BIOG1440 Week 2

  1. Homeostasis:
    • Active mechanisms keep our key measures at set points.
    • Deviations from set points are corrected by “feedback” control mechanisms with sensors signals and responders.
    • Involves mechanistically linked systems.
    • Failure to maintain homeostasis results in disease.
  2. What are the roles of membranes?
    • Life takes place at interfaces defined by membranes. Membranes define cell structure.
    • Separate cell from environment
    • Partition biological functions within a cell
    • Exchange materials
    • Reactions occur
    • Communication happens
  3. What is the plasma membrane?
    • Boundary that separates the cell from surroundings.
    • Enables interactions with surroundings, regulates the flow of materials and signals.
    • Cellular membranes are fluid mosaics of lipids and proteins.
  4. Structure of Membrane phospholipids:
    • Instantaneously forms a selective permeable bilayer in an aqueous environment.
    • They are amphipathic, meaning they contain two regions: hydrophilic (polar, charged, phosphate) heads and hydrophobic (non-polar, uncharged, lipid) tails.
    • Hydrophobic domains (fatty acid) domains mutually interact but repel water.
    • Hydrophilic domains (phosphate heads) interact with aqueous, polar surroundings, but repel water.
  5. Explain “fluid mosaic model”
    • Membrane is a fluid structure embedded with a mosaic of proteins. This is why they’re called a “fluid mosaic model.”
    • 50:50 protein to phospholipid based on mass.
  6. Explain selective permeability
    • Membranes are selectively permeable to different materials due to selectively structured pores.
    • Could be an active process, highly regulated, can be used to generate gradients and energy potentials, uses up energy.
    • Or a passive process, requiring no energy and along the concentration gradient.
  7. How is the fluidity of the plasma membrane controlled?
    • Lipid composition controls membrane fluidity.
    • Organisms regulate the composition of the membrane bilayer according to environmental conditions through adaptive measures.
    • Different melting points (based on saturation) of the various lipids that form the hydrophobic tail determine the fluidity of the cell membrane.
    • Unsaturated lipids limit packing density due to their kinks and have lower Van der Waals forces, increasing fluidity and decreasing melting temperature.
  8. How does plasma membrane react in hot and cold environments?
    • Hotter environment: more saturation occurs.
    • Colder environment: less saturated. Highly adaptive responses.
  9. Explain Cytosol, cytoplasm
    • Inside portion of the cell.
    • The site of many biochemical reactions
    • Subcellular structures in cytoplasm have membranes. (protein synthesis, energy synthesis etc.)
  10. Do all organisms have membranes made of glycero-phopholipids?
    • No.
    • Archaea have ether linked lipid monolayer (less permeable to ion leakage due to higher stability (no double bond), remember that they survive in extreme conditions so this is useful!)
    • Not a bilayer, but its width is the same as a bilayer.
    • It’s still amphipathic.
  11. Features of fluidity with regard to phospholipids and proteins:
    • Phospholipids move laterally in the bilayer, but do not flip from one side to the other
    • Proteins move laterally but do not flip.
    • Proteins are not randomly distributed and their movement is limited (anchored to intracellular protein network, cytoskeleton, or extracellular matrix, which impose a structure) and cannot readily diffuse within the bilayer.
  12. Is the movement of proteins in the fluid-mosaic bilayer random? Why is this important?
    • Proteins are not randomly distributed and their movement is limited (anchored to intracellular protein network, cytoskeleton, or extracellular matrix, which impose a structure) and cannot readily diffuse within the bilayer.
    • This is important for the catalysis of reactions that require the interaction between different proteins through phosphorylation.
    • Phosphorylation plays a critical role in the regulation of many cellular processes including cell cycle, growth, apoptosis and signal transduction pathways.
  13. Explain the fluid mosaic model in a sentence.
    The membrane is fluid structure embedded with a mosaic of proteins.
  14. How to measure fluidity?
    • Fluorescence recovery after photobleaching. (FRAP)
    • Fluorescent protein embedded in the layer.
    • Fluorescent area in the bilayer bleached using laser.
    • Bleached proteins do not regain fluorescence readily.
    • However, fluorescence is regained in the bleached area by diffusion of unbleached, mobile proteins.
  15. What are the controls of fluidity and viscosity?
    • Lipid composition
    • Unsaturated fats are more fluid
    • Saturated fats are more viscous
    • 2. Cholesterol buffers fluidity across temperature ranges
    • At warm temperatures (above 37), cholesterol limits excess fluidity by providing bonding opportunities.
    • At cold temperatures, cholesterol maintains fluidity by preventing tight packing.
  16. Explain cholesterol:
    • Cholesterol buffers fluidity across temperature ranges
    • At warm temperatures (above 37), cholesterol limits excess fluidity by providing bonding opportunities.
    • At cold temperatures, cholesterol maintains fluidity by preventing tight packing.
  17. Explain the structure of proteins
    • Polymers of 21 amino acids which can be neutral, polar, or non-polar.
    • They are in (integral) or on (peripheral) the membrane
  18. What are the 6 major functions of membrane proteins?
    • a) transport
    • b) cell-cell recognition (immunity)
    • c) intercellular joining
    • d) attachment to the extracellular matrix (ECM) or the cytoskeleton.
    • e) enzymatic activity
    • f) signal transduction
    • Membrane proteins typically undergo dynamic chemical and structural
    • changes when performing these functions.
  19. Explain proteins’ contribution to enzymatic activity
    G protein coupled receptors activate enzymes
  20. Explain proteins’ contribution to signal transduction
    • Receptor Tyrosine Kinases.
    • RTKs are vital to activating cell growth responses
  21. Explain peripheral proteins:
    • “On” the membrane
    • Have ionic interactions with phospholipids or other proteins through their exposed charged amino acids.
  22. Explain integral proteins:
    • Extends into or often through the bilayer.
    • Transmembrane proteins go through.
  23. Explain the interaction of proteins with the phospholipid bilayer:
    • Proteins often have charged surfaces due to charged amino-acids.
    • Charged proteins are hydrophilic and can’t be transmembrane proteins, as they can’t cross the hydrophobic fatty acid section.
    • Transmembrane proteins have hydrophobic amino acids on the region spanning the plasma membrane. They may have charged aminoacids elsewhere and thus be amphipathic.
  24. What is an aquaporin?
    • Integral membrane proteins that facilitate the movement of water.
    • Structure of channel prevents protons from leaking through due to placement of + charges in the lumen.
  25. Explain cell-cell recognition of proteins
    • Cytotoxic T cells recognize antigens present in infected cells, which is a recognition event critical for immune system responses.
    • This is done through glycoprotein structures on proteins.
  26. Explain attachment to cytoskeleton and extracellular matrix (ECM)
    • Gives structure to tissues.
    • Integrins (family of integral membrane proteins) bind ECM proteins, including collagen.
  27. Eukaryotic Cell Organelles with Membranes
    • Endoplasmic reticulum: Site of transmembrane protein synthesis
    • Golgi Apparatus: Site of protein modifications, membrane fusion, protein secretion.
  28. Explain prokaryotic cell organelle membranes
    Bacterial cells have limited or no intracellular membranes. Exceptions exist.
  29. What kind of membrane proteins pass through the plasma membrane most easily?
    Small and hydrophobic.
  30. What kind of materials are moved across the plasma membrane?
    • Nutrients
    • Waste
    • Gases
    • Signaling molecules
    • Solvents
    • Solutes
  31. What are the key issues in transport?
    • What is the nature of the barrier?
    • Permeable
    • Semipermeable
    • Impermeable
    • b) What are the concentrations (concentration gradient)
    • 2nd law of thermodynamics states that solutes flow instantaneously from high conc. to low conc.
    • However active transport may allow for solutes to flow against concentration gradient.
  32. What is osmosis?
    • The net movement of water (solvent) from high concentration to low concentration across a selectively permeable membrane according to the 2nd law of thermodynamics.
    • Water has a concentration too (just like solutes) and will thus flow.
  33. Explain osmotic pressure.
    • Tendency of the cell to take up water.
    • In hypotonic environments, lysis may occur due to excessive osmotic pressure within the cell.
  34. Explain Tonicity
    • Hypotonic: Lower osmotic pressure, lower solute concentration outside cells, cells explode (lysis)
    • Hypertonic: Higher osmotic pressure, higher solute concentration outside cell, cells shrivel
    • Isotonic: solute concentration is the same as the inside of the cell. No net water movement.
  35. How do organisms deal with water imbalances?
    • Cell wall resistant to hypotonicity: Plant cells are normally turgid due to hypotonic conditions. Cell wall prevents deplasmolysis. This is called Turgor pressure.
    • Contractile Vacuole: In paramecium, contractile vacuole pumps excess water out in hypotonic environment (freshwater.)
  36. Is diffusion active or passive?
  37. What is facilitated diffusion?
    • The presence of a pore or channel specific for a solute that can’t readily cross the lipid layer (polar or hydrophilic) facilitate the diffusion of molecules along the concentration gradient.
    • Transport proteins speed the passive movement of molecules.
    • SPECIFICITY is key in channel proteins and pores. Diameter of the ion has to be just right to open the gate.
    • Proteins typically change their shape as specific molecules (ligands) bond or specific charges occur. (gatedness) These regulate flow rate.
  38. Explain transporter proteins
    • Transporters are not static, they change their shape as their specific cargo is loaded. The induced shape change allows for them to release their cargo on the other side. This is called a conformational change.
    • Can be opened or closed by other bound molecules (ligands), voltage or pH to regulate flow rate.
    • Depends on an existing energy gradient
  39. What provides specificity of channels for their transported protein?
    • Size (diameter of atom)
    • Structure of channel (charge) For example, positive charge on the inside of aquaporins prevent K+ flow.
  40. What enables channels to be gated?
    • pH gradients across the membrane change K+ channel size.
    • The channel is open below pH 5.5 and closed above 5.5 pH. Higher proton concentrations keep the channel open.
  41. What is active transport?
    • Requires energy (typically ATP, but not necessarily) allowing for things to move against the concentration gradient.
    • This is not a violation of the 2nd law of thermodynamics, because you use energy.
  42. What is the alternating access model?
    • Important for active transport.
    • Membrane proteins act like a rocker switch.
    • ATP binding and hydrolysis toggles protein between open and closed state. This is a sequential process.
    • Alternating access model allows transport against concentration gradient.
  43. Explain endocytosis and exocytosis?
    • Bulk transport across the plasma membrane is possible when the membrane is remodeled to encapsulate cellular or extracellular materials.
    • Transport in vesicles (membrane bound packets, not as individual molecules) or large macromolecules. Proteins, polysaccharides.
    • Large quantities of materials can be moved
    • Exo: ER and Golgi
  44. What are the different kinds of endocytosis?
    • Phagocytosis: Particle uptake (macrophages)
    • Pinocytosis : liquid uptake (a way to deal with hypertonic environment)
    • Receptor-Mediated Endocytosis
  45. Explain passive diffusion:
    • No energy expenditure
    • Down the concentration gradient until an equilibrium is reached and forward and reverse rates are equal.
    • Permeable compounds such as O2.
    • Molecules move randomly.
    • Entropy increases according to the 2nd law of thermodynamics.
  46. Explain Sodium Potassium Pump
    • Energy from ATP produces a conformational change that translocates ions.
    • Causes accumulation of 3Na+ outside and 2K+ inside cells
    • Leads to net positive charge outside the cell.
    • Reaches equilibrium when charge and concentration gradient is too great for channel to increase gradient.
    • Charge separation creates a membrane (electrical) potential through difference in ion concentrations and chemical potential through differences in molecule concentrations. This is called electrochemical gradient.
  47. Explain electrochemical potential?
    • A chemical potential (due to ion’s concentration gradient)
    • An electrical potential (the effect of the ion’s movement on the membrane potential)
    • Exist and both store energy.
  48. What is chemical potential?
    A difference in the concentration of molecules across the membrane.
  49. What is membrane potential?
    Voltage difference across a membrane due to differences in ion concentrations across membrane.
  50. What is cotransport? Give an example.
    • Active transport of a solute indirectly drives transport of other substances.
    • Also called secondary active transport.
    • Example: Energy in Na+ gradient used to pump glucose against the concentration gradient (active transport)
    • Sodium pump creates Na+ gradient outside of membrane. The energy in the electrochemical gradient is used to pump glucose in.
  51. Differences between facilitated diffusion and active transport?
    • Facilitated is down the conc gradient. Active is against conc gradient.
    • Facilitated is passive. Active requires energy.
    • Facilitated acts on existing energy gradient. Active creates such graident.
  52. The sodium potassium pump is electrogenic because..
    It generates a voltage across the membrane.
  53. How is diffusion measured?
    • By calculating flux.
    • In biological systems, flux is defined as the diffusion or transport of molecules across a semi-permeable membrane, such as a lipid bilayer.
  54. What factors influence flux?
    • Concentration difference across the membrane (C1-C2) (change in concentration over distance)
    • Membrane thickness (x)
    • Temperature
    • Size, change, molecular weight of the diffusing molecule.
    • Lipid solubility of diffusing molecule
  55. What is the unit for flux
    micromoles cm-2 s-1
  56. Fick’s Law:
    • J = (Permeability coefficient of the solute across the membrane)*(Membrane area)*(Concentration gradient across the membrane)
    • J is the amount of substance that moves through a given surface area per unit of time.
    • P is a measure of how readily the molecule under consideration diffuses through the membrane. This depends on the properties of both the membrane and the solute.
Card Set
BIOG1440 Week 2
membranes and transportation