Transcript Cell Structure and Function

Molecular Movement
Across Membranes
What you need to know!
• How water will move if a cell is placed in
an isotonic, hypertonic, or hypotonic
solution
• How electrochemical gradients are formed.
Diffusion
• The movement of molecules from an area
of high concentration to an area of low
concentration
• Resulting in even distribution when
equilibrium is reached
• Reason: random movement of all
molecules driven by kinetic energy
(temperature/pressure)
• Cells do not invest energy (no ATP)
Diffusion in and out of cells
• Plasma membrane is selectively
permeable (semi-permeable)
• Permeable for very small, uncharged,
hydrophobic molecules and gases
• Impermeable for ions and large molecules
• Use of transport proteins/pores embedded
in the membrane
• No energy needed due to concentration
gradient
Examples of Diffusion
• O2/CO2 exchange between blood and
cells
• Sugar molecules/pores
• Small waste molecules-urea
• Amino acids/pores
• Lipids
Osmosis
• Definition: diffusion of water molecules
across a semi-permeable membrane
• All water movements in cells or organisms
through osmosis
• Water molecules use aquaporin proteins
• Water will always go in the direction of
lower water concentration (high solute
concentration) or lower water potential
Osmosis in Cells
• 2 parameters:
1. Solute concentration (salt, sugar, other
substances…)
2. Water concentration:
– High solute (salt, sugar, etc.) concentration
usually means low water concentration
– Low solute (salt, sugar, etc.) concentration
usually means higher water concentration
– Distilled water is solute free (100% water)
How to answer Osmosis
Questions
• Compare the inside water concentration
with the outside water concentration:
• Isotonic: inside and outside same
concentration of solute (and water)
• Hypertonic: solution with higher solute
concentration (lower water)
• Hypotonic: solution with lower solute
concentration (higher water)
Living Cells in solutions
a) Low salt water (same salt concentration
inside and outside the cell = isotonic) no
change in shape
b) Strong salt water (more salt outside the
cell = hypertonic; inside = hypotonic) cell
shrivles
c) Distilled water (no salt outside =
hypotonic; inside = hypertonic) cell
explodes