1. Membranes
Chapter 7

2. Membrane Structure
The Fluid Mosaic Model of membrane structure contends that membranes consist of:
-phospholipids arranged in a bilayer
-globular proteins inserted in the lipid bilayer

3. Fluid Mosaic Model of the cell membrane
Polar heads love water & dissolve.
Membrane movement animation
Non-polar tails hide from water.
Carbohydrate cell markers
Proteins

4. Membrane Structure Cellular membranes have 4 components:
1. phospholipid bilayer
2. transmembrane proteins
3. interior protein network
4. cell surface markers

5. Membrane Structure
Membrane structure is visible using an electron microscope.
Transmission electron microscopes (TEM) can show the 2 layers of a membrane.
Freeze-fracturing techniques separate the layers and reveal membrane proteins.

8. Phospholipids Phospholipid structure consists of
-glycerol – a 3-carbon polyalcohol acting as a backbone for the phospholipid
-2 fatty acids attached to the glycerol
-phosphate group attached to the glycerol

10. Phospholipids
The fatty acids are nonpolar chains of carbon and hydrogen.
-Their nonpolar nature makes them hydrophobic (“water-fearing”).
The phosphate group is polar and hydrophilic (“water-loving”).

11. Phospholipids
The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer:
-fatty acids are on the inside
-phosphate groups are on both surfaces of the bilayer

13. Phospholipids Phospholipid bilayers are fluid.
-hydrogen bonding of water holds the 2 layers together
-individual phospholipids and unanchored proteins can move through the membrane
-saturated fatty acids make the membrane less fluid than unsaturated fatty acids
-warm temperatures make the membrane more fluid than cold temperatures

14. Phospholipids

15. Membrane Proteins Membrane proteins have various functions:
1. transporters
2. enzymes
3. cell surface receptors
4. cell surface identity markers
5. cell-to-cell adhesion proteins
6. attachments to the cytoskeleton

17. Membrane Proteins Peripheral membrane proteins
-anchored to a phospholipid in one layer of the membrane
-possess nonpolar regions that are inserted in the lipid bilayer
-are free to move throughout one layer of the bilayer

19. Membrane Proteins Integral membrane proteins
- May span the lipid bilayer (transmembrane proteins) or only a portion.
-nonpolar regions of the protein are embedded in the interior of the bilayer
-polar regions of the protein protrude from both sides of the bilayer

21. Membrane Proteins
Integral proteins possess at least one transmembrane domain
-region of the protein containing hydrophobic amino acids
-spans the lipid bilayer

23. Membrane Proteins
Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane.
-b sheets in the protein secondary structure form a cylinder called a b-barrel
-b-barrel interior is polar and allows water and small polar molecules to pass through the membrane

25. Passive Transport
Passive transport is movement of molecules through the membrane in which
-no energy is required
-molecules move in response to a concentration gradient
Diffusion is movement of molecules from high concentration to low concentration

27. Passive Transport
Selective permeability: integral membrane proteins allow the cell to be selective about what passes through the membrane.
Channel proteins have a polar interior allowing polar molecules to pass through.
Carrier proteins bind to a specific molecule to facilitate its passage.

28. Passive Transport Channel proteins include:
-ion channels allow the passage of ions (charged atoms or molecules) which are associated with water
-gated channels are opened or closed in response to a stimulus
-the stimulus may be chemical or electrical

29. Passive Transport
Carrier proteins bind to the molecule that they transport across the membrane.
Facilitated diffusion is movement of a molecule from high to low concentration with the help of a carrier protein.
-is specific: Transport some materials but not all.
-is passive: No energy required.
-saturates when all carriers are occupied

30. Facilitated diffusion (Channel Protein) Diffusion (Lipid Bilayer)
Passive Transport: Facilitated Diffusion A B
Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane
Transports larger or charged molecules
Facilitated diffusion (Channel Protein)
Diffusion (Lipid Bilayer)
Carrier Protein

31. Passive Transport: 2. Facilitated Diffusion
Glucose
molecules
Cellular Transport From a-
High
High Concentration
Channel Proteins animations
Cell Membrane
Low Concentration
Protein
channel
Low
Transport Protein
Through a 
Go to Section:

32. Passive Transport In an aqueous solution -water is the solvent
-dissolved substances are the solutes
Osmosis is the movement of water from an area of high to low concentration of water
-movement of water toward an area of high solute concentration

34. Passive Transport
When 2 solutions have different osmotic concentrations
-the hypertonic solution has a higher solute concentration
-the hypotonic solution has a lower solute concentration
Osmosis moves water through aquaporins toward the hypertonic solution.

36. Passive Transport: 3. Osmosis
Osmosis animation
3.Osmosis: diffusion of water through a selectively permeable membrane
Water moves from high to low concentrations
Water moves freely through pores.
Solute (green) to large to move across.

37. Passive Transport
Organisms can maintain osmotic balance in different ways.
1. Some cells use extrusion in which water is ejected through contractile vacuoles.
2. Isosmotic regulation involves keeping cells isotonic with their environment.
3. Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid.

38. Active Transport Active transport
-requires energy – ATP is used directly or indirectly to fuel active transport
-moves substances from low to high concentration
-requires the use of carrier proteins

39. Active Transport Carrier proteins used in active transport include:
-uniporters – move one molecule at a time
-symporters – move two molecules in the same direction
-antiporters – move two molecules in opposite directions

40. Active Transport Sodium-potassium (Na+-K+) pump
-an active transport mechanism
-uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell
-ATP energy is used to change the conformation of the carrier protein
-the affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane

42. Active Transport Coupled transport
-uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule
-a symporter is used
-glucose-Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient

44. Bulk Transport Bulk transport of substances is accomplished by
1. endocytosis – movement of substances into the cell
2. exocytosis – movement of materials out of the cell

45. Bulk Transport
Endocytosis occurs when the plasma membrane envelops food particles and liquids.
1. phagocytosis – the cell takes in particulate matter
2. pinocytosis – the cell takes in only fluid
3. receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor

49. Bulk Transport
Exocytosis occurs when material is discharged from the cell.
-vesicles in the cytoplasm fuse with the cell membrane and release their contents to the exterior of the cell
-used in plants to export cell wall material
-used in animals to secrete hormones, neurotransmitters, digestive enzymes

51. Short distance Movement of water across a plasma membrane
Osmosis and Water Potential
Osmosis is the movement of water from High to Low Concentration.
Water Potential measures the concentration of free water molecules. It is a measure of the tendency of these molecules to diffuse to another area. The more free water molecules, the higher the Water Potential.

52. Understanding Water Potential
Water potential = Ψ : psi (sounds like “sigh”)
measure of pressure: megapascal (MPa) or “bars” (1 MPa=10 Bars)
Ψ = 0 MPa for pure water in an open container at sea level and at room temperature
Note: 1 MPa is about 10x atmospheric pressure at sea level

53. How Solutes and Pressure Affect Water Potential
Equation: Ψ = ΨS + ΨP
The solute potential “osmotic potential” (ΨS) of a solution is directly proportional to its molarity (pure water is 0)
Expresses as a negative (the more solute added, the more negative it becomes!!!)
Less “free” water
An increase in solute concentration has a negative effect on water potential
Pressure potential (ΨP) is the physical pressure on a solution (from cell wall)
Positive or negative
For example: A solution Drawn in, by a syringe, is under negative pressure; it is under positive pressure when it is being expelled by the syringe
Water in cells usually under positive pressure
Turgor pressure is the push out on the cell wall
Air in a tire

54. Water potential equation:
Water potential (ψ): H2O moves from high ψ  low ψ potential
Water potential equation:
ψ = ψS + ψP
Water potential (ψ) = free energy of water
Solute potential (ψS) = solute concentration (osmotic potential)
Pressure potential (ψP) = physical pressure on solution; turgor pressure (plants)
Pure water: ψP = 0 MPa
Plant cells: ψP = 1 MPa

55. Where will WATER move? From an area of:
higher ψ  lower ψ (more negative ψ)
low solute concentration  high solute concentration
high pressure  low pressure

57. Solute potential (ψs):
Water potential (ψ) = pressure potential (ψp) + solute (osmotic) potential (ψs)
Solute potential (ψs):
The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also.
*As solute is added, the water potential of a solution drops, and water will tend to move into the solution.

58. pressure potential (ψp ) + solute potential (ψs) (osmotic)
Water moves from a place of high water potential to a place of low water potential.
Water potential (ψ) =
pressure potential (ψp )
+ solute potential (ψs) (osmotic)
This is an open container, so the ψp = 0
This makes the ψ = ψs
The ψs =-0.23, so ψ is -0.23MPa, and water moves into the solution.

60. Pressure potential (ψp):
Water potential (ψ) = pressure potential (ψp) + solute (osmotic) potential (ψs)
Pressure potential (ψp):
In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake

62. Water Movement Across Plant Cell Membranes
Flaccid (limp) cell is placed in a hypertonic environment, the cell will lose water and undergo plasmolysis
Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall
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63. If a flaccid cell is placed in a hypotonic environment, the cell will gain water and become turgid
© 2017 Pearson Education, Inc.

64. Questions:
If a plant cell immersed in distilled water has a ΨS of -0.7 MPa and a Ψ of 0 MPa, what is the cell’s Ψp?
If you put it in an open beaker of solution that has a Ψ of -0.4 MPa what would be its Ψp at equilibrium?
0.7 MPa
0.3 MPa

65. Answer will be in Bars
10bars = 1MPa

66. Calculating Solute Potential (ψS)
ψS = -iCRT
i = ionization constant (# particles made in water)
C = molar concentration
R = pressure constant ( liter bars/mole-K)
T = temperature in K ( C)
The addition of solute to water lowers the solute potential (more negative) and therefore decreases the water potential.

67. So what is the solute potential of a 0.1 M solution of sucrose at 22 C?
Solute potential = -iCRT
i (ionization constant) = 1
R = (from handbook)
T = temp K (273 + C of solution)
Ωs = - (1) (0.1) (0.0831) (295)
Ωs = bars 67

68. Calculate the water potential of a solution of 0. 15 M sucrose
Calculate the water potential of a solution of 0.15 M sucrose. The solution is at standard temperature (273K) and pressure (.0831 L bars/mol K).
Ψ = Ψp + Ψs
= 0 bars + [-iCRT]
= 0 bars + [-(1) (0.15 mol/L) ( L bars/mol K) (273K)
= 0 bars bars
Ψ = bars

69. Calculate the solute potential of a 0.1M NaCl solution at 25°C.
If the concentration of NaCl inside the plant cell is 0.15M, which way will the water diffuse if the cell is placed in the 0.1M NaCl solution?

70. Which chamber has a lower water potential?
Which chamber has a lower solute potential?
In which direction will osmosis occur?
If one chamber has a Ψ of kPa, and the other kPa, which is the chamber that has the higher Ψ?