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Cell Membranes

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Title: Cell Membranes


1
Cell Membranes
2
6 Cell Membranes
  • 6.1 What Is the Structure of a Biological
    Membrane?
  • 6.2 How Is the Plasma Membrane Involved in Cell
    Adhesion and Recognition?
  • 6.3 What Are the Passive Processes of Membrane
    Transport?
  • 6.4 What Are the Active Processes of Membrane
    Transport?
  • 6.5 How Do Large Molecules Enter and Leave a Cell?

3
6 Cell Membranes
The cell membrane regulates what enters and
leaves the cytoplasm. Some cell membranes have
pores called aquaporins that allow water to pass
freely.
Opening Question Water purity is a worldwide
problem. Can aquaporin membrane channels be used
in water purification?
4
6.1 What Is the Structure of a Biological
Membrane?
  • The general structure of biological membranes is
    known as the fluid mosaic model.
  • Phospholipids form a bilayer, which is like a
    lake in which a variety of proteins float.

5
Figure 6.1 The Fluid Mosaic Model
6
6.1 What Is the Structure of a Biological
Membrane?
  • Phospholipids have a polar, hydrophilic head
    and hydrophobic fatty acid tails.
  • In an aqueous environment, phospholipids form a
    bilayer.

7
Figure 3.22 Phospholipids (Part 1)
8
Figure 6.2 A Phospholipid Bilayer
9
6.1 What Is the Structure of a Biological
Membrane?
  • Artificial bilayers can be made in the
    laboratory.
  • Lipids maintain a bilayer organization
    spontaneously. This helps membranes fuse during
    phagocytosis, vesicle formation, etc.

10
6.1 What Is the Structure of a Biological
Membrane?
  • Lipid composition of membranes vary.
  • Phospholipids vary in fatty acid chain length,
    degree of saturation, and phosphate groups.

11
6.1 What Is the Structure of a Biological
Membrane?
  • Animal cell membranes may be up to 25
    cholesterol, which is important for membrane
    integrity.

12
6.1 What Is the Structure of a Biological
Membrane?
  • The fatty acid tails make the interior somewhat
    fluid, allowing lateral movement of molecules.
  • Fluidity depends on temperature and lipid
    composition.

13
6.1 What Is the Structure of a Biological
Membrane?
  • Cholesterol and long-chain, saturated fatty acids
    pack tightly, making a less-fluid membrane.
  • As temperature decreases, movement of molecules
    and cellular processes slow. Some organisms
    change the lipid content of the cell membranes
    when they get cold.

14
6.1 What Is the Structure of a Biological
Membrane?
  • Membranes also contain proteins the number
    varies depending on membrane function.
  • Peripheral membrane proteins lack exposed
    hydrophobic groups and do not penetrate the
    bilayer.

15
6.1 What Is the Structure of a Biological
Membrane?
  • Integral membrane proteins have hydrophobic and
    hydrophilic regions or domains.
  • Some extend across the lipid bilayer others are
    partially embedded.

16
Figure 6.3 Interactions of Integral Membrane
Proteins
17
6.1 What Is the Structure of a Biological
Membrane?
  • Freeze-fracturing is a technique that reveals
    proteins embedded in the phospholipid bilayers of
    cellular membranes.

18
Figure 6.4 Membrane Proteins Revealed by the
Freeze-Fracture Technique
19
6.1 What Is the Structure of a Biological
Membrane?
  • The proteins and lipids interact noncovalently.
  • But some membrane proteins have lipid groups
    covalently attached and are tethered to the lipid
    bilayer.

20
6.1 What Is the Structure of a Biological
Membrane?
  • Transmembrane proteins extend all the way through
    the phospholipid bilayer.
  • They have one or more transmembrane domains, and
    the domains on the inner and outer sides of the
    membrane can have specific functions.
  • Peripheral membrane proteins are located on one
    side of the membrane.

21
6.1 What Is the Structure of a Biological
Membrane?
  • Some membrane proteins can move freely within the
    bilayer, while some are anchored to a specific
    region.
  • When cells are fused experimentally, some
    proteins from each cell distribute themselves
    uniformly around the membrane.

22
Figure 6.5 Rapid Diffusion of Membrane Proteins
23
Figure 6.5 Rapid Diffusion of Membrane Proteins
(Part 1)
24
Figure 6.5 Rapid Diffusion of Membrane Proteins
(Part 2)
25
6.1 What Is the Structure of a Biological
Membrane?
  • Membranes are dynamic and are constantly forming,
    transforming, fusing, and breaking down.

26
Figure 5.9 The Endomembrane System (Part 2)
27
6.1 What Is the Structure of a Biological
Membrane?
  • Membranes also have carbohydrates on the outer
    surface that serve as recognition sites for other
    cells and molecules.
  • Glycolipidscarbohydrate lipid
  • Glycoproteinscarbohydrate protein

28
Working with Data
  • A key experiment providing evidence for the fluid
    mosaic model used the technique of cell fusion to
    show that membrane proteins rapidly diffuse
    within the cell membrane.

29
Working with Data 6.1, Table 1
30
Working with Data 6.1 Rapid Diffusion of
Membrane Proteins
  • Question 1
  • Plot the percentage of fully mixed cells over
    time.
  • How long did it take for complete mixing?

31
Working with Data 6.1 Rapid Diffusion of
Membrane Proteins
  • Question 2
  • What does your answer to Question 1 indicate
    about the rate of diffusion of the mouse and
    human proteins?

32
6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?
  • Cells arrange themselves in groups by cell
    recognition and cell adhesion.
  • These processes can be studied in sponge
    cellsthe cells are easily separated and will
    come back together again.

33
Figure 6.6 Cell Recognition and Adhesion
34
6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?
  • Molecules involved in cell recognition and
    binding are glycoproteins.
  • Binding of cells is usually homotypic The same
    molecule sticks out from both cells and forms a
    bond.
  • Some binding is heterotypic The cells have
    different proteins.

35
6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?
  • Cell junctions are specialized structures that
    hold cells together
  • Tight junctions
  • Desmosomes
  • Gap junctions

36
Figure 6.7 Junctions Link Animal Cells Together
(Part 1)
  • Tight junctions help ensure directional movement
    of materials.

37
Figure 6.7 Junctions Link Animal Cells Together
(Part 2)
  • Desmosomes are like spot welds.

38
Figure 6.7 Junctions Link Animal Cells Together
(Part 3)
  • Gap junctions allow communication.

39
6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?
  • Cell membranes also adhere to the extracellular
    matrix.
  • The transmembrane protein integrin binds to the
    matrix outside epithelial cells, and to actin
    filaments inside the cells.
  • The binding is noncovalent and reversible.

40
6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?
  • Cells can move within a tissue by the binding and
    reattaching of integrin to the extracellular
    matrix.
  • This is important for cell movement within
    developing embryos and for the spread of cancer
    cells.

41
Figure 6.8 Integrins and the Extracellular Matrix
42
6.3 What Are the Passive Processes of Membrane
Transport?
  • Membranes have selective permeabilitysome
    substances can pass through, but not others.
  • Passive transportno outside energy required
    (diffusion).
  • Active transportenergy required.

43
6.3 What Are the Passive Processes of Membrane
Transport?
  • Energy for passive transport comes from the
    concentration gradient the difference in
    concentration between one side of the membrane
    and the other.

44
6.3 What Are the Passive Processes of Membrane
Transport?
  • Particles in a solution move randomly until they
    are evenly distributed.
  • At equilibrium, the particles continue to move,
    but there is no net change in distribution.

45
In-Text Art, Ch. 6, p. 113
46
6.3 What Are the Passive Processes of Membrane
Transport?
  • Diffusion the process of random movement toward
    equilibrium.
  • Net movement is directional until equilibrium is
    reached.
  • Diffusion is the net movement from regions of
    greater concentration to regions of lesser
    concentration.

47
6.3 What Are the Passive Processes of Membrane
Transport?
  • Diffusion rate depends on
  • Diameter of the molecules or ions
  • Temperature of the solution
  • Concentration gradient

48
6.3 What Are the Passive Processes of Membrane
Transport?
  • Diffusion works very well over short distances
    (e.g., within a cell).
  • Membrane properties affect the diffusion of
    solutes.
  • A membrane is permeable to solutes that move
    easily across it impermeable to those that
    cannot.

49
6.3 What Are the Passive Processes of Membrane
Transport?
  • Simple diffusion Small molecules pass through
    the lipid bilayer.
  • Water and lipid-soluble molecules can diffuse
    across the membrane.
  • Electrically charged and polar molecules can not
    pass through easily.

50
6.3 What Are the Passive Processes of Membrane
Transport?
  • Osmosis the diffusion of water.
  • It depends on the relative concentrations of
    water molecules on each side of the membrane.
  • Hypertonic higher solute concentration
  • Isotonic equal solute concentrations
  • Hypotonic lower solute concentration

51
Figure 6.9 Osmosis Can Modify the Shapes of
Cells (Part 1)
52
Figure 6.9 Osmosis Can Modify the Shapes of
Cells (Part 2)
53
Figure 6.9 Osmosis Can Modify the Shapes of
Cells (Part 3)
54
6.3 What Are the Passive Processes of Membrane
Transport?
  • If two solutions are separated by a membrane that
    allows water, but not solutes, to pass through
  • Water will diffuse from the region of higher
    water concentration (lower solute concentration)
    to the region of lower water concentration
    (higher solute concentration).

55
6.3 What Are the Passive Processes of Membrane
Transport?
  • Water will diffuse (net movement) from a
    hypotonic solution across a membrane to a
    hypertonic solution.
  • Animal cells may burst when placed in a hypotonic
    solution.
  • Plant cells with rigid cell walls build up
    internal pressure that keeps more water from
    enteringturgor pressure.

56
6.3 What Are the Passive Processes of Membrane
Transport?
  • Facilitated diffusion of polar molecules
    (passive)
  • Channel proteinsintegral membrane proteins that
    form a channel.
  • Carrier proteinsmembrane proteins that bind some
    substances and speed their diffusion through the
    bilayer.

57
6.3 What Are the Passive Processes of Membrane
Transport?
  • Ion channels Channel proteins with hydrophilic
    pores.
  • Most are gatedcan be closed or open to ion
    passage.
  • Gate opens when protein is stimulated to change
    shape by a chemical signal (ligand) or an
    electrical charge difference (voltage-gated).

58
Figure 6.10 A Gated Channel Protein Opens in
Response to a Stimulus
59
6.3 What Are the Passive Processes of Membrane
Transport?
  • The potassium channel allows K in the unhydrated
    state to pass through, but hydrated Na is too
    large to pass.

60
In-Text Art, Ch. 6, p. 116
61
6.3 What Are the Passive Processes of Membrane
Transport?
  • Water can cross a membrane by moving through
    special water channels called aquaporins.
  • The function of these proteins was determined by
    injecting the aquaporin proteins into a frog
    oocyte.

62
Figure 6.11 Aquaporins Increase Membrane
Permeability to Water
63
6.3 What Are the Passive Processes of Membrane
Transport?
  • Carrier proteins transport polar molecules such
    as glucose across membranes in both directions.
  • Glucose binds to the protein, causing it to
    change shape and release the glucose on the other
    side.

64
Figure 6.12 A Carrier Protein Facilitates
Diffusion (Part 1)
65
6.3 What Are the Passive Processes of Membrane
Transport?
  • In carrier-mediated transport, the rate of
    diffusion is limited by the number of carrier
    proteins in the cell membrane.
  • When all carriers are loaded with solute, the
    diffusion system is saturated.
  • Cells that need lots of energy (e.g., muscle
    cells) have many glucose transporters.

66
Figure 6.12 A Carrier Protein Facilitates
Diffusion (Part 2)
67
6.4 What Are the Active Processes of Membrane
Transport?
  • Active transport moves substances against a
    concentration and/or electrical gradient.
    Requires energy.
  • The energy source is often adenosine triphosphate
    (ATP).

68
Table 6.1
69
6.4 What Are the Active Processes of Membrane
Transport?
  • Active transport is directional. It involves
    three kinds of proteins
  • Uniportermoves one substance in one direction
  • Symportermoves two substances in one direction
  • Antiportermoves two substances in opposite
    directions

70
Figure 6.13 Three Types of Proteins for Active
Transport
71
6.4 What Are the Active Processes of Membrane
Transport?
  • Primary active transport requires direct
    hydrolysis of ATP.
  • Secondary active transport energy comes from an
    ion concentration gradient that is established by
    primary active transport.

72
6.4 What Are the Active Processes of Membrane
Transport?
  • The sodiumpotassium (NaK) pump is primary
    active transport.
  • Found in all animal cells.
  • The pump is an integral membrane glycoprotein (an
    antiporter).

73
Figure 6.14 Primary Active Transport The
SodiumPotassium Pump
74
6.4 What Are the Active Processes of Membrane
Transport?
  • In secondary active transport, energy can be
    regained by letting ions move across a membrane
    with the concentration gradient.
  • Aids in uptake of amino acids and sugars.
  • Uses symporters and antiporters.

75
Figure 6.15 Secondary Active Transport
76
6.5 How Do Large Molecules Enter and Leave a Cell?
  • Macromolecules (proteins, polysaccharides,
    nucleic acids) are too large to cross the
    membrane.
  • They can be taken in or secreted by means of
    membrane vesicles.

77
6.5 How Do Large Molecules Enter and Leave a Cell?
  • Endocytosis processes that brings molecules and
    cells into a eukaryotic cell.
  • The plasma membrane folds in or invaginates
    around the material, forming a vesicle.

78
Figure 6.16 Endocytosis and Exocytosis
79
6.5 How Do Large Molecules Enter and Leave a Cell?
  • Phagocytosis molecules or entire cells are
    engulfed. Some protists feed in this way. Some
    white blood cells engulf foreign substances in
    this way.
  • A food vacuole or phagosome forms, which fuses
    with a lysosome.

80
6.5 How Do Large Molecules Enter and Leave a Cell?
  • Pinocytosis a vesicle forms to bring small
    dissolved substances or fluids into a cell.
    Vesicles are much smaller than in phagocytosis.
  • Pinocytosis is constant in endothelial
    (capillary) cells.

81
6.5 How Do Large Molecules Enter and Leave a Cell?
  • Receptor mediated endocytosis is highly specific
  • Depends on receptor proteinsintegral membrane
    proteinsto bind to specific substances.
  • Sites are called coated pitscoated with other
    proteins such as clathrin.

82
Figure 6.17 Receptor-Mediated Endocytosis
83
6.5 How Do Large Molecules Enter and Leave a Cell?
  • Mammalian cells take in cholesterol by
    receptor-mediated endocytosis.
  • In the liver, cholesterol is packaged into
    low-density lipoprotein, or LDL, and secreted to
    the bloodstream.
  • Cells that need cholesterol have receptors for
    the LDLs in clathrin-coated pits.

84
6.5 How Do Large Molecules Enter and Leave a Cell?
  • Exocytosis material in vesicles is expelled from
    a cell.
  • Indigestible materials are expelled.
  • Other materials leave cells such as digestive
    enzymes and neurotransmitters.

85
Table 6.2
86
6 Answer to Opening Question
  • Aquaporins in both animals and plants are similar
    in structure.
  • Aquaporins are being inserted into synthetic
    membranes to purify drinking waterthey allow
    only water to pass through, not solutes.
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