1. Cell Membrane Structure & Function
Chapter 4
Cell Membrane Structure & Function

2. 4.1 – How Is the Structure of a Membrane Related to its Function?

3. The plasma membrane isolates the cell while allowing communication with its surroundings
Three General Functions:
Selectively Permeable
Regulates exchange
Communication

4. Membranes are Fluid Mosaics in Which Proteins Move
Fluid Mosaic Model was developed in 1972
Phospholipids act as grout for membrane proteins, which represent tiles

5. Phospholipid Bilayer is the Fluid Portion of the Membrane
Phospholipid Review
Have a polar head
Hydrophilic
Have 2 nonpolar tails
Hydrophobic
Double bonds in the tail increase fluidity of membrane

6. The Plasma Membrane is a Phospholipid Bilayer
The tails point inward
away from the watery environment
The heads point outward
toward the watery environment
Membrane is “fluid” because the phospholipds are not bonded together

7. Phospholipids bilayer selectively isolates internal environment from external environment
Most biological molecules are hydrophilic and cannot pass through the membrance easily
Some molecules can freely pass through the membrane

8. Cholesterol in animal cell membranes make the bilayer stronger, more flexible, less fluid & less permeable to water soluble substances
Flexibility and fluid nature of the bilayer is important to its function

9. The Plasma Membrane

10. A Mosaic of Proteins is Embedded in the Membrane
Proteins embedded within or attached to the surface of the bilayer regulate the movement of substance across the membrane and communicate with the environment
Many membrane proteins are glycoproteins

11. Three Categories of Membrane Proteins
Transport Proteins – regulate the movement of hydrophilic molecules through the membrane
Channel Proteins – form pores and channels for small water-soluble molecules
Carrier Proteins – bind molecules and move them across the membrane

12. Channel proteins do not change shape
Carrier proteins change shape

13. Three Categories of Membrane Proteins (con’t)
2. Receptor Proteins – bind molecules in the environment, triggering changes in the metabolism of the cell
3. Recognition Proteins – serve as identification tags and cell-surface attachment sites

14. 4.2 – How Do Substances Move Across the Membrane

15. Molecules Move in Response to Gradients
Characteristics of a fluid
Fluid – any substance that can move or change shape in response to external forces without breaking apart
Concentration – number of molecules in a given unit of volume
Gradient – physical difference in properties such as temperature, pressure, or concentration

16. Diffusion
The movement of molecules from regions of high concentration to regions of low concentration
Movement down the concentration gradient

17. Diffusion Con’t
Molecules move randomly and continuously, colliding with each other, until a dynamic equilibrium exists in which there is no concentration gradient
The greater the concentration gradient, the faster the rate of diffusion

18. Example of Diffusion
A drop of food coloring in a glass of water

19. Movement Across Membranes Occurs by Both Passive & Active Transport
There are significant concentration gradients of ions and molecules across the plasma membrane because the cytoplasm is very different from the extracellular fluid

20. Movement Across the Membrane Occurs by:
Passive Transport
Substances move down conc. gradient
No energy is required
Active Transport
Substance move up conc. gradient
Energy is required

21. Types of Passive Transport
Simple Diffusion
Facilitated Diffusion
Osmosis

22. Simple Diffusion
Membranes are selectively permeable to diffusion of molecules
Lipid-soluble molecules & very small molecules can easily diffuse across the membrane
Rate of simple diffusion depends on conc. gradient, the size of the molecule & its lipid solubility

23. Facilitated Diffusion
Molecules cross the membrane with the help of membrane transport proteins
Channel proteins
Carrier proteins
No energy required
molecules move down the conc. gradient

24. Osmosis Osmosis is the diffusion of water
Water moves down the conc. gradient across a selectively permeable membrane
Dissolved substances reduce the concentration of water molecules in solution

25. Osmosis is Important in the Life of Cells
Water balance between cells and their surroundings is crucial to organisms
Three environments exist due to varying water concentrations
Isotonic
Hypertonic
Hypotonic

26. Isotonic Cell Environment
Water concentration around the cell is the same as the water concentration inside the cell
No net movement of water occurs
Cell remains the same size
The type of dissolved particles does not have to be the same, but the total concentration of all dissolved particles is equal

27. Isotonic
Water is moving in to and out of the cell at an equal rate.

28. Hypertonic Cell Environment
The solution outside the cell has a higher concentration of solutes than the interior of the cell
Lower water concentration
Water will flow out of the cell by osmosis
Cells shrivel and shrink

29. Hypertonic
Net movement of water out of the cell
Cell shrinks

30. Hypotonic Cell Environment
The solution outside the cell has a lower solute concentration than the solution inside the cell
Higher water concentration
Water will flow into the cell by osmosis
Cells will swell and sometimes burst

31. Hypotonic
Net movement of water into the cell
Cells swell

32. Active Transport Uses Energy to Move Molecules
All cells need to move some substances against their conc. gradient
Membrane proteins that require energy are used to move molecules against their conc. gradient
Active transport proteins are sometimes called “pumps” because they move substances uphill

33. Active Transport Proteins
Active transport proteins span the width of the membrane and have 2 active sites
One site binds the substance to be transported
Second site binds an energy carrier molecule, usually ATP

34. Example of Active Transport

35. Cells Engulf Particles or Fluids by Endocytosis
Types of Endocytosis
Pinocytosis
Phagocytosis
Receptor-Mediated Endocytosis

36. Pinocytosis Moves liquids into the cell Means “cell drinking”
A small patch of membrane dimples inward to form a vesicle surrounding the fluid
The acquired material has the same concentration as extracellular fluid

37. Pinocytosis

38. Phagocytosis Moves large particles into the cell Means “cell eating”
Extensions of the membrane fuse around the large particle and carry it to the interior of the cell in a vacuole for intracellular digestion

39. Phagocytosis

40. Cells Move Material Out of the Cell by Exocytosis
A membrane-enclosed vesicle carrying the material to be expelled moves to the cell surface
The vesicle then fuses with the plasma membrane and releases its contents

41. Exocytosis

42. 4.3 – How Are Cell Surfaces Specialized?

43. Various Specialized Junctions Allow Cells to Connect and Communicate
Four types of connections occur between cells, depending on the organism and cell type
Desmosomes
Tight Junctions
Gap Junctions
Plasmodesmata

44. Desmosomes
Membranes of adjacent cells are held together by proteins and carbohydrates
Further strengthened by protein filaments that extend from inside the desmosome to the interior of each cell

45. Tight Junctions
Membranes of adjacent cells are fused together to create leak-proof junctions

46. Gap Junctions
Cell-to-cell cytoplasmic connections found in animal cells that need to communicate with each other

47. Plasmodesmata
Cell-to-cell cytoplasmic connections between plant cells