The Cell Membrane

1. The Cell Membrane Ms. Napolitano & Mrs. Haas CP Biology

2. The Cell Membrane • Thin, flexible barrier that surrounds the cell • AKA plasma membrane • Selectively permeable – allows some substances to cross more easily than others • Usually ~8nm thick

3. The Phospholipid Bilayer

4. Fluid Mosaic Model • Used to describe the structure of the membrane • Fluid structure of phospholipids with a mosaic of various proteinsembedded in it • Cholesterol helps maintain fluidity • Membrane becomes less fluid as temperature increases by restricting movement • Membrane becomes more fluid as temperature decreases by maintaining space

5. Structure of the Membrane

6. Membrane Proteins • Different types of cells contain different types of membrane proteins • Integral proteins – embedded into the hydrophobic core of the lipid bilayer • May or may not go all the way through the membrane • Peripheral proteins – on the bilayer surface Integral Peripheral

7. Types of Membrane Proteins(1 of 2) • Transport • Channel or carrier proteins (hydrophilic tunnels) allow polar molecules and ions to pass through the hydrophobic layer • Proteins are specific for the substance they transport • Enzymatic • Signal Transduction (chemical messages) • Receptor proteins transmit information from outside of the cell to inside of the cell

8. Types of Membrane Proteins(2 of 2) 4. Recognition • ID tags – glycolipids or glycoproteins 5. Intercellular Joining • Join together adjacent cells 6. Support • Attach to cytoskeleton or extracellular matrix for stability

9. The phospholipidbilayer is __________ in the middle and __________ on the outsides. • Hydrophobic, hydrophilic • Hydrophilic, hydrophobic • Hydrophobic, hydrophobic • Hydrophilic, hydrophilic

10. What is the function of cholesterol in the cell membrane? • Identification • Transport • Membrane fluidity • Recognition

11. The picture shows a peripheral protein. • True • False

12. Which of the following is NOT a function of membrane proteins? • Signal transduction • Identification • Transport • Support

13. Passive Transport • Passive Transport – the movement across the cell membrane that does not require energy • Types: • Diffusion • Osmosis • Facilitated Diffusion

14. Diffusion • Diffusion – the movement of particles from an area of high concentration to an area of low concentration until an equilibrium is reached • Transports small, nonpolar molecules such as CO2 and O2 • Concentration Gradient – the difference in the concentration of a substance across a space • Equilibrium – concentration of a substance is equal throughout a space

15. Osmosis • Osmosis – diffusion of free water across a selectively permeable membrane • Water diffuses across the cell membrane from the region of low solute concentration to that of a higher solute concentration until it reaches equilibrium

16. U-Tube (not YouTube!) Which way will water flow?

17. U-Tube (not YouTube!) This way!

18. Osmoregulation in Cells Without Cell Walls • Osmoregulation: control of water balance • Tonicity: the ability of a solution to cause a cell to gain/lose water • Isotonic solution: no net movement of water across the cell membrane • Hypertonic solution: more free water inside the cell (cells lose water & shrivel) • Hypotonic solution: less free water inside the cell (cells gain water & lyse) • Some cells have a contractile vacuole to pump water out of the cell http://www.youtube.com/watch?v=mTXRcbjuYGU

20. Osmoregulation of Cells WithCell Walls • Plant cells are healthiest in a hypotonic solution • Osmotic pressure keeps cell walls turgid (very firm) • Plant cells are flaccid (limp) in an isotonic solution • In a hypertonic solution, the cell membrane will shrink and pull away from the cell wall • AKA plasmolysis (wilting) http://www.youtube.com/watch?v=EsJ73x4ycp0

22. Facilitated Diffusion • Facilitated Diffusion: passive transport aided by proteins • Molecules travel from high  low concentration • Transports small, polar molecules • Glucose • Salts • Ions • Amino Acids • Water (aquaporins)

23. Channel Proteins • Hydrophilic passageways • Some are always open for diffusion • Rate of movement is determined by the concentration gradient • Some ion channels have gates that can be opened by: • Cell membrane stretching • Change in electrical charge • Binding of specific molecules

24. Carrier Proteins • Specific substance binds to carrier protein • Protein changes shape & transports substances across the cell membrane • Molecule is released into the cell, and carrier protein returns to its original shape http://www.youtube.com/watch?v=GFCcnxgXOhY

25. Active Transport • Uses energy to move solutes AGAINST the concentration gradient • Carrier proteins act as “pumps” powered by ATP • Examples: • Sodium Potassium Pump • Proton Pump • Cotransport

26. Sodium Potassium Pump • Pumps 3 Na+ out of the cell and 2 K+ into the cell • Actively transports both ions against their concentration gradient • Powered by ATP

27. http://www.youtube.com/watch?v=GTHWig1vOnY

28. Proton Pump • Actively transport protons (H+) through the internal membranes of mitochondria & chloroplasts

29. Cotransport (Coupled Transport) • Cotransport: The transport of one solute indirectly drives the transport of several other solutes (using ATP) • Example: As proton pump pumps H+ out, H+ ions diffuse back in pulling sucrose with it

30. Bulk Transport • Substances that are too large to be transported by carrier proteins • Example: proteins leaving/entering the cell • Uses vesicles • Types: • Exocytosis • Endocytosis

31. Exocytosis • Exo= Export (recall exo means outside!!) • Vesicles fuse with the cell membrane, releasing the contents outside of the cell

32. Endocytosis • Recall endo means inside!! • Cell membrane engulfs particles • Pinches off to form vesicles inside of the cell http://www.youtube.com/watch?v=ygkvXT1BmaA