Membrane Transport

Solute Movement across Lipid Bilayers

• Materials can move across the cell membrane in different ways.– Passive transport does not require an input of

energy.– Active transport requires energy to move

substances across the membrane.

• Small molecules and ions in solution are called solutes, have thermal energy, and are in constant, random motion.– This random movement is called diffusion.

• Diffusion is a form of passive transport.

Diffusion along a Concentration Gradient

• A difference in solute concentrations creates a concentration gradient.

Molecules and ions move randomly when a concentration gradient exists, but there is a net movement from high- concentration regions to low-concentration regions. Diffusion along a concentration gradient increases entropy and is thus spontaneous.

• Equilibrium is established once the molecules or ions are randomly distributed throughout a solution.– Molecules are still moving randomly but there

is no more net movement.

Diffusion

• The molecules and ions which are called solutes have thermal energy and are in constant motion.

• In motion they move from a higher concentration to one that is lower.

• The direct movement of molecules and ions (solutes) is called diffusion.

• Diffusion is an example of passive transport.

Osmosis: Passive or Active?• Water moves quickly across lipid

bilayers.– The movement of water is a special case of

diffusion called osmosis.

• Water moves from regions of low solute concentration to regions of high solute concentration.– This movement dilutes the higher

concentration, thus equalizing the concentration on both sides of the bilayer.

• Osmosis only occurs across a selectively permeable membrane.

Osmosis and Relative Solute Concentration

• The concentration of a solution outside a cell may differ from the concentration inside the cell.

– An outside solution with a higher concentration is said to be hypertonic to the inside of a cell.

– A solution with a lower concentration is hypotonic to the cell.

– If solute concentrations are equal on the outside and inside of a cell, solutions are isotonic to each other.

Osmosis in Hypertonic, Hypotonic, and Isotonic Solutions

• In a hypertonic solution, water will move out of the cell by osmosis and the cell will shrink.

• In a hypotonic solution, water will move into the cell by osmosis and the cell will swell.

• In an isotonic solution, there will be no net water movement and the cell size will remain the same.

Let’s practice

Hyper-, Hypo-, or Isotonic?(in your notes)

• Use arrows to show the direction of water movement into or out of each cell. Color and label the cell in an isotonic environment light blue, the hypotonic environment yellow, and the hypertonic environment light green.

Label the Plant Cell

• What type of solution are they in?

Label the Red Blood CellsWhy should you worry about this?

• What type of solution are they in?

Membrane Proteins Affect Ions and Molecules

• The transmembrane proteins that transport molecules are called transport proteins. There are three broad classes of transport proteins, each of which affects membrane permeability:

1. Channels2. Carrier proteins or transporters3. Pumps

Ion Channels and the Electrochemical Gradient

• Ion channels are specialized membrane proteins.– Ion channels circumvent the plasma membrane’s

impermeability to small, charged compounds.

• When ions build up on one side of a plasma membrane, they establish both a concentration gradient and a charge gradient, collectively called the electrochemical gradient.

• Ions diffuse through channels down their electrochemical gradients. This passive transport decreases the charge and concentration differences between the cell’s exterior and interior.

Facilitated Diffusion via Channel Proteins:(Passive transport)

• Cells have many different types of channel proteins in their membranes, each featuring a structure that allows it to admit a particular type of ion or small molecule.

• These channels are responsible for facilitated diffusion: the passive transport of substances that would not otherwise cross the membrane.

Facilitated Diffusion via Carrier Proteins

• Facilitated diffusion can occur through channels or through carrier proteins, or transporters, which change shape during the transport process.

• Facilitated diffusion by transporters occurs only down a concentration gradient, reducing differences between solutions.

• Glucose is a building block for important macromolecules and a major energy source, but lipid bilayers are only moderately permeable to glucose.– A glucose transporter named GLUT-1 increases membrane

permeability to glucose.

Active Transport by Pumps

• Cells can transport molecules or ions against an electrochemical gradient.

• This process requires energy in the form of ATP and is called active transport.

• Pumps are membrane proteins that provide active transport of molecules across the membrane.

– For example, the sodium-potassium pump, Na+/K+-ATPase, uses ATP to transport Na+ and K+ against their concentration gradients.

http://www.youtube.com/watch?v=IzuKhespz20&feature=related

Secondary Active Transport

• In addition to moving materials against their concentration gradients, pumps set up electrochemical gradients.

• These gradients make it possible for cells to engage in secondary active transport, or cotransport.– The gradient provides the potential energy

required to power the movement of a different molecule against its particular gradient.

Endocytosis and Exocytosis(Active transport)

• http://www.youtube.com/watch?v=1w10R9lv7eQ

a. phagocytosis?b. pinocytosis?

Summary of Membrane Transport

• There are three mechanisms of membrane transport: 1. 2. 3. • Diffusion and facilitated diffusion are forms of

________transport and thus move materials down their concentration gradient and ________require an input of energy.

• ______transport moves materials against their concentration gradient and _______energy provided by _____or an electrochemical gradient.

Summary

1. Intraveneous solutions must be prepared so they are isotonic to red blood cells. A 0.9% salt solution is isotonic to red blood cells.

a. Explain what would happen if you placed a red blood cell in a solution of 99.3% water and 0.7% salt.

b. Explain what would happen if you placed a red blood cell into a solution of 90% water and 10% salt.

2. What would happen to a cell if placed in the following solutions? Explain in detail and illustrate.

a. Hypotonicb. Isotonicc. Hypertonic