Transportacross cell membranes Flashcards
To have an understanding of: • The difference between passive and active transport of solutes • The difference between channel and carrier mediated transport • Differentiate between primary and secondary active transport • The properties driving water movement across cell membranes
What is Non-mediated transport?
does not directly use a transport
protein
What is Mediated transport?
moves materials with the help of a
transport protein
What is Passive transport?
moves substances down their
concentration or electrochemical gradients with only their kinetic energy
What is Active transport?
uses energy to drive substances against their concentration or electrochemical gradients
What is Vesicular transport?
move materials across membranes in small vesicles either by exocytosis or endocytosis – not discussed
Non-mediated transport is through the ……… layer
Non-mediated transport is through the lipid bilayer
Non-mediated transport is important for?
for absorption of nutrients – excretion of wastes
Is non-mediated transport nonpolar or polar?
Non polar
Does non-mediated transport involve Hydrophobic or Hydrophilic molecules?
Hydrophobic molecules
oxygen, carbon dioxide, nitrogen, fatty acids, steroids, small
alcohols, ammonia and fat-soluble vitamins (A, E, D and K)
Diffusion through ion channels
- Ions do not bind to channel pore. Therefore transport is very rapid
- The channel forms a water
filled pore that shields the ions
from the hydophobic core of the
lipid bilayer
Properties of channels: ionic selectivity
- Specific amino acids lining the pore determine the selectivity of the channel to ions
- By being selective to a particular ion the channel can
harness the energy stored in the different ion gradients
Properties of channels: gating
- Channels contain gates that control opening and closing
of the pore - Different stimuli can control (gate) channel opening
and closing - Stimuli include: voltage, ligand binding, cell volume
(stretch), pH, phosphorylation
How to measure ion channel function:
the patch clamp technique
Properties of ion channels: electrical current
- The diffusion of over 1 million ions per second through a channel generates a measurable current
(~10 -12 amp) - The current flowing through an individual channel can be recorded using the patch clamp technique
- Current fluctuations represent the opening and closing of single ion channels
- The current fluctuations represent the conformational changes in channel structure that are
associated with channel gating
Carrier mediated transport: mode of action
*The substrate to be transported directly interacts with the transporter protein
*Because the transporter undergoes a conformational change transport rates are slower than those obtained
for channels
Carrier mediated transport: properties
- These transport proteins have properties similar to
enzymes - They exhibit:
Specificity
Inhibition
Competition
Saturation (transport maximum) - Transport proteins do not catalyze chemical reactions
they mediate transport across the cell membrane at a
faster than normal rate - Mediated transport can be passive (facilitated) or
active
Transporter properties: saturation
*Transporters display enzyme kinetics
*Glucose transport occurs until all binding sites are
saturated
Facilitated diffusion of glucose
1) Glucose binds to transport
protein (GLUT)
2) Transport protein changes
shape. Glucose moves across cell
membrane (but only down
the concentration gradient)
3) Kinase enzyme reduces
glucose concentration inside
the cell by transforming
glucose into glucose-6-phosphate
* Conversion of glucose maintains concentration gradient
for glucose entry
What is Active transport?
An energy requiring process that moves molecules and
ions against their concentration or electrochemical gradients
What are the 2 types of Active transport?
Primary active transport:
- energy is directly derived from the hydrolysis of ATP
- A typical cell uses 30% of its energy (ATP) on primary active transport
Secondary active transport:
* energy stored in an ionic concentration gradient is used
to drive the active transport of a molecule against its gradient
Primary active transporters: Na/KATPase
. Na+ binding 2. ATP split/ Na+
pushed out
3. K + binding/
Phosphate release
4. K + is pushed in
* 3 Na+ ions removed from cell as 2 K + brought into cell.
*Therefore the pump generates a nett current and is
electrogenic.
Other examples: Ca/K ATPase (Muscle SR), H/K ATPase
(stomach)
Primary active transporters: Na pump
The Na pump maintains a low concentration of Na + and
a high concentration of K+ in the cytosol
This difference in ion concentrations is important for:
- Maintain resting membrane potential
- Electrical excitability
- Contraction of muscle
- Maintenance of steady state cell volume
- Uptake of nutrients via secondary active transporters
- Maintenance of intracellular pH by secondary active
transporters
Because Na and K are continually leaking back into the cell down their respective gradients the pump works continuously – known as the ……………p……
pump-leak hypothesis
