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Transcript
Physiology of Cells Passive Transport 1. Diffusion – – Tendency of small particles to spread out evenly within a given space Occurs down a concentration gradient until equilibrium is reached • Measurable difference between one area to another – – Diffusion or permeability dependent upon presence of specified channels Selectively permeable – channels open and close based on cell’s needs Diffusion Passive Transport 2. Dialysis (fig 4-4, pg. 94) – Selectively permeable membrane separates smaller and larger particles Passive Transport 3. Osmosis (fig 4-5, pg. 95) – Diffusion of water through a selectively permeable membrane Osmotic Pressure • Osmotic pressure – pressure that develops in a solution as a result of osmosis – Important concept for maintaining homeostasis • Isotonic – two fluids have the same osmotic pressure • Hypotonic – lower concentration of solute outside of the cell (ECF) • Hypertonic – higher concentration of solute outside of cell (ECF) Effects of Osmosis on a Cell Passive Transport 4. Facilitated Transport – (also carrier-mediated passive transport) – Movement of a molecule is facilitated by a carrier mechanism in the cell membrane – Still requires concentration gradient Passive Transport 5. Filtration – Passing of water and permeable solutes through a membrane by the force of hydrostatic pressure – Hydrostatic pressure: force or weight of a fluid pushing against a surface – Occurs down a hydrostatic pressure gradient – Most often associated with movement across a sheet of cells (ex: capillaries) Active Transport • Carrier-mediate process • Moves molecules “uphill” (against concentration gradients) • Ex: Sodium-potassium pump – Operates in all human cells Sodium-Potassium Pump • Transports Na+ out of the cell and K+ into the cell • Requires energy (sodium-potassium ATPase) – made in mitochondria • 3 Na+ bind on the intracellular side and are exchanged for 2 K+ from the ECF Sodium-Potassium Pump Endocytosis 1. Receptors in the plasma membrane bind to molecules in the ECF 2. Cytoskeleton pulls a portion of the membrane inward, creating a pocket 3. Edges of the “pocket” fuse forming a vesicle 4. Vesicle is pulled inside the cell by the cytoskeleton 5. Vesicle fuses with the membrane walls of a lysosome Exocytosis Process by which large molecules (proteins) exit the cell 1. Wastes/proteins enclosed by a membrane vesicle 2. Cytoskeleton moves vesicle to plasma membrane 3. Fuses with membrane & releases contents to ECF Cell Life Cycle • Two main processes: – Growth – Reproduction Cell Growth • During cell growth additional cytoplasm and plasma membrane are produce through protein synthesis – Includes structural proteins and enzymes – Anabolic process DNA Replication • Prior to cell reproduction, DNA must be replicated: 1. DNA unzips 2. Nucleotides attached to exposed base pairs according to base pair rule (A-T; G-C) 3. DNA polymerase binds nucleotides 4. Two identical DNA strands are formed DNA Replication Cell Reproduction • One parent cell (diploid) produces two identical daughter cells (also diploid) • Two steps: – Mitosis – Cytokinesis Mitosis • Cell organizes replicated DNA into two identical sets and distributes one set to each daughter cells • 4 phases of mitosis (PMAT) 1. Prophase – – – – Chromosomes shorten & thicken Centrioles move to poles of cell Spindle fibers appear Nuclear membrane disappears Mitosis 2. Metaphase – – Chromosomes lined up at the equator (middle) Spindle fibers attached to centromere 3. Anaphase – Centromeres break apart separating sister chromatids Mitosis 4. Telophase – Spindle fibers disappear – Nuclear envelope reappears Cytokinesis: • Completes cellular reproduction by splitting the plasma membrane and cytoplasm into two Meiosis • Cell division which occurs in primitive sex cells • The end result is mature gametes – Males sperm – Females ova (egg) • Gametes are haploid (contain half the number of chromosomes of a somatic cell) – Reduction division Deoxyribonucleic Acid (DNA) • Double Helix (twisted ladder) • Monomer: nucleotide 1. 5 Carbon sugar (deoxyribose) 2. Phosphate group 3. Nitrogen base • Nitrogen bases: • Purines: adenine, guanine • Pyrimidines: cytosine, thymine • Chargaff’s rule Deoxyribonucleic Acid (DNA) Deoxyribonucleic Acid (DNA) • Three base pairs = codon • Sequence of base pairs determines each gene • Genes control the production of proteins (therefore determine individual traits Ribonucleic Acid (RNA) • Single helix • Smaller than DNA • 5 carbon sugar = ribose • Uracil replaces thymine which binds with adenine Protein Synthesis • Occurs in 2 steps: – Translation: DNA RNA – Transcription: RNA Proteins Translation • Occurs in the nucleus • DNA “unzips” exposing base pairs • RNA nucleotides (already present in the nucleus) attach themselves to the exposed bases along one side of the DNA molecule • These RNA nucleotides bind to each other with the help of RNA polymerase • The chain that results is called messenger RNA (mRNA) Preparation of mRNA • Occurs in the nucleus • The mRNA pulles away from the DNA strand • The result is a copy or “transcript” of a gene • Non-coding portions of mRNA (introns) are removed • Remaining portions (exons) are spliced back together • mRNA travels to the nucleus via nuclear pores Translation • Occurs in the cytoplasm • Ribosomal subunits attach to one of the mRNA • Transfer RNA (tRNA) bring specific amino acids to the ribosome site – Which amino acid is based upon each mRNA codon – The tRNA must have a complementary anticodon to fit onto the binding site of each condon Translation • Amino acids are joined by peptide bonds • Forms long molecules called polypeptides • Several polypeptides are needed to make a protein After Translation… • Enzymes within the endoplasmic reticulum, Golgi apparatus and/or cytosol line polypeptides to form larger structures – May also form lipoproteins or glycoproteins Translation Video http://www.dnatube.com/video/1947/ProteinSynthesis-Translation-1