Passive TransportTopics 2.6 through 2.9

Passive Transport

• Passive transport is the net movement of molecules from high concentration to low concentration without the direct input of metabolic energy (ATP)

• Plays a primary role in the import of materials and the export of wastes

1. Simple Diffusion

2. Facilitated Diffusion

3. Osmosis

Simple Versus Facilitated Diffusion

Simple Diffusion

• Small, nonpolar molecules, such as N2, O2 and CO2 freely pass across the membrane

• Polar uncharged molecules, including water, pass through the membrane in small amounts

Facilitated Diffusion

• Membrane proteins are required for facilitated diffusion of charged and large polar molecules through a membrane

– Hydrophilic (polar) molecules such as amino acids and sugars

– Charged ions, including Na+ and K +, require channel proteins to move through the membrane

Proteins

• Transport proteins allow the passage of substances across the membrane that do not pass freely through the phospholipids

• There are many different types of transport protein, including channel and carrier proteins (highly specific)

Channel Proteins

• Two common groups of channel proteins included:

– Ligand-gated ion channels

–Voltage-gated ion channels

Ligand-gated Ion Channels

• Open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in response to the binding of a chemical messenger (ligand)

Voltage-gated Ion Channels

• A class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel protein

• Most commonly found in neurons

Osmosis• Water moves by osmosis from areas of high

water potential/low osmolarity/low solute concentration to areas of low water potential/high osmolarity/high solute concentration Diffusion of water across a selectively permeable membrane

• Large quantities of water pass through specialized channel proteins called aquaporins(hydrophilic channel)

Aquaporin

• 3 billion water molecules per second

Osmosis and Tonicity

• External environments can be hypotonic, hypertonic or isotonic to the internal environments of cells

Osmosis Terms

• Osmolarity: the concentration of a solution expressed as the total number of solute particles per liter

• Water potential: a measure of the potential energy in water as well as the difference between the potential in a given water sample and pure water

• Osmoregulation: is the active regulation of the osmotic pressure of an organism's body fluid

Osmolarity

Isotonic

• Concentration of dissolved solutes in the environment is equal to the cell

• Cell will neither gain nor lose water

Hypotonic

• Concentration of dissolved solutes in the environment is less than the cell (higher osmolarity and low water potential in the cell)

• Cells will gain water, swell, and possibly lyse(burst)

Hypertonic

• Concentration of dissolved solutes in the environment is more than the cell (lower osmolarity and higher water potential in the cell)

• Cell will lose water and shrivel

Plant Cell Osmosis

• Water moves into the cell creating turgorpressure in hypotonic environments

Plasmolysis Flaccid(Hypertonic) (Isotonic)

Osmoregulation

• Organisms must live in isotonic environments or have adaptations for osmoregulation

• Osmoregulation maintains water balance and allows organisms to control their internal solute composition/water potential

Paramecia have specialized organelles

called contractile vacuoles which

regulate water balance

Osmoregulation in Fish

Osmoregulation in Mammals

• Kidneys regulate the osmotic pressure of a mammal's blood through extensive filtration and purification

Water Potential

Ψ = Ψp + Ψs

Water Potential

• Used to describe the tendency of water to leave one place in favor of another

• Water always moves from an area of higher water potential to an area of lower water potential

Water Potential

• Affected by two factors: pressure and the amount of solute

• Measured in bars

1 bar = approximately 1 atmosphere

(unit of pressure)

• Water potential of pure water is 0 bars

Who cares about water potential?

• All living organisms, but let’s use plants as an example!

• Plants use differences in water potential to transport water to the leaves for photosynthesis

• Internal water potential of a plant cell is more negative than pure water – this causes water to move from the soil to the plant roots via osmosis

Water Potential and Transpiration

Plants

• Lose water (and turgor pressure) via transpiration through stomata in the leaves

• If the water potential outside the plant cells is lower than inside the cells, what happens to the plant?

Water Potential Formula

Ψ = Ψp + Ψs

Ψ = Water potential

Ψp = Pressure potential

Ψs = Solute potential

Solute and Pressure Potential

• Ψs = solute potential: solute potential is negative because solutes lower the water potential

• Ψp = pressure potential: physical pressure increases water potential

Osmolarity

• As solute concentration increases, osmolarityalso increases

Sample Problem 1

• If a plant cell’s Ψp = 2 bars and its Ψs = - 3.5 bars, what is the resulting Ψ?

Calculate Solute Potential (Ψs)

• Sometimes solute potential must be calculated first

• Solute potential becomes more negative as more solute is added

• Ψs = - iCRT i = ionization constantC = concentration (Molarity)

R = pressure constant

T = temperature in Kelvin

Sample Problem 2

• What is the solute potential of a 1.0 M sugar solution at 22 °C under standard atmospheric conditions?

Ψs = - iCRT

Sample Problem 3

• Zucchini cores are measured and determined to have a sucrose concentration of 0.36M. Calculate the solute potential using the same temperature and atmospheric conditions as the previous question.

Ψs = - iCRT

Sample Problem 4

• The zucchini core from Problem 3 is placed in a beaker of pure water. Will water diffuse into or out of the plant cell?