1' Principles of membrane transport

1
Chapter 5
A. The Movement of Substances Across Cell
Membranes
Learning Objectives
1. Principles of membrane transport 2. Passive
transport and active transport 3. Two main
classes of membrane transport proteins
Carriers and Channels 4. The ion transport
systems 5. Endocytosis and Phagocytosis
cellular uptake of macromolecules and
particles.
2
A motor neuron cell body in the spinal cord. (A)
Many thousands of nerve terminals synapse on the
cell body and dendrites. These deliver signals
from other parts of the organism to control the
firing of action potentials along the single axon
of this large cell. (B) Micrograph showing a
nerve cell body and its dendrites stained with a
fluorescent antibody that recognizes a
cytoskeletal protein (green). Thousands of axon
terminals (red) from other nerve cells (not
visible) make synapses on the cell body and
dendrites they are stained with a fluorescent
antibody that recognizes a protein in synaptic
vesicles.
3
1. Principles of membrane transport

• The plasma membrane is a selectively permeable
• barrier. It allows for separation and
  exchange of
• materials across the plasma membrane.

4
B. The protein-free lipid bilayers are highly
impermeable to ions.

• If uncharged solutes are small enough, they can
  move down their concentration gradients directly
  across the lipid bilayer by simple diffusion.
• Most solutes can cross the membrane only if
  there is a membrane transport protein to
  transfer them.
• Passive transport, in the same direction as a
  concentration gradient.
• Active transport, is mediated by carrier
  proteins, against a concentration gradient,
  require an input of energy.

Figure 11-1 The relative permeability of a
synthetic lipid bilayer to different classes of
molecules.    The smaller the molecule and, more
important, the fewer hydrogen bonds it makes with
water, the more rapidly the molecule diffuses
across the bilayer.
Diffusion of small molecules across phospholipid
bilayers
5

 Permeability coefficients (cm/sec) for the
passage of various molecules through synthetic
lipid bilayers.    
6
C. The energetics of solute movement

• Diffusion is the spontaneous movement of
  material from a region of high concentration to a
  region of low concentration.
• The free-energy change during diffusion of
  nonelectrolytes depends on the concentration
  grdient.
• The free-energy change during diffusion of
  electrolytes depends on the electrochemical
  grdient.

7
D. Transport processes within an eukaryotic cell
8
Simple diffusion
Osmosis
Facilitated transport
Transport
Sodium-Potassium Pump
Ca2 pump Proton Pump
Active transport
Symport
Cotransport
Aniport
Exocytosis and endocytosis
9
Passive transport

• Passive transport does not require energy
  expenditure, and occurs spontaneously
• Unaided movement through the phospholipid
  bilayer, in response to concentration gradients
• Large or charged molecules are unable to pass
  through the bilayer unassisted

10
Diffusion

• If a concentration gradient exists, there will
  be a net flow of material across the membrane

11
Diffusion

• Simple movement from regions of high
  concentration to low concentration,permeability
  depend on size of molecular and polarity
• Movement always is caused by internal thermal
  energy
• High concentration means high free energy low
  concentration means low free energy

12
Osmosis
Hypertonic
Hypotonic
13
Facilitated Diffusion

• Transport proteins
• Go along the concentration grade
• Require no energy

14

• TV is faster, Vmax exist,
• 1/2 Vmax Km
• Transport is specific
• Eg
• D-Glucose1.5mmol/L
• L-lucose30000mmol/L
• Membrane transport protein take part in
  transport

Kinetics of simple diffusion compared to
carrier-mediated diffusion.   
15
Transport Proteins are specific
16
A Carrier protein
Membrane Trans. Protein
B Channel protein
17
A Carrier protein
Carrier protein use for both passive and active
transport Material need bind with carrier
protein Specfic to material and conformation
change
18
(No Transcript)
19
B Channel protein

• Only for passive transport
• Material need not bind with channel protein
• Work by facilitated diffusion No Energy

20
(No Transcript)
21
Types of channel protein or ion channel
22
B Channel protein

• Work fast No conformation changes needed, No
  energy
• Specific to different ions (Na, K, Ca...)
• Gates control opening
• Drive power is concentration and electrochemical
  gradient

23

• Facilitate diffusion Protein-mediated movement,
• movement down the gradient

The carrier protein, the Glucose transporter
(GluT1 ) in the erythrocyte PM, alter
conformation to facilitate the transport of
glucose.
24
Brief summary of passive transport
25
Active transport

• Active transport requires that the cell use
  energy to move the molecules (or larger
  particles) through the cell membrane
• Transport from low concentration to high
  concentration
• Transporter protein take part in material transit

26
Active transport

• Proteins which transport against concentration
  gradient.
• Requires energy input

Why need cell active transport?
27
Charged particle cloaked with water molecules,
can not get through
Na Cl- K H
28
Large molecules, also cloaked with water
molecules, can not get through
Nucleotides
Amino acids
29

• Carrier proteins
• Go against the concentration
• Require Energy

30
Three ways of driving active transport.
31
Sodium-Potassium Pump (Na-K ATPase)
Ca2 pump Proton Pump
Active transport
symport
Cotransport
aniport
32
Sodium-Potassium Pump
33
Sodium-Potassium Pump

• Made of 2 large and 2 small subunits
• 2 large units span membrane
• -- inside region contains ATP binding site
• -- inside binding sites for Na
• -- outside binding site for K

• How does it work ?

34

• The Na-K ATPase
• ---A coupling active transport to
  ATP hydrolysis.

• The Na-K ATPase requires K outside, Na and
  ATP inside, and is inhibited by ouabain.
• The ratio of NaK pumped is 32 for each ATP
  hydrolyzed.
• The Na-K ATPase is a P-type pump.This ATPase
  seruentially phosphorylates and dephosphory-
  lates itself during the pumping cycle.
• The Na-K ATPase is found only in aniimals.

35

• The biological functions of Na/K pump

• The active transport of Na/K ATPase is used to
  maintains electrochemical ion gradients, and
  thereby maintains cells excitability.

• The Na/K pumo is required to maintain osmotic
  balance and stabilize cell volume

• forming a phosphorylated protein intermediate

36
Sodium-Potassium Pump
37
(No Transcript)
38
A Model Mechanism for the Na/K ATPase
39

• Other P-type pumps including H and Ca ATPases,
  
• and H/K ATPases

• Plant cells have a H-transporting plasma
  membrane pump .
• This proton pump plays a key role in the
  secondary transport of solutes, in the control of
  cytosolic pH, and possibly in control of cell
  growth by means of acidification of the plant
  cell wall.

• H/K ATPases (epithelial lining of the stomach)
  which secretes a solution of concentrated acid
  (up to 0.16N HCl) into the stomach chamber.

40
Question Compare with glucose transport
by facilitated transport and active transport
?
41
Ca2 pump

• Ca2 concentration
• exterior cell 10-3M
• interior cell 10-7M(free Ca2 )

42
Ca2 pump

• Ca2 pump Ca2-ATPase present in both the plasma
  membrane and
• the membranes of the ER. It contain 10
  transmembrane ? helices.
• This Ca2 pump functions to actively transport
  Ca2 out of the cytosol into either the
  extracellular space or the lumen of the ER.

43

• The difference between animal and plant cells
  to absorb nutrients

44
proton Pump

• p- proton Pump
• v -proton Pump
• H- ATP Pump

45

• The V-type pump utilize the energy of ATP
  without
• forming a phosphorylated protein intermediate.

• Vacuolar(V-type) pump actively transport H
  across the membranes of cytoplasmic organelles
  and vacuoles.

• They precent in lysosomes and plant cell
  vacuoles, have also been found in the plasma
  membranes of a variety of cells (kidney tubules).

46
Symport
Cotransport
Aniport
Sodium-Potassium Pump Carrier protein ATP
cooperation
47

• Indirect active transport is driven by Ion
  gradients ----- Cotransport

Gradients created by active ion pumping store
energy that can be coupled to other transport
processes.
A. Sugars, amino acids, and other organic
molecules into cells

• The inward transport of such molecules up their
  concentration gradients is often coupled to, and
  driven by, the concomitant inward movement of
  these ions down their electrochemical gradients
• Animal cells-----Sodium ions (Na/K ATPase)
• Plant, fungi, bacterium-----Protons(H ATPase)

48

• Cotransport Symport and antiport

Na-linked symporters import amino acids and
glucose into many animal cells
Na-linked antiporter exports Ca from cardiac
muscle cells
Medicine
Ouabain and digoxin increase the force of heart
muscle contraction by inhibiting the Na/K
ATPase. Fewer Ca ions are exported
49
(No Transcript)
50
A. Comparison of two classes of transport.
51
B. Two classes of membrane transport proteins

• Carrier proteins are responsible for both the
  passive and the active transport.
• Channel proteins are only responsible for passive
  transport.

52
Transport of Macromolecules

• Endocytosis (transport into cell)
• Exocytosis (transport out cell)
• Fusion model of transport vesicle and target
  membrane

53
Endocytosis

• Transports macromolecules and large particles
  into the cell.
• Part of the membrane engulfs the particle and
  folds inward to bud off.

active transport /bulk transport
54
Endocytosis

• A Pinocytosis
• Continuous progress
• Liquid material
• Diameter 150nm
• B Phagocytosis
• Trigger progress
• Large particles
• Diameter 250 nm
• vesicle forming mechanism is different

55
Receptor mediated endocytosis (clathrin coated
pit)
Endocytosis

• No-special endocytosis
• (cop-coated vesicle,
• regulate between golgi and ER)

56
No specific endocytosis
Receptor mediated endocytosis
57
Pinocytosis Endocytic vesicle forming mechanism

58
most Come back to original plasm position
Enter into lysosome (receptor down regulation)
Receptor fate
transcytosis transport into different plasm
region
59
Endocytosis
60
B Phagocytosis
Need microfilament and bind protein Special
cells have this function
61
Exocytosis
62
Exocytosis

• A Constitutive exocytosis
• Scope exsist in all type cell
• Mechanismdefault pathway
• B Regulated exocytosis pathway
• Scopeexist in some special cell

63
(No Transcript)
64
Fusion model of transport vesicle and target
membrane
Dynamin