Chapter 5 – Lipids, Membranes, & Cell CompartmentsPart 1 – Membrane Structure & FunctionI. Lipids (Phospholipids) A. Phospholipids are amphipathic (hydrophilic/phobic)B. When placed in water, the polar heads spontaneouslyorient to interact with water, and the tails orient tointeract with each other, making sphere C. Membranes are fluid; lipids move (rotate/migrate) within a layer, but neverswitch layers 1. Fluidity critical to function – cell changes membrane chem. to keepfluidity constant under changing conditionsD. Variable1. Tails of phospholipids can be variable in their saturation (bent vs.straight) – not always 1 bent & 1 straighta. Tighter packing due to more saturated (long/straight) fatty acidtails = increased weak interactions between phospholipids = lessmembrane fluidity2. Head groups may differ in size, shape, or chargeII. Lipids (Cholesterol) - Stabilize fluidity of animal membranesIII. Proteins A. Contained in most (99.9%) of membranesB. Bigger than phospholipids (& move less) C. Give membrane function. Classified by function:1. Allow materials to move across membrane (transporter)2. Receive info from environment & transmit it into the cell (receptor) (ex. senses)3. Catalyze reactions (enzymes) (could be flipped to other side of membrane)4. Anchor cell to other cells or help maintain cell shape (Anchors)D. Give the membrane structure. Classified by structure:1. Integral membrane proteins – irremovable unless membrane is destroyeda. Transmembrane proteins – many IMP’s that span the membraneb. Take form of alpha helices (secondary structure)2. Peripheral Membrane Proteins – temporarily associate with membrane lipid or proteins through weak, noncovalent interactionsIV. All membrane components are not uniformPart 2 – Membrane Function: Selective BarriersMembranes Are Selectively Permeable, Based On:I. Incoming molecule’s sizeII. Incoming molecule’s chargeIII. Hydrophilic natureIV. Transmembrane concentration gradient (diffusion force) A. Protein can facilitate diffusion 1. Channels – just open & let it flow2. Carriers - proteins that bind to a molecule & change shape, which transports molecules across the membrane (not the same thing as a channel)3. Active transport – use energy & protein transporters to move against [ ] gradientB. Osmosis : diffusion of H2O1. Ex. in kidneys – aquaporin (protein) utilized in absorbing excess H2O into nephrons2. Water diffuses across membranes in response to relative solute concentrationsa. Hypertonic environment: more concentrated than cellb. Isotonic: concentration = to cellc. Hypotonic: less concentrated than the celli. These 3 are describing environment relative to cell, but can be flipped to describe cell environment relative to outside environmentd. Effect of carrying tonicity – fig. 5.14i. Hypertonic environment (outside cell) – cells shrink (H2O leaves cell)ii. Isotonic – no change (H2O in = H2O out)iii. Hypotonic – cells swell (H2O rushes in) Part 3 – Introduction to Cells- Vary in size, & are much bigger the molecules they containI. Typical PROKARYOTIC Structure A. Plasma membraneB. Cell wall made of peptidoglycanC. Glycocalyx: outer gelatinous coveringD. Flagella (sometimes present) – can be grown/reabsorbed as neededE. Pilli: to attach or pick up DNA – covers surfaceF. Ribosome () – lots presentG. H. Transcription & translation both happen in prokaryotesII. Typical EUKARYOTIC Cell Structure A. More variable externally B. More complex internallyC. Everything inside the membrane is the cytoplasm1. Nucleus is often not considered part of the cytoplasm2. Cytosol: region inside the plasma membrane, but outside the organellesa. 20-50% of cell volumeb. Site of chemical reactionsc. Contains ribosomes (ribosomes are not organelles)d. Contains dense network of protein filaments (cytoskeleton)i. Provides cell shape, organization, & movementii. Dynamic & highly regulatediii. Protein constituting cytoskeleton is extremely dense (all over the cell)D. Plasma membrane is part of the endomembrane system (ES): a membrane network in eukaryotic cells1. ES – includes membrane & membrane-bound organelles, which all interactEndomembrane SystemI. Nuclear Envelope Nucleoid: region where circular chromosome is found A. 2 layers of protein-pore containing membrane that surrounds the nucleus B. Small molecules/ions diffuse through pores; large ones are actively transportedC. Chromatin – very well organized DNA wrapped around histones D. Nucleolus – sub-region where ribosomal subunits are assembled (small/large)E. Continuous with Endoplasmic ReticulumII. Endoplasmic Reticulum A. Network of membranes enclosing a lumenHas 2 subdivisions that can change into oneanother B. Rough E.R. – studded with ribosomes; activein protein synthesisC. Smooth E.R. – no ribosomes; lipid synthesisD. “Factory” of the cellE. Fig. 5.20:F. Amount of rough ER vs. smooth variesdepending on cell type & time1. Ex. Liver cells: lots of smooth for detox &metabolism. Tolerance to alcohol comesfrom increased production of smooth ER2. Ex. 2: Pancreatic cells: lots of rough ER toproduce insulin/digestive enzymes(proteins)G. Protein Sorting 1. Translation starts in free ribosomes in cytosol2. Proteins are then sorted based on their AA-sequencea. If translation is completed on a free ribosome, theprotein will do one of the following: (Fig. 5.23):i. Stay in the cytosolii. Go to nucleusiii. Go to chloroplast (plants)iv. Go to mitochondriab. If ribosome moves & associates with ER beforecompletion of translation (because of protein’s AA-sequence), the protein will stay in the ER’s lumen, stay in ER’s membrane, or move on to Golgi apparatusIII. Golgi Apparatus A. Vesicles bud from ER and bring product to Golgi: series of enclosed compartmentsB. Processes & sorts lipids/proteins from ER1. Glycosylation – adding sugar to a molecule2. Proteolysis (big proteins get cut into pieces)C. Synthesizes carbohydratesD. Vessicles bud from Golgi and go to membrane for exocytosis of productsIV. Summary of Excretory Pathway (“Biosynthetic Pathway”) A. ERGolgiinsertion to membraneexocytosis (out of cell)1. How insulin is made2. How proteins for membrane are made – instead of being released they implant3. *Material can also move in opposite direction (from Golgi back to ER)V.
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