~Osmosis
~Diffusion
Some basic definitions:
Concentration--The amount(mass) of substance (solute)/volume of fluid
so High concentration means more of the substance while LOW concentration means less of the substance is inside.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
It is a spontaneous (no external force/energy needed) process.
Factors affecting it are...
1) Temperature
2) Size of molecule/atom
Substances tend to spread from an area where they are more concentrated to an area where they are less concentrated.
Two or more substances can become evenly distributed (reach equilibrium) EVEN without external interventions.
Change in concentration between 2 regions is known as the concentration gradient.
Change in concentration is known as the concentration gradient.
The more the concentration, the lesser the distance. The particles diffuse downwards.
Particles diffuse down the concentration gradient.
The larger the concentration gradient, the faster the rate of diffusion.
Applications of diffusion in Biology:
for unicellular organisms, diffusion is used to obtain nutrients and remove waste via a membrane.
Chemical substances must be able to move from one place to another in order to keep the living organisms alive and growing.
E.G. Food substances that were absorbed need to
~Move from one cell to another.
~Move in and out of the cell.
~Move from 1 part of the cell to another.
2 Types of membranes:
Permeable: allows ALL substances to pass through.
Partially permeable: allow SOME substances to pass through only.
Conclusion:
Diffusion is an important process where substances are moved without use of energy.
It is the net movement of particles (or molecules or ions) from a region of higher concentration to a region of lower concentration.
Thus the movement is down a concentration gradient.
It is important to bear in mind that the movement is random and the greater the concentration gradient, the faster the rate of diffusion.
Osmosis
Osmosis is the net movement of water molecules down the concentration gradient through a partially permeable membrane.
Water/The solvent moves freely through pores in the partially permeable membrane
The solute is too large to pass through.
Diffusion vs. Osmosis
Diffusion:
The movement of particles in general.
Can occur both in the presence and
absence of a membrane.
Osmosis:
Movement of water molecules only.
Water molecules move across a partially
Water molecules move across a partially
permeable membrane.
Water potential (ψ phi)
Water potential (ψ) is a measure of the tendency
of water molecules to move from one area to
another.
Thus we can redefine Osmosis as:
The net movement of water through a selectively permeable membrane from a region of high water potential (dilute or hypotonic) to a region of low water potential (hypertonic).
Hypotonic--Dilute solution compared to surrounding
Hypertonic--Concentrated solution compared to surrounding
Isotonic--Same concentration as surrounding
In cells
When placed in a hypertonic solution, water from the cell enters the solution through osmosis and becomes plasmolysed (plant) [Means it shrinks]
When placed in a hypotonic solution water enters the animal cell through osmosis and under goes cytolysis (bursts).
Plant cell
Plant cells are structurally more complex.
They are surrounded by a cellulose cell wall
which is…
– Freely permeable to water--Permeable
– Not elastic
– Able to resist cell expansion
When placed in a hypertonic solution, water from the cell enters the solution through osmosis and becomes plasmolysed (plant) [Means it shrinks] Water leaves the cytoplasm and vacuole by osmosis. The cytoplasm and vacuole shrinks, pulling the cell membrane away from the cell wall. The cell is now plasmolysed or in a state of plasmolysis. The tissue becomes flaccid.
When placed in a hypotonic solution water enters the vacuole by osmosis. The vacuole swells, pushing the cytoplasm against the cell wall. The inelastic cell wall resists expansion and the becomes rigid, or turgid. It can be described as a state of turgor.
Young plants, which have little woody tissue, rely on turgor for support against wind and gravity.
Note that when the cell membrane is separated slightly, then it is flaccid. If it is severely separated, then it is plasmolysed.
No longer fully filled with water, the tissue loses support and becomes flaccid.
A non-woody plant which loses lots of water has many plasmolysed cells and as a result the plant wilts.
Plasmolysis vs. Crenation
Plasmolysis is only used for plant cells (the shrinking of plant cell cytoplasm) and cell membrane moves away from cell wall
Crenation is only used for animal cell
Isotonic solution
An isotonic solution has the same concentration of solutes as the cytoplasm.
-> No net movement of water molecules into or out of the cell (animal/plant cell)
Therefore, cells neither shrink nor expand when placed in isotonic solution.
Summary of the processes that occur due to osmosis
Crenation--when animal cells are placed in hypertonic solution and thus losing water to surroundings
Cytolysis--when animal cells are placed in hypotonic solution and thus bursting as gains water from surroundings
Plasmolysis--When plant cells are placed in a hypertonic solution and lose water, thus becoming flaccid
Turgor--When plant cells are place in a hypotonic solution and gain water from surroundings, and the cellulose cell wall prevents cell from bursting by exerting an outward force
