Responsible for diverse metabolic functions Synthesis of lipids Metabolism of carbohydrates Detoxification of drugs (enzymes for adding OH) Storage of Ca++ in muscle cells Rough ER Polypeptides assembled on ribosomes and extend into cisternal space Protein folds into native structure in the cisternal space Protein modification (glycosylation) done by enzymes in the cisternal space Secretory proteins are isolated inside transport vesicles from the cytosol RER also makes phospholipids and assembles membranes Membrane proteins are inserted into the membrane The Golgi apparatus finishes, sorts and ships cell products RER transport vesicles travel to the Golgi Golgi membranes have a polarity, the cis face and the trans face Cis face is located near RER, receiving side of the Golgi Glycosylation is modified in the golgi Molecules are modified in stages as they pass through the golgi Trans face buds off vesicles with the finished product for transport to the plasma membrane Golgi also synthesizes many cellular polysaccharides Lysosomes are digestive compartments Membrane bounded sac of hydrolytic enzymes Can hydrolyze all the macromolecule groups Optimal pH for these enzymes is 5 Lysosomal membrane pumps H+ into the organelle to keep pH acidic Explain why enzymes are not active if one lysosome breaks open Why doesn’t a lysosome digest itself Programmed destruction of cells by its own lysosomes is important in the development of many multicellular organisms. Examples tadpole tail cells, finger web cells, insect metamorphosis Lysosomal storage diseases are inherited disorders of lysosomal metabolism (Pompe’s disease, Tay Sachs) Vacuoles have diverse functions in cell maintenance Vacuoles are larger than vesicles Many types Food vacuoles Contractile vacuoles Central vacuole Content of a vacuole is different than the Cytosol Other membranous organelles Mitochondria and chloroplasts are the main energy transformers of cells These organelles are semi-autonomous Grow and reproduce inside the cell (binary fission) Contain DNA and ribosomes Proteins for these are NOT made in RER but in cytosol ribosomes and their own ribosomes Mitochondria Sites of cellular respiration Found in nearly all eukaryotic cells (1-10 microns long) Number per cell is correlates with the cells metabolic rate Two membranes Outside is smooth Inner convoluted (cristae) Gives rise to intermembrane space And the matrix Respiratory enzymes spatially separated some in the matrix some on the inner membrane Chloroplasts Sites of photosynthesis Found in green plants and algae (2-5 micron long, lens shaped) Two membranes Outside is smooth Inner is smooth, small intermembrane space Thylakoids stacked inside the inner membrane Fluid around the thylakoid is the stroma Peroxisomes generate and degrade H2O2 in performing various metabolic functions Single membrane encloses enzymes that produce H2O2 and destroy H2O2. Metabolic functions Use O2 to break fatty acids to smaller molecules Detoxify alcohol (OH) by transferring H to O2 In plants (glyoxysomes) covert fats to sugar for a seeding before it has a leaf The cytoskeleton Providing structural support for the cell and function in cell motility and regulation Network of proteins throughout the cell to: Provide structure, shape and support Motility for the cell and cell organelles Regulation of biochemical activities Three main types of fibers (based on diameter) Microtubules (largest diameter, 25 nm) Composed of dimer protein tubulin (alpha and beta tubulin) Shape and support cell, move chromosomes during mitosis, tracks for motor molecules Specialized organelles are made of microtubules Centrosomes and centrioles Microtubules scan grow out of a centrosome Centrioles are only in animal cells Cilia and Flagella Responsible for cell motion (sperm, move fluid over cell surfaces) Cilia short and numerous, flagella longer and fewer Ultrastructure is common 9 doublets of tubulin in a ring with a central pair ( 9 + 2 pattern) Doublets are attached to each other by a pair of motor molecules, the protein dynein and protein cross links Anchored to the cell by the basal body, ring of 9 triplet tubulin, no central pair Microfilaments (smallest diameter, 7 nm) Composed of protein actin Function to bear tension (pulling) in the cell Major role in cell motility Example is muscle cells (page 131) Make the pseudopodia in amoeba Function in cytoplasmic streaming in plants Intermediate filaments (diameter 8-12 nm) More permanent structure is cells Holds the nucleus in place Cell surfaces and junctions Plant cells are encased by cell walls Prevents excessive uptake of water Primary cell wall is thin, followed by secretion of the middle lamella made of pectin Secondary cell way is thicker and glued to the middle lamella Wood is secondary cell wall The ECM of animal cells functions in support, adhesion movement and regulation Animal cells have extensive extracellular matrix (ECM) Mostly glycoproteins, most abundant is collagen Collagen is embedded in proteoglycan another glycoprotein Fibronectin attaches the ECM proteins to the plasma membrane Integrins are receptor proteins that attach fibronectin to the microfilaments in the cytoplasm Intercellular junctions help integrate cells into higher levels of structure and function Cytosol of adjacent plant cells are connected through plasmodesmata, openings in the cell wall, plasma membrane is continuous Three types of cell junctions in animal cells Tight junctions: membranes of cells fused to prevent leakage of extracellular fluids across a layer of epithelial cells (intestinal epithelium) Desmosomes: attach cells together into sheets Gap junctions: allow cytosol to flow between cells The cell is a living unit greater than the sum of its parts