1. Membrane Structure and Function

2. Membrane Function Membranes organize the chemical activities of cells.
The outer plasma membrane
forms a boundary between a living cell and its surroundings
Exhibits selective permeability
Controls traffic of molecules in and out

3. Membrane Function
Internal membranes provide structural order for metabolism
Form the cell's organelles
Compartmentalize chemical reactions

4. Fluid Mosaic Model of the PM
A membrane is a mosaic
Proteins and other molecules are embedded in a framework of phospholipids
A membrane is fluid
Most protein and phospholipid molecules can move laterally

5. Membrane Structure
Phospholipid
Phospholipids are the major structural component of membranes.

6. Membrane Structure
All membranes are phospholipid bilayers with embedded proteins.
Phospholipid Bilayer
Label the:
Hydrophilic heads
Hydrophobic tails

7. Embedded in the bilayer are proteins
Most of the membrane’s functions are accomplished by the embedded proteins.
Integral proteins span the membrane
Peripheral proteins are on one side or the other of the membrane

8. Plasma Membrane Components
Glycoproteins and glycolipids are proteins/lipids with short chain carbohydrates attached on the extracellular side of the membrane.

9. Carbohydrate of glycoprotein Glycoprotein Glycolipid Integrin
Fig. 5-1a
Carbohydrate of
glycoprotein
Glycoprotein
Glycolipid
Integrin
Phospholipid
Microfilaments
of cytoskeleton
Cholesterol

10. Types of Membrane Proteins
Cell-cell recognition proteins
Integrins
Intercellular junction proteins
Enzymes
Signal transduction proteins
Aka - Receptor proteins
Transport proteins
Passive and active

11. Cell-cell recognition proteins - identify type of cell and identify a cell as “self” versus foreign
Most are glycoproteins
Carbohydrate chains vary between species, individuals, and even between cell types in a given individual.
Glycolipids also play a role in cell recognition

12. Integrins are a type of integral protein
The cytoskeleton attaches to integrins on the cytoplasmic side of the membrane
Integrins strengthen the membrane
Intercellular junction proteins - help like cells stick together to form tissues

13. Many membrane proteins are enzymes
This is especially important on the membranes of organelles.

14. Signal transduction (receptor) proteins bind hormones and other substances on the outside of the cell.
Binding triggers a change inside the cell.
Called signal transduction
Example: The binding of insulin to insulin receptors causes the cell to put glucose transport proteins into the membrane.

15. Fig. 5-1c
Messenger molecule
Receptor
Activated
molecule

16. Transport Proteins Passive Transport Proteins
allow water soluble substances (small polar molecules and ions) to pass through the membrane without any energy cost
Active Transport Proteins
The cell expends energy to transport water soluble substances against their concentration gradient

17. Fig. 5-1d

18. Transport of Substances Across the Plasma Membrane (PM)
Passive Transport
(Simple) Diffusion (5.3)
Facilitated diffusion (5.6)
Osmosis (5.4, 5.5)
Active Transport (5.8)
Bulk Flow (5.9)
Endocytosis
Exocytosis

19. Passive Transport
In passive transport substances cross the membrane by diffusion
Diffusion - net movement of substances from an area of high concentration to low concentration
no energy required

20. Factors Affecting Diffusion Rate
Steepness of concentration gradient
Steeper gradient, faster diffusion
Molecular size
Smaller molecules, faster diffusion
Temperature
Higher temperature, faster diffusion

21. Simple Diffusion
Nonpolar, hydrophobic molecules diffuse directly through the lipid bilayer
Simple diffusion does not require the use of transport proteins.
Examples: O2, CO2, steroids
Polar, hydrophilic substances cannot pass directly through the lipid bilayer
Examples: water, ions, carbohydrates

22. Simple Diffusion Polar molecules ions small, nonpolar molecules
(ex. Glucose, water)
ions
(ex. H+, Na+, K+)
small, nonpolar molecules
(ex. O2, CO2)
LIPID-SOLUBLE
WATER-SOLUBLE
LIPID-SOLUBLE

23. Facilitated Diffusion
In facilitated diffusion small polar molecules and ions diffuse through passive transport proteins.
No energy needed
Most passive transport proteins are solute specific
Example: glucose enter/leaves cells through facilitated diffusion

24. Facilitated Diffusion
Higher concentration of
Passive transport protein
Lower concentration

25. Osmosis
Osmosis – diffusion of water across a selectively permeable membrane
Water moves from an area of _______ water concentration to an area of _____ water conc.
Is energy required ?
Water travels in/out of the cell through aquaporins

26. Consider two solutions separated
Osmosis Terms
Consider two solutions separated
by a plasma membrane.
Hypertonic
solution with a relatively high concentration of solute
Hypotonic
solution with a relatively low concentration of solute
Isotonic
solutions with the same solute concentration

27. cluster of water molecules
Lower
concentration
of solute
Higher
concentration
of solute
Equal
concentration
of solute
H2O
Solute
molecule
Selectively
permeable
membrane
Water
molecule
Solute molecule with
cluster of water molecules
Net flow of water

28. Osmosis and Animal Cells

29. Osmosis and Plant Cells

30. Osmosis When a Cell is Placed in a Hypotonic Solution
Water concentration is _________ the cell.
Water flows ___________ the cell.

31. Osmosis When a Cell is Placed in a Hypertonic Solution
Water concentration is _________ the cell.
Water flows ___________ the cell.

32. See page 83 Isotonic solution Hypotonic solution Hypertonic solution
H2O
H2O
H2O
H2O
Animal
cell
(1) Normal
(2) Lysed
(3) Shriveled
H2O
H2O
H2O
Plasma
membrane
H2O
Plant
cell
(4) Flaccid
(5) Turgid
(6) Shriveled
(plasmolyzed)
See page 83

33. Osmosis Summary When a cell is placed in a Hypotonic solution:
Cell gains water through osmosis
Animal cell lyses; plant cell becomes turgid (firm)
When a cell is placed a Hypertonic solution:
Cell loses water through osmosis
Animal cell shrivels; plant cell plasmolyzes

34. Active Transport
Active transport proteins move substances across the PM against their concentration gradient.
Requires energy (ATP)
Active transport proteins are highly selective
Active transport is needed for proper functioning of nerves and muscles

35. Active Transport of “X”
Active transport proteins span the plasma membrane
They have openings for “X” on only one side of the membrane
“X” enters the channel and binds to functional groups inside the transport protein.
Cytoplasmic ATP binds to the transport protein

36. Active Transport of “X”
A phosphate group is transferred from ATP to the transport protein
protein is energized by the added –P.
The energized transport protein changes shape and releases “X” on the other side of the cell.
The phosphate group is released from the transport protein and it resumes its original shape.
Process repeats.

37. Fig
Transport
protein
Solute 1
Solute binding

38. Fig
Transport
protein
Solute 1
Solute binding 2
Phosphorylation

39. Transport protein Protein changes shape Solute 1 Solute binding 2
Fig
Transport
protein
Protein
changes shape
Solute 1
Solute binding 2
Phosphorylation 3
Transport

40. Transport protein Protein changes shape Phosphate detaches Solute 1
Fig
Transport
protein
Protein
changes shape
Phosphate
detaches
Solute 1
Solute binding 2
Phosphorylation 3
Transport 4
Protein reversion

41. Active Transport tell the story…
ATP P
ADP

42. Bulk Flow
Vesicles are used to transport large particles across the PM.
Requires energy
Types:
Exocytosis
Endocytosis
Phagocytosis, pinocytosis, receptor-mediated

43. Exocytosis
Fluid outside cell
Vesicle
Protein
Cytoplasm

44. Bulk Flow
Exocytosis
Cytoplasmic vesicle merges with the PM and releases its contents
Example:
Golgi body vesicles merge with the PM an release their contents
How nerve cells release neurotransmittors

45. Endocytosis Endocytosis can occur in three ways
Vesicle forming
Endocytosis can occur in three ways
Phagocytosis ("cell eating")
Pinocytosis ("cell drinking")
Receptor-mediated endocytosis

46. Endocytosis Endocytosis
PM sinks inward, pinches off and forms a vesicle
Vesicle often merges with Golgi for processing and sorting of its contents

47. Endocytosis - terms Phagocytosis – cell eating
Membrane sinks in and captures solid particles for transport into the cell
Examples:
Solid particles often include: bacteria, cell debris, or food
Pinocytosis – cell drinking
Cell brings in a liquid

48. Endocytosis - comments
Phagocytosis and pinocytosis are not selective
Membrane sinks inward and captures whatever particles/fluid present.
Vesicle forms and merges with the Golgi body…

49. Receptor Mediated Endocytosis
Receptor Mediated Endocytosis is a highly specific form of endocytosis.
Receptor proteins on the outside of the cell bind specific substances and bring them into the cell by endocytosis

50. Receptor Mediated Endocytosis
Receptor proteins on PM bind specific substances (vitamins, hormones..)
Membrane sinks in and forms a pit
Called a coated pit
Pit pinches closed to form a vesicle around bound substances
Cytoskeleton aids in pulling in the membrane and vesicle formation

51. Receptor-mediated endocytosis
Fig. 5-9c
Plasma membrane
Receptor-mediated endocytosis
Coat protein
Receptor
Coated
vesicle
Coated
pit
Coated
pit
Specific
molecule
Material bound
to receptor proteins

53. Fig. 5-9 Phagocytosis EXTRACELLULAR CYTOPLASM Food FLUID being
ingested
Pseudopodium
“Food” or
other particle
Food
vacuole
Pinocytosis
Plasma
membrane
Vesicle
Receptor-mediated endocytosis
Plasma membrane
Coat protein
Receptor
Coated
vesicle
Coated
pit
Coated
pit
Specific
molecule
Material bound
to receptor proteins