Cell Membrane: Gateway of Life

2. Chapter 8 RQ • What is the term for how the cell membrane “chooses” what enters and leaves the cell? • What kind of microscopes are used to study the cell membrane? • What is the term for the movement of a substance from a high to a low concentration? • What happens to an animal cell in a hypotonic solution? • What osmotic solution are most of your cells in (you hope)?

4. Boundary that separates the living cell from its nonliving surroundings  can discriminate in its chemical exchanges with the environment Lipids can enter rapidly  membranes are made of lipids Phospholipids form a membrane in water Phospholipid content is enough to cover cells 2X, creating a ‘bilayer’ Membranes contain protein and lipid  protein in membrane  1. Describe the function of the plasma membrane and explain how scientists used early experimental evidence to make deductions about its structure and function.

6. 2. Describe the Davson-Danielli membrane model and explain how it contributed to our current understanding of fluid-mosaic membrane structure. • Cell membrane is a phospholipid bilayer between 2 layers of globular proteins • Polar heads oriented toward the protein layers (philic zone) • Nonpolar tails are oriented in between heads (phobic zone) • Membrane is about 8um thick Other contributions: • Fluid mosaic model   • Proteins individually embedded 

8. Most membrane lipids and protein drift laterally Solidification would result in permeability changes Hydrocarbon tails kink and hinder the close packing of phospholipids Carbohydrates act as cell markers (to identify cells from one another)  3. Describe the fluid properties of the cell membrane and explain how membrane fluidity is influenced by membrane composition. Also, explain how hydrophobic interactions determine membrane structure and function.

9. 4. Describe how proteins are spatially arranged in the cell membrane and how they contribute to membrane function. • Proteins are individually embedded in the phospholipid layer • Hydrophilic portions are maximally exposed to water to promote stability • Membrane is a mosaic of proteins bobbing in fluid bilayer • Proteins drift move slowly than lipids 

10. 5. Describe the factors that affect selective permeability of membranes. • Regulation of the type and rate of traffic • Membrane solubility characteristics of phospholipid bilayer • Presence of specific integral transport proteins • Nonpolar will dissolve easily (O2, CO2, and hydrocarbons) • Polar  uncharged: (H2O, ethanol) & small will pass  large (glucose) & ions will not pass easily

12. 6. Define diffusion, concentration gradient, and passive transport. Also, explain what regulates the rate of active transport. Diffusion – the net movement of a substance down a concentration gradient - results from random movement - decreases free energy, increases entropy(random) Concentration gradient – the potential to move stuff across the membrane Passive transport – diffusion of a substance across a biological membrane - regulated by the permeability of the membrane 

13. 7. Explain why a concentration gradient across a membrane represents potential energy. • The concentration gradient is the POTENTIAL to move stuff across a membrane, therefore representing potential energy, until movement. • KINETIC energy is present when movement of the particles occurs across the membrane with the concentration gradient. 

14. 8. Define osmosis, hypertonic, hypotonic, and isotonic and predict the direction of water movement based upon differences in solute concentrations. Osmosis – the diffusion of WATER across a selectively permeable membrane (diffusion down a concentration gradient) Isotonic solution – equal concentrations of solutes inside and out of the cell 

15. Hypertonic solution – will shrivel a cell due to a higher concentration of solutes outside cell (water leaves cell) Hypotonic solution – will lyse a cell due to a lower concentration of solutes outside (water rushes into cell and bursts) Continued…

17. 9. Describe how living cells with and without walls regulate water balance. Animal cells  not tolerant of excessive uptake or loss of water - prefer isotonic solutions -can osmoregulate – pump in & out water Plant cells  must be hypoosmotic with the environment; allows cell to be ‘turgid’ - provides mechanical support to cells 

19. 10. Describe one model for facilitated diffusion. Example: Transport proteins • Transport protein most likely remains in place within the plasma membrane, alternating between 2 conformations • In 1, the transport protein binds to the solute and deposits it on the cell-side • Solute binding may trigger the conformational change • Diffusion of ions and polar molecules that is assisted by membrane proteins 

21. 11. Describe how transport proteins are like enzymes. • Transport proteins can be inhibited by molecules that resemble the solute normally carried by the protein (similar to competitive inhibitors in enzymes) • They are specific for the solutes they transport  specific binding site analogous to an enzyme’s active site 

22. 12. Explain how active transport differs from diffusion. Active transport – an energy-requiring process during which a transport protein pumps a molecule across a membrane AGAINST the concentration gradient • Different: 1. Requires energy 2. Is against gradient 

24. 13. Explain what mechanisms can generate a membrane potential or electrochemical gradient. Membrane potential = voltage across membranes Electrochemical gradient = the chemical force (the ion’s concentration gradient) and the electrical force (membrane potential) on the ion’s movement across the cell membrane • The concentration gradient of an ion • Negatively charged proteins inside the cell • Plasma membrane’s selective permeability to various ions • The Na – K pump 

26. 14. Explain how large molecules are transported across the cell membrane. Exocytosis • A transport vesicle buds from the Golgi apparatus and is moved by the cytoskeleton to the plasma membrane, where it fuses and the contents spill out Endocytosis • The cell takes in macromolecules and particles by forming new vesicles from the plasma membrane • Used to incorporate extracellular substances 

27. 15. Give an example of receptor-mediated endocytosis. • The process of importing specific macromolecules into the cell by the inward budding of vesicles formed from coated pits; occurs in response to the binding of specific ligands to receptors on the cell’s surface • Ligand  a molecule that binds to a specific receptor site of another • This is how cholesterol enters the cell - it enables cells to acquire bulk quantities of specific substances even if in low concentrations in the extracellular fluid 