area of the cell immediately surrounding the cytoplasm

1. CELL MEMBRANE • area of the cell immediately surrounding the cytoplasm • the most conserved structure in living cells. • Every living thing on this planet has some type of membrane

2. Anatomy of an animal cell

4. Anatomy of a plant cell

5. CELL MEMBRANE • are thin structures, measuring 8 nm thick. • the major barrier in the cell, separating the inside of the cell from the outside. • It is this structure which allows cells to selectively interact with their environment • highly organized and asymmetric having two faces with different topologies and different functions • Membranes are also dynamic, constantly adapting to changing environmental conditions

7. CELL MEMBRANE • not permeable to ionic or molecules that are non-polar. Only permeable if attached to specific proteins. • not rigid – can modify shape and size • durable – few months • trilaminar appearance – 2 dark lines separated with lighter space • contains proteins – enzyme activity

8. Trilaminar appearance

9. Membranes are vital because • Encloses the cell • defines its boundries • maintains essential differences between cytosol and extracellular environment • maintain the characteristic differences between contents of different organelles and the cytosol • synthesis of ATP through activities of specialized membrane proteins

10. Membranes are vital because • drive transmembrane movement of selected solutes’ • produce and transmit electrical signals – nerve and muscle cells • acts as sensors of external signals  change its behavior in response to environmental changes (transfer information, not ions)

11. All membranes • have similar chemical components • similar structural organization • similar general properties • However, different membranes have • different specialized lipids • different proteins • different carbohydrates Physiological interactions of the different molecules are similar

12. How did early cell biologists deduce • membrane structure from electron microscopic images and • the knowledge that membranes were lipoprotein complexes?

13. All biological membranes have a common general feature – a very thin film of lipid and proteinmolecules – major components • However, amounts differ depending on the types of membranes • e.g. mitochondria – 80% protein & 20% lipid ; myelin sheath – 80% lipid & 20% protein • Also contain carbohydrates - + protein = glycoprotein; + lipid =lipoprotein Universal basis for cell-membrane structure = lipid bilayer

15. LIPIDS • 2 main types • phospholipid • sterol phosphoglycerides (glycerophospholipid) sphingolipid The distribution varies depending on the types of membranes

16. Phosphoglycerides • The main type of phospholipid – derived from glycerol-3-phosphate • Simplest member = phosphatidic acid • The rest are derived from it e.g. phosphatidilethanolamine – from amine… choline, ….serine, …. inositol • etanolamina, choline, serine & inositol are polar

17. The main type of phospholipid • Also known as phosphoglycerides • Derived from glycerol-3-phosphate • Simplest = phosphatidic acid (phosphatidate)

18. Phosphatidylcholine with choline as polar head group, is an example of a glycerophospholipid. It is a common membrane lipid. Phosphatidylinositol, with inositol as polar head group, is another glycerophospholipid. In addition to being a membrane lipid, phosphatidylinositol has roles in cell signaling, to be discussed later.

20. Each glycerophospholipid has: • a polar region [glycerol, carbonyl oxygens of fatty acids, phosphate, and the polar head group (designated X above)] • two non-polar  hydrocarbon tails of fatty acids (designated R1, R2 above). 

21. SPHINGOLIPID • main compound – sphingosine = amino alcohol • if add phosphocholine to OH group sphingomyelin • various combinations – serine, ethanolamine etc   several types of sphingolipids • most abundant = sphingomyelin

23. SPHINGOMYELIN

24. 3. CHOLESTEROL • Compact molecules • Can be found free or as an ester The cholesterol molecule inserts itself in the membrane with the same orientation as the phospholipid molecules  polar head aligned with polar head of lipid

26. PROTEINS • 2 types • Peripheral (extrinsic) • Integral (intrinsic)

27. PERIPHERAL PROTEINS • Found on thesurface of proteins – exposed on both sides – internal & external – to cytosolic contents & external environment • proteins can be separated with salt concentrations – Y? – 30% a/acid residues are hydrophobic; 70% are hydrophilic or neutral – salt solution protects electrostatic interactions • EDTA (chelates Mg2+, Ca2+) ; extreme pH – can dissolve proteins • Most are enzymes with specific activities; soluble in water; free from lipid

29. Peripheral protein

30. 2. INTEGRAL PROTEINS • embedded in the bilayer • exposed to both sides of the membrane • involved in sending specialized substances or Messages through membrane

31. 2. INTEGRAL PROTEINS • difficult to isolate – necessary to use detergent or organic solvent – denature protein and loss of biological activity • How detergent works ? – disrupts bilayer – Detergents are amphiphatic molecules – form micelles- hydrophobic ends react with hydrophobic regions of membranes  dissociates the protein

32. Universal basis for cell-membrane structure = lipid bilayer • Why? How? • The bilayer is attributable to the special properties of the lipid molecules which cause them to spontaneously assemble into bilayers even in simple artificial conditions

33. In aqueous medium, amphiphilic molecules form micelle 1. Globular / ellipsoidal or elongated Soap and detergent

34. Amphiphilic One-tail (Soap) Form Spherical Or Ellipsoidal Micelle – Depends On The Tail Length • Hydrocarbon groups not in contact with water • Head group soluble in water

35. Two-tail hydrocarbon (glycerophospholipid, sphingolipid) Form extended micelle - known as lipid bilayer LIPID BILAYER - FORM THE BASIC STRUCTURE OF MEMBRANE

36. Glycerophospholipids suspension can form liposomes - closed vesicles surrounded by a phospholipid bilayer

37. Phospholipid bilayer • Hydrophilic group faces out towards both aqeous regions • Tail group (hydrophobic) faces towards inside the layer

40. At appropriate concentrations  spheres = micelles • tails =hydrophobic – inside – exclude water ; polar heads – outside of sphere They can also form bimolecular sheets or bilayers with the hydrophobic tails sandwiched between the hydrophilic heads  Basic structure for all membranes

42. MEMBRANE STRUCTURE – Historical Perspective How did early cell biologists deduce membrane structure from electron microscopic images and the knowledge that membranes were lipoprotein complexes?

43. In 1900 Overton – • Measured the permeability of various types of compounds across the membranes of a frog muscle • found that lipid molecules could readily cross this membrane, larger lipid insoluble molecules couldn't and small polar compounds could slowly cross the membrane. • He suggested that membranes were similar to lipids and that certain molecules (lipids) moved across membranes by dissolving in the membrane. • suggest that the biologic membrane is mainly lipid in nature but contains small aqueous channels or pores.

45. 1930's-40's, Danielli and Davson • studied triglyceride lipid bilayers over a water surface. • Found that they arranged themselves with the polar heads facing outward. • However, they always formed droplets (oil in water) and the surface tension was much higher than that of cells. • However, if you added proteins, the surface tension was reduced and the membranes flattened out. 

47. The Davson-Danielli Model

48. Later work (Danielli, 1975) suggested the presence of "active patches" and protein lining to pores in the membrane. Modified Davson-Danielli Model

49. Basic structural characteristics of membranes derived from the physiological properties of the major lipids • glycerophospholipids • sphingolipids • amphipathic • hydrophilic heads and hydrophobic tails • phospholipids tails = hydrophobic = 2 = hydrocarbon chains (12-14 C)