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The Cell Has Similar and Specialized Functions Cell or Plasma Membrane Structure - fluid mosaic hypothesis Phospholipid bilayer Cholesterol - stabilizes membrane Integral proteins - may be internal to the bilayer, extend through it, or extend beyond only one side Peripheral proteins - seem bound to integral proteins Cell Membrane Functions Enclose cell components Integral proteins form channels Integral proteins may act as carriers Integral proteins may act as receptors Peripheral proteins may act as enzymes and are involved in cell shape changes Both types of protein may attach to sugars for cell recognition; a glycocalyx Factors Affecting Permeability Size of molecules - small ones can fit thru channels Solubility in lipids increases permeability - steroids, alcohol Charges - proteins in membranes have charges; opposites attract Presence of protein carriers How Things Enter and Leave the Cell Passive processes - require no energy expenditure by the cell to move things Active processes - require the cell to expend some of its energy to get things into and out of the cell Due to Brownian motion – the random movement of all substances due to their charges Diffusion - movement of substance from where there is more of it to where there is less of it Osmosis - diffusion of water thru a semipermeable membrane Solvent – the substance present in greatest quantity Solute – the substance present in smallest quantity Hypotonic solution - contains fewer non-penetrating molecules than the cell Hypertonic solution - contains more non-penetrating molecules than the cell Isotonic solution - contains the same number of non-penetrating molecules as the cell Filtration - movement of solvent and solute thru a membrane via mechanical (hydrostatic) pressure; occurs at the capillary and in the kidney Dialysis - separates small from large molecules by diffusion and osmosis thru a semipermeable membrane Facilitated diffusion - diffusion of a substance via a carrier Active transport - movement of a substance from a lower concentration to a higher concentration using a carrier and energy Endocytosis - brings substances into the cells Phagocytosis - cell eating; important defense mechanism Pinocytosis - cell drinking; occurs in the kidney, bladder and intestine Receptor-Mediated Endocytosis large particle binds to a receptor, the membrane invaginates and forms a vesicle (coated pit). Exocytosis - vesicle fuses with the plasma membrane and then ruptures; used in hormone and neurotransmitter release Cell-to-Cell Communication Cells far apart - chemical acts on receptors Cells close together - gap junctions allow small molecules and ions to move between them Cytosol Organelles Inclusions Outside nucleus and inside cell membrane 75-90% water Sol-Gel Function - chemical reactions occur here I. Nucleus A. Structure 1. Nuclear membrane (envelope) 2. Karyolymph a gel 3. Nucleoli produce ribosomes 4. Chromatin chromosomes, genes B. Functions 1. Cell reproduction 2. Cell differentiation - also involves the environment 3. Directs cell metabolism II. Endoplasmic reticulum (ER) A. Smooth ER synthesizes lipids, stores Ca in muscle cells, breaks drugs down in the liver B. Rough ER makes new plasma membrane; contains ribosomes III. Ribosomes A. Free - make protein for the cell B. Attached - make protein for secretion from the cell IV. Golgi Complex (Apparatus) A. Receives protein from ER and remakes them into enzymes in lysosomes B. Synthesizes polysaccharides, glycolipids, glycoproteins, lipoproteins, mucous and secretes many of them from the cell V. Mitochondria - contain oxidative enzymes involved in cellular respiration to make ATP VI. Lysosomes - contain digestive enzymes, many are in osteoclasts, important in phagocytosis (Tay-Sachs disease is due to a lysosomal problem) VII. Centrosomes - 2 centrioles involved in cell division VIII. Flagellum - sperm movement IX. Cilia - oarlike motion moves substances across the cell surface X. Peroxisomes - contain enzymes to break down toxins; prevent harmful oxidation reactions from forming free radicals which are disruptive compounds XI. Cytoskeleton A. Microfilaments - solid; actin, myosin involved in muscle contraction, assist in cell movement B. Microtubules - hollow; in cilia and flagella, centrioles and mitotic spindles C. Intermediate filaments - act as guy wires Pigments - hemoglobin, melanin Fat Mucous Glycogen - in liver and muscle Mitosis - nuclear division Cytokinesis - division of the cytoplasm Why Do Cells Divide? To keep the surface area proportional to the volume of the cell Some cells divide continually RBC, skin cells Some lose the ability - CNS Nucleotides - P-S-NB; adenine, thymine, guanine, cytosine are the nitrogenous bases thus there are 4 types of nucleotides Double helix Protein Synthesis Transcription - m-RNA is made from one strand of unzipped DNA m-RNA moves out of the nucleus and attaches to the ribosomes Translation - t-RNA attaches to specific amino acids and aligns with m-RNA at the ribosomes At one end of t-RNA is a sequence of 3 bases (triplet) that code for a specific amino acid. This allows for 64 combinations (4 bases cubed) that may code for 20 amino acids the genetic code These 3 bases attach to the m-RNA in a complementary fashion, A-U, C-G, bringing the amino acids close together and allowing them to bond in a sequence determined by the DNA in the nucleus Interphase G1 - gap between cell divisions; mature cells stay in this stage S - DNA is synthesized (replicated) here prior to division G2 - cell prepares for division; enzymes needed for division are made Prophase Chromatin shortens into a chromosome and becomes visible Nucleoli become less distinctive Nuclear membrane disappears Centrioles move to the opposite poles and seem to form the mitotic spindle, some of which attaches to the centromeres of the chromosomes Chromosomes begin to move toward the equator Metaphase - chromosomes line up randomly on the equatorial plate Anaphase - centromeres divide and the replicated parts of each chromosome move to opposite poles Telophase - the reverse of prophase occurs Cytokinesis Division of the cytosol; occurs in late anaphase or early telophase via a cleavage furrow that pinches in Diploid -> haploid; 2N -> N (N = sets of chromosomes); each set has 23 chromosomes Meiosis I - reduction division Prophase I Synapsis - the lining up of homologous chromosome pairs in prophase I; the crossing over and exchange of chromosome parts allows for variation in the offspring Metaphase I Independent Assortment - one chromosome of a homologous pair assorts independently with another chromosome of a different homologous pair in metaphase I providing for more variation in the offspring. (Maternal #1 can assort with either maternal or paternal #2) Anaphase I - one chromosome of each homologous pair of chromosomes moves to the opposite pole Result - 2 haploid cells each containing 23 replicated chromosomes There is no interphase II as the chromosomes are already replicated Meiosis II - equatorial division, the same as mitosis; Results in 4 haploid cells each with 23 unreplicated chromosomes Turner’s Syndrome - XO 22 year old woman Webbed neck Widely spaced nipples Lack of sexual development