Chapter 2:

1. Chapter2: Cell membrane and cell surface

2. Outline 2.1 Components and structure of cell membrane 2.2 Transmembrane transport 2.3 Cell adhesion molecules and cell junction 2.4 Extracellular matrix and cell wall

3. 2.1 Components and structure of cell membrane • All cells are surrounded by a layer of membrane; • In eukaryote cell, membrane compartmentalizes the cell into sub-compartments termed organelles; • Prokaryote cell lacks sub-compartment.

4. eukaryote and prokaryote cell(See Chapter 1)

5. Common functions of plasma membrane • Act as permeability barrier • Intimately engaged in the assembly of cell walls • Form specific junctions between cells • Anchor components of the extracellular matrix • Contain receptor proteins that bind specific signaling molecules • Take part in the compartmentalization of cell • Energy transduction

6. The structure of plasma membrane

7. 2.1 Components and structure of cell membrane Basic compositions • lipids • proteins • saccharide

8. The basic compositions of some bio-membranes

9. 2.1.1 Lipids in biomembrane main types of membrane lipids: • Phospholipid • Phosphoglycerides • Sphingolipids • Cholesterol (steroids) amphipathic molecules hydrophilic head group + hydrophobic tail group

10. Phosphoglycerides PC: phosphatidylcholine X=choline PE: phosphatidylethanolamine X=ethanolamine PS: phosphatidylserine X=serine

11. phosphatidylcholine

12. The class of sphingolipids

13. Sphingomyelin

14. Structure of major phospholipid molecules

15. Structure of glycolipid molecules in plasma membrane

16. Cholesterol is smaller than the other lipids of the membrane and less amphipathic. • Cholesterol is absent from the plasma membranes of most plant. Cholesterol

17. 2.1.2 Proteins in biomembrane Three forms of proteins link to membrane • Integral proteins (Transmembrane proteins) • Lipid-anchored membrane proteins • Peripheral membrane proteins

18. Proteins associated with the lipid bilayer

19. proteins on cell membrane can be classed to : • Channel proteins: to form pores for the free transport of small molecules and ions across the membrane; • Carrier proteins: to facilitated diffusion and active transport of molecules and ions across the membrane; • Cell recognition proteins: to identifie a particular cell; • Receptor proteins: to bind specific molecules, such as hormones and cytokines, and mediate signal transduction; • Enzymatic proteins: that catalyze specific chemical reactions.

20. Integral proteins

21. Structural basis of integral proteins (A)α-helix model of bacteriorhodopsin (B): -barrel model of one subunit of OmpX

22. Lipid-anchored membrane proteins • covalent bound to lipid molecules of the phospholipidbilayer. • polypeptide chain does not enter the phospholipidbilayer.

23. Peripheral membrane proteins • bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid head groups. • localized to either the cytosolic or the exoplasmic face of the plasma membrane.

24. 2.1.3 Membrane carbohydrate • 2%~10% of membrane content depending on cell types; • covalently bound to membrane proteins and lipids to form glycoproteins or glycolipids; • all membrane carbohydrate pitch on the outside of plasma membrane.

25. Structure of glycolipid molecules in plasma membrane

26. Function of membrane carbohydrate • Protect cells against mechanical and chemical damage; • Preventing unwanted protein-protein interactions; • Help membrane proteins to form correct three-dimensional configures ; • Help to transfer of new proteins to correct position; • Cell recognition, cell adhension and cell junction.

27. 2.1.4 structure characters of plasma membrane • Fluid mosaic model • Lipid raft model • Membrane fluidity • Membrane asymmetry

28. Arrangement of lipid molecules in an aqueous environment

29. Fluid mosaic model • S.J. Singer and G.L. Nicolson in1972 • membranes as dynamic structures in which lipids and proteins are mobile • lipid bilayers form the basis of the membranes • proteins either span the bilayer or are attached to either side of the lipid membrane; • the membranes are asymmetrical

30. Lipid rafts model a complementation for the fluid mosaic model

31. Membrane fluidity The possible movements of phospholipids in a membrane

32. The physical state of the lipid of a membrane • phase transition • liquid-like state frozen crystalline gel • transition temperature: • the temperature point when the lipid phase transition appears.

33. Factors influence bilayer fluidity • unsaturation state of the fatty acids in the bilayer; • the length of the hydrocarbon chains of a lipid; • cholesterol molecules: • Decrease bilayer fluidity above the transition temperature • increase bilayer fluidity below transition temperature

34. Membrane fluidity Cell fusion technique reveals membrane protein mobility

35. Membrane fluidity Membrane protein mobility revealed by FRAP technique

36.  Factors influence membrane protein mobility • Integral proteins • Membrane lipid fluidity • ECM • Cell junctions • Ligand, antibody and drug molecules Restriction on membrane protein mobility by ECM

37. Membrane Asymmetry • The two halves of the bilayer often contain different types of phospholipids and glycolipids. • The proteins embedded in the bilayer have a specific orientation Freeze-fracture replication

38. Membrane Asymmetry • Lipid-digesting enzymes that cannot penetrate the plasma membrane and are subsequently only able to digest lipids that reside in the external monolayer of the bilayer. SM, sphingomyelin; PC, phosphatidylcholine; PS, phosphatidylserine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; Cl, cholesterol

39. 2.2 Transmembrane Transport • Plasma membrane is semipermeable a pure phospholipid bilayer

40. 2.2.1 Overview of trans-membrane transport

41. channel proteins transport proteins transporters Types of trans-membrane transport • Active transport • Passive transport

42. Three types of transporters • Uniport • symport • antiport

43. 2.2.2 Passive transport • Passive diffusion (simple diffusion) • no metabolic energy is expended; • no specific transport proteins needed; • molecules move down its chemical concentration gradient. • Diffusion rate is determined by: • concentration gradient across the layer • hydrophobicity • size • electric potential across the membrane

44. Facilitated Diffusion • polar molecules, ions and water, transport across membrane by a protein-mediated movement • exhibits the following distinguishing properties from passive diffusion: • The rate is far higher than passive diffusion • The partition coefficient K is irrelevant • Occurs via a limited number of uniporter molecules • Transport is specific.

45. A typical example of facilitated diffusion GLUT1 facilitates the unidirectional transport of glucose down its concentration gradient Uniporter mediates passive movement of a glucose solute.

46. Ion channel • assist diffusion • water, ions and hydrophilic small molecules • downconcentration or electric potential gradients • form a hydrophilic passageway across the membrane

47. Ion channel The structure and ion selectivity of a bacteria K+ channel.

48. Ion channel Channel proteins • nongatedchannels • gatedchannels

49. Some examples of ion channels

50. 2.2.3 Active transport • mediated by a specific membrane proteins • againsttheir concentration gradient • need the energy supply