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

Ion concentrations in the cytosol and the vacuole that controlled by passive (dashed arrow) and active (solid arrows) tyransport processes.

• 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

Diffusion potential and membrane potential

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).

C = concentration (mol/l)z = number of charge

F = 96400 J/Vmol = charge

R = 8.31 J/mol K, T = temperature (K)

Nernst equation :

= (RTlnC) + (zF)

< 0 , = 0 passive transport > 0, active transport

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 electrochemical-potential gradients

Examples of secondary active transport with a primary proton gradient

Hypothetical model for secondary active transport

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 plasmamembrane and tonoplast of plant cells

SODIUM-POTASSIUM PUMP• Helps maintain the electrochemical

gradient 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.

http://www.usm.maine.edu/~rhodes/Biochem/Images/Fig10-21imp.jpg

The Sodium-Potassium (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.

Mineral absorptionDirect process: K+ is present in the clay and soil

that surround the root. They can be actively taken up by

the active transport membrane pumps.

Through active transport the mineral ions pass through and enter the cell.

Indirect process: Proton pumps within the plasma

membrane pump out H+ ions into the soil.

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 sieveelement loading

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_phloem_loading.html