Solute Transport
Cell Membrane
Passive transport
Diffusion Across a Plasma Membrane
• Plasma membrane is semi-permeable
• Gases like O2, N2, diffuse easily through membrane
because they have no charge (partial or complete) to
interact with water
• Water, while polar, is small enough to freely move
across the plasma membrane
• Larger hydrophilic uncharged molecules; indirect
proportional to size
• Hydrophobic molecules (oils); direct proportional to
dilution
• Charged molecules cannot diffuse through lipid bi-layer
 ion channels and specific transporters are required
for charged molecules and larger, uncharged molecules
Diffusion of Hydrophilic Molecules Across a
Plasma Membrane
• Plasma membrane is semi-permeable
• Water, while polar, is small enough to freely move
across the plasma membrane
• Larger hydrophilic uncharged molecules, such as
sugars, do not freely diffuse
• Charged molecules cannot diffuse through lipid bilayer
• Ion channels and specific transporters are required for
charged molecules and larger, uncharged molecules
Active transport of solute across
membrane
Classification of membrane transport
process
The Nernst Equation
Diffusion potential and membrane potential
• K+ is accumulated passively by
both the cytosol and the vacuole,
except when extracellular K+
concentrations are very low
• Na+ and Ca2+ is pumped actively
out of the cytosol
• Excess H+ are actively extruded
from the cytosol
• All the anions are taken up
actively into the cytosol
Ion concentrations in the cytosol and the vacuole that controlled by passive
(dashed arrow) and active (solid arrows) tyransport processes.
ENERGY
• Non ionic solut : chemical gradient
• Ionic solut : electropotensial gradient (ion attraction
or repulsion)
Cells use energy

to pump proton, Na+, Ca+ out into the cell wall, loss of cation

cytosol become slightly negatively charged

cells attract cation, repel anion : electrochemical gradient
electrochemical gradient :
• a gradient composed of a chemical gradient (the
difference in H+ / pH) and an electrical gradient (the
difference in charge).
Nernst equation :
 = (RTlnC) + (zF)
 < 0 ,  = 0 passive transport
 > 0, active transport
C = concentration (mol/l)
z = number of charge
F = 96400 J/Vmol
 = charge
R = 8.31 J/mol K, T = temperature (K)
Membrane transport processes
Primary Active Transport Is Directly Coupled to Metabolic or Light Energy
Proton (H+) – ATPase :
 the most energy wasteful :
- it causes the pH of the cytosol to increase
- it causes the pH of the cell wall to decrease
- it causes the cytosol to become electronegative relative to the cell
wall as the cytosol loses H+ but retains OH-
Secondary active transport
Secondary active transport uses the energy stored in electrochemicalpotential gradients
Hypothetical model for secondary active transport
Examples of secondary active transport
with a primary proton gradient
Primary and secondary transports across the plasma membrane. The electrochemical
gradient created by H+-ATPase is used by secondary transporters (channels and carriers)
to move ions and organic compounds across the plasma membrane. Water transport
through aquaporins may not respond directly to the proton electrochemical gradient, but to
the osmotic potential and, thus, the solute movement.
Overview of the various transport processes on the plasma
membrane and tonoplast of plant cells
SODIUM-POTASSIUM
• Helps maintain the electrochemical
PUMPgradient in the cell.
• keeping a higher concentration of
potassium (K+) inside the cell than out
side, while maintaining a higher
concentration of sodium (Na+) outside
than in
• allows absorptive cells to transport
nutrients into the the cell via secondary
active transport
• For example, glucose is co-transported
(aka symported) with sodium into the
cell, this process actually uses no
energy, even though glucose is
transported against its concentration
gradient, because sodium flows down its
concentration gradient allow the glucose
symporter to function.
• Without the sodium-potassium pump the
transport of glucose would eventually
cease.
The SodiumPotassium (Na+/K+)
Pump
the ion transporter Na+/K+-ATPase
pumps sodium cations from the
inside to the outside, and potassium
cations from the outside to the inside
of the cell.
http://www.usm.maine.edu/~rhodes/Biochem/Images/Fig10-21imp.jpg
Mineral absorption
Direct process:
Indirect process:
 K+ is present in the clay and soil
that surround the root.
 Proton pumps within the plasma
membrane pump out H+ ions into the
soil.
 They can be actively taken up by
the active transport membrane
pumps.
 Through active transport the
mineral ions pass through and
enter the cell.
 These H+ ions combine with anions ( Cl-)
that allow the uptake of the ions against
the electrochemical gradient.
 H+ also displaces K+ from the clay
particles in the soil, which allows them to
travel through the electrochemical
gradient through facilitated diffusion.
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_mineral_uptake.html
Translocation in phloem
The phloem is the tissue that translocates the products of photosynthesis from
mature leaves to areas of growth and storage, including the roots.
Schematic diagram of pathways of phloem loading in
source leaves
ATP-dependent sucrose transport in sieve
element loading
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_phloem_loading.html