Transport Across Cell Membranes

1. Substances transported for
chemical reactions
electrical
potentials at the
membrane
Elimination from
the cells

2. Cell Membrane
• Proteins
Integral –carrier & channel
Peripheral-receptors & antigen

3. Methods of transport
Passive Active
Lipid bilayer
Protein
channels
Leaky channels
Gated channels
Diffusion
Simple facilitated
Osmosis Filtration
voltage gated
Ligand gated

4. Figure 5.10a
Sugar
molecule
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
H2O
Predict what you think will happen to
the water levels on either side of
the semi permeable membrane.

5. Figure 5.10a
Sugar
molecule
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
H2O
More similar concen-
trations of solute
hypotonic
hypertonic
isotonic

6. Isotonic Solution
Hypertonic Solution
Hypotonic solution

7. The Permeability of the Plasma Membrane
The plasma membrane is partially permeable.
Macromolecules cannot pass through because
of their size, and tiny charged molecules do
not pass through the nonpolar interior of the
membrane.
Small, uncharged molecules such as oxygen
and carbon dioxide pass through the
membrane, down their concentration
gradient.

8. Getting through cell membrane
• Diffusion
– Passive transport of small molecules
• high  low concentration gradient
• Facilitated Diffusion
– Passive transport of larger or polar molecules
– through a protein channel
• high  low concentration gradient
• Active transport
– diffusion against the concentration gradient
• low  high
– uses a protein channel (pump)
– requires ATP energy
S5

9. How molecules cross the plasma membrane

10. Diffusion
Diffusion is the passive movement of
molecules from a higher to a lower
concentration until equilibrium is reached.
Gases move through plasma membranes by
diffusion.

11. Gas exchange in the lungs occurs by diffusion

12. Semi-permeable membrane
• Will allow passage through the
membrane
• But need to control what gets in or
out
– membrane needs to be semi-permeable
So how does a semi-
permeable membrane work?
aa H2O
sugar lipid salt NH3
S3

13. Phospholipid bilayer
• What molecules can get through
directly?
inside cell
outside cell
Small lipids
can slip directly
through the
phospholipid cell
membrane,
lipid
salt
aa H2O
sugar
NH3
S4

14. Diffusion (passive transport)
• movement from high  low concentration
S6

15. Diffusion of two solutes
(passive)
• Each substance diffuses down its own
concentration gradient, independent of
concentration gradients of other substances
S7

16. 17
Simple Diffusion
• Requires NO
energy
• Molecules
move from
area of HIGH
to LOW
concentration

17. 18
DIFFUSION
Diffusion is a
PASSIVE process
which means no
energy is used to
make the
molecules move,
they have a
natural KINETIC
ENERGY

18. 19
Diffusion of Liquids

19. 20
Osmosis
• Diffusion of water
across a
membrane
• Moves from HIGH
water potential
(low solute) to
LOW water
potential (high
solute)
Diffusion across a membrane
Semipermeable
membrane

20. 21
Diffusion of H2O Across A
Membrane
High H2O potential
Low solute concentration
Low H2O potential
High solute concentration

21. Osmosis, the diffusion of
water
• Water goes from HIGH to LOW
concentration
– “passive transport”
– no energy needed (does not require ATP)
diffusion osmosis S8

22. Simple diffusion across
membrane
inside cell
outside cell
Which way will
lipid move?
low
high

lipid
lipid
lipid
lipid
lipid
lipid lipid
lipid
lipid
lipid
lipid
lipid
lipid
lipid

23. 24
Cell in Isotonic Solution
CELL
10% NaCL
90% H2O
10% NaCL
90% H2O
What is the direction of water movement?
The cell is at _______________.
equilibrium
ENVIRONMENT
NO NET
MOVEMENT

24. 25
Cell in Hypotonic Solution
CELL
10% NaCL
90% H2O
20% NaCL
80% H2O
What is the direction of water movement?

25. 26
Cell in Hypertonic Solution
CELL
15% NaCL
85% H2O
5% NaCL
95% H2O
What is the direction of water movement?
ENVIRONMENT

26. 27
Cells in Solutions

27. Osmosis works to equalize solute concentration
(especially when the membrane is impermeable to
solute)
• If you put a cell in water, what will happen to
it? That depends on the concentration of
water inside and outside the cell.
Classification of
Outside Fluid
Description Water diffuses… Affect on cell
Hypertonic Higher solute
concentration in
fluid outside cell
Out of the cell Shrinks (think
raisin)
Hypotonic Lower solute
concentration in
fluid outside cell
Into the cell Swells and/or
bursts
Isotonic Same solute
concentration
inside and outside
cell
No net movement None

28. 29
Cytolysis & Plasmolysis
Cytolysis Plasmolysis

29. 30
Osmosis in Red Blood Cells
Isotonic Hypotonic Hypertonic

30. 31
Three Forms of Transport Across the Membrane

31. 32
Passive Transport
Simple Diffusion
 Doesn’t require energy
 Moves high to low
concentration
 Example: Oxygen or
water diffusing into a
cell and carbon dioxide
diffusing out.

32. 33
Passive Transport
Facilitated diffusion
Doesn’t require energy
Uses transport
proteins to move high to
low concentration
Examples: Glucose or
amino acids moving from
blood into a cell.

33. Passive Transport
• Passive Transport – simple diffusion
across a membrane (does not require
a transport protein) – No energy
needed hence passive
• Examples – diffusion & Osmosis
• What substances can move across a
membrane without a transport
protein?

35. Types of Transport Proteins
• Channel proteins are embedded
in the cell membrane & have a
pore for materials to cross
• Carrier proteins can change
shape to move material from
one side of the membrane to
the other

36. Facilitated diffusion
During facilitated diffusion, substances pass
through a carrier protein following their
concentration gradients.
Facilitated diffusion does not require energy.
For example, the carrier protein for glucose
has two conformations and switches back and
forth between the two, carrying glucose across
the membrane.

37. Facilitated diffusion of glucose

38. 39
Facilitated Diffusion
Molecules will randomly move through
the pores in Channel Proteins.

39. 40
Facilitated Diffusion
• Some Carrier
proteins do not
extend through
the membrane.
• They bond and
drag molecules
through the lipid
bilayer and
release them on
the opposite side.

40. Facilitated Diffusion

41. The Movement of Substances
Across Cell Membranes (11)
• The Glucose Transporter: An Example
of Facilitated Diffusion
– The gradient for glucose entry into the
cell is maintained by phosphorylation of
glucose in the cytoplasm.
– Insulin stimulates glucose uptake by
causing the insertion into the cell
membrane of vesicles containing
preformed glucose transporters.

42. The Glucose Transporter

43. Facilitated Diffusion through a Channel
• Movement from high to low
inside cell
outside cell
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
Which way
will sugar
move?
low
high

sugar

44. 45
Carrier Proteins
• Other carrier
proteins
change shape
to move
materials
across the cell
membrane

45. Carrier protein

46. Carrier proteins involved in active transport
are called pumps.
The sodium-potassium pump is active in all
animal cells, and moves sodium ions to the
outside of the cell and potassium ions to the
inside.
The sodium-potassium pump carrier protein
exists in two conformations; one that moves
sodium to the inside, and the other that moves
potassium out of the cell.

47. Transport by Carrier Proteins
Some biologically useful molecules pass
through the plasma membrane through
channel proteins and carrier proteins that
span the membrane.
Carrier proteins are specific and combine with
only a certain type of molecule.
Facilitated diffusion and active transport
both require carrier proteins.

48. Movement of materials across a membrane
may be passive or active.
Passive transport does not use chemical
energy; diffusion and facilitated diffusion are
both passive.
Active transport requires chemical energy and
usually a carrier protein.

49. Active transport
During active transport, ions or molecules are
moved across the membrane against the
concentration gradient – from an area of lower
to higher concentration.
Energy in the form of ATP is required for the
carrier protein to combine with the
transported molecule and move it across the
membrane.

50. Active Transport
• Any process in which materials move
across the cell membrane WITH the
use of energy by the cell.
• Usually up the concentration gradient
(low concentration  high
concentration)
• like going up a hill

51. Passive Transport (diffusion)
Low Concentration
High Concentration
This is easy
– no energy
required!

52. Active Transport
Low Concentration
High Concentration
This is hard
– I have to
put in a lot
of energy!

53. Two Methods of Active
Transport:
• An example of Active Transport –
the cell membrane actually pinches
off to form a vesicle inside the cell
– Endocytosis – process of transporting
materials INTO a cell by means of a
pocket or pouch
– Exocytosis - process of transporting
materials OUT of a cell by means of a
pocket or pouch

54. Active transport

55. Active Transport (needs ATP energy)
“The Doorman”
shape change transports
high
low
• Membrane proteins act as a PUMP for specific
molecules
– shape change transports a substance from one side of
membrane to the other
– requires energy in the form of ATP
S9

56. Active transport (Low to High)
• Cells may need molecules to move
against concentration situation
protein pump
– requires energy
• ATP for NRG
Na+/K+ pump
in nerve cell
membranes
S10

57. 58
Active Transport
Requires energy or
ATP
Moves materials from
LOW to HIGH
concentration
AGAINST
concentration gradient

58. 59
Active transport
Examples: Pumping
Na+ (sodium ions)
out and K+
(potassium ions) in
against strong
concentration
gradients.
Called Na+-K+ Pump

59. 60
Sodium-Potassium Pump
3 Na+ pumped in for every 2 K+ pumped
out; creates a membrane potential

60. The sodium-potassium pump

65. Active transport
 Primary active transport
 Secondary active transport
 Endocytosis
 Pinocytosis
 Phagocytosis
 Exocytosis

66. Peculiarities of active transport
1) Carrier mediated transport
2) Rapid rate of transport
3) Transport takes place against electrochemical
gradient ( uphill )
4) Expenditure of energy by transport protein
which incorporates ATPase activity

67. 5) Carrier protein shows specificity, saturation
competitive inhibition, blocking
6) Substances transported – Na+ , K+, H+, Cl -, I - ,
Glucose, Amino acids

68. I. Primary active transport –
Examples - Na+ - K+ pump, Ca++ pump
H+-K+ pump
- Inner surface of carrier mol. has ATPase
which is activated by attachment of specific
ions and causes hydrolysis of ATP molecule
- Energy released from ATP causes
conformational change in the carrier which
transports ions to the opposite side.

69. a) Na+ -K + pump- electrogenic pump
- Attachment of 2K+ on outer side & 3 Na+ on inner side
Activation of ATPase
Conformational change
Efflux of 3 Na+ & influx of 2K+
Creates high K+ conc. & - vity inside the cell
Helps in maintaining cell volume
3Na+
2K+
ATPa
es

70. Na-K pump is one of the major energy using process in
the body & accounts for a large part of basal
metabolism.
Regulators of Na-K pump –
- Incraesed amount of cellular Na conc.
- Thyroid hormones increase pump activity by more # of
Na-K ATPase mol
- Aldosterone also increases # of pumps
- DOPamine inhibits pump
- Insulin increases pump activity
- Oubain or Digitalis inhibits ATPase (used when
weakness of cardiac muscle –maintains Ca conc. In ICF
of cardiac muscle

71. - Ca++ pump –
present in the membrane of ER, mitochondria
and cell membrane
- involves uniport carrier
- helps to maintain low Ca++conc. in ICF

72. II. Secondary active transport
Active transport depending upon conc.
gradient of Na+ from ECF to ICF created by
utilization of energy
_ carrier does not have ATPase activity
Substance is transported along with Na+
(Na increases affinity of carrier for gl.)
Na+ is transported only when glucose mol. is
attached

73. Examples – a) Reabsorption of glucose & amino acids in
PCT & Intestinal mucosa – Co-transport mechanism
b) H+ secretion by tubular epithelium –
counter transport mechanism
c)In heart Na-K ATPase indirectly affects Ca transport.
–antiport in the membrane of cardiac muscle
exchanges intracellular Ca for extracellular Na

74. Na+
K+
Glucose
lumen
basal
 Na + – K + pump on basal side
 Electrochemical gradient for Na + on luminal side
 Carrier mediated transport (SGLT-1)of Na+ along
with glucose ( or amino acid ) through the apical
membrane
 Transport of glucose by facilitated diffusion
( GLUT-2 ) through basal side

75. Transport summary

76. FACILITATED DIFFUSION
Movement from high to lower concentration
Use of carrier/channel protein
Proteins specific
Changes shape of protein and passes through
channel/membrane
No energy/ATP needed
ACTIVE TRANSPORT
Movement against concentration gradient
Use of carrier/intrinsic/pump proteins
Protein specific (to ion)
Energy/ATP required
Energy used to change shape of proteins/attach ion to
protein
Ions moved through membrane as proteins change
shape/position

77. Active transport Diffusion
May move
substances against
concentration
gradient;
Requires
ATP/energy;
Requires membrane
proteins/carriers
Substances moved
down concentration
gradient;
Does not require
ATP/energy;
Does not
(necessarily) require
membrane
proteins/carriers

78. Summary
The structure of the plasma membrane allows it to be
differentially permeable.
The fluid phospholipid bilayer, its mosaic of proteins,
and its glycocalyx make possible many unique
functions of the plasma membrane.
Passive and active methods of transport regulate
materials entering and exiting cells.

79. How about large molecules?
• Moving large molecules into & out of
cell
through vesicles & vacuoles
– Endocytosis (moving into cell)
• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
• receptor-mediated
endocytosis
– Exocytosis (moving out)
exocytosis

80. Endocytosis
Exocytosis

81. Extracellular material to be tackled by
lysosomes is brought into the cell by
endocytosis
3 types
pinocytosis
Receptor
mediated
endocytosis
phagocytosis
Specialised
cells
All cells
Requires ATPase, Ca, microfilaments

82. 83
Moving the “Big Stuff”
Molecules are moved out of the cell by vesicles that fuse
with the plasma membrane.
Exocytosis
- moving
things
out.
This is how many hormones are secreted and how nerve
cells communicate with one another.

84. 85
Exocytosis
Exocytic
vesicle
immediately
after fusion
with plasma
membrane.

85. 86
Moving the “Big Stuff”
Large molecules move materials into the cell by
one of three forms of endocytosis.

86. 87
Pinocytosis
Most common form of endocytosis.
Takes in dissolved molecules as a vesicle.

87. dynamin
Membrane deforming
coat protein
Endocytic
vesicle
ECF
Pinocytosis
ECF

88. 90
Pinocytosis
• Cell forms an
invagination
• Materials
dissolve in
water to be
brought into cell
• Called “Cell
Drinking”

89. 91
Example of Pinocytosis
pinocytic vesicles forming mature transport vesicle
Transport across a capillary cell (blue).

90. 92
Receptor-Mediated Endocytosis
Some integral proteins have receptors
on their surface to recognize & take in
hormones, cholesterol, etc.

91. 93
Exocytosis
The opposite of endocytosis is exocytosis. Large
molecules that are manufactured in the cell are
released through the cell membrane.
Inside Cell Cell environment

92. B. Receptor mediated endocytosis – highly
selective process to import imp. specific large
molecules. Requires energy & Ca++.
Coated pit
Clathrin, actin,
myosin
e.g. endocytosis of low density
lipoproteins
e.g. endocytosis of viruses such as
hepatitis, AIDS viruses & excess iron

93. 95
Receptor-Mediated Endocytosis

94. 96
Endocytosis – Phagocytosis
Used to engulf large particles such as
food, bacteria, etc. into vesicles
Called “Cell Eating”

95. C.
Phagocytosis
• Internalization of large
multimolecular particles, bacteria,
dead tissues by specialized cells
e.g. certain types of w.b.c.s (
Professional phagocytes)
• The material makes contact with
the cell membrane which then
invaginates.

96. Pseudopodia
internalization
Fusion
digestion
absorption
Residual
body
Phagoso-
some
Phagocytosis
bacterium

97. 99
Phagocytosis About to Occur

98. 100
Phagocytosis
- Capture
of a Yeast
Cell (yellow)
by
Membrane
Extensions
of an
Immune
System Cell
(blue)

99. .Diffusion can takes place through:
a) Lipid bilayer
i) Lipid soluble substances-
O2,CO2,alcohol, steriods etc
ii) Lipid insoluble – water (through
spaces bet lipid mol) urea, sugar
(less or no permeability)
iii) Electrolytes – impermeable
– charge on fatty acid chain
- Hydrated forms are larger

100. .
b) Protein Channels Open/leaky – Na+ channels,
K+ channels
Gated –channels open under specific conditions
Ligand gated
Na+
K+
Voltage gated
Na+,
K+
Ca++,
Mutation of ionic channels produce channelopathies –affecting
muscle and brain – paralysis or convulsions

101. Factors affecting rate of diffusion
• Lipid solubility
• Molecular size & wt.
• Temperature
• Thickness of membrane
• Surface area
• Concentration gradient
• Pressure gradient
• Electrical gradient
Molecular
Membrane related
Gradients

102. Passive transport Active
transport
• No expenditure of
energy molecules
• Takes place along
conc., electrical, &
pressure gradient
• Carrier may or may
not be required
• Rate is proportional
to conc. difference
• Expenditure of
energy mol. ( ATP )
• Can take place
against conc.
Gradient
• Carrier is always
required
• Rate is proportional
to availability of
carrier & energy.
(Vmax)

103. Simple Diffusion Facilitated
Diffusion
• Passive transport
• For small molecules
• No carrier required
• Rate of transport is
directly proportional
to conc. gradient
• Examples –
Lipid soluble –
O2, CO2, alcohol
Lipid insoluble –
urea, Na+, K+
• Passive transport
• For large molecules
• Carrier mediated
• Initially rate is
proportional to conc.
gradient till Vmax
( saturation of carriers)
• Examples –
glucose, amino acids