Cell Membranes

1

• Cell Membranes

2
Plasma Membrane

• Defines the cell as a distinct entity (p52)

Fig. 4-4, p.52
3
Plasma Membrane
enlarged area
cytoplasm
plasma membrane
Fig. 4-5b, p.53
4
Lipid Bilayer

• Main component of cell membranes
• Gives the membrane its fluid properties
• Two layers of phospholipids

5
Lipid Bilayer

• Membrane is a continuous boundary layer that
  selectively controls the flow of substances
  across it

6
Lipid Bilayer
hydrophilic parts
hydrophobic parts
fluid
fluid
one layer of lipids
one layer of lipids
b
cross-section through lipid bilayer
a
Fig. 5.3, pg. 76
7
Fluid Mosaic Model

• Membrane is a mosaic of
• Phospholipids
• Glycolipids
• Sterols
• Proteins
• Most phospholipids and some proteins can drift
  through membrane

8
Membrane Proteins

• Adhesion proteins
• Communication proteins
• Receptor proteins
• Recognition proteins

9
Fluid Mosaic Model
passive transporter
recognition protein
phospholipid
adhesion protein
cholesterol
receptor
Lipid bilayer
active transporter (ATPase pump)
active transporter (calcium pump)
Cytoplasm
Plasma Membrane
cytoskeletal proteins just beneath the plasma
membrane
Fig. 5.4, pg. 77
10
Studying Membranes
Stepped Art
Fig. 5.5a, pg. 77
11
Studying Membranes
Stepped Art
Fig. 5.5b, pg. 77
12
Overview of Membrane Proteins
Adhesion Proteins
Communication Proteins
Fig. 5.6, p.78
13
Overview of Membrane Proteins
Receptor Proteins
Recognition Proteins
Passive Transporters
Active Transporters
Fig. 5.6, p.79
14
Transport Proteins

• Span the lipid bilayer
• Interior is able to open to both sides
• Change shape when they interact with solute
• Play roles in active and passive transport

15
Concentration Gradient

• Means the number of molecules or ions in one
  region is different than the number in another
  region
• In the absence of other forces, a substance moves
  from a region where it is more concentrated to
  one one where its less concentrated - down
  gradient

16
Diffusion

• The net movement of like molecules or ions down a
  concentration gradient
• Although molecules collide randomly, the net
  movement is away from the place with the most
  collisions (down gradient)

17
Diffusion
Stepped Art
Fig. 5.7a, p.80
18
Diffusion
Stepped Art
Fig. 5.7b, p.80
19
Factors Affecting Diffusion Rate

• Steepness of concentration gradient
• Steeper gradient, faster diffusion
• Molecular size
• Smaller molecules, faster diffusion
• Temperature
• Higher temperature, faster diffusion
• Electrical or pressure gradients

20
Membrane Crossing Mechanisms

• Diffusion across lipid bilayer
• Passive transport
• Active transport
• Endocytosis
• Exocytosis

21
Cell Membranes Show Selective Permeability
oxygen, carbon dioxide, and other small,
nonpolar molecules some water molecules
glucose and other large, polar, water-soluble
molecules ions (e.g., H, Na, K, Ca, Cl)
water molecules
Fig. 5-8, p.80
22
Membrane Crossing Overview I
High
Concentration gradient across cell membrane
ATP
Low
Diffusion of lipid-soluble Substances across
bilayer
Passive transport of water- soluble
substances through channel protein no energy
input needed
Active transport through ATPase requires
energy input from ATP
Fig. 5-9, p.81
23
Membrane Crossing Overview II
Endocytosis (vesicles in)
Exocytosis (vesicles out)
Fig. 5-9, p.81
24
Passive Transport

• Flow of solutes through the interior of passive
  transport proteins down their concentration
  gradients
• Passive transport proteins allow solutes to move
  both ways
• Does not require any energy input

25
Passive Transport
glucose transporter
solute (glucose)
high
low
Stepped Art
Fig. 5.10, p.80
26
Active Transport

• Net diffusion of solute is against concentration
  gradient
• Transport protein must be activated
• ATP gives up phosphate to activate protein
• Binding of ATP changes protein shape and affinity
  for solute

27
Active Transport

• ATP gives up phosphate to activate protein
• Binding of ATP changes protein shape and affinity
  for solute

28
Active Transport
Stepped Art
Fig. 5-11, p.83
29
Osmosis

• Diffusion of water molecules across a selectively
  permeable membrane

• Direction of net flow is determined by water
  concentration gradient
• Side with the most solute molecules has the
  lowest water concentration

30
Osmosis
p.84
31
Tonicity

• Refers to relative solute concentration of two
  fluids
• Hypotonic - having fewer solutes
• Hypertonic - having more solutes
• Isotonic - having same amount

32
Tonicity and Osmosis
2 sucrose solution
1 liter of 10 sucrose solution
1 liter of 2 sucrose solution
1 liter of distilled water
33
Tonicity and Osmosis
2 sucrose solution
1 liter of 10 sucrose solution
1 liter of 2 sucrose solution
1 liter of distilled water
Hypotonic Conditions
Hypertonic Conditions
Isotonic Conditions
Fig. 5-13, p.85
34
Pressure and Osmosis

• Hydrostatic pressure
• Pressure exerted by fluid on the walls that
  contain it
• The greater the solute concentration of the
  fluid, the greater the hydrostatic pressure
• Osmotic pressure
• Amount of pressure necessary to prevent further
  increase of a solutions volume

35
Increase in Fluid Volume
second compartment
first compartment
hypotonic solution
hypertonic solution
membrane permeable to water but not to solutes
fluid volume rises in second compartment
Fig. 5.14, p.85
36
Endocytosis and Exocytosis

• Exocytosis A cytoplasmic vesicle fuses with the
  plasma membrane and contents are released outside
  the cell
• Endocytosis A small patch of plasma membrane
  sinks inward and seals back on itself, forming a
  vesicle inside the cytoplasm membrane receptors
  often mediate this process

37
endocytosis
exocytosis
a
Endocytosis and Exocytosis
coated pit
b
c
d
f
e
Fig. 5-15, p.86
38
Endocytosis of cholesterol
plasma membrane
cholesterol
39
Macrophage engulfing Leishmania mexicana
parasite
macrophage
Fig 5.17, p.87
40
Phagocytosis
bacterium
phagocytic vesicle
Fig. 5-17b, p.87
41
How Proteins Get to the Surface
vesicle membrane fuses with plasma membrane
Golgi body
endoplasmic reticulum
Fig. 5.18, pg. 87
42
Contractile Vacuole
contractile vacuole filled
contractile vacuole emptied
Fig. 5.21, pg. 89
43
adhesion protein
recognition protein
receptor protein
passive transporter
lipid bilayer
cytoskeletal proteins
cytoplasm
active transporter (calcium pump)
active transporter (ATPase pump)
Fig. 5-19, p.88