MEMBRANE TRANSPORT MECHANISMS

(Chapter 13, pp 132-137)

OVERVIEW• Ions and molecules must enter and exit

cells.

• Plasma membrane protects and isolates the cytoplasm from the environment and forms a barrier to the entry of molecules into the cell.

• This barrier is selectively permeable, allowing important molecules to enter and exit cells, while excluding (stop) other molecules.

• Proteins buried in plasma membrane like ion channels, transporters and pumps help in transport of ions and other molecules into the cell.

Diffusion

• Diffusion is random movement of molecules in a solution, where molecules spread out until they are equally distributed within the space they occupy.

• The process can continue to occur at the same rate as long as the concentration gradient (concentration difference) exists.

• Movement of a substance diffusing across the barrier depends on concentration gradientand then the permeability of the substance across the barrier.

Diffusion-Considerations• Permeability in a membrane is an important

consideration if a molecule is to cross it by diffusion.

• Phospholipids of the plasma membrane are amphipathic in nature.

• Hydrophobic molecules such as steroid hormones may be able to dissolve in the interior hydrophobic core of the plasma membrane.

• But the hydrophilic head groups of the phospholipids present an obstacle at the interface with the environment and with the cytosol.

• Movement of molecules from extracellular side into cytoplasm occur through membrane proteins that form channels.

• Some ions and small molecules may gain access to cytoplasm of cells through water pores.• Larger molecules require specific membrane transport proteins.

Osmosis• Osmosis is the transfer of liquid solvent

through a semipermeable membrane that does not permit dissolved solutes to pass.

• Water is the most important physiological solvent.

• By osmosis water passes through a plasma membrane from an area of high concentration of water to an area of lower concentration of water.

• Protein channels known as aquaporins enable water to pass through the hydrophobic core of the phospholipids of the membrane.

Osmosis- Net Movement of Water• Water constantly enters and exits our cells

by osmosis.• Rates of water entry and exit are usually

equal.

• Different solute concentrations result in different concentrations of free water.

• Solution with a high concentration of particles or solutes there is less free water than in a solution with low solute concentration. The result will be more water leaving the area of high free water concentration.

• The size or molecular weight of the solutesin water does not influence the net movement of water.

Osmotic Pressure

• Differences in solute concentration on opposing sides of a barrier such as plasma membrane generate an osmotic pressure.

• Osmotic pressure is defined as the measure of the tendency of a solution to take in water by osmosis.

• It is often used to express the concentration of the solution.

Osmosis- Cell Volume• When the solute concentration is same both inside and outside the cell, the

external solution surrounding the cell is said to be isotonic (same).• Cells maintain their volume in isotonic solutions.• A solution with less solute concentration than the cytosol is hypotonic.• Cell volume increases in hypotonic solutions because free water is at higher

concentrations outside the cell.• Cells placed in a solution with a higher solute concentration than their

cytosol, the solution is hypertonic.• Water will exit the cells and they will shrink.

RED BLOOD CELLS

Passive Transport

• Passive transport is used where molecules are moved across plasma membrane along their concentration gradients.

• The molecule flows from where it is at higher concentration to where it is at a low concentration.

• This process is also called catalyzed transport and sometimes referred to as facilitated diffusion.

• No direct source of energy is required to power the process.

Transport through Ion Channels

• Ions gain entry into cells through ion channels, which are transmembrane proteins within the plasma membrane that provide a route for an ion to pass through the membrane.

• Ion channels are selective, allowing only ions of certain size and charge to enter.

• For example, calcium channels are specific for calcium and sodium channels for sodium.

• Ions move through an ion channel from their region of higher concentration to a region of low concentration.

• Ion channels maybe open or closed.

• The opening and closing of gated channels is regulated by physical or chemical stimuli.

Transport through Transporters

• Transporters are transmembrane proteins bringing about movement of molecules in or out of cells.

• Uniport is the term used to describe transport of one molecule by a transporter.

• Passive transport via transporters requires a concentration gradient of the solute being transported.

• Molecules bind to their specific transport protein with a certain affinity.

• Maximum speed of transport will be achieved when all available transporter proteins are bound by their specific solute.

SUMMARY• Selective permeability of plasma membrane allows certain materials only to enter and

exit cells.• Diffusion of particles is powered by movement of particles within a solution and results

in a distribution of particles that move from their area of higher concentration to an area of lower concentration.

• The plasma membrane is always a barrier to access the cytoplasm and entry into cells require membrane proteins.

• Water enters and exits cells through osmosis and requires water channel proteins.• Ion channel facilitate movement specific ions into or out of cells along the

concentration gradient of ions.• Transport proteins catalyze the movement of specific solutes across plasma

membranes by passive transport .• Channels and transport proteins bind their solute with a certain affinity and catalyze the

movement of their solute across the membrane in a saturable manner.