STPM Form 6 Biology Cell membrane

1. The Cell Membrane
AP Biology

2. Overview
 Cell membrane separates living cell from
nonliving surroundings
 thin barrier = 8nm thick
 Controls traffic in & out of the cell
 selectively permeable
 allows some substances to cross more easily
than others
 hydrophobic vs hydrophilic
 Made of phospholipids, proteins & other
macromolecules
AP Biology

3. Phosphate
Phospholipids
 Fatty acid tails
 hydrophobic
 Phosphate group head
Fatty acid
 hydrophilic
 Arranged as a bilayer
Aaaah,
one of those
structure–function
examples
AP Biology

4. Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
AP Biology

5. More than lipids…
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
It’s like a fluid…
It’s like a mosaic…
It’s the
Fluid Mosaic Model!
AP Biology

6. Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Transmembrane
proteins
Peripheral
protein
Cytoplasm Filaments of
cytoskeleton
AP Biology

7. Membrane fat composition varies
 Fat composition affects flexibility
 membrane must be fluid & flexible
 about as fluid as thick salad oil
 % unsaturated fatty acids in phospholipids
 keep membrane less viscous
 cold-adapted organisms, like winter wheat
 increase % in autumn
 cholesterol in membrane
AP Biology

8. Membrane Proteins
 Proteins determine membrane’s specific functions
 cell membrane & organelle membranes each have
unique collections of proteins
 Membrane proteins:
 peripheral proteins
 loosely bound to surface of membrane
 cell surface identity marker (antigens)
 integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 transport proteins
 channels, permeases (pumps)
AP Biology

9. Fluid Mosaic Model
AP Biology

10. AP Biology

11. Membrane Fluidity
More kinks
1. More difficult to pack phospholipid together – membrane stays
fluid at lower temp
(Bacteria, yeast adjust the fatty acid composition according to
temp, to maintain membrane fluidity)
2. The kinks shorten the length of hydrocarbon chains, so that the
membrane is thinner.
AP Biology

12. 1. Lateral movement in the
plane of the lipid layer
2. Rotational movement around
the longitudinal axis of the
molecule
3.Transversion or “flip-flop” movement of
phospholipids from one layer to the other.
This movement is rare and requires
energy.
AP Biology

13. Cholesterol in Plasma Mambrane
Unique to plasma membrane
Stabilize membrane
AP Biology

14. AP Biology

15. The stereochemistry has a
profound effect on the lateral
packing of lipids in the membrane.
Unsaturated fatty acids decrease
the close packing of molecules in
the membrane
AP Biology

16. Cholesterol
 Makes the bilayer
– Stronger
– More flexible
– Less fluid at high temperatures
– Less solid at low temperatures
– Slows phospholipid movement
– Less permeable to water-soluble
substances
AP Biology

17. If the membrane consists mainly of saturated
fatty acid chains, it interdigitates with the
hydrocarbon chains making them more
loosely packed, thus increasing fluidity.
If the membrane contains several
unsaturated fatty acids, it fits into the
gaps caused by bending at the double
bonds and thus stabilizes the
membrane.
AP Biology

18.  Brain cell membranes in ground squirrels
become more solid during hibernation.
 Phospholipids containing more
polyunsaturated fatty acids are more fluid
than those with fewer polyunsaturated fatty
acids.
 Cholesterol in membranes reduces fluidity as
well.
 There are times when membranes need more
or less fluidity, and molecular composition
provides for sure membrane flexibility.
AP Biology

19. Why are
proteins the perfect
molecule to build structures
in the cell membrane?
AP Biology 2007-2008

20. Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
AP Biology

21. Classes of amino acids
What do these amino acids have in common?
I like the
polar ones
the best!
AP Biology
polar & hydrophilic

22. Proteins domains anchor molecule
 Within membrane Polar areas
of protein
 nonpolar amino acids
 hydrophobic
 anchors protein
into membrane
 On outer surfaces of
membrane
 polar amino acids
 hydrophilic
 extend into
extracellular fluid &
Nonpolar areas of protein
AP Biology
into cytosol

23. H+
Examples Retinal
chromophore
NH2
water channel
in bacteria
Porin monomer
b-pleated sheets
Bacterial Nonpolar
outer (hydrophobic) COOH
membrane a-helices in the
cell membrane H+
Cytoplasm
proton pump channel
in photosynthetic bacteria
function through
conformational change =
AP Biology shape change

24. Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter Enzyme Cell surface
activity receptor
Cell surface Cell adhesion Attachment to the
AP Biology identity marker cytoskeleton

25. Membrane carbohydrates
 Play a key role in cell-cell recognition
 ability of a cell to distinguish one cell
from another
 antigens
 important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system
AP Biology

26. Glycoprotein
Glycoprotein, a membrane protein with
a carbohydrate group attached to the
extracellular side.
• The carbohydrate acts as an
identification tag for the cell.
• Examples:
– Immune system and bacteria
– Red blood cells
AP Biology

27. Any Questions??
AP Biology

28. Movement across the
Cell Membrane
AP Biology 2007-2008

29. Diffusion
 2nd Law of Thermodynamics
governs biological systems
 universe tends towards disorder (entropy)
 Diffusion
AP Biology
 movement from high  low concentration

30. Diffusion
 Move from HIGH to LOW concentration
 “passive transport”
 no energy needed
movement of water
AP Biology diffusion osmosis

31. Diffusion across cell membrane
 Cell membrane is the boundary between
inside & outside…
 separates cell from its environment
Can it be an impenetrable boundary? NO!
IN OUT
food waste
carbohydrates OUT ammonia
sugars, proteins salts
amino acids CO2
IN
lipids H2O
salts, O2, H2O products
AP Biology
cell needs materials in & products or waste out

32. Diffusion through phospholipid bilayer
 What molecules can get through directly?
 fats & other lipids
lipid
 What molecules can
inside cell NOT get through
NH3 salt directly?
 polar molecules
 H 2O
 ions
 salts, ammonia
sugar aa H 2O
outside cell  large molecules
 starches, proteins
AP Biology

33. Channels through cell membrane
 Membrane becomes semi-permeable
with protein channels
 specific channels allow specific material
across cell membrane
inside cell H 2O aa sugar
NH3
AP Biology salt outside cell

34. Facilitated Diffusion
 Diffusion through protein channels
 channels move specific molecules across
cell membrane
facilitated = with help
 no energy needed
open channel = fast transport
high
low
AP Biology “The Bouncer”

35. Active Transport
 Cells may need to move molecules against
concentration gradient
 shape change transports solute from
one side of membrane to other
 protein “pump”
conformational change
 “costs” energy = ATP low
ATP
high
AP Biology “The Doorman”

36. Active transport
 Many models & mechanisms
ATP ATP
antiport symport
AP Biology

37. Getting through cell membrane
 Passive Transport
 Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 high  low concentration gradient
 Facilitated transport
 diffusion of polar, hydrophilic molecules
 through a protein channel
 high  low concentration gradient
 Active transport
 diffusion against concentration gradient
 low  high
 uses a protein pump ATP
 requires ATP
AP Biology

38. Transport summary
simple
diffusion
facilitated
diffusion
active ATP
transport
AP Biology

39. How about large molecules?
 Moving large molecules into & out of cell
 through vesicles & vacuoles
 endocytosis
 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”
 exocytosis
AP Biology exocytosis

40. Endocytosis
fuse with
phagocytosis lysosome for
digestion
pinocytosis non-specific
process
triggered by
receptor-mediated molecular
endocytosis signal
AP Biology

41. The Special Case of Water
Movement of water across
the cell membrane
AP Biology 2007-2008

42. Osmosis is diffusion of water
 Water is very important to life,
so we talk about water separately
 Diffusion of water from
high concentration of water to
low concentration of water
 across a
semi-permeable
membrane
AP Biology

43. Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations
 Hypertonic - more solute, less water
 Hypotonic - less solute, more water
 Isotonic - equal solute, equal water
water
hypotonic hypertonic
AP Biology
net movement of water

44. Managing water balance
 Cell survival depends on balancing
water uptake & loss
AP Biology freshwater balanced saltwater

45. Managing water balance
 Isotonic
 animal cell immersed in
mild salt solution
 example:
blood cells in blood plasma
 problem: none
 no net movement of water
 flows across membrane
equally, in both directions
 volume of cell is stable
AP Biology balanced

46. Managing water balance
 Hypotonic
 a cell in fresh water
 example: Paramecium
 problem: gains water,
swells & can burst
 water continually enters
Paramecium cell
 solution: contractile vacuole
ATP  pumps water out of cell
 ATP
 plant cells
 turgid
AP Biology freshwater

47. Water regulation
 Contractile vacuole in Paramecium
ATP
AP Biology

48. Managing water balance
 Hypertonic
 a cell in salt water
 example: shellfish
 problem: lose water & die
 solution: take up water or
pump out salt
 plant cells
 plasmolysis = wilt
AP Biology saltwater

49. 1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells
 evidence that there were water channels
Peter Agre Roderick MacKinnon
John Hopkins Rockefeller
AP Biology

50. Osmosis…
.05 M .03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
AP Biology

51. Any Questions??
AP Biology

52. Cytoplasm
Cytoplasm
Cytosol Organelles
Solution Microbody Compartments
AP Biology

53. AP Biology