1. Membrane Structure and Function

2. Plant and Animal cell

3. Plant and Animal cell

4. Plasma Membrane Edge of life
Separates living cell from its surroundings
8nm thick means 8000 membranes equal the thickness of thin page
Controls traffic into and out of the cell
Selectively permeable (make fundamentals of life)
Membranes form earlier in evolution of life
They enclose the solution different from its surroundings.
Membranes are vital  because they separate the cell from the outside world.  They also separate compartments inside the cell to protect important processes and events.

5. Composition of Membrane
Made up of lipids and proteins
Abundant lipids are phospholipids
Phospholipid is an amphipathic molecule having hydrophilic head and hydrophobic tail
Head consists of choline, phosphate and glycerol
Membrane proteins are also amphipathic having hydrophilic and hydrophobic region

6. Unsaturated Phospholipid molecule

7. Unsaturated Phospholipid molecule

8. Fluid mosaic model

9. History
1915: membranes isolated from RBCs were chemically analyzed, found to consist of lipids and proteins
1925: Two Dutch scientists suggested membranes are bilayer of phospholipids because they could exist as stable boundary between two aqueous compartments
: Danielli and Davson studied triglyceride lipid bilayer over a water surface with the polar heads facing outward. They always formed droplets (oil in water) and the surface tension was much higher than that of cells. However, by the addition of proteins, the surface tension was reduced and the membranes flattened out.  

10. Red Blood Cells

11. 1950s EM studies supports sandwich model
Surface of phospholipid bilayer adheres less strongly to water than the surface of biological membranes.
1935: Davson and Danielli suggested that membranes are coated on both sides with hydrophilic proteins (Sandwich model)
1950s EM studies supports sandwich model
Two problems with sandwich model:
Membranes with different functions differ in structure and chemical composition
Proteins dissolve in cytosol but membrane proteins are not very soluble in water because they are amphipathic
If amphipathic proteins are layered on the surface of membrane their hydrophobic part would be in aqueous surroundings

12. Sandwich model

13. Properties of phospholipid

14. Phospholipid bilayer

15. phosphoLipid bilayer

16. Sandwich Model

17. Plasma Membranes of two cells separated by intercellular space
Four Unit membranes , Two unit membranes form plasma membrane of each cell

18. Bacterial cell membrane

19. Fluid mosaic model
1972: S. J. Singer and G. Nicolson proposed that membranes proteins reside in the phospholipid bilayer with their hydrophilic regions protruding
This molecular arrangement maximize the contact of hydrophilic regions of proteins and phospholipids with water in cytosol and extracellular fluid
Membrane is a mosaic of protein molecule bobbing in a fluid bilayer of phospholipids
Confirmed by the freeze fracture split studies of membrane image EM

20. Membrane structure

21. Freeze Fracture Technique
A technique used to look at membranes that reveal the pattern of integral membrane proteins.
General outline of technique:
1. Cells are quickly frozen in liquid nitrogen (19°C), which immobilizes cell components instantly.
2. Block of frozen cells is fractured. This fracture is irregular and occurs along lines of weakness like the plasma membrane or surfaces of organelles.
3. Surface ice is removed by a vacuum (freeze etching)
4. A thin layer of carbon is evaporated vertically onto the surface to produce a carbon replica.
5. Surface is shadowed with a platinum vapor.
6. Organic material is digested away by acid, leaving a replica
7. Carbon-metal replica is put on a grid and examined by a transmission electron microscope.

22. Freeze Fracture Scheme

23. Image of membrane by freeze fracture

24. Freeze fracture drawing of membrane

25. EM Micrographs of Membrane

26. Functions of membranes
What is the main function of the cell membrane?
Diverse functions in the different regions and organelles of a cell. However, at EM level, they share a common structure
The cell membrane regulates what enters and leaves the cell and also provides protection and support.
The Lipid Bilayer gives cell membranes a flexible structure that forms a barrier between the cell and its surroundings.
Cell Membrane protein molecules embedded in the lipid bilayer, some of which have carbohydrate molecules attached to them.

27. Fluid mosaic model

28. Fluid mosaic model

29. Fluid mosaic model

30. Integral protein molecule

31. Fluidity of Membranes Membranes are not static sheets of molecules
Membranes molecules have hydrophobic interactions which are much weaker than covalent bonds
Lipids and proteins can shift laterally
Lipids can shift in flip-flop manner from one lipid layer to another is rare because hydrophilic part of the molecule has to pass through the hydrophobic region.
Lateral movement of phospholipid is rapid.
Adjacent phospholilipids switch positions about 107 times/sec: 2μm /sec

32. Unsaturated and saturated phospholipids and cholesterol molecule

33. Fluidity of membranes
Membranes remain fluid as temp decreases until the phospholipid become closely packed
Solidification temperature depends on the types of lipids it is made off
Unsaturated hydrocarbons have kinks at double bond and are more fluid
Saturated hydrocarbons have no double bonds and tightly packed are less fluid and more viscous
Steroid cholesterol is wedged shaped between phospholipid of animal cell at 37°C makes the membrane less fluid by restraining the movement of phospholipid. It hinders the close packing of phospholipid therefore lowers the temperature of membrane solidification (fluidity buffer)

34. Unsaturated and saturated phospholipids and cholesterol molecule

35. Unsaturated and saturated phospholipids and cholesterol molecule

36. Effect of Temperature Membranes work better when fluid.
Solidification: changes permeability and inactivate enzymatic proteins
Too fluid membranes can not support the protein function
Extreme environments pose a challenge for life and leads to evolutionary adaptation
Fishes in extreme cold environment have more unsaturated phospholipids
Bacteria at hot springs (90°C) have unusual phospholipids concentration
In Winter wheat % of unsaturated phospholipids increases in autumn

37. Functions of Membrane proteins
Proteins are mosaic part of membranes
Diverse proteins exist: 50 kinds of proteins in RBCs
Proteins determine most of the functions of membranes
Different types of cells contain different sets of proteins
Two types:
Integral proteins: in the hydrophobic interior and span the membrane
Peripheral protein: not embedded in lipid bilayer loosely bound on the surface of membrane

38. Some are immobile due to their attachments with cytoskeleton or extracellular matrix Fluid mosaic model

39. Proteins are larger and move more slowly

40. Functions of Membrane proteins
Transport Proteins: Spans around the membrane and provide hydrophilic channel across the membrane. Others shuttle a substance from one side to the other by changing shape (carrier protein), some may hydrolyze ATP as energy source to actively pump substances across the membrane

41. Transport protein

42. Functions of Membrane proteins
Enzymatic activity: A protein in the membrane may be an enzyme with its active site exposed to substances in adjacent solution. Several enzymes are arranged in series to carry out the various steps in metabolic pathways.

44. Proteins of photosynthesis Photosystem i