Happy Wonderful Wednesday!

1. Happy Wonderful Wednesday!

2. 2.4 Membranes Chapter 5

3. History of membranes • Davson-Danielli Model • 1935 -  Hugh Davson and James Danielli proposed the plasma membrane was a phospholipid bilayer that was sandwiched between two layers of globular proteins. • Determined this because of the observations on the surface tension of the lipid bilayers.

4. Singer-Nicolson model – 1972 • Known better as fluid-mosaic model • According to this model, biological membranes are composed largely of lipid bilayer with proteins penetrating either half way or all the way through the membrane. • These proteins are visualized as freely floating within a completely liquid bilayer

6. Membranes • The membranes of eukaryotic cells and their organelles are complex in structure: • Basic membrane is a phospholipid bilayer • Two layers of phosopholipids • Phospholipids are molecules that consist of a hydrophilic (“water loving”) phosphate group “head” and a hydrophobic (“water fearing”) lipid “tail” • Phosphate groups are therefore on the edges of the membrane while the lipid tails form the inner portion

7. Hydrophilic Phosphate groups Hydrophobic Lipid tails Hydrophilic Phosphate groups

8. P 57

9. Cholesterol and proteins are embedded within the plasma membrane: • Cholesterol - binds together lipids in plasma membrane, reducing fluidity; spaces lipids, preventing solidification. • Integral Proteins – Span from one side of the phospholipid bilayer to the other • Peripheral proteins – attached to the surface of the membrane • Glycoproteins – found on outside of bilayer; involved in cell recognition (immune sys) or cell communication (hormones)

11. Protein functions • Functions of membrane proteins • Hormone binding sites – glycoproteins; outside of membrane • Immobilized enzymes – peripheral; on the inside of the membrane • Cell adhesion – peripheral; on the outside of the membrane

12. Cell-to-cell communication - glycoproteins, outside of membrane • Channels for passive (non-energy requiring) transport of materials across the cell membrane - integral membrane proteins • Protein pumps for active (energy requiring) transport of materials across the membrane - integral membrane proteins **Note: The phospholipid bilayer only allows O2, CO2, and H2O to cross without the aid of membrane proteins. Because of this, it is sometimes referred to as being “semi-permeable”

13. Diffusion and Osmosis • Diffusion refers to the passive movement of particles from an area of high concentration to an area of low concentration • e.g. gas leaks and perfume

14. Osmosis • Osmosis is a specific type of diffusion: • The diffusion of water across a semi-permeable membrane (e.g. the plasma membrane) from an area of lower solute concentration to an area of higher solute concentration • solute refers to the substance that is dissolved in the water (e.g. salt)

16. Concentration on Solutions • There are three types of osmotic solutions: • Isotonic = solute concentration is the same inside the cell as outside the cell • Equal amount of water crossing in and out • Cell stays the same shape

18. Hypertonic = solute concentration is higher outside the cell than inside the cell(which means the water concentration is lower outside the cell than inside) • Water moves out of the cell • Plasma membrane shrinks • One example of this is plants wilting

20. Hypotonic = solute concentration is lower outside the cell than inside (which means water concentration is higher outside than inside) • Water moves into the cell • Plasma membrane swells • Can sometimes cause animal cells to burst

23. Transporting • Passive Transport • Passive transport refers to the movement of molecules down the concentration gradient (high conc -> low conc) • Passive transport does not require energy • Two types: simple diffusion and facilitated diffusion

24. Simple diffusion refers to the passive diffusion of molecules across the membrane without the aid of proteins (e.g. water, oxygen, carbon dioxide) • Facilitated diffusion refers to the passive diffusion of molecules through channel proteins

25. Active Transport • Refers to the transport of molecules across the membrane against the concentration gradient (low conc -> high conc) • Requires energy in the form of ATP • Transport is carried out by protein pumps (specialized integral proteins)

26. Transported molecules enter the protein pump • The release of energy from ATP causes a shape change in the protein that then allows the molecule to move through

27. What about HUGE items?? • Molecules are transported within the cell via vesicles • Primary example is the transport of proteins • Proteins are synthesized by ribosomes in the rough er Vesicles are essentially circular lipid membranes

28. Newly made proteins are packaged in vesicles and sent to the Golgi Apparatus where they fuse with the membrane • Proteins are modified and repackaged into vesicles • Vesicles travel to the cell membrane where they fuse, thus releasing the protein in the extracellular fluid (process called exocytosis)

30. Exocytosis and Endocytosis • Exocytosis is the fusing of a vesicle from inside the cell with the cell membrane • Purpose is to release a molecule made in the cell to the extracellular fluid

32. Endocytosis is pinching in of the plasma membrane to create a vesicle that contains molecules from the extracellular environment • This is how the cell obtains it’s nutrients

33. The ability of vesicles to fuse with the plasma membrane is due to the fluidity of it’s lipid structure (remember, though, the more cholesterol, the less fluid the membrane is) • Exocytosis enlarges the size of the plasma membrane • This is balanced by endocytosis, which reduces the size plasma membrane • Endocytosis is also known • Pinocytosis (liquids) or Phagocytosis(Solids)