osmosis 5.5 Flashcards
what is osmosis
Osmosis is a particular type of diffusion - specifically the diffusion of water across partially permeable membrane.
As with all types of diffusion it is a passive process and energy is not required
what is a solute
A solute is a substance dissolved in a solvent (for example water) forming a solution
what is water potential
Water potential is the pressure exerted by water molecules as they collide with a membrane or container.
It is measured in units of pressure pascals (Pa) or kilopascals (kPa).
The symbol for water potential is the greek letter psi Ψ
what is the water potential of pure water
0 kPa (at standard temperature and atmosphere pressure - 25°C and 100 kPa.
This is the highest possible value for water potential
what does a solute do to the water potential
the presence of a solute in water lowers the water potential below 0,
all solutions have negative water potentials - the more concentrated the solution the more negative the water potential
how does water move between water potentials
There will be a net movement of water from the solution with the higher water potential (less concentrated) to the solution with the lower water potential (more concentrated).
This will continue until the water potential is equal on both sides of the membrane (equilibrium)
what is hydrostatic pressure and what is its units
The diffusion of water into a solution leads to an increase in volume of this solution.
If the solution is in a closed system, such as a cell, this results in an increase in pressure.
This pressure is called hydrostatic pressure.
It has the same units as water potential, kPa.
At the cellular level this pressure is relatively large and potentially damaging
what is cytolysis (in terms on osmosis of an animal cell)
If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell.
These cells have thin cell-membranes (7nm) and no cell walls. The cell-surface membrane cannot stretch much and cannot withstand the increased pressure. It will break and the cell will burst, an event called cytolysis
what is crenation (in terms on osmosis of an animal cell)
If an animal cell is placed in a solution that has a lower water potential than the cytoplasm it will lose water to the solution by osmosis down the water potential gradient.
This will cause a reduction in the volume of the cell and the cell-surface membrane to ‘pucker’
how is cytolysis and crenation prevented within multicellular animals
multicellular animals usually have control mechanisms to make sure their cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic).
In blood the aqueous solution is blood plasma
whats the difference between animals and plants ability to control water potential
unlike animals, plants are unable to control the water potential of the fluid around them, for example, the roots are usually surrounded by almost pure water
what is turgor and what is meant by turgid
Plants cells have strong cellulose walls surrounding the cell-surface membrane. When water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell walls. This pressure against the cell wall is called turgor.
As the turgor pressure increases it resists the entry of further water and the cell is said to be turgid
what is meant by plasmolysed
When plant cells are placed in a solution with a lower water potential than their own, water is lost from the cells by osmosis.
This leads to a reduction in volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall - the cell is said to be plasmolysed
