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Cellular and Molecular Biology Chapter Summary 3

by: Jewelle Williams

Cellular and Molecular Biology Chapter Summary 3 Bio 300

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Jewelle Williams
Virginia Commonwealth University

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Book: Essential Cell Biology, 4th Edition Author: Bruce Alberts I summarized Chapters 13-15, and 17 in this packet. I took notes from the book, class and powerpoints and organized the informati...
Cellular and Molecular Biology
Dr Teshelle A. Ponteen Green
Cellular, Biology, Molecular, Respiration, Cell, catabolism, glycolysis, krebs cycle, electron transport chain, amino, acids, metabolism, phosphorylation, Chloroplasts
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This 23 page Bundle was uploaded by Jewelle Williams on Sunday July 24, 2016. The Bundle belongs to Bio 300 at Virginia Commonwealth University taught by Dr Teshelle A. Ponteen Green in Summer 2016. Since its upload, it has received 24 views. For similar materials see Cellular and Molecular Biology in Biology at Virginia Commonwealth University.

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Date Created: 07/24/16
Test 3 Chapters 13, 14, 15, 17 Chapter 13 How cells obtain Energy from Food (Cell Respiration) Breakdown of sugars and fats:  Step by step using enzymes to store as much free energy as possible  ATP Production: o Substrate Level Phosphorylation:  Direct Coupling of favorable catabolism of food molecules o Oxidative Phosphorylation Catabolism: Cellular Respiration Stages: 1. Stage 1 of Cell Respiration:  Digestion: Eat/Chew, Breakdown in the gut macromolecules rom polymers (protiens polysaccharides, fats) to monomers (sugars, amino acids, fatty acids)  Occurs in either the intestines/gut or lysosome 2. Stage 2 Glycolysis- The conversion of Glucose to 2 pyruvate molecules  (Step 1) Glucose is phosphorylated via hexokinase, trapping the glucose in the cell, using an ATP producing: Glucose 6-phophate, ADP and H+  (Step 2) Glucose 6-phophate is rearranged using phosphoglucose isomerase producing: Fructose 6-phophate  (Step 3) Fructose 6-phophate is phosphorylated using ATP and phosphofructokinase producing: Fructose 1, 6-bisphophate  (Step 4) Fructose 1, 6-bisphophate is cleaved via aldolase into 2 3 carbon molecules producing 2 Glyceraldehyde 3-phophate molecules  (Step 6) The glyceraldehyde 3-phophate is phosphorylated while NAD+ is reduced via Glyceraldehyde 3-phophate dehydrogenase producing: 1, 3- bisphosphoglycerate, NADH and H+  (Step 7) 1, 3-bisphosphoglycerate is dephosphorylated via phosphoglycerate kinase producing: 3 Phosphoglycerate and ATP  (Step 8-9) 3 phosphoglycerate is rearranged and dehydrated  (Step 10) Phosphoenolpyruvate is dephosphorylated via pyruvate kinase producing: pyruvate and ATP 2.5 Not step 3: Conversion of Pyruvate to Acetyl CoA Step 2.5 Alternate in anaerobic environment: Fermentation:  In Active muscle cells: o PyruvateLactic Acid o NAD+ is reduced it NADH in a cyclic manner o Used for making Cheese and yogurt  In Yeast: o Pyruvate Ethanol o Releases CO2 o Used for making liquor/wine/beer/baking 2.5 Dephosphorization of Pyruvate (or fats/protiens) as it passes through the Mitochondrial matrix via pyruvate dehydrogenase. This step produces Co2 (waste), NADH, and Acetyl CoA For fats 2 carbons are cleaved for each turn of the cycle to generate acetyl CoA 3. Stage 3 Krebs Cycle and ETS  Krebs Cycle/Tricarboxylic Cycle/Citric Acid Cycle- The Process that catalyzes the complete oxidation of the carbon atoms of the acetyl groups in acetyl CoA converting them to CO2 o 2/3 of the total oxidation (substrate level) o Products are: CO2, and NADH o Requires O2 but doesn’t actually use it up. The O2 is used as a final acceptor for the ETC allows NADH to get rid of its electrons and regenerate NAD+ to keep the cycle going. The oxygen used in the Krebs cycle is from the splitting of H2O to produce CO2 waste. o The acetyl group is not oxidized it is transferred to oxaloacetate o Each Rotation produces: 3 NADH, 1 FADH2, 1 GTP, 2 CO2 o Steps:  (1)  Electron Transport Cycle: o 1 glucose molecule~30 ATP via oxidative phosphorylation Other purposes of Glycolysis and the Citric Acid cycle:  Building blocks for anabolism are syphoned off during each step: o Glycolysis:  Nucleotides  Amino sugars/glycolipids/glycoproteins  Amino acids pyrimidines  Serine  Alanine o Krebs Cycle:  Cholesterol fatty acids  Glutamate, amino acids purines  Heme Chlorophyll  Aspartate, A.a. purines pyrimidines Regulation of Metabolism:  Food reserves when there is no sunlight/no food  Gluconeogenesis- Production of Glucose from pyruvate when glucose levels are low and food is scarce o Requires 4 ATP and 2 GTP  Binding of regulatory molecules to enzymes for positive and negative feedback tell the cell to catalyze or anabolize glucose  Fasting cells use glycogen, which is cleaved and the cleaving of glucose vs building of glycogen is regulated by glucose 6-phophate which activates glycogen synthetase (for synthesis of glycogen), and inhibits glycogen phosphorylase which breaks down glucose  Fat produces twice as much energy as glycogen (gram to gram), Fat is stored in adipocytes  Plants store glucose as starch instead of glycogen Chapter 14 Electron Transport System  Oxidative Phosphorylation o Occurs in the Mitochondria or Chloroplast o Requires a membrane and depends on electron transport process that drive transport of H+ (protons) across the inner mitochondrial membrane o 2 Stages  Stage 1: Electrons (from oxidation of food molecules) are transferred along electron carriers embedded on the membrane and release energy used to pump protons (from ubiquitous water) across the membrane and generate an electrochemical proton gradient This form of stored energy can be used to do work  Stage 2: Protons flow back according to the electrochemical gradient through ATP synthase which catalysis the formation of ATP from ADP and a P Group Using Chemosmotic Coupling.  Mitochondrion: Can adapt in shape size and location based on the cells needs and where ATP is needed most in the cell o Structure: Double membrane:  Outer membrane:  Has “porin” channels and acts as a sieve  Permeable to many molecules and some small protiens making the intermembrane space chemically equivalent to the cytosol  Inner membrane- Site of oxidative phosphorylation, holds proton pumps and ATP synthase  Impermeable to the passage of ions and most small molecules (like the plasma membrane) except through transport protiens  Inholdings called cristae to increase surface area (and therefore the amount of ATP to be produced)  2 compartments:  Intermembrane space between the outer and inner membrane  Matrix inside the Inner membrane o Conversion of Pyruvate to Acetyl CoA o Respiratory Enzyme Complex’s- Energetically favorable process that produces water (using oxygen we breathe)  NADH Dehydrogenase Complex-  Accepts Electrons from NADH (producing NAD+) in the form of a H- (hydride ion)  Ubiquinone ferry’s the H+ (from water) to the next protein  Picks up 1 or 2 e- and 1 H+ each time it ferries  It is between NADH dehydrogenase and Cytochrome c reductase in affinity for e- allowing the e- to be transferred easily to the next enzyme  Cytochrome c reductase complex  Cytochrome C ferries the H- to the next protein  Cytochrome c oxidase complex  Produces H2O o The Respiratory Enzyme complex produces an electrochemical proton gradient o ATP Synthase:  Allows the H+ protons to travel back through the inner membrane via a “rotor”  The rotation of the rotor alters the conformation and synthesizing ATP (3/rotation and ~100/second)  Can Operate in reverse also using ATP to generate the electrochemical gradient acting as an H+ pump o The negative membrane potential generated on the intermembrane side of the inner membrane by the H+ gradient is used by antiport carriers to export ATP from the Mitochondria and import ADP to the mitochondria o The electrochemical gradient is used to drive the formation, and transport of ATP as well as the transport of selected metabolites across the inner mitochondrial membrane  Import of Pyruvate, Pi-, ADP and H+  Export of ATP o The concentration of ATP in the cytosol must be ~10 times higher than the concentration of ADP o NADH passes their electrons to NADH dehydrogenase but FADH2 skips that enzyme and passes its electrons to the carrier ubiquinone as a result FADH2 moves less protons and makes less ATP than NADH o NADH: ~2ATP FADH2: ~1.5 o ~50% of the energy released is stored in ATP (gas/electric motors store 10-20%  The Proton Gradient How does it work? o H+ most abundant atom in living organisms (both carbon molecules and H2))  Very mobile dissociating from 1 H2O to another  Waters is a ready donator acceptor for H+ everywhere o Process:  (1) High energy e- move from their carriers to an enzyme then they are accepted in the next enzyme by H+ then H+ is released into the matrix and the e- (now a low energy electron) is moved to the final enzyme  Each e- acceptor complex has metal atoms that allow a greater affinity for e- o Metals with lower affinities are used earlier in the chain ex: Iron-sulfur centers (low affinity) are early in the chain o Only Iron atoms are used later (from the heme group) bound to the cytochrome c ENZYMES o Cytochrome c has an affinity to e- between cytochrome c reductase and cytochrome c oxidase o Cytochrome C Oxidase has the highest affinity (redox potential) to e- (as the final acceptor). It removes all electrons from itself (aka oxidase) and transfers them to an O2 molecule  4e- + 4H+ + O2 2H2O  O2- if highly radioactive so cytochrome C holds it very tightly until it gains its H+ molecules preventing DNA Protien a lipid membrane damage  4H+ molecules are pumped over the membrane during the transfer in the chemical equation above.  This is the step that consumes nearly all of the oxygen we breathe  The Intermediates between complex’s ubiquinone (animals), Quinone (plants), and cytochrome c diffuse readily through the inner membrane  Each transfer is a redox reaction  NADH NAD+ + H+ + 2e- o A strong e- donor that changes to a weak e- acceptor to make the High energy state favorable (Delta-G) < the tendency towards accepting/donating is call redox potential and can be measured experimentally o Electrons move from a redox with low potential to a compound with high redox potential light energy+CO +H O→sugars+O +heatenergy Photosynthesis: 2 2 2 o Chloroplast Structure:  Larger than mitochondrion, but similar in that they have a highly permeable outer membrane and a highly impermeable outer membrane  Stroma (like the matrix of mitochondrion)  Major Difference: The inner membrane does not contain photosynthetic machinery. The light-capturing sys, electron transport chain and ATP synthase is in the Thylakoid membrane (a third membrane) which are arranged in stacks (grana) that create the thylakoid space separated from the stroma o Stage 1 Light Reactions:  Similar to oxidative phosphorylation,  The electron transport chain in the thylakoid membrane uses the proton gradient and ATP synthase to make ATP  Where Photosynthesis starts to differ from cellular respiration is when the electrons make it down the chain they are donated to NADP+ producing NADPH instead of being donated to O2  Chlorophyll: It harness energy from sunlight by absorbing the energy (sunlight) which excites the electrons in the network. Since this high energy state is unstable the chlorophyll looks for a method to get rid of its excess electrons (electron transport chain) o Chlorophyll molecules in a chloroplast are able to convert light energy into useable energy because they are associated with a special set of photosynthetic protiens in the thylakoid membrane.  Chlorophyll molecules are held in photosystems in the thylakoid membranes  Each photosystem consists of antenna complexes. o Has hundreds of chlorophyll molecules arranged to collect light energy and transferred to the next molecule of chlorophyll within the antenna complex or to the next complex until it encounters the “special pair” o Photosystems work together:  Photosystem II The first photosystem: absorbs light and the reaction center passes the election (from water) to plastoquine (an electron carrier) which transfers the electron to the proton pumps generating a proton gradient producing ATP  Photosystem I The second photosystem: also captures sunlight energy and transfers the energy to reduce NADP+ to NADPH. In this step the lost electrons (from ____________________) are replaced by electrons form H2O. A set of 2 H2O are held together and when the 4 electrons are removed O2 is released (similar to How Cytochrome C oxidase transfers to O2 molecules). All oxygen on the earth is produced by the splitting of water molecules  This step acts as a “final acceptor” for electrons from photosystem II  They capture light and the reaction center converts the light energy to chemical energy o Stage 2 Dark Reactions/ carbon fixation reactions  ATP and NADPH from stage 1 are used to drive the production of sugars from CO2  They begin in the chloroplast stroma and generate a 3 carbon sugar that is exported to the cytosol and then used to produce sucrose and other organic molecules  Reaction centers are transmembrane complexes of protiens and pigments that perform the oxidative phosphorylation  This creates a charge separation that sets the whole process off in the electron transport chain  Carbon Fixation (Calvin) Cycle:  ATP and NADPH is used to regenerate ribulose 1,5- biphophate  3 CO2 molecules make: 1 molecule of glyceraldehyde 3- phohphate, 9 ATP and 6 NADPH  During photosynthetic activity glyceraldehyde 3-phophate is stored as starch or fat, during periods of low photosynthetic activity (night time) o Starch: a large polymer of glucose stored for times of low food in the plant cell  During periods of low photosynthetic activity (like at night) the fat or starch is fed in to the glycolytic pathway and converted to pyruvate then to the plant cells mitochondrion producing ATP for the cell  Sucrose is like glucose in plants Chapter 15 Intracellular Compartments and Protien Transport  Organelles and their functions: 1. Nucleus: DNA replication 2. Mitochondria: ATP synthesis 3. Chloroplasts (plants only)- ATP synthesis 4. Rough ER: Has Ribosomes that produces protiens that are transported into the ER  5. Smooth ER: no ribosomes 6. Lysosomes: 7. Endosomes: A sorting place 8. Golgi: Packaging area  Cis Face:  Adjacent to the ER  Receives protiens from the ER  Returns protiens with the ER retention to the ER  Trans Face:  Sorts protiens based on where they go (lysosome or membrane)  Modifies oligosaccharides before they exit 9. Cytosol: a mediator between membrane bound organelle 10.Peroxisomes: Oxidize molecules using H2O2 to prevent free radicals inside the cell  Contain enzymes that produces H2O2  Synthesize phospholipids  Most of their protiens are acquired via selective transport from the cytosol  # amino acid sequence  Protien translocator protiens are used to transport protiens  Protiens don’t need to be unfolded like in mitochondria and chloroplasts  Endomembrane System: Golgi, ER, Nucleus, Endosomes, Peroxisomes, Lysosomes  Methods of Membrane Transport Protiens know where to go because of signal sequences (~15-60 amino acids long) that are often removed from the Protien when it is transported where it needs to go. 1. Nuclear Transport (through nuclear pores)  Structure of nuclear membrane:  Inner membrane which has some Protien binding sites for chromosomes and others for anchorage of the nuclear lamina o The lamina is a meshwork of Protien filaments that gives the nucleus structure  Outer Membrane which is continuous with the ER  Nuclear Pores which allow selective transport of molecules in and out of the Nuclear envelope o Composed of ~30 protiens it is a complex gate system o The gate is like a “kelp forest” and only allows small water soluble molecules through  Pore Transport:  The molecule (Protien, RNA, etc.) must display a nuclear localization signal  The signal is recognized by the nuclear import receptors which direct them into the pore by interacting with the fibrils that extend from the pore into the cytosol  It is imported by a repeated grab and pull process  This process requires GTP and is mediated by Ran (a GTPase) 2. Membrane Transport (across membranes)  Signal on their N-terminus  Protiens are transported simultaneously across both the inner and outer membranes  Process:  The precursor Protien (that is not in its final conformation) encounters an import receptor Protien with its signal sequence  The Protien is unfolded and transported through the outer membrane where the signal sequence encounters the protein translocator in the inner membrane  Once it passes though the protein translocator in the inner membrane and is in the matrix chaperone protiens supervise the refolding of the protein into its final conformation and the signal peptide is cleaved form the protein 3. Vesicular Transport (through the endomembrane system)  Any protein gong to the Golgi, peroxisomes and lysosomes pass through the ER first from the cytosol  There are 2 types of protiens that go in the ER:  Water soluble proteins- which translocate completely through the ER to the lumen and are usually destined for secretion or for the lumen of an endomembrane system  Prospective Transmembrane Protiens which only partially translocate and become embedded in the membrane  All have an ER sequence of ~8 amino acids  The protiens that enter the ER must be synthesized as they are entering the ER. Ribosomes embedded in the ER allow this to happen o Ribosomes all work identically but the Protien the ribosome is encoding has an ER sequence it is directed to the ER o The growth of the polypeptide is enough to move the protein into the ER no other energy is required. o The Process:  SRP’s in the cytosol bind to the ribosome and the ER signal sequence  When the protein emerges from the ribosome an SRP receptor embedded in the ER membrane recognizes the SRP and binds to it  The SRP and SRP receptor act as molecular matchmakers  The SRP passes the ribosome to a protein translocator where synthesis recommences  The Signal sequence (which keeps the translocator open) stays bound to the Translocator and production continues. When the protein is complete it is cleaved by transmembrane signal peptidase and is released after the C-terminus passes through  For embedded protiens:  The same process occurs except a hydrophobic domain of the protein (called aa stop-transfer sequence) is synthesized and remains embedded in the membrane while the signal sequence is removed via transmembrane signal peptidase  Some work in pairs so they can pass through multiple times  Glycosylation: ___________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________  The process: Budding and Fusion  Outward is often secretory  Inward is often _________  Budding: o (1) Clatherin coated pit: The cargo molecule binds to the cargo receptors that have adaptin and clatherin attached o (2) Invagination: The bud slowly closes off with the help of dynamin until it is completely pinched from the membrane it comes from o The clatherin coat is shed and the bud encounters the membrane it will fuse with  Fusion: o When the vesicle reaches its target the Tethering protein encounters the rab on the vesicle o When it gets closer the v-SNARE encounters the t- SNARE and they o When they are within 1.5 nm of each other (to illuminate a change in concentration/entrance of unwanted particles) fusion occurs between the two bilayers. This unfavorable process is catalyzed by the SNARE’s  Secretory Pathways: o ER: Most protiens that enter the ER are modified with Disulfide bonds (between cysteine side chains), converted to glycoproteins via glycosylation, o Some protiens remain in the ER lumen (and are returned when they escape to the Golgi) and are identified by a four amino acid sequence on the C- terminal called an ER retention signal o Exit from the ER is highly selective o When protiens are folded improperly:  Repaired (via binding to a chaperone protein)  Destroyed o When unfolded protiens build up in the ER the Unfolded protein response (UPR) is triggered which triggers the production of more chaperone and quality control chaperones o Golgi: The Cis Face is Adjacent to the ER, receives protiens from the ER, returns protiens with the ER retention to the ER. The Trans Face Sorts protiens based on where they go (lysosome or membrane), Modifies oligosaccharides before they exit o Exocytosis: when vesicles from the trans face of the Golgi fuse with the plasma membrane  The constitutive exocytosis pathway supplies the plasma membrane with new lipids and protiens. Entry to this pathway does not require a specific sequence  Regulated Exocytosis: only occurs in specialize cells that have hormones/mucus/digestive enzymes that are stored in secretory vesicles for later release. These require an extracellular signal to fuse with the plasma membrane for release.  These protiens have special surface properties for sorting (selective aggregation), and allowing large amounts to build up for release o Endocytosis: ingestion of extracellular matter  Phagocytosis: Large ingested particles are enclosed in a phagosome inside the cell, the phagosome immediately fuses with the lysosome where it is degraded and the metabolites are released to the cytosol  Phagocytic cells are basically your immune system (white blood cells)  Pinocytosis: Clatherin coats pull in small amounts of extracellular fluid. The clatherin coat is shed and the pinocytotic vesicle fuses with the endosome  Receptor-mediated endocytosis: allows specific macromolecules to be taken into the cell  Example: LDL (bad cholesterol) is takin into the cell via receptor-LDL where it pulled into the cell taken to the endosome and degraded/processed in the lysosome  Endosomes an acidic purgatory for endocytotic vesicles  The receptors that bring molecules are usually recycled to the plasma membrane but some are degraded in the lysosomes, finally others transfer the cargo molecules from one extracellular space to another (Transcytosis)  Molecules that dissociate from receptors in the endosome are degraded in the lysosome o Lysosomes: organelles that use hydrolytic enzymes to digest extracellular and intracellular materials  ~40 types of hydrolytic enzymes for degrading everything from lipids to protiens to organelles to foreign matter  pH is ~5 (acidic) compared to the pH of 7.2 in the cytosol  The lysosomal membrane contains transporters for macromolecule to be transferred to the cytosol  An ATP driven pump keeps H+ protons in high concentration allowing the acidity o Autophagy: When cells eat/digest worn out organelles  An autophagosome is formed and the organelle is digested Chapter 17 Cytoskeleton:  Intermediate Filaments: o Main function: withstand mechanical stress from stretching o Toughest and most durable cytoskeletal filaments o They form the nuclear lamina, anchor to the plasma membranes at cell-cell junctions, and protect cells from tearing, and form a network within the cell o Structure:  Rope like with many strands for tensile strength  The rod domain is made up of extended alpha-helical regions that form pairs.  Levels of organization:  Filament of several groups of 8 tetramers o Association of eight tetramers  Tetramer (2 dimers liked together facing opposite ends)  Dimers (2 alpha helical monomers facing the same direction) o Alpha helical monomer  The ends of the intermediate filament are the same due to the antiparallel properties of the tetramers  The interactions are all non-covalent and depend on the combined strength of overlapping lateral interactions for their tensile strength o Location: prominent in cytoplasm of cells that have a lot of mechanical stress  Ex: nerve axons for reinforcement of the long thin projections o There are 4 classes:  Keratin (hair and nails) in epithelial cells  Most diverse form of Intermediate filament  Found in every type of epithelium in the vertebrate body  They typically span the length of cells and associate laterally through desmosomes in other cells they are also stabilized by plectin which cross-links filaments into bundles and link them to microtubules, actin filaments and desmosomes  Epidermolysis Bullosa Symplex- a mutation in keratin in which the skin ruptures with little pressure making the skin highly vulnerable to mechanical stress  Vimentin and Vimentin-related filaments- connective tissue cells, muscles cells and glial cells (cells of the nervous system)  Neurofiliments- nervous system  Nuclear lamins- nuclear envelope  They form a 2D meshwork that supports the nuclear membrane.  The phosphorylation of the meshwork causes it to fall apart during cell division and dephosphorylation after cell division allows the nuclear envelope to reform  Microtubules (largest): o Large hollow structures responsible for most of the transport in the cell. They have directionality and polarity due to the tubulin heterodimers. o They start at centrosomes in the cell and extend outward o They form the mitotic spindle during mitosis and form flagella and cilia in cells o Structure:  Hollow tube made of 13 profilaments  Profilaments with a plus and minus end o Tubulin Heterodimer made of alpha (-) and beta (+) sub units bound by noncovalent bonding  Heterodimers can be added to either end but add more rapidly to the beta (+) end  Due to dynamic instability the microtubule is always growing and shrinking in seemingly spontaneous ways  This trait of microtubules is important and allows the cell to remodel quickly.  When the plus end is anchored however the microtubule becomes fairly stable  Dynamic Instability is driven by GTP. Every free tubulin dyer has a GTP molecule bound to the beta (+) tubulin that hydrolyzes to GDP after it is added to the microtubule. The addition of tubulin subunits happens faster than the hydrolysis of GTP forming a “GTP cap” on the end of microtubules allowing the microtubule to continue growing because GTP-associated dimers bind stronger than those without GTP  When GTP hydrolyzes before the addition of another dimer then the GTP bound dimers bind very weakly causing disassembly of the microtubule o The GDP that is on the tubulin is exchanged in the cytosol for GTP so it can be bound again to the end of another microtubule o Growth:  Centrosome- made up of 2 centrioles surrounded by a centrosome matrix with y-tubulin ring complexes (starting point for a microtubule). The minus end is embedded in the y-tubulin o Motor Protiens  Dimers that have 2 globular ATP-binding heads and a single tail. They move in one direction only. The Tail binds to the cargo molecule and the Head binds to the microtubule  Kinesin (moves to the plus end or outward)  Dynein (move to the minus end or inward) o Flagella and Cilia:  Have 9 + 2 arrangement with 9 doublet microtubules and a pair of single tubules in the middle.  Ciliary Dynein is an accessory protein that attaches to one doublet with its tail and to a _____ doublet with the head allowing the sliding movement of cilia  Actin Filaments (smallest): o Very similar to Microtubules only smaller. They have polarity and actin subunits at more readily to the plus end. ATP bound actin subunits hydrolyze the ATP to ADP slower than the addition of subunits takes place so the strand grows. When ATP hydrolysis happens Treadmilling occurs. That process is very similar to Dynamic instability of microtubules the difference is treadmilling occurs on the minus end while addition of ATP-bound actin subunits occurs on the + end. Thus treadmilling because the filament never changes in length. o Actin makes up ~5% of the total protein. ~half is in filaments and the other half is free.  Thymisin and profiling mind to actin monomers in the cytosol to prevent them from being added on to the end of the filaments to regulate polymeration of actin filaments  Actin-binding Protiens:  Nucleating, severing, capping, side binding all secure/stabilize single strands  Cross-linking (cell cortex), bundling protiens and myosin all secure actin to each other  Monomer sequestering protiens keep free actin subunits from polymerizing  Cell Cortex: High concentration of cross-linked actin to support the plasma membrane and give it tensile strength  Cell crawling: Enabled by Actin polymerization.  The Process: o Actin polymerization pushes Lamellipodia (thin sheet like) or filipodia (pointy protrusions) out from the leading edge  Filpodia and lamellipodia are both exploratory and extend and recoil when necessary to find the correct path o The protrusions fuse to the surface over which the cell is moving via integrins that adhere to molecules in extracellular matrix or neighboring cells o The cell drags itself to the protrusion s and repeats this process  Phagocytosis is facilitated by the extensions of many actin filaments around the prey and fusion of the filaments and the membrane around it.  Myosin:  All actin dependent motor protiens belong to the myosin family.  Myosin-I found ______________________________________________ o has a head domain that binds to the actin and has ATP hydrolyzing abilities that allow it to “slide” along the actin filament and a tail domain that determines the type of cargo carried by the myosin o There are 2 states active GTP bound and inactive GDP bound  Myosin-II found _____________________________________________ o Structure:  2 subunits twisted together into a Myosin-II molecules  Those myosin-II molecules all linked in a bipolar manner so that there are many myosin heads and a “bare region” in the middle with each side proceeding opposite directions o Muscle Skeletal Muscle fibers MyofibrilsSarcomeres  Muscle Contraction: o The process:  (1) Skeletal muscle receives a signal from a motor nerve  (2) The neurotransmitter is released (due to excess Ca2+ in the neuron terminal)  (3) An action potential is triggered in the muscle cell plasma membrane which travels to the transvers tubules (T-tubules)  (4) The sarcoplasmic reticulum releases its store of Ca2+ which binds to tropomyosin  (5) The Myosin now can bind to Actin  (6) ATP binds to the myosin causing a conformational change allowing the myosin to move along the actin  (7) ATP is hydrolyzed to ADP and Pi allowing the myosin to effectively grab the Actin filament and pull it in o Smooth muscle contraction is largely involuntary


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