The Plasma Membrane and Cell Transport

1. The Plasma Membrane and Cell Transport

2. The Phospholipid Bilayer • The phospholipid bilayer is the basic structural unit of biological membranes • The membranes of the many organelles form the “cytomembrane system” in eukaryotes such as: • Plasma membrane • Endoplasmic reticulum • Golgi apparatus • Nuclear envelope • Vesicles

3. Phospholipids • Phospholipids in water will naturally form spherical shapes called micelles, as well as liposomes

4. Internal membranes • A nuclear membrane and the endoplasmic reticulum may have evolved from in-folding or invagination of the outer cell membrane. • Initially, such folding may have been an adaptation that permitted more efficient exchange of materials between the cell and its surroundings by increasing surface area, and it may also have provided more intimate chemical communication between the genetic material and the environment.

5. Internal membranes

7. Phospholipids

8. Properties of phospholipid bilayer • All membranes are surrounded by a watery environment • The hydrophilic heads allow water and anything dissolved into it to interact with internal and external surfaces of the cell membrane • externally: extracellular fluid (ECF) • internally: cytoplasm • The hydrophobic tails provide an effective barrier to diffusion of materials in or out

10. Phospholipid Bilayers • Along with the phospholipids, the membranes include other components including: • Proteins • Glycoproteins • Cholesterol • Most membrane proteins and lipids are laterally mobile, and add to the fluidity of the membrane, creating what is known as a “fluid mosaic”

11. The “Fluid Mosaic”

12. Membrane proteins and their functions

13. Transport • The movement of materials is determined by the permeability of the cell membrane and the characteristics of the molecules crossing it • There are 2 types of transport: • PASSIVE • ACTIVE

14. Passive transport – types

15. Simple Diffusion • Involves the movement of substances along a concentration gradient (two areas where the concentration is different) • Does NOT require energy (ATP)

16. Simple Diffusion • The spontaneous movement of particles from an area of high concentration to an area of low concentration • E.g.: Perfume, fresh-baked cookies, KMnO4 in water

17. Simple Diffusion • Molecules follows direction of their concentration gradients • Only some substances are able to diffuse directly through the phospholipid bilayer • Small uncharged polar molecules (water, urea, ethanol, glycerol) • Small non-polar gases (CO2, N2, O2) • Polar molecules (steroid hormones) • the plasma membrane is impermeable to other molecules that are water-soluble, and must rely on facilitated diffusion

18. Passive Transport

19. Factors that affect the rate of diffusion • Size of the molecule → smaller molecules diffuse faster than larger molecules • Concentration gradient → the higher the gradient, the faster the rate of diffusion • Temperature → as temp increases, molecules move faster and the rate of diffusion increases

20. Facilitated Diffusion • Metabolic processes may require molecules or ions much faster than simple diffusion can provide • Examples: glucose, amino acids, ions, charged polar molecules like ATP or amino acids • Facilitated diffusion uses transport proteins to help transport these ions and molecules across the membrane • This is still a type of passive transport because no energy is required on the part of the cells – it requires a concentration gradient to occur

21. Facilitated Diffusion • Channel proteins form hydrophilic pathways for water or ions to diffuse into or out of a cell • Carrier proteins are more selective because the solute must match the shape of the membrane protein

22. Osmosis • The diffusion of water across a selectively permeable membrane • Particles move from a high concentration to a low concentration • Ex: movement of substances in/out of the cell

23. Osmotic Conditions Hypertonic Solution • The fluid surrounding the cells has a HIGHER solute concentration than the cytoplasm • Water diffuses out of the cell by osmosis

24. Osmotic Conditions Isotonic Solution • The concentration of the fluid outside of the cell is the SAME as the concentration in the cell’s cytoplasm • The solute concentrations are at equilibrium

25. Osmotic Conditions Hypotonic Solution • The fluid surrounding the cells has a LOWER solute concentration than the cytoplasm • Water diffuses into the cell by osmosis

26. Osmotic Conditions

27. Osmotic Conditions

28. Active Transport • Sometimes cells need to move molecules or ions AGAINST a concentration gradient • Cannot use diffusion because diffusion moves particles from a high concentration to a low concentration

29. Active Transport

30. Active Transport • Like facilitated diffusion, active transport relies on membrane proteins • Molecules/ions are pumped across the membrane • If ATP is used directly, this is primary active transport (Na+/K+ pump) • Secondary active transport uses an electrochemical gradient as a source of energy to pump molecules across the membrane (H+/sucrose pump)

31. Primary and secondary transport

32. Sodium – potassium pump • A well-studied form of primary active transport • This pump ensures that nerve and muscle cells function properly

33. Secondary active transport in the hydrogen – sucrose pump http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect09.htm

34. Transport Requiring Vesicles

35. Endocytosis and Exocytosis

36. Endocytosis • The uptake of large particles or molecules by formation of a vesicle from the cell membrane • Requires energy from ATP

37. Endocytosis

38. Phagocytosis • Theingestion of solid particles • Cytoplasmic membrane invaginates and pinches off placing the particle in a phagocytotic vesicle • The vesicle fuses with lysosomes and the material is degraded

39. Phagocytosis

40. Pinocytosis • Theingestion of dissolved materials • The membrane invaginates and pinches off placing small droplets of fluid in a pinocytotic vesicle • The liquid contents of the vesicle is then slowly transferred to the cytosol

41. Pinocytosis

42. Exocytosis • The release of molecules from a vesicle that fuses with the cell membrane to export the molecules from the cell • Requires energy from ATP

43. Exocytosis • A transport vesicle moves to fuse with the cell membrane • The cell membrane rearranges, opens, and releases the contents of the vesicle outside the cell • Ex: insulin in the bloodstream