Passive Transport - nausetschools.org · Passive Transport •Passive transport is the net movement...
Transcript of Passive Transport - nausetschools.org · Passive Transport •Passive transport is the net movement...
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?