Roles of ER Rough Ribosomes synthesize excreted proteins  Stored in cisternae or vesicle Modify proteins  Glycosylation of proteins Delivery of membrane associated proteins Often interacts with the Golgi Smooth Tissue-specific uses Storage of carbohydrates Detoxification reactions in liver Synthesizes much of the new membrane material Modification of existing molecules

ER and Golgi Often adjacent in cytoplasm of cell Both are membrane producers  Membrane has sides  Vesicles are produced inside out  Vesicle fuses with surface and excreted proteins are released and integral proteins are added along with membrane ER often performs first steps of modification that is later finished in Golgi

Golgi apparatus Complex collection of membrane Has polarity  Cis and trans surfaces Responsible for secretion  Very prominent in cells that serve secretory functions-such as epithelial cells Modifies structures previously synthesized in the ER

Golgi Function Cis face is nearer the center of cell  Cis face is often adjacent to ER Trans face is nearer the cell membrane Activity is directional  Starts at cis face and moves to trans

Fig cis face (“receiving” side of Golgi apparatus) Cisternae trans face (“shipping” side of Golgi apparatus) TEM of Golgi apparatus 0.1 µm

ER and Golgi Synthesis in ER but modification in Golgi Similar to an assembly line  Options packages are added in Golgi Sequential passage through cisternae (cis to trans) Each cisternae contains different enzymes

Examples of Golgi Function Replace sugars placed on glycoproteins Modification of phospholipid acylgroups and head groups Molecule targeting and assembly Production of vesicles to delivery membrane associated molecules and excreted molecules to cell surface

Cell Renewal Cells and their molecules age and become less effective An important cellular function is renewal  Old molecules and organelles are recycled  New phospholipids are produced for the membrane  Toxic molecules need to be collected and detoxified Sometimes a cell needs to commit suicide  Called autolysis or apoptosis

Vesicle Organelles Perform many of the renewal functions Lysosome Peroxisome Glyoxisome

Lysosome Contains digestive enzymes used to degrade macromolecules or organelles Originates in ER but enzymes are activated in Golgi Important in macrophages for degradation of particles acquired via phagocytosis Used to degrade organelles-autophagy

Fig. 6-14a Nucleus 1 µm Lysosome Digestive enzymes Plasma membrane Food vacuole Digestion (a) Phagocytosis

Fig. 6-14b Vesicle containing two damaged organelles Mitochondrion fragment Peroxisome fragment Peroxisome Lysosome Digestion Mitochondrion Vesicle (b) Autophagy 1 µm

Cell Death Sometimes used in development of organism  Hand development  Certain cells are programmed to die at appropriate times Defense against viral or intracellular bacterial infections  Self-destruct  Program is started by T cells Apoptosis is often accomplished by leaky or popping lysosomes

Peroxisomes Contain enzymes to collect free hydrogen and oxygen and combine into H 2 O 2 (peroxide) Peroxide is also toxic so they contain enzymes to degrade into water and oxygen molecules  Catalase experiment in lab Peroxisomes are formed by aggregation of lipids and proteins (not formed in ER) Many are found in liver cells  Degrade alcohols into peroxide then into water and oxygen

Peroxisome pt 2 Also responsible for degradation of fatty acids in cytosol Modify fatty acids and phospholipids and incorporate into peroxisome membrane Divide by binary fission

Glyoxysomes Specialized peroxisomes found in plant seeds Recognize appropriate signals and begin to degrade stored fat in seed Fat is converted to sugars which allows seedling to sprout Once photosynthesis starts, glyoxysomes degrade