1. Lipids, Membranes, and the First Cells
6
Lipids, Membranes, and the First Cells
Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge

2. The Importance of Membranes
The plasma membrane, or cell membrane separates life from nonlife
The plasma membrane separates the cell’s interior from the external environment
Membranes function to
Keep damaging materials out of the cell
Allow entry of materials needed by the cell
Facilitate the chemical reactions necessary for life

3. Lipids: What Is a Lipid? Lipids are
Carbon-containing compounds
Found in organisms
Largely nonpolar and hydrophobic
Hydrocarbons are nonpolar molecules that contain only carbon and hydrogen

4. Lipids: What Is a Lipid? Lipids do not dissolve in water, because of
A major hydrocarbon component called a fatty acid
A hydrocarbon chain bonded to a carboxyl (–COOH) functional group
Fatty acids and isoprene are the key building blocks of lipids

5. Saturated and Unsaturated Fatty Acids
Saturated hydrocarbon chains consist of only single bonds between the carbons
Unsaturated hydrocarbon chains one or more double bonds exist in the hydrocarbon chains
Bond saturation also profoundly affects the physical state of lipids

6. Saturated and Unsaturated Fatty Acids
Highly saturated fats
Such as butter
Are solid at room temperature
Saturated lipids that have extremely long hydrocarbon tails
Such as waxes
Form particularly stiff solids at room temperature
Highly unsaturated fats are liquid at room temperature

7. (b) Saturated lipids with long hydrocarbon tails
Figure 6.2
(a) Saturated lipids
(b) Saturated lipids with long hydrocarbon tails
(c) Unsaturated lipids
Butter
Beeswax
Safflower oil
Figure 6.2 The Fluidity of Lipids Depends on the Length and Saturation of Their Hydrocarbon Chains. 7

8. Three Types of Lipids are Found in Cells
Lipid structure is characterized by a physical property
Their insolubility in water
Instead of a shared chemical structure
This insolubility is based on
The high proportion of nonpolar C–C and C–H bonds
Relative to polar functional groups
The three most important types of lipids found in cells:
Fats (triacylglycerols or triglycerides)
Steroids
Phospholipids

9. The Structure of Fats
Fats are composed of three fatty acids linked to glycerol
Also called triacylglycerols or triglycerides
When the fatty acids are polyunsaturated
They form liquid triacylglycerols called oils
The primary role of fats is energy storage

10. The Structure of Fats Fats form when
A dehydration reaction occurs between
A hydroxyl group of glycerol + the carboxyl group of a fatty acid
The glycerol and fatty acid molecules
Become joined by an ester linkage

11. (a) Fats form via dehydration reactions.
Figure 6.3
(a) Fats form via dehydration reactions.
(b) Fats consist of glycerol linked by ester linkages to three fatty acids.
Glycerol
Ester linkages
Dehydration reaction
Fatty acid
Figure 6.3 Fats Are One Type of Lipid Found in Cells. 11

12. The Structure of Steroids
Steroids are
A family of lipids
Distinguished by a bulky, four-ring structure
Steroids differ from one another by
The functional groups or side groups attached to different carbons in those hydrophobic rings

13. The Structure of Steroids
A steroid example is cholesterol
A hydrophilic hydroxyl group attached to the top ring and an isoprenoid “tail” attached at the bottom
Cholesterol is
An important component of plasma membranes in many organisms

14. The Structure of Membrane Lipids
Membrane-forming lipids contain
A polar, hydrophilic region
And a nonpolar, hydrophobic region
Phospholipids are amphipathic
The head region contains highly polar covalent bonds
Consists of a glycerol, a phosphate, and a charged group
The tail region is comprised of two nonpolar fatty acid or isoprene chains

15. The Structure of Membrane Lipids
When placed in solution, these lipids form membranes
The phospholipid heads interact with water
The tails do not

16. (a) A steroid (b) A phospholipid Figure 6.4
Schematic
Space-filling
(b) A phospholipid
Polar (hydrophilic)
Polar or charged group
Polar head (hydrophilic)
Phosphate
Glycerol
Steroid rings
Nonpolar (hydrophobic)
Isoprenoid
Nonpolar tail (hydrophobic)
Fatty acid
Fatty acid
Figure 6.4 Some Lipids Contain Hydrophilic and Hydrophobic Regions. 16

17. Phospholipids and Water
Phospholipids do not dissolve in water
Water molecules cannot form hydrogen bonds with the hydrocarbon tail
Water molecules interact
With the hydrophilic heads
Not with the hydrophobic tails
This drives the hydrophobic tails together

18. Phospholipids and Water
Upon contact with water, phospholipids form either
Micelles
Heads face the water and tails face each other
Phospholipid bilayers (lipid bilayers)

19. Phospholipid Bilayers
Phospholipid bilayers form when
Two sheets of phospholipid molecules align
Hydrophilic heads in each layer face a surrounding solution
The hydrophobic tails face one another inside the bilayer
Phospholipid bilayers form spontaneously
No outside input of energy is required

20. (a) Lipid micelles (b) Lipid bilayers
Figure 6.5
(a) Lipid micelles
Hydrophilic heads interact with water
Hydrophobic tails interact with one another
Water
Hydrophilic heads interact with water
(b) Lipid bilayers
Figure 6.5 Lipids Form Micelles and Bilayers in Solution.
Hydrophobic tails interact with one another 20

21. Selective Permeability of Lipid Bilayers
The permeability of a structure is its tendency to allow a given substance to pass across it
Lipid bilayers are highly selective
Phospholipid bilayers have selective permeability
Small or nonpolar molecules move across phospholipid bilayers quickly
Charged or large polar substances cross slowly, if at all

22. High permeability 100 102 104 106 108 1010 1012 Low permeability
Figure 6.8
High permeability
Small, nonpolar molecules
100
O2, CO2, N2
102
Small, uncharged polar molecules
H2O, glycerol
104
Permeability scale (cm/sec)
106
Glucose, sucrose
Large, uncharged polar molecules
108
Figure 6.8 Lipid Bilayers Show Selective Permeability.
1010
Cl, K, Na
Ions
1012
Low permeability
Phospholipid bilayer 22

23. Many Factors Affect Membrane Permeability
Many factors influence the behavior of the membrane
Number of double bonds between the carbons in the phospholipid’s hydrophobic tail
Length of the tail
Number of cholesterol molecules in the membrane
Temperature

24. Bond Saturation and Membrane Permeability
Double bonds in a hydrocarbon chain can cause a “kink” in the hydrocarbon chain
Prevents the close packing of hydrocarbon tails
Reduces hydrophobic interactions
Unsaturated hydrocarbon chains
Have at least one double bond
Membranes are much more permeable

25. Bond Saturation and Membrane Permeability
Saturated hydrocarbon chains
Are without double bonds
Have more chemical energy than unsaturated fats do
Are much less permeable

26. Higher permeability and fluidity
Figure 6.9
Lipid bilayer with short and unsaturated hydrocarbon tails
Higher permeability and fluidity
Lipid bilayer with long and saturated hydrocarbon tails
Lower permeability and fluidity
Figure 6.9 Fatty Acid Structure Changes the Permeability of Membranes. 26

27. Other Factors That Affect Permeability
Hydrophobic interactions become stronger
As saturated hydrocarbon tails increase in length
Membranes containing phospholipids with longer tails have reduced permeability
Adding cholesterol to membranes increases the density of the hydrophobic section
Cholesterol decreases membrane permeability

28. Other Factors That Affect Permeability
Membrane fluidity decreases
With temperature
When molecules in the bilayer are moving more slowly
Decreased membrane fluidity causes decreased permeability

29. Fluidity of the Membrane
Individual phospholipids can move laterally
Throughout the lipid bilayer
They rarely flip between layers
How quickly molecules move within and across membranes is a function of
Temperature
The structure of hydrocarbon tails
The number of cholesterol molecules in the bilayer

30. Figure 6.11
Phospholipids are in constant lateral motion, but rarely flip to the other side of the bilayer
Figure 6.11 Phospholipids Move within Membranes. 30

31. Solute Movement across Lipid Bilayers
Materials move across the cell membrane in different ways
Passive transport does not require an input of energy
Active transport requires energy to move substances across the membrane

32. Solute Movement across Lipid Bilayers
Small molecules and ions in a solution
Are called solutes
Have thermal energy
Are in constant, random motion
This random movement is called diffusion
Diffusion is a form of passive transport

33. Diffusion along a Concentration Gradient
A concentration gradient is created by a difference in solute concentrations
Molecules and ions move randomly when
A concentration gradient exists
There is a net movement from high-concentration regions to low-concentration regions
Diffusion along a concentration gradient
Increases entropy
Is spontaneous

34. Diffusion along a Concentration Gradient
Equilibrium occurs when the molecules or ions are randomly distributed
Molecules are still moving randomly
But there is no more net movement

35. 1. Separation of solutes by lipid bilayer 2. Diffusion 3. Equilibrium
Figure 6.12
Lipid bilayer
1. Separation of solutes by lipid bilayer
Figure 6.12 Diffusion across a Selectively Permeable Membrane Establishes an Equilibrium.
2. Diffusion
3. Equilibrium 35

36. Osmosis Water moves quickly across lipid bilayers
The movement of water is a special case of diffusion called osmosis
Water moves from regions of low solute concentration to regions of high solute concentration
This movement dilutes the higher concentration
It also equalizes the concentration on both sides of the bilayer
Osmosis only occurs across a selectively permeable membrane

37. 1. Unequal concentrations across membrane 2. Water movement
Figure 6.13
Lipid bilayer
Osmosis
Figure 6.13 Osmosis Is the Diffusion of Water.
1. Unequal concentrations across membrane
2. Water movement 37

38. Osmosis and Relative Solute Concentration
The concentration of a solution outside a cell may differ from the concentration inside the cell
An outside solution with a higher concentration is hypertonic to the inside of a cell
A solution with a lower concentration is hypotonic to the cell
If solute concentrations are equal on the outside and inside of a cell solutions are isotonic to each other

39. Osmosis in Hypertonic, Hypotonic, and Isotonic Solutions
In a hypertonic solution
Water will move out of the cell by osmosis
The cell will shrink
In a hypotonic solution
Water will move into the cell by osmosis
The cell will swell
In an isotonic solution
There will be no net water movement
The cell size will remain the same

40. Inside solution hypotonic to outside
Figure 6.14
Start with:
Inside solution hypotonic to outside
Inside solution hypertonic to outside
Inside and outside solutions isotonic
Lipid bilayer
Figure 6.14 Osmosis Can Shrink or Burst Membrane-Bound Vesicles.
Result:
Net flow of water out of vesicle; vesicle shrinks
Net flow of water into vesicle; vesicle swells or even bursts
No change 40

41. The Fluid-Mosaic Model of Membrane Structure
Phospholipids provide the basic membrane structure
Plasma membranes contain as much protein as phospholipid
The fluid-mosaic model of membrane structure suggests
Some proteins are inserted into the lipid bilayer
Thus making the membrane a fluid, dynamic mosaic of phospholipids and proteins

42. (b) Fluid-mosaic model
Figure 6.17
(a) Sandwich model
Cell exterior
Membrane proteins on cell exterior
Phospholipid bilayer
Membrane proteins on cell interior
Cell interior
(b) Fluid-mosaic model
Cell exterior
Peripheral membrane protein
Figure 6.17 Past and Current Models of Membrane Structure Differ in Where Membrane Proteins Reside.
Phospholipid bilayer
Integral membrane protein
Peripheral membrane protein
Cell interior 42

43. Membrane Proteins Integral proteins are Transmembrane proteins are
Amphipathic
Able to span a membrane
With segments facing both its interior and exterior surfaces
Transmembrane proteins are
Integral proteins that span the membrane
Involved in the transport of selected ions and molecules across the plasma membrane
Able to affect membrane permeability

44. Membrane Proteins Peripheral proteins are
Found only on one side of the membrane
Often attached to integral proteins

45. (a) Proteins can be amphipathic.
Figure 6.16
(a) Proteins can be amphipathic.
The polar and charged amino acid residues are hydrophilic
The nonpolar residues are hydrophobic
(b) Amphipathic proteins can integrate into lipid bilayers.
Outside cell
Figure 6.16 Amphipathic Proteins Are Anchored in Lipid Bilayers.
Inside cell 45

46. Membrane Proteins Affect Ions and Molecules
Transport proteins are transmembrane proteins that transport molecules
Three broad classes of transport proteins:
Channels
Carrier proteins or transporters
Pumps
Each affects membrane permeability

47. Ion Channels and the Electrochemical Gradient
Ion channels are specialized membrane proteins
They circumvent the plasma membrane’s impermeability to small, charged compounds
Electrochemical gradients occur when ions build up on one side of a plasma membrane
They establish both a concentration gradient and a charge gradient
Ions diffuse through channels down their electrochemical gradients

48. High concentration of Na Net  charge
Figure 6.20
High concentration of Na Net  charge
Outside cell
Electro- chemical gradient for sodium ions (Na)
Inside cell
Figure 6.20 Electrochemical Gradient Is a Combined Concentration and Electrical Gradient.
Net  charge
Low concentration of Na 48

49. Protein Structure Determines Channel Selectivity
Channel proteins are selective
Each channel protein has a structure
Permits only a particular type of ion or small molecule to pass through it

50. Key residues allow water to pass but block ions and larger molecules
Figure 6.22
Key residues allow water to pass but block ions and larger molecules
Outside cell
H2O
Figure 6.22 Membrane Channels Are Highly Selective.
Inside cell 50

51. Movement Through Membrane Channels Is Regulated
Gated channels
Open or close in response to a signal
Examples: The binding of a particular molecule, or a change in the electrical voltage across the membrane
The flow of ions and small molecules through membrane channels is carefully controlled

52. Movement Through Membrane Channels Is Regulated
The movement of substances through channels is passive
It does not require an input of energy
Passive transport is powered by diffusion along an electrochemical gradient

53. Figure 6.23 Some Membrane Channels Are Highly Regulated.
When the inside of the membrane is negatively charged relative to the outside, channel is closed.
If membrane charge asymmetry is reversed, channel opens
Filter allows only K ions to pass
Outside cell
Inside cell
Figure 6.23 Some Membrane Channels Are Highly Regulated.
Gate blocks ions from entering channel
Closed
Open 53

54. Facilitated Diffusion via Channel Proteins
Cells have many different types of channel proteins in their membranes
Each type of protein features a structure that allows it to admit a particular type of ion or small molecule
These channels are responsible for facilitated diffusion
The passive transport of substances that would not otherwise cross the membrane

55. Facilitated Diffusion via Carrier Proteins
Facilitated diffusion can occur through channels or through carrier proteins, or transporters
Change shape during the transport process
Move only down a concentration gradient, reducing differences between solutions
Glucose is
A building block for important macromolecules
A major energy source
Lipid bilayers are only moderately permeable to glucose
A glucose transporter named GLUT-1 increases membrane permeability to glucose

56. 3. Conformational change 4. Release
Figure 6.24
Outside cell
Glucose
GLUT-1
Inside cell
1. Unbound protein
2. Glucose binding
3. Conformational change
4. Release
Figure 6.24 Carrier Proteins Undergo Structural Changes to Move Substances. 56

57. Active Transport by Pumps
Cells can transport molecules or ions
They move against an electrochemical gradient
They require energy in the form of ATP
The process is called active transport
Pumps are membrane proteins that provide active transport of molecules across the membrane
The sodium–potassium pump (Na+/K+-ATPase) uses ATP to transport Na+ and K+
These ions move against their concentration gradients

58. Figure 6.25 ATP ADP 1. Unbound protein 2. Sodium binding
Outside cell
Inside cell
Phosphate group
ATP
ADP
1. Unbound protein
2. Sodium binding
3. Shape change
4. Release
Figure 6.25 The Sodium–Potassium Pump Depends on an Input of Chemical Energy Stored in ATP.
5. Unbound protein
6. Potassium binding
7. Shape change
8. Release 58

59. Secondary Active Transport
Pumps move materials against their concentration gradients
Pumps also set up electrochemical gradients
These gradients make it possible for cells to engage in secondary active transport, or cotransport
The gradient provides the potential energy required to power the movement of a different molecule against its particular gradient

60. Summary of Membrane Transport
Three mechanisms of membrane transport:
Diffusion
Facilitated diffusion
Active transport
Passive transport
Involves diffusion and facilitated diffusion
Moves materials down their concentration gradient
Does not require an input of energy

61. Summary of Membrane Transport
Active transport
Moves materials against their concentration gradient
Requires energy provided by ATP or an electrochemical gradient

62. Figure 6.26
Diffusion
Facilitated diffusion
Active transport
H2O
H2O
CO2
Outside cell
Inside cell
H2O
CO2
H2O
Description:
Passive movement of small, uncharged molecules along an electrochemical gradient, through a membrane
Passive movement of …
Active movement of …
Figure 6.26 Summary of the Passive and Active Mechanisms of Membrane Transport.
Protein(s) involved:
None 62

63. Plasma Membrane and the Intracellular Environment
Enables cells to create an internal environment that is much different from the external one
The selective permeability of the lipid bilayer
The specificity of the proteins involved in passive transport and active transport