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Cell theory was the first unifying theory of biology: • Cells are the fundamental units of life. • All organisms are composed of cells. • All cells come from preexisting cells. Most cells are tiny, in order to maintain a good surface area-to-volume ratio. The volume of a cell determines its metabolic activity per unit of time. The surface area of a cell determines the amount of substances that can enter or leave the cell. Figure 4.2 Why Cells Are Small As cells grow larger, metabolic activity and need for resources and rate of waste production increases faster than surface area. Some large cells increase surface area by folds in the cell membrane. The cytoskeleton: • Supports and maintains cell shape • Holds organelles in position • Moves organelles • Involved in cytoplasmic streaming • Interacts with extracellular structures to anchor cell in place The cytoskeleton has three components with very different functions: • Microfilaments • Intermediate filaments • Microtubules Microfilaments: • The thinnest microfilaments and most prevalent at the edge of the cell • Anchor the cytoskeleton to integral proteins in the plasma membrane • Help a cell or parts of a cell to move • Determine cell shape • Composed of actin and myosin • Provide mechanical support for microvilli Microvilli - non-moving, microscopic fingerlike projections of the plasma membrane that increase the surface area of the cell membrane Intermediate filaments: • Intermediate in size • At least 50 different kinds in six molecular classes • Anchor cell structures in place • Resist tension, maintain rigidity and found in parts of the cell subject to mechanical stress Microtubules: • Thickest cytoskeleton elements. • Form a rigid internal skeleton for some cells or regions • Act as a framework for motor proteins to move structures in the cell Cilia—short, usually many present; move stiffly to propel a cell, or move fluid over a stationary cell Flagella—longer, usually one or two present; move the entire cell by wiggling in a wavelike pattern. Ribosomes translate the nucleotide sequence of a messenger RNA molecule into a polypeptide. Each subunit consists of ribosomal RNA (rRNA) bound to smaller protein molecules. Ribosomes are not membrane-bound organelles. In eukaryotes, they are free in the cytoplasm, attached to the endoplasmic reticulum, or inside mitochondria. The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes. Tiny, membrane-surrounded vesicles shuttle substances between the various components, as well as to the cell membrane. Endoplasmic reticulum (ER)—network of interconnected membranes in the cytoplasm, with a large surface area Two types of ER: • Rough endoplasmic reticulum (RER) • Smooth endoplasmic reticulum (SER) Rough endoplasmic reticulum (RER) has ribosomes attached to its outer surface. Newly made proteins enter the RER lumen where they are chemically modified and tagged for delivery to specific locations. The proteins are transported in vesicles that pinch off from the ER. All secreted proteins and most membrane proteins pass through the RER. Polypeptides are transported into the RER lumen as they are being synthesized. In the lumen they are folded into their tertiary structures. Many are linked to carbohydrate groups, becoming glycoproteins. Many glycoproteins are important in recognition and interactions between cells. Smooth endoplasmic reticulum (SER)— more tubular, no ribosomes • Chemically modifies small molecules such as drugs and pesticides • Site of glycogen degradation in animal cells • Site of synthesis of lipids and steroids • Stores calcium ions, which trigger many cell responses Golgi apparatus: flattened sacs (cisternae) and small membrane-enclosed vesicles. • Receives proteins from the RER and can further modify them • Concentrates, packages, and sorts proteins • Adds carbohydrates to proteins • Site of polysaccharide synthesis for plant cell walls Golgi apparatus has three regions: • cis region: receives vesicles containing proteins from the ER • trans region: vesicles bud off from the Golgi apparatus and travel to the cell membrane or to lysosomes • medial region: in between trans and cis regions Primary lysosomes originate from the Golgi apparatus. They contain hydrolases (digestive enzymes), and are the site where macromolecules are hydrolyzed into monomers. Macromolecules may enter the cell by phagocytosis—part of the cell membrane encloses the material and a phagosome is formed. Phagosomes then fuse with primary lysosomes to form secondary lysosomes. Enzymes in the secondary lysosome hydrolyze the food molecules. • Autophagy – process by which worn out organelles are digested by lysosomes • Autophagosome – The name of the vesicle created when the organelle to be digested is enclosed by a membrane from the ER. This will fuse with a lysosome. • Autolysis – destruction of the entire cell by lysosomes Other organelles perform specialized functions. Peroxisomes similar to lysosomes but smaller (also called microbodies.) Oxidize various organic substances which can be toxic like fatty acids & alcohols. A by-product of this oxidation is peroxide which can also be toxic. So they also contain catalase which decomposes peroxide. They can self-replicate or be made from scratch. Proteasomes – barrel shaped and consist of 4 stacked proteins around a central core. These organelles digest and recycle used or damaged proteins in the cytosol. They contain large amounts of proteases. In eukaryotes, breakdown of energy-rich molecules begins in the cytosol. The partially digested molecules enter the mitochondria, where chemical energy is converted to energy-rich ATP. Cells that require a lot of energy often have more mitochondria. Mitochondria have two membranes: • Outer membrane—very porous • Inner membrane—extensive folds called cristae increase surface area The fluid-filled matrix contains enzymes, DNA, and ribosomes. Apoptosis – programmed cell death in response to cell damage, excess free radicals or DNA damage. Mitochondria initiate the process by releasing cytochrome c. The nucleus is usually the largest organelle: • Location of DNA and DNA replication • Site where DNA is transcribed to RNA • Contains the nucleolus (pl. nucleoli), where assembly of ribosomes from RNA and proteins begins The nucleus is surrounded by two membranes that form the nuclear envelope. Nuclear pores in the envelope control movement of molecules between nucleus and cytoplasm. In the nucleus, DNA combines with proteins to form chromatin in long, thin threads called chromosomes. Gene’s are the cells smaller hereditary units. There are many genes on individual chromosomes. The outer membrane of the envelope is continuous with the endoplasmic reticulum. Somatic cell – an cell of the body that is not a germ cell Germ cell – a gamete or any cell destined to become a gamete Interphase: G1 Phase – normal cell function time S Phase – DNA replicated G2 Phase – organelles replicated • Mitotic Phase- 4 stages during which nucleus divides • Cytokinesis