Biology 1113: Week 6 Notes
Biology 1113: Week 6 Notes Biology 1113
Popular in Biology 1113
verified elite notetaker
Popular in Department
This 19 page Class Notes was uploaded by Emily Notetaker on Monday October 3, 2016. The Class Notes belongs to Biology 1113 at Ohio State University taught by Dr. Ball and Dr. Weinstein in Fall 2016. Since its upload, it has received 5 views.
Reviews for Biology 1113: Week 6 Notes
Report this Material
What is Karma?
Karma is the currency of StudySoup.
Date Created: 10/03/16
Cell Gossip! Wednesday, September 21, 2016 9:56 AM • Concepts 11.1 - 11.5, pg. 210-229 • Learning Outcomes ○ Successful students will be able to Explain how a cell covertsan external signal into an internal response (from reception to response) Describe the molecules which relay messages Describe the three different types of membranerecepetors Explain the significance of signal transduction pathways Explain how the same signal can illicit different responses in different cells Chapter 11 Wednesday, September 21, 2016 9:58 AM Cellular Signaling • Cells must have a way to "talk" to one another and to respond to environmentalstimuli ○ Local and Long-Distance Signaling Local signaling □ Cell junctions allow signaling moleculesto pass freely between adjacent cells □ Cell-cell recognition via cell-surface molecules □ Paracrine signaling When a cell releases a signal, and nearby cells respond Autocrine signaling - cells respond to their own signal Long-distance signaling □ Uses hormones(endocrine signaling) Adrenaline □ Travel through bloodstream to reach target cells □ Ability of cell to respond depends on whether it has receptors that can bind the signaling molecule • 3 Stages of Cell Signaling 1. Reception - The signal is detected by the cell - A receptor protein on the surface of the cell (or inside the cell) bind the signaling molecule(ligand) and transmits the signal - Binding is very specific and usually causes the receptor to change its shape - 3 main types of membranereceptors □ G protein-coupledreceptors Plasma membrane receptorsare associated with G proteins ◊ G protein acts as on/off switch □ Receptor TyrosineKinases (RTKs) Membrane receptorswith enzymaticactivity which attach phosphates to tyrosine residues One RTK may activate 10 or more different pathways ◊ Abnormally activated RTKs are associated with many types of cancer Breast cancer patients have more than 10 RTK, so their enzymatic activity acts at a much faster rate □ Ion channel receptors Binding of ligand causes conformationalchange When gate opens specific ions can pass through Intracellular Receptors ◊ Found either in the cytoplasmor nucleus ◊ Signaling molecule must be able to move through the membrane Steroid hormones Thyroid hormones Nitric oxide 2. Transduction - The signal is convertedinto a form that can cause sometype of cellular response - The relay of signals from receptors to target moleculesin the cell - Often involvesmultiple steps □ Allows the signal to be amplified □ Also allows for more regulation - Message is often passed along through a change in □ Phosphorylation= protein kinases □ Phosphorylation= protein kinases □ Dephosphorylation= protein phosphatases - Second messengers □ Small, non-protein, water-soluble, molecules Can easily spread message throughout cell □ 2 most commonsecond messengers: cAMP ◊ Cyclic adenosine monophosphate ◊ Adenylyl cyclase + ATP --> cAMP Vibrio cholera produces a toxin that causes a G protein to be unable to hydrolyze GTP to GDP so it remains stuck in "ON" position … continually activating cAMP – Levels of cAMP causes salt to be excreted into the intestines and water follows = diarrhea Calcium ions (Ca2+) ◊ Used as a second messenger in both G-protein and tyrosine kinase receptor pathways ◊ Increasing Ca2+ concentrationscause a variety of responses Muscle contraction Secretion Cell division ◊ Level of Ca2+ is 10,000xhigher outside cell Ca2+ is actively transported out of the cell or into ER 3. Response - The cell responds in some way to the signal - Whole point of the signal is to tell the cell to do something □ Usually regulate gene expression (protein synthesis) □ Regulate the activity of a protein (i.e. open an ion channel) - Specificity and coordination of response □ All the cells in your body contain the same DNA so why is a heart cell different from a liver cell? Different responses are turned on in different cells Different cells therefore have different proteins - Apoptosis: To be or not to be □ Apoptosis (cell suicide) Eliminates cells that are damaged, infected or at the end of their lifespan □ Necessary for proper development i.e. hands and feet - DNA and organelles are chopped up and then packaged into vesicles which are engulfed and digested by scavenging cells - How could cancer result from a defect in apoptosis? Cancer can result from a failure of cell suicide; melanomahas been link to faulty forms of the human version of the Ced-4 protein Top Hat Questions Friday, September 23, 2016 9:28 AM A mutant form of a G protein lacks the ability to function as a GTPase. What effect do you think this would have? a. No effect, other proteins can perform the same function b. The G protein will remain inactive c. The G protein will be constitutivelyactive d. GDP will accumulate in the cell Which of the following would be inhibited by a drug that specifically blocks the addition of phosphate groups to proteins? a. Phosphatase activity b. Ligand-gated ion channel signaling c. Receptor tyrosinekinase activity d. Adenylyl cyclase activity e. G protein-coupledreceptor binding What characteristic allows some signaling molecules to pass through the membrane to deliver their message? a. Hydrophilic b. Hydrophobic c. Polar d. Negatively charged What is mostlikely to happen to an animal's target cells that lack receptors for local regulators? a. Hormoneswould not be able to interact with target cells b. They might not be able to multiply in response to growth factors from nearby cells c. They could develop normally in response to neurotransmittersinstead d. They might compensateby receiving nutrients via a factor e. They could divide but never reach full size Which of the following best describes a signal transduction pathway? a. Binding of a signal moleculeto a cell protein b. Catalysis mediated by an enzyme c. Sequence of changes in a series of moleculesresulting in a response d. Binding of a ligand on one side of a membrane that results in a change on the other side e. The cell's detection of a chemical or mechanical stimulus Which of the following is true of transcription factors? a. They initiate the epinephrine response in animal cells b. They control gene expression c. They regulate the synthesis of DNA in response to a signal d. They regulate the synthesis of lipids in the cytoplasm e. They transcribe ATP into cAMP Book Overview (pg. 210 - 229) Monday, September 26, 2016 3:21 PM Chapter 11: Cell Communication • Cellular Messaging ○ Cells signal to each other and interpret the signals from other cells and the environment These signals are often chemical, but can include light and touch Example: The "fight-or-flight" response is triggered by a signaling moleculecalled epinephrine or adrenaline Section 1.1 | External signals • External signals are converted to responses within the cell • What does a "talking" cell say to a "listening" cell, and how do they respond? ○ Cells of yeast have two different mating types and identify their mates by chemical signaling. This is used to make bread, wine, and beer. ○ This process is called signal transduction pathway and happens in both yeast and animal cells • Local and long-distance signaling ○ Local: Both plants and animals have cell junctions that directly connect the cytoplasmof adjacent cells □ Signaling substance dissolved in the cytosolcan pass freely between these adjacent cells Cell-cell recognition - Animal cells communicatethrough direct contact between membrane-bound cell-surface molecules □ This process is important in embryonic developmentand immune response Paracrine signaling - numerous cells can simultaneouslyreceive and respond to the moleculeof a factor produced by a single cell in their vicinity □ Messenger moleculesare secreted by signaling cells and travel short distances □ Growth factors - local regulators that stimulate nearby target cells to grow and divide Synaptic signaling - an electrical signal along a nerve cell triggers the secretion of neurotransmittermolecules.The molecules act as chemical signals, diffusing across the synapse, which triggers a response in the target cell □ Muscle or nerve cell **Because of their cell walls, plants use different mechanismsto communicatelocally (like plasmodesmata- plant cell junctions) (like plasmodesmata- plant cell junctions) ○ Long-distance: Hormones(vary greatly in size) □ Endocrine signaling - in animals, specialized cells release hormonemolecules, which travel via the circulatory system to other parts of the body, where they reach the target cells that recognize and respond to the hormones Insulin regulates sugar levels in the blood is a protein with thousands of atoms □ Plant growth regulators - plant hormones, travel in vessels or movethrough cells/ diffuse through the air as a gas to reach their target cells Ethylene is a gas hormone that promotesfruit ripening and regulates growth, very small and can pass through cell walls ○ What happens when a cell encounters a secreted signaling molecule? The ability of a cell to respond is determined by whether it has a specific receptor moleculethat can bind to the signaling molecule.The information must then be transduced or transformed into something it can read before it responds. • The three stages of cell signaling 1. Reception 2. Transduction 3. Response Section 11.2 | Reception • Reception is the target cell's detection of a signaling molecule coming from outside of the cell that then binds to a receptor protein • Reception of the signal depends on the receiver ○ Signals emitted by an a yeast cell are only "heard" by its mate, an alpha cell ○ Epinephrine will encounter many types of cells when circulating throughout the bloodstream,but only certain target cells will detect and react to the hormone ○ A receptor protein on or in the target cell allows for the cell to "hear" the signal and respond to it ○ A signaling moleculeacts as a ligand which is a molecule that specifically binds to another molecule Ligand binding generally causes a receptor protein to change in shape □ The changing of shape activatesthe receptor,enabling it to interact with other molecules For other receptors,ligands bonding causes the grouping of two or more receptor molecules • Receptorsin the plasma membrane ○ These receptorproteins are on the cell surface ○ Water-soluble signaling moleculesbind to specific sites on the transmembrane receptor proteins that transmit info from the extracellular environmentto the inside of the cell. There are three major types of the cell-surface transmembranereceptors: 1. G protein-coupledreceptors (GPCR) Works with a G protein ( a protein that binds GTP) Yeast mating factors, epinephrine, other hormones,and neurotransmittersuse GPCRs GPCR-based signaling systems include the roles of embryonic developmentand sensory reception (vision, smell, and taste) When G proteins malfunction, they cause cholera, whooping cough, botulism 2. Receptor tyrosinekinases (RTKs) Plasma membrane receptorscharacterized by having enzymaticactivity ◊ A kinase in the RTK is an enzyme that catalyzes the transfer of a phosphate group from ATP to the amino acid tyrosine on a phosphate group from ATP to the amino acid tyrosine on a substrate protein ◊ Summarized: RTKs are membraneacceptors that attach phosphates to tyrosines RTKs activatetransduction pathways which help the cell regulate and coordinate cell growth and cell reproduction ◊ The difference between GPCRs and RTKS, is that RTKs can activate many pathways, where GPCRs can only activate one Abnormal RTKs are associated with many types of cancers 3. Ion channel receptors(Ligand-gated and voltage-gated ion channel) When a signaling moleculebind as a ligand to the receptor,the gate opens and closes allowing/blocking specific ions to come through a channel Important in the nervous system ◊ Neurotransmittersreleased between two nerve cells bind as ligands to ion channels on receiving cells which causes the channels to open. ◊ Some of these gate ion channels are controlledby electrical signals instead of ligand Present on the ER ○ When cell-surface receptorsmalfunction, they are associated with cancer, heart disease, and asthma • Intracellular Receptors ○ These proteins are found in the cytoplasmor nucleus of target cells In order to reach them, the signaling molecule has to pass through the plasma membrane of the target cell ○ Once a hormonebinds to the protein, the receptor is changed into a hormone-receptorthat is able to cause a response that can turn on and off particular genes Ex: Aldosteroneis secreted by the cells of the adrenal gland. It travels through blood and enters cells all over the body, but only elicits a response in the kidney cells. It controls water and sodium flow in the kidneys and affects blood volume Section 11.3 | Transduction Transduction - cascades of molecular interactions relay signals from receptors to target moleculesin the cell Transduction is a multi-step pathway, which makes it possible for the cell signaling to amplified to multiple cells Signal Transduction Pathways • The first step in the chain of molecular interactions • It is like falling dominos - The binding of a specific signal to a receptor will activateone molecule, which will activateanother, and another, and so on until it reaches the protein that produces the final cellular response is activated. • The signaling molecule is not physically passed from protein to protein, only the informationis which can change the shape of the protein at each step Protein Phosphorylationand Dephosphorylation • Phosphorylationand dephosphorylationis a widespread cellular mechanism for regulating protein activity • The concept of activating a protein by adding or removing one or more phosphate groups ○ Protein kinase - and enzyme that transfers phosphate groups from ATP to a protein RTK is a specific type of protein kinase that phosphorylatestyrosines on the other RTK in a dimer Most kinases usually act on different proteins themselves Protein kinases are important to the regulation of cell division, abnormal kinases can Protein kinases are important to the regulation of cell division, abnormal kinases can cause cancer ○ Phosphorylationcascade - a series of different proteins in a pathway are phosphorylated in turn, each protein adding a phosphate group to the net one in line. Each encounter causes a shape change because of the interaction of the newly added phosphate groups with charged or polar amino acids on the protein □ The shape alteration usually activates the protein or alters the function ○ Protein phosphates - enzymes that rapidly removephosphate groups from proteins (dephosphorylation) Dephosphorylationallows the signal transduction pathway to be turned off which allows for the protein kinases to be reused The process of phosphorylation and dephosphorylationacts a molecularswitch that can be used to balance the active kinase moleculesand active phosphate molecules Small Molecules and Ions as Second Messengers • Ions (second messengers)are also a part of signal pathways Ions are called second messenger because of the pathway's "first messenger"is the extracellular signaling molecule or ligand that binds to the membrane receptor Second messengersare small and water soluble, so they can spread throughout the cell by diffusion Their pathways are initiated by GPCRs and RTKs ○ Cyclic AMP A second messenger that carries the signal initiated by epinephrine from the plasma membrane of the liver/musclecells into the cell's interior, where it signals to breakdown glycogen Adenylyl cyclase - an enzyme in the plasma membrane that converts ATP to cAMP in response to an extracellular signal (epinephrine) Regulation of the cell metabolism is provided by G protein systemsthat inhibit adenylyl cyclase, thereforemore cAMP cannot be made Diseases □ Cholera - a bacteria found in water when it is contaminatedby human feces Forms a biofilm on the small intestine to produce a toxin The toxin is an enzyme that modifies the G protein used in regulating salt and water secretion Adenylyl cyclase continues to make cAMP which results in high concentrationsof it which makes the intestinal cells secrete large amount of salt into the intestines then water This makes the patient have diarrhea which can deplete the body of water and nutrients ○ Calcium Ions Neurotransmitters,growth factors, and some hormonesinduce responses in their target cells that increase the cytosolicconcentrationof Ca2+ The increase of Ca2+ concentration causes muscle cell contraction, secretionof certain substances, cell division, greening in response to light in plants Cells use Ca2+ as a second messenger in pathways triggered by GPCRs and RTKs Can be used as a second messenger because its concentration in the cytosolis much lower than outside the cell Section 11.4 | Response Nuclear and CytoplasmicResponses • A signal transduction pathway ultimatelyleads to the regulation of cellular activities and the response at the end may occur in the nucleus or cytoplasm ○ Like in an activated steroid receptorthe final activated molecule in a signaling pathway functions as a transcription factor, and the response to this is transcription, the synthesis of one or more specific mRNA one or more specific mRNA • Nuclear responses to a signal: the activation of a specific gene by a growth factor • Signal receptors, relay molecules, and second messengers participate in a variety of pathways, leading to both nuclear and cytoplasmicresponses Regulation of the Response • The extent and specificity of the response are regulated: 1. Signaling pathways (enzyme cascades) amplify the cell's response to a single signaling event The degree of amplification depends on the function of the molecules in the pathway 2. The steps in a multistep pathway provide control points at which cell's response can be further regulated This allows coordinationwith other signaling pathways, and contributes to specificity of the response Some kinds of signals trigger responses in different types of cells but elicit different responses □ Epinephrine makesthe liver cell break down glycogen, while epinephrine makes the heart cells contract □ Why? Different kinds of cells have different collectionsof proteins which respond and handle the signal differently How cells respond differently: A. A different receptorprotein is used for the same signaling molecule,leading to a different response B. A pathway that is triggered by a single kind of signal diverges to produce two responses; such branched pathways often involve RTKs or second messengers C. Two pathways are triggered by separate signals that convergeto modulate a single response 3. The overall efficiency of the response is enhance by the presence of scaffolding proteins (large relay proteins to which several other relay proteins are simultaneouslyattached) This enhances the speed and accuracy of signal transfer, because the rate of protein- protein interaction is not limited Diseases: □ Wiskott - Aldrich syndrome The absence of a single relay protein (in the immune system) Leads to abnormal bleeding, eczema,predisposition to infections, leukemia 4. Termination of the signal For a cell to remain capable of responding to signals, each molecular change in a signaling pathway must las only a short time Plasma Membrane: The Moat of the Cell Wednesday, September 14, 2016 9:20 AM • Concepts 7.1 - 7.5, pg. 125-138 • Learning Outcomes ○ Successful students will be able to: Explain why the plasma membrane is referred to as a fluid mosaic Explain the various functions of membrane proteins Explain the differences between passive, facilitated and active transport Predict what will happen to a cell when it is placed in solutions of different tonicities Ch.7 Lecture Notes Wednesday, September 14, 2016 9:22 AM Plasma Membrane: What's it made of? • Lipids and Proteins ○ Phospholipids What property allows them to easily form membranes? □ Hydrophobic and hydrophilic ends • Fluid Mosaic model ○ Fluid structure with a varietyof proteins embedded or attached to it Membrane Proteins • Different types of cells contain different membrane proteins • 2 Major types: ○ Integral: embedded in the membrane Nonpolar portion of protein extends into the hydrophobic interior of the bilayer ○ Peripheral: looselybound to surface of membrane Often interact with exposed surface of integral proteins • Functions: ○ Transport ○ Enzymatic activity ○ Signal transduction ○ Cell-cell recognition ○ Intercellular joining ○ Attachment to the cytoskeletonand extracellular matrix **Don't need to memorizethese** Question: Which of the following is true of integral membraneproteins? Answer: They are usually transmembraneproteins (a portion inside and outside) ○ All proteins have a tertiary structure ○ They are mobile within the bilayer Question: In order for a protein to be embedded in the cell membraneit would have to be… Answer: Both hydrophobic and hydrophilic Importance of Carbohydrates • Necessary for cell-cell recognition ○ How liver cells distinguish themselvesfrom muscle cells ○ How your body recognizes foreign invaders This is why you can't just give someoneblood or a kidney • Role of MembraneProteins in HIV Infection: ○ HIV can infect a cell that has a co-receptor(CCR5) on its surface, as in most people. ○ HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals ○ Berlin patient - had leukemia and HIV, when he underwent a bone marrow transplant, the HIV was gone. This lead to the experimentalresearch of modificationof cells and the CCR5. How is traffic across the membrane regulated? • All moleculescan't just pass through whenever… ○ The membrane is selectivelypermeable Nonpolar molecules(hydrocarbons, CO2, O2) can cross easily Ions and polar molecules are impeded by the hydrophobic interior of the membrane □ Use transport proteins to cross membrane MovementAcross the Membrane • Diffusion (Passive Transport) ○ Movementof any molecule Down its concentration gradient (from HIGH to LOW) Each substance movesdown its OWN  gradient ○ Each substance movesdown its OWN  gradient Unaffected by  of other substances ○ Spontaneous (requires no energy) ○ Osmosis = diffusion of WATER across a selectivelypermeable membrane ○ Tonicity ○ Ability of a surrounding solution to cause a cell to gain/lose water Isotonic = environmentsame as the cell □ No net movement,cell is stable Hypertonic = more solutes in environment □ Water moves out of cell, cell shrivels Hypotonic = less solutes in environment □ Water moves into cell, cell swells (and may burst) • Facilitated Diffusion ○ Compositionof the lipid bilayer impedes the diffusion polar molecules and ions ○ Transport proteins help them diffuse passively across the membrane ○ No energy required (High  to Low ) ○ Channel Proteins ○ Provide door way through membrane ○ Aquaporins transport 3 billion water moleculesper second ○ Carrier Proteins ○ Change shape to shuttle moleculeacross the membrane ○ Cystinuria - defect in carrier protein that transports cysteine across membraneof kidney cells • Active Transport ○ Movementof moleculesagainst their  gradient requires energy ○ Incredibly important for nervous system ○ Uses carrier proteins ○ Ion pumps ○ Crucial for maintaining electrochemicalgradients and membrane potential for nerve impulses ○ Cotransport ○ Uses the  gradient created by an ATP-poweredpump to power the transport of a molecule against it  gradient • Bulk Transport ○ Allows large moleculesand particles to enter the cell ○ Requires energy ○ Endocytosis Phagocytosis Pinocytosis Receptor-mediateendocytosis ________________________________________________________________________ Top Hat Questions Monday, September 19, 2016 9:51 AM Which of the following would likely movethrough the lipid bilayer of a plasma membrane mostrapidly? a. CO2 - would move through the quickest b. K+ - it has a polar charge on it so it will not be able to pass through easily c. Starch - the rest are very large moleculesso they would need a transporter d. Glucose e. An amino acid Which of the following statementsabout osmosisis correct? a. If a cell is placed in an isotonic solution, more water will enter the cell than leaves the cell b. Osmoticmovementof water in a cell would likely occur if the cell accumulates water from its environment c. The presence of aquaporins (proteins that form water channels in the membrane)should speed up the process of osmosis d. If a solution outside the cell is hypertonic compared to the cytoplasm,water will moveinto the cell by osmosis e. Osmosis is the diffusion of water from a region o flower water concentrationto a region of higher water concentration The solution in the arms of a U-tube are separated at the bottom of the tube by a selectivelypermeable membrane. The membraneis permeable to sodium chloride but not to glucose. Side A is filled with a solution of 0.4 M glucose and 0.5 M sodium chloride, and side B is filled with a solution containing 0.8 M glucose and 0.4 M sodium chloride. Initially, the volume in both arms is the same. Refer to the figure to answer the following question. At the beginning of the experiment, a. Side A is hypotonic to side B b. Side A is hypertonic to side B c. Side A is hypertonic to side B with respect to glucose d. Side A is isotonic to side B e. Side A is hypotonic to side B with respect to sodium chloride An animal cell lacking oligosaccharides on the external surface of its plasma membrane would likely be impaired in which function? a. Transporting ions against an electrochemicalgradient b. Cell - cell recognition c. Maintaining fluidity of the phospholipid bilayer d. Attaching to the cytoskeleton e. Establishing the diffusion barrier to charged molecules **What would be observed by live-cell fluorescence microscopyimmediatelyafter HIV entry if HIV is endocytosedfirst, and then later fuses with the endocytoticvesicle membrane? a. A spot of red fluorescence will be visible on the infected cell's plasma membrane,marking the site of membrane fusion and HIV entry b. A spot of red fluorescence will remain outside the cell after delivering the viral capsid c. Fluorescence microscopydoes not have enough resolution to visualize fluorescentlylabeled HIV virus particles d. A spot of red fluorescence will diffuse in the infected cell's cytoplasm e. The red fluorescentdye-labeled lipids will appear in the infected cell's interior Book Overview (pg. 124-138) Tuesday, September 20, 2016 11:01 PM Section 7.1 | Cellular membranes are fluid mosaics of lipids and proteins • Membranes are mostlymade out of lipids (mostabundant is phospholipids due to their ability to form membranes from their amphipathic nature) and proteins (carbohydratesplay a smaller role) ○ Amphipathic - has both a hydrophilic region and a hydrophobic region A phospholipid bilayer can exist as a stable boundary between two aqueous compartments Most membrane proteins are amphipathic ○ Fluid mosaic model - the membrane is a mosaic of protein moleculesbobbing in a fluid bilayer of phospholipids The proteins are in groups that carry out commonfunctions • Fluidity of Membranes ○ Phospholipids move laterally in the membrane, proteins are larger than lipids so they move more slowly ○ The fluidity of a membrane affects permeability and the movementof proteins When membranes solidify, permeability changes and enzymatic proteins become inactive When membranes are too fluid, they cannot support protein function either ○ Membrane Lipid Composition Membranes remain fluid at all temperatures until phospholipids settle into a closely packed arrangement □ Depends on types of phospholipids (like unsaturated and saturated) Example: Fishes that live in extremecold have membraneswith a high proportion of unsaturated hydrocarbon tails which enables them to remain fluid in the cold temperatures Example: Winter wheat increases the percentage of unsaturated phospholipids in autumn so that the membranes do not solidify during winter • Membrane proteins ○ A membraneis like a collage of different proteins that are grouped and embedded in the fluid matrix of the lipid bilayer ○ The proteins determinethe function of the membrane ○ Two major types of proteins: Integral proteins - penetrate the hydrophobic interior of the lipid bilayer □ Most span the membrane,others extend partway into the hydrophobic interior □ Consist of one or more nonpolar amino acid helices in hydrophobic region □ Have hydrophilic channels that allow passage of hydrophilic substances like □ Have hydrophilic channels that allow passage of hydrophilic substances like water, passage through the membrane Peripheral proteins □ Are not embedded in the lipid bilayer, instead they are looselybound to the surface of the membrane On extracellular side, attached to fibers of extracellular matrix On cytoplasmicside, attached to cytoskeleton ○ Functions: Transportation through the cell membrane Enzymatic activity (may carry out the steps of a metabolic pathway) Attaching a cell to a neighbor cell/extracellularmatrix Signal relay to the inside of the cell Cell-cell recognition Intercellular joining ○ Example: A protein on the surface of immune cells called CD4 helps HIV infect cells leading to AIDS. People who have a protein called CCR5 attached to the cell surface is a co-receptor of HIV, so if there is an absence of this protein, the HIV virus cannot enter the cell. Now a drug is being developedthat will target CCR5 and mask the proteins presence so that HIV cannot enter a cell. • Membrane carbohydrates ○ Short, branched chains of fewer than 15 sugar units Glycolipids - when carbohydrates covalentlybond with lipids Glycoproteins - when carbohydrates covalentlybond with proteins ○ Cells recognize other cell by binding to moleculeswith carbohydratein the extracellular surface of the plasma membrane Cell to cell recognition - a cell's ability to distinguish one type of cell from another □ Helps to sort cells into tissue and organs □ Basis for rejecting foreign cells in the immune system Because the carbohydratesvary in location and moleculemake-up, cells are distinguishable • Membranes are made up of two lipid layers (with different lipid compositions)that are inside and outside faces 7.2 | Membrane structure results in selectivepermeability • The permeability of the lipid bilayer ○ Can pass through easily and rapidly: Nonpolar molecules(CO2 and O2) □ Because the moleculesare hydrophobic the can dissolve in the lipid bilayer of the membrane ○ Impedes movement,movesthrough slowly Polar molecules □ Pass through lipid bilayer very slowly Ions □ Are less likely to penetrate the hydrophobic interior of the membrane because they are hydrophilic • Transport proteins ○ Span the membrane and allow hydrophilic substances to avoid contact with lipid bilayer Channel proteins - have a hydrophilic channel that certain moleculesand ions can use as a tunnel through the membrane □ Aquaporins - channel proteins that facilitate water molecules Carrier proteins - hold on to passengers and change shape to shuttle them across membrane ○ Each protein is specific for a certain substance (so it only allows passage for that one type of substance) substance) Ex: A carrier protein in the red blood cells transports glucose through the membrane, but rejects fructose passage 7.3 | Passive transport • Diffusion - the movementof particles of any substance so that they spread out into available space ○ A result of the constant motion of molecules ○ Diffusion is directional ○ Rule of diffusion: a substance will diffuse from where it is moreconcentrated to where it is less concentrated Concentrationgradient - the region along which the density of a chemical substance increases or decreases □ Any substance will diffuse down its concentration gradient ○ Diffusion is a spontaneous process (requires no energy) ○ Ex: Cellular respiration where oxygen diffuses into the cell across the plasma membrane • Passive transport - diffusion of a substance across a membrane with no energy investment ○ The concentrationgradient is like potential energy which drives diffusion • Osmosis and water balance ○ Osmosis - diffusion of water across a selectivelypermeable membrane ○ Sugar moleculesare too big to pass through the pores in the membrane of the figure below Therefore,water moleculescluster around them and the free water molecules diffuse across the membranein order to make the concentrationon both sides equal ○ Tonicity - the ability of a surrounding solution to cause a cell to gain or lose water Depends upon concentration of solutes that cannot cross membrane relative to inside the cell 3 types of solutions: □ Isotonic - there will be no net movementof water across the plasma membrane The volume of an animal cell is stable □ Hypertonic - there will be movementout of the cell The cell will lose water, shrivel, and most likely die ◊ This is why an increase in saltiness of a lake can kill animals □ Hypotonic - water enters a cell faster than it is leaving The cell will swell and lyse (burst) A cell without rigid cell walls cannot tolerate excessiveuptake or loss of water □ Sea water is isotonic to marine invertebratestherefore they can live there □ Extracellular fluid is isotonic to the cells in land animals □ Osmoregulation- the control of solute concentration and water balance Example: paramecium lives in pond water which is hypotonic to the cell, Example: paramecium lives in pond water which is hypotonic to the cell, so the plasma membraneof this organism is much less permeable to water compared to other organism which slows the uptake of water. The cell also has a pump that rapidly pumps water out of the cell. A cell with cell walls like plants, prokaryotes,and fungi are able to maintain their water balance □ Cell walls exert turgor pressure that opposes water uptake which makes the cell turgid (very firm) which is a healthy state for cells Hypotonic solution - make turgid cells Hypertonic solution - make flaccid cells □ Cell walls however cannot protect cells if they are immersedin a hypertonic environment The plant cells will shrivel Plasmolysis- causes the plant to wilt and can lead to plant death • Facilitated diffusion - polar moleculescan pass through plasma membranewith the help of transport proteins ○ Channel proteins - provide corridors for specific molecules/ionsto cross Aquaporins □ Kidney cells have many aquaporins so that they can reclaim water from urine before it is excreted Ion channels - transport ions □ Act as gated channels which open and close in response to electrical stimulus Example: In a nerve cell, an ion channel opens in response to an electric stimulus which allows a stream of potassium ions to leave the cell □ Other gated channels open and close when a specific substance binds to the channel ○ Carrier proteins - change shape which is triggered b the binding and release of the transported molecule 7.4 | Active transport • Active transport - uses energy to move solutes against their gradients ○ The transport proteins that move solutes against the gradient are all carrier proteins Instead of picking them up though, they help them to diffuse ○ Allows a cell to maintain internal concentrationsof small solutes that differ from the environment Example: the plasma membranepumps Na+ out of the cell and K+ into the cell despite the lower concentration of Na+ ions in an animal cell ○ ATP powers active transport by transferring the terminal phosphate group directly to the transport proteins which allows the protein to change shape Example: The sodium phosphate pump exchanges Na+ for K+ across the plasma membrane of animal cells • Ion pumps ○ Membrane potential - the voltage across a membrane Is an energy source that affects the traffic of all charged substances across the membrane Electrochemicalgradient - the membranepotential favors the passive transport of cations into the cell and anions out of the cell because the inside of the cell is negative compared to the outside □ An ion diffuses down its "electrochemicalgradient" not concentrationgradient Some membrane proteins that actively transport ions contribute to the membrane potential ○ Electrogenic pump - transport protein that generates voltage across a membrane The sodium-potassium pump is the main electrogenic pump of animal cells The proton pump is the main electrogenicpump of plant, fungi, and bacteria cells The proton pump is the main electrogenicpump of plant, fungi, and bacteria cells □ Actively transports protons out of the cell Electrogenic pumps help store energy that is used for cellular work • Cotransport ○ A transports protein can couple the diffusion of the solute to the transport of a second substance against its own concentration gradient Example: a plant cell uses H+ generated by ATP-poweredproton pumps to drive the active transport of amino acids, sugars, and several other nutrients into the cell Since we know about cotransportproteins in animal cells, diarrhea in developing countries where the colon cannot reabsorb sodium which can cause death, is now being treated by drinking a solution that contains high concentrationsof salt and glucose. The solutes are taken up by sodium-glucosecotransporterson the surface of intestinal cells and passed through the cells into the blood. 7.5 | Exocytosis and endocytosis • Since small solutes and water enter and leave the cell by diffusion, how do large molecules like proteins and polysaccharides travel across the membrane? ○ They travel across the membranein bulk, packaged vesicles (bulk transport) through exocytosisand endocytosis • Exocytosis- the cell secretescertain molecules by the fusion of vesicles with the plasma membrane ○ When the membrane of a vesicle and the plasma membranecome into contact, proteins arrange the lipid moleculesof the bilayer so that the membranesfuse, and the contents of the vesicle will spill out of the cell and the vesicle becomesa part of the plasma membrane ○ Secretorycells use exocytosisto export products Example: pancreas secretesinsulin into the extracellular fluid Example: nerve cells release neurotransmittersthat signal other neurons or muscle cells Example: when plant cells are making cell walls, proteins and carbohydrates are delivered from the Golgi vesicle to the outside of the cell • Endocytosis- the cell takes in moleculesand particulate matter by forming new vesicle from the plasma membrane ○ A small area of the plasma membranewill sink inward to form a pocket that will pinch in to form a vesicle that contains material that was outside the cell Phagocytosis - a cell engulfs a particle by extending pseudopodia around it and packaging it within a membranoussac called a food vacuole. The particle will be packaging it within a membranoussac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome Pinocytosis - a cell continuously gulps droplets of extracellular fluid into tiny vesicles formed by infoldings of the plasma membrane. The cell will then obtain the molecules dissolved in the droplets. Receptor-mediatedendocytosis- a specialized type of pinocytosis that enables the cell to acquire the bulk quantities of specific substances, even those these substances may not be very concentratedin the extracellular fluid. Specific solutes bind to receptor sites within proteins. The receptor proteins then cluster in the coated pits and the pits form a vesicle contain the bound molecules.