1. “Keeping it all Together”
Cell Membrane
“Keeping it all Together”

3. Cell Membrane Defines the cell
About 8 nm thick (8,000 to equal the thickness of one page of paper)
Helps to regulate the cell (maintain equilibrium/homeostatic state)
Selectively permeable
Fluidity (lipids/proteins can shift laterally)

4. Gateway to the Cell

5. Cell Membrane Architecture
Lipids and Proteins – staple ingredients Most abundant lipids – phospholipids
Phospholipids are amphipathetic (philic/phobic)
Fluid Mosaic Model

6. Lipid Bilayer Structure cont…
Each lipid molecule contains a hydrophilic region or polar head region and a hydrophobic region or non polar tail region.
The hydrophilic region is attracted to aqueous water conditions while the hydrophobic region is repelled from such conditions

8. Cell Membrane

9. Freeze Fracture
A cell membrane can be split (with a knife) into it’s two layers revealing the ultrastructure of the membrane’s interior.

11. Membrane Proteins
While the lipid bilayer provides the structure for the cell membrane, membrane proteins allow for many of the interactions that occur between cells.
More than 50 kinds of proteins found (so far) in the plasma membrane of RBC’s for example.

12. Classifying Membrane Proteins
Proteins are generally broken down into the smaller classifications of integral proteins and peripheral proteins

13. Integral Proteins
Integral proteins are embedded within the lipid bilayer and partially penetrate the hydrophobic core.
Many are transmembrane proteins
Example of an integral protein is “integrins”. These attachments combine to give animal cells a stronger framework that the plasma membrane could alone.

14. Peripheral Proteins
Peripheral proteins are not embedded in the lipid bilayer at all
They are appendages loosely bound to the surface of the membrane, often to exposed parts of the integral proteins.
Some are attached to the cytoskeleton of the cell.

16. The Cell Membrane

18. Membrane Transport
We have discussed how the lipid bilayer acts as an efficient barrier by only allowing a very small number of non-polar molecules to freely enter or exit a cell.
While for the most part this selectivity is a valuable function and allows the cell to maintain its integrity, cells do need to move certain large, polar molecules such as amino acids, sugars, and nucleotides across their membranes.
As a result, cell membranes require specific structures that allow for the transport of certain molecules.

19. Membrane Transport
There are a number of different ways that molecules can pass from one side of a cell membrane to the other
Some such means, like diffusion and osmosis, are natural processes that require no expenditure of energy from the cell and are called passive transport.

20. Membrane Transport
Other methods of transport do require cellular energy and are called active transport
In addition to these two forms of transport, there exist other forms of transport such as endocytosis and exocytosis, which will be discuss later.

21. Passive Transport
Diffusion is the natural phenomenon in which nonpolar molecules naturally flow from an area of higher concentration to an area of lower concentration -
Diffusion DOES NOT require energy!
Osmosis is a similar process, but refers specifically to water molecules.

22. Diffusion of Liquids

24. Active Transport Requires energy or ATP
Moves materials from LOW to HIGH concentration
AGAINST concentration gradient

25. Transport Proteins
Both of passive and active transport are mediated with the help of transmembrane proteins that act as transporters
Two main classes of transport proteins are carrier proteins and channel proteins.

26. Transport Proteins cont…
For the most part, carrier proteins mediate active transport while channel proteins mediate passive transport

27. Transport Proteins cont…
Transport proteins are critical to cell life and cell interactions. They allow for the proper distribution of ions and molecules in multicellular organisms. Additionally, they can help to maintain proper intra- and extra-cellular pH levels, facilitate communication between cells, and are involved numerous other essential functions including protein sythesis.

28. Three Forms of Transport Across the Membrane

29. Checkpoint Quiz
QUESTION 1: The cell membrane is made up of a _____ bilayer. Carbohydrate Lipid Protein None of the Above

30. Answer to Question 1
Cell membranes are also described as lipid bilayers. There are two layers of phospholipids with proteins embedded in the layers. The membrane is able to move as needed by the cell.

31. Quiz
QUESTION 2: All eukaryotic cells are surrounded by a cell membrane. True False

32. Answer to Question 2
Eukaryotic cells have an organized nucleus and cell organelles. All of these cells have a membrane. Plant cells have a cell membrane and cell walls.

33. Quiz
QUESTION 3: The lipids in the bilayer have tails that are... Hydrophilic Hydrophobic

34. Answer to Question 3
The tail end of a phospholipid molecule is hydrophobic. The tails do not like to be near water and arrange themselves so that tails meet tails and the heads face the cellular fluids and exterior of the cell.

35. Quiz
Question 4: The heads of the lipids in the cell membrane are... Hydrophilic Hydrophobic

36. Answer to Question 4:
The head end of a phospholipid molecule is hydrophilic. The heads like to be near water and arrange themselves so that tails meet tails and the heads face the cellular fluids and exterior of the cell.

37. Quiz
QUESTION 5: Some molecules can pass through a cell membrane without help. True False

38. Answer to Question 5
Some molecules can pass through the cellular membrane by a process called diffusion. Other molecules are too large to pass through the membrane and must be helped across the lipid bilayer.

39. Answer to Question 6
There are proteins embedded in the cell membrane. Some of the proteins pass through the membrane and others are only attached to the surface. The proteins serve several functions.

40. Quiz
QUESTION 6: You will also find proteins in the lipid bilayer. True False

42. Overview of Membrane Transport
For a visual of the processes discussed, check out:
Assignment: Read section 7-3 and answer questions 1-6 on page 189
Copy Figure 7-16 into notes