Membrane Transport

1
Chapter 12

• Membrane Transport

2
Defintions

• Solution mixture of dissolved molecules in a
  liquid
• Solute the substance that is dissolved
• Solvent the liquid

3
Membrane Transport Proteins

• Many molecules must move back and forth from
  inside and outside of the cell
• Most cannot pass through without the assistance
  of proteins in the membrane bilayer
• Each cell has membrane has a specific set of
  proteins depending on the cell

4
Movement of Small Molecules
5
2 Major Classes

• Carrier proteins move the solute across the
  membrane by binding it on one side and
  transporting it to the other side
• Requires a conformation change
• Channel protein small hydrophilic pores that
  allow for solutes to pass through
• Use diffusion to move across
• Also called ion channels

6
Proteins
7
Ion Concentrations

• The maintenance of solutes on both sides of the
  membrane is critical to the cell
• Helps to keep the cell from rupturing
• Concentration of ions on either side varies
  widely
• Na and Cl- are higher outside the cell
• K is higher inside the cell
• Must balance the the number of positive and
  negative charges as well

8
Carrier Proteins

• Required for almost all small organic molecules
• Exception fat-soluble molecules and small
  uncharged molecules that can pass by simple
  diffusion
• Usually only carry one type of molecule
• Carriers can also be in other membranes of the
  cell such as the mitochondria

9
Carriers in the Cell
10
3-D of Carrier Protein
11
Carrier vs Channel

• Channels, if open, will let solutes pass if they
  have the right size and charge
• Trapdoor-like
• Carriers require that the solute fit in the
  binding site
• Turnstile-like
• Why carriers are specific like an enzyme and its
  substrate

12
Mechanisms of Transport

• Provided that there is a pathway, molecules move
  from a higher to lower concentration
• Doesnt require energy
• Passive transport of facilitated diffusion
• Movement against a concentration gradient
  requires energy
• Active transport
• Requires the harnessing of some energy source by
  the carrier protein

13
Passive vs Active Transport
14
Glucose Carrier in Liver Cells

• Glucose carrier crosses the membrane and has at
  least 2 conformations
• One conformation exposes the binding site on the
  outside of the cell and the other on the inside
  of the cell

15
How it Works (Passive)

• Glucose is high outside the cell so the
  conformation is open to take in glucose and move
  it to the cytosol where the concentration is low
• When glucose levels are low in the blood,
  glucagon (hormone) triggers the breakdown of
  glycogen, glucose levels are high in the cell and
  then the conformation moves the glucose out of
  the cell to the blood stream
• Glucose moves according to the concentration
  gradient across the membrane

16
Electrochemical Gradient

• Due to concentration gradient and the voltage
  across the membrane
• This gradient determines the direction of the
  solute during passive transport

17
Active Transport

• 3 main methods to move solutes against a gradient
• Coupled transporters
• ATP-driven pumps
• Light-driven pumps

18
Transporters are Linked

• The active transport proteins are linked together
  so that you can establish the electrochemical
  gradient
• Example
• ATP-driven pump removes Na to the outside of the
  cell (against the gradient) and then re-enters
  the cell through the Na-coupled transporter
  which can bring in many other solutes

19
Na-K ATPase (Na-K Pump)

• Requires ATP hydrolysis to maintain the Na-K
  equilibrium in the cell
• Transporter is also a ATPase (enzyme)
• This pump keeps the Na 10 to 30 times lower
  than extracellular levels and the K 10 to 30
  times higher than extracellular levels

20
Na-K Pump
21
Na and K Concentrations

• The Na outside the cell stores a large amount
  of energy, like water behind a dam
• Even if the Na-K pump is halted, there is
  enough stored energy to conduct other Na
  downhill reactions
• The K inside the cell does not have the same
  potential energy
• Electric force pulling K into the cell is almost
  the same as that pushing it out of the cell

22
Na-K Pump is a Cycle
23
Coupled Transport

• The energy in the Na-K pump can be used to move
  a second solute
• Couple the movement of 2 molecules in several
  ways
• Symport move both in the same direction
• Antiport move in opposite direction
• Carrier proteins that only carry one molecule is
  called uniport (not coupled)

24
Coupled Transporters
25
Na-Driven Symport

• If one molecule of the transport pair is missing,
  the transport of the second does not occur

26
2 Methods of Glucose Transport
27
Na-Driven Antiport

• Also very important in cells
• Na-H exchanger is used to move Na into the
  cell and then moves the H out of the cell
• Regulates the pH of the cytosol

28
Osmosis

• The movement of water from region of low solute
  concentration (high water concentration) to an
  area of high solute concentration (low water
  concentration)
• Driving force is the osmotic pressure caused by
  the difference in water pressure

29
Osmotic Solutions

• Isotonic equal solute on each side of the
  membrane
• Hypotonic less solute outside cell, water
  rushes into cell and cell bursts
• Hypertonic more solute outside cell, water
  rushes out of cell and cell shrivels

30
Osmotic Swelling

• Animal cells maintain normal cell structure with
  Na-K pump
• Plants have cell walls turgor pressure is the
  effect of osmosis and active transport of ions
  into the cell keeps leaves and stems upright
• Protozoans have special water collecting vacuoles
  to remove excess water

31
Calcium Pumps

• Calcium is kept at low concentration in the cell
  by ATP-driven calcium pump similar to Na-K pump
  with the exception that it does not transport a
  second solute
• Tightly regulated as it can influence many other
  molecules in the cytoplasm
• Influx of calcium is usually the trigger of cell
  signaling

32
H Gradients

• Drive the movement of molecule across the
  membranes of plants, fungi and bacteria
• Similar to animal Na-K pump

33
H Pumps
34
(No Transcript)
35
Channel Proteins

• Channel proteins create a hydrophilic opening in
  which small water-soluble molecules can pass into
  or out of the cell
• Gap junctions and porins make very large openings
• Ion channels are very specific with regards to
  pore size and the charge on the molecule to be
  moved
• Move mainly Na, K, Cl and Ca

36
Ion Channels

• Have ion selectivity allows some ions to pass
  and restricts others
• Based on pore size and the charges on the inner
  wall of the channel
• Ion channels are not always open
• Have the ability to regulate the movement of ions
  so that control can be maintained on the ion
  concentrations within the cell
• Channels are gated open or closed

37
Ion Channels
38
Membrane Potential

• Basis of all electrical activity in cells
• Active transport can keep ion concentration far
  from equilibrium in the cell
• Channels open and the ions rush in because of the
  gradient difference
• Allows for the electrical activity to move across
  the membrane

39
Patch Clamp

• Technique used to determine the electrical
  current in a cell
• Can measure the change in voltage that occurs
  when the channels are open or closed

40
3 Types of Channels

• Voltage-gated channels controlled by membrane
  potential
• Ligand-gated channels controlled by binding of
  a ligand to a membrane protein
• Stress activated channel controlled by
  mechanical force on the cell

41
Auditory Hair Cells

• Sound waves cause the stereocilia to tilt and
  this causes the channels to open and transport
  signal to the brain

42
(No Transcript)